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New bifunctional catalysts for ammonia-borane dehydrogenation, nitrile reduction, formic acid dehydrogenation, lactic acid synthesis, and carbon dioxide reduction
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New bifunctional catalysts for ammonia-borane dehydrogenation, nitrile reduction, formic acid dehydrogenation, lactic acid synthesis, and carbon dioxide reduction
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Content
NEW BIFUNCTIONAL CATALYSTS FOR AMMONIA-BORANE
DEHYDROGENATION, NITRILE REDUCTION, FORMIC ACID
DEHYDROGENATION, LACTIC ACID SYNTHESIS, AND CARBON
DIOXIDE REDUCTION
By
Zhiyao Lu
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
in Partial Fulfilment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMISTRY)
August 2016
i
I would like to thank my advisor Dr. Travis Williams for presenting the tremendous
opportunity to study as his PhD student. There is a lot to learn from him: his research approach
is scrupulous and tactical; above all he is fulfilled with a true passion for the pursuit of knowledge.
And he has a way to label his graduate students as “product of Williams”. I wish you a truckload of
success in the future.
Much gratitude must be given to my graduate committee members: Professors G. K. Surya
Prakash, Karl Christe, Charles McKenna, and John Petruska for their time, suggestion and support.
Furthermore, I would like to thank all the Williams group members I have overlapped with:
Brian Conley, Anna Dawsey, Vincent Li, Xinping Wu, Jeff Celaje, Xingyue Zhang, Ivan Demianets,
Valery Cherepakhin, Christina Ratto, Ana Victoria Flores, Brock Malinoski, Forrest Zhang, Elyse
Kedzie, Lisa Kam, Lena Foellmer, and Nicky Terrile. I appreciate your company: I enjoy
publishing papers and making jokes about Travis together.
To all the other excellent chemists I have worked with over the years – Ralf Haiges, Rasha
Hamze, Socrates Munoz, Matt Greaney – I would like to extend my appreciation. Without a doubt
I would not have overcome the challenges completing this thesis not having help from these
terrific friends.
Thanks to the excellent staff of LHI and USC Department of Chemistry: Carole Phillips,
Jessy May, David Hunter, Dr. Robert Anizfeld, Michele Dea, Marie de la Torre, Allan Kershaw,
Magnolia Benitez and Katie McKissick. In addition, I would like to thank the Sonosky families for
your support and generosity.
ii
To my supporting friends that are still striving in their PhD studies – Anna Batt, Hye Jung
Lee – I wish you a fun and fruitful journey.
Without a doubt no one deserves any more credit than Jialin Xiao, who has shown extreme
tolerance of my undoubtedly immature conduct and provided unconditional support towards my
finishing this challenging thesis. I am forever grateful and I look forward to our years ahead.
Particularly, my mother and father have made this all possible. I hope I have made you both
proud.
iii
List of Tables ................................................................................................................................................ viii
List of Figures .................................................................................................................................................. ix
List of Schemes ........................................................................................................................................... xvii
Abstract ........................................................................................................................................................... xx
Chapter 1. Recent Advances in Homogeneous Catalytic Hydrogenation and Dehydrogenation 1
1.1 Introduction: ......................................................................................................................................1
1.2. Hydrogenation: ...............................................................................................................................3
1.2.1. CO
2
Hydrogenation ........................................................................................................... 3
1.2.2. Ester, amide and nitrile hydrogenation .......................................................................... 9
1.3 Dehydrogenation: ......................................................................................................................... 13
1.3.1. Methanol dehydrogenation ............................................................................................ 13
1.3.2. Formic Acid Dehydrogenation ...................................................................................... 21
1.3.3. Ammonia-borane dehydrogenation ............................................................................. 24
1.4. Overview ........................................................................................................................................ 27
1.5. References ...................................................................................................................................... 28
Chapter 2. A Three-Stage Mechanistic Model for Ammonia−Borane Dehydrogenation by
Shvo’s Catalyst .............................................................................................................................................. 42
2.1 Introduction: ................................................................................................................................... 42
2.2 Results and Discussion ................................................................................................................. 45
iv
2.2.1. Catalyst Initiation (Case 1) ............................................................................................ 47
2.2.2. Fast Catalysis (Case 2) .................................................................................................... 48
2.2.3. Slow Catalysis (Case 3) .................................................................................................. 50
2.2.4. Semi-Site Protection Mechanism ................................................................................. 65
2.3 Conclusion ...................................................................................................................................... 67
2.4 Experimental Section.................................................................................................................... 69
2.4.1. General Procedures. ......................................................................................................... 69
2.4.2. Mechanistic Studies Utilizing
11
B and
1
H NMR Spectroscopy. .............................. 70
2.5 References ....................................................................................................................................... 90
Chapter 3. A Dual Site Catalyst for Mild, Selective Nitrile Reduction .............................................. 98
3.1 Introduction: ................................................................................................................................... 98
3.2. Results and Discussion ................................................................................................................ 99
3.2.1. Mechanism of Alcohol Oxidation by 3.2 ................................................................... 100
3.2.2. Synthesis of Ruthenium Complex 3.3 ........................................................................ 104
3.2.3. Nitrile Reduction ............................................................................................................ 106
3.2.4. Mechanistic Proposal .................................................................................................... 112
3.3. Conclusion ................................................................................................................................... 114
3.4. Experimental Section ................................................................................................................. 114
3.4.1. General Procedure ......................................................................................................... 114
3.4.2. Synthesis and Structural Data ...................................................................................... 116
3.4.3. Kinetic Data for Reduction of 3.3 ............................................................................... 142
v
3.5. References .................................................................................................................................... 143
Chapter 4. Mechanism of Neat Formic Acid Dehydrogenation by a Novel Dimeric Ir
Catalyst ......................................................................................................................................................... 149
4.1 Introduction: ................................................................................................................................. 149
4.2. Results and Discussion .............................................................................................................. 153
4.2.1. Kinetic Isotope Effects and Kinetic Dependence .................................................... 153
4.2.2. Spectroscopic Studies on Catalyst Initiation and Reactive Intermediate ........... 156
4.3. Conclusion on the Mechanism................................................................................................ 163
4.4. Formic Acid Dehydrogenation by Bidentate Ligand Supported Iridium Catalysts......165
4.5. Experimental Section. ................................................................................................................ 167
4.5.1. General Procedures ........................................................................................................ 167
4.5.2. Dehydrogenation Procedures ...................................................................................... 168
4.5.3. Synthesis and Characterization ................................................................................... 169
4.5.4. Observation of Catalyst Intermediates by NMR ..................................................... 173
4.5.5. FA Dehydrogenation by Various Iridium Catalysts ................................................ 174
4.6. References .................................................................................................................................... 174
Chapter 5. A Prolific Catalyst for Selective Conversion of Neat Glycerol to Lactic Acid ........... 179
5.1 Introduction .................................................................................................................................. 179
5.2. Results and Discussion .............................................................................................................. 180
5.2.1. Synthesis of Iridium Catalysts ...................................................................................... 180
5.2.2. Glycerol Dehydrogenation ........................................................................................... 182
vi
5.2.3. Further Utility Studies ................................................................................................... 185
5.2.4. Mechanistic Studies ....................................................................................................... 186
5.3. Conclusion ................................................................................................................................... 190
5.4. Experimental Section ................................................................................................................. 191
5.4.1. Materials and Methods .................................................................................................. 191
5.4.2. Dehydrogenation Procedures ...................................................................................... 192
5.4.3. Synthesis Procedures and Characterization Data .................................................... 194
5.4.4. Initial Condition Optimization ................................................................................... 205
5.4.5. Homogeneity Tests ........................................................................................................ 207
5.4.6. Conversion of Soybean Oil to FAMEs and Lactic Acid ......................................... 208
5.4.7. Mechanistic Study .......................................................................................................... 214
5.5. References .................................................................................................................................... 224
Chapter 6. Di(carbene)-Supported Nickel Systems for CO
2
Reduction Under Ambient
Conditions ................................................................................................................................................... 229
6.1. Introduction ................................................................................................................................ 229
6.2. Results and Discussion .............................................................................................................. 231
6.2.1. Design and Synthesis of Novel Nickel Catalysts ...................................................... 231
6.2.2. Room Temperature CO
2
Reduction to Methanol .................................................. 233
6.2.3 Mechanistic Discussion .................................................................................................. 235
6.3. Conclusion ................................................................................................................................... 239
6.4. Experimental Section ................................................................................................................. 240
vii
6.4.1. Materials and Methods .................................................................................................. 240
6.4.2. CO
2
Reduction Procedures .......................................................................................... 241
6.4.3. Synthesis Procedures and Characterization Data .................................................... 242
6.4.4. CO
2
Reduction conditions ........................................................................................... 261
6.4.5. Formic acid Reduction by NaBH
4
.............................................................................. 264
6.4.6. Kinetic Data .................................................................................................................. 271
6.5. References .................................................................................................................................... 277
Appendix. X-Ray Crystallography data .................................................................................................. 283
Crystal Structure of 3.3 ..................................................................................................................... 283
Crystal Structure of 4.3b ................................................................................................................... 294
Crystal Structure of 5.5 ..................................................................................................................... 303
Crystal structure of 5.9 ...................................................................................................................... 317
Crystal structure of 6.6 ...................................................................................................................... 326
Crystal structure of 6.1 ...................................................................................................................... 333
Crystal structure of 6.2 ...................................................................................................................... 343
Crystal structure of 6.9 ...................................................................................................................... 358
Crystal structure of 6.10 .............................................................................................. 365
viii
Table 2.1. Ammonia-Borane Consumption as a Function of [AB] and [Ru] in Slow Catalysis. 51
Table 2.2. Ammonia Borane Dehydrogenation Catalyzed by Borylated Complex 2.24. .............. 59
Table 2.3. Rate of Slow Catalysis in the Presence and Absence of BH
3
............................................ 61
Table 2.4. [AB] Dehydrogenation with 2.14 and 2.26 ......................................................................... 62
Table 3.1. Optimization of Nitrile Reduction Conditions ........................................................ 107
Table 3.2. Scope of 3.3-Catalyzed Nitrile Reduction .................................................................. 110
Table 3.3. Nitrile Reduction Condition Screening. ............................................................................ 130
Table 4.1. Kinetic Studies of 4.1 Catalyzed FA Dehydrogenation ................................................... 155
Table 5.1. Dehydrogenation of Neat Glycerol to Lactic Acid. .......................................................... 183
Table 5.2. Initial Conditions Attempted for a Conversion of Glycerol to Lactic Acid. ............... 206
Table 5.3. Homogeneity Test Using Various Poisons ........................................................................ 207
Table 6.1. Formate Conversion to Methoxides ................................................................................... 236
ix
Figure 1.1. Ruthenium Pincer Catalysts for Ester and Amide Hydrogenation ............................... 11
Figure 1.2. A Ru-pincer complex for nitrile hydrogenation ................................................................. 12
Figure 1.3. Noyori Type Hydrogenation Catalysts Used in Ester, Amide and Nitrile
Reductions ..................................................................................................................................................... 13
Figure 1.4. Methanol Dehydrogenation by Iron Complexes 1.47, 1.48 ........................................... 17
Figure 1.5. Equilibria among Complex 1.55, 1.30 and 1.56 ................................................................. 20
Figure 1.6. Equilibria among 1.57, and 1.58 and 1.59, Unreactive Iridium Complexes 1.60
and 1.61. ......................................................................................................................................................... 21
Figure 1.7. Hydrogen Evolution from Ammonia-Borane .................................................................... 25
Figure 1.8. Ammonia Borane Dehydrogenation by Iridium Pincer Complex 1.71. ....................... 25
Figure 1.9. Hydrogen Evolution from Ammonia-Borane by the Ni-NCH System ........................ 26
Figure 2.1. Ammonia-Borane Dehydrogenation Reactions and Possible Products ....................... 43
Figure 2.2. Transition-Metal Catalysts for AB dehydrogenation ....................................................... 44
Figure 2.3. Kinetic Profiles of AB Dehydrogenation by 2.14 .............................................................. 46
Figure 2.4. [AB] Dehydrogenation with 2.14 in presence of 1,10-phenanthroline ........................ 65
Figure 2.5 Reduction of 2.22 by ammonia borane ................................................................................ 71
Figure 2.6. Ruthenium Hydride Species in NMR Spectra ................................................................... 72
Figure 2.7. [AB] vs. Time Monitored by
11
B NMR in 2:1 Diglyme/Benzene-d
6
with Varying
Catalyst Concentrations in the Slow Catalysis Case ............................................................................. 74
x
Figure 2.8. [AB] vs. Time Monitored by
11
B NMR in 2:1 diglyme/benzene-d
6
with Varying
[AB] in the Slow Catalysis Case ................................................................................................................ 76
Figure 2.9. Kinetic Isotope Effects in Dehydrogenation by 5 mol % 2.14 ........................................ 78
Figure 2.10. H/D Exchange ....................................................................................................................... 79
Figure 2.11.
11
B NMR Spectrum in Presence of Added BH
3
· THF .................................................... 79
Figure 2.12. Kinetic Studies on the Effect of Added BH
3
-THF .......................................................... 80
Figure 2.13.
1
H and
13
C NMR of 2.26 taken in pyridine-d
5
at 25 ˚C ................................................. 82
Figure 2.14. Kinetic Studies of AB Dehydrogenation by 2.26 ............................................................ 83
Figure 2.15. Borazine hydroboration of 2.22 .......................................................................................... 84
Figure 2.16.
1
H NMR taken in benzene-d
6
at 25
o
C of a reaction between 2.14 and borazine ..... 85
Figure 2.17. Kinetic Studies of AB Dehydrogenation by 2.24 ............................................................ 86
Figure 2.18. AB Dehydrogenation by 2.14 in Presence of 1,10-Phenanthroline ............................ 88
Figure 2.19. AB Dehydrogenation by 2.24 in Presence or Absence of 1,10-Phenanthroline ....... 89
Figure 3.1. Dual Site Catalysts for Hydride Manipulation .......................................................... 99
Figure 3.2. Oxidation of 3.12 by Ruthenium Catalysts....................................................................... 103
Figure 3.3.
1
H-NMR spectrum for 3.4.................................................................................................... 117
Figure 3.4.
13
C-NMR spectrum for 3.4................................................................................................... 117
Figure 3.5.
11
B-NMR (
1
H coupled) spectrum for 3.4 ......................................................................... 117
Figure 3.6.
1
H-NMR spectrum for 3.3.................................................................................................... 120
Figure 3.7.
13
C-NMR spectrum for 3.3................................................................................................... 120
Figure 3.8.
11
B-NMR (
1
H coupled) spectrum for 3.3 ......................................................................... 120
xi
Figure 3.9.
19
F-NMR spectrum for 3.3 ................................................................................................... 120
Figure 3.10.
1
H-NMR spectrum for 3.19 ............................................................................................... 122
Figure 3.11.
13
C-NMR spectrum for 3.19 .............................................................................................. 122
Figure 3.12
11
B spectrum for 3.19 ........................................................................................................... 123
Figure 3.13. Graphical
1
H-
1
H COSY spectrum for 3.19 ..................................................................... 123
Figure 3.14. Time Course Graphs for Ammonia Borane Dehydrogenation and Water
Oxidation with Catalyst Precursors 3.2 and 3.3 ................................................................................... 126
Figure 3.15.
1
H-NMR Spectrum of 3.8a. ............................................................................................... 132
Figure 3.16.
13
C-NMR Spectrum of 3.8a. .............................................................................................. 132
Figure 3.17.
1
H-NMR Spectrum of 3.8b ................................................................................................ 133
Figure 3.18.
13
C-NMR Spectrum of 3.8b ............................................................................................... 133
Figure 3.19.
1
H-NMR Spectrum of 3.8d. ............................................................................................... 134
Figure 3.20.
13
C-NMR Spectrum of 3.8d. .............................................................................................. 134
Figure 3.21.
1
H-NMR Spectrum of 3.8e. ............................................................................................... 135
Figure 3.22.
13
C-NMR Spectrum of 3.8e. .............................................................................................. 135
Figure 3.23.
1
H-NMR Spectrum of 3.8f. ................................................................................................ 136
Figure 3.24.
13
C-NMR Spectrum of 3.8f. ............................................................................................... 136
Figure 3.25.
1
H-NMR Spectrum of 3.8g. ............................................................................................... 137
Figure 3.26.
1
H-NMR Spectrum of 3.8g. ............................................................................................... 137
Figure 3.27.
1
H-NMR Spectrum of 3.8h. ............................................................................................... 138
Figure 3.28.
13
C-NMR Spectrum of 3.8h. .............................................................................................. 138
xii
Figure 3.29.
1
H-NMR Spectrum of 3.9i ................................................................................................. 139
Figure 3.30.
13
C-NMR Spectrum of 3.9i ................................................................................................ 139
Figure 3.31.
1
H-NMR Spectrum of 3.9j ................................................................................................. 140
Figure 3.32.
13
C-NMR Spectrum of 3.9j ................................................................................................ 140
Figure 3.33. Kinetic Conversion of 3.3 to 3.10 ..................................................................................... 142
Figure 4.1. Formic Acid Dehydrogenation by Iridium Catalyst 4.1 ................................................ 151
Figure 4.2. Catalyst Initiation by
1
H-NMR ........................................................................................... 156
Figure 4.3.
1
H-NMR Hydride Region of the Extracted Iridium Catalyst ....................................... 157
Figure 4.4.
1
H-NMR of Hydrogenation of 4.1 ..................................................................................... 159
Figure 4.5. Hydride Region of Synthetic Ir Dimers ............................................................................ 159
Figure 4.6.
1
H NMR Hydride Region of FA ligated Ir Dimer. .......................................................... 160
Figure 4.7.
1
H-NMR Hydride Region. Top: Decompose Mixture from 4.12. Bottom: 4.4 ....... 160
Figure 4.8.
1
H-NMR of Acetate or Formate Bridged Iridium Dimers ............................................ 162
Figure 4.9.
1
H NMR spectrum of complex 4.3b at 25 ° C in CD
2
Cl
2
. ............................................... 170
Figure 4.10.
13
C NMR spectrum of complex 4.3b at 25 ° C in CD
2
Cl
2
............................................ 171
Figure 4.11.
31
P NMR spectrum of complex 4.3b at 25 ° C in CD
2
Cl
2
............................................. 171
Figure 4.12.
19
F NMR spectrum of complex 4.3b at 25 ° C in CD
2
Cl
2
............................................. 172
Figure 5.1. Apparatus Setup for Dehydrogenation Reactions and Typical Kinetic Profile......... 193
Figure 5.2.
1
H NMR spectrum of complex 5.5 at 25 ° C in CD
2
Cl
2
.................................................. 196
Figure 5.3.
13
C NMR spectrum of complex 5.5 at 25 ° C in CD
2
Cl
2
................................................. 196
Figure 5.4.
19
F NMR spectrum of complex 5.5 at 25 ° C in CD
2
Cl
2
.................................................. 197
xiii
Figure 5.5.
1
H NMR spectrum of complex 5.6 at 25 ° C in CD
2
Cl
2
.................................................. 199
Figure 5.6.
13
C NMR spectrum of complex 5.6 at 25 ° C in CD
2
Cl
2
................................................. 200
Figure 5.7.
19
F NMR spectrum of complex 5.6 at 25 ° C in CD
2
Cl
2
.................................................. 200
Figure 5.8. Infrared Spectrum of Complex 5.6 ..................................................................................... 201
Figure 5.9.
1
H NMR Spectrum of Complex 5.9 at 25 ° C in CD
2
Cl
2
................................................ 204
Figure 5.10.
13
C NMR Spectrum of Complex 5.9 at 25 ° C in CD
2
Cl
2
. ............................................ 204
Figure 5.11.
19
F NMR Spectrum of Complex 5.9 at 25 ° C in CD
2
Cl
2
.............................................. 205
Figure 5.12.
1
H-NMR Spectrum of the Glycerol Isolated from a Transesterification Product
of Wesson Soybean Oil. ............................................................................................................................ 208
Figure 5.13. A Snapshot of Reaction Mixture after 3 Days. ............................................................... 209
Figure 5.14.
1
H NMR of Reaction Mixture after 7 days, at 90% Conversion ................................. 210
Figure 5.15.
1
H NMR of Isolated Lactic Acid in D
2
O ......................................................................... 211
Figure 5.16.
1
H NMR of Poly(Lactic Acid) Oligomer in DMSO-d
6
. .............................................. 212
Figure 5.17.
1
H NMR “Zoom-in” on "Methine" Region of the Poly(Lactic Acid) Oligomer .... 212
Figure 5.18.
1
H NMR of rac-Lactide in DMSO-d
6
. ............................................................................. 213
Figure 5.19. H
2
Evolution at Room Temperature from Dehydrogenation of Isopropanol
by Iridium Catalyst 5.5 in Presence of KOH ........................................................................................ 215
Figure 5.20. H
2
Evolution at Room Temperature from Dehydrogenation of Isopropanol
by Iridium Catalyst 5.9 in Presence of KOH ........................................................................................ 216
Figure 5.21. Formation of Cyclooctane from Ligand COD in Iridium Compound 5.5 .............. 216
xiv
Figure 5.22. Room Temperature Deprotonation at Ligand –CH
2
– to Iridium Catalyst 5.9
in CD
3
OD .................................................................................................................................................... 217
Figure 5.23. Room Temperature Deprotonation at Ligand –CH
2
– to Iridium
Catalyst 5.9 in CD
3
CN .............................................................................................................................. 218
Figure 5.24. Glycerol Conversion to Glyceraldehyde by Iridium Catalyst 5.9 in CD
3
CN ......... 219
Figure 5.25. KIE Study of Phenylethyl Alcohol Dehydrogenation by Catalyst 5.9 ...................... 221
Figure 5.26. Kinetic Dependence of Phenylethyl Alcohol ................................................................. 222
Figure 5.27. Kinetic Profile of Dehydrogenation of 4-Methoxy-Phenylethyl Alcohol ................ 223
Figure 6.1. Catalytic CO
2
Reduction with Silanes and Boranes ....................................................... 230
Figure 6.2. Dual Center Manifold for CO
2
Reduction ....................................................................... 231
Figure 6.3. ORTEP Diagrams of 6.1 and 6.2 ........................................................................................ 233
Figure 6.4. Kinetic Profile of CO
2
Reduction by NaBH
4
Catalyzed by 1 and 2 ............................ 234
Figure 6.5.
1
H-NMR of CO
2
(1 atm) Reduction in THF by NaBH
4
and Catalyst 6.1 ................ 235
Figure 6.6. Reduction of Formates to Methoxides Monitored by NMR. ....................................... 237
Figure 6.7. Nickel Complexes 6.9, 6.10 and Their Crystal Structures ............................................ 239
Figure 6.8. Apparatus Set-up for the Synthesis of Borate Ligand 6.5 and 6.6 ................................ 243
Figure 6.9.
1
H NMR Spectrum of 6.5 at 25 ° C in CD
3
CN ................................................................. 244
Figure 6.10.
13
C NMR Spectrum of 6.5 at 25 ° C in CD
3
CN .............................................................. 244
Figure 6.11.
11
B NMR Spectrum of 6.5 at 25 ° C in CD
3
CN .............................................................. 245
Figure 6.12.
1
H NMR Spectrum of 6.6 at 25 ° C in CD
3
CN ............................................................... 248
Figure 6.13.
13
C NMR Spectrum of 6.6 at 25 ° C in CD
2
CN .............................................................. 248
xv
Figure 6.14.
11
B NMR Spectrum of 6.6 at 25 ° C in CD
3
CN .............................................................. 249
Figure 6.15.
1
H NMR Spectrum of 6.7 at 25 ° C in C
6
D
6
.................................................................... 251
Figure 6.16.
1
H NMR Spectrum of 6.1 at 25 ° C in CD
3
CN ............................................................... 253
Figure 6.17.
13
C NMR Spectrum of 6.1 at 25 ° C in CD
3
CN .............................................................. 254
Figure 6.18.
11
B NMR Spectrum of 6.1 at 25 ° C in CD
3
CN .............................................................. 254
Figure 6.19.
1
H NMR Spectrum of 6.9 at 25 ° C in CD
3
CN ............................................................... 256
Figure 6.20.
13
C NMR Spectrum of 6.2 at 25 ° C in CD
3
CN .............................................................. 257
Figure 6.21.
1
H NMR Spectrum of 6.9 at 25 ° C in CD
2
Cl
2
. ............................................................... 259
Figure 6.22.
13
C NMR Spectrum of 6.9 at 25 ° C in CD
2
Cl
2
. .............................................................. 259
Figure 6.23.
11
B NMR Spectrum of 6.9 at 25 ° C in CD
2
Cl
2
............................................................... 260
Figure 6.24. Stacked
1
H NMR Spectra of CO
2
Reduction in THF-d
8
............................................. 261
Figure 6.25.
1
H NMR Spectrum of CO
2
Reduction in THF after the First 14 h ........................... 263
Figure 6.26.
1
H NMR Spectrum of Reaction between Formic Acid and NaBH
4
in dry
Acetonitrile-d
3
after 1 d. ............................................................................................................................ 264
Figure 6.27.
1
H NMR Spectrum of Hydrolyzed Product of This Reaction. .................................. 265
Figure 6.28.
13
C NMR Spectrum of Hydrolyzed Product of This Reaction .................................. 265
Figure 6.29.
11
B-NMR Spectrum of Reaction between 6.2 and NaBH
4
. ......................................... 267
Figure 6.30.
1
H-NMR Spectrum of Reaction between 6.2 and NaBH
4
. ......................................... 267
Figure 6.31. Treatment of the Ni-H species with CO
2
in
1
H-NMR................................................. 268
Figure 6.32.
1
H-NMR Spectrum of Reaction between Ni(acac)
2
and NaBH
4
. ............................. 269
Figure 6.33.
11
B-NMR Spectrum of Reaction between Ni(acac)
2
and NaBH
4
. ............................ 269
xvi
Figure 6.34. Reduction of DCOONa to Methoxide by
2
H-NMR. ................................................... 270
Figure 6.35. The Kinetic Profile of Formate Reduction 1 ................................................................. 271
Figure 6.36. The Kinetic Profile of Formate Reduction 2 ................................................................. 272
Figure 6.37. The Kinetic Profile of Formate Reduction 3 ................................................................. 272
Figure 6.38. The Kinetic Profile of Formate Reduction 4 ................................................................. 273
Figure 6.39. The Kinetic Profile of Formate Reduction 5 ................................................................. 273
Figure 6.40. The Kinetic Profile of Formate Reduction 6 ................................................................. 274
Figure 6.41. The Kinetic Profile of Formate Reduction 7 ................................................................. 274
Figure 6.42. The Log-Log Plot for Formate Concentration Dependence ..................................... 275
Figure 6.43. The Log-Log Plot for Nickel Catalyst 6.1 Concentration Dependence .................. 275
Figure 6.44. The Kinetic Profile of Formate Reduction ..................................................................... 276
xvii
Scheme 1.1. Famous Hydrogenative Processes ....................................................................................... 2
Scheme 1.2 Proposed Catalytic Cycle for Hydrogenation of CO
2
by Ir catalyst 1.1 ........................ 4
Scheme 1.3. CO
2
to formate Hydrogenation by Iridium Catalyst 1.10. ............................................. 5
Scheme 1.4. Proposed Mechanism for CO
2
hydrogenation by 1.15 ................................................... 7
Scheme 1.5. Cascade Methanol Synthesis from CO
2
Hydrogenation ................................................ 8
Scheme 1.6. Direct CO
2
to Methanol Hydrogenation by Ru(triphos) Catalyst 1.18 ...................... 9
Scheme 1.7. Ru-triphos Systems for Hydrogenation of Esters and Amides .................................... 11
Scheme 1.8. Hydrogen Activation by Ru-Pincer Complexes. ............................................................. 11
Scheme 1.9. a) Methanol reforming; b) Envisioned Catalytic Dehydrogenation of Methanol .. 14
Scheme 1.10. Methanol Dehydrogenation by Beller’s Ruthenium Catalysts 1.39 and1.40. ........ 16
Scheme 1.11. Proposed Catalytic Cycle of Methanol Dehydrogenation by Ruthenium
Catalyst 1.49. ................................................................................................................................................. 19
Scheme 1.12. Proposed Catalytic Cycle of FA Decomposition by Iron Catalyst 1.62 .................. 23
Scheme 1.13. Proposed Mechanism for 1.69 Catalyzed FA Dehydrogenation. ............................. 24
Scheme 1.14. Ligand Metal Bifunctional Hydrogenation/Dehydrogenation Equilibria
among 1.72, 1.73, and 1.74 ......................................................................................................................... 27
Scheme 2.1. Dehydrogenation of AB by Shvo’s Catalyst ..................................................................... 44
Scheme 2.2. Catalyst Initiation .................................................................................................................. 48
Scheme 2.3. Proposed Mechanism of Fast Catalysis ............................................................................ 49
xviii
Scheme 2.4. H/D Exchange Experiments. .............................................................................................. 54
Scheme 2.5. Proposed Borazine-Mediated Hydroboration. ............................................................... 55
Scheme 2.6. Hydroboration and Hydrosilylation of Tol-2.22. ........................................................... 55
Scheme 2.7. Synthesis of a Mechanistic Analog for the Proposed Deactivated Catalyst
Complex. ........................................................................................................................................................ 58
Scheme 2.8. Formation of 2.6. ................................................................................................................... 60
Scheme 2.9. Synthesis of Ammonia Adduct 2.26. ................................................................................. 61
Scheme 2.10. Semi-Site Protection Mechanism .................................................................................... 66
Scheme 2.11. Mechanistic Proposal of Slow Catalysis. ........................................................................ 69
Scheme 3.1. Three Possible Mechanisms for Alcohol Oxidation by 3.2 ........................................ 101
Scheme 3.2. Labiality of μ-OR Groups in 3.2. ...................................................................................... 102
Scheme 3.3. Synthesis of Precursor 3.3 and Complex 3.6 ......................................................... 105
Scheme 3.4. Tandem Nitrile Reduction-Cyclization .................................................................. 112
Scheme 3.5. Mechanistic Template for Nitrile Reductions. ..................................................... 113
Scheme 4.1. Synthesis of Acetate Bridged Ir Dimer 4.3b ................................................................... 161
Scheme 4.2. Addition of a Formate Ligand to 4.3a ............................................................................. 162
Scheme 4.3. Proposed Mechanism for FA Dehydrogenation by Iridium Catalyst 4.1 ................ 164
Scheme 4.4. Synthesis of Iridium Complexes and Their FA Dehydrogenation
Reactivity Compared to 4.1. ..................................................................................................................... 166
Scheme 4.5. Formation of Iridium Dimer 4.3a .................................................................................... 173
xix
Scheme 5.1. (A) Formic Acid Dehydrogenation System 5.1/5.2, (B) Syntheses, and (C)
Molecular Structures of Novel Iridium Complexes 5.5 and 5.9 ....................................................... 181
Scheme 5.2. A Mechanistic Model for Catalytic Glycerol Dehydrogenation with 5.9. ............... 229
Scheme 6.1. Synthesis of nickel compounds 6.1 and 6.2. .................................................................. 232
Scheme 6.2. A Plausible Mechanism for CO
2
Reduction by NaBH
4
and Catalyst 6.1. ............... 239
xx
This thesis focuses on new catalytic chemistry of hydride manipulation for processes such
as hydrogenation and dehydrogenation. The research projects mostly involve design and
synthesis of novel catalysts as well as mechanistic analysis of catalytic processes. Such mechanistic
understandings lead to improvement in catalysis in terms of efficiency, selectivity, and catalyst
longevity. These are introduced in Chapter one.
Chapter two proposes and provides evidence for a mechanistic model for three-stage
dehydrogenation of ammonia−borane (AB) catalyzed by Shvo’s cyclopentadienone-ligated
ruthenium complex. In particular, we characterized the mechanism of catalyst deactivation: a
borazine-mediated hydroboration of the active catalyst to afford less reactive ruthenium species.
We are inspired by these findings to discover catalysts that do not suffer from the same
deactivation chemistry So doing, we either use a “semi-site protection strategy” or introduce a
borate ligand into the catalyst.
Chapter three shows complex {[(κ
3
-(N,N,O)-py
2
B(Me)OH)Ru(NCMe)
3
]}
+
TfO
−
is not
a ligand-metal bifunctional catalyst and in case of alcohol oxidation, the mechanism most likely
involves reactivity only at the ruthenium center. Further we present a novel ruthenium
bis(pyrazolyl)borate scaffold that enables cooperative reactivity in which boron and ruthenium
centers work in concert to effect selective nitrile reduction. The pre-catalyst compound {[κ
3
-(1-
pz)
2
HB(N=CHCH
3
)]Ru(cymene)}
+
TfO
−
(pz = pyrazolyl) was synthesized using readily-
xxi
available materials through a straightforward route, thus making it an appealing catalyst for a
number of reactions.
In chapter four, a prolific homogeneous iridium catalyst is introduced for selective
dehydrogenation of neat formic acid under mild conditions. Mechanistic study on this catalysis
reveals that the promising reactivity is enabled by in situ formation of a novel dimeric iridium
species {[(P-N)Ir(CH
2
Cl
2
)(H)]
2
(μ-H)(μ
2
-OCHO)
2
}
+
TfO
−
.
Chapter five describes the synthesis and reactivity of a very robust iridium catalyst for
glycerol to lactate conversion. The high reactivity and selectivity of this catalyst enable a sequence
for the conversion of biodiesel waste stream to lactide monomers for the preparation of poly(lactic
acid). Furthermore, experimental data collected with this system provide a general understanding
of its reactive mechanism.
Chapter six recounts the design and synthesis of structurally novel di(carbene)-supported
nickel species, which are efficient catalysts for the room-temperature reduction of CO
2
to
methanol in the presence of sodium borohydride. The catalysts feature unprecedented stability,
particularly for a base metal catalyst, enabling > 1,100,000 turnovers of CO
2
. Moreover, while
other systems involve more expensive or air-sensitive borane reagents, sodium borohydride is
inexpensive and easily handled. Further, effecting reduction in the presence of water enables
direct access to methanol without a subsequent hydrolysis step. Preliminary mechanistic data
collected for the catalysis are most consistent with a mononuclear nickel active species that
xxii
mediates rate-determining reduction of a boron formate.
1
1.1. Introduction
Hydrogen exists ubiquitously among almost all types of molecules, from substance as
simple as water, ammonia to such large, complicated biopolymers as protein and nucleic acid.
After all, it is the most abundant element in the universe. Processes that are primarily
hydrogenation or dehydrogenation are practiced every day both in nature and in industry. One of
the world’s most famous hydrogenations is photosynthesis. Plants turn carbon dioxide into
carbohydrates storing 4 × 10
18
kJ energy per year.
1
Another example for hydrogenation is the
Haber process. The named artificial nitrogen fixation reaction serves as the main nitrogen source
in agriculture around the world, which ultimately accounts for nearly 80% of the total nitrogen
atoms in our body.
2
Dehydrogenation, naturally, is a class of equally valuable transformations. An
example in industry is the dehydrogenation of ethylbenzene to styrene. Styrene is an
indispensable industrial feedstock and is widely used as a precursor in many polymer syntheses
(Scheme 1.1).
3
Over the past few decades, research to improve performance of hydrogenation
and dehydrogenation processes have been exceptionally popular in science laboratories. New
technologies that can contribute to preserve energy or the environment are highly anticipated for
a sustainable future.
2
Scheme 1.1. Hydrogenative Processes: A) Calvin cycle in photosynthesis; B) ammonia formation
from N
2
hydrogenation; C) styrene production.
Catalysis might be the solution. Generally, a good catalyst offers faster reaction rate, lower
energy requirement, besides a useful longevity of itself.
4
In nature, enzymes enable photosynthesis
to occur smoothly at ambient temperature, and the Haber process uses transition metal catalysts
for higher efficiency.
5
Two approaches diverge in catalysis, namely the heterogeneous catalysis
and the homogeneous catalysis. On one hand, heterogeneous catalysts are cheaper, easier to
prepare and to recover, and are resistant to decomposition by heat and moisture: desirable
qualities for large scale production.
6
On the other hand, homogeneous catalyses are usually more
selective and efficient, and more significantly they provide opportunities to elude the mechanism
of a catalytic transformation for scientific purposes.
6
For example, catalytic hydride activation and
hydride transfer are often key steps in hydrogenation and dehydrogenation events. The most
convenient way to study the fate of a hydride species is by using well defined small molecules. The
results of such research usually enable the discovery of new reactions and the invention of new
catalysts. As researchers in basic science, we will be focused on homogeneous catalysis in the
following discussion.
3
1.2. Hydrogenation
1.2.1. CO
2
Hydrogenation
Carbon dioxide is a C1 small molecule that naturally exists in the atmosphere, as one of
the most important forms in the carbon cycle.
7
However, in the past 150 years, the CO
2
concentration has reached the highest level in the past 420000 years due to human activities,
8
and is still escalating acceleratively from mass fossil fuel combustion. The greenhouse effect
from CO
2
has caused global concerns of climate change, rising sea levels, and etc. Thus, new
technology that can enable a more balanced carbon cycle appears to be an impending research
target.
9
More specifically, CO
2
hydrogenation to store energy chemically has become an intense
research area.
10
1.2.1.1. CO
2
Hydrogenation to Formic Acid (FA)
In the last few years, a number of catalysts have been developed for CO
2
to formic acid
conversion.
11
In 2009 the Nozaki group reported one of the most prolific catalyst for
hydrogenation of CO
2
to formic acid to date.
11d
The PNP pincer ligand supported Ir-H
3
complex
1.1, impressively tolerates high temperatures (120–200
o
C) and high pressures (50 -60 bar) to
deliver 150000 h
-1
TOF and 3500000 TON in optimized aqueous reactions. In a follow-up,
detailed mechanistic study, they demonstrated a plausible ligand-metal cooperative process for
H
2
activation. With the aid of computational study they proposed a hydrogenation mechanism
(Scheme 1.2).
4
Computational results as well as experimental observation suggest that, there are two
competing catalytic cycles: 1.1-1.2-1.3-1.4-1.5-1.6-1.7-1.8-1.1 and/or 1.1-1.2-1.3-1.4-1.9-1.1.
The rate determining steps (RDS) were believed both to be deprotonation, from 1.5 to 1.6 or
from 1.9 to 1.1, respectively. However, the deprotonation seems less likely to be the RDS because
they also observed that a higher concentration of KOH resulted in lower reactivity. More
interestingly, they noticed that the reaction rate has kinetic dependence on H
2
pressure and CO
2
pressure, but according to their proposed mechanism the pressures should have little effect on the
RDS. Despite the unconvincing mechanistic proposal, this work presents an outstanding Ir system
for CO
2
hydrogenation to formate.
Scheme 1.2 Proposed Catalytic Cycle for Hydrogenation of CO
2
by Ir catalyst 1.1.
5
In 2011, Hull, Himeda, and Fujida reported an iridium complex that is modulated by the
solution pH to catalyze CO
2
to formic acid hydrogenation, or formic acid dehydrogenation.
11t
To
the best of our knowledge, this is the first homogeneous system that is active under 1 atm total
pressure for CO
2
hydrogenation.
Scheme 1.3. CO
2
to formate Hydrogenation by Iridium Catalyst 1.10.
Iridium catalyst 1.11 features a bipyrimidine backbone with four hydroxyl groups (Scheme
1.3). The authors rationalize that the iridium and one deprotonated hydroxyl can heterolytically
split H
2
to yield Ir-H complex 1.11. Furthermore, the depronated hydroxyl groups push electron
density towards the metal center to generate more electron rich hydride species, which is
6
beneficial to the hydrogenation. Gas-phase free energy calculations give negative values to both
H
2
activation and formate dissociation, but a positive value to CO
2
insertion, which suggests a
CO
2
insertion RDS. This is in consistence with the observation of the Ir-H species as the long-
lived species in the reaction solution. To further rationalize their design, similar iridium complexes
1.13 and 1.14 were synthesized and subjected to the same catalytic conditions. In contrast, these
two species lack in number of hydroxyl groups for the catalyst deprotonation, and are
consequently showed much less reactive.
In 2013, research in first row transition metals continues to rise. Highlighted was a cobalt
based catalyst 1.15 for CO
2
hydrogenation to formate, contributed by Linehan and coworkers at
PNNL.
11i
Remarkably, catalyst 1.15 works efficiently (TOF = 3400 h
-1
) at 21
o
C, making it one of
the fastest catalyst at lower temperatures. This work sheds light on a possible solution of carbon
neutral fuel cycle enabled by first-row transition metals. The downside of this contribution is that
the application scope might be limited by an expensive base (2,8,9-triisopropyl-2,5,8,9-tetraaza-
1-phosphabicyclo[3,3,3]undecane) and a moderate TON (2000). Unimpressively, the authors
offered very limited characterization data, which makes it almost impossible to comprehend how
the catalyst possesses its reactivity. In their proposed mechanism, catalyst 1.15 under goes CO
2
insertion and then releases a formate to give 1.16 (Scheme 1.4). 1.16 can then activate H
2
to afford
dihydrido cobalt compound 1.17, which is acidic enough to be depronated back to 1.15.
7
Scheme 1.4. Proposed Mechanism for CO
2
hydrogenation by 1.15.
1.2.1.2. CO
2
Hydrogenation to Methanol (methoxide)
Direct hydrogenation of CO
2
to methanol proves to be difficult and sparse in literature. A
sophisticated approach towards CO
2
to methanol hydrogenation employs a cascade strategy, i.e.
breaking the reaction into multiple mechanistically independent steps, and use a different catalyst
for each step. Sanford and co-workers reported in 2011 such a cascade conversion of CO
2
to
methanol.
12
This strategy involves three steps in sequence of: 1) CO
2
hydrogenation to formic
acid, 2) Lewsi acid catalyzed formate ester formation, and 3) formate ester hydrogenation to
methanol (Scheme 1.5). In their report, each step was developed individually by screening a
number of catalyst candidates. Then they tried to combine the optimized steps to work together
8
as a cascade. However, the cascade showed lower reactivity because the reaction conditions are
not all compatible. After many attempts a highest TON of 21 (21 based on hydrides delivered, or
7 based on CO
2
) was achieved. Although the reported system is only marginally reactive, it clearly
delivers a message that the cascade strategy has potential in this difficult transformation.
Scheme 1.5. Cascade Methanol Synthesis from CO
2
Hydrogenation.
Ru(triphos) 1.18 might be one of the best single catalysts for CO
2
hydrogenation known to
date. In their original study published in 2012,
13
at 140
o
C, under 80 bar H
2
/CO
2
(3:1), the
Ru(triphos) (triphos = 1,1,1-tris(diphenylphosphinemethyl)ethane) catalyst 1.18 hydrogenates
CO
2
to methanol with a TON of 221 (based on hydrides delivered) in 24 hours (Scheme 1.6).
Because The reduction of CO
2
to methanol goes through multiple oxidation states/intermediates,
it might be out of the reaction scope of a single catalyst. The success of this work presents much
value, especially to the understanding of the working mechanism. In their follow-up study, they
observed formation of a formate adduct 1.19 from the precatalyst 1.18 immediately after
pressurized under H
2
and CO
2
.
14
1.19 is a reasonable intermediate in 1.18 catalyzed CO
2
to
methanol hydrogenation, because 1) metal-formate adducts are common intermediates in CO
2
hydrogenation, 2) 1.19 readily gets reduced to release a methanol if heated under H
2
. Notably to
this catalysis, the resting state of the catalyst is a dihydride bridged dimer 1.20.
9
Scheme 1.6. Direct CO
2
to Methanol Hydrogenation by Ru(triphos) Catalyst 1.18.
1.2.2 Ester, amide and nitrile hydrogenation
Reductions of esters, amide and nitriles are useful processes in fine chemical synthesis
industry. In addition to conventionally available methods using metal hydrides as the reductant,
hydrogenation with a selective molecular catalyst offers an environmentally friendly, atom
economic alternative.
15
Here we only focus on a few representative catalytic systems.
1.2.2.1 The Ru-triphos system
As early as 1997, Elsevier developed a catalytic system for hydrogenation of dimethyl
oxalate, based on the combination of ruthenium precursor and triphos ligand (Scheme 1.7a).
16
This appears to be the first efficient homogeneous catalytic system for direct ester hydrogenation.
In 2003, Crabtree and co-workers reported the first example of homogeneous hydrogenation of
amides bases on a ruthenium catalyst in presence of the triphos ligand.
17
In this work, a mixture of
products can be formed, e. g. amines, alcohols, and esters. This is a result of uncontrolled
hydrogenation to the substrate (Scheme 1.7b). In 2007, Cole-Hamilton and co-workers showed
that they could fully reduce the carbonyl groups in a N-phenylamide substrate with the
Ru/triphos catalyst, to yield only amines and alcohols (Scheme 1.7c).
18
In 2010 and 2011, Leiner
10
and Klankermayer described hydrogenation of levulinic acid with a precomplexed Ru-triphos
catalyst 1.21.
19
Under their standard hydrogenation conditions, levulinic acid is first reduced to
valerolactone, a cyclic ester. Further hydrogenation with the same catalyst takes it to 1,4-
pentanediol, which upon dehydration affords 2-methyltetrahydrofuran (Scheme 1.7d). To note,
acidic ionic liquid improves the catalytic reactivity dramatically: the hydrogenation of
valerolactone affords in one step the 2-methyltetrahydrofuran in high yields. With the same ionic
liquid, a mixture of triphos and Ru(acac)
3
showes similar catalytic reactivity as complex 1.21.
1.2.2.2. Pyridine-centered Pincer Catalysts
In the early 2000s, Milstein and co-workers demonstrated hydrogen activation on a family
of ruthenium complexes bearing non-innocent pincer ligands.
20
(Figure 1.1) The working
mechanism can be summarized as following: deprotonation of the ligand –CH
2
- in a precatalyst
(1.22/1.23) leads to dearomatization of the pyridine ring (1.24/1.25), and the resulting complex
can split H
2
under mild conditions to regain aromaticity.
21
(Scheme 1.8) Using these complexes,
in 2006 they reported the first hydrogenation of esters without a co-catalyst, e. g. acids or bases.
22
Specifically, in this report, PNN ligated complex 1.24 showed somewhat higher reactivity than the
PNP complex 1.28. Under similar conditions, they reported amide hydrogenation to alcohols and
amines by PNN complex 1.30.
23
More recently, enhanced reactivity was observed on CNN
carbene pincer complexes such as 1.31. In this design, a strong carbene donor replaces a
phosphine group to enable a more electron rich Ru-H species for higher hydrogenation
reactivity.
24
Similar pincer complex is also tested for nitrile reduction. Notably, ruthenium hydride
11
complex 1.32 delivers excellent conversion of subject nitriles (Figure 1.2), although higher H
2
pressure and higher temperature are required.
25
Scheme 1.7. Ru-Triphos Systems for Hydrogenation of Esters and Amides.
Figure 1.1. Ruthenium Pincer Catalysts for Ester and Amide Hydrogenation.
Scheme 1.8. Hydrogen Activation by Ru-Pincer Complexes.
12
Figure 1.2. A Ru-Pincer Complex for Nitrile Hydrogenation
1.2.2.3 Noyori Type Catalysts
Noyori reaction was first introduced a few decades ago for catalytic enantioselective
hydrogenation of ketones, aldehydes, and imines.
26
This Nobel prize winning breakthrough
centers around a family of ruthenium catalysts that are capable of hydrogen splitting and hydride
delivery. In a simplified expression, the Noyori catalytic motif is composed of a ruthenium center
atom and ligand with a labile N-H bond. The N-H bond is often acidic enough that it can be
deprotonated by a range of bases. Once deprotonated, the nitrogen can split H
2
with the Ru center
in a cooperative fashion.
In past few years, many research groups gain focus on hydrogenation reactions based on
the Noyori type ruthenium catalysts, targeting some more difficult-to-reduce substrates (Figure
1.3). An pioneering work was contributed by Saudan and co-workers, where they used ruthenium
catalysts 1.33 and 1.34 for amide and ester reduction. Under 30-50 bar H
2
pressure, these catalysts
successfully yielded corresponding amines and alcohols with thousands of turnovers.
27
Similarly,
catalysts 1.35-1.37 show hydrogenation reactivity on a range of ester substrates.
27a,28
Particularly,
13
one of the original Noyori’s asymmetric catalyst 1.37 also successfully hydrogenates esters and
lactones to alcohols, as reported by Bergens and co-workers.
8e
In 2011, a Noyori type catalyst 1.38
was reported by Gunanathan et al. for nitrile reduction. 1.38 can be activated by KO
t
Bu (base) to
hydrogenate benzonitrile to benzyl amine with good yields.
29
Figure 1.3. Noyori Type Hydrogenation Catalysts Used in Ester, Amide and Nitrile Reductions.
1.3 Dehydrogenation
1.3.1. Methanol dehydrogenation
Methanol has gained much research interest as a potential energy carrier and hydrogen storage
material (12.6 wt. %) in the context of methanol economy.
30
There are appealing advantages using
methanol as a fuel. Firstly, methanol only has H
2
O and CO
2
in the exhaust stream. Besides,
methanol can be reformed from CO
2
and solar H
2
as a solar fuel.
10
Moreover, methanol is a liquid
under standard conditions, much like gasoline and diesel fuel, which means it can be stored,
14
transported and dispensed using the existing infrastructure. Because hydrogen fuel cells are highly
efficient at converting chemical energy to electricity, on-demand hydrogen release from methanol
have value in such applications as transportation.
Heterogeneous methanol reforming yields three molecules of H
2
, one molecule of CO
2
from one molecule of methanol and one H
2
O. However, traditional methanol reforming requires
forcing conditions, high temperatures and high pressures for instance.
31
Thus it suffers from high
energy consumption and crippling inconvenience for on-site hydrogen production. As we enter
the 21
st
century, technologies on dehydrogenative oxidation of alcohols blooms with prosperity,
which provides a more mature platform to develop catalysts that dehydrogenates methanol under
reasonable conditions. Since the early 2000s, it has been rationally envisioned a stepwise, water-
assisted dehydrogenation of methanol as shown in Scheme 1.9. According to the scheme, three
H
2
evolve from three catalytic steps: 1) methanol dehydrogenation to formaldehyde followed by
H
2
O addition to methanediol, 2) dehydrogenation of methanediol to FA, 3) FA dehydrogenation
to H
2
and CO
2
.
Scheme 1.9. a) Methanol reforming; b) Envisioned Catalytic Dehydrogenation of Methanol.
15
In 2011, Beller and co-workers reported dehydrogenation of methanol by two ruthenium
pincer complexes. The reaction proceeds in aqueous solution under mild conditions with >
350000 TON and about 30% conversion.
32
H
2
and CO
2
are observed as the products. This is the
first homogeneous system we can find in literature that efficiently dehydrogenates aqueous
methanol to H
2
and CO
2
at a temperature under 100
o
C. To better illustrate the working
mechanism, the catalytic cycle is demonstrated in Scheme 1.10 along with some possible
intermediates. First, there is an initiation of the catalyst, in which ruthenium complexes 1.39 or
1.40 are activated in alkali medium to form the active species 1.41. This reactive species 1.41 has
an empty ligation site on the ruthenium, and a deprotonated secondary amine group in the pincer
backbone. The deprotonated ligand can cooperatively activate an alcohol substrate with the
ruthenium center. Based on their previous studies on alcohol oxidation,
33
the authors believe the
first dehydrogenation is an outer-sphere concerted process shown as 1.42. The product of this
step, formaldehyde, possibly undergoes a few steps of transformation before it dissociates from
the metal complex. For example, it may react with H
2
O or hydroxide to give the gem-diolate 1.43,
which is dehydrogenated to formate. The decarboxylation of formate releases one equivalent of
CO
2
to afford a diydrido complex 1.45. 1.45 then undergoes another H
2
evolution to release one
more equivalent of H
2
to form 1.41 again and to finish the catalytic cycle.
16
Scheme 1.10. Methanol Dehydrogenation by Beller’s Ruthenium Catalysts 1.39 and1.40.
Taking advantage of the same ligand scaffold, in 2013 the Beller group reported the iron
variant of catalyst 1.40, specifically Fe
II
complexes 1.47 and 1.48 (Figure 1.4). 1.47 and 1.48 also
catalyze methanol dehydrogenation under similar aqueous conditions for 1.40.
34
Remarkably, in
Ru N
PR
2
PR
2
H
Cl
H
CO
1.39 R = Ph
1.40 R =
i
Pr
Ru N
PR
2
PR
2
H
CO
ROH
Base
Ru N
PR
2
PR
2
H
OR
H
CO
Ru N
PR
2
PR
2
H
CO
H
O CH
2
H
2+
Ru N
PR
2
PR
2
H
CO
H
O CH
H
2+
O
H Sol
Ru N
PR
2
PR
2
H
CO
H
O C
H
2+
O
Outer-sphere
Ru N
PR
2
PR
2
H
H
H
CO
O
O H
H
2
H
2
CO
2
H
2
O
H
2
Outer-sphere
fast
1.41
1.42
1.43
1.44
1.45
1.46
MeOH + H
2
O
1.39 or 1.40
4 : 1
to
neat MeOH
H
2
+ C
1
residue
89-95
o
C
17
the first hour, these iron complexes show comparable dehydrogenative reactivity in methanol
compared to their noble metal analogues. For example, when 4.18 μmol catalyst (19 ppm) is
added to a methanol/water mixture (9:1) in presence of 8 M KOH at 91
o
C, iron catalyst 1.48
delivers a turnover frequency (TOF) of 702 h
-1
, which is only ~30% slower than 1.40’s TOF at
1023 h
-1
. This result offers the first example demonstrating iron’s catalytic potential in methanol
dehydrogenation. Notably, complex 1.48 can stay active for two days delivering a maximum TON
of 9834. 1.48’s more modest longevity compared to ruthenium complex 1.40 was suspected to be
catalyst decomposition. In NMR experiments, free ligand can be observed in the reaction solution,
which marks for complex degradation. In an attempt to have the catalysts last longer, five
equivalents of external pincer ligand is added to the reaction solution. As a result, the catalysis
proceeded for three more days. Regarding the dehydrogenation mechanism, the authors suggest
that the rate limiting step may in fact be a β–hydride elimination, as it is more consistent with
density functional theory (DFT) calculations as well as other new evidence.
35
Figure 1.4. Methanol Dehydrogenation by Iron Complexes 1.47, 1.48.
Fe N
P
i
Pr
2
P
i
Pr
2
H
Br
H
CO
Fe N
P
i
Pr
2
P
i
Pr
2
H
HBH
3
H
CO
1.47 1.48
MeOH + H
2
O
1.47 or 1.48
4.18 mmol, 19 ppm
8 M KOH
9 : 1
H
2
+ C
1
residue
91
o
C
18
Another successful catalysis in methanol dehydrogenation also relies on metal-ligand
cooperation. In 2013, Rodrí guez-Lugo et al. designed ruthenium catalyst 1.49 that features a non-
innocent tetra-dentate ligand and showed that it effectively dehydrogenates methanol.
