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Ruthenium catalysis for ammonia borane dehydrogenation and dehydrative coupling
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Ruthenium catalysis for ammonia borane dehydrogenation and dehydrative coupling
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Content
Copyright 2016 Xingyue Zhang
RUTHENIUM CATALYSIS FOR AMMONIA BORANE DEHYDROGENATION
AND DEHYDRATIVE COUPLING
by
Xingyue Zhang
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMISTRY)
August 2016
ii
Dedication
To Mom, Dad, Ben, Noodles, and Cheeto
iii
Acknowledgments
When I joined the Williams’ Group, I was not sure what I wanted out of my Ph.D.
career. I didn’t know much about catalysis and my organic chemistry needed serious
brushing up, but I knew one thing: I wanted a supportive lab environment that’s
conducive to learning. That is exactly what I received in my time with Professor Travis
J. Williams as my advisor. I am extremely grateful for his guidance and encouragement
throughout my years here. I ended up learning much more than just chemistry from him,
such as how to become a great mentor and a productive member of society.
I would also like to thank the Williams lab members past and present for their
help in chemistry and in life, and their friendship. I have made valuable friends here and
my Ph.D. wouldn’t have been the same without them. Thanks to Dr. Brian C. Conley
and Dr. Megan Pennington-Boggio for their continued guidance on chemistry and GC-
MS even after their departure from USC. Thanks to Dr. Anna Dawsey and Dr. Vincent
Li for the best time sharing an office and lab, bobble heads, and Sunday pig-outs. Thanks
to Zhiyao Lu, Jeff Celaje, Ivan Demianets, and Valery Cherepakhin for all the Happy
Monday’s and out of lab excursions. Thanks to the undergraduates Blaine Bolibol,
Forrest Zhang, Elyse Kedzie, Lisa Kam, Lena Foellmer, and Ryan Trerise for keeping me
amused by telling me about USC undergrad antics and joining our antics. A very special
thanks to Lisa Kam and Lena Foellmer for putting up with me as their mentor. Thanks to
Kim Nguyen and Jonathan Lo for being honorary Williams lab members and keeping my
cats alive.
iv
I would not be where I am today without the unconditional support of my parents
who have always let me choose my own path and trusted me to make the correct
decisions in life. Thanks for all your sacrifice and hard work to bring me to the U.S. to
give me a more successful life. I could not have gone through a Ph.D. without my fiancé
Ben Decato at my side going through the same difficulties as me. His upbeat personality
and unwavering comfort lifts me up from the most stressful of times. A furry thanks to
my cats Noodles and Cheeto for still loving me after long days of failed experiments.
I would like to thank my committee members Professor G. K. Surya Prakash,
Professor Karl Christe, and Professor Noah Malmstadt for their time and valuable
discussions. Especially Professor Smaranda Marinescu who played a huge role in getting
me employed. A special thanks to Allan Kershaw for teaching me the ins and outs of the
NMR’s and being there to solve all my NMR problems. A big thanks to the staff of LHI
and the USC Department of Chemistry including Dr. Robert Anizfeld, David Hunter,
Jessie May, Carole Phillips, Marie de la Torre, Dr. Frank Devlin, Dr. Ralf Haiges,
Michele Dea, Susan Peterson and Magnolia Benitez for keeping things running smoothly.
I would like to thank Dr. Kyung Jung and Dr. Jennifer Moore for overseeing my time as a
teaching assistant in undergraduate organic chemistry, and Jessica Margot Dutton for
organizing the Sonosky Summer Fellowship.
v
Table of Contents
Dedication ii
Acknowledgments iii
List of Tables viii
List of Figures ix
List of Schemes xii
Preparative Procedures xiv
Abstract xx
Chapter 1. Introduction of Hydrogen Fuel, Ammonia Borane, and Shvo’s Catalyst 1
1.1 Alternative Energy and Hydrogen as a Clean Fuel Source 1
1.2 Ammonia Borane- A Chemical Storage Carrier of Hydrogen 2
1.3 Shvo’s Catalyst 5
1.4 Conclusions 6
1.5 References 6
Chapter 2. Ammonia Borane Dehydrogenation by Shvo’s Catalyst and its
Derivatives: Reactivity and Mechanism 12
2.1 Introduction 12
2.2 Reactivity of Shvo’s Catalyst with Ammonia Borane 12
2.3 Mechanism of Shvo Catalyzed AB Dehydrogenation 14
2.4 Shvo Pyridine and Amine Derivatives and Their Reactivity with AB 24
2.5 Conclusions 31
2.6 References 32
vi
Chapter 3. Breaking the Third Equivalent Barrier in Ammonia Borane 38
3.1 Introduction to the Dehydrogenation of the Third Equivalent of AB 38
3.2 Borazine Introduction 39
3.3 Effects of TMEDA, Calcium, and Water on the Third Equivalent of Hydrogen in AB
Dehydrogenation Catalyzed by TMEDA-Supported Shvo Analog 40
3.4 Development and Execution of (phen)Ru(CO)2(OAc)2-Catalyzed AB Dehyrogenation
through the Third Equivalent 48
3.5 Conclusions 55
3.6 References 56
Chapter 4. Transition Metal Catalyzed Decarboxylation of Formic Acid and
Ammonium Boroformate 60
4.1 Introduction to Formic Acid: A Method for Spent Fuel Regeneration 60
4.2 AB Spent Fuel Regeneration Cycle Using Formic Acid 61
4.3 Derivatization of Borazine by Formic Acid 62
4.4 Catalytic Decarboxylation of Formic Acid 64
4.5 Catalytic Decarboxylation of Ammonia Boroformate 69
4.6 Conclusions 78
4.7 References 78
Chapter 5. Acceptorless Dehydrogenation and Dehydrative Coupling by 2-((di-tert-
butylphosphino)methyl)pyridine-Supported Ru and Ir Catalysts 81
5.1 Introduction to Acceptorless Dehydrogenation and Dehydrative Coupling