36
Particularly, the ligand trop
2
dad (1,4-bis(5H-dibenzo[a,d]cyclohepten-5-yl)-1,4-diazabuta-1,3-
diene) is key to this chemistry, because 1) isomerization of the ligand allowing the Ru center to
adapt to two different oxidation states without significant structural change of the complex; 2) it
can reversibly store up to four hydrogen atoms in one ligand. More mechanistic insight was
collected from observation of possible intermediates by running stoichiometric reactions
(Scheme 1.11). For example, 1.50 forms from 1.49 if treated with a slight excess of water. 1.50 is
a hydrogen acceptor under the reaction conditions. It is found to split alcohols to give the alcoxide
adduct 1.51. Because of the large hydrogen storage capacity of the ligand, 1.51 can further react
with another equivalent of alcohol to give 1.52, which is followed by the formation of 1.53, and
eventually 1.54. Alternatively, the hydrogen rich amino compounds such as 1.54 could evolve H
2
under basic conditions to regenerate reactive species such as 1.49. Thereafter, a catalytic cycle can
be depicted as shown in Scheme 1.11. In virtue of the reactive ligand-metal cooperative
mechanism, catalyst 1.49 converts a MeOH/H
2
O mixture selectively to CO
2
/H
2
gas products,
allowing the evolution of almost the entire hydrogen content (12 wt. %).
19
Scheme 1.11. Proposed Catalytic Cycle of Methanol Dehydrogenation by Ruthenium Catalyst
1.49.
A few other examples have been reported more recently on methanol dehydrogenation. In
2014, Milstein and co-workers found their ruthenium PNN-pincer complex 1.30, an alcohol to
ester conversion catalyst, is also reactive for methanol dehydrogenation. Although 1.30 works at
a higher temperature (115
o
C), but in general it goes through a higher conversion than Beller’s
catalyst (71% yield for H
2
). In addition, under their optimized condition 1.30 reaches an
impressive maximum TON of 29000. Possible catalyst intermediates are also studied (Figure 1.5).
20
Under NMR, 1.55 forms instantly upon addition of FA to 1.30. Adding excessive H
2
O to a
solution of 1.55 gives the hydrido-hydroxo complex 1.56 quantitatively. Also it is known that 1.56
is the product between 1.30 and H
2
O.
37
Based on these observations, an equilibria among 1.55,
1.30 and 1.56 can be drawn in Figure 1.5. Both 1.55 and 1.56 are stable compounds under inert
atmosphere, which makes them both candidates for the intermediates in catalytic methanol
dehydrogenation.
Figure 1.5. Equilibria among Complex 1.55, 1.30 and 1.56.
In 2015, Fujita and Yamaguchi reported iridium complexes 1.57 and 1.58 that are active
catalysts for H
2
evolution from methanol.
38
In the initial catalyst screening, 1.57 or 1.58 can release
61% and 60 % of H
2
, respectively. Further condition optimization enabled up to 99% hydrogen
yield and up to 10510 TON. These Ir-Cp* compounds feature functional bipyridine or
bipyridonate ligands and a hydrate (Figure 1.6). To obtain mechanistic insight, stoichiometric
conversion of 1.57 and 1.58 were conducted under acidic or basic conditions. Reversible
deprotonation of 1.57 to 1.58, then to 1.59 can be observed simply by manipulating the solution
pH. Considering the reaction condition is highly basic, it is reasonable to suggest that 1.58 or 1.59
21
should be the active form of 1.57. The authors also show that structurally highly analogous iridium
complexes 1.60 and 1.61 are completely unreactive under same condition. This result provides
convincing evidence for that the oxo-group in 1.58 or 1.59 is involved in the working catalysis.
Although the mechanism is still not fully clear, a ligand-metal cooperative mechanism seems to
critical to this reaction.
Figure 1.6. Equilibria among 1.57, and 1.58 and 1.59. Unreactive Iridium Complexes 1.60 and
1.61.
1.3.2. Formic Acid Dehydrogenation
Formic acid (FA) has received much research interest as a hydrogen carrier. Despite a lower
hydrogen storage density (4.4 wt %) than methanol, formic acid has its own prominent
advantages. First of all, it is a potential solar fuel because efficient synthesis of formic acid from
CO
2
and H
2
is known.
11
In addition, hydrogen evolution from FA can be achieved under mild
conditions, which is a crucial premise for on-demand hydrogen release. Furthermore, formic acid
and its spent fuel are environmentally benign chemicals. Much progress has been achieved in
formic acid decomposition,
39
regretfully we will only be able to focus on a few representative
examples.
22
In 2008, Beller and co-workers found that mixing ruthenium complex and phosphine
ligands makes an active catalyst for FA decomposition.
40
A simple phosphine supported
ruthenium complex RuCl
2
(PPh
3
)
3
is much reactive for dehydrogenation of formic acid/amine
adduct. Under their conditions, RuCl
2
(PPh
3
)
3
is used to dehydrogenate a 5:2 HCO
2
H/NEt
3
adduct. At 40
o
C, the RuCl
2
(PPh
3
)
3
can convert up to 90% of formic acid to H
2
and CO
2
in 2 hours
with a TON of 834. Even at as low as 26.5
o
C, 362 turnovers can be observed after 2 hours. Simply
mixture of RuCl
3
· xH
2
O and three equivalents triphenylphosphine can also be used in
decomposition of the 5:2 HCO
2
H/NEt
3
adduct. In 2 hours, this catalyst delivers a TON of 691,
which is only ~20% less reactive than the pre-complexed RuCl
2
(PPh
3
)
3
.
In 2011, the Beller group reported a more reactive catalytic system for FA decomposition
and it is based on iron catalysts.
41
To our knowledge, this is the first homogeneous iron catalyst
for mild, efficient FA decomposition. In a typical run, a mixture of iron precursor Fe(BF
4
)· 6 H
2
O
and phosphine ligands (P(CH
2
CH
2
PPh
2
)
3
, PPh
3
) are added to a propylene carbonate solution of
FA. Particulatrly, when Fe(BF
4
)
2
· 6 H
2
O and 4 equivalents PPh
3
are used as the catalyst, the
authors observed more than 92000 turnovers at 80
o
C. The true identity of the working catalyst
remains unknown. However, because phosphine ligands are required for this reaction, it is
believed that a phosphine supported iron complex is the active catalyst. A kinetic study shows that
the reaction has 1
st
order dependence on [Ir] and half order on [FA]. Because it is difficult to
explain these observations, DTF calculation is employed to gain more mechanistic insight. For
the calculations, two possible pathways are subjected to the tests (Scheme 1.12). In the left cycle,
23
the rate determining step is β-hydride elimination, from 1.65 to 1.67, with a small energy hurdle
(22-49 kJ/mol) associated with it. RDS for the right cycle is also found to be β-hydride elimination,
from 1.63 to 1.64, however, this step is uphill by over 400 kJ/mol. Thus, the left cycle is plausibly
closer to the working mechanism, though it fails to explain why the reaction has half order
dependence on [FA].
Scheme 1.12. Proposed Catalytic Cycle of FA Decomposition by Iron Catalyst 1.62.
In 2014, Hazari and Schneider reported a molecular-defined iron catalyst 1.68.
40
To date,
1.68 is one of the most reactive homogeneous catalysts for formic acid dehydrogenation. In
dioxane in presence of 50 mol % NEt
3
, when 1.68 is used a co-catalyst (a Lewis acid, e.g. NaCl,
NaBF
4
, or LiBF
4
), a TON of about 1 million can observed. The authors also proposed a catalytic
mechanism based on computation studies as well as experimental data (Scheme 1.13). If 1.68 is
exposed to 1 atm of H
2
, it transforms to 1.69 almost quantitatively. Because H
2
is one of the major
products from FA decomposition, it is possible that 1.69 is an intermediate in the catalysis. The
24
decarboxylation is arguably the slow step (RDS) in the catalysis, as evidenced by that 1.68 is the
resting state of the catalyst, and a Lewis acid profoundly accelerates the overall reaction. The
authors also suggested that the mechanism for decarboxylation is β-hydride elimination. This
conclusion is in fine agreement with DFT calculations, and the fact that Lewis acid facilitate the
catalysis.
Scheme 1.13. Proposed Mechanism for 1.69 Catalyzed FA Dehydrogenation.
1.3.3 Ammonia-borane dehydrogenation
Ammonia-borane (H
3
N· BH
3
, AB) is an excellent material for hydrogen storage in form of
chemical bonds, given 19.6 % of its weight is constituted by hydrogen atoms. Despite its inherent
limitations, it was once one of the “hottest” candidates for clean, renewable transportation fuel.
42
Although thermo-hydrolysis of ammonia borane is generally fast and it releases multi-equivalents
H
2
, researchers seek to water-free dehydrogenative conditions in order to avoid formation of a
25
“spent fuel” that has “unbreakable” B-O bonds. Since one molecule of AB contains three
equivalents of H
2
, ideally a good AB dehydrogenation catalyst should liberate > 2 equiv. H
2
, and
the by-product is polyborazylene (Figure 1.7). Because of the opposite polarities of NH
3
and BH
3
groups, the research to activate both N-H bond and B-H bond lead to much contemporary
discovery on bifunctional catalysis. In the following paragraphs, we will show a few representative
homogeneous catalysts for AB dehydrogenation.
Figure 1.7. Hydrogen Evolution from Ammonia-Borane.
In 2006, Heinekey and Goldberg reported AB dehydrogenation by a iridium pincer
complex 1.71.
43
The catalyst, 1.71, exhibits very high reactivity for AB even at room temperature.
For instance, 0.5 mol % of 1.71 quantitatively converts AB to a B, N oligomer in ~14 minutes
liberating 1 equivalent of H
2
(Figure 1.8). Unfortunately, the catalysis ceases after the first
equivalent of H
2
is evolved.
Figure 1.8. Ammonia Borane Dehydrogenation by Iridium Pincer Complex 1.71.
N B
H
H
H
H
H
H
d+ d-
D
catalyst
n H
2
+ [N
x
B
x
H
y
]
extent of reaction and by-products are
determined by the catalyst
26
Another early breakthrough in AB dehydrogenation was contributed by Baker and co-
workers in 2007.
44
In a communication, they reported a nickel-carbene system that can evolve 2.5-
2.7 equivalents H
2
from AB under mild conditions (Figure 1.9). This result was believed to be the
first AB dehydrogenation catalyzed by an earth-abundant metal. Although a Ni(NHC)
2
complex
was originally suspected to be a part of the “metal catalyzed” reaction, the authors did not observe
expected irreversible oxidative addition at the Ni center. In a computational study, Yang el al.
suggested that mechanism may involve hydrogen transfer from AB to the N-heterocyclic carbene
(NHC), followed by nickel activation of the C-H bond.
45
Later on, Zimmerman et al. reported
that, according to their DFT calculations, a ligated NHC is likely to dissociate from Ni(NHC)
2
in
presence of excess AB.
46
In the same year, they further showed an alternative viable mechanism,
where the dehydrogenation happens first at a free NHC, which can be regenerated by Ni through
C-H bond insertion.
47
Figure 1.9. Hydrogen Evolution from Ammonia-Borane by the Ni-NCH System.
27
Scheme 1.14. Ligand Metal Bifunctional Hydrogenation/Dehydrogenation Equilibria among
1.72, 1.73, and 1.74.
In 2009, Schenider and co-workers reported a ruthenium based ligand-metal bifunctional
catalyst for AB dehydrogenation.
48
Catalyst 1.73 evolves 1 equivalent H
2
from a THF solution of
AB at room temperature, with a TON up to 8300. Catalyst 1.73’s hydrogenative reactivity is
believed to function via reversible hydrogen uptake/release between the amino complex 1.72 and
enamido complex 1.74 (Scheme 1.14). It is noteworthy that, this is the first direct observation of
hydrogenation/dehydrogenation of an ethylene bridge in amido chelate complexes.
1.4 Overview
Catalytic hydrogenation and dehydrogenation reactions are powerful tools when
interconversion between energy and chemical bonds takes place. Homogeneous catalysis are
adopted in research to help understand the working mechanism of a catalysis, and further to help
design an improved process. Thanks to the hard work done by our chemistry community, many
exciting new progresses have been introduced to us in the last few years, especially in the research
realm of energy storage.
28
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Amides to Alcohols and Amines under Mild Conditions. J. Am. Chem. Soc. 2010, 132, 16756-
16758.
36
24. (a) Fogler, E.; Balaraman, E.; Ben-David, Y.; Leitus, G.; Shimon, S. J. W.; Milstein, D. New
CNN-Type Ruthenium Pincer NHC Complexes. Mild, Efficient Catalytic Hydrogenation of
Esters. Organometallics 2011, 30, 3826-3833; (b) Balaraman, E.; Fogler, E.; Milstein, D.
Efficient Hydrogenation of Biomass-Derived Cyclic Di-Esters To 1,2-Diols. Chem. Commun.
2012, 1111-1113.
25. Gunanathan, C.; Ho ̈lscher, M.; Leitner, W. Reduction of Nitriles to Amines with H
2
Catalyzed
by Nonclassical Ruthenium Hydrides – Water-Promoted Selectivity for Primary Amines and
Mechanistic Investigations. Eur. J. Inorg. Chem. 2011, 3381-3386.
26. For a representative examples see: Noyori, R.; Ohkuma, T.; Kitamura, M.; Takaya, H.; Sayo,
N.; Kumobayashi, H.; Akutagawa, S. Asymmetric Hydrogenation of .Beta.-Keto Carboxylic
Esters. A Practical, Purely Chemical Access To .Beta.-Hydroxy Esters in High Enantiomeric
Purity. J. Am. Chem. Soc. 1987, 109, 5856-5858.
27. General selection of manuscripts by Saudan et al.: (a) Saudan, L.; Dupau, P.; Riedhauser, J.;
Wyss, P. Hydrogenation of Esters With Ru/Tetradentate Ligands Complexes. PCT Int. Pat.
Appl. WO/2006/ 106484A1, 2006; (b) Saudan, L.; Saudan, C. M.; Debieux, C.; Wyss, P.
Dihydrogen Reduction of Carboxylic Esters to Alcohols under the Catalysis of Homogeneous
Ruthenium Complexes: High Efficiency and Unprecedented Chemoselectivity. Angew. Chem.
Int. Ed. 2007, 46, 7473-7476; (c) Saudan, L.; Saudan, C. PCT Int. Pat. Appl.
WO/2008/065588A1, 2008; (d) Saudan, L.; Saudan, C. PCT Int. Pat. Appl. WO/2010/
37
038209A1, 2010.
28. (a) Kuriyama, W.; Ino, Y.; Ogata, O.; Noboru, S.; Saito, T. A Homogeneous Catalyst for
Reduction of Optically Active Esters to the Corresponding Chiral Alcohols without Loss of
Optical Purities. Adv. Synth. Catal. 2010, 352, 92-96; (b) Kuriyama, W.; Matsumoto, T.;
Ogata, O.; Ino, Y.; Aoki, K.; Tanaka, S.; Ishida, K.; Kobayashi, T.; Sayo, N.; Saito, T. Catalytic
Hydrogenation of Esters. Development of an Efficient Catalyst and Processes for Synthesising
(R)-1,2-Propanediol and 2-(l-Menthoxy)ethanol. Org. Process Res. Dev. 2012, 16, 166-171;
(c) Carpenter, I.; Eckelman, S. C.; Kuntz, M. T.; Fuentes, J. A.; France, M. B.; Clarke, M. L.
Convenient and Improved Protocols for the Hydrogenation of Esters Using Ru Catalysts
Derived From (P,P), (P,N,N) and (P,N,O) Ligands. Dalton Trans. 2012, 41, 10136-10140;
(d) Saudan, L. A. Hydrogenation Processes in the Synthesis of Perfumery Ingredients. Acc.
Chem. Res. 2007, 40, 1309-1319; (e) Takebayashi, S.; Bergens, S. H. Facile Bifunctional
Addition of Lactones and Esters at Low Temperatures. The First Intermediates in
Lactone/Ester Hydrogenations. Organometallics 2009, 28, 2349-2351; (f) Clarke, M. L.;
Belén Diaz-Valenzuela, M. B.; Slawin, A. M. Z. Hydrogenation of Aldehydes, Esters, Imines,
and Ketones Catalyzed by a Ruthenium Complex of a Chiral Tridentate Ligand.
Organometallics 2007, 26, 16-19.
29. Li, T.; Bergner, I.; Haque, F. N.; Zimmer-De Iuliis, M.; Song, D.; Morris, R. Hydrogenation of
Benzonitrile to Benzylamine Catalyzed by Ruthenium Hydride Complexes with
P−NH−NH−P Tetradentate Ligands: Evidence for a Hydridic−Protonic Outer Sphere
38
Mechanism. Organometallics 2007, 26, 5940-5949.
30. (a) Olah, G. A. Beyond Oil and Gas: The Methanol Economy. Angew. Chem. Int. Ed., 2005,
44, 2636–2639; (b) Olah, G. A.; Prakash, S. G. K.; Goeppert A. Chemical Recycling of Carbon
Dioxide to Methanol and Dimethyl Ether: From Greenhouse Gas to Renewable,
Environmentally Carbon Neutral Fuels and Synthetic Hydrocarbons. J. Org. Chem. 2009, 74,
487-498; (c) Olah, G. A.; Goeppert, A.; Prakash, S. G. K.Beyond Oil and Gas: The Methanol
Economy, Wiley-VCH, 2006.
31. for a review on this topic, see: Palo, D. R.; Dagle, R. A.; Holladay, J. D. Methanol Steam
Reforming for Hydrogen Production. Chem. Rev. 2007, 107, 3992–4021; for a representative
example, see: Cortright, R. D.; Davda, R. R.; Dumesic, J. A. Hydrogen from Catalytic
Reforming of Biomass-Derived Hydrocarbons in Liquid Water. Nature 2002, 418, 964–967.
32. Nielsen, M.; Alberico, E.; Baumann, W.; Drexler, H.-J.; Junge. H.; Gladiali, S.; Beller, M. Low-
Temperature Aqueous-Phase Methanol Dehydrogenation to Hydrogen and Carbon Dioxide.
Nature, 2013, 495, 85-89.
33. Nielsen, M.; Alberico. E.; Baumann, W.; Drexler, H.-J.; Junge, H.; Gladiali, S.; Beller, M.
Efficient Hydrogen Production from Alcohols under Mild Reaction Conditions. Angew.
Chem. Int. Ed. 2011, 50, 9593–9597.
34. Alberico, E.; Sponholz, P.; Cordes, C.; Nielsen, M.; Drexler, H.-J.; Baumann, W.; Junge, H.;
39
Beller, M. Selective Hydrogen Production from Methanol with a Defined Iron Pincer Catalyst
under Mild Conditions. Angew. Chem. Int. Ed. 2013, 52, 14162–14166.
35. (a) X. Yang, Hydrogenation of Carbon Dioxide Catalyzed by PNP Pincer Iridium, Iron, and
Cobalt Complexes: A Computational Design of Base Metal Catalysts. ACS Catal. 2011, 1,
849–854; (b) X. Yang, Unexpected Direct Reduction Mechanism for Hydrogenation of
Ketones Catalyzed by Iron PNP Pincer Complexes. Inorg. Chem. 2011, 50, 12836–12843; (c)
Yang, X. A Self-Promotion Mechanism for Efficient Dehydrogenation of Ethanol Catalyzed by
Pincer Ruthenium and Iron Complexes: Aliphatic versus Aromatic Ligands. ACS Catal. 2013,
3, 2684–2688.
36. Rodrí guez-Lugo, R. E.; Trincado, M.; Vogt, M.; Tewes, F.; Santiso-Quinones, G.;
Grützmacher, H. A Homogeneous Transition Metal Complex for Clean Hydrogen
Production from Methanol–Water Mixtures. Nat. Chem. 2013, 5, 342-347.
37. Balaraman, E.; Khaskin, E.; Leitus, G.; Milstein, D. Catalytic Transformation of Alcohols to
Carboxylic Acid Salts and H
2
Using Water as the Oxygen Atom Source. Nat. Chem. 2013, 5,
122−125.
38. Fujita, K.; Kawahara, R.; Aikawa, T.; Yamaguchi, R. Hydrogen Production from a Methanol–
Water Solution Catalyzed by an Anionic Iridium Complex Bearing a Functional Bipyridonate
Ligand under Weakly Basic Conditions. Angew. Chem. Int. Ed. 2015, 54, 9057–9060.
40
39. for a review on this topic, see: Johnson, T. C.; Morris, D. J.; Wills, M. Hydrogen Generation
from Formic Acid and Alcohols Using Homogeneous Catalysts. Chem. Soc. Rev. 2010, 39,
81–88.
40. Loges, B.; Boddien, A.; Junge, H.; Beller, M. Controlled Generation of Hydrogen from Formic
Acid Amine Adducts at Room Temperature and Application in H
2
/O
2
Fuel Cells. Angew.
Chem. Int. Ed. 2008, 47, 3962 –3965.
41. Bielinski, E. A.; Lagaditis, P. O.; Zhang, Y.; Mercado, B. Q.; Würtele, C.; Bernskoetter, W. H.;
Hazari, N.; Schneider, S. Lewis Acid-Assisted Formic Acid Dehydrogenation Using a Pincer-
Supported Iron Catalyst. J. Am. Chem. Soc. 2014, 136, 10234−10237.
42. Marder, Todd B. Will We Soon Be Fueling our Automobiles with Ammonia–Borane? Angew.
Chem. Int. Ed. 2007, 46, 8116–8118.
43. Denny, M. C.; Pons, V.; Hebden, T. J.; Heinekey, D. M.; Goldberg, K. I. Efficient Catalysis of
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44. Keaton, R. J.; Blacquiere, J. M.; Baker, R. T. Base Metal Catalyzed Dehydrogenation of
Ammonia−Borane for Chemical Hydrogen Storage. J. Am. Chem. Soc. 2007, 129, 1844–1845.
45. Yang, X.; Hall, M. B. The Catalytic Dehydrogenation of Ammonia-Borane Involving an
Unexpected Hydrogen Transfer to Ligated Carbene and Subsequent Carbon−Hydrogen
Activation. J. Am. Chem. Soc. 2008, 130, 1798–1799.
41
46. Zimmerman, P. M.; Paul, A.; Musgrave, C. B. Catalytic Dehydrogenation of Ammonia Borane
at Ni Monocarbene and Dicarbene Catalysts. Inorg. Chem. 2009, 48, 5418–5433.
47. Zimmerman, P. M.; Paul, A.; Zhang, Z.; Musgrave, C. B. The Role of Free N-Heterocyclic
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Int. Ed. 2009, 48, 905–907.
42
2.1 Introduction
This chapter in part reprints published work.
1
I would like to take this opportunity to
acknowledge my co-authors. Dr. Brian Conley contributed to experimental design throughout the
project. He also studied of the mechanism of the catalyst initiation and fast catalysis. Moreover,
he acquired a crystal structure of ruthenium complex 2.26.
Hydrogen is an attractive alternative transportation fuel that has appeal because it is carbon-
free, is easily oxidized in fuel cells, and is potentially available from water electrolysis.
2
Although
pressurized hydrogen gas currently has some use in vehicles, its practicality in cars is limited by
fuel range, convenience, and safety concerns.
3
Particularly, hydrogen has low volumetric energy
density (5.6 MJ/L at 700 bar) despite high mass energy density (120 MJ/kg for hydrogen).
4
Thus,
a highly weight efficient strategy to store hydrogen as condensed matter might enable its
translation more broadly into transportation applications and consumer products.
Ammonia−borane (AB) is a promising material from which to build a practical hydrogen storage
system, because it has high hydrogen density (19.6 wt %, ca. 9.9 MJ/L, 12.6 MJ/kg) and it can
release hydrogen under mild conditions (thermolysis, hydrolysis, and catalysis; Figure 2.1).
5
Transition-metal-catalyzed dehydrogenation of ammonia−borane, particularly, is an area of active
research interest because it may enable a more efficient fuel cycle than the well-known catalytic
43
hydrolysis reaction that forms ammonia, which is poisonous to fuel cells, and strong B−O bonds,
which are energetically costly to rereduce.
6
Several active transition-metal-based catalysts have
been reported for AB dehydrogenation reactions, in either heterogeneous or homogeneous
systems; these involve rhodium,
7
iridium,
8
ruthenium,
9−11
nickel,
12
palladium,
13
and iron
catalysts,
14
among others,
15
(Figure 2.2). Our laboratory’s studies on catalytic AB
dehydrogenation have been focused on the reactivity of Shvo’s catalyst (2.14, Scheme 2.1)
16
and
its relatives. By analogy to the established mechanism for alcohol oxidation with 2.14, we
presumed that the coordinative saturation of the reduced form of the Shvo system would preclude
coordination of aminoborane, NH
2
BH
2
, to the catalyst, and thus disfavor the formation of
insoluble oligomers, [NH
2
BH
2
]
n
, which limits the hydrogen production of some catalysts
8,9
for
ammonia−borane dehydrogenation to 1 equiv.
17
Although this proposal seems to hold true,
10
the
catalyst begins to deactivate after ca. 25% conversion in the first pass, which renders this system
irrelevant to practical implementation.
Figure 2.1. Ammonia-Borane Dehydrogenation Reactions and Possible Products.
N B
H
H
H
H
H
H
d+ d-
D
catalyst
n H
2
+ [N
x
B
x
H
y
]
H
2
N BH
2
2.1
iminoborane
2.6
m-amidodiborane
H
2
B
H
2
N
B
H
2
NH
2
BH
2
H
2
N
2.5
cyclotriborazane
H
2
N
B
H
2
H
2
N
B
H
2
n
2.4
linear
polyaminoborane
2.2
branched
cyclotetraborazane
HB
HN
B
H
NH
BH
H
N
2.3
borazine
B
HN
B
N
B
H
N
B
N
B
N NH
BH
H
N
HB
N
B
N
B
NH
B
HB
N
B
H
N
B
N
HN
N
NH
BH
2.7
cross-linked
polyborazylene
A. Catalytic dehydrogenation of ammonia borane, NH
3
BH
3
, AB
B. Boron, nitrogen intermediates and products
extent of reaction and by-products are
determined by the catalyst
- H
2
- H
2
H
2
N
H
2
B
H
BH
2
H
2
B
H
2
N
B
H
2
NH
2
H
B
H
2
N
H
2
N
BH
3
44
Figure 2.2. Transition-Metal Catalysts for AB dehydrogenation.
Scheme 2.1. Dehydrogenation of AB by Shvo’s Catalyst.
45
This text examines the molecular events that deactivate the Shvo catalyst in
ammonia−borane dehydrogenation. These include hydroboration of the active, oxidizing form of
the catalyst by borazine, which involves the borylation of the catalyst’s ligand oxygen atom so that
the turnover-limiting H−H bond-forming step is no longer accessible. Ultimately, we addressed
this problem by designing a second-generation system, 2.15
18
and 2.17 that does not rely on an
oxygen center as a proton acceptor in the same way as the Shvo catalyst. This second generation
system then enables access to high weight efficiency dehydrogenation of ammonia−borane.
11
2.2 Results and Discussion
The kinetic profile of AB dehydrogenation with 2.14 shows complicated behavior that we
deconvoluted into a sequence of three limiting cases (Figure 2.3).
10
Catalyst initiation is the case
of low conversion and zero [borazine] with ruthenium beginning in its dimeric form. This can be
easily studied in isolation, because these are the conditions at the beginning of the reaction and
because initiation occurs quickly at 55 ° C, where the catalysis is slow. The second case is the one
in which AB conversion and [borazine] are low and the ruthenium in the system is no longer in
the form of its dimeric precursor. This case can easily be studied in isolation by either (1) allowing
the reaction to incubate at room temperature until it is initiated, i.e. 2.14’s characteristic μ-H peak
(δ(
1
H) = −17.7 ppm) is consumed, or (2) delivering the ruthenium as dimer 2.22 (vide infra,
Scheme 2.2). The third case, slow catalysis, occurs under the condition that [borazine] ≈ [Ru
atoms]. In this case the rate of dehydrogenation catalysis drops precipitously and the catalyst
46
“dies”. This case can be generated in isolation by adding a catalytic portion (1 equiv. versus
[Ru
atom
]) of borazine to the reaction mixture at the outset of dehydrogenation.
10
Figure 2.3. Kinetic Profiles of AB Dehydrogenation by 2.14. (left)
11
B NMR data showing
consumption of AB in the presence of 2.5 mol% 2.14 in a sealed J-Young NMR tube. (right)
Eudiometer data showing production of hydrogen gas in the presence of 5.0 mol% 2.14 and 2.0
mol% ethanol in 2:1 diglyme/benzene-d
6
at 70 ° C.
Figure 2.3 shows (left) AB consumption and (right) H
2
generation as functions of time for
trials of dehydrogenation that cycle through all three of its mechanistic cases.
10
This is particularly
evident from AB consumption: AB is consumed slowly as the catalyst initiates (case 1) and then
it is consumed more quickly once dimer 2.14 is cleaved. Ultimately, [borazine] increases and the
catalysis slows down.
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0 5 10 15 20
Time (x 10
3
sec)
[AB] (M)
47
Understanding the mechanism of dehydrogenation in the high [borazine] case and the
catalyst deactivation process is particularly important for the following reasons: (1) The kinetic
profile of the AB consumption in the high [borazine] case is that observed when the catalyst is
reused, and reusability is essential for practical applications. (2) Borazine is an unavoidable
intermediate in AB dehydrogenation if ≥ 2 mol equiv. of hydrogen is released, yet the
coordination chemistry of borazine is not well studied in the context of catalytic AB
dehydrogenation systems.
4
Understanding the mechanism of this deactivation is important for
the design of more efficient catalytic systems and, ultimately, a broadly useful solution to
reversible hydrogen storage on ammonia−borane.
2.2.1. Catalyst Initiation (Case 1).
A short period in the beginning of the reaction (ca. 2% conversion) is an initiation period
in which AB consumption is slow. This situation can be studied in isolation by monitoring the
reaction at a temperature well below that needed for fast catalysis, 70 ° C. Thus, upon heating to
55 ° C, the bridging hydride in 2.14 (δ(1H) = −17.7 ppm) is replaced by the hydride of monomeric
species 2.21 (δ(
1
H) = −10.0 ppm) at a rate of 7.96(21) × 10
−4
s
−1
.
10
This indicates that Shvo’s
catalyst dissociates to its reduced monomer 2.21 and (presumably) oxidized monomer 2.20
(Scheme 2.2A).
1
H NMR integrations show that 2 equiv. of 2.21 is formed with consumption of
2.14; thus, the reduction of 2.20 is rapid relative to the dissociation of 2.14.
48
Scheme 2.2. Catalyst Initiation. A: Catalyst Initiation. B: Rapid Formation of 2.21 from 2.22. [AB]
is 0.42 M in benzene-d
6
solution, [Ru]
2
is 5 mol% to AB.
The finding of rapid reduction of 2.21 by ammonia−borane is supported by the relatively rapid
rate of reduction of 2.22, a stable dimer of 2.21, under analogous conditions (Scheme 2.2B). In
this experiment we see that conversion of 2.22 to 2.20 reaches completion within 5 min at 50 ° C.
This corresponds to a rate constant of >10
−2
s
−1
at 50 ° C, which is faster than the rate of catalyst
initiation (10
−3
s
−1
at 55 ° C). Thus, the dissociation of dimer 2.14 is rate-limiting in catalyst
initiation. This result is in accordance with Shvo’s considerable dissociation enthalpy: 28.8
kcal/mol in toluene in the absence of AB.
19b
2.2.2. Fast Catalysis (Case 2).
After the catalyst initiation, the kinetic profile of AB consumption displays fast, linear
kinetics through ca. 20−30% conversion. In these conditions, the reaction has a zero-order
dependence on [AB] and first-order dependence on the catalyst’s [Ru], as determined by the
Shvo's catalyst
O
Ph
Ph
Ph
Ph
Ru
OC
OC
Ru
H
CO
CO
Ph
Ph
Ph Ph
HO
+
k
init
NH
3
BH
3
fast
2.14 2.20 2.21
O
Ph
Ph
Ph
Ph
Ru
CO
OC
O
Ph
Ph
Ph
Ph
Ru
CO
CO
2.22
Ru
H
OC
OC
Ph
Ph
Ph Ph
OH
2.21
NH
3
BH
3
A. Catalyst initiation
B. Direct synthesis of 2.21
Benzene-d
6
50
o
C, < 5 min
49
respective zero and unity slopes of plots of ln k
obs
versus ln [AB] and ln [Ru].
10
Throughout this
case,
1
H NMR shows a persistent monomeric ruthenium hydride at δ(
1
H) = −10 ppm, which is
plausibly the resting state of the catalyst. This is consistent with H−H bond formation as the
turnover-limiting step in fast catalysis. This assignment is consistent with the observed kinetic
dependencies of [AB]
0
and [Ru]
1
. We further observe first-order dependence on [EtOH], which
is consistent with the transition state model established by Casey for stoichiometric hydrogen loss
from 2.21.
19
Thus, we adopt Casey and Cui’s geometry for ethanol-mediated H−H bond
formation from 2.21 as the turnover-limiting transition state of this catalysis (Scheme 2.3). In sum,
the observed rate law in this case of the reaction is −d[AB]/dt = k
obs
[Ru][EtOH].
Scheme 2.3. Proposed Mechanism of Fast Catalysis (Case 2).
50
2.2.3. Slow Catalysis (Case 3).
2.2.3.1 Kinetics of Ammonia−Borane Dehydrogenation in the Slow Catalysis Case.
Onset of the slow catalysis conditions, i.e. catalyst deactivation, is characterized by the
appearance of curvature in the time course plot of [AB], and it becomes more likely as [borazine]
rises. The conditions of slow catalysis cause the emergence of multiple κ
1
-Ru−H hydride peaks
(from δ(
1
H) = −9 to −10 ppm), which occurs simultaneously with exponential decay behavior in
[AB]. We believe that these correspond, respectively, to (a) new resting state(s) of the catalyst
and ammonia−borane’s role in a new turnover-limiting step. Understanding this mechanism is
essential to our studies on catalyst reuse and, we infer, spent fuel regeneration.
An essential feature of the high [borazine] case of the reaction is that the catalyst is reusable
within the limits of its kinetics. Thus, if a completed reaction mixture is treated with a new aliquot
of ammonia−borane, dehydrogenation will recommence upon heating, and the reaction’s kinetic
profile will follow slow catalysis behavior wherein the rate of hydrogen production is too slow to
be useful.
10
It is easy to believe that, at the conclusion of the reaction, the concentration of [2.20]
is very small, so that dimer 2.14 is not reformed, and it is unnecessary to repeat catalyst initiation
(case 1) in catalyst reuse experiments. However, the absence of fast catalysis must result from a
practically irreversible deactivation of the catalyst during the transition from fast catalysis to slow
catalysis in the first run. The exponential decay shape of the kinetic profile of AB consumption
suggests that the reaction is now first order in [AB] in the slow catalysis regime (Table 2.1, left),
51
as verified by a plot of ln k
obs
versus ln [AB] with a slope of 1.03(4) as recorded in the case of high
[borazine]. This draws a significant contrast to fast catalysis, where such a plot was of zero slope,
and indicates that, unlike fast catalysis, once the catalyst deactivates, the turnover-limiting step for
further turnover involves conversion of the resting species by 1 equiv. of ammonia−borane.
Table 2.1. Ammonia-Borane Consumption as a Function of [AB] and [Ru] in Slow Catalysis.
a
AB (M) Rate (k
obs
, s
-1
)
b
0.42 5.99(12) × 10-5
0.52 7.09(19) × 10-5
0.73 1.05 (2) × 10-4
0.94 1.35(4) × 10-4
a
Data calculated from
11
B NMR-monitored kinetic studies at 70 ˚C.
b
[Ru
atom
]
0
is 42.0 mM.
c
[AB]
0
is 0.42 M.
-2.8
-2.6
-2.4
-2.2
-2
-1.8
-1 -0.8 -0.6 -0.4 -0.2 0
ln (k
obs
)
ln ([AB])
y = m1 + m2 * M0
Error Value
0.02438 -1.9394 m1
0.043047 1.0334 m2
NA 0.0014173 Chisq
NA 0.99654
R
2
-3.2
-3
-2.8
-2.6
-2.4
-2.2
-3.8 -3.6 -3.4 -3.2 -3 -2.8 -2.6 -2.4
ln (k
obs
)
ln (mol % 14)
y = m1 + m2 * M0
Error Value
0.069051 -1.2939 m1
0.021816 0.50538 m2
NA 0.00061383 Chisq
NA 0.99814 R
52
The kinetic order in ruthenium also changes upon the onset of slow catalysis conditions.
[Ru] is first order in the fast catalysis case, but it becomes half order in the high [borazine] case:
rate constants were measured for a series of [Ru] concentrations in AB dehydrogenation in the
presence of borazine, which gave a plot of ln kobs versus ln [Ru] with a slope of 0.50(2) (Table
2.1, right). This is analogous to 2.14-catalyzed alcohol oxidation,
20
wherein apparent half-order
dependence on [Ru] is a result of equilibrium between 2.20 + 2.21 and dimer 2.14, for which a
plot of ln k
obs
versus ln [Ru
atom
] has a slope of 0.40(6).
21
2.2.3.2 Isotope Effects.
Kinetic isotope effects were determined for dehydrogenation of selectively deuterium-
labeled ammonia− borane isotopologues
22
ND
3
BH
3
, NH
3
BD
3
, and ND
3
BD
3
in high [borazine]
conditions. Comparison of measured rate constants for parallel runs at 70 ° C gave kinetic isotope
effects of k
NHBH
/k
NDBH
= 1.46(3), k
NHBH
/k
NHBD
= 1.07(5), and k
NHBH
/k
NDBD
= 2.30(4) (see the
experimental section). The KIE in NH is suggestive of a catalyst reactivation involving
participation of the NH in its turnover-limiting step. This might be akin to a protonation of the
resting state of the catalyst by an acidic NH proton. These data present a conundrum, however,
which is that the product of the two single-label KIEs should equal the double-label KIE; in this
case we have 1.46(3) × 1.07(5) = 1.56(6), which is well below the observed value of 2.30(4).
Along these lines, Casey’s group has reported H/D exchange of the Ru−H group in Shvo’s catalyst
with D
2
(or vice versa) of 2.21-Tol in THF without substituting the corresponding ligand O−H.
19
Similarly, in two parallel runs of ND
3
BH
3
dehydrogenation, similar portions of HD and H
2
were
53
formed. The presence of H
2
(and by symmetry D
2
) implies the availability of a mechanism for
proton/hydride exchange under our catalytic conditions. On the basis of our observations and
their result, we conducted an experiment of ND
3
BD
3
dehydrogenation with 1 atm of H
2
gas
applied to the solution. HD was formed during this reaction (Scheme 2.4A), which necessitates
an H/D crossover mechanism involving the final product, H
2
. We believe that the mechanism of
this is the same as Casey’s H/D exchange mechanism, except we suggest that this mechanism is
available to the resting state(s) of our catalyst. To test this latter hypothesis, we treated borylated
ruthenium complex 2.24 with 1 atm of D
2
under conditions analogous to our catalytic reactions.
We observed the formation of HD at room temperature in 5 min and complete deuteration in the
hydride position of 2.24 within 1 h at 60 ° C (Scheme 2.4B). This shows us that there is a
mechanism for H/D exchange of the ruthenium hydride in an O-borylated homologue of the
Shvo system. This result provides an explanation for the small experimental k
NHBH
/k
NHBD
value
and the mismatch between our observed k
NHBH
/k
NDBD
and the value predicted by the separate
values for proton and hydride: because there is a facile mechanism for H/D exchange, an isotopic
kinetic resolution is possible.
54
Scheme 2.4. H/D Exchange Experiments.
2.2.3.3. Mechanistic Proposal.
We propose, on the basis of NMR observations, that fast catalysis ends (i.e., catalyst
deactivaiton occurs) because borazine undergoes a hydroboration with ruthenium intermediate
2.20 to give the deactivated complex 2.33, which further converts to other derivatives (Scheme
2.5).
10
Reactions analogous to the addition of 2.3 to 2.20 are known from the Casey
18a
and Clark
22
laboratories (Scheme 2.6). Casey has shown hydrosilylation of the Shvo scaffold by triethylsilane.
This adduct, Tol-2.27, has a δ(
1
H) value of −9.20 ppm in benzene-d
6
. Similarly, Clark has shown
hydroboration of the Shvo complex with pinacol−and catechol−boranes in high yield at mild
temperature. These adducts have δ(
1
H) values of −9.33 and −9.26 ppm in benzene-d
6
,
respectively.
2.14 (cat), 70
o
C, H
2
(1 atm)
diglyme/benzene-d
6
HD D
3
N BD
3
O
Ph
Ph
Ph
Ph
Ru
CO
OC
H
H
Ru
CO
CO
O
Ph
Ph
Ph
Ph
2.14
Ru
H
OC
OC
Ph
Ph
Ph
Ph
O B
O
O
2.24
70
o
C, D
2
(1 atm)
diglyme/benzene-d
6
Ru
D
OC
OC
Ph
Ph
Ph
Ph
O B
O
O
+ HD
A: H/D exchange in a d
6
-AB dehydrogenation
B: Direct H/D exchange on a hydroborated Shvo catalyst's Ru site
2.24-d
55
Scheme 2.5. Proposed Borazine-Mediated Hydroboration.
Scheme 2.6. Hydroboration and Hydrosilylation of Tol-2.22.
O
Ph
Tol
Tol
Ph
Ru
CO
OC
O
Ph Tol
Tol
Ph
Ru
CO
CO
Ru
H
OC
OC
Ph
Tol
Ph
Tol
OSiEt
3
Ru
H
OC
OC
Ph
Tol
Ph
Tol
O B
O
Ph
Tol
Tol
Ph
Ru
CO
OC
O
Ph Tol
Tol
Ph
Ru
CO
CO
O
Ph
Tol
Tol
Ph
Ru
CO
OC
O
Ph Tol
Tol
Ph
Ru
CO
CO
Et
3
Si-H, CH
2
Cl
2
4h, rt, 40%
pinB-H, toluene
2 h, 50
o
C, 74%
catB-H, benzene-d
6
18 h, 50
o
C, 75%
O
O
Tol-2.22
Tol-2.22
Tol-2.22
Ru
H
OC
OC
Ph
Tol
Ph
Tol
O B
O
O
Tol-2.27
Tol-2.28
Tol-2.24
56
We propose that this hydroborated species can dimerize to form a O−B−O and Ru−H−Ru
bridged dimer (2.32, Scheme 2.5) akin to the parent Shvo complex and Clark’s [(μ-(cat)B-
(C
4
Ar
4
O)
2
)Ru
2
(CO)
4
(μ-H)] dimer, which accounts for the observed half-order kinetic
dependence on [Ru]. These species can re-enter the catalytic cycle if the B−O bond affixing
borazine to the catalyst is cleaved in the presence of ammonia−borane, which accounts for its first-
order kinetic dependence. We do not know the mechanism of ammonia−borane’s involvement in
this step.
2.2.3.4 Catalyst Deactivation.
We conducted a series of experiments directly to interrogate our proposal for the
mechanism of slow catalysis, yet we observe that the proposed complex 2.24 is not stable to
isolation. Borazine was added to dimer 2.22 at room temperature, and a bridging hydride peak
formed at the beginning of the reaction (δ(
1
H) = −18.3) was then consumed in 2 min (Scheme
2.5). This was replaced by a set of 13 hydride peaks from δ(
1
H) = −9.3 to −10.0 ppm, which
correspond to κ
1
Ru−H groups such as 2.25. We suspect that these signals correspond to multiple
hydroboration events on a single borazine or ring-opened borazine derivatives.
We can create slow catalysis case conditions at the beginning of a dehydrogenation reaction
very simply by adding 1 mol equiv. of borazine relative to [Ru
atom
] to the reaction mixture prior to
heating. Under these conditions the kinetic profile of the reaction does not show any properties
of initiation or fast catalysis but proceeds directly to the rate and rate law of slow catalysis.
10
This
57
is strong evidence indicating that borazine is the agent that causes catalyst deactivation, and it
aptly accounts for the instant slow catalysis situation that is observed in catalyst reuse experiments.
Because of their self-reactive nature, we are unable to isolate these complexes directly, but when a
mixture of these materials is collected and excess borazine is quantitatively removed under
reduced pressure, the resulting material can be isolated through aqueous workup. This treatment
cleaves any borazine rings remaining in the borazine−catalyst complex(es) and affords a
rutheniumcontaining adduct, ammonia complex 2.26,
24
which can be isolated in 42% yield. This
observation gives strong evidence that the deactivated catalyst, the one present in the slow
catalysis case, is covalently bound to a borazine moiety, because NH
3
could not have been
delivered in any other plausible way.
2.2.3.5. An Analogue of the Deactivated Catalyst.
Because our efforts to isolate and characterize our proposed deactivated catalyst were
frustrated by its reactivity, we set about to devise a borazine analogue with which we could
hydroborate 2.22 and generate a stable surrogate of the catalyst of the slow catalysis case. The
premise of this design was our hypothesis that multiple equivalents of 2.20 are hydroborated by 1
equiv. of borazine to yield multiple κ
1
-Ru−H signals in the
1
H NMR spectrum. Along these lines,
we prepared diazaborane 2.31 and treated it with dimer 2.22. The result was near-quantitative
formation of 2.14 under rigorously anhydrous conditions (Scheme 2.6). We infer from this result
that proposed dimer 2.29, if formed, apparently loses a borabenzoimidazole rapidly to regenerate
58
2.14. In contrast, hydroboration of 2.22 with catecholborane to form 2.24 is facile and gives an
oxygen substituted analogue of our proposed deactivated catalyst that is free of N−H groups.
In situ preparation of 2.24 in a benzene/diglyme solution affords an opportunity to
compare the rate and kinetic profile of 2.24-catalyzed ammonia−borane dehydrogenation with
those of the slow catalysis case (Scheme 2.7). The kinetic profiles each appear first order in AB,
but the rate for dehydrogenation with catalyst precursor 2.24 is faster than slow catalysis by a
factor of ca. 3-fold. This faster rate could be a result of a more labile O−B bond between the
catalyst’s hydroxycyclopentadiene and the corresponding boranes. A plot of ln k
obs
versus ln [Ru]
gave a slope of 0.51(3) (Table 2.2), which is in agreement with the measured [Ru] dependence
for slow catalysis. These data show us that the dimerization behavior that we see in the slow
catalysis case is effectively recreated in borylated analogue 2.24. Taken together, these data
provide good anecdotal evidence that the deactivated catalyst is an O-borylated form of the
precatalyst.
59
Scheme 2.7. Synthesis of a Mechanistic Analog for the Proposed Deactivated Catalyst Complex.
Table 2.2. Ammonia Borane Dehydrogenation Catalyzed by Borylated Complex 2.24.
amount of 2.24 (mol %) Rate (s
-1
)
5.0 1.26(3) × 10
-4
7.5 1.50(2) × 10
-4
10 1.82(2) × 10
-4
15 2.18(2) × 10
-4
-2.5
-2
-1.5
-1
-4 -3.5 -3 -2.5 -2
ln (k
obs
)
ln ([AB])
y = m1 + m2 * M0
Error Value
0.10197 -0.19603 m1
0.032218 0.51045 m2
NA 0.0013387 Chisq
NA 0.99604 R
60
2.2.3.6. Other Reaction Products and Intermediates in the Slow Catalysis Case.
Ammonia−borane dehydrogenation catalyzed by 2.14 generates multiple B, N
intermediates throughout the reaction. In fast catalysis the intermediates detected by
11
B NMR
are the same as those observed for other catalysts that are known to liberate multiple equivalents
of hydrogen from ammonia−borane:
10−12
AB → branched cyclotetraborazane (2) → borazine
(3) (Figure 2.1A). In the slow catalysis case, however, a new species appears, aminodiborane 2.6
(Scheme 2.8). This species should be dehydrogenated to borazine, and in fact, in reactions in
which this species is an intermediate, borazine remains the only product upon completion of the
reaction. We believe that 2.6 is an adduct formed from BH
3
, dissociated from ammonia−borane,
and NH
2
BH
2
, generated transiently after the first dehydrogenation of ammonia−borane.
6a,17
We
propose that formation of 2.6 is reversible, and this is only a mechanistic cul de sac, rather than an
in-line intermediate in the dehydrogenation sequence. To test this hypothesis, we added (a) 0.5
equiv of 1 M BH
3
· THF and (b) a comparable volume of THF to two otherwise identical runs of
ammonia−borane dehydrogenation with 2.14 (Table 2.3). A strong signal for 6 was observed by
11
B NMR in tube a in the beginning of the reaction, much earlier than the first emergence of 2.6’s
peak in the THF control experiment (tube b). Both reactions proceeded through fast catalysis at
about the same rate (Figure 2.3). Furthermore, the rates of AB consumption in slow catalysis case
are similar, ca. 25% difference, which shows that although there is a large excess of 2.6 in tube a in
comparison to the amount in tube b, this has a disproportionately small effect on the rate of AB
consumption. We therefore know that 2.6 goes on to dehydrogenate to borazine and does not
61
significantly interfere with the rates of the steps in slow catalysis as it forms and disappears. It
further appears that BH
3
does not hydroborate and deactivate the catalyst in the same way as
borazine or catecholborane.
Scheme 2.8. Formation of 2.6.
Table 2.3. Rate of Slow Catalysis in the Presence and Absence of BH
3
.
a
conditions slow catalysis k
obs
(s
-1
)
BH
3
· THF 1.57(7) × 10
-4
THF only 7.0(13) × 10
-5
Parent Conditions
b
6.3(6) × 10
-5
a
Data calculated from
11
B NMR kinetic studies at 70 ˚C. Smoothed curves are empirical fits; k
obs
values shown are for slow catalysis, not the entire curve. See Experimental Section.
b
A parallel run
under parent conditions has k
obs
in statistical agreement with others reported herein.
If free BH
3
from the dissociation of ammonia−borane is impacting the course of the
reaction in the slow catalysis case, then free NH
3
must also be present, and NH
3
is known to
0
0.1
0.2
0.3
0.4
0 5 10 15 20
BH
3
¥ T H F
THF only
Parent Conditions
[A-B] (M)
Time (x 10
3
s)
62
modulate the reactivity of the Shvo system.
24
Thus, we propose a second mechanism of catalyst
deactivation, which is reversible formation of 2.26 by NH
3
ligation to the catalyst.
Scheme 2.9. Synthesis of Ammonia Adduct 2.26.
Table 2.4. [AB] Dehydrogenation with 2.14 and 2.26.
a
fast catalysis slow catalysis
cat. rate (M s
-1
) cat. k
obs
(s
-1
)
2.14 1.47(9) × 10
-4
2.14 3.06(39) × 10
-4
2.26 5.12(3) × 10
-5
2.26 4.23(33) × 10
-4
a
Data calculated from
11
B NMR kinetic studies. 0.25 mol AB and 0.035 mol [Ruatom] are added
to 0.6 mL diglyme/benzene-d
6
. Both reactions were run at 70
o
C. Black circles and diamonds are
kinetic profiles of reactions catalyzed by 2.26 and 2.14 respectively.
0
0.1
0.2
0.3
0.4
0.5
0 5 10 15
[A-B] (M)
Time (x 10
3
s)
63
To interrogate directly the reactivity of ammonia adduct 2.26, we prepared it independently
through the addition of ammonia gas to 2.22 (Scheme 2.9). Analogous to the case for 2.14, the
11
B NMR kinetic profile of AB dehydrogenation with 2.26 appears to have two distinct kinetic
cases, one linear case, with a reaction rate of 5.12 × 10
−5
M s
−1
, and one exponential decay case,
with a rate constant of 4.22 × 10
−4
s
−1
(Table 2.4). This is similar to the second and third cases
(fast and slow catalysis) of dehydrogenation with 2.14, but since 2.26 is monomeric, it stands to
reason that there should not be an initiation delay analogous to that observed in reactions
featuring 2.14. We account for this behavior by proposing that NH
3
reversibly can ligate 2.20 as
previously documented
24
and thereby temporarily sequester it from its catalytic roles. Thus, NH
3
ligation provides a second mechanism for catalyst deactivation, although this one appears to be
less deleterious than hydroboration of 2.20.
2.2.3.7. Homogeneous versus Heterogeneous Catalysis.
We propose that this reaction is homogeneous throughout its duration on the basis on four
observations. First, the reactor maintains its homogeneous appearance through the duration of
the reactions. No metallic residue is observed. Second, the rate of catalysis is not impacted by the
addition of Hg(0). In contrast, a mercury drop does inhibit the catalytic hydrogenation of benzene
based on the [Ru
3
(μ
2
-H)
3
(η
6
-C
6
H
6
)(η
6
-C
6
Me
6
)
2
(μ
3
-O)]
+
catalyst precursor, which is part of the
evidence for heterogeneous reduction in that system.
25
This result is germane to the present
discussion because it shows a documented system wherein the mercury drop experiment was
effective with ruthenium. Still, the best evidence we have (third point) for homogeneous catalysis
64
remains the foregoing kinetics data, in which the data remain pseudo first order in the slow
catalysis case through > 90% conversion (> 3 half-lives). A fourth piece of evidence favoring
homogeneous catalysis comes from a quantitative poisoning experiment wherein the reaction is
run in the presence of a small portion of 1,10-phenanthroline.
Quantitative poisoning is an experiment in which less than 1 molar equiv (relative to the
proposed monomeric catalyst) is introduced into the reaction, and one monitors the rate to see if
it is affected proportionally to the concentration of the poison. If the drop in rate upon poisoning
is disproportionally large, this can be evidence for heterogeneous catalysis, because < 100% metal
atoms (and often ≤ 50%)
26
are on the surface of a nanoparticle and thus ≤50% are available
to be poisoned. In this present case, two quantitative poisoning experiments were conducted
wherein 0.1 and 0.5 equiv of 1,10-phenanthroline (phen) relative to [Ru
atom
] were added to two
otherwise standard ammonia−borane dehydrogenation runs with catalyst 2.14 (0.25 mol of AB,
70 ° C, diglyme/benzene-d
6
). Although these are not first-order reactions, generally we see that
phen accelerates the reaction, apparently by prolonging the fast catalysis portion of the reaction
(Figure 2.4). In contrast, 0.5 equiv of phen (relative to ruthenium atoms) completely quenches
catalytic heterogeneous hydrogenation of benzene based on the [Ru
3
(μ
2
-H)
3
(η
6
-C
6
H
6
)(η
6
-
C
6
Me
6
)
2
(μ
3
-O)]
+
catalyst precursor.
25
Thus, we take our evidence to argue against the formation
of a ruthenium nanoparticle. The origin of the acceleration behavior seems as if the
phenanthroline “poison” is protecting the catalyst from borazine-mediated deactivation.
65
Figure 2.4. [AB] Dehydrogenation with 2.14 in presence of 1,10-phenanthroline. Data calculated
from
11
B NMR kinetic studies. 0.25 mol AB and 5 mol% 2.14 are added to 0.6 mL
diglyme/benzene-d
6
at 70 ˚C.
A tetranuclear catalyst (e.g., Ru
4
L
n
) is unlikely but cannot be rigorously eliminated. Such a
hypothesis is disfavored because an analogous Rh
4
Cp*
2.4
Cl
4
H
c
cluster has been shown to be the
likely active catalyst in the hydrogen of benzene with [Cp*RhCl
2
]
2
, and this Rh
4
cluster is
deactivated by Hg(0).
27
Further, that Rh
4
-based system is deactivated by 4 equiv. of 1,10-
phenanthroline (i.e., 1:1 phen:Rh
atom
), while ours is not.
2.2.4. Semi-Site Protection Mechanism
To account for phanenthroline accelerated catalysis, Scheme 2.10 suggests a possible
explanation for this observation. Conceptually, we speculate that phenanthroline is protecting the
0
0.1
0.2
0.3
0.4
0.5
0 5 10 15 20
1,10-Phenanthroline Poisoning Experiment on Shvo System
0.5 equiv 1,10-phenanthroline (versus [Ru])
0.1 equiv 1,10-phenanthroline (versus [Ru])
No 1,10-phenanthroline
[A-B] (M)
Time (x10
3
sec)
66
reactive site on ruthenium from hydroboration. This can enable a hydrogen bond from 2.34’s
ligand oxygen to ammonia borane, analogous to those well documented for imine reduction with
the Shvo system. We suspect the reaction process is akin to 2.26 catalyzed ammonia borane
dehydrogenation, however the equilibrium between 2.20 and 2.34 is slower (relative to the
equilibrium between 2.20 and 2.26) and thus enables a faster catalysis mechanism.
Scheme 2.10. Semi-Site Protection Mechanism
Following this lead, we ran ammonia borane dehydrogenation in presence of several
different nitrogen ligands.
10b
These catalytic systems show faster reaction rates and larger
conversion before catalyst deactivation. For example, amine and pyridine ligands can accelerate
the 2.14-catalyzed dehydrogenation of AB. Monodentate pyridines enable accelerated
dehydrogenation of AB without cyclopentadienone (CPD) displacement. We propose that the
67
reason for this is that pyridines can protect the catalyst against hydroboration by borazine.
Bidentate (N−N) ligands such as phenanthroline appear to accelerate catalysis through an
additional mechanism where a relatively more reactive ruthenium species is formed upon loss of
CPD. These catalytic systems release over 2 equiv. of H
2
.
2.3 Conclusion
In conclusion, we were able to propose a full picture of the catalyst deactivation mechanism
(Scheme 2.11). A full scheme showing the origin of the slow catalysis case is illustrated in Scheme
2.11. We propose that catalyst deactivation is caused by the introduction of borazine, the product
of selective dehydrogenation, into solution. When the rate of reduction of oxidized catalyst 2.20
by ammonia−borane becomes competitive with its hydroboration, AB consumption ceases to be
linear. When hydroboration of 2.20 becomes fast relative to reduction of 2.20, then 2.24 becomes
a resting state of [Ru], and the catalysis “dies”, as sketched in the lower-left half of the cycle in
Scheme 2.11. In this phase of the reaction, conversion has first order dependence on [AB],
apparently because ammonia−borane is needed to convert borazine species 2.24 back into an
active species.
Reversible ammonia ligation of 2.20 is almost certainly happening throughout the reaction.
We know, however, from the rates of (a) fast catalysis and (b) slow catalysis with ammonia
complex 2.26 that 2.26 is a minor contributor to the total [Ru
atoms
] in fast catalysis because (i) the
addition of 1 equiv. of NH
3
relative to ruthenium slows fast catalysis and (ii) the rate of AB
68
consumption is constant throughout the fast catalysis case. We also know that NH
3
ligation to
ruthenium does not significantly alter the rate of slow catalysis.
In summary, the mechanism of AB dehydrogenation catalyzed by Shvo catalyst 2.14 was
investigated. This reaction initiates with dissociation of the dimeric precatalyst 2.14 and then goes
through a fast dehydrogenation reaction wherein the H−H bond formation is the turnover-
limiting step. As the concentration of borazine increases, it adds to the reactive form of the catalyst
to give ruthenium species, which are not as reactive as their mechanistic predecessor in
ammonia−borane dehydrogenation. Presumably, these deactivated ruthenium species are
reactivated by ammonia−borane itself and proceed to further ammonia−borane dehydrogenation.
This observation gives us insight into the higher reactivity of our second generation catalyst 3.3,
which will be discussed in the next chapter.
69
Scheme 2.11. Mechanistic Proposal of Slow Catalysis.
2.4 Experimental Section
2.4.1. General Procedures.
All air- and water-sensitive procedures were carried out either in a Vacuum Atmospheres
glovebox under nitrogen (0.5−10 ppm of O
2
for all manipulations) or using standard Schlenk
techniques under nitrogen. Deuterated NMR solvents were purchased from Cambridge Isotopes
70
Laboratories. Benzene-d
6
and diethylene glycol dimethyl ether (diglyme, J. T. Baker) were dried
over sodium benzophenone ketyl and distilled prior to use. Shvo’s catalyst was purchased from
Strem Chemicals. Ammonia−borane (NH
3
BH
3
, AB) was purchased from Sigma Aldrich.
Borazine was synthesized and purified by the method used by Wideman and Sneddon.
28
1
H and
11
B NMR spectra were obtained on a Varian 600 spectrometer (600 MHz in
1
H, 192 MHz in
11
B)
with chemical shifts reported in units of ppm. All
1
H chemical shifts are referenced to the residual
1
H solvent (relative to TMS). All
11
B chemical shifts are referenced to BF
3
· OEt
2
in diglyme in a
coaxial external standard (0 ppm). NMR spectra were taken in 8 in. J. Young tubes (Wilmad) with
Teflon valve plugs. The NMR tubes were shaken vigorously for several minutes with
chlorotrimethylsilane and then dried in vacuo on a Schlenk line prior to use.
Caution! Extreme caution should be used when carrying out these reactions, as the release of
hydrogen can lead to sudden pressurization of reaction vessels.
2.4.2. Mechanistic Studies Utilizing
11
B and
1
H NMR Spectroscopy.
In a typical reaction, 7.7 mg of AB was combined with Shvo’s catalyst (2.14, 13.6 mg, 5
mol %) in a J. Young NMR tube while in a glovebox under nitrogen. The AB and catalyst
concentrations may be varied. Diglyme (0.4 mL) and benzene-d
6
(0.2 mL) were added to the tube,
as was the BF
3
insert. The sample tube was immediately inserted into a preheated NMR (70 ° C),
and the kinetic monitoring commenced after quickly locking and shimming. Disappearance of AB
in the solution was monitored by the relative integration of its characteristic peak in the
11
B
71
spectrum (~22 ppm) and the BF
3
· OEt
2
standard. All spectra were processed using VNMRJ
(version 2.3). The acquisition involved a 1.67 s pulse sequence in which 4096 complex points
were recorded, followed by 1 s relaxation delay. To eliminate B−O peaks from the borosilicate
NMR tube and probe, the
11
B FIDs were processed with back linear prediction, ca. 5−15 points.
2.4.2.1 Reduction of 2.22 with Ammonia Borane.
In this experiment we show spectra for the following reaction (Figure 2.5).
Figure 2.5 Reduction of 2.22 by ammonia borane
In two J. Young tubes (a and b), 7.7 mg (0.25 mmol) AB and 13.5 mg (5 mol%) 2.22 were
dissolved in 0.6 mL benzene-d
6
. A
1
H-NMR of tube (a) was taken quickly (ca. 1 min) in a pre-
locked and shimmed instrument and some reduction reaction already appears (Figure 2.6 top).
Tube (b) was placed in a preheated oil bath at 50
o
C for 4 min before quickly taken a
1
H-NMR of.
The NMR spectrum of tube (b) shows completion of conversion of 2.22 in this system in 5 min
(Figure 2.6 bottom).
50
o
C
benzne-d
6
< 5 min
O
Ph
Ph
Ph
Ph
Ru
CO
OC
O
Ph Ph
Ph
Ph
Ru
CO
CO
2.22
BH
3
NH
3
Ru
H
OC
OC
Ph
Ph
Ph Ph
OH
2.21
72
Figure 2.6. Ruthenium Hydride Species in NMR Spectra. 0.42 M AB with 5 mol% 2.22 in 0.6 mL
benzene-d
6
. Top: tube (a) at room temperature in 5 min, some conversion of 2.22 by AB can be
observed. Bottom: in tube (b) 2.22 is fully reduced to Ru—H hydride species in 5 min at 50
o
C,
suggesting a rate constant > 10
-2
s
-1
.
2.4.2.2 Determination of Catalyst Order in Conversion of AB in Case 3 (Slow Catalysis).
The rate values for the slow catalysis case were determined using
11
B NMR in sealed NMR
tubes, as described above. The amount of AB was 7.7 mg (0.25 mmol), and catalyst
concentrations were varied (6.8, 10.2, 13.6, and 20.3 mg of 2.14, (2.5, 3.75, 5.0, and 7.5 mol %)).
73
The results were plotted as a ln/ln relationship to determine the order in catalyst (Table 2.1, right).
These graphs show raw kinetics data for [Ru] dependence in the slow catalysis case (Figure 2.7).
74
2.5 mol % 2.14. k
obs
= 4.31(1) × 10
-5
s
-1
. 3.75 mol % 2.14. k
obs
= 5.13(6) × 10
-5
s
-1
.
5.0 mol % 2.14. k
obs
= 5. 99(12) × 10-5 s-1. 7.5 mol % 2.14. k
obs
= 7.49(12) × 10
-5
s
-1
.
Figure 2.7. [AB] vs. Time Monitored by
11
B NMR in 2:1 Diglyme/Benzene-d
6
with Varying
Catalyst Concentrations in the Slow Catalysis Case. Exponential coefficients represent the
observed rate constants in s
-1
(× 10
-3
).
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 5 10 15 20 25 30 35 40
[A-B] ( M)
Time (x10
3
sec)
y = m1 + m2*exp(-m3*x)
Error Value
0.0033949 0.013772 m1
0.0029075 0.40053 m2
0.00072476 0.043066 m3
NA 0.00059351 Chisq
NA 0.99915
R
2
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 5 10 15 20 25 30 35
y = m1 + m2*exp(-m3*x)
Error Value
0.0022868 0.029866 m1
0.0019276 0.38818 m2
0.00063443 0.051333 m3
NA 0.00031765 Chisq
NA 0.99951
R
2
[A-B] ( M)
Time (x10
3
sec)
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 5 10 15 20 25 30
y = m1 + m2*exp(-m3*x)
Error Value
0.0036195 0.028159 m1
0.0030517 0.39569 m2
0.0011637 0.059868 m3
NA 0.0001628 Chisq
NA 0.99947
R
2
[A-B] ( M)
Time (x10
3
sec)
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 5 10 15 20 25
[A-B] ( M)
Time (x10
3
sec)
y = m1 + m2*exp(-m3*x)
Error Value
0.002878 0.029471 m1
0.0024244 0.38963 m2
0.0011802 0.074869 m3
NA 0.00028072 Chisq
NA 0.99941
R
2
75
2.4.2.3. Determination of Order in AB in Case 3 (Slow Catalysis).
The rate values for the slow catalysis case were again determined using
11
B NMR in sealed
NMR tubes. Data treatments are shown in the Supporting Information. The amount of 2.14 was
13.6 mg (0.013 mmol), and AB concentrations were varied (7.7, 8.7, 13.5, and 17.4 mg AB (0.42,
0.53, 0.73, and 0.94 M)). The results were plotted as a ln/ln relationship to determine the order
in ammonia-borane (Table 2.1, left). The following graphs show raw kinetics data fits for [AB]
dependence in the slow catalysis case (Figure 2.8)
76
0.42 M AB. kobs = 5.99(12) × 10
-5
s
-1
. 0.53 M AB. kobs = 7.09(12) × 10
-5
s
-1
.
0.73 M AB. kobs = 1.05(2) × 10
-4
s
-1
. 0.94 M AB. kobs = 1.35(4) × 10
-4
s
-1
.
Figure 2.8. [AB] vs. Time Monitored by
11
B NMR in 2:1 diglyme/benzene-d
6
with Varying [AB]
in the Slow Catalysis Case. Exponential coefficients represent k
obs
.
0
0.1
0.2
0.3
0.4
0 5 10 15 20 25 30
[A-B] (M)
Time (x 10
3
s)
y = m1 + m2*exp(-m3*x)
Error Value
0.0036195 0.028159 m1
0.0030517 0.39569 m2
0.0011637 0.059868 m3
NA 0.0001628 Chisq
NA 0.99947
R
2
0
0.1
0.2
0.3
0.4
0.5
0.6
0 5 10 15 20 25
[A-B] (M)
Time (x 10
3
s)
y = m1 + m2*exp(-m3*x)
Error Value
0.0038688 0.048224 m1
0.0031036 0.49978 m2
0.001245 0.070942 m3
NA 0.00031871 Chisq
NA 0.99943
R
2
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 5 10 15 20
[A-B] (M)
Time (x 10
3
s)
y = m1 + m2*exp(-m3*x)
Error Value
0.0039966 0.092647 m1
0.0031449 0.68155 m2
0.0015246 0.10463 m3
NA 0.00023381 Chisq
NA 0.99968
R
2
0.2
0.4
0.6
0.8
1
0 5 10 15
[A-B] (M)
Time (x 10
3
s)
y = m1 + m2*exp(-m3*x)
Error Value
0.009882 0.11633 m1
0.007813 0.89594 m2
0.0039395 0.1353 m3
NA 0.0012115 Chisq
NA 0.99887
R
2
77
2.4.2.4. Kinetic Isotope Effects in Case 3 (Slow Catalysis).
To determine the kinetic isotope effects on the reaction rate, 8.5 mg of ND
3
BH
3
, 8.5 mg of
NH
3
BD
3
, or 9.2 mg ND
3
BD
3
(0.25 mmol) was added to a J. Young NMR tube. To each was added
13.6 mg of 2.14 (5 mol %), diglyme (0.4 mL), and benzene-d
6
(0.2 mL). Again, we analyzed [AB]
vs time for the third case of the reaction conditions. (Figure 2.9) KIEs were determined from the
quotient of protic AB k
obs
(5.99(12) × 10
−5
s
−1
) divided by the deuterated AB k
obs
. The HD signal
found in the
1
H NMR resulted from H-D exchange and is shown in Figure 2.10, J
HD
= 42.4 Hz
These graphs (Figure 2.9) show raw data for kinetic isotope effect experiments done on the
reaction in case 3.
78
ND
3
BH
3
, k
obs
= 4.10(10) × 10
-5
s
-1
NH
3
BD
3
, k
obs
= 5.59(25) × 10
-5
s
-1
KIE = 1.46(3) KIE = 1.07(5)
ND
3
BD
3
, k
obs
= 2.60(8) × 10
-5
s
-1
KIE = 2.30(4)
Figure 2.9. Kinetic Isotope Effects in Dehydrogenation by 5 mol % 2.14.
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 10 20 30 40
[A-B] (M)
Time (x 10
3
s)
y = m1 + m2*exp(-m3*x)
Error Value
0.0046094 0.03027 m1
0.0038619 0.38176 m2
0.0010569 0.041047 m3
NA 0.0023103 Chisq
NA 0.99857 R
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 5 10 15 20 25
[A-B] (M)
Time (x 10
3
s)
y = m1 + m2*exp(-m3*x)
Error Value
0.0099233 -0.007968 m1
0.00873 0.41251 m2
0.0024862 0.055855 m3
NA 0.0015061 Chisq
NA 0.99839 R
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 10 20 30 40 50 60 70
[A-B] (M)
Time (x 10
3
s)
y = m1 + m2*exp(-m3*x)
Error Value
0.0067945 0.0022702 m1
0.0058925 0.43203 m2
0.00083385 0.026026 m3
NA 8.0723e-5 Chisq
NA 0.99975 R
79
Figure 2.10. H/D Exchange.
2.4.2.4. Kinetics for AB Dehydrogenation in the Presence of Added BH
3
· THF.
To examine the role of aminodiborane (2.6) in the mechanism of AB dehydrogenation, we
manipulated its concentration by adding (tube A) 0.125 mL of 1 M BH
3
· THF (0.125 mmol, 0.5
equiv. to AB) or (tube B) 0.125 mL of THF to otherwise typical AB dehydrogenation reactions
(7.7 mg (0.25 mmol) of AB, 13.6 mg (5 mol %) of 2.14, 0.6 mL of 2/1 diglyme/benzene-d
6
).
11
B
NMR showed 2.6 (−26.7 ppm) in tube A at the beginning of the reaction. Figure 2.11 shows an
illustration of the
11
B{
1
H} NMR spectrum acquired for the reaction above at 70 ° C after 300 s
reaction time. The peaks in this
11
B spectrum are 2.6 (-26.7 ppm), AB (-22 ppm), and BF
3
∙OEt
2
in dyglyme co-axial external standard (0 ppm).
Figure 2.11.
11
B NMR Spectrum in Presence of Added BH
3
· THF.
80
The k
obs
values for these runs were determined using
11
B NMR; data fits of these kinetic runs
and their control reactions are shown in Figure 2.12.
0.5 eq. BH
3
· THF. k
obs
= 1.57(7) × 10
-4
s
-1
THF only. k
obs
= 7.0(13) × 10
-5
s
-1
Parent conditions with added THF. k
obs
= 6.3(6) × 10
-5
s
-1
Figure 2.12. Kinetic Studies on the Effect of Added BH
3
-THF. Top: [AB] vs. time for kinetic runs
wherein BH
3
∙THF or THF were added to 5 mol% 2.14 in 2:1 diglyme/benzene-d
6
at 70
o
C as
monitored by
11
B NMR. Exponential coefficients in exponential decay plots represent k
obs
.
0
0.02
0.04
0.06
0.08
0.1
0.12
6 8 10 12 14 16 18 20 22
y = m1 + m2*exp(-m3*x)
Error Value
0.0022402 -0.0064918 m1
0.01634 0.36971 m2
0.0073418 0.15736 m3
NA 2.275e-5 Chisq
NA 0.99915 R
[A-B] (M)
Time (x 10
3
s)
0.08
0.1
0.12
0.14
0.16
0.18
0.2
6 8 10 12 14 16 18 20 22
y = m1 + m2*exp(-m3*x)
Error Value
0.013197 0.060298 m1
0.0051603 0.18679 m2
0.012611 0.069693 m3
NA 3.244e-5 Chisq
NA 0.9973 R
[A-B] (M)
Time (x 10
3
s)
0.1
0.12
0.14
0.16
0.18
0.2
6 8 10 12 14 16 18 20 22
y = m1 + m2*exp(-m3*x)
Error Value
0.0075048 0.072721 m1
0.0034884 0.17927 m2
0.0061856 0.062723 m3
NA 7.7924e-6 Chisq
NA 0.99929 R
[A-B] (M)
Time (x 10
3
s)
81
2.4.2.5. Kinetics for AB Dehydrogenation by NH
3
-Ligated Species 2.26.
The Ru−NH
3
adduct 2.26 was prepared by delivering ammonia gas to a benzene (5 mL)
solution of 2.22 (50 mg, 0.046 mmol). The reaction mixture was stirred at room temperature for
15 min. A black precipitate was filtered out and successively washed with deionized water, acetone,
benzene, and hexanes. The solid was then dried under vacuum to give a pale gray powder in 59%
yield (30 mg).
1
H NMR (pyridine-d
5
, 600 MHz): δ 8.05 (d, J
HH
= 7.1 Hz, 4H, Ph), 7.47 (d, J
HH
= 7.1 Hz, 4H, Ph),
7.18 (t, J
HH
= 7.1 Hz, 4H, Ph), 7.13 (m, 6H, Ph), 7.07 (t, J
HH
= 7.1 Hz, 4H, Ph), 4.25 (br. s, 3H,
NH3).
13
C{
1
H} NMR (pyridine-d
5
, 150 MHz): δ 202.5 (CO), 165.5 (C1 of Cp), 134.9 (Ph), 133.4 (Ph),
133.2 (Ph), 131.3 (Ph), 128.5 (Ph), 128.4 (Ph), 128.3 (Ph), 126.9 (Ph), 104.3 (C2,5 of Cp), 83.0
(C3,4 of Cp). Data are consistent with a known compound.
24
82
Figure 2.13.
1
H and
13
C NMR of 2.26 taken in pyridine-d
5
at 25 ˚C.
Rate values for 2.26-catalyzed dehydrogenation run at 70 ° C were determined using
11
B
NMR as shown vide infra. In this reaction 7.7 mg of AB (0.25 mmol) was combined with (tube
A) 20.7 mg of 2.26 (35 μmol, 14 mol %) and (tube B) 38.0 mg of 2.14 (35 μmol, 14 mol %). Data
are shown in Table 2.4. Treatment of the data for 2.26-catalyzed dehydrogenation is shown in
Figure 2.14.
83
Fast catalysis, rate = 5.12(3) × 10
-5
M s
-1
Slow catalysis case, k
obs
= 4.23(33) × 10
-4
s
-1
Figure 2.14. Kinetic Studies of AB Dehydrogenation by 2.26. [AB] vs. time for kinetic run
monitored by
11
B NMR with 5 mol % 2.26 in 2:1 diglyme/benzene-d
6
at 70 ˚C. The slope in the
linear plot represents reaction rate, and the exponential coefficient in the exponential decay plot
represent k
obs
.
2.4.2.6. Reaction of Complex 2.22 with Borazine.
Figure 2.15 shows the reaction of 2.22 with borazine. Figure 2.15 (middle) shows the
hydride region of the
1
H NMR spectrum of a benzene solution of 2.22 in a sealed tube after 1 eq.
borazine is added and incubated for 5 min at 25
o
C. The taller peak in the κ
1
–Ru–H hydride region
(δ = -9.7 ppm) corresponds to borazine adduct 2.33, and the peak in the μ
2
-Ru–H hydride region
(δ = -18.2 ppm) corresponds to borazine adduct 2.32. Multipleκ
1
–Ru–H peaks are observed after
2 hours (shown in Figure 2.15, bottom). The μ
2
-Ru–H hydride peak is no longer at presence in
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 1 2 3 4 5 6
[A-B] (M)
Time (x 10
3
s)
y = m1 + m2 * M0
Error Value
0.0010252 0.42331 m1
0.00034313 -0.051177 m2
NA 5.3512e-5 Chisq
NA 0.99937
R
2
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
7 8 9 10 11 12
[A-B] (M)
Time (x 10
3
s)
y = m1 + m2*exp(-m3*x)
Error Value
0.0028337 0.0085337 m1
0.35622 1.6827 m2
0.032901 0.42252 m3
NA 3.4841e-5 Chisq
NA 0.99542
R
2
84
the spectrum, and 2.33’s peak becomes the tallest in hydride region. Others correspond to other
O-borylated homologs of 2.21, as sketched in Scheme 2.5.
Figure 2.15. Borazine hydroboration of 2.22. Top: reaction scheme. Middle:
1
H NMR in 5 min at
25
o
C. Bottom: 2 hours at 25
o
C.
2.4.2.7. Reaction of Complex 2.14 with Borazine.
A reaction between 2.14 and borazine affords multiple κ
1
–Ru–H complexes. Upon removal
of all free borazine under vacuum, the reaction mixture was re-dissolved in benzene-d
6
. The
relative integrations of theκ
1
–Ru–H hydride signals and the aryl region in the resulting spectrum
Ru
H
OC
OC
Ph
Ph
Ph Ph
O
N
H
B
H
NH
H
B
H
N
B
HB
HN
B
H
NH
BH
H
N
25
o
C
benzne-d 6
further
hydroboration
2.33
multiple hydroborations
and borazine opening
are possible
O
Ph
Ph
Ph
Ph
Ru
OC
OC NH 3
2.26
(X-ray)
O
Ph
Ph
Ph
Ph
Ru
CO
OC
O
Ph Ph
Ph
Ph
Ru
CO
CO
2.3
1
H d =
-9.7 ppm 2.22
Ru
H
OC
OC
Ph
Ph
Ph Ph
O
N
H
B
NH
B
H
N
B
O
O
2.25
O
Ph
Ph
Ph
Ph
Ru
CO
OC
H
Ru
CO
CO
O
Ph Ph
Ph
Ph
HB
HN
B
NH
BH
H
N
another
molecule of 2.3
1
H d = -18.3 ppm
2.32
see text
85
show that the products of this reaction comprise > 90% κ
1
–Ru–H complexes (Figure 2.16).
Aqueous work-up on this sample affords 2.26 in 42% isolated yield. Therefore, because theseκ
1
–
Ru–H complexes convert to 2.26, they must be covalently bound to borazine, because it is the
only plausible source of NH
3
.
Figure 2.16.
1
H NMR taken in benzene-d
6
at 25
o
C of a reaction between 2.14 and borazine after
reaction completion and removal of the volatiles (mostly solvent and borazine) under vacuum.
2.4.2.8. Synthesis and Reactions of 2.24.
2.24 was prepared in situ by a method introduced by Clark.
23
Data treatments for 2.24-
catalyzed AB dehydrogenation are shown in Figure 2.17.
86
5.0 mol % 2.24. k
obs
= 1.26(3) × 10
-4
s
-1
. 7.5 mol % 2.24. k
obs
= 1.50(2) × 10
-4
s
-1
.
10.0 mol % 2.24. k
obs
= 1.82(2) × 10
-4
s
-1
. 15.0 mol % 2.24. k
obs
= 2.18(2) × 10
-4
s
-1
.
Figure 2.17. Kinetic Studies of AB Dehydrogenation by 2.24. [AB] vs. time monitored by 11B
NMR in 2:1 diglyme/benzene-d6 with varying catalyst 2.24 concentrations. Exponential
coefficients represents k
obs
.
0
0.1
0.2
0.3
0.4
0.5
0 5 10 15 20
[A-B] (M)
Time (x10
3
sec)
y = m1 + m2*exp(-m3*x)
Error Value
0.0029056 0.073269 m1
0.0023751 0.35695 m2
0.0027019 0.12619 m3
NA 0.00061467 Chisq
NA 0.99923 R
0
0.1
0.2
0.3
0.4
0.5
0 5 10 15 20
[A-B] (M)
Time (x10
3
sec)
y = m1 + m2*exp(-m3*x)
Error Value
0.001499 0.063836 m1
0.0014164 0.37576 m2
0.0018406 0.14966 m3
NA 0.00026698 Chisq
NA 0.99969 R
0
0.1
0.2
0.3
0.4
0.5
0 5 10 15 20
[A-B] (M)
Time (x10
3
sec)
y = m1 + m2*exp(-m3*x)
Error Value
0.0015449 0.012266 m1
0.0018672 0.42693 m2
0.0023506 0.18214 m3
NA 0.00056933 Chisq
NA 0.99953 R
0
0.1
0.2
0.3
0.4
0.5
0 5 10 15 20
[A-B] (M)
Time (x10
3
sec)
y = m1 + m2*exp(-m3*x)
Error Value
0.0007167 0.0077085 m1
0.0011452 0.42125 m2
0.001521 0.21824 m3
NA 0.00083062 Chisq
NA 0.99965 R
87
2.4.2.8. 1,10-Phenanthroline Poisoning Experiments.
Fractional poisoning experiments employing separately 0.5, 0.1, and 0 mol equiv of 1,10-
phenanthroline relative to [Ru
atom
] were done under conditions otherwise identical with our
standard conditions for ammonia−borane dehydrogenation by 2.14.
1,10-Phenanthroline (2.2 mg, 12 μmol, 50 mol % versus [Ruatom]) was added to a solution
of 7.7 mg of ammonia−borane (0.25 mmol) and 13.6 mg of 2.14 (25 μmol, 5 mol % versus AB) in
0.6 mL of 2:1 diglyme:benzene-d
6
. The rates for both the fast and slow catalysis cases at 70 ° C
were determined using
11
B NMR. (Figure 2.18)
1,10-Phenanthroline (2.2 mg) was dissolved in 0.5 mL of diglyme to make a 1,10-
phenanthroline stock solution. A portion of this solution (0.1 mL, 12 μmol, 10 mol % 1,10-
phenanthroline versus [Ru
atom
]) was added to a solution of 7.7 mg of ammonia−borane (0.25
mmol) and 13.6 mg of 2.14 (25 μmol, 5 mol % versus AB) in 0.5 mL of 2/1 diglyme/benzene-d
6
.
The rates for both the fast and slow catalysis cases at 70 ° C were determined using
11
B NMR.
88
Conv. of AB in fast catalysis case: ca. 66%. Conv. of AB in fast catalysis case: ca. 40%.
Rate = 1.35 (5) × 10
-4
M s
-1
Rate = 1.16(1) × 10
-4
M s
-1
Conversion of AB in fast catalysis case: ca. 28%. Rate = 1.06(3) × 10
-4
M s
-1
Figure 2.18. AB Dehydrogenation by 2.14 in Presence of 1,10-Phenanthroline. [AB] vs. time for
kinetic runs monitored by
11
B NMR. The slope in the linear plots represents reaction rate, and the
exponential coefficient in the exponential decay plots represent k
obs
.
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0 0.5 1 1.5 2 2.5
0.5 equiv 1,10-phenanthroline fast catalysis
y = m1 + m2 * M0
Error Value
0.0062722 0.43005 m1
0.0045219 -0.13536 m2
NA 0.00082528 Chisq
NA 0.99447 R
[A-B] (M)
Time (x10
3
sec)
0.25
0.3
0.35
0.4
0.6 0.8 1 1.2 1.4 1.6
0.1 equiv 1,10-phenanthroline fast catalysis
y = m1 + m2 * M0
Error Value
0.0013042 0.45423 m1
0.0011264 -0.1162 m2
NA 9.0236e-6 Chisq
NA 0.99972 R
[A-B] (M)
Time (x10
3
sec)
0.3
0.35
0.4
0.2 0.4 0.6 0.8 1 1.2 1.4
no 1,10-phenanthroline fast catalysis
[A-B] (M)
y = m1 + m2 * M0
Error Value
0.0027485 0.44036 m1
0.003038 -0.10591 m2
NA 3.647e-5 Chisq
NA 0.99795 R
Time (x10
3
sec)
89
A further fractional poisoning regarding the role of 1,10-phenanthroline in the 2.24-
catalyzed dehydrogenation of ammonia−borane was also conducted. These data show little
change in reactivity upon addition of the poison (Figure 2.14). 2.24 was prepared in situ by
dissolving 6.8 mg 2.22 (12 μmol) and 1.2 μL catechol borane in a 0.6 mL 2:1 diglyme/benzene-
d
6
solution and heated at 55
o
C for 30 min. The resulting solution was checked by
1
H-NMR before
proceeding to next step. Ammonia borane (7.7 mg, 0.25 mmol) and 2.2 mg (12 μmol, 50 mol %
versus 2.24) were then added to this solution and dehydrogenation was started and monitored at
70
o
C. AB consumption throughout the duration of the reaction was determined using
11
B NMR
and is shown in Figure 2.19. These data show that although the rate is not proportionally affected
by the presence of phenanthroline, so the reaction is likely homogeneous.
Figure 2.19. AB Dehydrogenation by 2.24 in Presence or Absence of 1,10-Phenanthroline. [AB]
vs. time for kinetic runs monitored by
11
B NMR. 1,10-phenanthroline doesn’t impact the reaction
rate significantly.
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0 5 10 15 20
1,10-Phenanthroline Poisoning Chatacholborane Hydroborated [Ru]
No 1,10-phenanthroline
0.5 equiv 1, 10-phenanthroline (versus [Ru])
[A-B] (M)
Time (x10
3
sec)
90
2.5 References
1. Lu, Z.; Conley, B. L.; Williams, T. J. Ruthenium Complexes with Cooperative PNP Ligands:
Bifunctional Catalysts for the Dehydrogenation of Ammonia–Borane. Organometallics, 2012,
31, 6705-6714.
2. (a) Imarisio, G. Int. J. Hydrogen Energy 1981, 6, 153-158; (b) Zoulias, E.; Varkaraki, E.;
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Medium. Int. J. Hydrogen Energy 2009, 34, 2616-2621. Regarding borane-Rh coordination,
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Ammonia-Borane Involving an Unexpected Hydrogen Transfer to Ligated Carbene and
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Centers. Angew. Chem. Int. Ed. 2010, 49, 7170-7179.
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Int. Ed. 2009, 48, 905-907.
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Independently Measured Rates of Elementary Reactions. J. Am. Chem. Soc. 2008, 130, 2285–
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98
3.1. Introduction
This chapter contains published work.
1
I would like to take this opportunity to
acknowledge my co-authors. In the study of the mechanism of alcohol oxidation by ruthium
catlysts 3.1 and 3.2, Brock Malinoski, Ana Flores contributed to kinetic data acquisition of
alcohol dehydrogenations; Dr. Brian Conley and Denver Guess conducted study on the
role of acid/base in alcohol dehydrogenation by catalyst 3.2. This mechanistic discovery
lead to the design and synthesis of a new ruthenium complex 3.3, which proves to be a
nitrile reduction catalyst.
As part of our ongoing studies of dual site ruthenium, boron-containing catalysts for
the manipulation of hydride groups, we have recently reported a series of
[di(pyridyl)borate]ruthenium complexes (3.1, 3.2)
2
that exhibit remarkable reactivity in a
number of applications,
3
notably including dehydrogenation of ammonia borane.
4
Although they are successful catalysts, these di(pyridyl)dimethylborate-derived
complexes are cumbersome to prepare, due largely to dependence on an expensive and
reactive BrBMe
2
starting material and high water and oxygen sensitivity of intermediate
complexes in their syntheses. Further, despite their catalytic utility, no direct evidence has
been collected to show a mechanistic account of the cooperative roles, if any, that boron
99
and ruthenium are playing in the reactive mechanisms of 3.1 or 3.2.
5
We suspect this is
partially due to the robustness of the bridging μ-OH ligand between the boron and
ruthenium centers, which inhibits access to a free borane in catalytic reactions.
Figure 3.1. Dual Site Catalysts for Hydride Manipulation
3.2. Results and Discussion
We have acquired experimental evidence for that the μ-OH ligand between the
boron and ruthenium in 3.2 is not labile under alcohol dehydrogenation conditions, and
that the mechanism for alcohol oxidation is most likely β–hydride elimination at the
ruthenium center. We further present a conveniently prepared, borate-pendant ruthenium
complex (3.3) that retains much of the reactivity of the original di(pyridyl)borate complexes (e.g.
3.1, 3.2), show that it is an efficient and selective catalyst for nitrile reduction, and provide
evidence for the cooperative role that boron and ruthenium play in the reaction, as hydride donor
and activating group, respectively.
100
3.2.1. Mechanism of Alcohol Oxidation by 3.2
We designed di(pyridyl)borate complexes 3.1 and 3.2 to have a boron, ruthenium dual
active site that can work in convert to position and oxidize a particular substrate. Such a strategy
is used prolifically in ligand, metal bifunctional catalysts, such as those pioneered by Noyori
6
and
Shvo
7
Our general strategy of devising a catalyst with a built-in Lewis acid to direct the metal to a
particular substrate is analogous to these.
We use alcohol oxidation by 3.2 as a mechanistic probe to study the 3.2’s ligand-metal
cooperative potential in catalytic applications. In general, there are three likely candidates for the
C–H bond cleavage steps in the mechanism of alcohol oxidation with 3.2.(Scheme 3.1) They
include (A) a β-hydride elimination from a coordinated alkoxide. This type of mechanism is
common for transfer dehydrogenation of alcohols with phosphine-ligated ruthenium(II)
catalysts.
8
A second possibility (B) is a Shvo-like mechanism wherein the bridging hydroxyl
directs the alkoxide to the metal center.
9
Our designed mechanism (C) is also possible.
101
Scheme 3.1. Three Possible Mechanisms for Alcohol Oxidation by 3.2.
Our stoichiometric studies of this mechanism revealed key insights into the reactivity of the
proposed intermediates (Scheme 3.2A). First we observed that substitution of the hydroxide
bridge of 3.2 is slow: heating 3.2 in isopropanol did not result in hydroxide substitution at neutral
pH. Conversely, the desired bridged isopropoxy complex 3.19 (independently prepared) is inert
to reaction with neutral water in acetonitrile solution. This exchange is facile, however, in the
presence of aqueous acid. Treatment of alcohol 3.12 with 3.19 results in divergent outcomes
depending on conditions (Scheme 3.2B): when the catalyst is activated with acid, no oxidative
pathways are observed. In these conditions, we see elimination and apparent carbocation
chemistry with or without 3.2. In the presence of base, however, selective and high yielding
oxidation of the alcohol to the corresponding ketone is possible (with concurrent formation of
N
Ru
N
B
O
H
Ar
N
Ru
N
B
OH
O
O
Ar
H
Ar
N
Ru
N
B
OH
H
O
Ar
N
Ru
N
B
O
H
H
N
Ru
N
B
O
H
Ar
A. b-Hydride elimination B. H
+
-coordination directed
C. Boron coordination directed
3.14 3.15 3.16
3.17 3.18
OH
MeO
O
MeO
[Ru] catalyst
Acetone
3.12 3.13
N
Ru
N
B
OH
3.2
OH
Ar
102
iPrOH-d
6
). Taken together, these observations indicate that 3.2’s μ-OH group is not labile under
the conditions of alcohol oxidation catalysis.
Scheme 3.2. Labiality of μ-OR Groups in 3.2.
In catalytic experiments, we have shown that cymene adduct 3.1 is a more reactive catalyst
for alcohol oxidation than 3.2 (Figure 3.2). This provides further data refuting a (B) or (C) class
mechanism (Scheme 3.1): a class (B) mechanism would require the presence of the μ-OH group
of 3.2, and a class (C) mechanism would necessitate a free Lewis acid site on boron. Neither of
these is available in catalyst precursor 3.1, yet alcohol oxidation with 3.1 is far faster than alcohol
oxidation with 3.2 under analogous conditions.
In analyzing the above data, one must consider a scenario in which a methylborate group in
3.1 is cleaved in the course of the reaction to reveal a boron hydroxide or a free Lewis acid. To test
this hypothesis, we compared the
11
B NMR chemical shifts of the materials formed in situ at the
103
conclusion of the reaction of 3.1 with 3.12 with the
11
B chemical shifts of 3.1 and 3.2. The catalytic
species in this reaction appear to speciate into three boron-containing moieties, which appear at
11
B δ = −14.1, −14.7, and −15.4 ppm. By comparison, the corresponding
11
B NMR chemical shifts
for 3.1 and 3.2 are −12.0 and −8.1 ppm in acetone, respectively. Thus, because the
11
B species of
this reaction are upfield of 3.1 and downfield of 3.2, we surmise that if a methylborate group is
cleaved in 3.1 in the course of this reaction, it is not converted to a boron hydroxide.
Figure 3.2. Oxidation of 3.12 by Ruthenium Catalysts. 3.12 (10 μL, 71 μmol) by 3.2 (4.1 mg, 8.7
μmol, 12 mol%) and 3.1 (4.6 mg, 8.7 μmol, 12 mol%) in acetone-d
6
(0.7 mL) in the presence of
potassium tert-butoxide (1.6 mg, 14 μmol, 20 mol%) at 100 °C in a sealed J-Young tube. Points
are plotted for the concentration of 3.12 vs. time, where circles and squares respectively represent
reactions catalyzed by 3.1 and 3.2.
0
0.02
0.04
0.06
0.08
0.1
0 5 10 15 20 25 30
1 as the catalyst
8 as the catalyst
Conc of [16] (M)
Time (x100 min)
■ 3.2 as the catalyst
□ 3.1 as the catalyst
104
3.2.2. Synthesis of Ruthenium Complex 3.3
Ruthenium complex 3.3 is designed to be a more versatile boron-metal dual-center
catalyst. Unlike 3.1 and 3.2, a boron-hydride bond is pre-installed in the
bis(pyrazole)borate (Bp) ligand. The synthesis of 3.3 (Scheme 3.3) proceeds from
potassium di(pyrazolyl)borohydride
10-12
and commercially-available (cymene)ruthen-
ium dichloride dimer to give intermediate chloride 3.4. Although 3.4 can be isolated, it is
easily converted in situ to 3.3 by treatment with 1 equiv. thallium (or silver) triflate in
nitrile solution. The synthesis proceeds in two smooth steps without the need for materials
that are cost-prohibitive or difficult to manipulate: all materials are amenable to handling
using standard Schlenk techniques and/or a glove box. 3.3 can be crystallized from
isopropanol and hexanes; its molecular structure was determined by single crystal X-ray
diffraction (Scheme 3.3). The crystal structure of 3.3 shows the borate ligand in a
tetrahedral geometry at boron, which has analogy to the popular
tris(pyrazolyl)borohydride (Tp) ligand series.
11-13
105
Scheme 3.3. Synthesis of Precursor 3.3 and Complex 3.6.
The synthesis of 3.3 revealed an important insight into the mechanism of its catalytic
reactivity. In the conversion of 3.3 to 3.3, a hydride is transferred from a ligand B-H group
to 5’s coordinated nitrile in > 90% NMR yield (Scheme 3.3). This reaction is the first
example of our envisioned cooperative reactivity of ruthenium and boron. Contrary to the
design concept from which we originally prepared 3.2,
3
boron in 3.3 is not behaving as a
Lewis acid. The structure of 3.3 shows that B-H addition to the nitrile proceeds in a cis
fashion, and NMR evidence reveals that the selectivity for this geometry is exclusive. Thus
we believe that the mechanism for this reaction involves intramolecular hydride transfer
from boron to carbon, rapidly followed by (or concerted with) boron-nitrogen
coordination. In an attempt to commercialize 3.3 with Strem Chemicals, we conducted
106
larger scale synthesis. In the process, we find that 3.3 can be prepared on a scale of several
grams with high yields using the same procedures.
Catalyst 3.3 has similar reactivity to 3.2 in ammonia borane dehydrogenation and
water oxidation. For example, 3.3 is a modestly improved catalyst for ammonia borane
dehydrogenation, generating 2.1 equivalents of H
2
in 6 hr, where 3.2 would generate 2.1
equiv H
2
in 7 hr. Both 3.2 and 3.3 have reactivity in water oxidation: O
2
could be produced
at 70 ° C at a rate of ca. 10
-3
s
-1
in the presence of aqueous cerium(IV). While this is slow,
the juxtaposition of successful reactions in both oxidative and reductive conditions
highlight the unique ability of these catalysts to support both electron rich and poor
catalytic transition states.
3.2.3. Nitrile Reduction
The stoichiometric reduction of acetonitrile observed in the synthesis of 3 can be
made catalytic by treating 3 with nitrile and sodium borohydride,
14
thus enabling a mild
and selective approach to the synthesis of primary amines from nitriles, which remains a
contemporary topic in bifunctional catalysis.
15
Known methods for nitrile reduction, e.g.
excess LiAlH
4
, borohydride reduction of a nitrilium salt,
16
metal hydride reagents,
17
or
high-pressure hydrogenation
18
can be incompatible with important synthetic handles, such
as aryl bromide and nitro groups.
19
More mild conditions compatible with groups like this
require a stoichiometric portions a borane reagent,
20
which in some cases must be
107
independently prepared. The new catalytic mechanism reported here enables high
functional group tolerance while incorporating an inexpensive reducing agent.
Table 3.1. Optimization of Nitrile Reduction Conditions.
Entry Catalyst NaOtBu 5 h conversion
a
NMR Yield
b
1 3.3 none 50 % c
2 3.3 1 equiv. > 95% > 90%, 5 hr
3 3.2 1 equiv. 46% 42%, 245 hr
4 3.6 1 equiv. 41% trace, 245 hr
5 no catalyst 1 equiv. 38% 43%, 245 hr
a
Starting material consumption by NMR in 5 hours.
b
Product formed upon consumption
of nitrile and subsequent addition of water.
c
Not recorded.
Table 3.1 shows the discovery and optimization of our conditions for nitrile
reduction. In presence of 5 mol% 3.3, 2.0 equiv. NaBH
4
can reduce 4-
trifluoromethylbenzonitrile (3.7a) to the corresponding benzylamine (3.8a) in 50%
conversion by NMR in 5 hours (entry 1). When 1.0 equiv. of NaO
t
Bu was added, the
analogous reaction reached >90% yield (>95% conversion) in the same time. With no
108
catalyst, the reaction was much slower, reaching completion in 7 days with a 43% overall
yield (entry 5). If 3.3’s Tp-ligated homologue (3.6, Scheme 3.3) is used in this reaction,
much of the starting material decomposed under the reaction conditions. This illustrates
that the μ-acetimine ligand in 3.3 plays an essential role in nitrile reduction.
The substrate scope of nitriles that can be reduced using our optimized conditions is
broad. As shown in Table 3.2, aromatic, aliphatic, and heterocyclic nitriles are smoothly
reduced to amines under optimized conditions. Both electron poor and electron rich
substrates can be reduced in high yield. For example, In presence of 5 mol% 3.3, 4.0 equiv.
NaBH
4
can reduce 4-trifluorobenzonitrile 3.7a to the corresponding benzylamine 3.8a in
82% isolated yield (entry 1). Electron rich nitrile 3.7b can be reduced with similar facility
in 87% yield (entry 2). We believe that this startling result is attributable to selective
binding and activation of the nitrile over the nitro by the catalyst’s ruthenium center.
Furthermore, we see this as additional evidence that nitrile binding to ruthenium is
important to the catalytic mechanism.
109
Table 3.2. Scope of 3.3-Catalyzed Nitrile Reduction.
Entry Nitrile Conditions Product Yield
a
1
3.7a
5 mol% 3.3
4 eq NaBH
4
, 1 eq NaO
t
Bu
MeOH, reflux,12 h
3.8a
82%
2
3.7b
5 mol% 3.3
8 eq NaBH
4
, 1 eq NaO
t
Bu
MeOH, reflux,12 h
3.8b
87%
3
3.7d
5 mol% 3.3
8 eq NaBH
4
, 1 eq NaO
t
Bu
MeOH, reflux,12 h
3.8d
85%
4
3.7e
5 mol% 3.3
4 eq NaBH
4
, 1 eq NaO
t
Bu
MeOH, reflux,12 h
3.8e
80%
5
3.7f
5 mol% 3.3
4 eq NaBH
4
, 1 eq NaO
t
Bu
MeOH, reflux,12 h
3.8f
84%
110
6
3.7g
5 mol% 3.3
8 eq NaBH
4
, 1 eq NaO
t
Bu
MeOH, reflux,12 h
3.8g
64%
7
3.7h
5 mol% 3.3
4 eq NaBH
4
, 1 eq NaO
t
Bu
MeOH, reflux,12 h
3.8h
60%
8
3.7i
5 mol% 3.3
4 eq NaBH
4
, 1 eq NaO
t
Bu
MeOH, reflux,12 h
3.9i
56%
9
3.7j
5 mol% 3.3
4 eq NaBH
4
, 1 eq NaO
t
Bu
MeOH, reflux,12 h
3.9j
77%
10
3.7k
5 mol% 3.3
4 eq NaBH
4
, 1 eq NaO
t
Bu
MeOH, reflux,12 h
3.8k
72%
a
Reported yields are isolated yields.
Despite the presence of highly reducing conditions, an aryl bromide group in nitrile
3.7d is not derivitized in the course of amine synthesis. This is an important result because
aryl bromides such as this are high value substrates for cross-coupling and amination
reactions relevant to the synthesis of medicinally-relevant compounds.
21
This example
111
further illustrates high-yielding reduction of an alkyl nitrile. Whereas ketone groups are
known to react with NaBH
4
, the reduction of 3.7e (entry 5) illustrates high-yielding double
reduction for the synthesis of aminoachohol 8e. Similar double reduction is observed in
the reaction of cinnamonitrile (3.7g, entry 7) to give alkyl amine 8g in 60% yield.
Reactions of nitriles appended to aromatic heterocycles afforded complicated results.
For example, pyridine 3.7h is compatible with the conditions and resulted in the formation
of aminomethylpyridine 3.8h in 64% yield (entry 7). By remarkable contrast, more
electron rich heterocycle systems are not reduced. For example, 2-cyanofuran 3.7i and 2-
cyanothiophene 3.7j are selectively monohydrated as opposed to reduced. Thus, amides
3.9ij were isolated as the main products (entries 9 and 10). We suspect that the mechanism
for these reactions involves the addition of methanol (solvent) to a ruthenium-coordinated
nitrile, and that the amide products are formed upon aqueous work-up. We do not
currently have a proposal to account for the selectivity of hydration versus reduction.
Particularly, in case of the 2-(phenylethynyl)benzonitrile (3.7k) as the substrate, a tandem
alkyne insertion takes place to afford cyclized product (Z)-1-benzylideneisoindoline 3.8k.
3.8k appears to be the only product by NMR, and the only isolated product. We propose
this reaction goes in a cascade of the following steps: 1) reduction of the nitrile to an amine;
2) the alkynyl group activation by ruthenium catalyst; 3) amine insertion to the alkynyl
group to close a five-membered ring. Because the ring formation is also controlled by the
112
ruthenium catalyst, it has very high selectivity for both (Z)-alkene and five-membered ring
(Scheme 3.4).
Scheme 3.4. Tandem Nitrile Reduction-Cyclization.
3.2.4. Mechanistic Proposal
According to the insight gained from the stoichiometric synthesis of 3.3 from 3.4, we
propose the following template mechanism for catalysis (Scheme 3.5). We suspect that the
bridging imine 3.3 is reduced by borohydride to produce the amine product. We do not
have direct evidence for the intermediacy of 3.10; however, treatment of 3.3 with a
stoichiometric portion of NaBH
4
in methanol-d
4
results in clean desymmeteriation of the
catalyst’s cymene and pyrazole C—H groups, consistent with the formation of
diastereomeric protons, as expected with the pyramidalization of the bridging nitrogen
ligand (see experimental section for graphical spectra). Still, the intermediacy of 3.10
remains a proposal because we have not established the kinetic role of this transient
material.
113
Scheme 3.5. Mechanistic Template for Nitrile Reductions.
We propose that the bridging amine ligand is replaced by incoming substrate, and
the borohydride group of 5 is regenerated by a hydride from NaBH
4
, although we do not
know the details of these steps. We do observe that treatment of 3 with a stoichiometric
portion of NaBH
4
in methanol-d
4
results in the formation of (MeO)
4
B
-
, unreacted BH
4
-
and a catalyst doublet, by
1
H-coupled
11
B NMR, which indicates a (pz)
2
BH
2
intermediate,
if formed, is transient. Thus, we suspect that X in Scheme 3.5 is methoxide.
1
H NMR
studies of the working catalyst reveal that once ligated, the reductions of the nitrile groups
and imine groups are very facile. Thus, the rate-determining step could be amine for nitrile
substitution of the nitrile substrate to the ruthenium center.
114
3.3. Conclusion
In conclusion we report here a conveniently-prepared homologue of our successful
di(pyridyl)borate-ligated ruthenium complexes. The new catalytic scaffold has
comparable reactivity to the old in several key reactions and introduces new reactivity to
the cooperative ruthenium, boron catalytic motif by enabling selective and high yielding
nitrile reduction under mild conditions. Furthermore, this platform has yielded new
insight into the cooperative reactivity of ruthenium and boron by showing a plausible
scenario of how these two centers can work together respectively as activating group
(ruthenium) and hydride donor (boron). Ongoing work in our laboratory regards the
application of this system to the reduction of other high-value pi systems and the
elucidation of the mechanistic details of these reactions.
3.4. Experimental Section
3.4.1. General Procedure
All air and water sensitive procedures were carried out either in a Vacuum Atmosphere
glove box under nitrogen (0.1-10 ppm O
2
for all manipulations) or using standard Schlenk
techniques under nitrogen. Deuterated NMR solvents were purchased from Cambridge Isotopes
Laboratories. Acetonitrile-d
3
and acetonitrile were dried over calcium hydride, methanol-d
4
and
methanol was dried over sodium. These solvents were distilled prior to use. Methylene chloride
and hexanes were obtained from a J. C. Meyer Solvent Dispensing System (SDS) and used
115
without further purification. Deionized water was purchaced from Arrowhead. Dichloro(p-
cymene)ruthenium(II) dimer
and thallium triflate were purchased from Strem Chemicals. 2-
Methyl-6-nitrobenzonitrile, 4-bromophenylacetonitrile, 4-(dimethylamino)benzonitrile, and 4-
cyanopyridine were purchased from Sigma Aldrich. Cinnamyl nitrile was purchased from
Eastman Chemical Company. 4-(Trifluoromethyl)benzonitrile was purchased from Matrix
Scientific. 3,4-Dimethoxybenzonitrile and 4-acetyl-benzonitrile were purchased from Alfa Aeser.
Potassium bis(pyrazolyl)borate was synthesized and purified by the method used by Hill.
8
1
H,
11
B,
13
C,
19
F NMR spectra were obtained on Varian 400MR, VNMRS 500, or VNMRS 600
spectrometers with chemical shifts reported in units of ppm. All
1
H chemical shifts are referenced
to the residual
1
H solvent (relative to TMS). All
11
B chemical shifts are referenced to a BF
3
•OEt
2
in diglyme co-axial external standard (0 ppm). All
19
F chemical shifts are referenced to the
trichlorofluoromethane standard (0 ppm). All air and water sensitive NMR spectra were acquired
using 8” J-Young tubes (Wilmad) with Teflon valve plugs. The J-Young NMR tubes were shaken
vigorously for several minutes with chlorotrimethylsilane then dried in vacuo on a Schlenk line
prior to use. ESI-HRMS data were acquired on an Agilent LC-TOF (2006). The GC-MS data
were acquired on a Thermo Scientific Focus DSQ II GC-MS system.
Safety Notes.
Extreme caution should be used when treating any borohydride reagent with a ruthenium catalyst,
because this can result in the release of hydrogen gas and lead to sudden pressurization of a
reaction vessel.
116
3.4.2. Synthesis and Structural Data
3.4.2.1. Complex Synthesis
[κ
2
-(1-pz)
2
BH
2
]Ru(cymene)Cl 3.4: A solution of potassium
bis(pyrazolyl)borate (1.64 mmol, 305 mg) in 30 mL acetonitrile was
added dropwise to a stirred solution of [Ru(cym)Cl
2
]
2
(0.82 mmol, 500
mg) in 30 mL acetonitrile. During the addition, a color change from dark
red to light red was observed. The reaction was stirred at room temperature (~10 °C) for 15
minutes. The solvent was then removed and the residue was dried under vacuum for overnight.
This residue was dissolved in a minimal amount of benzene, and insoluble materials were filtered
off. The benzene solution was then saturated with hexane and concentrated under reduced
pressure. A red precipitate was collected and identified as 3.4 in 40% yield (273 mg).
1
H NMR (600 MHz in methylene chloride-d
2
at 25 ∘C): 7.69 (d, pyrazolyl 2H, J = 2.2 Hz), 7.55
(d, pyrazolyl 2H, J = 2.3 Hz), 6.24 (t, pyrazolyl 2H, J = 2.3 Hz), 5.56 (d, cym aromatic 2H, J = 6.3
Hz), 5.38 (d, cym aromatic 2H, J = 6.3 Hz), 3.8~3.2 (d, BH
2
, J
BH
= 120 Hz), 2.90 (septet, cym
methine 1H, J = 6.9 Hz), 1.94 (s, cym methyl, 3H), 1.26 (d, sym isopropyl methyls 6H, J = 6.9
Hz).
13
C NMR (150 MHz in methylene chloride-d
2
at 25 ∘C): δ = 144.19 (pyrazolyl), 136.73
(pyrazolyl), 105.73 (pyrazolyl), 105.21 (ipso cymene), 99.15 (ipso cymene), 84.95 (cymene
Ru
N
N
N
N
B
H
H
Cl
117
aromatic), 83.79 (cymene aromatic), 30.65 (cym methine), 22.1(cym isopropyl methyls), 18.03
(cym methyl).
11
B NMR (192 MHz in methylene chloride-d
2
at 50
∘
C): δ = -7.88 (d, J
B,H
= 100 Hz)
ESI-HRMS for C
16
H
23
N
4
RuCl: calculated [MH]
+
419.0742, found 419.0736.