Reactions 81
5.2 Milstein Pincer Catalysts for AD and Coupling Reactions 82
vii
5.3 Bidentate Phosphorous-Nitrogen Ligand Design and Catalyst Syntheses 84
5.4 AD of Alcohols to Ketones and Esters, and Dehydrative Coupling of Alcohols to
Ethers 86
5.5 Iridium Catalyzed Guerbet-like Coupling of Alcohols to Longer Chain Alcohols 88
5.6 Dehydrative Coupling of Alcohols and Amines to Alkylated Amines 89
5.7 Mechanistic Insight into Dehydrative Coupling of Alcohols and Amines 94
5.8 Conclusions 95
5.9 References 95
Chapter 6. Experimental Procedures and Spectral Data 98
6.1 General Procedures 98
6.2 Chapter 2 Experimental and Spectral Data 100
6.3 Chapter 3 Experimental and Spectral Data 142
6.4 Chapter 4 Experimental and Spectral Data 162
6.5 Chapter 5 Experimental and Spectral Data 168
6.6 References 189
viii
List of Tables
Table 3.1. List of solvent conditions that affect AB dehydrogenation. 46
Table 5.1. Secondary benzyl alcohol (1-phenylethanol, 5.8) and amine
(5.12a-16a) coupling. 92
Table 5.2. Primary benzyl alcohol (5.9) and amine (5.12a-18a) coupling. 93
Table 6.1. Synthetic optimization for 2.20. 111
Table 6.2. Crystal data and structure refinement for 2.15. 128
Table 6.3. Bond lengths (Å) for 2.15. 129
Table 6.4. Bond angles (°) for 2.15. 130
Table 6.5. Crystal data and structure refinement for 2.21. 136
Table 6.6. Bond lengths (Å) for 2.21. 137
Table 6.7. Bond angles (°) for 2.21. 138
Table 6.8. Amount of Catalyst, FA, and ACN for FA dehydrogenation. 164
Table 6.9. Amount of Catalyst, FA, and DMSO for ABF dehydrogenation. 167
ix
List of Figures
Figure 1.1. Shvo-derived ruthenium catalysts from the Williams Group that
produce 2 or more equivalents of H2 from AB. 4
Figure 2.1. (left) Eudiometer data showing production of hydrogen gas in the
presence of 5.0 mol% 2.1 and 2.0 mol% ethanol in 2:1 diglyme/benzene-d6 at 70
°C. (right)
11
B NMR data showing consumption of AB in the presence of 2.5
mol% 2.1 in a sealed J-Young NMR tube. 14
Figure 2.2. AB dehydrogenation with 2.1 and 2.12. 0.25 mol AB and 0.035 mol
[Ruatom] are added to 0.6 mL diglyme/benzene-d6. 20
Figure 2.3. 1,10-phenanthroline 2.13, a catalyst poison. 22
Figure 2.4. [AB] Dehydrogenation with 2.1 in the presence of 1,10-
phenanthroline, 2.13. 23
Figure 2.5. Catalytic AB dehydrogenation Left: AB consumption (
11
B NMR)
catalyzed by 2.1, 2.15, 2.16, and 2.17. Right: H2 release by eudiometry with 2.17. 27
Figure 2.6. Left: Metal catalyzed dehydrogenation of 2.18. Right:
Dehydrogenation of AB (blue squares) and 2.18 (red circles) by Shvo’s catalyst
2.1. 28
Figure 2.7. Left: dehydrogenation of AB by 2.1 (green circles),
(phen)RuCl2(CO)2 (2.23) with 2 equiv. TlOTf (blue diamonds), and 2.15 (red
squares). Right: dehydrogenation of AB by 2.1 (green circles), 2.1 with 1 eq. of
phen (black diamonds), and (phen)RuCl2(CO)2 with 2 equiv. TlOTf (blue
diamonds). 31
Figure 3.1. Catalysts that dehydrogenate AB over 2.5 equiv. 39
Figure 3.2. H2 release from 0.21 M (AB) catalyzed by 2.8% 3.3 (5.6% Ru atom)
with 1.1 equivalents of TMEDA relative to AB in ketyl dried tetraglyme. 42
Figure 3.3. H2 release from 0.21 M (AB) catalyzed by 2.8% 3.3 (5.6% Ru atom)
with 1.1 equivalents of TMEDA relative to AB in tetraglyme containing 4% Ca
+2
(by ICP) (Solvent 1). 43
Figure 3.4.
11
B spectra comparison of AB dehydrogenation with 0.21 M (AB),
2.8 mol% 3.3, and 1.1 equivalents of TMEDA under various solvents. 45
Figure 3.5. Comparison of H2 release in three solvent systems at 0.21 M (AB),
3.8 mol% 3.3 and 1.1 equiv. of TMEDA. 47
x
Figure 3.6. Left: H2 production from 1% of 3.6 at 70 C in diglyme releasing 2.7
equiv. Right:
11
B kinetic profile of the consumption of AB catalyzed by 1% of 3.6
in 2:1 diglyme/benzene-d6. 50
Figure 3.7. Reaction of 3.6 with borazine (3.1) in 2:1 diglyme/benzene-d6. 52
Figure 3.8. a. Comparison of end-of-reaction
11
B NMR spectra under
representative conditions. b. Peak height of borazine over time catalyzed by 10%
Ru atom of 3.6 (left) and 3.7 (right). 53
Figure 3.9. Hg addition homogeniety test in 2:1 diglyme/benzene-d6 at 70 C.
Black circles: 10 mol% of 3.6 with Hg. Blue squares: 10% of 3.6. 55
Figure 4.1.
1
H NMR of borazine reaction with FA in CD3CN at room
temperature immediately following addition. 63
Figure 4.2.
1
H NMR of 4.8 (20 mol %) catalyzed FA decarboxylation in CD3CN
for 3 hours and 50 min at 70 °C. 66
Figure 4.3. Graph summarizing optimal amount of ACN for the decarboxylation
of FA by 1.0 mol % of catalyst 4.8 at 70 °C. 67
Figure 4.4. Catalysts screened for the decarboxylation of FA in addition to 4.8. 68
Figure 4.5. Catalyst screening for best conversion of FA at 1% catalyst loading, 1
mL of ACN at 70 °C. 69
Figure 4.6. NMR spectra of ABF in CD3OH. Top:
1
H NMR. Middle:
13
C NMR.
Bottom:
11
B NMR. 70
Figure 4.7. Catalysts screened to decarboxylate ABF. 71
Figure 4.8. ABF decarboxylation by various catalysts (a-e) at 5% loading, 70 °C
in DMSO-d6. 72-76
Figure 5.1. a. Pincer scaffold on a metallic species where M = metal. b. Examples
of Milstein’s ruthenium pincer catalysts. 83
Figure 5.2. Iridium (5.4 and 5.5) and ruthenium (5.6) complexes supported with
PN ligand. 85
Figure 5.3. Synthesis of ruthenium 5.6. 86
Figure 6.1.
1
H NMR spectra of syntheses of 2.21 taken in C6D6. 112
xi
Figure 6.2. Reaction between 2.8 and 2,2’-bipyridine. 120
Figure 6.3. Reaction between 2.8 and 1,10-phenanthroline. 121
Figure 6.4. Reaction between 2.8 and TMEDA. 122
Figure 6.5.
19
F spectra of end of AB dehydrogenation reaction catalyzed by 2.17. 123
Figure 6.6.