Figure 3.3.
1
H-NMR spectrum for 3.4
Figure 3.4.
13
C-NMR spectrum for 3.4
Figure 3.5.
11
B-NMR (
1
H coupled) spectrum for 3.4
118
{[κ
3
-(1-pz)
2
HB(N=CHCH
3
)]Ru(cymene)}
+
TfO
-
3.3: A
solution of potassium bis(pyrazolyl)borate (1.30 mmol, 244
mg) in 30 mL acetonitrile was added dropwise to a stirred
solution of [Ru(cym)Cl
2
]
2
(0.65 mmol, 400 mg) in 30 mL
acetonitrile. During the addition, a color change from dark red to light red was observed. The
reaction was stirred at room temperature for 15 minutes. An aliquot of thallium triflate (2.7 mmol,
970 mg) was added to the solution. A white precipitate was observed immediately. The resulting
mixture was stirred for another 15 minutes before it was filtered though celite. The filtrate was
decanted, extracted with methylene chloride, concentrated, and dried under vacuum overnight.
The residue can be washed with a
i
PrOH to hexane 4:1 solvent to afford a bright yellow powder
which was later identified as 3.3 (320 mg, 43%). The crude product can also be recrystallized from
an
i
PrOH/hexane solvent system to form crystals for X-ray diffraction analysis.
Alternatively, 3.3 can be made using the less toxic AgOTf in place of TlOTf. The synthetic
procedure is very similar: a solution of potassium bis(pyrazolyl)borate (0.65 mmol, 122 mg) in
10 mL acetonitrile was added dropwise to a stirred solution of [Ru(cym)Cl
2
]
2
(0.32 mmol, 200
mg) in 10 mL acetonitrile. During the addition, a color change from dark red to light red was
observed. The reaction was stirred at room temperature for 15 minutes. An aliquot of silver triflate
(1.3 mmol, 353 mg) was added to the solution. A brownish white precipitate was observed
immediately. The resulting mixture was stirred for another 15 minutes before it was filtered
though celite. The filtrate was decanted, extracted with methylene chloride, concentrated, and
Ru
N
N
N
N
B
H
N
OTf
119
dried under vacuum overnight. The residue can be washed with an
i
PrOH to hexane 4:1 solvent
to afford 3.3 (108 mg, 29%).
1
H NMR (600 MHz in acetonitrile-d
3
at 25 ∘C): δ = 8.28 (q, N=CHCH
3
1H, J = 5.3 Hz), 8.10
(d, pyrazolyl 2H, J = 2.2 Hz), 7.66 (d, pyrazolyl 2H, J = 2.3 Hz), 6.31 (t, pyrazolyl 2H, J = 2.3
Hz), 6.15 (d, cym aromatic 2H, J = 6.4 Hz), 6.02 (d, cym aromatic 2H, J = 6.4 Hz), 3.72 (q, BH,
J
BH
= 120 Hz) 2.83 (septet, cym methine 1H, J = 6.9 Hz), 2.34 (d, N=CHCH
3
, 3H, J = 5.3 Hz)
2.22 (s, cym methyl, 3H), 1.09 (d, sym isopropyl methyls 6H, J = 6.9 Hz).
13
C NMR (150 MHz in acetonitrile-d
3
at 25 ∘C): δ = 177.5 (N=CHCH
3
), 142.89 (pyrazolyl),
132.6 (pyrazolyl), 107.6 (pyrazolyl), 105.7 (ipso cymene), 100.0 (ipso cymene), 83.99 (cymene
aromatic), 83.6 (cymene aromatic), 30.6 (cym methine), 24.1 (N=CHCH
3
), 21.5(cym isopropyl
methyls), 17.8 (cym methyl).
11
B NMR (192 MHz in acetonitrile-d
3
at 50 ∘C): δ = 1.53 (d, J
B,H
= 120 Hz).
19
F NMR (562 MHz in acetonitrile-d
3
at 25 ∘C): δ = -78.96 (s, triflate).
ESI-HRMS for C
18
H
25
N
5
Ru: calculated [M]
+
424.1241, found 424.1256.
120
Figure 3.6.
1
H-NMR spectrum for 3.3
Figure 3.7.
13
C-NMR spectrum for 3.3.
Figure 3.8.
11
B-NMR (
1
H coupled) spectrum for 3.3.
Figure 3.9.
19
F-NMR spectrum for 3.3.
121
Synthesis for {[κ
3
-(1-pz)
3
BH]Ru(cymene)}
+
PF
6
-
3.6 was
adopted from known methods.
11-13
Potassium
tris(pyrazolyl)borate (0.36 mmol, 92 mg) was added to a stirred
methylene chloride solution of [Ru(cym)Cl
2
]
2
(0.18 mmol, 112
mg). The reaction mixture was stirred at room temperature for an hour with a color change from
dark red to orange and eventually to yellow. Then thallium triflate (0.76 mmol, 267 mg) was
added and the reaction was stirred overnight. This mixture was filtered through celite, dried under
vacuum overnight, then triturated with Et
2
O. The expected product 3.6 was collected with a 55%
overall yield (120 mg).
3.6 was identified by NMR, and data are consistent with a previously-reported sample.
11
3.19 was generated in situ according to the following procedure.
In a glove box, 3.2 (10.0 mg, 11 μmol) was dissolved in
dichloromethane (0.6 mL) in a 8 inch J-Young tube that had
been pre-treated with chlorotrimethylsilane. Silane residue had
been removed under reduced pressure before use. Trifluoroacetic anhydride (5.0 μL, 38 μmol, 2.0
eq) was added to the tube. The mixture was then heated to 60 ° C for 20 h and cooled to room
temperature, then anhydrous
i
PrOH (7.3 μL, 96 μmol, 5 eq) was added to the J-Young tube in the
glove box. The reaction mixture was heated at 60 ° C for 20 h. All volatiles were removed under
reduced pressure on the Schlenk line. Recrystallization from dichloromethane/hexane yielded a
pale yellow solid 2.3 mg (21%).
Ru
N
N
N
N
B
H
N
N
PF
6
122
1
H NMR (600 MHz in dichloromethane-d
2
at 25 °C): δ = 8.57 (d, pyridyl 2H, J = 5.6 Hz), 7.59
(dt, pyridyl 2H, J = 1.6, 7.6 Hz), 7.39 (d, pyridyl 2H, J = 7.8 Hz), 7.07 (dt, pyridyl 2H, J = 1.6, 6.6
Hz), 3.82 (m, (CH
3
)
2
CH-O, 1H), 2.66 (s, CH
3
CN axial, 3H), 2.55 (s, CH
3
CN equatorial, 6H),
0.80 (d, (CH
3
)
2
CH–O 6H, J = 6.4 Hz), 0.43 (s, CH
3
–B 3H).
13
C{
1
H} NMR (150.84 MHz in dichloromethane-d
2
at 25 °C) δ = 184.33 (broad s, ipso pyridyl-
B), 152.87 (pyridyl), 134.81 (pyridyl), 125.73 (axial CH
3
CN), 125.53 (equatorial CH
3
CN),
125.39 (pyridyl), 121,48 (pyridyl), 70.49 ((CH
3
)
2
CH–O), 23.42 ((CH
3
)
2
CH–O), 5.27 (axial
CH
3
CN), 4.70 (equatorial CH
3
CN), 1.33 (broad s, CH
3
-B).
11
B NMR (192 MHz in dichloromethane-d
2
at 50 °C): δ = 2.17.
Figure 3.10.
1
H-NMR spectrum for 3.19.
Figure 3.11.
13
C-NMR spectrum for 3.19.
123
Figure 3.12
11
B spectrum for 3.19. The bigger peak corresponds to a BF
3
etherate standard.
Figure 3.13. Graphical
1
H-
1
H COSY spectrum for 3.19.
124
A mixture of 1.8 μL 3.12 (13 μmol), 0.6 μL HBF
4
(4.2 μmol) and 2.4 mg 3.2
(4.2 μmol) was dissolved in 0.6 mL acetonitrile-d
3
in a sealed J-Young tube.
The tube was then heated in a regulated oil bath at 100 ° C for 18 h. The
structure of 3.20 was determined based on NMR data and previously reported reference
22
.
125
3.4.2.2. Ammonia-Borane Dehydrogenation and Water Oxidation
Catalyst 3.3 has comparable reactivity to 3.2 in ammonia borane dehydrogenation and
water oxidation (Figure 3.7). While 3.3 is modestly faster initially, 3.2 generates a somewhat
higher extent of dehydrogenation. Further, both 3.2 and 3.3 have reactivity in water oxidation. For
example, in the presence of aqueous cerium(IV), O
2
could be produced at 70 ° C at a rate of 7.2 ×
10
-4
s
-1
(entry 3). The juxtaposition of successful reactions in both oxidative and reductive
conditions highlight the unique ability of 3’s novel borate scaffold to support both electron rich
and poor catalytic transition states.
126
(left) H
2
production from AB with 5.0 mol % [Ru] (3.2 or 3.3) under air at 70
o
C under optimized
conditions in tetraglyme (3.3) or diglyme (3.2) solution. (middle) AB consumption by
11
B NMR
at 70 ˚C in 2:1 diglyme/benzene solution. Data are fit to exponential decay (first order reaction
kinetics with respect to AB). (right) O
2
production from water with 6 μmol [Ru] (3.2 or 3.3) at
70
o
C in presence of Ce(IV).
Figure 3.14. Time Course Graphs for Ammonia Borane Dehydrogenation and Water Oxidation
with Catalyst Precursors 3.2 and 3.3.
a
0
0.5
1
1.5
2
2.5
0 5 10 15 20 25
3
2
Equivalents H
2
Time (x 10
3
sec)
0
0.1
0.2
0.3
0.4
0.5
0 2 4 6 8 10
3
2
[AB] (M)
y = m1 + m2*exp(-m3*x)
Error Value
0.011739 -0.08269 m1
0.009348 0.53427 m2
0.023057 0.48688 m3
NA 0.00040365 Chisq
NA 0.99926 R
y = m1 + m2*exp(-m3*x)
Error Value
0.00274 0.005722 m1
0.0025839 0.42864 m2
0.0067926 0.37437 m3
NA 0.0003462 Chisq
NA 0.99967 R
Time (x 10
3
sec)
0
5
10
15
0 0.5 1 1.5 2 2.5
2
3
TON
Time (x 10
3
sec)
127
3.4.2.2.1. Ammonia Borane Dehydrogenation
In a typical
11
B NMR reaction, 7.7 mg AB was combined with ruthenium catalyst (3.2 or
3.3, 5 mol %) in a J. Young NMR tube while in a glovebox under nitrogen. Diglyme (0.4 mL) and
benzene-d
6
(0.2 mL) were added to the tube. Because 3.3 is not soluble in the diglyme solution,
3.3 (7.1 mg, 5 mol % to 7.7 mg of AB) and 1.0 equiv. of AB (0.4 mg) was heated at 70
o
C for 10-
15 minutes until there is no undissolved solid in the tube. Then of AB (7.7 mg) was added to the
tube in a glovebox under nitrogen. The sample tube was immediately inserted into a preheated
NMR (70
o
C) and the kinetic monitoring commenced after quick locking and shimming.
Disappearance of AB in the solution was monitored by the relative integration of its characteristic
peak in the
11
B spectrum (-22 ppm) and the BF
3
· OEt
2
standard. All spectra were processed using
VNMRJ (v. 2.3). The acquisition involved a 1.67 sec pulse sequence in which 4,096 complex
points were recorded, followed by 1 sec relaxation delay. To eliminate B-O peaks from the
borosilicate NMR tube and probe, the
11
B FIDs were processed with back linear prediction, ca. 5-
15 points.
In a typical eudiometry reaction, 7.7 mg AB (0.25 mmol) was combined with catalyst (1,
7.2 mg, 5.0 mol %) in a 2 mL Schlenk tube equipped with a Teflon stir bar while in a glovebox
under nitrogen. tetraglyme (0.60 mL) was added to the tube. The side arm of the valve was
connected to a piece of Tygon tubing, which was adapted to 20 gauge (0.03”) Teflon tubing with
a needle. For reactions run under N
2
, a three-way valve was inserted between the 20 gauge Teflon
tubing and the Tygon tubing so that the tubing could be thoroughly purged with N
2
gas after
128
assembling the entire apparatus. In either case, the 20 gauge tubing was threaded through a porous
septum that was sized to fit over the open end of a buret that was flame sealed on the other end.
The role of the septum is to help keep the tubing inside the buret during the reaction. The buret
was filled with water and the septum was attached. The entire apparatus was then inverted into a
one-liter cylinder filled with water and clamped onto a metal ring stand. The reactor’s valve was
opened to release gas from the reactor headspace while heating in a regulated oil bath at 70
o
C.
The volume of liberated gas was recorded periodically until gas evolution ceased. Liberated
hydrogen was quantified by recording its volume displacement in the eudiometer and correcting
its volume for water content.
3.4.2.2.2. Water Oxidation
In a typical reaction, 6 μmol ruthenium complex (1 or 3) was suspended in 2 mL deionized
water in a 15 mL Schlenk flask connected with a eudiometer. To the solution, 1.1 g ceric
ammonium nitrate (2 mmol) was added. The reaction was then heated in a 70 ° C oil bath. The
reaction was monitored by observing O
2
formation by eudiometry.
No gas evolution was observed in an analogous reaction that was free of ruthenium.
129
3.4.2.3. Optimization of Nitrile Reduction Conditions
General procedures for screening reaction conditions for nitrile reductions: 4-trifluoromethyl
benzonitrile (0.05 mmol), NaBH
4
(0.1 mmol), NaO
t
Bu (0.05 mmol), and ruthenium catalyst (5
mol % if used) were dissolved in methanol-d
4
(0.6 mL) in a J. Young NMR tube in a dry box. Four
parallel runs were set up using (1) no ruthenium catalyst (2) 3.2, (3) 3.3, and (4) 3.6 respectively.
The tubes were then heated in an oil bath at 70 ° C and
1
H NMR spectra were taken at several time
points. The nitrile conversion against time is plotted in Table 3.3. The reaction involving catalyst
3.3 reached completion much faster than the three other reaction with an NMR yield > 90% upon
hydrolysis, whereas some other reactions did reach a reaction completion after 85 h and had lower
NMR yields upon hydrolysis (see Table 3.3). The integration standard for NMR conversions was
residual solvent, CHD
2
OD, in a sealed J-Young NMR tube. NMR yields were similarly
determined after the specified extended reaction time upon addition of water to the tube.
130
Table 3.3. Nitrile Reduction Condition Screening.
entry catalyst base NMR yield
1 3.3 no base N/A
2 3.3 1 equiv. > 90 %
3 3.2 1 equiv. 42 % (245 hr)
4 3.6 1 equiv. Trace (245 hr)
5 none 1 equiv. 43 % (245 hr)
0
0.01
0.02
0.03
0.04
0.05
0 20 40 60 80
3
no catalyst
2
Tp catalyst
Time (h)
Nitrile (mmol)
131
3.4.2.4. Nitrile Reduction
General procedure for nitrile reduction: Catalyst 3 (0.05 mmol, 28.6 mg) and sodium tert-
butoxide (1 mmol, 96.1 mg) were dissolved in 15 mL methanol. The solution was heated in an oil
bath at 70 ° C for about 2 minutes until all the solids was dissolved. Then nitrile substrate (1 mmol),
sodium borohydride (4 mmol, 150.4 mg) were added to the solution. The solution was again
heated in a 70 ° C oil bath and the reaction was monitored by TLC (typically eluting with
CH
2
Cl
2
:MeOH:Et
3
N = 50:1:1). Once the reaction was finished, the methanol solvent was
removed under reduced pressure. The residue was extracted with EtOAc (20 mL × 3) from H
2
O
(20 mL), dried over anhydrous sodium sulfate, and purified by column chromatography (typically
eluting with CH
2
Cl
2
100 mL, then CH
2
Cl
2
:MeOH = 50:1 250 mL, then CH
2
Cl
2
:MeOH:Et
3
N =
50:1:1 250 mL).
132
(4-(Trifluoromethyl)phenyl)methanamine 3.8a: 3.7a was subjucted to
the conditions described above. This reaction afforded the product as a
white solid (143 mg, 82%).
1
H NMR (400 MHz in acetonitrile-d
3
at 25
o
C): δ = 7.67 (d, 2H, J = 8.0 Hz), 7.57 (d, 2H, J = 8.0
Hz), 4.00 (s, 2H), 2.65 (br, NH
2
2H).
13
C NMR (100 MHz in acetonitrile-d
3
at 25
o
C): δ = 143.9, 135.0, 129.7, 124.4, 124.3, 72.2, 45.5.
IR (neat cm
-1
): 3344, 3096, 1434, 1325, 1166.
GC-MS for C
8
H
8
F
3
N: calculated [M] 175.06, found 175.14.
Figure 3.15.
1
H-NMR Spectrum of 3.8a.
Figure 3.16.
13
C-NMR Spectrum of 3.8a.
NH
2
F
3
C
133
(3,4-Dimethoxyphenyl)methanamine 3.8b: 3.7b was subjucted to the
conditions described above, except more sodium borohydride was used
(8 mmol, 300.7 mg). This reaction yielded a white solid (145 mg, 87%).
1
H NMR (600 MHz in methanol-d
4
at 25 ∘C): δ = 7.01 (d, 1H, J = 2.0 Hz), 6.93 (s, 1H), 6.91 (d,
2H, J = 2.0 Hz), 3.86 (s, 3H), 3.83 (s, 3H). 3.81 (s, 2H).
13
C NMR (150 MHz in methanol-d
4
at 25 ∘C): δ = 150.7, 150.1, 134.1, 121.5, 113.2, 113.0, 56.6,
56.5, 45.9.
IR (neat cm
-1
): 3370, 3303, 2998, 2835, 1639, 1515, 1264, 1027.
GC-MS for C
9
H
13
NO
2
: calculated [M] 167.19, found 167.09.
Figure 3.17.
1
H-NMR Spectrum of 3.8b.
Figure 3.18.
13
C-NMR Spectrum of 3.8b.
NH
2
MeO
MeO
134
2-(4-Bromophenyl)ethanamine 3.8d: 3.7d was subjucted to the
conditions described above, except sodium borohydride loading (8
mmol, 300.7 mg in total). The reaction afforded product as a white solid (170 mg, 85%).
1
H NMR (600 MHz in acetonitrile-d
3
at 25 ∘C): δ = 7.46 (d, 2H, J = 8.1 Hz), 7.24 (d, 2H, J = 8.1
Hz), 3.14 (t, 2H, J = 8.4 Hz), 3.01 (t, 2H, J = 8.4 Hz), 2.49 (br, NH
2
2H).
13
C NMR (150 MHz in acetonitrile-d
3
at 25 ∘C): δ = 138.8, 132.6, 132.1, 120.9, 41.4, 33.4.
IR (neat cm
-1
): 3391, 2994, 2900, 1619, 1467, 1378, 1178, 816, 563.
GC-MS for C
8
H
10
BrN: calculated [M] 199.00, found 199.01.
Figure 3.19.
1
H-NMR Spectrum of 3.8d.
Figure 3.20.
13
C-NMR Spectrum of 3.8d.
Br
NH
2
135
1-(4-(Aminomethyl)phenyl)ethanol 3.8e: 3.7e was subjucted to the
conditions described above. This reaction afforded the product as a
white solid (121 mg, 80%).
1
H NMR (600 MHz in methanol-d
4
at 25 ∘C): δ = 7.47 (d, 2H, J = 8.3 Hz), 7.43 (d, 2H, J = 8.3
Hz), 4.11 (s, 2H), 3.49 (q, 1H, J = 7.0 Hz), 1.44 (d, 2H, J = 6.4 Hz).
13
C NMR (150 MHz in methanol-d
4
at 25 ∘C): δ = 149.2, 133.4, 130.2, 127.5, 70.5, 44.3, 25.9.
IR (neat cm
-1
): 3280, 3220, 2969, 1377, 1081, 1010.
GC-MS for C
9
H
13
NO: calculated [M] 151.10, found 151.17.
Figure 3.21.
1
H-NMR Spectrum of 3.8e.
Figure 3.22.
13
C-NMR Spectrum of 3.8e.
NH
2
HO
136
4-(aminomethyl)-N,N-dimethylaniline 3.8f: 3.7f was subjucted to the
conditions described above. This reaction afforded the product as a white
solid (126 mg, 84%).
1
H NMR (600 MHz in acetonitrile-d
3
at 25 ∘C): δ = 7.18 (d, 2H, J = 8.8 Hz), 6.71 (d, 2H, J = 8.8
Hz), 3.65 (s, 2H), 3.31 (br, NH
2
2H), 2.89 (s, 6H).
13
C NMR (150 MHz in acetonitrile-d
3
at 25 ∘C): δ = 151.2, 130.4, 128.1, 113.6, 52.8, 41.0.
IR (neat cm
-1
): 3371, 2885, 2801, 1614, 1523, 1349, 1164, 808.
GC-MS for C
9
H
14
N
2
: calculated [M] 151.12, found 151.19.
Figure 3.23.
1
H-NMR Spectrum of 3.8f.
Figure 3.24.
13
C-NMR Spectrum of 3.8f.
NH
2
N
137
3-Phenylpropan-1-amine 3.8g: 3.7g was subjucted to the conditions
described above. This reaction afforded the product as a white solid (81
mg, 60%).
1
H NMR (600 MHz in acetonitrile-d
3
at 25 ∘C): δ = 7.29 (t, 2H, J = 7.8 Hz), 7.22 (d, 2H, J = 7.8
Hz), 7.19 (t, 1H, J = 7.6 Hz), 4.16 (br, NH
2
2H), 2.79 (t, 2H, J = 7.6 Hz), 2.66 (t, 2H, J = 7.6 Hz),
1.88 (tt, 2H, J = 7.6 Hz).
13
C NMR (150 MHz in acetonitrile-d
3
at 25 ∘C): δ = 142.7, 129.5, 129.4, 127.0, 41.03, 33.5, 32.3.
IR (neat cm
-1
): 3102, 3024, 2927, 1576, 1379.
GC-MS for C
9
H
13
N: calculated [M] 135.10, found 135.15.
Figure 3.25.
1
H-NMR Spectrum of 3.8g.
Figure 3.26.
1
H-NMR Spectrum of 3.8g.
H
2
N
138
Pyridin-4-ylmethanamine 3.8h: 3.7h was subjucted to the conditions described
above, except more sodium borohydride was used (8 mmol, 300.7 mg). This
reaction afforded the product as a white solid (70 mg, 65%).
1
H NMR (500 MHz in methanol-d
4
at 25 ∘C): δ = 8.61 (s, 2H), 7.52 (d, 2H, J = 4.4 Hz), 4.19 (s,
2H).
13
C NMR (126 MHz in methanol-d
4
at 25 ∘C): δ = 151.0, 146.2, 125.0, 123.9, 43.5.
IR (neat cm
-1
): 3351, 3151, 3030, 2929, 1680, 1602, 1414, 1325, 1030, 638.
GC-MS for C
6
H
8
N
2
: calculated [M] 108.06, found 108.16.
Figure 3.27.
1
H-NMR Spectrum of 3.8h.
Figure 3.28.
13
C-NMR Spectrum of 3.8h.
N
NH
2
139
Furan-2-carboxamide 3.9i: 3.7i was subjucted to the conditions described above.
This reaction afforded the product as a white solid (58 mg, 53%).
1
H NMR (600 MHz in acetonitrile-d
3
at 25 ∘C): δ = 7.60 (d, 1H, J = 1.8 Hz), 7.05 (d, 1H, J = 3.5
Hz), 6.56 (dd, 1H, J = 1.8, 3.5 Hz), 6.63 (br, NH
2
1H), 6.02 (br, NH
2
1H).
13
C NMR (150 MHz in acetonitrile-d
3
at 25 ∘C): δ = 160.8, 149.2, 146.1, 115.2, 113.1.
IR (neat cm
-1
): 3347, 3168, 1661, 1480, 1374, 1227, 1105.
GC-MS for C
5
H
5
NO
2
: calculated [M] 108.06, found 108.16.
Figure 3.29.
1
H-NMR Spectrum of 3.9i.
Figure 3.30.
13
C-NMR Spectrum of 3.9i.
O
O
NH
2
140
Thiophene-2-carboxamide 3.9j: 3.7j was subjucted to the conditions described
above. This reaction afforded the product as a white solid (90 mg, 77%).
1
H NMR (600 MHz in acetonitrile-d
3
at 25 ∘C): δ = 7.62 (dd, 1H, J = 1.2, 5.2 Hz), 7.59 (dd, 1H,
J = 1.2, 3.6 Hz), 7.12 (dd, 1H, J = 3.6, 5.2 Hz), 6.72 (br, NH
2
1H), 6.07 (br, NH
2
1H).
13
C NMR (150 MHz in acetonitrile-d
3
at 25 ∘C): δ = 164.4, 140.4, 132.0, 129.8, 129.0.
IR (neat cm
-1
): 3363, 3174, 1651, 1607, 1432, 1395, 1243, 1123, 712.
GC-MS for C
5
H
5
NO
2
: calculated [M] 127.01, found 127.05.
Figure 3.31.
1
H-NMR Spectrum of 3.9j.
Figure 3.32.
13
C-NMR Spectrum of 3.9j.
S
O
NH
2
141
In the reduction of 2-methyl-6-nitrobenzonitrile, we
found 2-amino-6-methylbenzamide as a product to
this reaction. When we originally reported the
catalysis in Chem. Commun., we overlooked this product by mistake.
CN
NO
2
NH
2
O
NH
2
142
3.4.3. Kinetic Data for Reduction of 3.3
3.3 (5.7 mg, 10 μmol), NaBH
4
(1.0 mg, 26 μmol) and NaO
t
Bu (1.0 mg, 10 μmol) were dissolved
in methanol-d
4
(0.6 mL) in a J.-Young NMR tube. The sample tube was immediately inserted into
a NMR (25 ˚C) and the kinetic monitoring commenced after quick locking and shimming. All
spectra were processed using VNMRJ (v. 2.3).
Figure 3.33. Kinetic Conversion of 3.3 to 3.10.
5 min
15 min
25 min
35 min
45 min
143
3.5. References
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Dehydrogenation with a Dual Site Ruthenium, Boron Catalyst Occurs at Ruthenium.
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Nitrile Reduction. Chem. Commun. 2014, 50, 5391-5393.
2. Conley, B. L.; Williams, T. J. Thermochemistry and Molecular Structure of a Remarkable
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3. Conley, B. L.; Williams, T. J. Dual Site Catalysts for Hydride Manipulation. Comments Inorg.
Chem. 2011, 32, 195-218.
4. Conley, B. L.; Guess, D.; Williams, T. J. A Robust, Air-Stable, Reusable Ruthenium Catalyst for
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5. Lu, Z.; Malinoski, B.; Flores, A. V.; Guess, D.; Conley, B. L.; Williams, T. J. Alcohol
Dehydrogenation with a Dual Site Ruthenium, Boron Catalyst Occurs at Ruthenium.
Catalysts, 2012, 2, 412-421.
6. Noyori, R.; Sandoval, C.A.; Muñ iz, K.; Ohkuma, T. Metal –Ligand Bifunctional Catalysis
for Asymmetric Hydrogenation. Phil. Trans. R. Soc. A 2005, 363, 901 –912.
144
7. (a) Conley, B.L.; Pennington-Boggio, M.K.; Boz, E.; Williams, T.J. Discovery, Applications,
and Catalytic Mechanisms of Shvo’s Catalyst. Chem. Rev. 2010, 110, 2294–2312; (b)
Karvembu, R.; Prabhakaran, R.; Natarajan, K. Shvo's Diruthenium Complex: A Robust
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Andersson, P.G.; Brandt, P. Mechanistic Aspects of Transition Metal-Catalyzed Hydrogen
Transfer Reactions. Chem. Soc. Rev. 2006, 35, 237–248; (d) Casey, C.P.; Guan, H.
Cyclopentadienone Iron Alcohol Complexes: Synthesis, Reactivity, and Implications for the
Mechanism of Iron-Catalyzed Hydrogenation of Aldehydes. J. Am. Chem. Soc. 2009, 131,
2499–2507.
8. (a) Kaneda, K.; Mori, K.; Hara, T.; Mizugaki, T.; Ebitani, K. Design of Hydroxyapatite-Bound
Transition Metal Catalysts for Environmentally-Benign Organic Syntheses. Catal. Surv. Asia 2004,
8, 231–239; Martí n-Mature, B.; Å berg, J.B.; Edin, M.; Bä ckvall, J.-E. Racemization of Secondary
Alcohols Catalyzed by Cyclopentadienylruthenium Complexes: Evidence for an Alkoxide
Pathway by Fast β-Hydride Elimination–Readdition. Chem. Eur. J. 2007, 13, 6063–6072.
9. Casey, C.P.; Beetner, S.E.; Johnson, J.B. Spectroscopic Determination of Hydrogenation Rates
and Intermediates during Carbonyl Hydrogenation Catalyzed by Shvo's
Hydroxycyclopentadienyl Diruthenium Hydride Agrees with Kinetic Modeling Based on
Independently Measured Rates of Elementary Reactions. J. Am. Chem. Soc. 2008, 130, 2285–
2295; (b) Takao, I.; Masakatsu, S. Bifunctional Molecular Catalysis, 1st ed.; Springer: Berlin,
Germany, 2011; Volume 37.
145
10. Trofimenko, S. Scorpionates: The Coordination of Polypyrazolylborate Ligands; Imperial
College Press: London, 1999.
11. For recent reviews of organometallic chemistry of poly(pyzalolyl)borate liganted complexes
see: (a) Caldwell, L. M. Alkylidyne Complexes Ligated by Poly(pyrazolyl)borates. Adv.
Organomet. Chem. 2008, 56, 1-94; (b) Lail, M.; Pittard, K. A.; Gunnoe, T. B. Chemistry
Surrounding Group 7 Complexes that Possess Poly(pyrazolyl)borate Ligands. Adv.
Organomet. Chem. 2008, 56, 95-153; (c) Becker, E.; Pavli, S.; Kirchner, K. The
Organometallic Chemistry of Group 8 Tris(pyrazolyl)borate Complexes. Adv. Organomet.
Chem. 2008, 56, 155-197; (d) Crossly, I. R. The Organometallic Chemistry of Group 9
Poly(pyrazolyl)borate Complexes. Adv. Organomet. Chem. 2008, 56, 199-321.
12. Abernethy, R. J.; Hill, A. F.; Smith, M. K.; Willis, A. C. Boron Functionalization of
Bis(pyrazolyl)borate Ligands: Molecular Structures of [RuX(PPh
3
)
2
{(MeO)
2
B(pz)
2
}] (X =
H, Cl; pz = pyrazol-1-yl). Organometallics 2009, 28, 6152-6159.
13. Bhambri, S.; Tocher, D. A. Synthesis And Characterisation of Ruthenium(II) Arene
Complexes Containing κ
3
- And κ
2
-Poly(pyrazolyl)borates and Methanes. J. Chem. Soc.,
Dalton Trans. 1997, 3367-3372.
14. A ruthenium-ligated acetonitrile was reduced by NaBH
4
, however the system produced
unidentified products.
146
15. Vogt, M.; Nerush, A.; Iron, M. A.; Leitus, G.; Diskin-Posner, Y.; Shimon, L. J. W.; Ben-David,
Y.; Milstein, D. Activation of Nitriles by Metal Ligand Cooperation. Reversible Formation of
Ketimido- and Enamido-Rhenium PNP Pincer Complexes and Relevance to Catalytic Design.
J. Am. Chem. Soc. 2013, 135, 17004–17018.
16. Borch, R. F. The Reduction of Nitrilium Salts. Chem. Commun. 1968, 442-443.
17. For example, stoichiometric portions of Ni
II
, Co
II
, Os
IV
, Ir
III
, Pt
II
, are known to mediate nitrle,
nitro and amide group reduction in the presence of excess NaBH
4
. Suzuki, Y.; Miyaji, Y.; Imai,
Z. Reduction of Organic Compounds With Sodium Borohydride-Transition Metal Salt
Systems: Reduction of Organic Nitrile, Nitro and Amide Compounds to Primary Amines.
Tetrahedron Lett. 1969, 52, 4555-4558.
18. Selected examples see: (a) Paul, H.; Bhaduri, S.; Lahiri, G. K. Platinum Carbonyl Cluster
Derived Catalyst of Superior Activity in Ketone and Unusual Selectivity in Nitrile
Hydrogenation Reactions. Organometallics 2003, 22, 3019-3021; (b) Chase, P. A.; Jurca, T.;
Stephan, D. W. Lewis Acid-Catalyzed Hydrogenation: B(C
6
F
5
)
3
-Mediated Reduction of
Imines and Nitriles with H
2
. Chem. Comm. 2008, 1701-1703; (c) Reguillo, R.; Grellier, M.;
Vautravers, N.; Vendier L. Sabo-Etienne S. Ruthenium-Catalyzed Hydrogenation of Nitriles:
Insights into the Mechanism. J. Am. Chem. Soc. 2010, 132, 7854-7855; (d) Miao, X.;
Fischmeister, C.; Bruneau, C.; Dubois, J.-L.; Couturier, J.-L. Tandem Catalytic Acrylonitrile
147
Cross-Metathesis and Hydrogenation of Nitriles with Ruthenium Catalysts: Direct Access to
Linear α,ω-Aminoesters from Renewables. ChemSusChem, 2012, 5, 1410-1414.
19. Magnus, P.; Sane, N.; Fauber, B. P.; Lynch, V. Concise Syntheses of (−)-Galanthamine and
(±)-Codeine via Intramolecular Alkylation of a Phenol Derivative. J. Am. Chem. Soc. 2009,
131, 16045-16047.
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and Aromatic Nitriles to Primary Amines with Diisopropylaminoborane. J. Org. Chem. 2009,
74, 1964-1970.
21. For selected reviews of utilization of aryl bromides in medicinal synthesis see: (a) Hassan, J.;
Sévignon, M.; Gozzi, C.; Schulz, E.; Lemaire M. Aryl−Aryl Bond Formation One Century after
the Discovery of the Ullmann Reaction. Chem. Rev. 2002, 102, 1359-1470; (b) Alberico, D.;
Scott, M. E.; Lautens, M. Aryl−Aryl Bond Formation by Transition-Metal-Catalyzed Direct
Arylation. Chem. Rev. 2007, 107, 174-238; (c) Zeni, G.; Larock, R. C. Synthesis of
Heterocycles via Palladium-Catalyzed Oxidative Addition. Chem. Rev. 2006, 106, 4644-4680;
(d) Corbet, J.-P.; Mignani, G. Selected Patented Cross-Coupling Reaction Technologies.
Chem. Rev. 2006, 106, 2651-2710; (e) Han F.-S. Transition-Metal-Catalyzed Suzuki–
Miyaura Cross-Coupling Reactions: A Remarkable Advance From Palladium to Nickel
Catalysts. Chem. Soc. Rev. 2013, 42, 5270-5298; (f) Fleckenstein, C. A.; Plenio, H. Sterically
Demanding Trialkylphosphines for Palladium-Catalyzed Cross Coupling Reactions—
148
Alternatives to PtBu
3
. Chem. Soc. Rev. 2010, 39, 694-711; (g) McGlacken G. P.; Bateman, L.
M. Recent Advances in Aryl–Aryl Bond Formation by Direct Arylation. Chem. Soc. Rev. 2009,
38, 2447-2464; (h) Rauws T. R. M.; Maes, B. U .W. Transition Metal-Catalyzed N-Arylations
of Amidines And Guanidines. Chem. Soc. Rev. 2012, 41, 2463-2497.
22. Das, R.N.; Sarma, K; Pathak, M.G.; Goswami, A. Silica-Supported KHSO4: An Efficient
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149
4.1 Introduction
This chapter contains published work.
1
I would like to take this opportunity to acknowledge
my co-authors. Jeff Celaje designed and synthesized iridium catalyst 4.1, and measured kinetic
data. Elyse Kedzie and Nicky Terrile carried out some kinetic experiments. Jon Lo contributed to
IR data acquisition.
Ongoing research in the area of sustainable solutions to alleviate petrochemical
dependence includes hydrogen release from solar fuels, the effort to synthesize liquid fuel
materials based on harnessed solar energy. This work is important, because it enables us to provide
commodity chemicals and energy carriers without reliance on petroleum products. Many of these
strategies involve electrocatalytic (or photocatalytic) cleavage of water to form hydrogen and
oxygen. The reducing equivalent, H
2
, is thus an energy carrier because it can be re-oxidized, either
by combustion to give heat or catalytically in a fuel cell to give electricity. There is a disabling
problem with large-scale utilization of hydrogen as a fuel, since it is a gas under ambient conditions,
thus limiting its volume-energy density (0.013 MJ L
-1
). As a result, physical-based hydrogen
storage technologies (compression, cryogenic liquefaction, adsorption) involve low capacity,
high costs, or safety issues.
2
Therefore, the discovery of highly weight-efficient strategies for on-
demand hydrogen release from hydrogen-rich liquids has potential value toward enabling
150
hydrogen (with an appropriate H
2
oxidation fuel cell) as a renewable fuel for light vehicles. Formic
acid (HCO
2
H, FA, 7.5 MJ L
-1
) is a hydrogen carrier, owing to its ability to release hydrogen under
mild conditions with only CO
2
as a by-product,
2-5
which can then be recycled, in principle, to give
a carbon-neutral fuel cycle.
6-8
To date, many efficient homogeneous
9-13
and heterogeneous
14-21
catalysts for FA
dehydrogenation have been developed. Heterogeneous catalysts have advantages of separability
and reusability,
10
while homogeneous catalysts are generally more efficient, the latter exhibiting
the best turnover number
15
and turnover frequency
16
. Moreover, homogeneous catalysts generally
are more selective, producing less carbon monoxide, a common byproduct of FA
dehydrogenation. This is essential, because CO is a fuel cell catalyst poison. Still, no known system
is stable and reactive through multiple uses, air and water tolerant, selective against CO formation,
and functions in neat formic acid liquid. Each of these is critical to achieving a usable hydrogen
generation system based on formic acid. This chapter focuses on a novel iridium-based catalytic
system that meets all of these criteria.
Complex 4.1, decomposes formic acid (500 μL, 12.7 mmol) with NaO
2
CH co-catalyst (5
mol%) at 50 ppm loading and 90 ᵒC.(Figure 1) In a particular, representative single run, we
converted FA (2 mL) to gaseous products in 97% isolated yield with 140 ppm 4.1 and 280 ppm
sodium formate. The rate of the reaction is constant through ca. 20% of conversion before it
accelerates as FA disappears. At the end of the reaction, a pale orange solid (the catalyst system:
an iridium complex and sodium formate), remains at the bottom of the reaction vessel.
151
Recharging the reaction flask with formic acid and reheating to 90 ᵒC results in continued H
2
production without any catalyst regeneration.
Figure 4.1. Formic Acid Dehydrogenation by Iridium Catalyst 4.1. In ORTEP diagram of 4.1,
ellipsoids drawn at the 50% probability level. CDCC #1415049 (4.1) contain supplementary
crystallographic data available via www.ccdc.cam.ac.uk/data_request/cif.
The reaction requires base as co-catalyst, but the source of the base is not specific: the
reaction rates are similar when 5 mol% NaO
2
CH, KO
2
CH, KOH, NaOH, LiOH, or nBu
4
NOH or
2.5 mol% of Na
2
CO
3
or K
2
CO
3
is used. Any of these are converted rapidly to the corresponding
formate, which comprises the bulk of the catalytic material and gives it its pale color. Moreover,
water does not affect the rate of dehydrogenation significantly.
The catalysts are air stable. Although dehydrogenation is slower when the catalysts are
prepared in air, the system remains active, even when the solution is allowed to sit on the bench
top for two weeks before dehydrogenation rates are measured. Under these conditions, the
catalysts can be re-loaded in an air atmosphere and re-used repeatedly. For example, a reaction
flask containing iridium 4.1 (6.1 mg, 8.9 μmol) and NaO
2
CH (185 mg, 2.72 mmol) was charged
152
with formic acid through 50 cycles. In this experiment 28.85 L of gas were produced from 25 mL
of formic acid, corresponding to a turnover number of 66,403 and 89% conversion. Over the
course of 50 loadings, we measured the initial rates and maximum turnover frequencies during
certain runs (Table 1). Interestingly, these increased over the course of ten cycles before slowing
over time.
The iridium catalyst delivers very high turnover numbers at low loading with repeated re-
use. For example, we prepared in the drybox a reaction flask containing 4.1 (90 μg, 0.13 μmol) and
NaO
2
CH (184 mg, 2.65 mmol) and repeatedly charged it with formic acid, which was
decomposed until a pale yellow solid remained at the bottom of the flask. After 40 cycles over a
period of four months, 13.71 L of gas were produced, which corresponds to a turnover number of
2.16 million. Although unoptimized, this is the best turnover number for a formic acid
dehydrogenation catalyst known to date to our knowledge. Under these conditions, the maximum
TOF was measured to be 3.7 s
-1
.
The reaction is operationally simple. The catalytic materials are weighed out in a reactor,
which is attached to a vent line for the gaseous products. Liquid FA is added, and the reaction is
initiated by heating. Upon completion, the catalyst system remains as a pale-colored precipitate
at the bottom of the vessel for re-use.
To be useful in fuel cells, FA decomposition must be selective for H
2
and CO
2
over H
2
O
and CO, because CO is a poison for PEM fuel cell catalysts such as platinum. The composition of
153
gas produced from our conditions was determined by gas chromatography, which showed only
H
2
and CO
2
(1:1 ratio) and no detectable CO (< 1 part per thousand).
We find that the active catalyst is homogeneous on the basis of physical appearance, clean
kinetics, tolerance of liquid mercury, and proportional inhibition by phenanthroline.
22
Accordingly, the system exhibits the reactivity and selectivity advantages of homogeneous
catalysis. Nonetheless, because the catalytic materials are deposited cleanly in the reactor at the
end of the reaction, the system enjoys many of the catalyst separability and reusability benefits of
heterogeneous conditions.
4.2. Results and Discussion
Here we present a novel mechanism for this catalytic formic acid decomposition. It is truly
remarkable that the mechanism features a new, unique, two iridium dimer through which it
operates. We successfully isolated, characterized this dimeric complex 4.3, which enabled us to
depict the full catalytic cycle as shown in Scheme 4.3.
4.2.1. Kinetic Isotope Effects and Kinetic Dependence
Kinetic isotope effect data indicate that both the C—H and O—H groups of formic acid
are involved in (or before) the rate-determining transition state. Table 4.1 (left) summarizes the
reaction rates for four selectively labeled formic acid isotopologues. The combined isotope effect
(k
CHOH
/k
CDOD
= 6.5(2)) is comparable to the product of the average separate C—H and O—H
154
isotope effects (6.5(4)). This is consistent with a mechanism in which bonds to proton and
hydride are transformed in a single kinetically relevant step. Hydrogen loss from 4.4 involves
protonation of an iridium hydride (which comes from FA’s C—H group) by a formic acid group.
Further, we observe that in a sample of formic acid-(O)-d
1
, NMR reveals HD as the catalytic
product. This indicates that there is separation of proton and hydride groups throughout the
mechanism and refutes the possibility of an iridium dihydride species in the mechanism, because
such a species would be likely to enable proton/hydride scrambling via reversible reductive
elimination of dihydrogen. This observation also shows that the reaction is irreversible at ambient
pressure, so we assign the isotope effects as kinetic. The observed rate law for FA dehydrogenation
has rate ~ [Ir]
1
[base]
0.5
[FA]
-1
, which is based on the slopes of double logarithmic plots recorded
both in neat formic acid and dilute in tetraglyme solution (Table 2, left). This rate law requires
that two sites of the catalyst are activated by a single equivalent of formate, thus causing half order
dependence on base.
155
Table 4.1. Kinetic Studies of 4.1 Catalyzed FA Dehydrogenation.
KIE (observed)
kinetic dependence
neat solution
c
k
CHOH
/k
CHOD
1.8(3) [ir] 0.95(3)
a
0.96(4)
d
k
CDOH
/k
CDOD
1.65(3) [base] 0.64(5)
b
0.44(2)
e
k
CHOH
/k
CDOH
3.9(2) [fa] - -0.94(9)
f
k
CHOD
/k
CDOD
3.6(2)
k
CHOH
/k
CDOD
6.5(2)
Reaction Kinetics. Left: isotope effect data. Conditions are 50 ppm 4.1, 5 mol% base, 86 ° C. Right:
reaction order. Data were collected at 86 ° C as an average of two runs.
a
Data collected using 0.63
M [NaO
2
CH] (2.5 mol%) and [Ir] ranging among 0.63, 1.86, 2.59, 3.25, and 4.41 mM.
b
Data
were collected using 0.66 mM [Ir], and [NaO
2
CH] ranging among 0.26, 0.53, 1.06, 1.59, 2.11,
and 2.65 M.
c
Tetraglyme was used as solvent. Base was delivered as (n-Bu)
4
NOH to generate
soluble (n-Bu)
4
N(O
2
CH).
d
Data were collected using 13.2 mM [(n-Bu)
4
N(O
2
CH)] (5 mol%)
and [Ir] concentration ranging among 0.066, 0.13, 0.20, 0.26, and 0.33 mM.
e
Data were collected
using 0.066 mM [Ir] and [(n-Bu)
4
N(O
2
CH)] ranging among 13.2, 26.4, 39.6, 52.8, and 66.0 mM.
f
Data were collected using 0.026 [Ir], 13.2 mM [(n-Bu)
4
N(O
2
CH)], and [FA] ranging among
265, 331, 398, 530 and 662 mM.
156
4.2.2. Spectroscopic Studies on Catalyst Initiation and Reactive Intermediate
We used NMR experiments to directly visualize any derivatization on the catalyst through
the reaction. We believe there is an initiation to the catalyst through the loss of cyclooctadiene
(COD) ligand, as we can observe the rapid dissociation of COD at room temperature when pre-
catalyst 4.1 is exposed to formate or H
2
. For example, when a CD
2
Cl
2
solution of 4.1 is treated
with the catalytic conditions: FA and formate, we observe formation of new species upon loss of
COD ligand (Figure 4.2).
Figure 4.2. Catalyst Initiation by
1
H-NMR.
Because FA can be all converted to gaseous products, we believe the solid remains from a
turned-over reaction is a mixture of sodium formate and iridium catalyst. In attempts to isolate
after initiation, free COD peaks
before initiation, ligated COD peaks
157
the working catalyst, we extracted organometallic species 4.4 from the solid remains after full
conversion (> 50 turnovers). This species 4.4 shows three distinct hydride peaks at (δ(1H) -19.5
ppm, -24.9 ppm, -26.5 ppm) (Figure 4.3).
Figure 4.3.
1
H-NMR Hydride Region of the Extracted Iridium Catalyst.
The
1
H-NMR signals in 4.4 are quite uncharacteristic of monomeric Ir-H species, however,
they are analogous to the hydrides in iridium dimer {[(P-N)Ir(CH
2
Cl
2
)(H)]
2
( μ
2
-H)
2
}
2+
prepared by hydrogenation of [(P-N)Ir(COD)] (P-N = SimplePHOX).
23
We used synthetic
approach to prepare iridium dimers and compared them to 4.4. The hydrogenation of 4.1 in gives
iridium dimer 4.9 that has only two hydride peaks (δ(
1
H) -16.9 ppm, -29.5 ppm) (Figure 4.4).
Although we wanted to study 4.9 under our FA dehydrogenation condition, it undergoes
158
decomposition rapidly even at room temperature. This is possibly due to the fact that
dichloromethane is a poor ligand for iridium, which dissociates and causes the complex to degrade.
In order to show this type of dimer such as 4.9 is kinetically relevant, we introduced binding
ligands for synthesis of iridium dimers to prepare stable, isolabe dimeric species. For example, in
presence of benzonitrile or pyridine, we prepared iridium dimers 4.10 and 4.11 that each has a set
of two hydride signals in
1
H-NMR (Figure 4.5). Unfortunately, 4.10 and 4.11 degrades upon
removal of the solvent, thus we could not isolate them as crystalline solid. Nonetheless, they are
stable enough in solution for us to use the solutions in situ for FA dehydrogenation reaction. In
this case, 4.10 and 4.11 both show comparable reactivity as 4.1. This encouraging result suggests
the working catalyst might actually be a dimer analogous to our proposed 4.4. Therefore, further
research effort was conducted to directly observe the resting state of the working catalyst. In so
doing, we hydrogenated 4.1 in presence of FA, simulating the catalytic condition. The reaction
yielded the supposedly FA supported dimer 4.12 (Figure 4.6). 4.12 decomposed at room
temperature in a few hours resulting in a mixture of multiple Ir-H species, including the three
hydrides in 4.4 (Figure 4.6).
159
Figure 4.4.
1
H-NMR of Hydrogenation of 4.1
Figure 4.5. Hydride Region of Synthetic Iridium Dimers.
160
Figure 4.6.
1
H NMR Hydride Region of FA ligated Iridium Dimer.
Figure 4.7.
1
H-NMR Hydride Region. Top: Decompose Mixture from 4.12. Bottom: 4.4.
161
The very limited thermos-stability of 4.12 makes it impossible to obtain useful structure
information of the working catalyst. Seeking for a more stable surrogate to FA ligate iridium dimer,
we synthesized an acetic acid ligated iridium dimer by hydrogenation of 4.1 in presence of acetic
acid. This experiment leads to the isolation of a acetate bridged iridium dimer 4.3b. 4.3b is stable
at room temperature for weeks, allowing us to get full characterization data on it, including its
crystal structure. (Scheme 4.8) We then show that we can observe 4.3b’s formate homologue 4.3a
under similar conditions, even though 4.3a is unstable and decomposes in 10 minutes. (Figure 4.8)
4.3a is observed to be in equilibrium with 4.4, and the equilibrium favors the 4.4 in presence of
formate. Thus, we rationalize that 4.4 can be generated from 4.3a by adding one equiv. of formate
as a ligand to iridium (Scheme 4.2).
Scheme 4.1. Synthesis of Acetate Bridged Ir Dimer 4.3b. In ORTEP diagram of 4.3b, ellipsoids
drawn at the 50% probability level. CDCC #1415050 (4.3b) contain supplementary
crystallographic data available via www.ccdc.cam.ac.uk/data_request/cif.
162
Figure 4.8.
1
H-NMR of Acetate or Formate Bridged Iridium Dimers
Scheme 4.2. Addition of a Formate Ligand to 4.3a.
163
4.3. Conclusion on the Mechanism
Based on the experimental evidence, we believe the catalysis has two stages:1) catalyst
ignition, which is rapid even at room temperature, 2) a FA dehydrogenation catalytic cycle around
an Ir dimer catalyst (Scheme 4.3).
Our rationale is as follows: species 4.1 is a catalyst precursor from which an active catalyst
is generated. Species 4.1 loses its cyclooctadiene ligand as cyclooctene in a solution of either H
2
or buffered formic acid and dimerizes to form 4.2. In buffered formic acid conditions, 4.2 is then
converted to a formate-bridged species 4.3a. While this species is observable by NMR, it is not
amenable to isolation in our hands. Species 4.3a is relevant in catalysis: we observe it by NMR as
the minor form of the working catalyst. We see a second, major resting species by NMR, which
has a spectrum consistent with structure 4.4, featuring three differentiated metal hydride groups.
164
Scheme 4.3. Proposed Mechanism for FA Dehydrogenation by Iridium Catalyst 4.1.
After the first equivalent of H
2
is lost in the conversion of 4.4 to 4.6, a second equivalent
forms from the iridium hydride on the complementary metal center. We propose that the latter is
more rapid than the former, and that the single equivalent of formate enables both by opening a
formate bridge in dimer 4.3a. The rate law also has [Ir] first order, which indicates a dimeric
165
iridium species that does not dissociate once formed. Inverse order in [FA] implies inhibition, but
the origins of this inhibition are unclear. Acid is known to favor closure of carboxylate bridges in
ruthenium species similar to ours,
24
which enables several opportunities for formic acid inhibition
in our mechanism. Moreover formic acid has roles in the conversion of 4.4 and as solvent. We are
currently studying this complex system of interactions.
4.4. Formic Acid Dehydrogenation by Bidentate Ligand Supported Iridium Catalysts
The dimeric iridium catalyst, either in form of 4.3, 4.4, is unique in structure, and its structure
enables its superior reactivity in FA dehydrogenation. We prepared a small library of Ir pre-
catalysts and tested their potential in FA dehydrogenation under the same conditions for 4.1
(Scheme 4.4). We show that these iridium complexes, although bearing significant similarity to
4.1, they have little reactivity for neat FA dehydrogenation compared to 4.1 (see more discussion
on the iridium carbene complexes in next chapter).
166
Scheme 4.4. Synthesis of Iridium Complexes and Their FA Dehydrogenation Reactivity
Compared to 4.1.
0 10 20 30
0
500
1000
1500
Reaction Time (h)
Gas Volume (mL)
Gas Evolution
bipy-Ir
dppe-Ir
Py-P
t
Bu2
completion (dryness)
Py-carbene-Mes
Py-carbene-Me
Kinetic profile of 4.1 catalyzed FA
dehydrogenation
Kinetic profile of FA dehydrogenation by
other iridium complexes
167
In summary, we show here a new catalytic system for the repeated conversion of formic acid
to CO
2
and hydrogen. This reaction features a novel two iridium mechanism through which it
works. We understand the mechanism because we were able to characterize the key intermediate,
a dimeric iridium species, in addition to kinetic and spectroscopic studies.
4.5. Experimental Section
4.5.1. General Procedures
All air and water sensitive procedures were carried out either in a Vacuum Atmospheres
glove box under nitrogen (2-10 ppm O
2
for all manipulations) or using standard Schlenk
techniques under nitrogen. Dichloromethane-d
2
, acetonitrile-d
3
, methanol-d
4
, water-d
2
, formic
acid-d
1
(C–D), and formic acid-d
1
(O–D) NMR solvents and reagents were purchased from
Cambridge Isotopes Laboratories. Formic acid-d
2
was purchased from SynQuest Laboratories.
Dichloromethane, ethyl ether, and hexanes were purchased from VWR and dried in a J. C. Meyer
solvent purification system with alumina/copper(II) oxide columns; chloro(1,5-
cyclooctadiene)iridium(I) dimer (Strem), sodium trifluoromethanesulfonate (Sigma-Aldrich),
and formic acid (Sigma-Aldrich) were purchased and used as received; 2-((di-t-
butylphosphino)methyl)pyridine was synthesized using a literature procedure.
25
NMR spectra were recorded on a Varian VNMRS 500 or VNMRS 600 spectrometer. All
chemical shifts are reported in units of ppm and referenced to the residual
1
H or
13
C solvent peak
and line-listed according to (s) singlet, (bs) broad singlet, (d) doublet, (t) triplet, (dd) double
168
doublet, etc.
13
C spectra are delimited by carbon peaks, not carbon count.
31
P chemical shifts are
referenced to an 85% phosphoric acid external standard. Air-sensitive NMR spectra were taken
in 8” J-Young tubes (Wilmad or Norell) with Teflon valve plugs. MALDI mass spectra were
obtained on an Applied Biosystems Voyager spectrometer using the evaporated drop method on
a coated 96 well plate. The matrix was 2,5-dihydroxybenzoic acid. In a standard preparation, ca.
1 mg analyte and ca. 10 mg matrix was dissolved in methanol and spotted on the plate with a glass
capillary. Infrared spectra were recorded on Bruker OPUS FTIR spectrometer. X-ray
crystallography data were obtained on a Bruker APEX DUO single-crystal diffractometer
equipped with an APEX2 CCD detector, Mo fine-focus and Cu micro-focus X-ray sources. Gas
chromatography data were obtained on a Thermo gas chromatograph (Supelco Carboxen® -1010
plot, 30 m × 0.53 mm) equipped with a TCD detector (detection limit: 0.099 v/v %) and with a
Jasco FT-IR instrument. Elemental analysis data were obtained on a Thermo Flash 2000 CHNS
Elemental Analyzer.
4.5.2. Dehydrogenation Procedures
The dehydrogenation of formic acid can generally be performed by preparing a stock
solution of the catalysts. In the drybox, formate and the iridium precatalyst are dissolved in either
formic acid or tetraglyme solvent. The resulting orange solution slowly turns pale yellow over the
course of ca. 1 hour. The solution is allowed to sit for several hours or overnight before the catalyst
is used for dehydrogenation reactions.
169
4.5.3. Synthesis and Characterization
4.5.3.1 Complex 4.3b
In the drybox under nitrogen, complex 1 (10 mg, 14.9 μmol)
was dissolved in 0.6 mL dichloromethane-d
2
in a J-Young
NMR tube. Dry acetic acid (8.6 μL, 149 μmol) was added to
this solution. The tube was then degassed, put under 1 atm
head pressure of H
2
gas, and shaken. After ca. 5 minutes, a
1
H NMR spectrum of the crude reaction
mixture was obtained, which confirmed the formation of 2. The solution was then poured into a
dry dram vial. Hexane was carefully layered on top of this DCM solution, and the vial was left in a
desiccator for 1 week. A crystal suitable for x-ray diffraction was isolated from the vial. Although
the crystal of 3b is stable for days, the pure crystal of 3b redissolved in dichloromethane-d
2
appears
to be in equilibrium with 2 and potentially other form of the iridium complex.
1
H NMR (600 MHz, methylene chloride-d
2
): δ = 9.23 (dd, J = 5.9, 1.6 Hz, py 2H), 7.93 (tt, J =
7.7, 1.4 Hz, py 2H), 7.70 (d, J = 7.9 Hz, py 2H), 7.36 (ddd, J = 7.4, 5.8, 1.5 Hz, py 2H), 3.58 (dd,
J = 16.7, 9.6 Hz, methylene 2H), 3.43 (dd, J = 16.7, 10.9 Hz, methylene 2H), 2.07 (d, J = 1.6 Hz,
μ-acetate 6H) 1.25 (d, J = 14.0 Hz, tBu 18H), 1.19 (d, J = 13.9 Hz, tBu 18H), -26.30 (dd, J = 19.0,
3.1 Hz, Ir-H 2H), -28.76 (m, μ-H 1H).
13
C NMR (150 MHz , methylene chloride-d
2
): δ = 184.63 (acetate), 163.84 (py), 149.59 (py),
138.94 (py), 123.42 (py), 123.16 (py), 36.78 (d, J = 30.0 Hz, P-C), 36.38 (d, J = 27.5 Hz, P-C),
170
34.88 (d, J = 26.9 Hz, P-C), 29.79 (d, J = 25.6 Hz, P-C), 28.95 (tBu methyl), 28.69 (tBu methyl),
24.27 (acetate methyl). Other small peaks on the spectrum appeared overtime due to the
instability of this compound in solution.
1
P NMR (243 MHz, methylene chloride-d
2
): δ = 44.97.
19
F NMR (564 MHz, methylene chloride-d
2
): δ = -79.43.
IR (thin film/cm
-1
) ν 3584, 3441, 2956, 2917, 2238, 2078, 1589, 1479, 1433, 1392, 1371, 1264,
1224, 1158, 1031, 822, 770, 638.
MS (MALDI) calc’d for [C
30
H
53
Ir
2
N
2
O
4
P
2
]
+
951.3 g/mol, found 951.1 g/mol.
Figure 4.9.
1
H NMR spectrum of complex 4.3b at 25 ° C in CD
2
Cl
2
.
171
Figure 4.10.
13
C NMR spectrum of complex 4.3b at 25 ° C in CD
2
Cl
2
.
Figure 4.11.
31
P NMR spectrum of complex 4.3b at 25 ° C in CD
2
Cl
2
.
172
Figure 4.12.
19
F NMR spectrum of complex 4.3b at 25 ° C in CD
2
Cl
2
.
173
4.5.4. Observation of Catalyst Intermediates by NMR
Data regarding the elementary steps of the conversion of the pre-catalyst to the active dimer
catalyst were obtained using NMR. We can observe the formation of a Pfaltz-type dimer 4.2 from
pre-catalyst 4.1 in two different ways: 1) reaction of 4.1 with formic acid in a coordinating solvent
such as acetonitrile or 2) reaction of 4.1 with H
2
in various solvents. In formic acid solvent,
iridium dimer 4.2 is converted further into the (di- μ-formate)iridium dimer 4.3a.
Scheme 4.5. Formation of Iridium Dimer 4.3a.
We studied the catalyst resting state(s) in formic acid by dissolving, in the drybox, 10.0 mg
of the iridium pre-catalyst and 10.0 mg of sodium formate in 1.0 mL of formic acid in a J-Young
tube. Examination of the hydride region reveals two resting states: (A) a minor resting state with
two different hydrides at -27.24 and -28.51 ppm that integrate in a 2:1 ratio (consistent with the
hydrides in the
1
H NMR of 4.3b, whose X-ray structure is known). We formulate this species as
dimer 4.3a. (B) The major resting state contains three different hydrides at -19.41, -25.05, and -
27.13 ppm. We formulate this species as dimer 4.4.
174
4.5.5. FA Dehydrogenation by Various Iridium Catalysts
We studied the kinetics of formic acid dehydrogenation by various iridium catalyst using
the following standard procedures: in the drybox, dissolve 140 ppm of the iridium pre-catalyst and
280 ppm of sodium formate in 2.0 mL of formic acid in a Schlenk bomb. The Schlenk bomb was
taken out of the drybox and was connected to a eudiometer (gas buret). The reactions are
monitored for 48 hours or all FA is consumed.
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5.1. Introduction
This chapter contains published work.
1
I would like to take this opportunity to acknowledge
my co-authors. Ivan Demianets converted lactic acid to lactide. Rasha Hamze synthesized 1-
mesityl imidazole. Nicky Terrile contributed to biodiesel synthesis.
Glycerol is a byproduct of biodiesel production and of other fine chemical syntheses, such
as those of perfumes, fragrances, and pharmaceuticals.
2
Currently the biodiesel industry in the
United States produces 2.0 billion gallons of glycerol each year,
3
with an increase projected in the
future.
4
Because glycerol constitutes about 10% of the weight of crude biodiesel, the utilization of
this “waste” is an opportunity for new technology.
5
Significant effort has been invested in catalytic
conversion of glycerol to value-added products.
6
Selective dehydrogenation of glycerol to lactic
acid is particularly appealing, because lactic acid is both a valuable feedstock for organic synthesis
and a precursor for poly(lactic acid) (PLA), a biodegradable polymer. The market demand of
PLA is estimated at 150,000 metric tons by 2017 and 400,000 metric tons by 2022.
7
Moreover,
when such conversions are conducted by acceptorless dehydrogenation, the byproduct H
2
is a
readily separable, energy carrier that has value as such. In these regards, homogeneous conversion
of glycerol to lactic acid has shown promising reactivity and good selectivity.
8
180
This chapter intoduces the most robust and selective catalyst to date for the conversion of
glycerol and new insights into its reactive mechanism. Our system enables high conversion of neat
glycerol, even if isolated crude from biodiesel production, to sodium lactate with > 99% selectivity.
We also show that lactic acid can be easily isolated from our reaction mixture and then converted
to rac- and meso-lactides, the precursors for PLA synthesis.
5.2. Results and Discussion
5.2.1. Synthesis of Iridium Catalysts
Our entry into glycerol dehydrogenation stemmed from our diiridium catalyst (5.2) for
formic acid dehydrogenation (Scheme 5.1a).
9
In this prior study we found that 5.2 forms from
monomer 5.1, and that the (pyridyl)methylphosphine ligand plays a vital part in enabling dimer
formation and catalyst longevity: other P—N and C—N ligands did not efficiently dimerize or
display the reactivity of 5.1/5.2. We further observed that once dimerized, complex 5.2 had little
dehydrogenation reactivity with substrates other than formic acid. On these bases, we designed
complexes 5 and 9 (Scheme 1bc), which feature a bidentate (pyridyl)methylcarbene ligand that
apparently inhibits an analogous dimerization and enables more general dehydration reactivity.
181
Scheme 5.1. (A) Formic Acid Dehydrogenation System 5.1/5.2, (B) Syntheses, and (C)
Molecular Structures
10
of Novel Iridium Complexes. Ellipsoids drawn at the 50% probability level.
CDCC #1438246 and #1438247 contain supplementary crystallographic data for compound 5.5
and 5.9, respectively. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
182
5.2.2. Glycerol Dehydrogenation
Heating compound 5.5 in air with KOH and glycerol results in the selective formation of
H
2
and lactate (> 95%, table 5.1) with no other products detectable. For example, in a particular
run, we observe absence of common side products in glycerol oxidation, such as propyl alcohol,
ethelene glycol, proplene glycol. The robustness of the catalyst is evident from an experiment in
which we observe over 1 million turnovers in 8 days (entry 2). This TON is higher than any other
homogeneous system reported to date. For example, maximum turnover number (TON) for the
Crabtree (Ir)
8a
and Beller (Ru)
8b
systems are 30,100 and 256,326, respectively. In a case of a
polymeric iridium catalyst, the maximum TON is 124,000.
8c
Further, our system is robust at
higher temperatures: at 180 ° C the reaction time is shortened from days to hours (entries 5-9).
We think that the greater stability and longevity of catalyst 5.9 is due, in part, to the bidentate
architecture of the (pyridyl)carbene. This appears to inhibit ligand scrambling processes, which
are observed in the Crabtree system.
8a
183
Table 5.1. Dehydrogenation of Neat Glycerol to Lactic Acid.
Entry
Catalyst
(ppm)
Temp.
(° C)
Time Base (mol%) TON Conversion
1 20 (5.5) 145 3 d 25 12964 25.9%
2
a
0.1 (5.5) 145 8 d 25 1057172 10.5%
3 20 (5.5) 145 6 d 50 14901 29.8% (22%)
b,c
4 20 (5.5) 145 6 d 100 17285 34.6%
5 100 (5.5) 180 1 h 1 119 1.2%
6 100 (5.5) 180 1.5 h 10 1162 11.6%
7 20 (5.5) 180 10 h 10 5215 10.4%
8 20 (5.5) 180 15 h 25 13113 26.2%
9 20 (5.5) 180 15 h 50 15894 31.9%
10
d
200 (5.5) 145 5 d 50 1909 38.2%
11
d
200 (5.5) 145 5 d 100 3445 68.9%
12 20 (5.6) 145 6 d 50 7490 15.0%
13 20 (5.6) 145 6 d 100 10785 21.6%
14 20 (5.9) 145 3 d 50 25015 50.0% (37%)
b
15 20 (5.9) 145 3 d 100 37083 74.2% (54.0%)
b
16
a
0.1 (5.9) 145 32 d 100 4557487 45.6 %
17
d,f
20 (5.9) 145 7 d 100 40889 81.7% (55.6%)
b
18
e,f
140 (5.9) 145 7 d 100 45010 90.0% (61.2%)
b
Typical reaction conditions are 5 mL glycerol, Ir catalyst, and base (KOH/ NaOH weighed and
mixed in air). Reaction progress was monitored by gas evolution.
a
The reaction started with 100
mL glycerol, and was active when quenched.
b
Isolated yield.
c
73% based on conversion.
d
The
volume of glycerol is 2 mL in this reaction.
e
9.3 g glycerol isolated from biodiesel
transesterification was used.
f
NaOH is used in place of KOH.
OH HO
OH
Ir catalyst, MOH
145
o
C, M = K, Na
OM
OH
O
OH HO
O
184
Because these reactions are free of solvent, the medium is very viscous, and the hydroxide
base is a partially-dissolved suspension. Upon completion, the reaction mixture comprises mostly
lactate salt. Thus, the reaction reaches a solid, unstirrable state at its end, when H
2
ceases to evolve.
At this point the reaction system is no longer a fluid. The reaction rate slows after ca. 25-30%
conversion. We expect that this is a result of the very high viscosity of the reaction mixture limiting
mixing and heat flow, rather than chemical deactivation of the catalyst itself. Accordingly, higher
catalyst loading will affect higher conversion (compare entries 4 and 11).
While 5.5 is very robust, we sought a faster and more efficient catalyst. Unlike Crabtree’s
iridium systems,
8a
our CO-coordinated catalyst precursor (5.6) shows a mild decrease in catalytic
reactivity relative to 5.5 (compare table 1, entries 4 and 13). We find, however, that the less
sterically hindered pyridine-carbene complex 5.9 enables more rapid reactions than 5.5. For
example, in a typical run with a catalyst loading as low as 20 ppm, over 80% of glycerol can be
converted to lactate (entries 17, 18). This conversion is higher than any other homogeneous
catalyst in neat glycerol. In a particular run, 5.9 remains reactive over 32 days delivering a total
TON of over 4.5 million (table 1, entry 14). The catalysis is very fast at 145
o
C, with a turn-over-
frequency (TOF) of up to 4 × 10
4
h
-1
in the first hour, and the reaction also takes place at as low
as 110
o
C, with a TOF up to 190 h
-1
in the first hour. By switching the base to NaOH, the reaction
eudiometry kinetic profile appeared a little slower yet steadier through a higher conversion;
further the sodium salt enabled a more facile product isolation (vide infra).
185
While our reaction mixtures are suspensions because of the sparing solubility of the
hydroxide base, we find that that dehydrogenation catalysis is most likely homogeneous on the
basis of (1) physical appearance, (2) clean kinetics, and (3) tolerance of liquid mercury.
Quantitative poisoning results are less useful with this reaction:
8,11
surprisingly, 1,10-
phenanthroline, a popular catalyst poison that quantitatively deactivates 5.2 in formic acid
dehydrogenation, was found to have no significant impact on the reaction kinetics, even when
present in large excess (35 equiv. to [Ir]). We thus find that 5.9 is tolerant of nitrogen-containing
compounds. Triphenylphosphine, another strong poison for homogenous iridium catalysts, was
also used in our glycerol dehydrogenation reaction. With a substoichiometric amount of the
poison (0.5 eq. to [Ir]), the reaction rate is within error of the parent. With 600 equiv. of
triphenylphosphine to catalyst, the reaction stopped after ca. 3% conversion, 1500 TON.
5.2.3. Further Utility Studies
Key to the value of this contribution is the ability to convert crude output from biodiesel
production to value added material. Along these lines, we have demonstrated the conversion of
soybean oil to fatty acid methyl esters (FAMEs, a biodiesel component) and crude glycerol, then
further conversion of the resulting crude glycerol to lactate salt. Thus, we treated 100 mL (93.2 g)
Wesson soy bean oil with sodium methoxide and successfully isolated 100 mL of FAMEs and 9.3
g glycerol, the latter with > 95% NMR purity. With no purification other than solvent removal,
this glycerol was catalytically converted to an isolated aliquot of 5.6 g of lactic acid.
186
Of further importance to the utility of this technology is a facile route to convert the crude
lactate salt to rac- and meso-lactide monomers for use in poly(lactic acid) synthesis. We have
achieved this using a simple pH extraction followed by known transformations for lactide
preparation (see experimental section). Thus, lactic acid can be thermally oligomerized directly
from our concentrated extract to yield a prepolymer, which can then be treated with SnO to
convert the material to crude rac- and meso-lactide mixture. Recrystallization of lactides mixture
successfully afforded rac-lactide with high purity and a yield of 69% from crude lactic acid, with a
small fraction of meso-lactide available from the mother liquor.
Beyond glycerol conversion, we find that 5.9 is a catalyst for general alcohol
dehydrogenation. For example, we can effect methanol dehydrogenation in a refluxing alkaline
solution of 25% aqueous methanol. From the reaction solution, we evolve hydrogen with 461
turnovers of H
2
. in 12 hours and isolate a crystal Na
2
CO
3
· NaHCO
3
· H
2
O as the by-product.
5.2.4. Mechanistic Studies
Although we do not yet have a complete understanding of the mechanism of our reaction,
we do have a working model (Scheme 5.2). The fate of the organic species is known:
12
an initial,
rate-determining dehydrogenation of either of the alcohol positions of glycerol enables facile
dehydration and rearrangement according to scheme 5.2a. We know that catalytic oxidation is the
slow step in this sequence for us, because we see no organic species other than glycerol and lactate >
187
1% by NMR under the catalytic conditions. Further, conversion of glyceraldehyde to lactate is
known to be rapid at temperatures as low as 25
o
C in alkali media.
13
More interesting to us is the mechanism of the catalytic oxidation cycle (scheme 5.2b). We
propose that the active catalytic species is monomeric: unlike species 5.1, species 5.5 and 5.9 do
not undergo dimerization in the presence of buffered formic acid and lack 5.1’s reactivity in formic
acid dehydrogenation. Particularly, under comparable conditions (140 ppm [Ir], 280 ppm base
in 2 mL HCOOH, 3.5 h), 5.1 catalyzed reaction undergoes > 97% conversion, whereas the
conversion is below 3% when 5.5 or 5.9 is used. Conversely, 5.1 does not lead to efficient glycerol
to lactate conversion under the conditions used in table 5.1. In a representative run, when 200
ppm 5.1 is the catalyst < 5% lactate was detected in the reaction mixture after the reaction ceases
to proceed.
Unfortunately, study of this mechanism is frustrated by a complicated network of
exchangeable protons and rapidly substituting labile oxygen ligands. We propose that catalysis
initiates with the formation of an iridium-glycerol adduct. One possibility for this structure is 5.11.
We believe that formation of 5.11 from 5.9 proceeds by solvent displacement of the
cyclooctadiene ligand of 5.9, which we observe to be rapid, even at room temperature. We then
believe that our ligand is deprotonated to make a charge neutral complex. This deprotonated form
of 5.9 is deeply purple in color, which is observed at room temperature only when 5.9 is treated
with base in the absence of glycerol.
14
If 5.9 is treated with base in the presence of glycerol, the red
solution of 5.9 assumes a light yellow color, which is characteristic of our working catalyst. We
188
therefore expect that the deprotonated catalyst cleaves glycerol’s O—H bond cooperatively,
rather than by simple proton transfer, because we observe that 5.9 is more acidic than glycerol.
One possibility for this O—H cleavage is illustrated as 5.10.
Scheme 5.2. A Mechanistic Model for Catalytic Glycerol Dehydrogenation with 5.9.
We used dehydrogenation of 1-phenylethanol as simplified model to probe reaction
kinetics. The turnover-limiting step of catalysis appears to be β-hydride elimination from an
iridium alkoxide. Three key data points support this finding: (1) we observe a first order
dependence on the concentration of the alcohol substrate, which is inconsistent with rate-
189
determining H—H bond formation or H
2
loss. (2) We find an insignificant KIE
OH/OD
of 1.1(1),
which is inconsistent with kinetic relevance of any transition state involving O—H cleavage or
H—H formation. (3) A more electron rich substrate, 1-(4-methoxyphenyl)ethanol,
dehydrogenates with a rate ca. 3 times faster than 1-phenethylethanol. This indicates a negative
(electrophilic) Hammett reaction parameter, which is better fit to β-hydride elimination than
H—H bond formation or ligand substitution as a turnover-limiting step.
15
We cannot observe a single structure for 11, because it has multiple possible isomeric forms
differing in the placement of labile oxygen ligand(s): one possibility is illustrated. We believe that
11 proceeds through β-hydride elimination of glycerol’s central hydroxyl group. In addition to
being the more homolytically weak C—H bond in the substrate, selectivity for this position is
indicated by two parallel experiments: when a toluene solution of
i
PrOH is dehydrogenated with
9 and base, we see > 90% conversion to acetone at room temperature in 12 hours; when a toluene
solution of
n
PrOH is dehydrogenated with 9 and base, we see no reaction in 12 hours. We
therefore find that the central C—H is the first to cleave. We also know that glyceraldehyde is
present when glycerol is treated with 9 and base in an NMR experiment: this we expect to be the
result of rapid isomerization of initially formed dihydroxyacetone intermediate according to
scheme 2a.
Hydrogen is likely released from hydride 12 by protonation with an alcohol O—H group.
One such protonation might involve intramolecular O—H cleavage of a coordinated
dihydroxyacetone ligand. Another involves substitution of dihydroxyacetone for glycerol,
190
followed by intramolecular O—H cleavage. These are not kinetically differentiable, because this
step is fast in the catalytic cycle. While we see no evidence for an iridium hydride under the
catalytic conditions, we can observe a diversity of iridium hydride species at room temperature
when 9 is treated with a stoichiometric portion of isopropanol in alkaline solution. We therefore
find that an iridium hydride is a plausible intermediate, although not a resting state of catalysis.
5.3. Conclusion
In conclusion, we present here a high-utility technique for the conversion of crude glycerol
to value-added lactides based on the oxidative conversion of glycerol to lactate. This oxidation
utilizes a structurally novel iridium catalyst supported by a bidentate (pyridylmethyl)imidazolium
carbene ligand. The new catalyst system enables unprecedented efficiency, longevity, and
conversion in the oxidation of glycerol to lactic acid and thus enables a very practical alternative
to fermentation compared to those currently available for lactic acid preparation. The reactive
mechanism of this new system is proposed on the basis of experimental evidence: oxidation
involves turnover-limiting β-hydride elimination to form dihydroxyacetone, which is converted
rapidly to lactate.
191
5.4. Experimental Section
5.4.1. Materials and Methods
All air and water sensitive procedures were carried out either in a Vacuum Atmosphere
glove box under nitrogen (2-10 ppm O
2
for all manipulations) or using standard Schlenk
techniques under nitrogen. Dichloromethane-d
2
, methanol-d
4
, D
2
O, acetonitrile-d
3
, benzene-d
6
and any other NMR solvents were purchased from Cambridge Isotopes Laboratories.
Dichloromethane-d
2
, acetonitrile-d
3
, benzene-d
6
and methanol-d
4
, THF-d
8
are carefully dried
prior to use. Dichloromethane-d
2
is stirred over CaH
2
for 1 day then vapor transferred into a dry
flask; acetonitrile-d
3
is stirred over CaH
2
for 1 day then vapor transferred into a dry flask; benzene-
d
6
is stirred over Na/benzophenone for 1 day then vapor transferred into a dry flask; methanol-d
4
is stirred over Na for 1 day then vapor transferred into a dry flask; THF-d
8
is stirred over
Na/benzophenone for 1 day then vapor transferred into a dry flask. Dichloromethane, ethyl ether,
THF and hexanes are purchased from VWR and dried in a J. C. Meyer solvent purification system
with alumina/copper(II) oxide columns; bulk methanol in methyl FAME synthesis was dried by
stirring over activated molecular sieves over night; chloro(1,5-cyclooctadiene)iridium(I) dimer
(Strem), sodium trifluoromethanesulfonate (Sigma-Aldrich), potassium tert-butoxide (Sigma-
Aldrich) were purged with nitrogen and stored under nitrogen atmosphere; glycerol (EMD
Millipore) was used as received; Wesson vegetable oil was purchased from a local grocery store
and used without purification; pyridine-imidazolium ligands and corresponding silver carbenes
were synthesized using a literature procedure.
16
192
NMR spectra were recorded on a Varian VNMRS 500 or VNMRS 600 spectrometer,
processed using MestroNova. All chemical shifts are reported in units of ppm and referenced to
the residual
1
H or
13
C solvent peak and line-listed according to (s) singlet, (bs) broad singlet, (d)
doublet, (t) triplet, (dd) double doublet, etc.
13
C spectra are delimited by carbon peaks, not
carbon count.
19
F chemical shifts are referenced to a trichlorofluoromethane external (coaxial
insert tube) standard (0 ppm). Air-sensitive NMR spectra were taken in 8” J-Young tubes
(Wilmad or Norell) with Teflon valve plugs. MALDI mass spectra were obtained on an Applied
Biosystems Voyager spectrometer using the evaporated drop method on a coated 96 well plate.
The matrices used for MALDI are 2,5-dihydroxybenzoic acid or anthracene. In a standard
preparation, ca. 1 mg analyte and ca. 10 mg matrix was dissolved in methanol and spotted on the
96-well MALDI plate with a glass capillary. Infrared spectra were recorded on Bruker OPUS FTIR
spectrometer. X-ray crystallography data were obtained on a Bruker APEX DUO single-crystal
diffractometer equipped with an APEX2 CCD detector, Mo fine-focus and Cu micro-focus X-ray
sources. Elemental analysis data were obtained on a Thermo Flash 2000 CHNS Elemental
Analyzer. Lyophilization was accomplished with a Millrock benchtop freeze dryer.
5.4.2. Dehydrogenation Procedures
The iridium catalysts for glycerol dehydrogenation are stored in a glovebox for long term
purpose. In a typical reaction, iridium catalyst, base (i.e. KOH, NaOH), are weighed outside the
glovebox, added to a round bottom flask equipped with a magnetic stir bar. Glycerol is measured
and added to the same flask with a disposable plastic syringe. The flask then is connected to an air
193
condenser, which has an 8 mm Tygon tubing gas outlet immersed in a water eudiometer (inverted
burette, Figure 5.1). The reaction progress is monitored by eudiometry. An oil bath is used for
reactions at 145 ° C; a sand bath is used for reactions at 180 ° C. Bath temperature is monitor using
an alcohol thermometer. Normally < ±2.5 ° C temperature fluctuation is observed for oil baths,
and < ±10 ° C fluctuation is observed for sand baths.
Figure 5.1. Apparatus Setup for Dehydrogenation Reactions and Typical Kinetic Profile.
0 10000 20000 30000 40000 50000
0
5000
10000
15000
20000
kinetic profile
time (h)
H
2
evolution (mL)
194
5.4.3. Synthesis Procedures and Characterization Data
Note: all the procedures for syntheses of iridium complexes are performed in the glovebox.
5.4.3.1. Complex 5.5
In the glovebox under nitrogen, in a 100 mL in a Schlenk flask, dichloro-di(1-(2,4,6-
trimethylphenyl)-3-(2-picolyl)-imidazol-2-ylidene)-disilver(I)
16
5.4 (251 mg, 0.298 mmol) was
added in small portions to a stirring solution of chloro(1,5-cyclooctadiene) Iridium(I) dimer
(200.0 mg, 0.298 mmol) in 20 mL dry acetonitrile. After 30 minutes, sodium
trifluoromethanesulfonate (102.5 mg, 0.596 mmol) was also added to the mixture. After stirring
for another 30 minutes, the solution was filtered through a dry pad of celite to remove the sodium
chloride byproduct. The solvent was evaporated under reduced pressure to yield a red glassy solid.
This red solid was dissolved in 10 mL dry dichloromethane, and 20 mL dry hexanes was added to
the solution to facilitate a precipitation. A red crystalline solid was acquired and dried under
vacuum (400 mg, 93%). This sample was later determined to be spectroscopically pure under
NMR. Slow recrystallization from dichloromethane and hexanes produced crystals suitable for X-
ray crystallography.
195
1
H NMR
1
H NMR (600 MHz, methylene chloride-d
2
) δ 8.50 (ddd, J = 7.7, 1.6, 0.8 Hz, py 1H),
8.08 (ddd, J = 7.8, 1.6, 0.7 Hz, py 1H), 8.01 (td, J = 7.7, 1.5 Hz, py 1H), 7.70 (d, J = 1.9 Hz, Im
1H), 7.48 (ddd, J = 7.4, 5.6, 1.5 Hz, py 1H), 7.04 (s, mesityl-ar 1H), 7.00 (s, mesityl-ar 1H), 6.88
(d, J = 1.9 Hz, Im 1H), 5.77 (s, methylene 1H), 5.74 (s, methylene 1H), 4.15 (s, COD sp
2
1H),
4.07 (s, COD sp
2
1H), 3.92 (s, COD sp
2
1H), 3.21 (s, COD sp
2
1H), 2.37 (s, mesityl-para-methyl
3H), 2.25-1.85 (m, COD sp
3
6H), 2.06 (s, mesityl-ortho-methyl 3H), 1.90 (s, mesityl-ortho-
methyl 3H), 1.63 (s, COD sp
3
1H) 1.47 (s, COD sp
3
1H).
13
C NMR (151 MHz, methylene chloride-d
2
) δ 174.64 (carbene C), 153.38 (py), 151.40 (py),
140.51 (py), 140.16 (mesityl), 135.32 (py), 129.68 (mesityl-ar), 129.46 (mesityl-ar), 126.88 (py),
126.74 (mesityl-ar), 123.51 (Im), 122.54 (Im), 86.04 (COD sp
2
), 82.80 (COD sp
2
), 66.08 (COD
sp
2
), 64.43 (COD sp
2
), 55,34 (py-CH2), 34.99 (COD sp
3
), 31.89 (COD sp
3
), 31.44 (COD sp
3
),
28.17 (COD sp
3
), 21.23 (mesityl-CH
3
), 19.13 (mesityl-CH
3
), 17.94 (mesityl-CH
3
).
19
F NMR (470 MHz, methylene chloride-d
2
) δ -79.43.
Elemental Analysis (CHNS) calc’d for C
27
H
31
F
3
IrN
3
O
3
S: C, 44.62; H, 4.30; N, 5.78; S, 4.41.
Found: C, 44.55; H, 4.24; N, 5.84; S, 4.24.
IR (thin film/cm
-1
) ν 3584, 3441, 2918, 2849, 2362, 1734, 1608, 1444, 1411, 1263, 1223, 1070,
1030, 853, 804, 636, 628.
MS (MALDI) calc’d for [C
26
H
31
IrN
3
]
+
578.2, found 577.9.
196
Figure 5.2.
1
H NMR spectrum of complex 5.5 at 25 ° C in CD
2
Cl
2
.
Figure 5.3.
13
C NMR spectrum of complex 5.5 at 25 ° C in CD
2
Cl
2
.
197
Figure 5.4.
19
F NMR spectrum of complex 5.5 at 25 ° C in CD
2
Cl
2
.
198
5.4.3.2. Complex 5.6
In the glovebox under nitrogen, 5.5 (330 mg, 0.453 mmol) was dissolved in 15 mL CH
2
Cl
2
in a 100 mL Schlenk flask. The flask was sealed and frozen in liquid nitrogen outside the glovebox.
The headspace of the flask was evacuated for 1 minute under dynamic vacuum whilst the solution
remains frozen, then the headspace was refilled with 1 atm CO gas. The solution was allowed to
warm up to room temperature and was further stirred for 30 minutes. During this time, the
reaction turned from red to yellow. Slowly adding 30 mL hexanes afforded a yellow crystalline
solid, which was dried under vacuum (300 mg, 98%). Compound 5.6 is stable under air.
1
H NMR (600 MHz, methylene chloride-d
2
) δ 8.83 (dd, J = 5.7, 1.5 Hz, py 1H), 8.30 (dd, J =
7.9, 1.4 Hz, py 1H), 8.19 (ddd, J = 7.8, 6.5, 1.3 Hz, py 1H), 7.99 (dd, J = 2.0, 0.9 Hz, imi 1H),
7.59 (td, J = 6.5, 1.1 Hz, py 1H), 7.08 (dd, J = 1.9, 0.9 Hz, imi 1H), 7.05 (s, mesityl-ar 2H), 5.77
(s, methylene 2H), 2.38 (s, mesityl-para-methyl 3H), 2.00 (s, mesityl-ortho-6H).
13
C NMR (151 MHz, methylene chloride-d
2
) δ 182.71, 173.05, 169.67, 156.68, 154.71, 143.23,
141.09, 135.87, 134.40, 129.84, 128.50, 127.04, 124.25, 123.78, 54.52, 21.28, 18.38.
19
F NMR (470 MHz, methylene chloride-d
2
) δ -79.4.
199
Elemental Analysis (CHNS) Anal. calc’d for C
21
H
20
F
3
IrN
3
O
5
S: C, 37.39; H, 2.84; N, 6.23; S, 4.75.
Found: C, 37.44; H, 3.02; N, 6.34; S, 4.34.
IR (thin film/cm
-1
) ν 3442, 3126, 2921, 2077 (CO), 2011 (CO), 1612, 1488, 1451, 1420, 1364,
1309, 1225, 1157, 1072, 704.
MS (MALDI) calc’d for [C
20
H
19
IrN
3
O
2
]
+
526.1, found 525.9.
Figure 5.5.
1
H NMR spectrum of complex 5.6 at 25 ° C in CD
2
Cl
2
.
200
Figure 5.6.
13
C NMR spectrum of complex 5.6 at 25 ° C in CD
2
Cl
2
Figure 5.7.
19
F NMR spectrum of complex 5.6 at 25 ° C in CD
2
Cl
2
.
201
Figure 5.8. Infrared Spectrum of Complex 5.6.
202
5.4.3.3. Complex 5.9
In the glovebox under nitrogen, in a 100 mL in a Schlenk flask, dichloro-di(1-methyl-3-(2-
picolyl)-imidazol-2-ylidene)-disilver(I) 5.8 (89.5 mg, 0.141 mmol) was added in small portions
to a stirring solution of chloro(1,5-cyclooctadiene) iridium(I) dimer (95.0 mg, 0.141 mmol) in
20 mL dry dichloromethane. After 30 minutes, sodium trifluoromethanesulfonate (49 mg, 0.285
mmol) was also added to the mixture. After stirring for another 30 minutes, the solution was
filtered through a dry pad of celite to remove the sodium chloride byproduct. The solvent was
evaporated under reduced pressure to yield a red glassy solid. This red solid was dissolved in 5 mL
dry dichloromethane, and 10 mL dry hexanes was slowly added to the solution to facilitate a
precipitation. A red crystalline solid was acquired and dried under vacuum (150 mg, 85%). This
sample was later determined to be spectroscopically pure under NMR. Slow recrystallization from
dichloromethane and hexanes produced crystals suitable for X-ray crystallography. 5.9 is mildly
air-sensitive and is stored under N
2
.
1
H NMR (600 MHz, methylene chloride-d
2
) δ 8.55 (d, J = 5.2 Hz, py 1H), 7.79 (t, J = 7.7 Hz, py
1H), 7.64 (d, J = 7.9 Hz, py 1H), 7.33 (t, J = 6.4 Hz, py 1H), 7.14 (d, J = 2.0 Hz, imi 1H), 6.89
(d, J = 2.0 Hz, imi 1H), 5.74 (d, J = 15.0 Hz, methylene 1H), 5.56 (d, J = 14.8 Hz, methylene
203
1H), 4.48 (s, COD sp
2
1H), 4.28 (s, COD sp
2
1H), 3.89 (s, imi-methyl 3H), 3.48 (s, COD sp
2
1H), 3.29 (s, COD sp
2
1H), 2.33 (s, COD sp
3
2H), 2.14 (s, COD sp
3
2H), 1.81 (s, COD sp
3
3H), 1.61 (s, COD sp
3
1H).
13
C NMR (151 MHz, methylene chloride-d
2
) δ 174.54 (carbene C), 153.26 (py), 151.42 (py),
140.12 (py), 137.58 (py), 126.40 (py), 123.16 (Im), 122.31 (Im), 79.57 (COD sp
2
), 75.42
(COD sp
2
), 65.59 (COD sp
2
), 58.86 (COD sp
2
), 55.30 (py-CH
2
), 37.87 (Im-CH
3
), 33.38
(COD sp
3
), 33.24 (COD sp
3
), 30.14 (COD sp
3
), 29.77 (COD sp
3
).
19
F NMR (470 MHz, methylene chloride-d
2
) δ -79.41.
Elemental Analysis (CHNS) Anal. calc’d for C
19
H
23
F
3
IrN
3
O
3
S: C, 36.65; H, 3.72; N, 6.75; S, 5.15.
Found: C, 37.04; H, 3.75; N, 6.41; S, 4.97.
MS (MALDI) calc’d for [C
18
H
23
IrN
3
]
+
474.2, found 473.9.
204
Figure 5.9.
1
H NMR Spectrum of Complex 5.9 at 25 ° C in CD
2
Cl
2
.
Figure 5.10.
13
C NMR Spectrum of Complex 5.9 at 25 ° C in CD
2
Cl
2
.
205
Figure 5.11.
19
F NMR Spectrum of Complex 5.9 at 25 ° C in CD
2
Cl
2
.
5.4.4. Initial Condition Optimization
In a typical reaction, iridium catalyst (20 ppm with regard to glycerol), base (i.e. KOH,
NaOH, 1 eq. to glycerol), are weighed outside the glovebox in air, added to a round bottom flask
equipped with a magnetic stir bar. Glycerol (5 mL) is measured and added to the same flask with
a disposable plastic syringe. The flask then is connected to a short path distilling head, which has
an 8 mm Tygon tubing gas outlet put in a water eudiometer (inverted burette, Figure 5.1). The
flask is then placed in an oil bath set to 145 ° C, and the reaction progress is monitored by
eudiometry. The results are summarized as following.
206
Table 5.2. Initial Conditions Attempted for a Conversion of Glycerol to Lactic Acid.
entry Catalyst Temp.
(° C)
Time Base or Acid TON Conversion
1 − 145 1 d 0.5 eq. KOH − < 1 %
2 20 ppm 5.5 145 1 d 10 mol % TFA − < 1 %
3 100 ppm 5.5 145 1 d − − < 1 %
4 20 ppm 5.5 145 1 d 0.5 eq.
Ca(OH)
2
899 1.8 %
5 20 ppm 5.5 145 1 d 0.5 eq. CaO − < 1 %
6 20 ppm 5.5 145 1 d 10 mol % DBU − < 1 %
7 20 ppm 5.5 145 4 d 1 eq. KO
t
Bu 16777 33.6 %
8 50 ppm 5.14 145 3 d 0.5 eq. KOH 839
a
8.4 %
9 50 ppm 5.15 145 3 d 0.5 eq. KOH 1198 6.0 %
a
TON calculated based on iridium atom.
207
5.4.5. Homogeneity Tests
In these reactions, based on our typical conditions (5 mL glycerol, iridium catalyst 5.9, 20 ppm;
base KOH, 0.5 eq. to glycerol at 145 ° C) mercury drops, phenanthroline (phen) poison or PPh
3
poison are also added to the reaction flask in parallel runs. The reactions were monitored by
eudiometry. The results are summarized in table S2. For a typical kinetic profile ,please see figure
5.1.
Table 5.3. Homogeneity Test Using Various Poisons.
Entry Poison equiv. to [Ir] Time TON Conversion
1 − − 1 d 18125 36.3 %
2 Hg (l) − 1 d 18574 37.1 %
3 250 ppm phen 15 1 d 17376 34.8 %
4 700 ppm phen 35 1 d 18724 37.4 %
5 10 ppm PPh
3
0.5 1 d 14080 28.2 %
6 1.2 mol % PPh
3
600 3 h 1498 3.0 %
208
5.4.6. Conversion of Soybean Oil to FAMEs and Lactic Acid
5.4.6.1. From soybean oil to FAMEs and glycerol
In a typical reaction soybean oil (100 mL, 93 g) is added to a stirring solution of dilute NaOMe
(20 mg) in dry methanol (100 mL) under nitrogen. The mixture is heated in a water bath set to
50 ° C for 5 hours. After stirring is stopped and the mixture is returned to room temperature, the
reaction mixture settles to two layers. On the top is the SoyFAME layer, in it ca. 100 mL of
biodiesel material. The bottom is a methanol solution of glycerol, which was subsequently
concentrated by rotary evaporation, then dried on a lyophilizer overnight, and finally on a high-
vacuum Schlenk line to afford 9.3 g of NMR pure (> 95%) glycerol. We believe that the glycerol
is mostly free of methanol or water. For
1
H-NMR of the glycerol product, see figure 5.12.
Figure 5.12.
1
H-NMR Spectrum of the Glycerol Isolated from a Transesterification Product of
Wesson Soybean Oil.
209
5.4.6.2. From glycerol to lactic acid
Iridium catalyst 5.9 (5.8 mg, 140 ppm to glycerol) and NaOH (4.0 g, 1.0 eq. to glycerol),
are weighed in air and added to a round bottom flask equipped with a magnetic stir bar. Glycerol
(9.3 g from soybean oil, above) is added to the same flask. The flask then is connected to a short
path distilling head, which has an 8 mm Tygon tubing gas outlet put in a water eudiometer. The
flask is then placed in an oil bath set to 145 ° C, and the reaction progress is monitored by
eudiometry and
1
H-NMR. A snapshot of the reaction mixture is taken in
1
H-NMR, which shows
well-controlled selectivity for lactate.
Figure 5.13. A Snapshot of Reaction Mixture after 3 Days.
H
2
O
(solvent)
glycerol
lactate
}
210
After 7 days, eudiometry shows that the reaction has ca. 90% conversion. The reaction flask
was cooled to room temperature.
1
H NMR shows only lactate product and a small amount of
glycerol.
Figure 5.14.
1
H NMR of Reaction Mixture after 7 days, at 90% Conversion. The solvent is D
2
O.
We find that using NaOH instead of KOH yielded much more soluble solid at the end of
the reaction, which enabled a much facile isolation of lactic acid. Accordingly, conc. hydrochloric
acid (1 M, 70 mL) was added the reaction flask until the pH was < 1, then the solution was
extracted with ethyl acetate (25 mL × 5). The organic solvent was evaporated under a constant
flow of air, a colorless liquid left at the bottom of the flask was identified as NMR pure lactic acid
(5.6 g, 61.5%).
lactate
}
glycerol
211
Figure 5.15.
1
H NMR of Isolated Lactic Acid in D
2
O.
5.4.6.3. Lactide synthesis
The reactions for lactic acid to lactide conversion are based on known procedures.
17
5.4.6.3.1. From lactic acid to polylactic acid oligomers
In a typical run, 2.50 g of lactic acid obtained from the previously described extractions is
weighed in air and added to a round bottom flask equipped with a magnetic stirring bar. The flask
is connected to a Dean-Stark trap with condenser on top of it. The flask is then placed in a wax
bath set to 210 ° C. The reaction is carried out under nitrogen for 6 hours.
212
Figure 5.16.
1
H NMR of Poly(Lactic Acid) Oligomer in DMSO-d
6
.
Figure 5.17.
1
H NMR “Zoom-in” on "Methine" Region of the Poly(Lactic Acid) Oligomer.
213
5.4.6.3.2. Lactide formation
In a typical run, 1.0 wt % of SnO (25 mg, 0.186 mmol) is added to the flask containing our
synthetic lactic acid oligomers (2.5 g). The flask is equipped with distillation apparatus, and the
receiving flask is placed into an oil bath set to 80 ° C. The reaction flask is placed in a wax bath set
to 210 ° C, and is stirred for 3 hours. During this time, lactide mixture condensed in the receiving
flask. This lactide mixture is dissolved in ethyl acetate (3 mL) and washed with 2 × 3 mL ethyl
acetate, then transferred into a 50 mL beaker. The beaker is stored at -15° C for 72 h. White crystals
of rac-lactide recrystallized from the solution is filtrated, dried, weighted and analyzed by
1
H-
NMR (0.69 g, 69% yield). The rac-lactide is > 90% pure by
1
H-NMR.
Figure 5.18.
1
H NMR of rac-Lactide in DMSO-d
6
.
214
5.4.7. Mechanistic Study
5.4.7.1. Catalyst Initiation
In a glovebox, iridium compound 5.9 (20 mg, 0.032 mmol) and
i
PrOH (3.7µL, 1.5 eq) are
added to a J. Young tube. 0.6 mL dichloromethane-d
2
solvent is added to the same tube.
1
H NMR
shows no reaction at room temperature over 24 hours. The J. Young tube is then gently evacuated,
refilled with 1 atm H
2
, heated to 60
o
C for 10 min.
1
H NMR shows COD fully reduced to
cyclooctane. Also a number of Ir-H species formed.
215
Figure 5.19. H
2
Evolution at Room Temperature from Dehydrogenation of Isopropanol by
Iridium Catalyst 5.5 in Presence of KOH.
In a glovebox, iridium compound 5.9 (20 mg, 0.027 mmol) and KOH (15.4 mg, 0.27 mmol)
are added to a J. Young tube. 0.6 mL 1:1 isopropanol to benzene-d
6
(0.3 mL : 0.3 mL) is added to
dissolve the solid.
1
H NMR shows H
2
formation at room temperature. The J. Young tube is then
closed and heated in an oil bath set to 90
o
C for 10 minutes.
1
H-NMR shows full reduction of
COD to cyclooctane.
cyclooctane
216
Figure 5.20. H
2
Evolution at Room Temperature from Dehydrogenation of Isopropanol by
Iridium Catalyst 5.9 in Presence of KOH.
Figure 5.21. Formation of Cyclooctane from Ligand COD in Iridium Compound 5.5
benzene-d
6
isopropanol
cyclooctane
acetone
H
2
H
2
217
5.4.7.2. Probing Catalytic Mechanism
5.4.7.2.1. Reversible Ligand Deprotonation in Alcohol Medium
In a glovebox, iridium compound 5.9 (20 mg, 0.032 mmol) and KOH (1.8 mg, 0.032
mmol) are added to a J. Young tube. 0.6 mL methanol-d
6
is added to dissolve the solid.
1
H NMR
shows rapid deuteration at ligand methylene –CH
2
– group at room temperature.
Figure 5.22. Room Temperature Deprotonation at Ligand –CH
2
– to Iridium Catalyst 5.9 in
CD
3
OD.
time = 1 minute
time = 2 hours
–CH
2
– in ligand
fully deuterated
218
5.4.7.2.2. COD Displacement
In a glovebox, iridium compound 5.9 (20 mg, 0.032 mmol) and KOH (1.8 mg, 0.032 mmol)
are added to a J. Young tube. 0.6 mL acetonitrile-d
3
is added to dissolve the solid.
1
H NMR shows
rapid deuteration at ligand methylene –CH
2
– group at room temperature within a few minutes.
Figure 5.23. Room Temperature Deprotonation at Ligand –CH2– to Iridium Catalyst 5.9 in
CD
3
CN.
free COD
fully deuterated
–CD
2
–
219
5.4.7.2.2. Formation of Glycraldehyde
In a glovebox, iridium compound 5.9 (20 mg, 0.032 mmol) and KOH (1.8 mg, 0.032
mmol) are added to a J. Young tube. 0.6 mL acetonitrile-d
3
is added to dissolve the solid. Then
glycerol (10 μL, 12.6 mg, 0.136mmol) is added to the tube and the tube is heated to 60
o
C.
1
H
NMR shows formation of glyceraldehyde as a doublet at 9.4 ppm. This compares well with the
experimental DMSO-d
6
spectrum, which shows a doublet at 9.61 ppm.
18
Figure 5.24. Glycerol Conversion to Glyceraldehyde by Iridium Catalyst 5.9 in CD
3
CN.
aldehyde –CHO from
glycerol
dehydrogenation
with glycerol
40 min at 60
o
C
with glycerol
16 h at 60
o
C
220
5.4.7.2.3. Acidity Comparison between Ligand in 5.9 and Glycerol via Base Titration
In a glovebox, in a dry vial iridium catalyst 5.9 (1 mg, 0.0016 mmol) is
dissolved in 1.0 mL dry THF to give a red solution. In a parallel reaction vial, 5.9
(1 mg, 0.0016 mmol) and glycerol (1 mg, 0.011 mmol) are also dissolved in 1.0
mL dry THF to give a similar red solution. A stock solution of KO
t
Bu (10 mg,
0.089 mmol) in 10 mL THF is prepared to titrate the two red THF solutions. In both cases, a
color change appeared after 0.9 mL of addition of the KO
t
Bu solution. In the vial of 5.9 in THF,
the color turned from red to purple; while the vial of 5.9 and glycerol, the color turned into light
yellow. The red colored solution, apparently the deprotonated form of 5.9, reverts to the yellow
color when warmed in the presence of glycerol.
5.4.7.3. Kinetic Experiments
5.4.7.3.1. OH/OD KIE Study
In a glovebox, phenylethyl alcohol (OH or OD, 10 µL, 0.083 mmol), iridium catalyst 5.9 (1
mg, 0.0016 mmol, 2 mol %) and KO
t
Bu (9.3 mg, 0.083 mmol) are added to a J. Young tube. 0.6
mL dichloromethane-d
2
is added to dissolve the solid mixture. The NMR tube is quickly taken to
a pre-lock-and-shimmed NMR instrument for an overnight kinetic study at 25
o
C. Rate constant
221
of each kinetic run is calculated based on the consumption of the alcohol substrate. For the –OH
experiment, a rate constant of 1.27(5) 10
-4
s
-1
could be obtained, while for the –OD run, we
observed a rate constant of 1.15(6) 10
-4
s
-1
. This gives us a KIE
OH/OD
= 1.12(11).
Figure 5.25. KIE Study of Phenylethyl Alcohol Dehydrogenation by Catalyst 5.9.
0 20000 40000 60000
0.00
0.05
0.10
0.15
KIE Study
Time (s)
Conc. (mol/L)
phenylethyl alcohol -OD
phenylethyl alcohol -OH
222
5.4.7.3.2. Kinetic Dependence on Alcohol Concentration
In a glovebox, phenylethyl alcohol (10 µL, 0.083 mmol, or 20 µL, 0.166 mmol, or 40 µL,
0.332 mmol, or 80 µL, 0.664 mmol), iridium catalyst 5.9 (1 mg, 0.0016 mmol, 2 mol %) and
KO
t
Bu (0.9 mg, 0.008 mmol) are added to a J. Young tube. 0.6 mL toluene-d
8
is added to dissolve
the solid mixture. The NMR tube is taken to a pre-heated NMR instrument for a kinetic run at
100
o
C. Rate constant of each kinetic run is calculated based on the consumption of the alcohol
substrate. Rate constants are calculated to be 2.5(5) 10
-3
s
-1
, 5.8(6) 10
-3
s
-1
, 9.8(3) 10
-3
s
-1
,
2.0(2) 10
-2
s
-1
. A log-log plot gives us a slop of 1.004(46), indicating the reaction is first order on
the alcohol substrate.
Figure 5.26. Kinetic Dependence of Phenylethyl Alcohol.
-0.2 0.0 0.2 0.4 0.6 0.8 1.0
-2.5
-2.0
-1.5
log (k
obs
)
log ([phenylethyl alcohol])
Kinetic Dependence on Alcohol Conc.
Slope
log (k
obs
)
1.004 ± 0.04572
223
5.4.7.3.3. Kinetic Run on (4-Methoxyphenyl)ethyl Alcohol Dehydrogention
In a glovebox, (4-methoxyphenyl)ethyl alcohol (12 µL, 0.083 mmol), iridium catalyst 5.9
(1 mg, 0.0016 mmol, 2 mol %) and KO
t
Bu (0.9 mg, 0.008 mmol) are added to a J. Young tube.
0.6 mL toluene-d
8
is added to dissolve the solid mixture. The NMR tube is taken to a pre-heated
NMR instrument for a kinetic run at 100
o
C. Rate constant of each kinetic run is calculated
based on the consumption of the alcohol substrate. Rate constant is calculated to be 7.3(7) 10
-
3
s
-1
.
Figure 5.27. Kinetic Profile of Dehydrogenation of 4-Methoxy-Phenylethyl Alcohol.
0 100 200 300 400
0.35
0.40
0.45
0.50
0.55
0.60
0.65
4-OMe-PhEtOH Consumption
Time (min)
Conc. (mol/L)
224
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18. Garcia-Jimenez, F; Zuniga, O. C.; Garcia, Y. C.; Cardenas, J.; Cuevas, G. Experimental and
Theoretical Study of the Products from the Spontaneous Dimerization of DL- and D-
Glyceraldehyde. J. Braz. Chem. Soc. 2005, 16, 467-476.
229
6.1. Introduction
The global atmospheric CO
2
concentration has recently (May, 2015) passed the 400 ppm
threshold for the first time on the NOAA record.
1
This index has risen from a pre-industrial level
of ca. 280 ppm by more than 30% in the last 150 years largely due to anthropomorphic CO
2
emission.
2
CO
2
, a greenhouse gas, causes concerns about climate change and rising sea levels as
its concentration escalates.
3
Among the options to transition from fossil fuel to more sustainable
alternatives, solar fuel generated from CO
2
reduction is promising. Thus, methods for CO
2
conversion are important targets for the catalysis community.
4
Most CO
2
reduction products,
such as methanol, formic acid, CO, etc., are useful C1 feedstocks in chemical synthesis; among
them methanol has the highest volume energy density (and stored hydrogen density) at room
temperature and is thus an outstanding product.
5
However, it is also a challenging target, because
CO
2
is thermodynamically robust, so its activation requires a strong thermodynamic driving
force.
6
Also, selective reduction of CO
2
is problematic; some known catalytic CO
2
reduction
systems afford a mixture of products.
7
Regarding synthetic routes to methanol from CO
2
, direct
hydrogenation (with H
2
) has been observed with a few ruthenium catalysts; these catalytic
systems adopt forcing conditions or a multiple catalyst cascade and they have limited longevity.
8
Certain silanes or boranes can effect CO
2
reduction under much milder conditions.
9
For example,
in figure 6.1 we show known, selective catalytic systems for CO
2
to methoxide reduction under
230
low temperatures. These catalysts show markedly enhanced longevity compared to direct
hydrogenation catalysts. At the first glance, these systems differ significantly: the catalysts are
respectively a free N-heterocyclic carbene (NHC),
7a
a nickel pincer complex,
7c
a ruthenium
complex,
7d
and a frustrated Lewis pair.
7f
More recently, an example appeared wherein BH
3
-THF
reduces CO
2
to methoxide in THF at room temperature with NaBH
4
as the catalyst.
10
However,
all of these low temperature CO
2
reductions have a common feature: they rely on stronger
reductants, i. e. boranes and silanes. The inexpensive and more easily handled NaBH
4
was only
sparsely investigated for CO
2
reduction in the last century,
11
and was reported by some to have no
reactivity with CO
2
.
12
More recently Cummins and co-workers have shown that CO
2
is reduced
to triformatoborohydride, HB(OCHO)
3
-
by NaBH
4
in anhydrous acetonitrile.
13
Figure 6.1. Catalytic CO
2
Reduction with Silanes and Boranes. TONs are based on the number of
hydrides delivered.
231
6.2. Results and Discussion
6.2.1. Design and Synthesis of Novel Nickel Catalysts
We have developed a new method for reduction of small molecule targets using boron-
metal cooperative hydride delivery.
14
For example, boron-ruthenium cooperation enables 6.4 as a
nitrile reduction catalyst under mild conditions.
11
Particularly, we have observed modest
reactivity for CO
2
conversion to methoxide with catalysts 6.3 and 6.4 (Figure 6.2). We believe
that the key to these CO
2
reduction is boron-metal cooperative hydride delivery. In order to
improve catalytic reactivity, we designed a family of bis(NHC)-ligated complexes, conceptually
sketched as 6.11 in figure 6.2, which retain the feature of a potentially reactive boron group while
introducing significantly stronger donor ligands to the metal. The latter were selected based on a
rationale that more donating ligands would enable more reducing metal hydride intermediates
that would be needed for facile CO
2
reduction.