11
B spectra of end of AB dehydrogenation reaction catalyzed by
2.17. 123
Figure 6.7. AB dehydrogenation by catalytic amount of 4-DMAP (10%, 1:2
C6D6: diglyme) at 70 C. 124
Figure 6.8. AB dehydrogenation by catalytic amount of TMEDA (5%, 1:2 C6D6:
diglyme) at 70 C 124
Figure 6.9. AB dehydrogenation by catalytic amount of 1,10-phenanthroline (5%,
1:2 C6D6: diglyme) at 70 C. 125
Figure 6.10. X-Ray ORTEP of 2.15 (50% probability). 127
Figure 6.11. X-Ray ORTEP of 2.21 (50% probability). 135
Figure 6.12. Kinetics for AB dehydrogenation catalyzed by 3.6. 153
Figure 6.13. Kinetics for AB dehydrogenation catalyzed by 3.6 after
catalyst/solvent system was exposed to air. 155
Figure 6.14.
11
B spectra borazine in diglyme/benzene-d6, control reaction of no
catalyst 3.6. 156
Figure 6.15.
11
B spectra of end of AB dehydrogenation reaction catalyzed by 3.6. 157
Figure 6.16.
1
H spectra of end of AB dehydrogenation reaction catalyzed by 3.6. 157
Figure 6.17. Top:
11
B spectra of end of AB dehydrogenation reaction catalyzed
by 3.6 after 3.6 and solvents were exposed to air and sonicated for 20 min.
Bottom:
11
B spectra of end of AB dehydrogenation reaction catalyzed by 3.6. 158
Figure 6.18.
1
H spectra of end of AB dehydrogenation reaction catalyzed by 3.6
after 3.6 and solvents were exposed to air and submerged in an ultrasonic cleaning
bath for 20 min. 159
Figure 6.19. NMR spectra of ABF in CD3OH. 166
xii
List of Schemes
Scheme 1.1. Non-hydrolytic hydrogen release from AB. 4
Scheme 1.2. Shvo’s catalyst 1.7 and its oxidizing monomer 1.13 and reducing
monomer 1.14 carrying out transfer hydrogenation. 6
Scheme 2.1. Dehydrogenation of AB with Shvo’s Catalyst, 2.1. 13
Scheme 2.2. AB dehydrogenation by Shvo’s Catalyst 2.1 and the boron-nitrogen
byproducts produced. 13
Scheme 2.3. A mechanistic scheme for catalyst initiation. B: Rapid formation of
2.7 from 2.8. 15
Scheme 2.4. Mechanistic cycle of fast catalysis of AB dehydrogenation aided by
EtOH. 16
Scheme 2.5. Deactivation of the Shvo’s catalyst by borazine via hydroboration. 17
Scheme 2.6. A: Proposed formation of μ-aminodiborane 2.2. B: Separate
synthesis of Shvo-NH3 ligated species 2.12. 19
Scheme 2.7. Shvo catalyzed AB dehydrogenation mechanism. 21
Scheme 2.8. 1,10-phenanthroline “semi-site protection” mechanism. 24
Scheme 2.9. A. Synthesis of pyridine complexes 2.15-2.17. B. ORTEP diagram
of 2.15. 25
Scheme 2.10. Synthesis and Structure of 2.21. 30
Scheme 3.1. Mechanism of hydrogen evolution from AB. 38
Scheme 3.2. Synthesis of soluble calcium adduct Ca(OTf)2 Tetraglyme 3.4. 46
Scheme 3.3. Shvo’s oxidized form’s loss of CPD ligand in the presence of a
bidentate ligand such as phen. 49
Scheme 3.4. Synthesis of 3.6. 49
Scheme 4.1. Dehydrogenation/Decarboxylation of formic acid into H2 and CO2. 60
Scheme 4.2. Proposed spent fuel regeneration cycle using FA. 61
xiii
Scheme 4.3. Goal of treating borazine and polyborazylene with FA to form ABF. 62
Scheme 4.4. Proposed ruthenium catalyst appended boron Lewis acid assisted FA
decarboxylation. 64
Scheme 4.5. Hypothesized 4.8-catalyzed FA decarboxylation. 65
Scheme 4.6. Synthesis of ABF (4.2). 69
Scheme 4.7. Hypothesized decarboxylation and decomplexation of ABF, and
eventual dehydrogenation of AB. 71
Scheme 5.1. AD versus traditional dehydrogenation or oxidations. 82
Scheme 5.2. “Borrowing Hydrogen” mechanism of an alcohol-amine couple
reaction. 82
Scheme 5.3. Hydrogen splitting of pincer type catalysts in the presence of base. 84
Scheme 5.4. Synthesis of PN ligand supported iridium Cp* complex 5.5. 85
Scheme 5.5. Reactions of Catalyst 5.6 with 1-Phenylethanol (5.8). 87
Scheme 5.6. Coupling reaction of benzyl alcohol (5.9) without base. 87
Scheme 5.7. Reactions of Catalyst 5.6 with 1-octanol (5.10). 87
Scheme 5.8. C-C coupling of 1-octanol catalyzed by 5.5. 88
Scheme 5.9. Guerbet reaction. 89
Scheme 5.10. Guerbet reaction of ethanol to butanol catalyzed by 5.5. 89
Scheme 5.11. Over-alkylation of benzyl alcohol at higher temperatures. 90
Scheme 5.12. Proposed Ru-benzyl mechanism. 94
xiv
Preparative Procedures
Ruthenium complex 2.15 101
Ruthenium complex 2.16 103
Ruthenium complex 2.17 105
Ruthenium complex 2.20 and 3.3 108
xv
Ca(OTf)2 Tetraglyme 3.4 144
Ruthenium polymer [RuCl2(CO)2]n 146
[(phen)RuCl2(CO)2] 3.5 147
[(phen)Ru(OAc)2(CO)2] 3.6 148
Ammonium Boroformate 4.2 165
xvi
Iridium complex 5.5 168
Ruthenium complex 5.6 169
Oxidation of 5.8 to acetophenone 170
Coupling of 5.8 to ethers and acetophenone 171
Coupling of 5.9 to benzylether 172
Coupling of 5.10 to octyl octanoate ester 173
xvii
Coupling of 5.10 to dioctyl ether 174
Guerbet Coupling of 1-butanol 175
Guerbet Coupling of EtOH to 1-butanol 176
Amine 5.12b 177
Amine 5.13b 178
Amine 5.14b 179
xviii
Amine 5.15b 180
Amine 5.16b 181
Amine 5.12c 182
Amine 5.13c 183
Amine 5.14c 184
Amine 5.15c 185
xix
Amine 5.16c 186
Amine 5.17c 187
Amine 5.18c 188
xx
Abstract
In the field of alternative energy, hydrogen gas has become a popular fuel source
because of its clean waste stream and abundance. However, one of the greatest challenges
in using H2 as a fuel source is finding a safe, efficient, and inexpensive method for its
storage. Ammonia borane (AB) is a solid hydrogen storage material that has garnered
attention for its high hydrogen weight density of 19.6%. Shvo’s catalyst is a ruthenium
dimer capable of hydrogenation and dehydrogenation of substrates via metal-ligand
cooperation. AB dehydrogenation with Shvo’s catalyst releases three possible equivalents
of H2, and the mechanism of this reaction has been extensively studied, leading to a
detailed understanding of the role of borazine in the dehydrogenation. Borazine is a
poison not only for Shvo’s catalyst, but also for fuel cells.
Through the close study of independent syntheses of Shvo derivatives, a
protective mechanism was presented wherein catalyst deactivation by borazine was
prevented by coordination of a potentially inhibiting ligand. These studies showed how
bidentate nitrogen ligands can transform Shvo’s catalyst into more reactive species for
dehydrogenation of AB. Further studies with a tetramethylethylenediamine supported
Shvo derivative and optimized ruthenium catalyst phenRu(OAc)2(CO)2 provided highly
efficient H2 release while avoiding the accumulation of borazine.
While hydrogen release from AB was optimized, the issue of fuel release was also
of interest. After an AB dehydrogenation cycle, the most environmentally friendly
solution to would be to reuse the spent fuel generated by the reaction. A means to
regenerate spent fuel was to use formic acid and a transition metal catalyst to digest the
B-N byproducts produced at the end of AB dehydrogenation into formates. Therefore, the
xxi
dehydrogenation of ammonium boroformate by various catalysts was studied by
1
H
NMR.
In the development of other green, catalytic reactions, acceptorless
dehydrogenations (AD) and its dehydrative coupling reactions were studied. An
environmentally friendly reaction involving hydrogen transfer catalyzed by a
pyridylphosphine-supported ruthenium complex has rapidly developed into a powerful
method for coupling of alcohols and amines to alkylated amines. Alkylation of amines is
of great importance in synthetic routes because they are often intermediates in a wide
range of useful compounds in the pharmaceutical and agriculture industries. Moreover,
this reaction is atom economical and environmentally friendly because it can be
accomplished in the absence of solvent and produces only water as a byproduct.
1
Chapter 1. Introduction of Hydrogen Fuel, Ammonia Borane, and Shvo’s Catalyst
1.1 Alternative Energy and Hydrogen as a Clean Fuel Source
As the world continues to burn away at the planet’s fossil fuels generating
greenhouse gases such as carbon dioxide (CO2), carbon monoxide (CO) as well as other
pollutants, there has been a surge in the search for new fuels. While the types of
alternative energy include hydroelectricity, nuclear energy, wind energy, solar energy,
geothermal energy, and biofuel and ethanol, hydrogen has attracted attention for being a
clean and abundant energy carrier.
Hydrogen (H2) is one of the most abundant elements in nature, making up more
than 90% of all known matter, and can be produced from many feedstocks anywhere in
the world. Once the hydrogen gas is produced, it can be used in a fuel cell to generate
electric energy that powers electrical devices such as computers or electric cars. One of
the most attractive features of H2 as a fuel source is that using hydrogen does not produce
carbon monoxide or carbon dioxide, the greenhouse gases that contribute to pollution and
global warming.
One of the greatest challenges to H2 as a fuel source is finding a safe, efficient,
and inexpensive method of storage. If hydrogen were ever to be economized and used in
society, it must be stored either as a high-pressure gas, a liquid, or a solid. High-pressure
gas poses the dangers of explosions from pressurized vessels, especially next to an
internal combustion engine in vehicles, as hydrogen gas is extremely flammable. Storing
hydrogen gas in its liquid form can only be achieved at cryogenic temperatures of -253
°C and the use of expensive materials that withstand these temperature conditions.
1
Both
of these methods are not very efficient, since hydrogen in its gaseous and liquid forms are
2
not dense and would require large amount of materials and physical space to store a
comparable amount to common fuels. Therefore, attention has shifted to chemical
storage where hydrogen atoms are stored in the form of an air and water stable chemical
until its controlled release. One of the most efficient materials for chemical storage of H2
is ammonia borane, and the majority of this work will describe efforts to efficiently
release hydrogen from this material.
1.2 Ammonia Borane- A Chemical Storage Carrier of Hydrogen
Ammonia borane (AB, 1.1) or H3N-BH3 is a material that has garnered attention
for its high hydrogen weight density of 19.6 wt.%
2
that far surpasses the United States
Department of Energy’s 2015 target of 5.5 wt.%.
3
A solid at room temperature, AB is
easy to handle and stable enough to store and transport. AB is highly polarized with the
hydrogens on nitrogen having acidic character and the three hydrogens on boron having
hydritic character. Theoretically, up to three equivalents of hydrogen gas can be released
from one equivalent of AB although a catalyst is required to increase the rate of hydrogen
release because the thermal decomposition of AB itself is too slow for efficient use of the
gas.
4
There are two methods catalytically to release H2 gas from AB. The first involves
the presence of water in the reaction system, or hydrolysis of AB. Often, water is chosen
to solvate AB, and many catalysts have been discovered to release 2.8-3.0 equivalents
from AB in minutes via hydrolysis. Transition metal complexes of cobalt, iron, copper,
molybdenum, vanadium, rhodium and iridium,
5
and nanoparticles of cobalt, nickel, iron,
copper, gold, ruthenium, rhodium, palladium, and platinum are popular catalysts for mild
AB hydrolysis, often proceeding at low temperatures of 25 °C and low catalyst
3
loadings.
2c,6
However, the hydrolytic method of hydrogen production from AB poses
several drawbacks. One of the potential byproducts of the reaction is the formation of
NH4OH, which can decompose to NH3, ammonia gas. A stream of ammonia gas in the
H2 product eluent will poison a proton exchange membrane (PEM) fuel cell and thus
must be removed prior to reaching the fuel cell.
7
The other major concern is the multiple
borate byproducts (B-O bond containing compounds) that are inevitably formed in the
presence of water byproducts. If regeneration and recycling of spent fuel is the ultimate
goal for efficient implementation of AB in vehicles, then the end byproducts of the
reaction should consist of compounds where rehydrogenation is possible.
8
Therefore, the non-hydrolytic method of dehydrogenating AB is more conducive
for spent-fuel use because the borate byproducts are avoided. Instead, the
dehydrogenation pathway of AB is shown below in Scheme 1.1. The thermal
dehydrogenation follows a pathway that goes through a compound called borazine (1.5)
after two equivalents of H2 are produced, which then can be dehydrogenated to
polyborazylene. A drawback to this pathway is the production of 1.5. Borazine has a low
boiling point of 55 °C, a detrimental trait because it may then travel with the eluent
stream of H2 into the PEM fuel cell where then it will poison the fuel cell and decrease
energy output efficiency. However, these drawbacks can be bypassed if borazine is
further dehydrogenated into polyborazylene (1.6), a gelatin-like solid under reaction
conditions. For spent fuel reuse, rehydrogenation of polyborazylene and other N xBxHx
byproducts is possible under the right catalytic conditions.