Figure 6.2. Dual Center Manifold for CO
2
Reduction.
232
Accordingly, we prepared bis(imidazolium)borate monocations 6.5 and 6.6 (Scheme 1),
featuring diverse steric environments. Cation 6.5 or 6.6 can be doubly deprotanated to form
bis(imidazolium carbene)borate monoanions, respectively 6.7 and 6.8, which can be treated in
situ with Ni(acac)
2
to form structurally novel nickel complexes 6.1 and 6.2 in preparatively useful
yields over 2 steps. Formation of 6.1’s nickel iodide bond results from persistence of the iodide
counterion that accompanies 6.5.
Scheme 6.1. Synthesis of nickel compounds 6.1 and 6.2.
Nickel(II) complexes 6.1 and 6.2 are bimetallic and trimetallic compounds, respectively
(Figure 6.3). In the solid state structure of complex 6.1, one of three bidentate borate ligands
bridges the two nickels. More curiously, a B—H bond from one nickel’s ligand interacts with
another nickel in an agostic fashion. Quite unlike compound 6.1, compound 6.2 is free of all the
233
boron atoms that are introduced in its synthesis. However, it retains the potential for dual site
reactivity in that the three nickel atoms are held in close proximity by bridging imidizole groups.
While not designed based on the enzymes, these multi-metallic structures are reminiscent of
nickel-dependent hydrogenases that are reactive catalysts for CO
2
reduction under ambient
conditions.
15
Figure 6.3. ORTEP Diagrams of 6.1 and 6.2. Ellipsoids are drawn at the 50% probability level.
6.2.2. Room Temperature CO
2
Reduction to Methanol
Both 6.1 and 6.2 exhibit high reactivity as catalysts for room-temperature reduction of CO
2
by NaBH
4
. For example, in two weeks, 6.1 and 6.2 can deliver more than 72000 or 143000
turnovers of CO
2
(TONs of hydride are 216k or 429k), respectively, without apparent loss of
reactivity (Figure 6.4). In a longevity experiment, catalyst 6.2 reached to a CO
2
TON of 1.1
million over 2 months (3.3 M TONs of hydride), and was still reactive when the reaction mixture
was quenched. This TON is over three orders of magnitude more than the highest of CO
2
reduction by metal catalysts and borohydride in literature (Figure 6.1). Also noteworthy in this
234
reaction is that > 90% of the total hydride groups in NaBH
4
were converted to C—H bonds, which
is superior in productivity to a typical NaBH
4
reduction of a carbonyl group.
Figure 6.4. Kinetic Profile of CO
2
Reduction by NaBH
4
Catalyzed by 1 and 2 in 2 Weeks. TONs
are based on CO
2
. Yield is based on NaBH
4
. Loadings of Ni catalysts 1 and 2 and BH
3
· SMe
2
are
1.9 μmol, 1.3 μmol, and 20 μmol, respectively.
0 100 200 300 400
0
20
40
60
80
100
Time (h)
yield (%)
catalyst 1
catalyst 2
BH
3
-SMe
2
no catalyst
TON = 143000
TON = 72000
235
Figure 6.5.
1
H-NMR of CO
2
(1 atm) Reduction in THF by NaBH
4
and Catalyst 6.1.
Nickel catalysts derived from 6.1 and 6.2 are exceptionally robust: they work in air and they
have high tolerance for water. We take advantage of this fact and reduce CO
2
in presence of a small
amount of water to directly synthesize methanol. In representative NMR experiment, in presence
of 1 vol% H
2
O, the reaction yielded methanol, instead of boron methoxides, without a hydrolysis
step (Figure 6.5).
6.2.3 Mechanistic Discussion
Both nickel and the ligand are important to the mechanism (Table 1), because none of
Ni(acac)
2
, 6.5, or a mixture of Ni(acac)
2
with 6.5 or 6.6 or imidazole effect reduction. When 6.7
is used to affect the same transformation, we observe quantitative conversion of formate to
formates
methanol
H
2
1% H
2
O
solvent
solvent
5 minutes
25 minutes
benzene
236
methoxide. We suspect this reactivity is from borohydride activation by NHCs, as suggested by
computational study on this topic.
16
Table 6.1. Formate Conversion to Methoxides.
entry Catalyst conversion TON
1 6.1 61% 99
2 6.2 64% 104
3 Ni(acac)
2
< 5% N/A
4 Ni(acac)
2
+ methylimidazole < 5% N/A
5 Ni(acac)
2
+ 6.5 < 5% N/A
6 6.5 < 5% N/A
7 Ni(acac)
2
+ 6.6 < 5% N/A
8
a,b
6.7 6.8% 1
c
9
d
6.9 27% 20
Conditions: formates are generated from the reaction between NaBH
4
(8.0 mg, 210 μmol) and
HCO
2
H (8.0 µL, 210 μmol) in CD
3
CN (0.6 mL) in a J. Young tube. For the conversion of
formates, catalyst (1.3 µmol) is added. Reaction progress is monitored by disappearance of
formate peaks in
1
H NMR.
a
CO
2
is used instead of FA to avoid protonation of 6.7.
b
The initial
[formate] is 0.30 M.
c
TON is based on two carbenes per molecule of 6.7.
d
2.6 µmol catalyst is
added.
237
Kinetic studies were conducted on nickel complex 6.1. We find that the CO
2
pressure has
little impact on the reduction: in parallel runs under different CO
2
pressures ranging from 15 to
45 psi, same yields of methoxides were observed. Complexes 6.1 and 6.2 are also effective catalysts
for reduction of formic acid by NaBH
4
to methoxides. Sequential protonations of borohydride
with formic acid give the same formates as those generated from CO
2
: dissolving formic acid and
NaBH
4
in acetonitrile affords identical
1
H-NMR spectra (Figure 6.6). This enables formic acid as
a convenient liquid surrogate for CO
2
for kinetic studies. The formate compounds are stable at
room temperature and do not undergo further reduction until nickel catalyst 6.1 or 6.2 is
introduced. Thereafter, methoxides form until all formates are consumed. We find this formate
reduction to have first order dependence on [formate] and first order dependence on [6.1].
Figure 6.6. Reduction of Formates to Methoxides Monitored by NMR. Left: formate
1
H-NMR
peaks from NaBH
4
reaction with CO
2
or FA. The two major peaks are: the peak at δ(
1
H) = 8.29
ppm is [HB(OCHO)
3
]
-
; the peak at δ(
1
H) = 8.22 ppm is [H
2
B(OCHO)
2
]
-
.
13,16
Right: conversion
of formates to methoxides by NMR.
methoxides
formates from CO
2
formates
formates from FA
5 min
5 min
3 h
6 h
9 h
12 h
15 h
18 h
238
When 6.2 is treated with a stoichiometric amount of NaBH
4
, we see rapid conversion of
BH
4
-
to a new borane species (
11
B-NMR, Figure 6.29) and a new, broad hydride peak in
1
H-NMR
at δ(
1
H) = - 13.8 ppm, suggesting the formation of Ni-H species, which is consistent with a
hydrogen bridging Ni and B (Figure 6.30).
8c
This Ni—H species, if charged with 1 atm CO
2
,
yields formate and methoxide peaks in
1
H-NMR (Figure 6.31). Similarly, in an isotope labelling
experiment, sodium formate-d
1
(DCOONa) was clearly reduced by this Ni-H system to a
methoxide-d
1
product (Figure 6.34). These data show us that our conditions result in the
formation of a nickel hydride intermediate that is capable of reduction of both CO
2
and our
formate species. While we don’t know that it is a resting state, we propose that this is part of our
catalytic cycle. In a control experiment, Ni(acac)
2
reacts with NaBH
4
to afford a different hydride
species companying formation a new borate species. This result is taken as evidence for the center
nickel atom in 6.2 not being the only reactive center.
We have isolated two nickel(II) species from the working reaction conditions,
tetra(carbene) species 9 and 10 (Figure 6.7). Complex 6.9 shows only modest reactivity in
formate reduction: its reaction rate is ca. 5 times slower than 6.1, and we have only observed a
modest TON (72) with it. Compound 6.10 is not long-lived: while we were extraordinarily
fortunate to obtain crystallography data; it was not sufficiently robust to test in catalysis. We
suspect that the active catalyst is a monomeric nickel carbene complex with a reactive nickel
hydride in its reducing form. We base this on the observation of first order kinetic dependence on
[6.1], the isolations of 6.9 and 6.10, and the observation of a hydride in stoichiometric model
239
reactions. We propose that dimer 6.1 cleaves to make the inert species 6.9 and a
bis(carbene)nickel active species 6.11, which is reduced to give an active hydride (Scheme 6.2).
We expect that 6.9 can lose a ligand (slowly) to become 6.11, which then reduces formate using
hydrides from BH
4
-
.
Figure 6.7. Nickel Complexes 6.9, 6.10 and Their Crystal Structures. ORTEP ellipsoids drawn at
the 50% probability level.
Scheme 6.2. A Plausible Mechanism for CO
2
Reduction by NaBH
4
and Catalyst 6.1.
6.3. Conclusion
In conclusion, we report the synthesis and structural characterization of two novel NHC
supported nickel complexes, 6.1 and 6.2. These nickel complexes can catalyze CO
2
reduction to
methoxides with NaBH
4
under ambient conditions. The catalysis features unprecedented stability,
240
enabling a stunning > 1 million turnovers. The reaction features sodium borohydride, which has
superior cost and convenience relative to more complex and sensitive borane and silane reagents.
6.4. Experimental Section
6.4.1. Materials and Methods
All air and water sensitive procedures were carried out either in a Vacuum Atmosphere
glove box under nitrogen (2-10 ppm O
2
for all manipulations) or using standard Schlenk
techniques under nitrogen. Acetonitrile-d
3
, tetrahydrofuran-d
8
, D
2
O and any other NMR solvents
were purchased from Cambridge Isotopes Laboratories. Acetonitrile-d
3
, and tetrahydrofuran-d
8
are dried over CaH
2
and sodium benzophenone ketyl, respectively, and vapor-transferred prior to
use. Dichloromethane, ethyl ether, tetrahydrofuran and hexanes were purchased from VWR and
dried in a J. C. Meyer solvent purification system with alumina/copper(II) oxide columns;
toluene was stirred over sodium benzophenone ketyl and distilled; nickel(II) 2,4-pentanedionate
(Alfa Aesar), iodine (Sigma-Aldrich), 1-methylimidazole (Sigma-Aldrich) were purged with
nitrogen and stored under nitrogen atmosphere; 1-mesitylimidazole was synthesized following
literature procedures.
17
Dry CO
2
gas is purchased from Gilmore Liquid Air Company; dry ice is
purchased from Airgas, Inc.
NMR spectra were recorded on a Varian VNMRS 500 or VNMRS 600 spectrometer,
processed using MestreNova. All chemical shifts are reported in units of ppm and referenced to
the residual
1
H or
13
C solvent peak and line-listed according to (s) singlet, (bs) broad singlet, (d)
241
doublet, (t) triplet, (dd) double doublet, etc.
13
C spectra are delimited by carbon peaks, not
carbon count. Air-sensitive NMR spectra were taken in 8” J-Young tubes (Wilmad or Norell)
with Teflon valve plugs. MALDI mass spectra were obtained on an Applied Biosystems Voyager
spectrometer using the evaporated drop method on a coated 96 well plate. The matrices used
for MALDI are 2,5-dihydroxybenzoic acid or anthracene. In a standard preparation, ca. 1 mg
analyte and ca. 10 mg matrix was dissolved in methanol and spotted on the 96-well MALDI plate
with a glass capillary. Infrared spectra were recorded on Bruker OPUS FTIR spectrometer. X-ray
crystallography data were obtained on a Bruker APEX DUO single-crystal diffractometer
equipped with an APEX2 CCD detector, Mo fine-focus and Cu micro-focus X-ray sources.
Elemental analysis data were obtained on a Thermo Flash 2000 CHNS Elemental Analyzer.
6.4.2. CO
2
Reduction Procedures
In a glovebox, NaBH
4
and nickel catalyst are weighed and added to a Schlenk flask equipped
with a magnetic stir bar. After solvent (THF or acetonitrile) is measured and transferred with a
disposable plastic syringe, the flask is sealed with a vacuum septum then firmly taped. The flask is
taken out of the glovebox, connected to a Schlenk line, gently purged with 1 atm CO
2
. A needle
attached to an empty balloon then pierces the septum to allow the balloon to be filled with CO
2
.
Usually there is no difference if we purge the balloon three times or just fill it up. A few seconds
after the solution is exposed to CO
2
, it turns obviously cloudy and feels warm in hands. The
reaction mixture is stirred at room temperature until analyses are performed, where typically a
small aliquot of the solution is transferred with a microsyringe.
242
When the same reaction is performed in wet benchtop solvents (ACS grade), all the
chemicals are handled in air, and dry ice is used in place of high purity CO
2
gas unless otherwise
specified.
6.4.3. Synthesis Procedures and Characterization Data
6.4.3.1. Ligand 6.5
This synthesis is slightly modified from known precedures.
18
In the glovebox under nitrogen,
in a 500 mL in a Schlenk flask, iodine (8.33 g, 32.8 mmol) is added in small portions to a stirring
solution of borane trimethylamine complex (4.79 mg, 65.6 mmol) in 100 mL dry toluene. After
30 minutes, 1-methylimidazole (10.46 mL, 10.77 g, 131.2 mmol) is also added to the mixture.
The flask is then taken out of the glovebox, carefully attached to a reflux condenser. The top of the
reflux condenser is connected to a gas outlet, which is further connected to an eudiometer
(inverted graduated cylinder) through an oil bubbler.(see Figure 6.8) The reaction flask is placed
in an oil bath set to 110 º C to bring the reaction to reflux. After reflux for 14 hours, the eudiometer
shows > 99% conversion. A white precipitate has formed at bottom of the flask. The flask is
removed from the hot bath letting the reaction mixture cool to room temperature and the flask is
taken into the glovebox again. In the glovebox, the white precipitate is collected by filtration, and
243
washed with cold toluene (15 mL × 3). The white solid is then dried under vacuum for afford pure,
desired product (19.16 g, 96%).
Figure 6.8. Apparatus Set-up for the Synthesis of Borate Ligand 6.5 and 6.6.
1
H NMR (500 MHz, Acetonitrile-d
3
) δ 8.39 (s, ar, 1H), 7.25 (d, J = 1.4 Hz, ar, 1H), 7.18 (d, J =
1.3 Hz, ar, 1H), 3.80 (s, ar-CH
3
, 3H), 3.32 (q, J = 110 Hz, BH
2
, 2H).
13
C NMR (126 MHz, Acetonitrile-d
3
) δ 139.77, 125.87, 124.16, 35.96.
11
B NMR (160 MHz, Acetonitrile-d
3
) δ -8.81(t, J = 108.8 Hz).
Elemental Analysis (CHNS) calc’d for C
8
H
14
BIN
4
: C, 31.61; H, 4.64; N, 18.43; S, 0. Found: C,
31.22; H, 4.91; N, 18.86; S, 0.
IR (thin film/cm
-1
) ν 3584, 3745, 3584, 3443, 3070, 2918, 2850, 2425, 1617, 1558, 1539, 1419,
1262, 1126, 1074, 804, 630.
244
Figure 6.9.
1
H NMR Spectrum of 6.5 at 25 ° C in CD
3
CN.
Figure 6.10.
13
C NMR Spectrum of 6.5 at 25 ° C in CD
3
CN.
245
Figure 6.11.
11
B NMR Spectrum of 6.5 at 25 ° C in CD
3
CN.
246
6.4.3.2. Ligand 6.6
The synthesis of 6.6 follows the same procedure for the synthesis of 6.5. In the glovebox
under nitrogen, in a 500 mL in a Schlenk flask, iodine (341.0 mg, 1.34 mmol) is added in small
portions to a stirring solution of borane trimethylamine complex (234.6 mg, 3.22 mmol) in 30
mL dry toluene. After 30 minutes, 1-mesitylimidazole (1000 mg, 5.37 mmol) is also added to the
mixture. The flask is then taken out of the glovebox, carefully attached to a reflux condenser. The
top of the reflux condenser is connected to a gas outlet, which is further connected to a eudiometer
(inverted graduated cylinder) through an oil bubbler (see figure 6.8). The reaction flask is placed
in an oil bath set to 110 º C to bring the reaction to reflux. After reflux for 24 hours, eudiometer
shows > 99% conversion. A white precipitate has formed at bottom of the flask. The flask is
removed from the hot bath letting the reaction mixture cool to room temperature and the flask is
taken into the glovebox again. In the glovebox, the white precipitate is collected upon filtration,
and washed with cold ether (10 mL × 3). The white solid is then dried under vacuum for afford
pure, desired product (1.3 g, 95%). Slow recrystallization from THF and hexanes afforded crystals
suitable for X-ray crystallography.
247
1
H NMR (500 MHz, Acetonitrile-d
3
) δ 8.43 (d, J = 1.7 Hz, imidazole 2H), 7.48 (dd, J = 1.5, 0.8
Hz, imidazole 2H), 7.33 (dd, J = 1.6, 0.6 Hz, imidazole 2H), 7.10 (t, J = 0.6 Hz, mesityl-ar 4H),
3.54 (d, J = 110 Hz, BH
2
2H), 2.35 (s, mesityl-4-CH
3
6H), 1.99 (s, mesityl-2,6-CH
3
, 12H).
13
C NMR (126 MHz, Acetonitrile-d
3
) δ 141.74, 139.95, 135.87, 132.49, 130.32, 126.74, 124.87,
21.14, 17.44.
11
B NMR (160 MHz, Acetonitrile-d
3
) δ -2.52 (t, J = 110 Hz).
Elemental Analysis (CHNS) Anal. calc’d for C
24
H
30
BIN
4
: C, 56.27; H, 5.90; N, 10.94; S, 0. Found:
C, 56.39; H, 5.94; N, 11.33; S, 0.
IR (thin film/cm
-1
) ν 3854, 3745, 3584, 3442, 3119, 2918, 2850, 2429, 2362, 1734, 1700, 1599,
1558, 1539, 1457, 1419, 1262, 1127, 1069, 888.
248
Figure 6.12.
1
H NMR Spectrum of 6.6 at 25 ° C in CD
3
CN.
Figure 6.13.
13
C NMR Spectrum of 6.6 at 25 ° C in CD
2
CN.
249
Figure 6.14.
11
B NMR Spectrum of 6.6 at 25 ° C in CD
3
CN.
6.4.3.3. Deprotonation of 6.5
The deprotonation procedure is modified from an existing preparation.
19
In this synthesis,
it is recommended that LDA is prepared and used fresh. Diisopropylamine is dried over CaH
2
and
distilled prior to use. Typically, diisopropylamine (0.46 mL, 3.3 mmol) is dissolved in hexanes in
a Schlenk flask protected under N
2
. The solution is stirred and chilled in acetone/dry ice bath and
a recently titrated n-butyllithium solution (1.5 M, 2.2 mL, 3.3 mmol) is slowly added to the
250
solution. The acetone/dry ice bath is then removed to allow slow warming of the solution and the
solution is stirred for another 15 min. And then the solution is stirred in warm water bath for a few
minutes until all is dissolved and the solution appears clear and homogeneous. The flask is
removed from the water bath and put in an acetone/dry ice bath without stirring for 12 hours.
White crystalline LDA forms abundant precipitate at the bottom of the flask. The precipitate is
very carefully filtered and dried in a glovebox to yield pure solid LDA (152 mg). This reaction is
repeated a few times to accumulate enough material for the deprotonation of 5. LDA is packed
under N
2
and stored in a freezer when not being used.
In the glovebox, 6.5 (500 mg, 1.65 mmol) and LDA (353.5 mg, 3.3 mmol) are weighed and
added to a 50 mL Schlenk flask equipped with a stir bar. The flask is chilled in a dry ice/acetone
bath and cold diethyl ether (-78
o
C) is added to the flask via cannula transfer. The mixture is stirred
and allowed to slowly warm up to room temperature over 2 h. Then volatiles are removed under
vacuum and residue is redissolved in THF and filtered through celite. Upon slow addition of
hexanes, an off-white powder precipitated and can be collected by filtration. NMR study suggests
this powder is a THF adduct of the lithium salt 6.7. This THF adduct proves to be very reactive
in ligation.
1
H NMR (600 MHz, Benzene-d
6
) δ 7.25 (s, imidazole, 2H), 6.33 (s, imidazole, 2H), 4.51 (q, J =
115, BH
2
, 2H), 3.46 (td, J = 6.3, 2.9 Hz, THF, 4H), 3.17 (s, ar-CH
3
, 6H), 1.28 (td, J = 6.4, 3.1
Hz, THF, 4H).
251
Figure 6.15.
1
H NMR Spectrum of 6.7 at 25 ° C in C
6
D
6
252
6.4.3.4. Nickel Complex 6.1
Although 6.7 was found to be reactive with Ni(acac)
2
, the reaction was difficult to scale up.
After a number of attempts, we decided to more conveniently prepare 6.7 in situ for the ligation.
Typically, 6.5 (887.3 mg, 2.92 mmol) and LDA (626,7 mg, 5.85 mmol) are stirred in a dry Schlenk
flask chilled in a dry ice/acetone bath. Cold THF (-78
o
C, 50 mL) is added to the flask via cannula
transfer. The reaction is allowed to slowly warm up to room temperature and is stirred for another
hour after reached room temperature. This solution is then added dropwise to a stirred THF
solution of Ni(acac)
2
(500 mg, 1.95 mmol in 150 mL THF). After stirring for an hour at room
temperature, volatiles are removed under vacuum to yield a yellow precipitate. This precipitate
from THF/hexanes recrystallized 6.1 as a pale yellow solid (535 mg, 71%). Slower
recrystallization over a week afforded crystals suitable for X-ray crystallography.
1
H NMR (600 MHz, Acetonitrile-d
3
) δ 6.99 (m, ar 3H), 6.66 (dd, J = 6.5, 3.0 Hz, 4H), 5.50 (dd,
J = 6.3, 2.5 Hz, 2H), 5.46 (s, 1H), 5.37 (d, J = 5.2 Hz, 1H), 5.21 (s, 1H), 4.00-3.57 (m, B-H,
6H), 2.23 (s, ar- CH
3
, 3H), 2.18 (s, ar- CH
3
, 15H).
253
13
C NMR (151 MHz, Acetonitrile-d
3
) δ 148.00, 147.55, 143.03, 142.42, 126.39, 108.26, 68.08,
35.37, 35.04, 30.64, 29.72, 24.58, 24.14.
11
B NMR (192 MHz, Acetonitrile-d
3
) δ 17.98, -2.43.
Elemental Analysis (CHNS) Anal. calc’d for C
24
H
36
B
3
IN
12
Ni
2
: C, 37.47; H, 4.22; N, 21.85; S, 0.
Found: C, 37.09; H, 4.40; N, 21.59; S, 0.
IR (thin film/cm
-1
) ν 3437, 2919, 2850, 2422, 1598, 1517, 1448, 1260, 1072, 919, 864, 804, 630.
MS (MALDI) Anal. calc’d for [C
24
H
36
B
3
N
12
Ni
2
+
]: 641.2. Found: 641.4.
Figure 6.16.
1
H NMR Spectrum of 6.1 at 25 ° C in CD
3
CN.
254
Figure 6.17.
13
C NMR Spectrum of 6.1 at 25 ° C in CD
3
CN.
Figure 6.18.
11
B NMR Spectrum of 6.1 at 25 ° C in CD
3
CN.
255
6.4.3.5. Nickel Complex 6.2
6.2 is synthesized using a similar procedure as the one for 6.1. Typically, 6.6 (996.9 mg, 1.95
mmol) is suspended and stirred in 50 mL cold THF (-78
o
C). To this THF solution, nBuLi (1.5
M, 1.3 mL, 1.95 mmol) is added dropwise to via a plastic syringe. The reaction is then allowed to
slowly warm up to room temperature and is stirred for another 2 hours after reached room
temperature. This resulted solution is added dropwise to a stirred THF solution of Ni(acac)
2
(500
mg, 1.95 mmol). After stirring for an hour at room temperature, volatiles are removed under
vacuum to yield a yellow precipitate. This precipitate from THF/hexanes recrystallized 6.2 as a
pale yellow solid (283 mg, 39%). Slower recrystallization over a week afforded crystals suitable for
X-ray crystallography.
1
H NMR (600 MHz, Acetonitrile-d
3
) δ 7.57 (s, imidazole, 8H), 7.44 (s, mesityl-ar, 2H), 7.06 (s,
mesityl-ar, 2H), 6.96 (s, mesityl-ar, 2H), 6.93 (s, mesityl-ar, 2H), 5.46 (s, CH
3
COCHCOCH
3
,
1H), 5.11 (s, CH
3
COCHCOCH
3
, 1H), 3.25 (s, CH
3
COCHCOCH
3
, 3H), 2.74 (s, mesityl,
12H), 2.35 (s, mesityl 24H), 2.30 (s, CH
3
COCHCOCH
3
, 3H), 2.20 (s, CH
3
COCHCOCH
3
,
3H), 1.31 (s, CH
3
COCHCOCH
3
, 3H).
256
13
C NMR (151 MHz, Acetonitrile-d
3
) δ 187.54, 150.15, 139.24, 136.89, 135.96, 134.21, 130.68,
130.01, 129.27, 127.46, 126.43, 101.92, 50.73, 24.48, 21.00, 20.32, 19.06, 17.67.
Elemental Analysis (CHNS) Anal. calc’d for C
58
H
66
N
8
Ni
3
O
4
: C, 62.46; H, 5.97; N, 15.79; S, 0.
Found: C, 62.20; H, 5.90; N, 15.46; S, 0.
IR (thin film/cm
-1
) ν 3854, 3436, 2918, 2850, 2428, 2362, 1591, 1539, 1517, 1436, 1261, 1066,
861, 804, 630.
MS (MALDI) Anal. calc’d for [C
53
H
59
N
8
Ni
3
O
2
+
]: 1015.3. Found:1015.6.
Figure 6.19.
1
H NMR Spectrum of 6.9 at 25 ° C in CD
3
CN.
257
Figure 6.20.
13
C NMR Spectrum of 6.2 at 25 ° C in CD
3
CN.
258
6.4.3.6. Nickel Complex 6.9
A number of attempts were made to find a reactive intermediate for the catalytic reaction.
In a particular one, we were able to isolate 6.9 after a few turnovers. In this reaction, 6.1 (20 mg,
0.026 mmol) was added to a stirred suspension of NaBH
4
(9.8 mg, 0.26 mmol) in 30 mL THF.
The reaction flask was gently evacuated for 10 seconds, then refilled with 1 atm CO
2
. The reaction
was stirred for 3 hours before filtered through celite. This clear THF solution was added 30 mL
hexanes and reduced volume to approximately 20 mL under vacuum when it appeared to be a
little cloudy. The solution was stored in a -20
o
C freezer for a week; small crystals formed in the
flask were found to be suitable for X-ray crystallography.
1
H NMR (600 MHz, Methylene Chloride-d
2
) δ 7.00 (d, J = 1.6 Hz, imidazole, 2H), 6.65 (d, J =
1.6 Hz, imidazole, 2H), 4.13 (q, J = 120 Hz, BH
2
, 2H), 3.44 (d, J = 110 Hz, BH
2
, 2H), 3.00 (s, ar-
CH
3
12H).
13
C NMR (151 MHz, Methylene Chloride-d
2
) δ 180.88, 124.90, 120.18, 35.21.
11
B NMR (192 MHz, Methylene Chloride-d
2
) δ -6.76 (t, J = 110 Hz).
259
Figure 6.21.
1
H NMR Spectrum of 6.9 at 25 ° C in CD
2
Cl
2
.
Figure 6.22.
13
C NMR Spectrum of 6.9 at 25 ° C in CD
2
Cl
2
.
260
Figure 6.23.
11
B NMR Spectrum of 6.9 at 25 ° C in CD
2
Cl
2
.
261
6.4.4. CO
2
Reduction conditions
6.4.4.1. NMR Kinetic Run of CO
2
Reduction in wet THF
In a typical reaction, in a glovebox NaBH
4
(8 mg, 0.21 mmol), nickel catalyst 6.1 (0.6 mg,
0.78 μmol) and dry THF (0.6 mL) are added to a J. Young tube. 6 μL H
2
O is added to increase
the reaction rate, and 1 μL benzene is added as internal standard for chemical shifts. The tube is
gently evacuated under vacuum for 10 seconds then refilled with 1 atm CO
2
. The tube is quickly
taken to an NMR spectrometer to observe reactivity. In
1
H-NMR, we see quick formation of
formates in the first spectrum. Over time, the formates are converted into methoxides.
Figure 6.24. Stacked
1
H NMR Spectra of CO
2
Reduction in THF-d
8
. The above spectra are focus
in on formate region.
formates
1380 s
1080 s
780 s
540 s
300 s
262
6.4.4.2. CO
2
Reduction Over Two Weeks
In a typical reaction, in a glovebox NaBH
4
(6 g, 159 mmol), nickel catalyst 6.1 (1.5 mg,
0.0019 mmol) or nickel catalyst 6.2 (1.5 mg, 0.0013 mmol) are suspended in wet THF (ACS
grade, 30 mL) in a 250 mL Schlenk flask. The flask is gently evacuated under vacuum for 5 seconds,
then refilled with 1 atm CO
2
. Also a CO
2
purged balloon is attached to the flask and filled up with
ca. 1 ft
3
of CO
2
gas. The reaction is then stirred overnight, analyzed and recharged with CO
2
on
the next day. When doing quantitative analysis, 50 μL reaction solution is dissolved in 0.6 mL D
2
O,
and 1 mg 2,3-dihydroxybenzoic acid is added as an external standard. The reaction conversions
are calculated based on NMR integrations. During the two weeks, both of the reactions catalyzed
by 6.1 or 6.2 were recharged and analyzed 9 times. By the end of the two weeks, we observed TON
= 72,000 on 6.1 and TON = 143,000 on 6.2. The NMR yields are 67.7% and 93.2%, respectively,
based on the total hydrides in NaBH
4
. Most significantly, no apparent reactivity loss was observed.
263
Figure 6.25.
1
H NMR Spectrum of CO
2
Reduction in THF after the First 14 h.
6.4.4.3. Longevity Experiment: CO
2
Reduction Over Two Months
In a typical reaction, in a glovebox NaBH
4
(60 g, 1.59 mol), nickel catalyst 6.2 (1.0 mg, 0.9
μmol) are suspended in wet THF (ACS grade, 100 mL) in a 500 mL Schlenk flask. The flask is
gently evacuated under vacuum for 5 seconds, then refilled with 1 atm CO
2
. Also a CO
2
purged
balloon is attached to the flask and filled up with ca. 2 ft
3
of CO
2
gas. The reaction is then stirred
overnight, analyzed and recharged with CO
2
after the balloon is deflated. The quantitative analysis
is perform using the same method as previous described. The reaction conversions are calculated
based on NMR integrations. During the two months, the reaction was recharged over 40 times.
By the end of the two moth period, we observed TON > 1,100,000 for the catalyst. The NMR
H
2
O THF
THF
HCOOH
2,3-dihydrooxybenzoic
acid
MeOH
264
yields are 46.7%, based on the total hydrides in NaBH
4
. By the time the reaction mixture was
quenched, the catalyst was still active.
6.4.5. Formic acid Reduction by NaBH
4
We first set up a control reaction, in a glovebox NaBH
4
(10 mg, 0.26 mmol) and HCOOH
(10 μL, 12 mg, 0.26 mmol) were added to a J. Young NMR tube with 0.6 mL dry acetonitrile-d
3
in it. The tube was observed in an NMR kinetic run at room temperature for 22 hours, then
sonicated for 2 hours and another
1
H-NMR was acquired. During all this time, we only saw
formation of formatoborates in this reaction. Then we added the nickel catalyst 6.1 (1 mg, 1.3
μmol) to the solution. As soon as 6.1 is added, bubles evolved from the solution. We then quickly
ran an NMR kinetic experiment on the sample, observing conversion of formates to methoxides
under room temperature. After 18 h, 0.6 mL of D
2
O was added to the tube and the tube was
sonicated briefly to fully hydrolyze methoxides to methanol.
Figure 6.26.
1
H NMR Spectrum of Reaction between Formic Acid and NaBH
4
in dry Acetonitrile-
d
3
after 1 d.
formates
H
2
acetonitrile borohydride
265
Figure 6.27.
1
H NMR Spectrum of Hydrolyzed Product of This Reaction.
Figure 6.28.
13
C NMR Spectrum of Hydrolyzed Product of This Reaction.
HCOOH
water
MeOH
acetonitrile
CH
3
CH
2
NH
2
CH
3
CH
2
NH
2
borohydride
HCOOH
acetonitrile
MeOH CH
3
CH
2
NH
2
acetonitrile
266
6.4.6. Mechanistic NMR Experiments
The fate of the nickel catalyst was studied with NMR experiments. In a glovebox NaBH
4
(0.7 mg, 18 µmol) and 6.2 (20 mg, 18 µmol) were added to a J. Young NMR tube with 0.6 mL dry
THF-d
8
in it. The tube was immersed in a sonic cleaning bath at room temperature for ca. 5
minutes to to form a light yellow solution with a modest amount of undissolved material. During
this time, the color of the solution appeared to have darkened. The solution was quickly taken for
1
H-NMR (Figure 6.30)
11
B-NMR analyses (Figure 6.29). The results are consistent with the
formation of a nickel hydride species.
The J. Young tube prepared was then gently evacuated and refilled with 1 atm CO
2
. The
tube was shaken at room temperature for a few minutes before a
1
H-NMR spectrum was acquired.
Multiple formates and a prominent methoxide peak could be observed from this sample.
267
Figure 6.29.
11
B-NMR Spectrum of Reaction between 6.2 and NaBH
4
.
Figure 6.30.
1
H-NMR Spectrum of Reaction between 6.2 and NaBH
4
.
268
Figure 6.31. Treatment of the Ni-H species with CO
2
in
1
H-NMR.
Similarly, in a glovebox NaBH
4
(3.0 mg, 78 µmol) and Ni(acac)
2
(20 mg, 78 µmol) were
added to a J. Young NMR tube with 0.6 mL dry THF-d
8
in it. The tube was immersed in a sonic
cleaning bath at room temperature for ca. 5 minutes to to form a light blue solution with a modest
amount of undissolved material. During this time, the color of the solution appeared to have
darkened. The solution was quickly taken for
1
H-NMR (Figure 6.32)
11
B-NMR analyses (Figure
6.33). The results are consistent with the formation of a nickel hydride species. A different nickel
hydride species formed.
11
B NMR shows formation of new borate species instead of a borane.
269
Figure 6.32.
1
H-NMR Spectrum of Reaction between Ni(acac)
2
and NaBH
4
.
Figure 6.33.
11
B-NMR Spectrum of Reaction between Ni(acac)
2
and NaBH
4
.
270
6.4.7. Reduction of Formate-d
1
to Methoxide-d
1
In a glovebox NaBH
4
(5 mg, 0.13 mmol), nickel catalyst 2 (2.0 mg, 1.8 μmol) and THF
(not deuterated, ACS grade, 0.6 mL) are added to a J. Young tube. The tube is gently shaken for
a few minutes at room temperature. Formate-d
1
(20 mg, 0.29 mmol) is added to the same tube
and the tube is sonicated for 30 min at room temperature.
2
H-NMR reveals conversion of the
formate cleanly to methoxide without any other intermediate.
Figure 6.34. Reduction of DCOONa to Methoxide by
2
H-NMR.
formate
methoxide
271
6.4.8. Kinetic Data
6.4.6.1. The Kinetic Dependence Study of Formate and Nickel Catalyst
In these reactions, based on our typical conditions (0.6 mL dry acetonitrile-d
3
, 1 mg nickel
catalyst 6.1, 8 mg NaBH
4
, 8 μL dry HCOOH in a J. Young tube at room temperature), the
concentration of formates or nickel 6.1 is varied in these runs. Each of the kinetic run is acquired
on an NMR spectrometer; typically 25 data points are used in a fit to exponential decay to
calculate a reaction rate. For each variable (formate or 6.1), after four rates are analyzed, a
logarithm-logarithm plot of the correlation between rates and concentration is generated to
predict the kinetic dependence of the variable in the catalysis. The kinetic data acquired is plotted
as below.
Figure 6.35. The Kinetic Profile of Formate Reduction, 0.6 mL Dry Acetonitrile-d
3
, 1 mg Nickel
Catalyst 6.1, 8 mg NaBH
4
, 8 μL Dry HCOOH in a J. Young tube at room temperature.
0 20000 40000 60000 80000
0.0
0.1
0.2
0.3
0.4
Time (s)
Formate Conc. (mol/L)
k
obs
= 1.1(3) × 10
-3
272
Figure 6.36. The Kinetic Profile of Formate Reduction, 0.6 mL dry acetonitrile-d
3
, 1 mg nickel
catalyst 6.1, 8 mg NaBH
4
, 6 μL dry HCOOH in a J. Young tube at room temperature.
Figure 6.37. The Kinetic Profile of Formate Reduction, 0.6 mL dry acetonitrile-d
3
, 1 mg nickel
catalyst 6.1, 8 mg NaBH
4
, 4 μL dry HCOOH in a J. Young tube at room temperature.
0 20000 40000 60000
0.0
0.1
0.2
0.3
Time (s)
Formate Conc. (mol/L)
0 20000 40000 60000
0.00
0.05
0.10
0.15
0.20
Time (s)
Formate Conc. (mol/L)
k
obs
= 8.0(5) × 10
-4
k
obs
= 5.2(7) × 10
-4
273
Figure 6.38. The Kinetic Profile of Formate Reduction, 0.6 mL dry acetonitrile-d
3
, 1 mg nickel
catalyst 6.1, 8 mg NaBH
4
, 2 μL dry HCOOH in a J. Young tube at room temperature.
Figure 6.39. The Kinetic Profile of Formate Reduction, 0.6 mL dry acetonitrile-d
3
, 1.5 mg nickel
catalyst 6.1, 8 mg NaBH
4
, 8 μL dry HCOOH in a J. Young tube at room temperature.
0 20000 40000 60000
0.00
0.02
0.04
0.06
0.08
0.10
Time (s)
Conc. of formates (mol/L)
0 20000 40000 60000
0.0
0.1
0.2
0.3
0.4
Time (s)
Conc. of formates (mol/L)
k
obs
= 2.4(6) × 10
-4
k
obs
= 1.7(2) × 10
-3
274
Figure 6.40. The Kinetic Profile of Formate Reduction, 0.6 mL dry acetonitrile-d
3
, 2.3 mg nickel
catalyst 6.1, 8 mg NaBH
4
, 8 μL dry HCOOH in a J. Young tube at room temperature.
Figure 6.41. The Kinetic Profile of Formate Reduction, 0.6 mL dry acetonitrile-d
3
, 0.5 mg nickel
catalyst 6.1, 8 mg NaBH
4
, 8 μL dry HCOOH in a J. Young tube at room temperature.
0 20000 40000 60000
0.0
0.1
0.2
0.3
0.4
Time (s)
Formate Conc. (mol/L)
0 20000 40000 60000
0.0
0.1
0.2
0.3
0.4
Time (s)
Conc. of formates (mol/L)
k
obs
= 2.5(4) × 10
-3
k
obs
= 4.1(3) × 10
-3
275
Figure 6.42. The Log-Log Plot for Formate Concentration Dependence.
Figure 6.43. The Log-Log Plot for Nickel Catalyst 6.1 Concentration Dependence.
6.4.6.2. Reaction Catalyzed by Nickel Complex 6.9
In this reaction, based on our typical conditions (0.6 mL dry acetonitrile-d
3
, 1 mg nickel
catalyst 6.1, 8 mg NaBH
4
, 8 μL dry HCOOH in a J. Young tube at room temperature), the reaction
-1.2 -1.0 -0.8 -0.6 -0.4
-4.0
-3.8
-3.6
-3.4
-3.2
-3.0
-2.8
log [formate]
log (rate)
Y = 1.096*X - 2.460
-3.2 -3.0 -2.8 -2.6 -2.4 -2.2 -2.0
-4.0
-3.5
-3.0
-2.5
-2.0
log (conc. 1)
log (rate)
Y = 1.193*X + 0.1772
276
rate for formate conversion catalyzed by 6.9 (1.2 mg) was measured also using a kinetic NMR
study. The data acquired is shown below:
Figure 6.44. The Kinetic Profile of Formate Reduction, 0.6 mL dry acetonitrile-d
3
, 1.2 mg nickel
catalyst 6.9, 8 mg NaBH
4
, 8 μL dry HCOOH in a J. Young tube at room temperature.
0 20000 40000 60000 80000
0.15
0.20
0.25
0.30
0.35
0.40
Time (s)
Formate Conc. (mol/L)
k
obs
= 5.0(7) × 10
-4
277
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15, 401−413; (f) Eisenberg, F., Jr.; Bolden, A. H. Formate Contamination in Borohydride
Reduction. Carbohydr. Res. 1967, 5, 349−350.
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via the
Formation of the Formate Complex C5Me5Ru(PCy3)(.eta.2-OCHO). Organometallics
1995, 14, 2616−2617.
13. Knopf, I.; Cummins C. C. Revisiting CO2 Reduction with NaBH
4
under Aprotic Conditions:
Synthesis and Characterization of Sodium Triformatoborohydride. Organometallics 2015, 34,
1601-1603.
14. Lu, Z., Williams, T. J. A Dual Site Catalyst for Mild, Selective Nitrile Reduction. Chem.
Commun. 2014, 50, 5391-5393.
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Crystal Structure of the Nickel–Iron Hydrogenase from Desulfovibrio Gigas. Nature, 1995,
373, 580-587.
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in a Metal-Free Conversion of Carbon Dioxide into Methanol: A Computational Mechanism
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of 1-Arylimidazoles. Synthesis, 2003, 2661-2666.
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Contrasting Structures of Homoleptic Nickel(II) Bis(pyrazolyl)borate and
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Bis(carbene)borate Complexes. Eur. J. Inorg. Chem. 2008, 2476-2480.
19. (a) Rüther, T.; Huynh, T. D.; Huang, J.; Hollenkamp, A. F.; Salter, E. A.; Wierzbicki, A.;
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Complexes. J. Am. Chem. Soc. 2014, 136, 6056-6068.
283
Crystal Structure of 3.3
A clear light yellow prism-like specimen compound 3 were grown by slow evaporation of a
saturated DCM/hexane solution at ambient temperature. Diffraction data for
C
19
H
25
BF
3
N
5
O
3
RuS were collected at 100.1(2) K on a Bruker APEX II CCD system equipped
with a TRIUMPH curved-crystal monochromator and a MoKα fine-focus tube (λ = 0.71073 Å).
A total of 2520 frames were collected. The total exposure time was 7.00 hours. The frames
were integrated with the Bruker SAINT software package using a SAINT V8.18C algorithm. The
integration of the data using a monoclinic unit cell yielded a total of 112504 reflections to a
maximum θ angle of 30.02° (0.71 Å resolution), of which 13663 were independent (average
redundancy 8.234, completeness = 99.9%, Rint = 5.29%, Rsig = 3.24%) and 10943 (80.09%) were
284
greater than 2σ(F
2
). The final cell constants of a = 13.1714(7) Å , b = 13.8135(7) Å , c =
25.8235(13) Å, β = 95.3220(10)°, volume = 4678.2(4) Å
3
, are based upon the refinement of the
XYZ-centroids of 9664 reflections above 20 σ(I) with 4.667° < 2θ < 60.82°. Data were corrected
for absorption effects using the multi-scan method (SADABS). The ratio of minimum to
maximum apparent transmission was 0.857. The calculated minimum and maximum
transmission coefficients (based on crystal size) are 0.8149 and 0.8869.
The structure was solved and refined using the Bruker SHELXTL Software Package, using
the space group P 1 21/c 1, with Z = 8 for the formula unit, C
19
H
25
BF
3
N
5
O
3
RuS. The final
anisotropic full-matrix least-squares refinement on F
2
with 621 variables converged at R1 = 3.77%,
for the observed data and wR2 = 8.83% for all data. The goodness-of-fit was 1.025. The largest
peak in the final difference electron density synthesis was 1.763 e
-
/Å
3
and the largest hole was -
1.356 e
-
/Å
3
with an RMS deviation of 0.100 e
-
/Å
3
. On the basis of the final model, the calculated
density was 1.625 g/cm
3
and F(000), 2320 e
-
.
Table A.1. Sample and Crystal Data.
Chemical formula C
19
H
25
BF
3
N
5
O
3
RuS
Formula weight 572.38
Temperature 100(2) K
Wavelength 0.71073 Å
Crystal size 0.150 x 0.170 x 0.260 mm
Crystal habit clear light yellow prism
Crystal system monoclinic
285
Space group P 1 21/c 1
Unit cell dimensions a = 13.1714(7) Å α = 90°
b = 13.8135(7) Å β = 95.3220(10)°
c = 25.8235(13) Å γ = 90°
Volume 4678.2(4) Å
3
Z 8
Density (calculated) 1.625 g/cm
3
Absorption coefficient 0.814 mm
-1
F(000) 2320
Table A.2. Data Collection and Structure Refinement.
Diffractometer Bruker APEX II CCD
Radiation source fine-focus tube, MoKα
Theta range for data collection 1.55 to 30.02°
Index ranges -18 ≤ h ≤ 18, -19 ≤ k ≤ 19, -36 ≤ l ≤ 36
Reflections collected 112504
Independent reflections 13663 [R(int) = 0.0529]
Coverage of independent reflections 99.9%
Absorption correction multi-scan
Max. and min. transmission 0.8869 and 0.8149
Structure solution technique direct methods
Structure solution program SHELXS-97 (Sheldrick, 2008)
Refinement method Full-matrix least-squares on F
2
Refinement program SHELXL 2012-4 (Sheldrick, 2012)
286
Function minimized Σ w(F
o
2
- F
c
2
)
2
Data / restraints / parameters 13663 / 64 / 621
Goodness-of-fit on F
2
1.025
Δ/σ
max
0.002
Final R indices
10943
data;
I>2σ(I)
R1 = 0.0377, wR2 = 0.0815
all data R1 = 0.0540, wR2 = 0.0883
Weighting scheme
w=1/[σ
2
(F
o
2
)+(0.0317P)
2
+10.5182P]
where P=(F
o
2
+2F
c
2
)/3
Largest diff. peak and hole 1.763 and -1.356 eÅ
-3
R.M.S. deviation from mean 0.100 eÅ
-3
Table A.3. Bond Lengths (Å )
B1-N4 1.548(4) O5-S2 1.437(2)
B1-N5 1.578(4) O1A-S1A 1.419(11)
B2-N7 1.534(4) O3A-S1A 1.425(11)
B2-N10 1.549(4) B1-N2 1.549(4)
C1-C6 1.403(4) B1-H1B 1.11(3)
C1-C7 1.499(4) B2-N9 1.545(4)
C2-C3 1.408(4) B2-H2B 1.15(3)
C3-Ru1 2.198(3) C1-C2 1.434(4)
C4-C5 1.409(4) C1-Ru1 2.233(3)
287
C4-Ru1 2.201(3) C2-Ru1 2.197(3)
C5-Ru1 2.198(3) C3-C4 1.439(4)
C6-Ru1 2.211(3) C4-C8 1.516(4)
C8-C9 1.522(4) C5-C6 1.429(4)
C11-N2 1.349(4) C8-C10 1.518(4)
C14-C15 1.380(4) C11-C12 1.375(5)
C15-C16 1.400(4) C12-C13 1.389(5)
C16-N3 1.339(3) C13-N1 1.339(3)
C17-N5 1.267(4) C14-N4 1.342(3)
C19-C20 1.406(4) C17-C18 1.480(4)
C19-C25 1.501(4) C19-C24 1.436(4)
C20-C21 1.429(4) C19-Ru2 2.213(3)
C21-Ru2 2.205(2) C20-Ru2 2.184(2)
C22-C23 1.438(4) C21-C22 1.400(4)
C22-Ru2 2.229(3) C22-C26 1.519(4)
C23-Ru2 2.195(3) C23-C24 1.400(4)
C24-Ru2 2.195(2) C26-C28A 1.497(12)
C26-C27A 1.510(12) C26-C27 1.534(5)
C26-C28 1.535(5) C29-C30 1.373(5)
C29-N7 1.347(4) C30-C31 1.398(4)
C32-C33 1.374(4) C31-N6 1.339(3)
C33-C34 1.397(4) C32-N9 1.342(3)
C34-N8 1.336(3) C35-C36 1.488(4)
C35-N10 1.271(3) C37-F2 1.365(10)
288
C37-F1 1.323(10) C37-S1 1.838(10)
C37-F3 1.373(10) C38-F6 1.331(4)
C38-F5 1.328(4) C38-S2 1.824(3)
C38-F4 1.340(4) C37A-F3A 1.341(13)
C37A-F1A 1.311(12) C37A-S1A 1.822(13)
C37A-F2A 1.346(12) N1-Ru1 2.107(2)
N1-N2 1.362(3) N3-Ru1 2.106(2)
N3-N4 1.362(3) N6-N7 1.363(3)
N5-Ru1 2.138(2) N8-N9 1.365(3)
N6-Ru2 2.086(2) N10-Ru2 2.134(2)
N8-Ru2 2.108(2) O2-S1 1.448(8)
O1-S1 1.443(9) O4-S2 1.438(2)
O3-S1 1.459(9) O6-S2 1.437(2)
O2A-S1A 1.424(11)
Table A.4. Bond angles (° ).
N4-B1-N2 106.8(2) N4-B1-N5 102.6(2)
N2-B1-N5 102.2(2) N4-B1-H1B 112.1(17)
N2-B1-H1B 112.5(17) N5-B1-H1B 119.5(17)
N7-B2-N9 106.6(2) N7-B2-N10 103.5(2)
N9-B2-N10 102.4(2) N7-B2-H2B 114.5(16)
N9-B2-H2B 111.3(16) N10-B2-H2B 117.3(16)
289
C6-C1-C2 117.9(2) C6-C1-C7 120.4(3)
C2-C1-C7 121.7(3) C6-C1-Ru1 70.77(16)
C2-C1-Ru1 69.80(15) C7-C1-Ru1 130.5(2)
C3-C2-C1 121.2(2) C3-C2-Ru1 71.34(15)
C1-C2-Ru1 72.45(15) C2-C3-Ru1 71.30(15)
C2-C3-C4 120.7(2) C5-C4-C8 121.1(2)
C4-C3-Ru1 71.02(15) C5-C4-Ru1 71.21(15)
C5-C4-C3 117.8(2) C8-C4-Ru1 128.86(19)
C3-C4-C8 121.1(2) C4-C5-Ru1 71.44(15)
C3-C4-Ru1 70.80(15) C1-C6-Ru1 72.42(15)
C4-C5-C6 121.1(2) C4-C8-C9 109.6(2)
C6-C5-Ru1 71.60(15) C11-C12-C13 105.6(3)
C1-C6-C5 121.2(2) N4-C14-C15 108.5(3)
C5-C6-Ru1 70.60(15) N3-C16-C15 109.5(3)
C4-C8-C10 113.0(2) C20-C19-C24 118.0(2)
C10-C8-C9 111.6(3) C24-C19-C25 122.0(2)
N2-C11-C12 108.3(3) C24-C19-Ru2 70.30(14)
N1-C13-C12 109.6(3) C19-C20-C21 120.9(2)
C14-C15-C16 105.2(2) C21-C20-Ru2 71.83(14)
N5-C17-C18 128.2(3) C22-C21-C20 121.2(2)
C20-C19-C25 120.0(2) C20-C21-Ru2 70.19(14)
C20-C19-Ru2 70.23(14) C21-C22-C23 118.1(2)
C25-C19-Ru2 129.72(18) C23-C22-C26 119.2(2)
C19-C20-Ru2 72.49(14) C23-C22-Ru2 69.75(14)
290
C22-C21-Ru2 72.53(14) C24-C23-C22 120.8(2)
C21-C22-C26 122.8(2) C22-C23-Ru2 72.31(14)
C21-C22-Ru2 70.67(15) C23-C24-C19 121.0(2)
C26-C22-Ru2 130.90(18) C19-C24-Ru2 71.67(14)
C24-C23-Ru2 71.37(15) C28A-C26-C27A 113.7(8)
C23-C24-Ru2 71.42(14) C27A-C26-C22 108.1(7)
C28A-C26-C22 112.4(7) C27A-C26-C27 22.3(6)
C28A-C26-C27 129.2(8) C28A-C26-C28 24.4(7)
C22-C26-C27 109.0(3) C22-C26-C28 113.7(3)
C27A-C26-C28 91.4(7) C29-C30-C31 105.4(3)
C27-C26-C28 110.2(3) N9-C32-C33 108.3(3)
N7-C29-C30 108.6(3) N8-C34-C33 109.6(2)
N6-C31-C30 109.4(3) F1-C37-F2 104.3(8)
C32-C33-C34 105.5(2) F2-C37-F3 113.1(9)
N10-C35-C36 128.1(3) F2-C37-S1 114.7(8)
F1-C37-F3 104.9(8) F5-C38-F6 107.3(3)
F1-C37-S1 117.4(8) F6-C38-F4 106.4(3)
F3-C37-S1 102.3(8) F6-C38-S2 112.0(2)
F5-C38-F4 107.7(3) F1A-C37A-F3A 105.4(11)
F5-C38-S2 112.3(2) F3A-C37A-F2A 105.2(11)
F4-C38-S2 110.7(2) F3A-C37A-S1A 123.1(12)
F1A-C37A-F2A 113.3(12) C13-N1-N2 107.1(2)
F1A-C37A-S1A 105.0(11) N2-N1-Ru1 114.24(16)
F2A-C37A-S1A 105.0(10) C11-N2-B1 136.8(3)
291
C13-N1-Ru1 137.9(2) C16-N3-N4 107.2(2)
C11-N2-N1 109.3(3) N4-N3-Ru1 113.93(16)
N1-N2-B1 113.9(2) C14-N4-B1 135.5(3)
C16-N3-Ru1 138.86(19) C17-N5-B1 119.4(3)
C14-N4-N3 109.6(2) B1-N5-Ru1 102.25(17)
N3-N4-B1 114.8(2) C31-N6-Ru2 138.47(19)
C17-N5-Ru1 138.4(2) C29-N7-N6 109.2(2)
C31-N6-N7 107.3(2) N6-N7-B2 114.4(2)
N7-N6-Ru2 114.12(17) C34-N8-Ru2 139.36(19)
C29-N7-B2 136.2(3) C32-N9-N8 109.7(2)
C34-N8-N9 106.9(2) N8-N9-B2 113.7(2)
N9-N8-Ru2 113.71(16) C35-N10-Ru2 138.2(2)
C32-N9-B2 135.9(2) N3-Ru1-N1 82.67(9)
C35-N10-B2 119.4(2) N1-Ru1-N5 77.82(9)
B2-N10-Ru2 102.39(16) N1-Ru1-C2 117.93(10)
N3-Ru1-N5 77.08(9) N3-Ru1-C3 122.99(9)
N3-Ru1-C2 97.05(9) N5-Ru1-C3 156.91(9)
N5-Ru1-C2 162.72(9) N3-Ru1-C5 156.07(9)
N1-Ru1-C3 92.64(10) N5-Ru1-C5 99.02(10)
C2-Ru1-C3 37.36(10) C3-Ru1-C5 67.38(10)
N1-Ru1-C5 119.98(9) N1-Ru1-C4 92.81(9)
C2-Ru1-C5 79.67(10) C2-Ru1-C4 68.46(10)
N3-Ru1-C4 160.71(9) C5-Ru1-C4 37.35(10)
N5-Ru1-C4 120.43(9) N1-Ru1-C6 157.75(10)
292
C3-Ru1-C4 38.18(10) C2-Ru1-C6 66.92(10)
N3-Ru1-C6 119.11(9) C5-Ru1-C6 37.81(10)
N5-Ru1-C6 101.43(10) N3-Ru1-C1 95.13(9)
C3-Ru1-C6 79.49(10) N5-Ru1-C1 125.77(9)
C4-Ru1-C6 68.12(10) C3-Ru1-C1 67.92(10)
N1-Ru1-C1 155.37(10) C4-Ru1-C1 81.18(10)
C2-Ru1-C1 37.76(10) N6-Ru2-N8 82.63(9)
C5-Ru1-C1 67.69(10) N8-Ru2-N10 77.51(9)
C6-Ru1-C1 36.81(10) N8-Ru2-C20 157.04(9)
N6-Ru2-N10 77.17(9) N6-Ru2-C24 93.94(9)
N6-Ru2-C20 119.34(9) N10-Ru2-C24 158.87(9)
N10-Ru2-C20 99.80(9) N6-Ru2-C23 119.36(9)
N8-Ru2-C24 120.83(9) N10-Ru2-C23 161.36(9)
C20-Ru2-C24 67.61(10) C24-Ru2-C23 37.20(9)
N8-Ru2-C23 95.27(9) N8-Ru2-C21 119.57(9)
C20-Ru2-C23 80.09(10) C20-Ru2-C21 37.99(9)
N6-Ru2-C21 157.17(9) C23-Ru2-C21 67.17(10)
N10-Ru2-C21 101.20(9) N8-Ru2-C19 158.47(9)
C24-Ru2-C21 79.53(9) C20-Ru2-C19 37.28(9)
N6-Ru2-C19 93.49(9) C23-Ru2-C19 68.13(10)
N10-Ru2-C19 122.43(9) N6-Ru2-C22 157.03(9)
C24-Ru2-C19 38.04(9) N10-Ru2-C22 124.67(9)
C21-Ru2-C19 67.86(9) C24-Ru2-C22 67.82(9)
N8-Ru2-C22 94.60(9) C21-Ru2-C22 36.80(9)
293
C20-Ru2-C22 67.89(10) O1-S1-O2 112.0(7)
C23-Ru2-C22 37.94(9) O2-S1-O3 111.6(7)
C19-Ru2-C22 80.72(9) O2-S1-C37 102.4(9)
O1-S1-O3 111.5(8) O5-S2-O6 115.09(15)
O1-S1-C37 109.3(7) O6-S2-O4 115.02(15)
O3-S1-C37 109.5(7) O6-S2-C38 103.17(15)
O5-S2-O4 115.36(13) O1A-S1A-O2A 120.4(9)
O5-S2-C38 102.64(14) O2A-S1A-O3A 115.5(9)
O4-S2-C38 102.94(15) O2A-S1A-C37A 102.7(12)
O1A-S1A-O3A 121.0(9)
O1A-S1A-C37A 89.3(10)
O3A-S1A-C37A 96.0(10)
294
Crystal Structure of 4.3b
A clear orange plate-like specimen of C
33
H
57
F
3
Ir
2
N
2
O
7
P
2
S, approximate dimensions 0.041
mm x 0.087 mm x 0.137 mm, was used for the X-ray crystallographic analysis. The X-ray
intensity data were measured on a Bruker APEX DUO system equipped with a TRIUMPH
curved-crystal monochromator and a MoKα fine-focus tube (λ = 0.71073 Å).