9
4
Scheme 1.1. Non-hydrolytic hydrogen release from AB.
There are many catalyst systems that dehydrogenate AB non-hydrolytically up to
one equivalent, but because this work is mainly focused on homogeneous transition metal
catalysis of AB, only some of the homogeneous catalysts are highlighted here. AB can
be dehydrogenated by iron,
10
molybdenum,
11
iridium,
12
rhodium,
13
nickel,
14
and
palladium complexes.
15
However, many of these homogenous metal catalysts are only
capable of releasing one equivalent of H2 from AB. Few can produce 2 equivalents and
subsequently borazine and polyborazylene. Most notably, the ruthenium catalyst systems
that dehydrogenate AB from the Williams group all produce at least two equivalents of
H2.
16
This work will mostly focus on Shvo’s catalyst 1.7 and its derivatives (1.8-1.12)
shown in Figure 1.1.
Figure 1.1. Shvo-derived ruthenium catalysts from the Williams Group that produce 2 or
more equivalents of H2 from AB.
5
1.3 Shvo’s Catalyst
Shvo’s catalyst (1.7) was first synthesized from the reaction of
tritutheniumdodecacarbonyl and tetraphenylcyclopentadienone by Youval Shvo in the
1980’s.
17
This complex and its analogs have been studied extensively for reactions such
as hydrogenation of aldehydes, ketones, alkynes, and alkenes; transfer hydrogenation;
disproportionation of aldehydes to esters; isomerization of allylic alcohols; dynamic
kinetic resolution (DKR); amine-amine coupling; and hydroboration reactions.
18
Shvo’s
catalyst (1.7) was chosen to dehydrogenate AB because of its bifunctional nature that
matches AB’s protons and hydrides. 1.7 structurally contains two ruthenium atoms
connected by a bridging hydride and a bridging proton between the two
tetraphenylcyclopentadienone ligands. It is commonly used as a hydrogen transferring
catalyst that invokes metal-ligand cooperation in its mechanism shown in Scheme 1.2.
The dimer splits heterolytically into an oxidizing and a reducing fragments. The
oxidizing monomer 1.13 is implied in theory based on evidence supporting the existence
of the reducing monomer 1.14. 1.13 would quickly pick up hydrogens to form 1.14. 1.14,
the reducing fragment has a metal hydride as well as a proton in the substituted phenol
ligand, thus giving the catalyst its dual reactivity. Although there is no crystal structure
for either 1.13 or 1.14, solution NMR data, mechanistic probes, and trapping experiments
have been utilized to establish their structures. The structure of 1.14 is well characterized
by NMR, while the structure of 1.13 is proposed to be a coordinatively unsaturated
intermediate based on the characterization of trapped derivatives.
6
Scheme 1.2. Shvo’s catalyst 1.7 and its oxidizing monomer 1.13 and reducing monomer
1.14 carrying out transfer hydrogenation.
1.4 Conclusions
In sum, hydrogen gas is popular energy carrier and its drawbacks are the
difficulties of its storage. Chemical storage of hydrogen gas in solid ammonia borane is a
safe, and efficient way to bypassing the dangers of high-pressure and cryogenic hydrogen
storage. Therefore, efforts are focused on how to efficiently release the hydrogen atoms
from ammonia borane through homogeneous catalysis. The catalyst chosen to produce
hydrogen from ammonia borane is Shvo’s catalyst because its chemical structure
complements ammonia borane’s well making Shvo’s an important starting point in
catalyst development for ammonia borane dehydrogenation.
1.5 References
1
The Florida Solar Energy Center. Hydrogen: Hydrogen Basics.
http://www.fsec.ucf.edu/en/consumer/hydrogen/basics/storage.htm (accessed Apr. 17,
2016).
2
(a) Staubitz, A.; Robertson, A. P. M.; Manners, I. Ammonia-Borane and Related
compounds as Hydrogen Donors. Chem. Rev. 2010, 110, 4079-4124. (b) Stephens, F. H.;
7
Pons, V.; Baker, R. T. Ammonia-Borane: The Hydrogen Source Par Excellence? Dalton
Trans. 2007, 2613-2626. (c) Marder, T. B. Will We Soon Be Fueling our Automobiles
with Ammonia–Borane? Angew. Chem. Int. Ed. 2007, 46, 8116-8118. (d) Hamilton, C.
W.; Baker, R. T.; Staubitz, A.; Manners, I. B-N Compounds for Chemical Hydrogen
Storage. Chem. Soc. Rev. 2009, 38, 279-293. (e) Baitalow, F.; Baumann, J.; Wolf, G.;
Jaenicke-Rößler, K.; Leitner, G. Thermal Decomposition of B–N–H Compounds
Investigated by using Combined Thermoanalytical Methods. Thermochim. Acta 2002,
391, 159-168. (f) Wolf, G.; Baumann, J.; Baitalow, F.; Hoffmann, F. P. Calorimetric
Process Monitoring of Thermal Decomposition of B–N–H Compounds. Thermochim.
Acta 2000, 343, 19-25. (g) Wang, J. S.; Geanangel, R. A. B NMR Studies of the Thermal
Decomposition of Ammonia Borane in Solution. Inorg. Chim. Acta 1988, 148, 185-190.
(h) Bluhm, M. E.; Bradley, M. G.; Butterick, R. III; Kusari, U.; Sneddon, L. G.
Amineborane-Based Chemical Hydrogen Storage: Enhanced Ammonia Borane
Dehydrogenation in Ionic Liquids. J. Am. Chem. Soc. 2006, 128, 7748-7749. (i) Rassat,
S. D.; Aardahl, C. L.; Autrey, T.; Smith, R. S. Thermal Stability of Ammonia Borane: A
Case Study for Exothermic Hydrogen Storage Materials. Energy Fuels 2010, 24, 2596-
2606.
3
Targets for Onboard Hydrogen Storage Systems for Light-Duty Vehicles. US
Department of Energy Office of Energy Efficiency and Renewable Energy and The
FreedomCAR and Fuel Partnership, U.S. Government Printing Office: Washington, DC,
2009.
4
Fazen, P. J.; Remsen, E. E.; Beck, J. S.; Carroll, P. J.; McGhie, A. R.; Sneddon, L. G.
Synthesis, Properties, and Ceramic Conversion Reactions of Polyborazylene. A High-
8
Yield Polymeric Precursor to Boron Nitride. Chem. Mater. 1995, 7, 1942–1956. (b)
Lynch, A. T. Transition Metal Catalyzed Reactions of Borazine: New Synthetic Routes to
Boron Nitride Ceramics. Ph.D. Dissertation, University of Pennsylvania, Philadelphia,
PA, 1989.
5
(a) Nelson, D. J.; Truscott, B. J.; Egbert, J. D.; Nolan, S. P. Exploring the Limits of
Catalytic Ammonia–Borane Dehydrogenation Using a Bis(N-heterocyclic carbene)
Iridium(III) Complex. Organometallics 2013, 32, 3769−3772. (b) San Nacianceno, V.;
Azpeitia, S.; Ibarlucea, L.; Mendicute-Fierro, C.; Rodríguez-Diéguez, A.; Seco, J. M.;
San Sebastian, E.; Garralda, M. A. Stereoselective Formation and Catalytic Activity of
Hydrido(acylphosphane)(chlorido)(pyrazole)rhodium(III) Complexes. Experimental and
DFT Studies. Dalton Trans 2015, 44, 13141–13155. (c) Lapin, N. V.; D’yankova, N. Y.
Hydrogen Evolution Kinetics during Transition Metal Oxide-Catalyzed Ammonia Borane
Hydrolysis. Inorg. Mater. 2013, 49, 975–979.
6
(a) (a) Jiang, H.-L.; Xu, Q. Catalytic Hydrolysis of Ammonia Borane for Chemical
Hydrogen Storage. Catal. Today 2011, 170, 56–63. (b) Yan, J. M.; Zhang, X. B.; Akita,
T.; Haruta, M.; Xu, Q. One-Step Seeding Growth of Magnetically Recyclable Au@Co
Core−Shell Nanoparticles: Highly Efficient Catalyst for Hydrolytic Dehydrogenation of
Ammonia Borane. J. Am. Chem. Soc. 2010, 132, 5326−5327. (c) Jiang, H. L.; Umegaki,
T.; Akita, T.; Zhang, X. B.; Haruta, M.; Xu, Q. Bimetallic Au–Ni Nanoparticles
Embedded in SiO2 Nanospheres: Synergetic Catalysis in Hydrolytic Dehydrogenation of
Ammonia Borane. Chem. Eur. J. 2010, 16, 3132−3137. (d) Ramachandran, P. V.;
Gagare, P. D. Preparation of Ammonia Borane in High Yield and Purity, Methanolysis,
and Regeneration. Inorg. Chem. 2007, 46, 7810− 7817.
9
7
Halseid, R.; Vie, P. J. S.; Tunold, R. Effect of Ammonia on the Performance of Polymer
Electrolyte Membrane Fuel Cells. J. Power Sources 2006, 154, 343–350.
8
Mohring, R. M.; Wu, Y. Hydrogen Generation Via Sodium. Borohydride. AIP Conf.
Proc. 2003, 671, 90–100.
9
Sutton, A. D.; Burrell, A. K.; Dixon, D. A.; Garner, E. B.; Gordon, J. C.; Nakagawa, T.;
Ott, K. C.; Robinson, J. P.; Vasiliu, M. Regeneration of Ammonia Borane Spent Fuel by
Direct Reaction with Hydrazine and Liquid Ammonia. Science 2011, 331, 1426–1429.
10
(a) Vance, J. R.; Robertson, A. P. M.; Lee, K.; Manners, I. Photoactivated, Iron-
Catalyzed Dehydrocoupling of Amine–Borane Adducts: Formation of Boron–Nitrogen
Oligomers and Polymers. Chem. Eur. J. 2011, 17, 4099−4103. (b) Baker, R. T.; Gordon,
J. C.; Hamilton, C. W.; Henson, N. J.; Lin, P. H.; Maguire, S.; Murugesu, M.; Scott, B.
L.; Smythe, N. C. Iron Complex-Catalyzed Ammonia–Borane Dehydrogenation. A
Potential Route toward B–N-Containing Polymer Motifs Using Earth-Abundant Metal
Catalysts. J. Am. Chem. Soc. 2012, 134, 5598− 5609. (c) Bhattacharya, P.; Krause, J. A.;
Guan, H. Mechanistic Studies of Ammonia Borane Dehydrogenation Catalyzed by Iron
Pincer Complexes. J. Am. Chem. Soc. 2014, 136, 11153−11161.
11
Buss, J. A.; Edouard, G. A.; Cheng, C.; Shi, J.; Agapie, T. Molybdenum Catalyzed
Ammonia Borane Dehydrogenation: Oxidation State Specific Mechanisms. J. Am.
Chem. Soc. 2014, 136, 11272−11275.
12
Denny, M. C.; Pons, V.; Hebden, T. J.; Heinekey, M.; Goldberg, K. I. Efficient
Catalysis of Ammonia Borane Dehydrogenation. J. Am. Chem. Soc. 2006, 128,
12048−12049.
10
13
(a) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I. Transition Metal-Catalyzed
Formation of Boron−Nitrogen Bonds: Catalytic Dehydrocoupling of Amine-Borane
Adducts to Form Aminoboranes and Borazines. J. Am. Chem. Soc. 2003, 125,
9424−9434. (b) Jaska, C. A.; Manners, I. Catalytic Dehydrocoupling of Amine-Borane
and Phosphine-Borane Adducts: The Mechanism Is Heterogeneous in One Case and
Homogeneous in the Other. J. Am. Chem. Soc. 2004, 126, 1334−1335. (c) Shrestha, R. P.;
Diyabalanage, H. V. K.; Semelsberger, T. A.; Ott, K. C.; Burrell, A. K. Catalytic
Dehydrogenation of Ammonia Borane in Non-Aqueous Medium. Int. J. Hydrogen
Energy 2009, 34, 2616−2621. (d) Douglas, T. M.; Chaplin, A. B.; Weller, A. S. Amine-
Borane σ-Complexes of Rhodium. Relevance to the Catalytic Dehydrogenation of
Amine-Boranes. J. Am. Chem. Soc. 2008, 130, 14432−14433. (e) Alcaraz, G.; Sabo-
Etienne, S. Coordination and Dehydrogenation of Amine–Boranes at Metal Centers.
Angew. Chem. Int. Ed. 2010, 49, 7170−7179.
14
Keaton, R. J.; Blacquiere, J. M.; Baker, R. T. Base Metal Catalysts for
Dehydrogenation of Ammonia-Borane for Chemical Hydrogen Storage. J. Am. Chem.
Soc. 2007, 129, 1844−1845.