Table A.5. Data collection details.
Axis dx/mm 2θ/° ω/° φ/° χ/°
Width
/°
Frame
s
Time/
s
Wavelengt
h/Å
Voltag
e/kV
Curre
nt/mA
Temp
eratur
e/K
Omega 50.456 26.83 192.83 353.34 54.75 0.50 416 45.00 0.71073 50 30.0 100
Omega 50.456 26.83 192.83 235.33 54.75 0.50 416 45.00 0.71073 50 30.0 100
Omega 50.456 26.83 192.83 87.24 54.75 0.50 243 45.00 0.71073 50 30.0 100
Omega 50.456 26.83 192.83 88.82 54.75 0.50 164 45.00 0.71073 50 30.0 100
295
A total of 1239 frames were collected. The total exposure time was 15.49 hours. The frames
were integrated with the Bruker SAINT software package using a narrow-frame algorithm. The
integration of the data using a monoclinic unit cell yielded a total of 26491 reflections to a
maximum θ angle of 28.34° (0.75 Å resolution), of which 5228 were independent (average
redundancy 5.067, completeness = 99.5%, R
int
= 9.96%, R
sig
= 9.85%) and 3567 (68.23%) were
greater than 2σ(F
2
). The final cell constants of a = 8.5078(12) Å , b = 15.172(2) Å , c = 16.298(2)
Å, β = 92.454(2)°, volume = 2101.8(5) Å
3
, are based upon the refinement of the XYZ-centroids
of 9649 reflections above 20 σ(I) with 5.003° < 2θ < 56.55°. Data were corrected for absorption
effects using the multi-scan method (SADABS). The ratio of minimum to maximum apparent
transmission was 0.756. The calculated minimum and maximum transmission coefficients (based
on crystal size) are 0.4690 and 0.7760.
The structure was solved and refined using the Bruker SHELXTL Software Package, using
the space group P 1 2/n 1, with Z = 2 for the formula unit, C
33
H
57
F
3
Ir
2
N
2
O
7
P
2
S. The final
anisotropic full-matrix least-squares refinement on F
2
with 205 variables converged at R1 = 5.79%,
for the observed data and wR2 = 16.54% for all data. The goodness-of-fit was 1.037. The largest
peak in the final difference electron density synthesis was 3.425 e
-
/Å
3
and the largest hole was -
2.480 e
-
/Å
3
with an RMS deviation of 0.304 e
-
/Å
3
. On the basis of the final model, the calculated
density was 1.784 g/cm
3
and F(000), 1104 e
-
.
296
Table A.6. Sample and crystal data.
Chemical formula C
33
H
57
F
3
Ir
2
N
2
O
7
P
2
S
Formula weight 1129.20 g/mol
Temperature 100(2) K
Wavelength 0.71073 Å
Crystal size 0.041 x 0.087 x 0.137 mm
Crystal habit clear orange plate
Crystal system monoclinic
Space group P 1 2/n 1
Unit cell dimensions a = 8.5078(12) Å α = 90°
b = 15.172(2) Å β = 92.454(2)°
c = 16.298(2) Å γ = 90°
Volume 2101.8(5) Å
3
Z 2
Density (calculated) 1.784 g/cm
3
Absorption coefficient 6.507 mm
-1
F(000) 1104
297
Table A.7. Data collection and structure refinement.
Diffractometer Bruker APEX DUO
Radiation source fine-focus tube, MoKα
Theta range for data collection 1.84 to 28.34°
Index ranges -11 ≤ h ≤ 11, -20 ≤ k ≤ 20, -21 ≤ l ≤ 21
Reflections collected 26491
Independent reflections 5228 [R(int) = 0.0996]
Coverage of independent reflections 99.5%
Absorption correction multi-scan
Max. and min. transmission 0.7760 and 0.4690
Structure solution technique direct methods
Structure solution program SHELXTL XT 2014/4 (Bruker AXS, 2014)
Refinement method Full-matrix least-squares on F
2
Refinement program SHELXL-2014/6 (Sheldrick, 2014)
Function minimized Σ w(F
o
2
- F
c
2
)
2
Data / restraints / parameters 5228 / 0 / 205
Goodness-of-fit on F
2
1.037
Δ/σ
max
0.001
Final R indices 3567 data; I>2σ(I) R1 = 0.0579, wR2 = 0.1496
all data R1 = 0.0980, wR2 = 0.1654
Weighting scheme
w=1/[σ
2
(F
o
2
)+(0.0903P)
2
+6.1533P]
where P=(F
o
2
+2F
c
2
)/3
Largest diff. peak and hole 3.425 and -2.480 eÅ
-3
R.M.S. deviation from mean 0.304 eÅ
-3
298
Table A.8. Bond lengths (Å )
C1-N1 1.364(12) C1-C2 1.378(15)
C1-H1 0.95 C2-C3 1.381(17)
C2-H2 0.95 C3-C4 1.389(16)
C3-H3 0.95 C4-C5 1.383(15)
C4-H4 0.95 C5-N1 1.392(13)
C5-C6 1.504(14) C6-P1 1.850(12)
C6-H6A 0.99 C6-H6B 0.99
C7-C9 1.484(16) C7-C8 1.522(15)
C7-C10 1.550(15) C7-P1 1.891(11)
C8-H8A 0.98 C8-H8B 0.98
C8-H8C 0.98 C9-H9A 0.98
C9-H9B 0.98 C9-H9C 0.98
C10-H10A 0.98 C10-H10B 0.98
C10-H10C 0.98 C11-C14 1.501(16)
C11-C13 1.540(18) C11-C12 1.565(16)
C11-P1 1.905(10) C12-H12A 0.98
C12-H12B 0.98 C12-H12C 0.98
C13-H13A 0.98 C13-H13B 0.98
C13-H13C 0.98 C14-H14A 0.98
C14-H14B 0.98 C14-H14C 0.98
C15-O2 1.241(12) C15-O1 1.262(13)
299
C15-C16 1.505(15) C16-H16A 0.98
C16-H16B 0.98 C16-H16C 0.98
Ir1-N1 2.070(9) Ir1-O1 2.151(7)
Ir1-O2 2.185(7) Ir1-P1 2.253(3)
Ir1-Ir1 2.8428(8) Ir1-H17 1.5398
Ir1-H18 1.4214 O2-Ir1 2.185(7)
S1-O5 1.3726 S1-O3 1.4676
S1-O4 1.4936 S1-C17 1.8596
C17-F3 1.3384 C17-F2 1.3385
C17-F1 1.3385
Table A.9. Bond angles (° ).
N1-C1-C2 123.5(10) N1-C1-H1 118.2
C2-C1-H1 118.2 C1-C2-C3 118.9(10)
C1-C2-H2 120.6 C3-C2-H2 120.6
C2-C3-C4 120.0(12) C2-C3-H3 120.0
C4-C3-H3 120.0 C5-C4-C3 118.6(11)
C5-C4-H4 120.7 C3-C4-H4 120.7
C4-C5-N1 122.6(9) C4-C5-C6 122.2(10)
N1-C5-C6 115.1(9) C5-C6-P1 110.6(7)
C5-C6-H6A 109.5 P1-C6-H6A 109.5
C5-C6-H6B 109.5 P1-C6-H6B 109.5
300
H6A-C6-H6B 108.1 C9-C7-C8 109.1(9)
C9-C7-C10 110.3(10) C8-C7-C10 106.6(10)
C9-C7-P1 110.1(9) C8-C7-P1 107.7(8)
C10-C7-P1 112.9(7) C7-C8-H8A 109.5
C7-C8-H8B 109.5 H8A-C8-H8B 109.5
C7-C8-H8C 109.5 H8A-C8-H8C 109.5
H8B-C8-H8C 109.5 C7-C9-H9A 109.5
C7-C9-H9B 109.5 H9A-C9-H9B 109.5
C7-C9-H9C 109.5 H9A-C9-H9C 109.5
H9B-C9-H9C 109.5 C7-C10-H10A 109.5
C7-C10-H10B 109.5 H10A-C10-H10B 109.5
C7-C10-H10C 109.5 H10A-C10-H10C 109.5
H10B-C10-H10C 109.5 C14-C11-C13 108.7(10)
C14-C11-C12 107.6(9) C13-C11-C12 111.2(10)
C14-C11-P1 109.2(8) C13-C11-P1 110.8(7)
C12-C11-P1 109.3(8) C11-C12-H12A 109.5
C11-C12-H12B 109.5 H12A-C12-H12B 109.5
C11-C12-H12C 109.5 H12A-C12-H12C 109.5
H12B-C12-H12C 109.5 C11-C13-H13A 109.5
C11-C13-H13B 109.5 H13A-C13-H13B 109.5
C11-C13-H13C 109.5 H13A-C13-H13C 109.5
H13B-C13-H13C 109.5 C11-C14-H14A 109.5
C11-C14-H14B 109.5 H14A-C14-H14B 109.5
C11-C14-H14C 109.5 H14A-C14-H14C 109.5
301
H14B-C14-H14C 109.5 O2-C15-O1 126.0(10)
O2-C15-C16 117.9(10) O1-C15-C16 116.1(9)
C15-C16-H16A 109.5 C15-C16-H16B 109.5
H16A-C16-H16B 109.5 C15-C16-H16C 109.5
H16A-C16-H16C 109.5 H16B-C16-H16C 109.5
N1-Ir1-O1 85.0(3) N1-Ir1-O2 84.7(3)
O1-Ir1-O2 82.4(3) N1-Ir1-P1 83.2(2)
O1-Ir1-P1 168.0(2) O2-Ir1-P1 94.0(2)
N1-Ir1-Ir1 162.1(2) O1-Ir1-Ir1 79.95(19)
O2-Ir1-Ir1 83.7(2) P1-Ir1-Ir1 111.14(7)
N1-Ir1-H17 107.0 O1-Ir1-H17 99.1
O2-Ir1-H17 168.3 P1-Ir1-H17 86.8
Ir1-Ir1-H17 85.1 N1-Ir1-H18 162.1
O1-Ir1-H18 79.9 O2-Ir1-H18 83.7
P1-Ir1-H18 111.1 Ir1-Ir1-H18 0
H17-Ir1-H18 85.1 C1-N1-C5 116.3(9)
C1-N1-Ir1 122.8(7) C5-N1-Ir1 120.7(7)
C15-O1-Ir1 127.9(6) C15-O2-Ir1 122.1(7)
C6-P1-C7 105.6(6) C6-P1-C11 102.9(5)
C7-P1-C11 110.4(5) C6-P1-Ir1 98.2(4)
C7-P1-Ir1 115.0(3) C11-P1-Ir1 121.6(4)
O5-S1-O3 118.8 O5-S1-O4 117.0
O3-S1-O4 111.3 O5-S1-C17 105.0
O3-S1-C17 101.1 O4-S1-C17 100.1
302
F3-C17-F2 107.0 F3-C17-F1 107.0
F2-C17-F1 107.0 F3-C17-S1 114.1
F2-C17-S1 110.3 F1-C17-S1 111.2
303
Crystal Structure of 5.5
A specimen of C
27
H
31
F
3
IrN
3
O
3
S, approximate dimensions 0.186 mm x 0.208 mm x 0.264
mm, was used for the X-ray crystallographic analysis. The X-ray intensity data were measured on
a Bruker APEX DUO system equipped with a TRIUMPH curved-crystal monochromator and a
MoKα fine-focus tube (λ = 0.71073 Å).
The total exposure time was 7.00 hours. The frames were integrated with the Bruker
304
SAINT software package using a SAINT V8.34A (Bruker AXS, 2013) algorithm. The integration
of the data using a monoclinic unit cell yielded a total of 144457 reflections to a maximum θ angle
of 30.66° (0.70 Å resolution), of which 17712 were independent (average redundancy 8.156,
completeness = 99.2%, R
int
= 4.31%, R
sig
= 2.76%) and 14863 (83.91%) were greater than 2σ(F
2
).
The final cell constants of a = 11.0313(4) Å , b = 26.4631(9) Å , c = 19.9865(7) Å, β =
98.4150(10)° , volume = 5771.7(4) Å
3
, are based upon the refinement of the XYZ-centroids of
9726 reflections above 20 σ(I) with 4.398° < 2θ < 60.74°. Data were corrected for absorption
effects using the multi-scan method (SADABS). The ratio of minimum to maximum apparent
transmission was 0.795. The calculated minimum and maximum transmission coefficients (based
on crystal size) are 0.3670 and 0.4720.
The structure was solved and refined using the Bruker SHELXTL Software Package, using
the space group P 1 21/c 1, with Z = 8 for the formula unit, C
27
H
31
F
3
IrN
3
O
3
S. The final anisotropic
full-matrix least-squares refinement on F
2
with 638 variables converged at R1 = 4.70%, for the
observed data and wR2 = 12.43% for all data. The goodness-of-fit was 1.028. The largest peak in
the final difference electron density synthesis was 7.169 e
-
/Å
3
and the largest hole was -3.738 e
-
/Å
3
with an RMS deviation of 0.198 e
-
/Å
3
. On the basis of the final model, the calculated density
was 1.673 g/cm
3
and F(000), 2864 e
-
.
Table A.10. Sample and crystal data.
Chemical formula C
27
H
31
F
3
IrN
3
O
3
S
305
Formula weight 726.81 g/mol
Temperature 100(2) K
Wavelength 0.71073 Å
Crystal size 0.186 x 0.208 x 0.264 mm
Crystal system monoclinic
Space group P 1 21/c 1
Unit cell dimensions a = 11.0313(4) Å α = 90°
b = 26.4631(9) Å β = 98.4150(10)°
c = 19.9865(7) Å γ = 90°
Volume 5771.7(4) Å
3
Z 8
Density (calculated) 1.673 g/cm
3
Absorption coefficient 4.750 mm
-1
F(000) 2864
Table A.11. Data collection and structure refinement.
Diffractometer Bruker APEX DUO
Radiation source fine-focus tube, MoKα
Theta range for data collection 1.29 to 30.66°
Index ranges -15 ≤ h ≤ 15, -37 ≤ k ≤ 37, -28 ≤ l ≤ 28
Reflections collected 144457
Independent reflections 17712 [R(int) = 0.0431]
Coverage of independent reflections 99.2%
Absorption correction multi-scan
Max. and min. transmission 0.4720 and 0.3670
306
Structure solution technique direct methods
Structure solution program SHELXTL XT 2013/1 (Bruker AXS, 2014)
Refinement method Full-matrix least-squares on F
2
Refinement program SHELXTL XL 2014/7 (Bruker AXS, 2014)
Function minimized Σ w(F
o
2
- F
c
2
)
2
Data / restraints / parameters 17712 / 64 / 638
Goodness-of-fit on F
2
1.028
Δ/σ
max
0.001
Final R indices 14863 data; I>2σ(I) R1 = 0.0470, wR2 = 0.1179
all data R1 = 0.0591, wR2 = 0.1243
Weighting scheme
w=1/[σ
2
(F
o
2
)+(0.0565P)
2
+53.1775P]
where P=(F
o
2
+2F
c
2
)/3
Largest diff. peak and hole 7.169 and -3.738 eÅ
-3
R.M.S. deviation from mean 0.198 eÅ
-3
Table A.12. Bond lengths (Å ).
C1-N1 1.351(6) C1-C2 1.385(7)
C1-H1 0.95 C2-C3 1.383(8)
C2-H2 0.95 C3-C4 1.392(8)
307
C3-H3 0.95 C4-C5 1.385(7)
C4-H4 0.95 C5-N1 1.357(7)
C5-C6 1.511(7) C6-N2 1.467(7)
C6-H6A 0.99 C6-H6B 0.99
C7-N3 1.362(6) C7-N2 1.364(7)
C7-Ir1 2.029(5) C8-C9 1.344(8)
C8-N2 1.386(7) C8-H8 0.95
C9-N3 1.398(7) C9-H9 0.95
C10-C11 1.401(7) C10-C15 1.405(7)
C10-N3 1.442(7) C11-C12 1.389(8)
C11-C16 1.503(8) C12-C13 1.389(8)
C12-H12 0.95 C13-C14 1.390(8)
C13-C17 1.511(8) C14-C15 1.389(7)
C14-H14 0.95 C15-C18 1.510(7)
C16-H16A 0.98 C16-H16B 0.98
C16-H16C 0.98 C17-H17A 0.98
C17-H17B 0.98 C17-H17C 0.98
C18-H18A 0.98 C18-H18B 0.98
C18-H18C 0.98 C19-C20 1.392(7)
C19-C26 1.516(7) C19-Ir1 2.156(5)
C19-H19 1.0 C20-C21 1.527(8)
C20-Ir1 2.197(5) C20-H20 1.0
C21-C22 1.535(8) C21-H21A 0.99
C21-H21B 0.99 C22-C23 1.517(8)
308
C22-H22A 0.99 C22-H22B 0.99
C23-C24 1.413(8) C23-Ir1 2.129(5)
C23-H23 1.0 C24-C25 1.526(8)
C24-Ir1 2.140(5) C24-H24 1.0
C25-C26 1.534(8) C25-H25A 0.99
C25-H25B 0.99 C26-H26A 0.99
C26-H26B 0.99 Ir1-N1 2.111(4)
C27-N4 1.357(7) C27-C28 1.382(8)
C27-H27 0.95 C28-C29 1.377(8)
C28-H28 0.95 C29-C30 1.392(8)
C29-H29 0.95 C30-C31 1.372(8)
C30-H30 0.95 C31-N4 1.361(7)
C31-C32 1.516(7) C32-N5 1.472(6)
C32-H32A 0.99 C32-H32B 0.99
C33-N5 1.346(6) C33-N6 1.360(6)
C33-Ir2 2.046(5) C34-C35 1.352(7)
C34-N5 1.383(6) C34-H34 0.95
C35-N6 1.396(6) C35-H35 0.95
C36-C41 1.391(7) C36-C37 1.408(7)
C36-N6 1.438(6) C37-C38 1.393(8)
C37-C42 1.520(8) C38-C39 1.387(8)
C38-H38 0.95 C39-C40 1.389(8)
C39-C43 1.511(8) C40-C41 1.389(7)
C40-H40 0.95 C41-C44 1.509(7)
309
C42-H42A 0.98 C42-H42B 0.98
C42-H42C 0.98 C43-H43A 0.98
C43-H43B 0.98 C43-H43C 0.98
C44-H44A 0.98 C44-H44B 0.98
C44-H44C 0.98 C45-C46 1.383(9)
C45-C52 1.525(9) C45-Ir2 2.154(5)
C45-H45 1.0 C46-C47 1.537(9)
C46-Ir2 2.187(5) C46-H46 1.0
C47-C48 1.517(10) C47-H47A 0.99
C47-H47B 0.99 C48-C49 1.502(9)
C48-H48A 0.99 C48-H48B 0.99
C49-C50 1.406(8) C49-Ir2 2.138(6)
C49-H49 1.0 C50-C51 1.526(8)
C50-Ir2 2.141(6) C50-H50 1.0
C51-C52 1.514(9) C51-H51A 0.99
C51-H51B 0.99 C52-H52A 0.99
C52-H52B 0.99 Ir2-N4 2.110(5)
C53-F3 1.324(9) C53-F1 1.325(9)
C53-F2 1.334(9) C53-S1 1.802(8)
O1-S1 1.452(7) O2-S1 1.434(7)
O3-S1 1.468(8) C54-F4 1.304(11)
C54-F6 1.351(12) C54-F5 1.369(12)
C54-S2 1.804(10) O4-S2 1.459(8)
O5-S2 1.471(10) O6-S2 1.434(9)
310
C54A-F4A 1.326(12) C54A-F5A 1.333(12)
C54A-F6A 1.384(13) C54A-S2A 1.788(11)
O4A-S2A 1.450(9) O5A-S2A 1.436(10)
O6A-S2A 1.435(10)
Table A.13. Bond angles (° ).
N1-C1-C2 122.6(5) N1-C1-H1 118.7
C2-C1-H1 118.7 C3-C2-C1 118.9(5)
C3-C2-H2 120.5 C1-C2-H2 120.5
C2-C3-C4 118.9(5) C2-C3-H3 120.5
C4-C3-H3 120.5 C5-C4-C3 119.5(5)
C5-C4-H4 120.2 C3-C4-H4 120.2
N1-C5-C4 121.5(5) N1-C5-C6 117.0(4)
C4-C5-C6 121.4(5) N2-C6-C5 109.0(4)
N2-C6-H6A 109.9 C5-C6-H6A 109.9
N2-C6-H6B 109.9 C5-C6-H6B 109.9
H6A-C6-H6B 108.3 N3-C7-N2 103.8(4)
N3-C7-Ir1 138.3(4) N2-C7-Ir1 117.8(4)
C9-C8-N2 106.4(5) C9-C8-H8 126.8
N2-C8-H8 126.8 C8-C9-N3 107.0(5)
C8-C9-H9 126.5 N3-C9-H9 126.5
C11-C10-C15 122.0(5) C11-C10-N3 119.4(5)
311
C15-C10-N3 118.6(4) C12-C11-C10 117.8(5)
C12-C11-C16 120.5(5) C10-C11-C16 121.7(5)
C11-C12-C13 121.9(5) C11-C12-H12 119.1
C13-C12-H12 119.1 C12-C13-C14 118.9(5)
C12-C13-C17 121.5(5) C14-C13-C17 119.6(5)
C15-C14-C13 121.6(5) C15-C14-H14 119.2
C13-C14-H14 119.2 C14-C15-C10 117.9(5)
C14-C15-C18 120.5(5) C10-C15-C18 121.6(5)
C11-C16-H16A 109.5 C11-C16-H16B 109.5
H16A-C16-H16B 109.5 C11-C16-H16C 109.5
H16A-C16-H16C 109.5 H16B-C16-H16C 109.5
C13-C17-H17A 109.5 C13-C17-H17B 109.5
H17A-C17-H17B 109.5 C13-C17-H17C 109.5
H17A-C17-H17C 109.5 H17B-C17-H17C 109.5
C15-C18-H18A 109.5 C15-C18-H18B 109.5
H18A-C18-H18B 109.5 C15-C18-H18C 109.5
H18A-C18-H18C 109.5 H18B-C18-H18C 109.5
C20-C19-C26 125.5(5) C20-C19-Ir1 73.0(3)
C26-C19-Ir1 109.9(3) C20-C19-H19 113.7
C26-C19-H19 113.7 Ir1-C19-H19 113.7
C19-C20-C21 123.6(5) C19-C20-Ir1 69.7(3)
C21-C20-Ir1 112.5(4) C19-C20-H20 114.4
C21-C20-H20 114.4 Ir1-C20-H20 114.4
C20-C21-C22 111.9(5) C20-C21-H21A 109.2
312
C22-C21-H21A 109.2 C20-C21-H21B 109.2
C22-C21-H21B 109.2 H21A-C21-H21B 107.9
C23-C22-C21 113.2(5) C23-C22-H22A 108.9
C21-C22-H22A 108.9 C23-C22-H22B 108.9
C21-C22-H22B 108.9 H22A-C22-H22B 107.8
C24-C23-C22 125.2(5) C24-C23-Ir1 71.1(3)
C22-C23-Ir1 111.1(4) C24-C23-H23 113.9
C22-C23-H23 113.9 Ir1-C23-H23 113.9
C23-C24-C25 123.3(5) C23-C24-Ir1 70.3(3)
C25-C24-Ir1 113.1(4) C23-C24-H24 114.2
C25-C24-H24 114.2 Ir1-C24-H24 114.2
C24-C25-C26 112.6(5) C24-C25-H25A 109.1
C26-C25-H25A 109.1 C24-C25-H25B 109.1
C26-C25-H25B 109.1 H25A-C25-H25B 107.8
C19-C26-C25 112.3(5) C19-C26-H26A 109.1
C25-C26-H26A 109.1 C19-C26-H26B 109.1
C25-C26-H26B 109.1 H26A-C26-H26B 107.9
C7-Ir1-N1 84.27(19) C7-Ir1-C23 97.3(2)
N1-Ir1-C23 160.0(2) C7-Ir1-C24 93.7(2)
N1-Ir1-C24 161.4(2) C23-Ir1-C24 38.7(2)
C7-Ir1-C19 149.51(19) N1-Ir1-C19 90.83(18)
C23-Ir1-C19 97.4(2) C24-Ir1-C19 81.5(2)
C7-Ir1-C20 173.01(19) N1-Ir1-C20 94.94(18)
C23-Ir1-C20 81.0(2) C24-Ir1-C20 89.3(2)
313
C19-Ir1-C20 37.30(19) C1-N1-C5 118.4(5)
C1-N1-Ir1 121.9(4) C5-N1-Ir1 119.6(3)
C7-N2-C8 111.9(4) C7-N2-C6 121.5(4)
C8-N2-C6 126.5(5) C7-N3-C9 110.9(4)
C7-N3-C10 125.7(4) C9-N3-C10 122.8(4)
N4-C27-C28 122.5(5) N4-C27-H27 118.8
C28-C27-H27 118.8 C29-C28-C27 119.1(5)
C29-C28-H28 120.4 C27-C28-H28 120.4
C28-C29-C30 119.2(5) C28-C29-H29 120.4
C30-C29-H29 120.4 C31-C30-C29 119.1(5)
C31-C30-H30 120.4 C29-C30-H30 120.4
N4-C31-C30 122.4(5) N4-C31-C32 115.7(5)
C30-C31-C32 121.8(5) N5-C32-C31 108.7(4)
N5-C32-H32A 110.0 C31-C32-H32A 110.0
N5-C32-H32B 110.0 C31-C32-H32B 110.0
H32A-C32-H32B 108.3 N5-C33-N6 104.5(4)
N5-C33-Ir2 118.0(3) N6-C33-Ir2 137.4(4)
C35-C34-N5 106.1(4) C35-C34-H34 126.9
N5-C34-H34 126.9 C34-C35-N6 106.8(4)
C34-C35-H35 126.6 N6-C35-H35 126.6
C41-C36-C37 122.4(5) C41-C36-N6 119.4(4)
C37-C36-N6 118.3(4) C38-C37-C36 116.9(5)
C38-C37-C42 121.5(5) C36-C37-C42 121.6(5)
C39-C38-C37 122.4(5) C39-C38-H38 118.8
314
C37-C38-H38 118.8 C38-C39-C40 118.5(5)
C38-C39-C43 120.9(5) C40-C39-C43 120.6(5)
C41-C40-C39 121.7(5) C41-C40-H40 119.1
C39-C40-H40 119.1 C40-C41-C36 118.1(5)
C40-C41-C44 120.3(5) C36-C41-C44 121.6(5)
C37-C42-H42A 109.5 C37-C42-H42B 109.5
H42A-C42-H42B 109.5 C37-C42-H42C 109.5
H42A-C42-H42C 109.5 H42B-C42-H42C 109.5
C39-C43-H43A 109.5 C39-C43-H43B 109.5
H43A-C43-H43B 109.5 C39-C43-H43C 109.5
H43A-C43-H43C 109.5 H43B-C43-H43C 109.5
C41-C44-H44A 109.5 C41-C44-H44B 109.5
H44A-C44-H44B 109.5 C41-C44-H44C 109.5
H44A-C44-H44C 109.5 H44B-C44-H44C 109.5
C46-C45-C52 125.9(6) C46-C45-Ir2 72.7(3)
C52-C45-Ir2 109.2(4) C46-C45-H45 113.8
C52-C45-H45 113.8 Ir2-C45-H45 113.8
C45-C46-C47 123.3(6) C45-C46-Ir2 70.1(3)
C47-C46-Ir2 112.2(4) C45-C46-H46 114.4
C47-C46-H46 114.4 Ir2-C46-H46 114.4
C48-C47-C46 112.6(5) C48-C47-H47A 109.1
C46-C47-H47A 109.1 C48-C47-H47B 109.1
C46-C47-H47B 109.1 H47A-C47-H47B 107.8
C49-C48-C47 114.7(5) C49-C48-H48A 108.6
315
C47-C48-H48A 108.6 C49-C48-H48B 108.6
C47-C48-H48B 108.6 H48A-C48-H48B 107.6
C50-C49-C48 125.1(5) C50-C49-Ir2 70.9(3)
C48-C49-Ir2 111.1(4) C50-C49-H49 113.9
C48-C49-H49 113.9 Ir2-C49-H49 113.9
C49-C50-C51 122.3(5) C49-C50-Ir2 70.7(3)
C51-C50-Ir2 113.9(4) C49-C50-H50 114.2
C51-C50-H50 114.2 Ir2-C50-H50 114.2
C52-C51-C50 111.6(5) C52-C51-H51A 109.3
C50-C51-H51A 109.3 C52-C51-H51B 109.3
C50-C51-H51B 109.3 H51A-C51-H51B 108.0
C51-C52-C45 113.9(5) C51-C52-H52A 108.8
C45-C52-H52A 108.8 C51-C52-H52B 108.8
C45-C52-H52B 108.8 H52A-C52-H52B 107.7
C33-Ir2-N4 83.96(18) C33-Ir2-C49 98.1(2)
N4-Ir2-C49 159.9(2) C33-Ir2-C50 93.7(2)
N4-Ir2-C50 161.7(2) C49-Ir2-C50 38.4(2)
C33-Ir2-C45 149.1(2) N4-Ir2-C45 91.38(19)
C49-Ir2-C45 96.6(2) C50-Ir2-C45 81.4(2)
C33-Ir2-C46 173.4(2) N4-Ir2-C46 94.4(2)
C49-Ir2-C46 81.2(2) C50-Ir2-C46 89.9(2)
C45-Ir2-C46 37.1(2) C27-N4-C31 117.7(5)
C27-N4-Ir2 122.0(4) C31-N4-Ir2 120.2(4)
C33-N5-C34 112.1(4) C33-N5-C32 121.0(4)
316
C34-N5-C32 126.6(4) C33-N6-C35 110.6(4)
C33-N6-C36 125.5(4) C35-N6-C36 123.8(4)
F3-C53-F1 105.6(7) F3-C53-F2 108.6(8)
F1-C53-F2 104.4(7) F3-C53-S1 112.6(6)
F1-C53-S1 113.6(6) F2-C53-S1 111.6(6)
O2-S1-O1 115.5(5) O2-S1-O3 111.7(6)
O1-S1-O3 113.4(5) O2-S1-C53 105.6(5)
O1-S1-C53 107.2(4) O3-S1-C53 102.0(5)
F4-C54-F6 105.7(9) F4-C54-F5 108.9(9)
F6-C54-F5 105.1(9) F4-C54-S2 112.4(7)
F6-C54-S2 110.5(8) F5-C54-S2 113.6(8)
O6-S2-O4 114.6(5) O6-S2-O5 117.4(6)
O4-S2-O5 112.0(6) O6-S2-C54 103.3(5)
O4-S2-C54 105.2(5) O5-S2-C54 102.3(6)
F4A-C54A-F5A 113.6(10) F4A-C54A-F6A 97.6(9)
F5A-C54A-F6A 106.0(10) F4A-C54A-S2A 116.6(9)
F5A-C54A-S2A 112.5(8) F6A-C54A-S2A 108.7(8)
O6A-S2A-O5A 114.6(7) O6A-S2A-O4A 115.6(7)
O5A-S2A-O4A 112.6(7) O6A-S2A-C54A 100.8(6)
O5A-S2A-C54A 108.1(7) O4A-S2A-C54A 103.3(6)
317
Crystal structure of 5.9
A clear orange-red plate-like specimen of C
19
H
23
F
3
IrN
3
O
3
S, approximate dimensions
0.070 mm x 0.228 mm x 0.422 mm, was used for the X-ray crystallographic analysis. The X-ray
intensity data were measured on a Bruker APEX DUO system equipped with a TRIUMPH
curved-crystal monochromator and a MoKα fine-focus tube (λ = 0.71073 Å).
318
Table A.14. Data collection details.
Axis dx/mm 2θ/° ω/° φ/° χ/°
Width
/°
Frames Time/s
Wavel
ength
/Å
Voltage/kV
Current/
mA
Omega 50.390 30.00 30.00 0.00 54.73 0.50 360 10.00 0.71073 50 30.0
Omega 50.390 30.00 30.00 72.00 54.73 0.50 360 10.00 0.71073 50 30.0
Omega 50.390 30.00 30.00 144.00 54.73 0.50 360 10.00 0.71073 50 30.0
Omega 50.390 30.00 30.00 216.00 54.73 0.50 360 10.00 0.71073 50 30.0
Omega 50.390 30.00 30.00 288.00 54.73 0.50 360 10.00 0.71073 50 30.0
Phi 50.390 30.00 0.00 0.00 54.73 0.50 720 10.00 0.71073 50 30.0
A total of 2520 frames were collected. The total exposure time was 7.00 hours. The frames
were integrated with the Bruker SAINT software package using a SAINT V8.34A (Bruker AXS,
2013) algorithm. The integration of the data using a monoclinic unit cell yielded a total of 34687
reflections to a maximum θ angle of 30.57° (0.70 Å resolution), of which 6447 were independent
(average redundancy 5.380, completeness = 99.1%, R
int
= 4.96%, R
sig
= 3.88%) and 5285 (81.98%)
were greater than 2σ(F
2
). The final cell constants of a = 14.9292(12) Å , b = 12.2212(10) Å , c =
12.2965(10) Å, β = 109.5060(10)°, volume = 2114.8(3) Å
3
, are based upon the refinement of the
XYZ-centroids of 9381 reflections above 20 σ(I) with 4.414° < 2θ < 61.11°. Data were corrected
for absorption effects using the multi-scan method (SADABS). The ratio of minimum to
maximum apparent transmission was 0.782. The calculated minimum and maximum
transmission coefficients (based on crystal size) are 0.1710 and 0.6600.
The structure was solved and refined using the Bruker SHELXTL Software Package, using
319
the space group P 1 21/c 1, with Z = 4 for the formula unit, C
19
H
23
F
3
IrN
3
O
3
S. The final anisotropic
full-matrix least-squares refinement on F
2
with 297 variables converged at R1 = 2.41%, for the
observed data and wR2 = 5.27% for all data. The goodness-of-fit was 1.014. The largest peak in
the final difference electron density synthesis was 1.375 e
-
/Å
3
and the largest hole was -1.018 e
-
/Å
3
with an RMS deviation of 0.138 e
-
/Å
3
. On the basis of the final model, the calculated density
was 1.956 g/cm
3
and F(000), 1208 e
-
.
Table A.15. Sample and crystal data.
Chemical formula C
19
H
23
F
3
IrN
3
O
3
S
Formula weight 622.66 g/mol
Temperature 100(2) K
Wavelength 0.71073 Å
Crystal size 0.070 x 0.228 x 0.422 mm
Crystal habit clear orange-red plate
Crystal system monoclinic
Space group P 1 21/c 1
Unit cell dimensions a = 14.9292(12) Å α = 90°
b = 12.2212(10) Å β = 109.5060(10)°
c = 12.2965(10) Å γ = 90°
Volume 2114.8(3) Å
3
Z 4
Density (calculated) 1.956 g/cm
3
Absorption coefficient 6.464 mm
-1
320
F(000) 1208
Table A.16. Data collection and structure refinement.
Diffractometer Bruker APEX DUO
Radiation source fine-focus tube, MoKα
Theta range for data collection 2.21 to 30.57°
Index ranges -21 ≤ h ≤ 21, -17 ≤ k ≤ 17, -17 ≤ l ≤ 17
Reflections collected 34687
Independent reflections 6447 [R(int) = 0.0496]
Coverage of independent
reflections
99.1%
Absorption correction multi-scan
Max. and min. transmission 0.6600 and 0.1710
Structure solution technique direct methods
Structure solution program SHELXTL XT 2014/4 (Bruker AXS, 2014)
Refinement method Full-matrix least-squares on F
2
Refinement program SHELXTL XL 2014/7 (Bruker AXS, 2014)
Function minimized Σ w(F
o
2
- F
c
2
)
2
Data / restraints / parameters 6447 / 19 / 297
Goodness-of-fit on F
2
1.014
Δ/σ
max
0.002
Final R indices 5285 data; I>2σ(I) R1 = 0.0241, wR2 = 0.0492
321
all data R1 = 0.0359, wR2 = 0.0527
Weighting scheme
w=1/[σ
2
(F
o
2
)+(0.0201P)
2
+1.3199P]
where P=(F
o
2
+2F
c
2
)/3
Largest diff. peak and hole 1.375 and -1.018 eÅ
-3
R.M.S. deviation from mean 0.138 eÅ
-3
Table A.17. Bond lengths (Å ).
C1-C2 1.405(5) C1-C8 1.515(4)
C1-Ir1 2.136(3) C1-H1 1.0
C2-C3 1.517(5) C2-Ir1 2.134(3)
C2-H2 1.0 C3-C4 1.528(5)
C3-H3A 0.99 C3-H3B 0.99
C4-C5 1.524(5) C4-H4A 0.99
C4-H4B 0.99 C5-C6 1.391(4)
C5-Ir1 2.206(3) C5-H5 1.0
C6-C7 1.505(5) C6-Ir1 2.167(3)
C6-H6 1.0 C7-C8 1.535(5)
C7-H7A 0.99 C7-H7B 0.99
C8-H8A 0.99 C8-H8B 0.99
C9-N1 1.353(3) C9-C10 1.380(4)
C9-H9 0.95 C10-C11 1.380(4)
C10-H10 0.95 C11-C12 1.393(4)
322
C11-H11 0.95 C12-C13 1.379(4)
C12-H12 0.95 C13-N1 1.357(3)
C13-C14 1.514(4) C14-N2 1.461(3)
C14-H14A 0.99 C14-H14B 0.99
C15-C16 1.349(4) C15-N2 1.390(3)
C15-H15 0.95 C16-N3 1.394(4)
C16-H16 0.95 C17-N3 1.360(3)
C17-N2 1.369(4) C17-Ir1 2.036(3)
C18-N3 1.462(4) C18-H18A 0.98
C18-H18B 0.98 C18-H18C 0.98
C19-F1 1.319(5) C19-F2 1.339(5)
C19-F3 1.347(7) C19-S1 1.806(6)
O1-S1 1.448(4) O2-S1 1.449(4)
O3-S1 1.450(4) C19'-F1' 1.307(15)
C19'-F3' 1.326(16) C19'-F2' 1.352(16)
C19'-S1' 1.808(15) O1'-S1' 1.447(15)
O2'-S1' 1.446(15) O3'-S1' 1.430(15)
Ir1-N1 2.104(2)
Table A.18. Bond angles.
C2-C1-C8 123.8(3) C2-C1-Ir1 70.75(17)
C8-C1-Ir1 113.9(2) C2-C1-H1 113.7
323
C8-C1-H1 113.7 Ir1-C1-H1 113.7
C1-C2-C3 125.4(3) C1-C2-Ir1 70.84(17)
C3-C2-Ir1 111.0(2) C1-C2-H2 113.9
C3-C2-H2 113.9 Ir1-C2-H2 113.9
C2-C3-C4 113.3(3) C2-C3-H3A 108.9
C4-C3-H3A 108.9 C2-C3-H3B 108.9
C4-C3-H3B 108.9 H3A-C3-H3B 107.7
C5-C4-C3 111.7(3) C5-C4-H4A 109.3
C3-C4-H4A 109.3 C5-C4-H4B 109.3
C3-C4-H4B 109.3 H4A-C4-H4B 107.9
C6-C5-C4 124.0(3) C6-C5-Ir1 69.96(16)
C4-C5-Ir1 112.7(2) C6-C5-H5 114.1
C4-C5-H5 114.1 Ir1-C5-H5 114.1
C5-C6-C7 125.8(3) C5-C6-Ir1 72.96(17)
C7-C6-Ir1 109.2(2) C5-C6-H6 113.7
C7-C6-H6 113.7 Ir1-C6-H6 113.7
C6-C7-C8 112.8(3) C6-C7-H7A 109.0
C8-C7-H7A 109.0 C6-C7-H7B 109.0
C8-C7-H7B 109.0 H7A-C7-H7B 107.8
C1-C8-C7 111.8(3) C1-C8-H8A 109.3
C7-C8-H8A 109.3 C1-C8-H8B 109.3
C7-C8-H8B 109.3 H8A-C8-H8B 107.9
N1-C9-C10 122.7(3) N1-C9-H9 118.6
C10-C9-H9 118.6 C11-C10-C9 119.4(3)
324
C11-C10-H10 120.3 C9-C10-H10 120.3
C10-C11-C12 118.4(3) C10-C11-H11 120.8
C12-C11-H11 120.8 C13-C12-C11 119.6(3)
C13-C12-H12 120.2 C11-C12-H12 120.2
N1-C13-C12 122.1(2) N1-C13-C14 116.8(2)
C12-C13-C14 121.2(2) N2-C14-C13 109.4(2)
N2-C14-H14A 109.8 C13-C14-H14A 109.8
N2-C14-H14B 109.8 C13-C14-H14B 109.8
H14A-C14-H14B 108.2 C16-C15-N2 106.4(2)
C16-C15-H15 126.8 N2-C15-H15 126.8
C15-C16-N3 107.0(2) C15-C16-H16 126.5
N3-C16-H16 126.5 N3-C17-N2 103.8(2)
N3-C17-Ir1 138.5(2) N2-C17-Ir1 117.62(19)
N3-C18-H18A 109.5 N3-C18-H18B 109.5
H18A-C18-H18B 109.5 N3-C18-H18C 109.5
H18A-C18-H18C 109.5 H18B-C18-H18C 109.5
F1-C19-F2 105.5(4) F1-C19-F3 107.1(4)
F2-C19-F3 107.9(4) F1-C19-S1 113.6(4)
F2-C19-S1 111.8(4) F3-C19-S1 110.6(4)
O1-S1-O2 114.6(4) O1-S1-O3 115.7(3)
O2-S1-O3 113.3(3) O1-S1-C19 103.7(4)
O2-S1-C19 104.4(3) O3-S1-C19 103.0(3)
F1'-C19'-F3' 108.1(16) F1'-C19'-F2' 112.4(16)
F3'-C19'-F2' 106.0(14) F1'-C19'-S1' 114.4(13)
325
F3'-C19'-S1' 111.4(14) F2'-C19'-S1' 104.3(13)
O3'-S1'-O2' 115.1(16) O3'-S1'-O1' 119.3(18)
O2'-S1'-O1' 111.0(18) O3'-S1'-C19' 102.9(14)
O2'-S1'-C19' 101.6(13) O1'-S1'-C19' 104.2(15)
C17-Ir1-N1 84.33(10) C17-Ir1-C2 95.47(11)
N1-Ir1-C2 167.82(12) C17-Ir1-C1 95.68(11)
N1-Ir1-C1 153.77(11) C2-Ir1-C1 38.41(13)
C17-Ir1-C6 155.15(12) N1-Ir1-C6 88.09(10)
C2-Ir1-C6 96.81(12) C1-Ir1-C6 80.97(12)
C17-Ir1-C5 167.64(11) N1-Ir1-C5 97.11(10)
C2-Ir1-C5 80.52(12) C1-Ir1-C5 88.43(12)
C6-Ir1-C5 37.09(11) C9-N1-C13 117.8(2)
C9-N1-Ir1 121.88(19) C13-N1-Ir1 120.28(18)
C17-N2-C15 111.6(2) C17-N2-C14 121.7(2)
C15-N2-C14 126.2(2) C17-N3-C16 111.2(2)
C17-N3-C18 126.2(2) C16-N3-C18 122.4(2)
326
Crystal structure of 6.6
A clear colorless plate-like specimen of C
24
H
30
BIN
4
, approximate dimensions 0.122 mm
x 0.187 mm x 0.529 mm, was used for the X-ray crystallographic analysis. The X-ray intensity data
were measured on a Bruker APEX DUO system equipped with a TRIUMPH curved-crystal
monochromator and a MoKα fine-focus tube (λ = 0.71073 Å).
327
Table A.19. Data collection details for 6.6.
Axis dx/mm 2θ/° ω/° φ/° χ/°
Widt
h/°
Frames
Time/
s
Wavelengt
h/Å
Voltag
e/kV
Current
/mA
Temper
ature/K
Omega 50.502 30.00 30.00 0.00 54.73 0.50 360 2.00 0.71073 50 30.0 100
Omega 50.502 30.00 30.00 72.00 54.73 0.50 360 2.00 0.71073 50 30.0 100
Omega 50.502 30.00 30.00 144.00 54.73 0.50 360 2.00 0.71073 50 30.0 100
Omega 50.502 30.00 30.00 216.00 54.73 0.50 360 2.00 0.71073 50 30.0 100
Omega 50.502 30.00 30.00 288.00 54.73 0.50 360 2.00 0.71073 50 30.0 100
Phi 50.502 30.00 0.00 0.00 54.73 0.50 720 2.00 0.71073 50 30.0 100
A total of 2520 frames were collected. The total exposure time was 1.40 hours. The frames
were integrated with the Bruker SAINT software package using a SAINT V8.34A (Bruker AXS,
2013) algorithm. The integration of the data using a monoclinic unit cell yielded a total
of 28153reflections to a maximum θ angle of 30.52° (0.70 Å resolution), of which 3959 were
independent (average redundancy 7.111, completeness = 99.3%, R
int
= 3.51%, R
sig
= 2.16%)
and 3634 (91.79%) were greater than 2σ(F
2
). The final cell constants
of a = 8.2687(7) Å, b = 19.3361(15) Å, c =8.7596(7) Å, β = 115.0290(10)° , volume
= 1269.01(18) Å
3
, are based upon the refinement of the XYZ-centroids of 121 reflections above
20 σ(I) with 5.662° < 2θ < 48.50° . Data were corrected for absorption effects using the multi-scan
method (SADABS). The ratio of minimum to maximum apparent transmission was 0.836.
328
The structure was solved and refined using the Bruker SHELXTL Software Package,
using the space group P 1 21/m 1, with Z = 2 for the formula unit, C
24
H
30
BIN
4
. The final
anisotropic full-matrix least-squares refinement on F
2
with 146 variables converged at R1 = 3.51%,
for the observed data and wR2 = 7.89% for all data. The goodness-of-fit was 1.288. The largest
peak in the final difference electron density synthesis was 1.099 e
-
/Å
3
and the largest hole was -
1.607 e
-
/Å
3
with an RMS deviation of 0.094 e
-
/Å
3
. On the basis of the final model, the calculated
density was 1.341g/cm
3
and F(000), 520 e
-
.
Table A.20. Sample and crystal data.
Chemical formula C
24
H
30
BIN
4
Formula weight 512.23 g/mol
Temperature 100(2) K
Wavelength 0.71073 Å
Crystal size 0.122 x 0.187 x 0.529 mm
Crystal habit clear colourless plate
Crystal system monoclinic
Space group P 1 21/m 1
Unit cell dimensions a = 8.2687(7) Å α = 90°
b = 19.3361(15) Å β = 115.0290(10)°
c = 8.7596(7) Å γ = 90°
Volume 1269.01(18) Å
3
Z 2
329
Density (calculated) 1.341 g/cm
3
Absorption coefficient 1.278 mm
-1
F(000) 520
Table A.21. Data collection and structure refinement.
Diffractometer Bruker APEX DUO
Radiation source fine-focus tube, MoKα
Theta range for data collection 2.11 to 30.52°
Index ranges -11 ≤ h ≤ 11, -27 ≤ k ≤ 27, -12 ≤ l ≤ 12
Reflections collected 28153
Independent reflections 3959 [R(int) = 0.0351]
Coverage of independent
reflections
99.3%
Absorption correction multi-scan
Structure solution technique direct methods
Structure solution program SHELXTL XT 2014/4 (Bruker AXS, 2014)
Refinement method Full-matrix least-squares on F
2
Refinement program SHELXL-2014/6 (Sheldrick, 2014)
Function minimized Σ w(F
o
2
- F
c
2
)
2
Data / restraints / parameters 3959 / 0 / 146
Goodness-of-fit on F
2
1.288
Final R indices 3634 data; I>2σ(I) R1 = 0.0351, wR2 = 0.0777
330
all data R1 = 0.0395, wR2 = 0.0789
Weighting scheme
w=1/[σ
2
(F
o
2
)+(0.0161P)
2
+1.9591P]
where P=(F
o
2
+2F
c
2
)/3
Largest diff. peak and hole 1.099 and -1.607 eÅ
-3
R.M.S. deviation from mean 0.094 eÅ
-3
Table A.22. Bond lengths (Å ).
B1-N1 1.572(3) B1-N1 1.572(3)
B1-H1B 1.12(4) B1-H2B 1.10(4)
C1-C2 1.360(3) C1-N1 1.387(3)
C1-H1 0.95 C2-N2 1.384(3)
C2-H2 0.95 C3-N1 1.331(3)
C3-N2 1.343(3) C3-H3 0.95
C4-C9 1.396(3) C4-C5 1.402(3)
C4-N2 1.448(3) C5-C6 1.396(3)
C5-C10 1.512(3) C6-C7 1.394(3)
C6-H6 0.95 C7-C8 1.400(3)
C7-C11 1.515(3) C8-C9 1.401(3)
C8-H8 0.95 C9-C12 1.513(3)
C10-H10A 0.98 C10-H10B 0.98
C10-H10C 0.98 C11-H11A 0.98
C11-H11B 0.98 C11-H11C 0.98
331
C12-H12A 0.98 C12-H12B 0.98
C12-H12C 0.98
Table A.23. Bond angles (° ).
N1-B1-N1 107.4(3) N1-B1-H1B 109.1(12)
N1-B1-H1B 109.1(12) N1-B1-H2B 107.9(12)
N1-B1-H2B 107.9(12) H1B-B1-H2B 115.(3)
C2-C1-N1 107.80(19) C2-C1-H1 126.1
N1-C1-H1 126.1 C1-C2-N2 106.96(19)
C1-C2-H2 126.5 N2-C2-H2 126.5
N1-C3-N2 109.80(18) N1-C3-H3 125.1
N2-C3-H3 125.1 C9-C4-C5 123.1(2)
C9-C4-N2 119.09(19) C5-C4-N2 117.78(19)
C6-C5-C4 117.6(2) C6-C5-C10 120.6(2)
C4-C5-C10 121.78(19) C7-C6-C5 121.7(2)
C7-C6-H6 119.2 C5-C6-H6 119.2
C6-C7-C8 118.5(2) C6-C7-C11 120.1(2)
C8-C7-C11 121.4(2) C7-C8-C9 122.1(2)
C7-C8-H8 118.9 C9-C8-H8 118.9
C4-C9-C8 116.9(2) C4-C9-C12 121.9(2)
332
C8-C9-C12 121.2(2) C5-C10-H10A 109.5
C5-C10-H10B 109.5 H10A-C10-H10B 109.5
C5-C10-H10C 109.5 H10A-C10-H10C 109.5
H10B-C10-H10C 109.5 C7-C11-H11A 109.5
C7-C11-H11B 109.5 H11A-C11-H11B 109.5
C7-C11-H11C 109.5 H11A-C11-H11C 109.5
H11B-C11-H11C 109.5 C9-C12-H12A 109.5
C9-C12-H12B 109.5 H12A-C12-H12B 109.5
C9-C12-H12C 109.5 H12A-C12-H12C 109.5
H12B-C12-H12C 109.5 C3-N1-C1 107.56(18)
C3-N1-B1 125.4(2) C1-N1-B1 127.0(2)
C3-N2-C2 107.87(18) C3-N2-C4 126.43(18)
C2-N2-C4 125.61(18)
333
Crystal structure of 6.1
A clear orange prismlike specimen of C
25
H
38
B
3
Cl
2
IN
12
Ni
2
, approximate dimensions
0.105 mm x 0.131 mm x 0.247 mm, was used for the X-ray crystallographic analysis. The X-ray
intensity data were measured on a Bruker APEX DUO system equipped with a TRIUMPH
curved-crystal monochromator and a MoKα fine-focus tube (λ = 0.71073 Å).