15
(a) Kim, S.-K.; Han, W.-S.; Kim, T.-J.; Kim, T.-Y.; Nam, S. W.; Mitoraj, M.; Piecos,
Ł.; Michalak, A.; Hwang, S.-J.; Kang, S. O. Palladium Catalysts for Dehydrogenation of
Ammonia Borane with Preferential B−H Activation. J. Am. Chem. Soc. 2010, 132,
9954−9955. (b) Kim, S.-K.; Hong, S.-A.; Son, H.-J.; Han, W.-S.; Michalak, A.; Kang, S.
O. Dehydrogenation of Ammonia-Borane by Cationic Pd(II) and Ni(II) Complexes in a
Nitromethane Medium: Hydrogen Release and Spent Fuel Characterization. Dalton
Trans. 2015, 44, 7373−7381.
11
16
(a) Conley, B. L.; Williams, T. J. Dehydrogenation of Ammonia-Borane by Shvo's
Catalyst. Chem. Commun. 2010, 46, 4815−4817. (b) Lu, Z.; Conley, B. L.; Williams, T.
J. A Three-Stage Mechanistic Model for Ammonia–Borane Dehydrogenation by Shvo’s
Catalyst. Organometallics 2012, 31, 6705−6714. (c) Zhang, X.; Lu, Z.; Foellmer, L. K.;
Williams, T. J. Nitrogen-Based Ligands Accelerate Ammonia Borane Dehydrogenation
with the Shvo Catalyst. Organometallics 2015, 34, 3732–3738. (d) Zhang, X.; Kam, L.;
Williams, T. J. Dehydrogenation of Ammonia Borane through the Third Equivalent.
Dalton Trans. 2016, 45, 7672 - 7677.
17
Shvo, Y.; Czarkie, D.; Rahamim, Y. A New Group of Ruthenium Complexes: Structure
And Catalysis. J. Am. Chem. Soc. 1986, 108, 7400-7402.
18
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.
12
Chapter 2. Ammonia Borane Dehydrogenation by Shvo’s Catalyst and its
Derivatives: Reactivity and Mechanism
2.1 Introduction
Shvo’s catalyst (2.1, Scheme 2.1) was chosen to dehydrogenate ammonia borane
(AB, H3N-BH3) because of AB’s dual composition of both protons and hydrides, which
enable a bifunctional catalytic mechanism for dehydrogenation that complements the
nature of 2.1. This approach is similar to Noyori-type bifunctional catalysts studied
previously by Schneider and Fagnou
1
as well as “frustrated Lewis-pair” chemistry
reported by Zou and Bercaw.
2
Dr. Brian Conley
3
carried out the initial AB
dehydrogenation reactions by Shvo’s catalyst.
4
Zhiyao Lu
3
and Dr. Brian Conley
investigated the mechanism of the dehydrogenation reaction ending at the homogeneity
tests.
5
2.2 Reactivity of Shvo’s Catalyst with Ammonia Borane
Shvo’s catalyst enables liberation of 2.0 equivalents of H2 from AB. Under certain
conditions, the hydrogen release completes in 2 hours at 70 °C in a 2:1 diglyme/benzene
solution (Scheme 2.1). The solution changes color from bright orange to yellow/brown
when hydrogen pressure evolves from the reaction vessel. Hydrogen gas evolution and
consumption of AB are quantified by eudiometry and
11
B NMR respectively. Under
optimized conditions, the reaction of 5.0 mol% 2.1 and 2.0 mol% ethanol with AB (0.42
M) liberated 1.0 equivalent hydrogen in 30 minutes and reached full conversion after 120
min to yield 2.0 equivalents hydrogen gas (Figure 2.1, left) when run in a reaction vessel
open to a eudiometer—an inverted beret that measures H2 output. Alcohol aids the
13
release of hydrogen in 2.1 catalyzed AB dehydrogenation. This is consistent with H2 loss
from Casey’s tolyl-analogue of 2.1 in which the alcohol shuttles the ligand hydroxyl
proton to the Ru–H to form a labile dihydrogen complex.
6
Scheme 2.1. Dehydrogenation of AB with Shvo’s Catalyst, 2.1.
At 70 °C, the disappearance of AB is observed via
11
B NMR along with formation
of various boron containing species such as μ-aminodiborane (2.2), cyclotriborazane
(2.3), branched cyclotetraborazane (2.4), and borazine (2.5) (Scheme 2.2). The
11
B NMR
spectrum of this solution at completion revealed borazine as the exclusive unsaturated
boron product. This indicates that cross-linking of borazine to form polyborazylene is not
efficient under these conditions.
A kinetic profile for AB consumption was generated by plotting [AB] against
time via
11
B NMR (Figure 2.1, right). The data feature three rate regimes: 1. a brief
initiation period (ca. 2% conversion), 2. a fast linear regime (through ca. 30%
conversion), and 3. a slower regime that fits to first order exponential decay. The
mechanisms of each regime were studied intensively and are presented in Section 2.2 in
detail.
Scheme 2.2. AB dehydrogenation by Shvo’s Catalyst 2.1 and the boron-nitrogen
byproducts produced.
14
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)
Figure 2.1. (left) Eudiometer data showing production of hydrogen gas in the presence of
5.0 mol% 2.1 and 2.0 mol% ethanol in 2:1 diglyme/benzene-d6 at 70 °C. (right)
11
B NMR
data showing consumption of AB in the presence of 2.5 mol% 2.1 in a sealed J-Young
NMR tube.
2.3 Mechanism of Shvo Catalyzed AB Dehydrogenation
2.2.1 Catalyst Initiation Period
As prefaced earlier, the mechanism of Shvo catalyzed AB dehydrogenation is
broken down into three distinct regimes of 1) a slow initial Ru dimer cleavage, 2) fast
catalysis, and 3) slow catalysis. A short period in the beginning 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 2.1 (
1
H =
-17.7 ppm) is replaced by the hydride of monomeric species 2.7 (
1
H = -10.0 ppm) at a
rate of 7.96(21) × 10
-4
s
-1
. This indicates that Shvo’s catalyst, 2.1, dissociates to its
reduced monomer 2.7 and presumably oxidized monomer 2.6 (Scheme 2.3A).
1
H NMR
integrations show that 2 equivalents of 2.7 are formed with consumption of 2.1, so the
15
reduction of 2.6 by AB is rapid. 2.7 could also be independently synthesized from AB
reduction of the Shvo oxidized dimer (2.8) (Scheme 2.3B).
Scheme 2.3. A. Mechanistic scheme for catalyst initiation. B. Rapid formation of 2.7
from 2.8. [AB] is 0.42 M in benzene-d6 solution, [Ru]2 is 5 mol% to AB.