334
Table A.24. Data collection details.
Axis dx/mm 2θ/° ω/° φ/° χ/° Width/° Frames Time/s
Wavelen
gth/Å
Volt
age/kV
Curr
ent/mA
Temper
ature/K
Omega 50.000 30.00 30.00 0.00 54.75 -0.50 360 5.00 0.71073 50 30.0 100
Omega 50.000 30.00 30.00 72.00 54.75 -0.50 181 5.00 0.71073 50 30.0 100
Omega 50.000 30.00 30.00 0.00 54.75 -0.50 360 10.00 0.71073 50 30.0 100
Omega 50.000 30.00 30.00 72.00 54.75 -0.50 360 10.00 0.71073 50 30.0 100
Omega 50.000 30.00 30.00 144.00 54.75 -0.50 360 10.00 0.71073 50 30.0 100
Omega 50.000 30.00 30.00 216.00 54.75 -0.50 360 10.00 0.71073 50 30.0 100
Omega 50.000 30.00 30.00 288.00 54.75 -0.50 360 10.00 0.71073 50 30.0 100
Phi 50.000 30.00 0.00 0.00 54.75 -0.50 720 10.00 0.71073 50 30.0 100
A total of 3061 frames were collected. The total exposure time was 7.75 hours. The frames
were integrated with the Bruker SAINT software package using a SAINT V8.34A (Bruker AXS,
2013) algorithm. The integration of the data using a triclinic unit cell yielded a total
of 58909 reflections to a maximum θ angle of 30.54° (0.70 Å resolution), of which 11486 were
independent (average redundancy 5.129, completeness = 98.6%, R
int
= 3.01%, R
sig
= 2.41%)
and 9852 (85.77%) were greater than 2σ(F
2
). The final cell constants
of a = 12.3061(10) Å, b = 12.6455(10) Å, c =13.1675(10) Å, α = 78.8847(12)°, β
= 71.0045(12)°, γ = 87.4156(12)° , volume = 1900.8(3) Å
3
, are based upon the refinement of the
XYZ-centroids of reflections above 20 σ(I). Data were corrected for absorption effects using the
multi-scan method (SADABS). The ratio of minimum to maximum apparent transmission
335
was 0.895. The calculated minimum and maximum transmission coefficients (based on crystal
size) are 0.6410 and 0.8190.
The structure was solved and refined using the Bruker SHELXTL Software Package,
using the space group P-1, with Z = 2 for the formula unit, C
25
H
38
B
3
Cl
2
IN
12
Ni
2
. The final
anisotropic full-matrix least-squares refinement on F
2
with 441 variables converged at R1 = 2.44%,
for the observed data and wR2 = 5.84% for all data. The goodness-of-fit was 1.032. The largest
peak in the final difference electron density synthesis was 0.777 e
-
/Å
3
and the largest hole was -
0.806 e
-
/Å
3
with an RMS deviation of 0.076 e
-
/Å
3
. On the basis of the final model, the calculated
density was1.493 g/cm
3
and F(000), 860 e
-
.
Table A.25. Sample and crystal data.
Chemical formula C
25
H
38
B
3
Cl
2
IN
12
Ni
2
Formula weight 854.32 g/mol
Temperature 100(2) K
Wavelength 0.71073 Å
Crystal size 0.105 x 0.131 x 0.247 mm
Crystal habit clear orange prism
Crystal system triclinic
Space group P -1
Unit cell dimensions a = 12.3061(10) Å α = 78.8847(12)°
b = 12.6455(10) Å β = 71.0045(12)°
c = 13.1675(10) Å γ = 87.4156(12)°
336
Volume 1900.8(3) Å
3
Z 2
Density (calculated) 1.493 g/cm
3
Absorption coefficient 1.977 mm
-1
F(000) 860
Table A.26. Data collection and structure refinement.
Diffractometer Bruker APEX DUO
Radiation source fine-focus tube, MoKα
Theta range for data collection 1.64 to 30.54°
Index ranges -17 ≤ h ≤ 17, -18≤ k ≤ 18, -18 ≤ l ≤ 18
Reflections collected 58909
Independent reflections 11486 [R(int) = 0.0301]
Absorption correction multi-scan
Max. and min. transmission 0.8190 and 0.6410
Structure solution technique direct methods
Structure solution program SHELXTL XT 2013/1 (Bruker AXS, 2014)
Refinement method Full-matrix least-squares on F
2
Refinement program SHELXTL XL 2014/7 (Bruker AXS, 2014)
Function minimized Σ w(F
o
2
- F
c
2
)
2
Data / restraints / parameters 11486 / 3 / 441
Goodness-of-fit on F
2
1.032
337
Δ/σ
max
0.001
Final R indices 9852 data; I>2σ(I) R1 = 0.0244, wR2 = 0.0555
all data R1 = 0.0328, wR2 = 0.0584
Weighting scheme
w=1/[σ
2
(F
o
2
)+(0.0257P)
2
+1.2161P]
where P=(F
o
2
+2F
c
2
)/3
Largest diff. peak and hole 0.777 and -0.806 eÅ
-3
R.M.S. deviation from mean 0.076 eÅ
-3
Table A.27. Bond lengths (Å ).
B1-N2 1.564(2) B1-N3 1.565(2)
B1-H30 1.11(2) B1-H31 1.13(2)
B2-N7 1.561(2) B2-N6 1.565(3)
B2-H32 1.16(2) B2-H33 1.15(2)
B3-N9 1.536(2) B3-N12 1.537(2)
B3-Ni2 2.3420(18) B3-H34 1.24(2)
B3-H35 1.10(2) C1-N2 1.358(2)
C1-N1 1.359(2) C1-Ni1 1.9051(16)
C2-C3 1.350(3) C2-N1 1.387(2)
C2-H2 0.95 C3-N2 1.390(2)
C3-H3 0.95 C4-N1 1.456(2)
C4-H4A 0.98 C4-H4B 0.98
C4-H4C 0.98 C5-N3 1.352(2)
338
C5-N4 1.360(2) C5-Ni2 1.9136(16)
C6-C7 1.353(3) C6-N3 1.389(2)
C6-H6 0.95 C7-N4 1.383(2)
C7-H7 0.95 C8-N4 1.454(2)
C8-H8A 0.98 C8-H8B 0.98
C8-H8C 0.98 C9-N6 1.353(2)
C9-N5 1.356(2) C9-Ni2 1.8746(16)
C10-C11 1.343(3) C10-N5 1.386(2)
C10-H10 0.95 C11-N6 1.386(2)
C11-H11 0.95 C12-N5 1.457(2)
C12-H12A 0.98 C12-H12B 0.98
C12-H12C 0.98 C13-N7 1.353(2)
C13-N8 1.359(2) C13-Ni2 1.9032(16)
C14-C15 1.356(2) C14-N7 1.381(2)
C14-H14 0.95 C15-N8 1.384(2)
C15-H15 0.95 C16-N8 1.462(2)
C16-H16A 0.98 C16-H16B 0.98
C16-H16C 0.98 C17-N10 1.354(2)
C17-N9 1.358(2) C17-Ni1 1.9243(16)
C18-C19 1.347(2) C18-N10 1.387(2)
C18-H18 0.95 C19-N9 1.384(2)
C19-H19 0.95 C20-N10 1.456(2)
C20-H20A 0.98 C20-H20B 0.98
C20-H20C 0.98 C21-N12 1.352(2)
339
C21-N11 1.362(2) C21-Ni1 1.8755(16)
C22-C23 1.348(2) C22-N11 1.391(2)
C22-H22 0.95 C23-N12 1.385(2)
C23-H23 0.95 C24-N11 1.454(2)
C24-H24A 0.98 C24-H24B 0.98
C24-H24C 0.98 I1-Ni1 2.5457(3)
Ni2-H34 1.58(2) C25-Cl1 1.766(4)
C25-Cl2 1.832(10) C25-H25A 0.99
C25-H25B 0.99 C25'-Cl1' 1.747(15)
C25'-Cl2' 1.790(17) C25'-H25C 0.99
C25'-H25D 0.99
Table A.28. Bond angles.
N2-B1-N3 111.22(14) N2-B1-H30 109.5(11)
N3-B1-H30 109.6(11) N2-B1-H31 107.5(10)
N3-B1-H31 107.9(11) H30-B1-H31 111.1(15)
N7-B2-N6 104.96(14) N7-B2-H32 110.0(11)
N6-B2-H32 109.5(11) N7-B2-H33 110.3(11)
N6-B2-H33 111.2(11) H32-B2-H33 110.7(15)
N9-B3-N12 106.78(13) N9-B3-Ni2 127.78(11)
N12-B3-Ni2 116.32(11) N9-B3-H34 102.8(9)
N12-B3-H34 106.2(9) Ni2-B3-H34 38.8(9)
340
N9-B3-H35 113.1(10) N12-B3-H35 111.8(10)
Ni2-B3-H35 77.3(10) H34-B3-H35 115.4(14)
N2-C1-N1 106.00(14) N2-C1-Ni1 126.80(12)
N1-C1-Ni1 127.13(12) C3-C2-N1 105.98(15)
C3-C2-H2 127.0 N1-C2-H2 127.0
C2-C3-N2 108.41(15) C2-C3-H3 125.8
N2-C3-H3 125.8 N1-C4-H4A 109.5
N1-C4-H4B 109.5 H4A-C4-H4B 109.5
N1-C4-H4C 109.5 H4A-C4-H4C 109.5
H4B-C4-H4C 109.5 N3-C5-N4 105.95(14)
N3-C5-Ni2 126.50(12) N4-C5-Ni2 127.38(12)
C7-C6-N3 107.94(15) C7-C6-H6 126.0
N3-C6-H6 126.0 C6-C7-N4 106.17(15)
C6-C7-H7 126.9 N4-C7-H7 126.9
N4-C8-H8A 109.5 N4-C8-H8B 109.5
H8A-C8-H8B 109.5 N4-C8-H8C 109.5
H8A-C8-H8C 109.5 H8B-C8-H8C 109.5
N6-C9-N5 106.39(14) N6-C9-Ni2 122.14(12)
N5-C9-Ni2 131.32(13) C11-C10-N5 106.81(16)
C11-C10-H10 126.6 N5-C10-H10 126.6
C10-C11-N6 107.86(16) C10-C11-H11 126.1
N6-C11-H11 126.1 N5-C12-H12A 109.5
N5-C12-H12B 109.5 H12A-C12-H12B 109.5
N5-C12-H12C 109.5 H12A-C12-H12C 109.5
341
H12B-C12-H12C 109.5 N7-C13-N8 106.15(13)
N7-C13-Ni2 122.47(12) N8-C13-Ni2 130.99(12)
C15-C14-N7 107.89(15) C15-C14-H14 126.1
N7-C14-H14 126.1 C14-C15-N8 106.27(15)
C14-C15-H15 126.9 N8-C15-H15 126.9
N8-C16-H16A 109.5 N8-C16-H16B 109.5
H16A-C16-H16B 109.5 N8-C16-H16C 109.5
H16A-C16-H16C 109.5 H16B-C16-H16C 109.5
N10-C17-N9 105.38(13) N10-C17-Ni1 135.18(12)
N9-C17-Ni1 119.06(11) C19-C18-N10 106.73(14)
C19-C18-H18 126.6 N10-C18-H18 126.6
C18-C19-N9 107.22(15) C18-C19-H19 126.4
N9-C19-H19 126.4 N10-C20-H20A 109.5
N10-C20-H20B 109.5 H20A-C20-H20B 109.5
N10-C20-H20C 109.5 H20A-C20-H20C 109.5
H20B-C20-H20C 109.5 N12-C21-N11 105.41(13)
N12-C21-Ni1 120.04(11) N11-C21-Ni1 134.51(12)
C23-C22-N11 106.53(14) C23-C22-H22 126.7
N11-C22-H22 126.7 C22-C23-N12 107.46(15)
C22-C23-H23 126.3 N12-C23-H23 126.3
N11-C24-H24A 109.5 N11-C24-H24B 109.5
H24A-C24-H24B 109.5 N11-C24-H24C 109.5
H24A-C24-H24C 109.5 H24B-C24-H24C 109.5
C1-N1-C2 110.63(14) C1-N1-C4 125.28(14)
342
C2-N1-C4 123.98(14) C1-N2-C3 108.98(14)
C1-N2-B1 127.07(14) C3-N2-B1 123.48(14)
C5-N3-C6 109.40(14) C5-N3-B1 125.16(14)
C6-N3-B1 124.78(14) C5-N4-C7 110.53(14)
C5-N4-C8 125.17(14) C7-N4-C8 124.30(15)
C9-N5-C10 109.76(16) C9-N5-C12 125.96(15)
C10-N5-C12 124.28(16) C9-N6-C11 109.17(15)
C9-N6-B2 123.98(14) C11-N6-B2 126.81(15)
C13-N7-C14 109.51(14) C13-N7-B2 122.86(14)
C14-N7-B2 127.60(14) C13-N8-C15 110.17(13)
C13-N8-C16 125.62(13) C15-N8-C16 124.18(14)
C17-N9-C19 110.17(13) C17-N9-B3 120.82(13)
C19-N9-B3 128.89(14) C17-N10-C18 110.47(14)
C17-N10-C20 125.95(14) C18-N10-C20 123.54(14)
C21-N11-C22 110.26(14) C21-N11-C24 125.82(15)
C22-N11-C24 123.89(15) C21-N12-C23 110.32(13)
C21-N12-B3 121.60(13) C23-N12-B3 128.06(13)
C21-Ni1-C1 90.75(7) C21-Ni1-C17 87.94(7)
C1-Ni1-C17 172.25(7) C21-Ni1-I1 174.90(5)
C1-Ni1-I1 88.12(5) C17-Ni1-I1 92.52(5)
C9-Ni2-C13 87.71(7) C9-Ni2-C5 94.05(7)
C13-Ni2-C5 177.63(7) C9-Ni2-B3 152.92(7)
C13-Ni2-B3 88.91(6) C5-Ni2-B3 88.80(6)
C9-Ni2-H34 177.5(7) C13-Ni2-H34 91.6(7)
343
C5-Ni2-H34 86.7(7) B3-Ni2-H34 29.4(7)
Cl1-C25-Cl2 111.4(4) Cl1-C25-H25A 109.3
Cl2-C25-H25A 109.3 Cl1-C25-H25B 109.3
Cl2-C25-H25B 109.3 H25A-C25-H25B 108.0
Cl1'-C25'-Cl2' 91.4(8) Cl1'-C25'-H25C 113.4
Cl2'-C25'-H25C 113.4 Cl1'-C25'-H25D 113.4
Cl2'-C25'-H25D 113.4 H25C-C25'-H25D 110.7
Crystal structure of 6.2
344
A clear pale orange prism-like specimen of C
59
H
68
Cl
2
N
8
Ni
3
O
4
, approximate
dimensions 0.126 mm x 0.302 mm x 0.753 mm, was used for the X-ray crystallographic analysis.
The X-ray intensity data were measured on a Bruker APEX DUO system equipped with a
TRIUMPH curved-crystal monochromator and a MoKα fine-focus tube (λ = 0.71073 Å).
345
Table A.29. Data collection details.
Axis dx/mm 2θ/° ω/° φ/° χ/°
Width
/°
Frame
s
Time/
s
Wavelengt
h/Å
Voltag
e/kV
Curren
t/mA
Tempera
ture/K
Omega 50.395 30.00 30.00 0.00 54.73 0.50 360 1.00 0.71073 50 30.0 100
Omega 50.395 30.00 30.00 72.00 54.73 0.50 360 1.00 0.71073 50 30.0 100
Omega 50.395 30.00 30.00 144.00 54.73 0.50 360 1.00 0.71073 50 30.0 100
Omega 50.395 30.00 30.00 216.00 54.73 0.50 360 1.00 0.71073 50 30.0 100
Omega 50.395 30.00 30.00 288.00 54.73 0.50 360 1.00 0.71073 50 30.0 100
Phi 50.395 30.00 0.00 0.00 54.73 0.50 720 1.00 0.71073 50 30.0 100
A total of 2520 frames were collected. The total exposure time was 0.70 hours. The frames
were integrated with the Bruker SAINT software package using a SAINT V8.34A (Bruker AXS,
2013) algorithm. The integration of the data using a monoclinic unit cell yielded a total of 141899
reflections to a maximum θ angle of 30.54° (0.70 Å resolution), of which 17841 were independent
(average redundancy 7.954, completeness = 99.6%, R
int
= 5.87%, R
sig
= 3.94%)
and 12343 (69.18%) were greater than 2σ(F
2
).The final cell constants
of a = 22.9867(14) Å, b = 8.2956(5) Å, c =32.6131(19) Å, β = 109.9220(10)° , volume
= 5846.8(6) Å
3
, are based upon the refinement of the XYZ-centroids of 9862 reflections above 20
σ(I) with 5.259° < 2θ < 61.03° .Data were corrected for absorption effects using the multi-scan
method (SADABS). The ratio of minimum to maximum apparent transmission was 0.818. The
calculated minimum and maximum transmission coefficients (based on crystal size)
346
are 0.4910 and 0.8740.
The structure was solved and refined using the Bruker SHELXTL Software Package,
using the space group P 1 21/c 1, with Z = 4 for the formula unit, C
59
H
68
Cl
2
N
8
Ni
3
O
4
.The final
anisotropic full-matrix least-squares refinement on F
2
with 704 variables converged at R1 = 4.86%,
for the observed data and wR2 = 13.16% for all data. The goodness-of-fit was 1.030. The largest
peak in the final difference electron density synthesis was 0.874 e
-
/Å
3
and the largest hole was -
1.156 e
-
/Å
3
with an RMS deviation of 0.093 e
-
/Å
3
. On the basis of the final model, the calculated
density was 1.364 g/cm
3
and F(000), 2512 e
-
.
Table A.30. Sample and crystal data.
Chemical formula C
59
H
68
Cl
2
N
8
Ni
3
O
4
Formula weight 1200.24 g/mol
Temperature 100(2) K
Wavelength 0.71073 Å
Crystal size 0.126 x 0.302 x 0.753 mm
Crystal habit clear pale orange prism
Crystal system monoclinic
Space group P 1 21/c 1
Unit cell dimensions a = 22.9867(14) Å α = 90°
b = 8.2956(5) Å β = 109.9220(10)°
347
c = 32.6131(19) Å γ = 90°
Volume 5846.8(6) Å
3
Z 4
Density (calculated) 1.364 g/cm
3
Absorption coefficient 1.100 mm
-1
F(000) 2512
Table A.31. Data collection and structure.
Diffractometer Bruker APEX DUO
Radiation source fine-focus tube, MoKα
Theta range for data collection 1.33 to 30.54°
Index ranges -32 ≤ h ≤ 32, -11 ≤ k ≤ 11, -46 ≤ l ≤ 46
Reflections collected 141899
Independent reflections 17841 [R(int) = 0.0587]
Coverage of independent reflections 99.6%
Absorption correction multi-scan
Max. and min. transmission 0.8740 and 0.4910
Structure solution technique direct methods
Structure solution program SHELXTL XT 2013/1 (Bruker AXS, 2014)
Refinement method Full-matrix least-squares on F
2
Refinement program SHELXTL XL 2014/7 (Bruker AXS, 2014)
Function minimized Σ w(F
o
2
- F
c
2
)
2
348
Data / restraints / parameters 17841 / 0 / 704
Goodness-of-fit on F
2
1.030
Δ/σ
max
0.007
Final R indices
12343 data;
I>2σ(I)
R1 = 0.0486, wR2 = 0.1171
all data R1 = 0.0783, wR2 = 0.1316
Weighting scheme
w=1/[σ
2
(F
o
2
)+(0.0509P)
2
+10.2519P]
where P=(F
o
2
+2F
c
2
)/3
Largest diff. peak and hole 0.874 and -1.156 eÅ
-3
R.M.S. deviation from mean 0.093 eÅ
-3
Table A.32. Bond lengths (Å ).
C1-C2 1.353(3) C1-N1 1.384(3)
C1-H1 0.95 C2-N2 1.388(3)
C2-H2 0.95 C3-N1 1.347(3)
C3-N2 1.374(3) C3-Ni2 1.887(2)
C4-C9 1.390(3) C4-C5 1.408(3)
C4-N2 1.434(3) C5-C6 1.393(3)
C5-C10 1.501(4) C6-C7 1.393(4)
C6-H6 0.95 C7-C8 1.383(4)
C7-C11 1.515(4) C8-C9 1.398(3)
C8-H8 0.95 C9-C12 1.498(4)
C10-H10A 0.98 C10-H10B 0.98
349
C10-H10C 0.98 C11-H11A 0.98
C11-H11B 0.98 C11-H11C 0.98
C12-H12A 0.98 C12-H12B 0.98
C12-H12C 0.98 C13-O1 1.270(3)
C13-C15 1.403(4) C13-C14 1.507(4)
C14-H14A 0.98 C14-H14B 0.98
C14-H14C 0.98 C15-C16 1.396(4)
C15-H15 0.95 C16-O2 1.274(3)
C16-C17 1.508(4) C17-H17A 0.98
C17-H17B 0.98 C17-H17C 0.98
C18-C19 1.353(3) C18-N3 1.379(3)
C18-H18 0.95 C19-N4 1.386(3)
C19-H19 0.95 C20-N3 1.345(3)
C20-N4 1.385(3) C20-Ni2 1.899(2)
C21-C22 1.392(3) C21-C26 1.403(3)
C21-N4 1.439(3) C22-C23 1.404(4)
C22-C27 1.500(4) C23-C24 1.380(4)
C23-H23 0.95 C24-C25 1.392(4)
C24-C28 1.515(4) C25-C26 1.388(3)
C25-H25 0.95 C26-C29 1.508(3)
C27-H27A 0.98 C27-H27B 0.98
C27-H27C 0.98 C28-H28A 0.98
C28-H28B 0.98 C28-H28C 0.98
C29-H29A 0.98 C29-H29B 0.98
350
C29-H29C 0.98 C30-C31 1.355(3)
C30-N5 1.382(3) C30-H30 0.95
C31-N6 1.390(3) C31-H31 0.95
C32-N5 1.348(3) C32-N6 1.374(3)
C32-Ni4 1.883(2) C33-C38 1.395(3)
C33-C34 1.400(3) C33-N6 1.436(3)
C34-C35 1.395(3) C34-C39 1.505(4)
C35-C36 1.390(4) C35-H35 0.95
C36-C37 1.386(4) C36-C40 1.512(3)
C37-C38 1.402(3) C37-H37 0.95
C38-C41 1.499(4) C39-H39A 0.98
C39-H39B 0.98 C39-H39C 0.98
C40-H40A 0.98 C40-H40B 0.98
C40-H40C 0.98 C41-H41A 0.98
C41-H41B 0.98 C41-H41C 0.98
C42-C43 1.506(4) C42-H42A 0.98
C42-H42B 0.98 C42-H42C 0.98
C43-O3 1.277(3) C43-C44 1.390(4)
C44-C45 1.390(4) C44-H44 0.95
C45-O4 1.277(3) C45-C46 1.511(4)
C46-H46A 0.98 C46-H46B 0.98
C46-H46C 0.98 C47-C48 1.354(3)
C47-N7 1.385(3) C47-H47 0.95
C48-N8 1.387(3) C48-H48 0.95
351
C49-N7 1.350(3) C49-N8 1.382(3)
C49-Ni4 1.897(2) C50-C55 1.390(3)
C50-C51 1.400(3) C50-N8 1.440(3)
C51-C52 1.394(4) C51-C56 1.505(4)
C52-C53 1.390(4) C52-H52 0.95
C53-C54 1.388(4) C53-C57 1.512(4)
C54-C55 1.396(3) C54-H54 0.95
C55-C58 1.504(3) C56-H56A 0.98
C56-H56B 0.98 C56-H56C 0.98
C57-H57A 0.98 C57-H57B 0.98
C57-H57C 0.98 C58-H58A 0.98
C58-H58B 0.98 C58-H58C 0.98
C59-Cl2 1.733(4) C59-Cl1 1.775(4)
C59-H59A 0.99 C59-H59B 0.99
N1-Ni1 1.8659(18) N3-Ni1 1.8759(19)
N5-Ni3 1.8655(18) N7-Ni3 1.8807(19)
Ni1-N1 1.8660(18) Ni1-N3 1.8758(19)
Ni2-O2 1.8912(16) Ni2-O1 1.8938(17)
Ni3-N5 1.8654(18) Ni3-N7 1.8807(19)
Ni4-O4 1.8931(17) Ni4-O3 1.8944(17)
352
Table A.33. Bond angles (° ).
C2-C1-N1 108.2(2) C2-C1-H1 125.9
N1-C1-H1 125.9 C1-C2-N2 106.3(2)
C1-C2-H2 126.9 N2-C2-H2 126.9
N1-C3-N2 106.08(19) N1-C3-Ni2 124.38(16)
N2-C3-Ni2 129.36(17) C9-C4-C5 121.8(2)
C9-C4-N2 119.2(2) C5-C4-N2 118.9(2)
C6-C5-C4 117.7(2) C6-C5-C10 121.4(2)
C4-C5-C10 120.9(2) C5-C6-C7 121.8(2)
C5-C6-H6 119.1 C7-C6-H6 119.1
C8-C7-C6 118.6(2) C8-C7-C11 121.0(3)
C6-C7-C11 120.4(3) C7-C8-C9 121.9(2)
C7-C8-H8 119.1 C9-C8-H8 119.1
C4-C9-C8 118.1(2) C4-C9-C12 120.6(2)
C8-C9-C12 121.3(2) C5-C10-H10A 109.5
C5-C10-H10B 109.5 H10A-C10-H10B 109.5
C5-C10-H10C 109.5 H10A-C10-H10C 109.5
H10B-C10-H10C 109.5 C7-C11-H11A 109.5
C7-C11-H11B 109.5 H11A-C11-H11B 109.5
C7-C11-H11C 109.5 H11A-C11-H11C 109.5
H11B-C11-H11C 109.5 C9-C12-H12A 109.5
C9-C12-H12B 109.5 H12A-C12-H12B 109.5
C9-C12-H12C 109.5 H12A-C12-H12C 109.5
H12B-C12-H12C 109.5 O1-C13-C15 124.7(2)
353
O1-C13-C14 116.4(2) C15-C13-C14 118.9(2)
C13-C14-H14A 109.5 C13-C14-H14B 109.5
H14A-C14-H14B 109.5 C13-C14-H14C 109.5
H14A-C14-H14C 109.5 H14B-C14-H14C 109.5
C16-C15-C13 123.1(2) C16-C15-H15 118.4
C13-C15-H15 118.4 O2-C16-C15 125.1(2)
O2-C16-C17 115.7(2) C15-C16-C17 119.2(2)
C16-C17-H17A 109.5 C16-C17-H17B 109.5
H17A-C17-H17B 109.5 C16-C17-H17C 109.5
H17A-C17-H17C 109.5 H17B-C17-H17C 109.5
C19-C18-N3 108.5(2) C19-C18-H18 125.7
N3-C18-H18 125.7 C18-C19-N4 106.4(2)
C18-C19-H19 126.8 N4-C19-H19 126.8
N3-C20-N4 106.07(19) N3-C20-Ni2 123.30(16)
N4-C20-Ni2 130.63(16) C22-C21-C26 121.5(2)
C22-C21-N4 120.6(2) C26-C21-N4 117.8(2)
C21-C22-C23 117.5(2) C21-C22-C27 122.3(2)
C23-C22-C27 120.1(2) C24-C23-C22 122.2(2)
C24-C23-H23 118.9 C22-C23-H23 118.9
C23-C24-C25 118.5(2) C23-C24-C28 121.6(3)
C25-C24-C28 119.9(2) C26-C25-C24 121.5(2)
C26-C25-H25 119.3 C24-C25-H25 119.3
C25-C26-C21 118.5(2) C25-C26-C29 120.8(2)
C21-C26-C29 120.7(2) C22-C27-H27A 109.5
354
C22-C27-H27B 109.5 H27A-C27-H27B 109.5
C22-C27-H27C 109.5 H27A-C27-H27C 109.5
H27B-C27-H27C 109.5 C24-C28-H28A 109.5
C24-C28-H28B 109.5 H28A-C28-H28B 109.5
C24-C28-H28C 109.5 H28A-C28-H28C 109.5
H28B-C28-H28C 109.5 C26-C29-H29A 109.5
C26-C29-H29B 109.5 H29A-C29-H29B 109.5
C26-C29-H29C 109.5 H29A-C29-H29C 109.5
H29B-C29-H29C 109.5 C31-C30-N5 108.4(2)
C31-C30-H30 125.8 N5-C30-H30 125.8
C30-C31-N6 106.2(2) C30-C31-H31 126.9
N6-C31-H31 126.9 N5-C32-N6 106.39(19)
N5-C32-Ni4 126.76(16) N6-C32-Ni4 126.85(17)
C38-C33-C34 121.7(2) C38-C33-N6 120.0(2)
C34-C33-N6 118.2(2) C35-C34-C33 118.2(2)
C35-C34-C39 120.5(2) C33-C34-C39 121.3(2)
C36-C35-C34 121.8(2) C36-C35-H35 119.1
C34-C35-H35 119.1 C37-C36-C35 118.5(2)
C37-C36-C40 121.4(2) C35-C36-C40 120.1(2)
C36-C37-C38 122.0(2) C36-C37-H37 119.0
C38-C37-H37 119.0 C33-C38-C37 117.8(2)
C33-C38-C41 121.4(2) C37-C38-C41 120.7(2)
C34-C39-H39A 109.5 C34-C39-H39B 109.5
H39A-C39-H39B 109.5 C34-C39-H39C 109.5
355
H39A-C39-H39C 109.5 H39B-C39-H39C 109.5
C36-C40-H40A 109.5 C36-C40-H40B 109.5
H40A-C40-H40B 109.5 C36-C40-H40C 109.5
H40A-C40-H40C 109.5 H40B-C40-H40C 109.5
C38-C41-H41A 109.5 C38-C41-H41B 109.5
H41A-C41-H41B 109.5 C38-C41-H41C 109.5
H41A-C41-H41C 109.5 H41B-C41-H41C 109.5
C43-C42-H42A 109.5 C43-C42-H42B 109.5
H42A-C42-H42B 109.5 C43-C42-H42C 109.5
H42A-C42-H42C 109.5 H42B-C42-H42C 109.5
O3-C43-C44 124.8(2) O3-C43-C42 115.4(2)
C44-C43-C42 119.8(2) C43-C44-C45 123.2(2)
C43-C44-H44 118.4 C45-C44-H44 118.4
O4-C45-C44 125.6(2) O4-C45-C46 115.7(3)
C44-C45-C46 118.8(2) C45-C46-H46A 109.5
C45-C46-H46B 109.5 H46A-C46-H46B 109.5
C45-C46-H46C 109.5 H46A-C46-H46C 109.5
H46B-C46-H46C 109.5 C48-C47-N7 108.5(2)
C48-C47-H47 125.8 N7-C47-H47 125.8
C47-C48-N8 106.3(2) C47-C48-H48 126.8
N8-C48-H48 126.8 N7-C49-N8 106.02(19)
N7-C49-Ni4 122.45(16) N8-C49-Ni4 131.41(16)
C55-C50-C51 121.8(2) C55-C50-N8 120.2(2)
C51-C50-N8 117.8(2) C52-C51-C50 117.7(2)
356
C52-C51-C56 121.4(2) C50-C51-C56 120.9(2)
C53-C52-C51 121.9(2) C53-C52-H52 119.0
C51-C52-H52 119.0 C54-C53-C52 118.5(2)
C54-C53-C57 120.5(3) C52-C53-C57 121.0(3)
C53-C54-C55 121.7(3) C53-C54-H54 119.2
C55-C54-H54 119.2 C50-C55-C54 118.2(2)
C50-C55-C58 121.2(2) C54-C55-C58 120.6(2)
C51-C56-H56A 109.5 C51-C56-H56B 109.5
H56A-C56-H56B 109.5 C51-C56-H56C 109.5
H56A-C56-H56C 109.5 H56B-C56-H56C 109.5
C53-C57-H57A 109.5 C53-C57-H57B 109.5
H57A-C57-H57B 109.5 C53-C57-H57C 109.5
H57A-C57-H57C 109.5 H57B-C57-H57C 109.5
C55-C58-H58A 109.5 C55-C58-H58B 109.5
H58A-C58-H58B 109.5 C55-C58-H58C 109.5
H58A-C58-H58C 109.5 H58B-C58-H58C 109.5
Cl2-C59-Cl1 112.1(2) Cl2-C59-H59A 109.2
Cl1-C59-H59A 109.2 Cl2-C59-H59B 109.2
Cl1-C59-H59B 109.2 H59A-C59-H59B 107.9
C3-N1-C1 109.63(19) C3-N1-Ni1 122.85(15)
C1-N1-Ni1 127.34(16) C3-N2-C2 109.77(19)
C3-N2-C4 127.66(19) C2-N2-C4 122.54(19)
C20-N3-C18 109.65(19) C20-N3-Ni1 123.48(16)
C18-N3-Ni1 126.66(16) C20-N4-C19 109.38(19)
357
C20-N4-C21 126.96(19) C19-N4-C21 121.69(19)
C32-N5-C30 109.42(19) C32-N5-Ni3 121.94(15)
C30-N5-Ni3 127.96(16) C32-N6-C31 109.56(19)
C32-N6-C33 128.13(19) C31-N6-C33 121.74(19)
C49-N7-C47 109.47(19) C49-N7-Ni3 125.75(16)
C47-N7-Ni3 124.72(16) C49-N8-C48 109.71(19)
C49-N8-C50 128.5(2) C48-N8-C50 121.6(2)
N1-Ni1-N1 180.0 N1-Ni1-N3 91.35(8)
N1-Ni1-N3 88.65(8) N1-Ni1-N3 88.65(8)
N1-Ni1-N3 91.35(8) N3-Ni1-N3 180.00(12)
C3-Ni2-O2 176.26(9) C3-Ni2-O1 87.56(9)
O2-Ni2-O1 93.18(7) C3-Ni2-C20 88.88(9)
O2-Ni2-C20 90.20(8) O1-Ni2-C20 175.55(8)
N5-Ni3-N5 180.0 N5-Ni3-N7 91.13(8)
N5-Ni3-N7 88.87(8) N5-Ni3-N7 88.87(8)
N5-Ni3-N7 91.13(8) N7-Ni3-N7 180.0
C32-Ni4-O4 174.48(9) C32-Ni4-O3 87.99(9)
O4-Ni4-O3 93.06(8) C32-Ni4-C49 88.61(9)
O4-Ni4-C49 91.12(8) O3-Ni4-C49 170.97(9)
C13-O1-Ni2 127.10(16) C16-O2-Ni2 126.72(16)
C43-O3-Ni4 126.79(17) C45-O4-Ni4 125.84(18)
358
Crystal structure of 6.9
A clear colourless rod-like specimen of C
16
H
24
B
2
N
8
Ni, approximate dimensions
0.058 mm x 0.066 mm x 0.332 mm, was used for the X-ray crystallographic analysis. The X-ray
intensity data were measured on a Bruker APEX DUO system equipped with a TRIUMPH
curved-crystal monochromator and a MoKα fine-focus tube (λ = 0.71073 Å).
359
Table A.34. Data collection details.
Axis dx/mm 2θ/° ω/° φ/° χ/°
Width
/°
Frames
Time
/s
Wavelength
/Å
Voltage
/kV
Current
/mA
Temper
ature/K
Omega 50.390 30.00 30.00 0.00 54.73 0.50 360 15.00 0.71073 50 30.0 100
Omega 50.390 30.00 30.00 72.00 54.73 0.50 360 15.00 0.71073 50 30.0 100
Omega 50.390 30.00 30.00 144.00 54.73 0.50 360 15.00 0.71073 50 30.0 100
Omega 50.390 30.00 30.00 216.00 54.73 0.50 360 15.00 0.71073 50 30.0 100
Omega 50.390 30.00 30.00 288.00 54.73 0.50 360 15.00 0.71073 50 30.0 100
Phi 50.390 30.00 0.00 0.00 54.73 0.50 720 15.00 0.71073 50 30.0 100
A total of 2520 frames were collected. The total exposure time was 10.50 hours. The
frames were integrated with the Bruker SAINT software package using a SAINT V8.34A (Bruker
AXS, 2013) algorithm. The integration of the data using a monoclinic unit cell yielded a total
of 22807reflections to a maximum θ angle of 30.65° (0.70 Å resolution), of which 2888 were
independent (average redundancy 7.897, completeness = 99.3%, R
int
= 5.21%, R
sig
= 3.36%)
and 2287 (79.19%) were greater than 2σ(F
2
). The final cell constants of a = 6.8569(6) Å, b =
16.0355(13) Å, c = 8.6680(7) Å, β = 98.9260(10)° , volume = 941.54(14) Å
3
, are based upon the
refinement of the XYZ-centroids of 246 reflections above 20 σ(I) with 6.982° < 2θ < 58.68° . Data
were corrected for absorption effects using the multi-scan method (SADABS). The ratio of
minimum to maximum apparent transmission was 0.821. The calculated minimum and maximum
transmission coefficients (based on crystal size) are 0.7220 and 0.9420.
360
The structure was solved and refined using the Bruker SHELXTL Software Package,
using the space group P 1 21/n 1, with Z = 2 for the formula unit, C
16
H
24
B
2
N
8
Ni. The final
anisotropic full-matrix least-squares refinement on F
2
with 132 variables converged at R1 = 3.13%,
for the observed data and wR2 = 7.38% for all data. The goodness-of-fit was 1.037. The largest
peak in the final difference electron density synthesis was 0.434 e
-
/Å
3
and the largest hole was -
0.382 e
-
/Å
3
with an RMS deviation of 0.068 e
-
/Å
3
. On the basis of the final model, the calculated
density was 1.442g/cm
3
and F(000), 428 e
-
.
Table A.35. Sample and crystal data.
Chemical formula C
16
H
24
B
2
N
8
Ni
Formula weight 408.76 g/mol
Temperature 100(2) K
Wavelength 0.71073 Å
Crystal size 0.058 x 0.066 x 0.332 mm
Crystal habit clear colourless rod
Crystal system monoclinic
Space group P 1 21/n 1
Unit cell dimensions a = 6.8569(6) Å α = 90°
b = 16.0355(13) Å β = 98.9260(10)°
c = 8.6680(7) Å γ = 90°
Volume 941.54(14) Å
3
361
Z 2
Density (calculated) 1.442 g/cm
3
Absorption coefficient 1.049 mm
-1
F(000) 428
Table A.36. Data collection and structure refinement.
Diffractometer Bruker APEX DUO
Radiation source fine-focus tube, MoKα
Theta range for data collection 2.54 to 30.65°
Index ranges -9 ≤ h ≤ 9, -22 ≤ k ≤ 22, -12 ≤ l ≤ 12
Reflections collected 22807
Independent reflections 2888 [R(int) = 0.0521]
Coverage of independent reflections 99.3%
Absorption correction multi-scan
Max. and min. transmission 0.9420 and 0.7220
Structure solution technique direct methods
Structure solution program SHELXTL XT 2013/1 (Bruker AXS, 2014)
Refinement method Full-matrix least-squares on F
2
Refinement program SHELXTL XL 2014/7 (Bruker AXS, 2014)
Function minimized Σ w(F
o
2
- F
c
2
)
2
Data / restraints / parameters 2888 / 0 / 132
Goodness-of-fit on F
2
1.037
362
Final R indices 2287 data; I>2σ(I) R1 = 0.0313, wR2 = 0.0682
all data R1 = 0.0481, wR2 = 0.0738
Weighting scheme
w=1/[σ
2
(F
o
2
)+(0.0298P)
2
+0.5497P]
where P=(F
o
2
+2F
c
2
)/3
Largest diff. peak and hole 0.434 and -0.382 eÅ
-3
R.M.S. deviation from mean 0.068 eÅ
-3
Table A.37. Bond lengths (Å ).
B1-N1 1.565(2) B1-N3 1.566(2)
B1-H1B 1.16(2) B1-H2B 1.10(2)
C1-N1 1.3537(19) C1-N2 1.3632(18)
C1-Ni1 1.9038(14) C2-C3 1.351(2)
C2-N1 1.3817(19) C2-H2 0.95
C3-N2 1.3884(19) C3-H3 0.95
C4-N2 1.461(2) C4-H4A 0.98
C4-H4B 0.98 C4-H4C 0.98
C5-N3 1.3539(19) C5-N4 1.3679(19)
C5-Ni1 1.9003(15) C6-C7 1.346(2)
C6-N4 1.385(2) C6-H6 0.95
C7-N3 1.382(2) C7-H7 0.95
C8-N4 1.460(2) C8-H8A 0.98
C8-H8B 0.98 C8-H8C 0.98
363
Ni1-C5 1.9004(15) Ni1-C1 1.9038(14)
Table A.38. Bond angles (° ).
N1-B1-N3 104.57(12) N1-B1-H1B 109.1(10)
N3-B1-H1B 111.3(9) N1-B1-H2B 108.2(10)
N3-B1-H2B 108.5(10) H1B-B1-H2B 114.7(14)
N1-C1-N2 105.73(12) N1-C1-Ni1 122.37(11)
N2-C1-Ni1 131.89(11) C3-C2-N1 107.71(13)
C3-C2-H2 126.1 N1-C2-H2 126.1
C2-C3-N2 106.52(13) C2-C3-H3 126.7
N2-C3-H3 126.7 N2-C4-H4A 109.5
N2-C4-H4B 109.5 H4A-C4-H4B 109.5
N2-C4-H4C 109.5 H4A-C4-H4C 109.5
H4B-C4-H4C 109.5 N3-C5-N4 105.73(13)
N3-C5-Ni1 121.77(11) N4-C5-Ni1 132.48(11)
C7-C6-N4 106.46(14) C7-C6-H6 126.8
N4-C6-H6 126.8 C6-C7-N3 108.17(14)
C6-C7-H7 125.9 N3-C7-H7 125.9
N4-C8-H8A 109.5 N4-C8-H8B 109.5
H8A-C8-H8B 109.5 N4-C8-H8C 109.5
H8A-C8-H8C 109.5 H8B-C8-H8C 109.5
C1-N1-C2 109.97(13) C1-N1-B1 121.97(12)
364
C2-N1-B1 127.85(13) C1-N2-C3 110.07(13)
C1-N2-C4 125.67(12) C3-N2-C4 124.26(13)
C5-N3-C7 109.57(13) C5-N3-B1 122.64(13)
C7-N3-B1 127.48(13) C5-N4-C6 110.05(13)
C5-N4-C8 126.19(13) C6-N4-C8 123.75(14)
C5-Ni1-C5 180.0 C5-Ni1-C1 87.17(6)
C5-Ni1-C1 92.83(6) C5-Ni1-C1 92.83(6)
C5-Ni1-C1 87.17(6) C1-Ni1-C1 180.0
365
Crystal structure of 6.10
A clear colourless plate-like specimen of C
16
H
20
I
2
N
8
Ni was used for the X-ray
crystallographic analysis. The X-ray intensity data were measured on a Bruker APEX DUO
system equipped with a multi-layer optics monochromator and a CuKα IuS microsource (λ =
1.54178 Å ).
366
Table A.39. Data collection details.
Axis dx/mm 2θ/° ω/° φ/° χ/°
Width
/°
Frame
s
Time/
s
Wavelengt
h/Å
Volt
age/
kV
Curren
t/mA
Tempe
rature/
K
Omega 40.489 103.16 -79.84 120.00 54.74 1.00 186 20.00 1.54184 45 0.6 100
Omega 40.489 -73.16 -257.16 270.00 54.74 1.00 188 20.00 1.54184 45 0.6 100
Omega 40.489 103.16 -79.84 0.00 54.74 1.00 186 20.00 1.54184 45 0.6 100
Omega 40.489 -31.86 -214.87 153.00 54.74 1.00 186 20.00 1.54184 45 0.6 100
Omega 40.489 103.16 -79.84 -150.00 54.74 1.00 186 20.00 1.54184 45 0.6 100
Omega 40.489 103.16 -79.84 60.00 54.74 1.00 186 20.00 1.54184 45 0.6 100
Omega 40.489 103.16 -79.84 -60.00 54.74 1.00 186 20.00 1.54184 45 0.6 100
Omega 40.489 -31.86 -214.87 0.00 54.74 1.00 186 20.00 1.54184 45 0.6 100
Omega 40.489 103.16 -79.84 -180.00 54.74 1.00 186 20.00 1.54184 45 0.6 100
Omega 40.489 103.16 -79.84 90.00 54.74 1.00 186 20.00 1.54184 45 0.6 100
Omega 40.489 103.16 -79.84 150.00 54.74 1.00 186 20.00 1.54184 45 0.6 100
Omega 40.489 103.16 -79.84 30.00 54.74 1.00 186 20.00 1.54184 45 0.6 100
Omega 40.489 -31.86 -214.87 -156.00 54.74 1.00 186 20.00 1.54184 45 0.6 100
Omega 40.489 103.16 -79.84 -30.00 54.74 1.00 186 20.00 1.54184 45 0.6 100
Omega 40.489 88.16 -95.84 90.00 54.74 1.00 188 20.00 1.54184 45 0.6 100
A total of 2794 frames were collected. The total exposure time was 15.52 hours. The frames
were integrated with the Bruker SAINT software package using a SAINT V8.34A (Bruker AXS,
2013) algorithm. The integration of the data using a triclinic unit cell yielded a total
of 7145 reflections to a maximum θ angle of 68.36° (0.83 Å resolution), of which 1881 were
independent (average redundancy 3.799, completeness = 92.8%, R
int
= 4.48%, R
sig
= 4.05%)
367
and 1719 (91.39%) were greater than 2σ(F
2
). The final cell constants
of a = 8.347(2) Å, b = 8.4576(14) Å, c = 8.7980(12) Å, α = 113.849(11)°, β = 101.699(15)°, γ
= 91.550(18)° , volume = 552.13(19) Å
3
, are based upon the refinement of the XYZ-centroids
of 1122 reflections above 20 σ(I) with 11.30° < 2θ < 136.3° . Data were corrected for absorption
effects using the multi-scan method (SADABS).
The structure was solved and refined using the Bruker SHELXTL Software Package, using
the space group P -1, with Z = 1 for the formula unit, C
16
H
20
I
2
N
8
Ni. The final anisotropic full-
matrix least-squares refinement on F
2
with 126 variables converged at R1 = 4.35%, for the
observed data and wR2 = 12.33% for all data. The goodness-of-fit was 1.144. The largest peak in
the final difference electron density synthesis was 1.968 e
-
/Å
3
and the largest hole was -0.709 e
-
/Å
3
with an RMS deviation of 0.228 e
-
/Å
3
. On the basis of the final model, the calculated density
was 1.915 g/cm
3
and F(000), 306 e
-
.
Table A.40. Sample and crystal data.
Chemical formula C
16
H
20
I
2
N
8
Ni
Formula weight 636.91 g/mol
Temperature 100(2) K
Wavelength 1.54178 Å
Crystal habit clear colourless plate
Crystal system triclinic
Space group P -1
368
Unit cell dimensions a = 8.347(2) Å α = 113.849(11)°
b = 8.4576(14) Å β = 101.699(15)°
c = 8.7980(12) Å γ = 91.550(18)°
Volume 552.13(19) Å
3
Z 1
Density (calculated) 1.915 g/cm
3
Absorption coefficient 23.362 mm
-1
F(000) 306
Table A.41. Data collection and structure refinement.
Diffractometer Bruker APEX DUO
Radiation source IuS microsource, CuKα
Theta range for data collection 5.45 to 68.36°
Index ranges -10 ≤ h ≤ 9, -10 ≤ k ≤ 10, -10 ≤ l ≤ 10
Reflections collected 7145
Independent reflections 1881 [R(int) = 0.0448]
Coverage of independent
reflections
92.8%
Absorption correction multi-scan
Structure solution technique direct methods
Structure solution program SHELXTL XT 2014/4 (Bruker AXS, 2014)
Refinement method Full-matrix least-squares on F
2
369
Refinement program SHELXTL XL 2014/7 (Bruker AXS, 2014)
Function minimized Σ w(F
o
2
- F
c
2
)
2
Data / restraints / parameters 1881 / 0 / 126
Goodness-of-fit on F
2
1.144
Δ/σ
max
0.001
Final R indices 1719 data; I>2σ(I) R1 = 0.0435, wR2 = 0.1199
all data R1 = 0.0473, wR2 = 0.1233
Weighting scheme
w=1/[σ
2
(F
o
2
)+(0.0790P)
2
+1.0839P]
where P=(F
o
2
+2F
c
2
)/3
Largest diff. peak and hole 1.968 and -0.709 eÅ
-3
R.M.S. deviation from mean 0.228 eÅ
-3
Table A.42. Bond lengths (Å )
C1-N1 1.347(8) C1-N2 1.356(8)
C1-Ni1 1.907(6) C2-C3 1.327(10)
C2-N1 1.383(9) C2-H2 0.95
C3-N2 1.379(9) C3-H3 0.95
C4-N1 1.462(8) C4-H4A 0.98
C4-H4B 0.98 C4-H4C 0.98
C5-N4 1.353(8) C5-N3 1.358(8)
C5-Ni1 1.896(6) C6-C7 1.336(10)
370
C6-N3 1.391(8) C6-H6 0.95
C7-N4 1.394(8) C7-H7 0.95
C8-N3 1.446(8) C8-H8A 0.98
C8-H8B 0.98 C8-H8C 0.98
Ni1-C5 1.896(6) Ni1-C1 1.907(6)
Table A.43. Bond angles (° ).
N1-C1-N2 103.5(5) N1-C1-Ni1 128.6(4)
N2-C1-Ni1 127.9(4) C3-C2-N1 106.7(6)
C3-C2-H2 126.6 N1-C2-H2 126.6
C2-C3-N2 106.7(6) C2-C3-H3 126.6
N2-C3-H3 126.6 N1-C4-H4A 109.5
N1-C4-H4B 109.5 H4A-C4-H4B 109.5
N1-C4-H4C 109.5 H4A-C4-H4C 109.5
H4B-C4-H4C 109.5 N4-C5-N3 104.1(5)
N4-C5-Ni1 127.6(4) N3-C5-Ni1 128.2(4)
C7-C6-N3 107.2(6) C7-C6-H6 126.4
N3-C6-H6 126.4 C6-C7-N4 106.2(5)
C6-C7-H7 126.9 N4-C7-H7 126.9
N3-C8-H8A 109.5 N3-C8-H8B 109.5
H8A-C8-H8B 109.5 N3-C8-H8C 109.5
H8A-C8-H8C 109.5 H8B-C8-H8C 109.5
371
C1-N1-C2 111.6(5) C1-N1-C4 125.2(6)
C2-N1-C4 123.1(6) C1-N2-C3 111.5(5)
C5-N3-C6 110.9(5) C5-N3-C8 125.0(5)
C6-N3-C8 124.0(5) C5-N4-C7 111.5(5)
C5-Ni1-C5 180.0 C5-Ni1-C1 90.5(2)
C5-Ni1-C1 89.5(2) C5-Ni1-C1 89.5(2)
C5-Ni1-C1 90.5(2) C1-Ni1-C1 180.0
Abstract (if available)
Abstract
This thesis focuses on new catalytic chemistry of hydride manipulation for processes such as hydrogenation and dehydrogenation. The research projects mostly involve design and synthesis of novel catalysts as well as mechanistic analysis of catalytic processes. Such mechanistic understandings lead to improvement in catalysis in terms of efficiency, selectivity, and catalyst longevity. These are introduced in Chapter one. ❧ Chapter two proposes and provides evidence for a mechanistic model for three-stage dehydrogenation of ammonia−borane (AB) catalyzed by Shvo’s cyclopentadienone-ligated ruthenium complex. In particular, we characterized the mechanism of catalyst deactivation: a borazine-mediated hydroboration of the active catalyst to afford less reactive ruthenium species. We are inspired by these findings to discover catalysts that do not suffer from the same deactivation chemistry So doing, we either use a “semi-site protection strategy” or introduce a borate ligand into the catalyst. ❧ Chapter three shows complex {[(κ³-(N,N,O)-py₂B(Me)OH)Ru(NCMe)₃]}⁺ TfO⁻ is not a ligand-metal bifunctional catalyst and in case of alcohol oxidation, the mechanism most likely involves reactivity only at the ruthenium center. Further we present a novel ruthenium bis(pyrazolyl)borate scaffold that enables cooperative reactivity in which boron and ruthenium centers work in concert to effect selective nitrile reduction. The pre-catalyst compound {[κ³-(1-pz)₂HB(N=CHCH₃)]Ru(cymene)}⁺ TfO⁻ (pz = pyrazolyl) was synthesized using readily-available materials through a straightforward route, thus making it an appealing catalyst for a number of reactions. ❧ In chapter four, a prolific homogeneous iridium catalyst is introduced for selective dehydrogenation of neat formic acid under mild conditions. Mechanistic study on this catalysis reveals that the promising reactivity is enabled by in situ formation of a novel dimeric iridium species {[(P-N)Ir(CH₂Cl₂)(H)]₂(μ-H)(μ²-OCHO)₂}+ TfO⁻. ❧ Chapter five describes the synthesis and reactivity of a very robust iridium catalyst for glycerol to lactate conversion. The high reactivity and selectivity of this catalyst enable a sequence for the conversion of biodiesel waste stream to lactide monomers for the preparation of poly(lactic acid). Furthermore, experimental data collected with this system provide a general understanding of its reactive mechanism. ❧ Chapter six recounts the design and synthesis of structurally novel di(carbene)-supported nickel species, which are efficient catalysts for the room-temperature reduction of CO₂ to methanol in the presence of sodium borohydride. The catalysts feature unprecedented stability, particularly for a base metal catalyst, enabling > 1,100,000 turnovers of CO₂. Moreover, while other systems involve more expensive or air-sensitive borane reagents, sodium borohydride is inexpensive and easily handled. Further, effecting reduction in the presence of water enables direct access to methanol without a subsequent hydrolysis step. Preliminary mechanistic data collected for the catalysis are most consistent with a mononuclear nickel active species that mediates rate-determining reduction of a boron formate.
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Asset Metadata
Creator
Lu, Zhiyao
(author)
Core Title
New bifunctional catalysts for ammonia-borane dehydrogenation, nitrile reduction, formic acid dehydrogenation, lactic acid synthesis, and carbon dioxide reduction
School
College of Letters, Arts and Sciences
Degree
Doctor of Philosophy
Degree Program
Chemistry
Publication Date
06/20/2016
Defense Date
05/10/2016
Publisher
University of Southern California
(original),
University of Southern California. Libraries
(digital)
Tag
catalysis,dehydrogenation,hydrogenation,OAI-PMH Harvest,synthesis,transition-metal catalysts
Format
application/pdf
(imt)
Language
English
Contributor
Electronically uploaded by the author
(provenance)
Advisor
Williams, Travis J. (
committee chair
), Petruska, John (
committee member
), Prakash, G. K Surya (
committee member
)
Creator Email
areslv@gmail.com,zhiyaolu@usc.edu
Permanent Link (DOI)
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Tags
catalysis
dehydrogenation
hydrogenation
synthesis
transition-metal catalysts