2.2.2 Fast Catalysis
After the catalyst initiation, the kinetic profile of AB consumption displays a fast
linear kinetics through ca. 20-30% conversion. In these conditions, the reaction has a zero
order dependence on [AB] and first order on [Ru] as determined by the respective zero
and unity slopes of plots of ln [kobs] versus ln [AB] and ln [kobs] versus ln [Ru].
Throughout this case,
1
H NMR shows a persistent monomeric ruthenium hydride at
1
H
= -10 ppm, which is plausibly the resting state of the catalyst. This is consistent with H-H
bond formation as the rate-determining step in fast catalysis. This assignment is
consistent with the observed kinetic dependencies of [AB]
0
and [Ru]
1
.
We further observed first-order dependence on [EtOH], which is consistent with a
transition state model established by Casey for stoichiometric hydrogen loss from 2.7.
4,7
Thus, we adopt Casey and Cui’s geometry for ethanol-mediated H-H bond formation
16
from 2.7 as the rate determining transition state of this catalysis. Scheme 2.4 summarizes
the fast catalysis portion of 2.1 catalyzed AB dehydrogenation.
Scheme 2.4. Mechanistic cycle of fast catalysis of AB dehydrogenation aided by EtOH.
2.2.3 Slow Catalysis
Onset of the slow catalysis conditions, i.e. catalyst death, is characterized by the
appearance of curvature in the time course plot of [AB] and a rise in [borazine] (2.5). The
conditions of slow catalysis cause the emergence of multiple
1
-Ru—H hydride peaks
(from
1
H = -9 to -10 ppm), which occurs simultaneously with exponential decay
behavior in [AB]. These peaks correspond, respectively, to (a) new resting state(s) of the
catalyst and ammonia borane’s role in a new rate-determining step. The absence of fast
catalysis must result from a practically irreversible deactivation of the catalyst during the
transition from fast catalysis to slow catalysis.
17
Fast catalysis ends (i.e. catalyst death occurs) because borazine (2.5) undergoes a
hydroboration with ruthenium intermediate 2.6 to give the deactivated complex 2.10
(Scheme 2.5), which further converts to other derivatives. This catalyst’s deactivation
was speculated to occur in this way because of analogous reactions to the addition of 2.5
to 2.6 are known from the Casey
6a
and Clark
8
labs. Casey has shown hydrosilylation of
the Shvo scaffold at the cyclopentadienone oxygen by triethylsilane. This adduct has a
1
H
of -9.20 in benzene-d6. Similarly, Clark has shown hydroboration of the Shvo complex
with pinacol and catecholboranes in high yield at mild temperature. These adducts have
1
H of -9.33 and -9.26 in benzene-d6, respectively.
Scheme 2.5. Deactivation of the Shvo’s catalyst by borazine via hydroboration.
The proposal for the mechanism of slow catalysis was investigated, however, the
proposed complex 2.10 was not stable to isolation. Borazine 2.5 was added to dimer 2.8
at room temperature, and a bridging hydride peak formed at the beginning the reaction
(
1
H = -18.3) was then consumed in 2 minutes (Scheme 2.5). This was replaced by a set
18
of 13 hydride peaks from
1
H δ = -9.3 – -10.0 ppm, which correspond to
1
-Ru—H groups
such as 2.11. We suspect that these signals correspond to multiple hydroboration events
on a single borazine or ring-opened borazine derivative.
Slow catalysis case conditions can be created at the beginning of a
dehydrogenation reaction by adding 0.3 molar equivalents of borazine 2.5 relative to
[Ruatom] to the reaction mixture containing 2.1 prior to heating. In 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. This is strong evidence to
indicate that borazine is the agent that causes catalyst deactivation and aptly accounts for
the instant slow catalysis situation that is observed in catalyst reuse experiments.
A new species, aminodiborane 2.2, in the
11
B NMR during slow catalysis. 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. 2.2 is a dimerization product of BH3, dissociated from ammonia borane, and
[NH2BH2], generated transiently after the first dehydrogenation of ammonia borane
(Scheme 2.6A).
9
If free BH3 from the dissociation of ammonia borane is impacting the
course of the reaction in the slow catalysis case, then free NH3 must also be present, and
NH3 is known to modulate the reactivity of the Shvo system.
10
Thus, we speculated that
reversible formation of 2.12 by NH3 ligation to the catalyst may be a second mechanism
of catalyst deactivation.
19
Scheme 2.6. A. Proposed formation of μ-aminodiborane 2.2. B. Separate synthesis of
Shvo-NH3 ligated species 2.12.
To directly study the reactivity of ammonia adduct 2.12, we prepared it
independently through the addition of ammonia gas to 2.8 (Scheme 2.6B). Analogous to
2.1, the
11
B NMR kinetic profile of AB dehydrogenation with 2.12 appears to have 2
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
. This is similar to the
second and third cases (fast and slow catalysis) of dehydrogenation with 2.1, but since
2.12 is monomeric, it stands to reason that there should not be an initiation delay
analogous to the one observed in reactions featuring 2.1. We account for this behavior by
proposing that NH3 reversibly can ligate 2.6 as previously documented
9
and thereby
temporarily sequester 2.6 from its catalytic roles. Thus, NH3 ligation provides a second
mechanism for catalyst deactivation, although this one appears to be less deleterious than
hydroboration of 2.7. Scheme 2.7 summarizes the entire Shvo catalyzed AB
dehydrogenation mechanistic cycle including initiation, fast catalysis, slow catalysis, and
the potential deactivating adducts formed outside the cycle.
20
0
0.1
0.2
0.3
0.4
0.5
0 5 10 15
[A-B] (M)
Time (x 10
3
s)
Fast Catalysis Slow Catalysis
Catalyst Rate (M s
-1
) Catalyst kobs (s
-1
)
2.1 1.47(9) × 10
-4
2.1 3.06(39) × 10
-4
2.12 5.12(3) × 10
-5
2.12 4.23(33) × 10
-4
Figure 2.2. AB dehydrogenation with 2.1 and 2.12. 0.25 mol AB and 0.035 mol [Ruatom]
are added to 0.6 mL diglyme/benzene-d6. Both reactions were run at 70 ˚C. Black circles
and diamonds are kinetic profiles of reactions catalyzed by 2.12 and 2.1 respectively.
21
Scheme 2.7. Shvo catalyzed AB dehydrogenation mechanism.
2.2.4 Homogeneity of the Shvo Catalyzed AB Dehydrogenation
We proposed that the Shvo catalyzed AB dehydrogenation 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).
The best evidence for homogeneous catalysis is point three: the foregoing kinetics data in
which the data remain pseudo-first order in the slow catalysis case through > 90%
conversion (> 3 half-lives).
The 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 (2.13, Figure 2.3). Quantitative poisoning is an
experiment in which less than one molar equivalent (relative to the proposed monomeric
catalyst) is introduced into the reaction, and the rate is monitored to see if it is affected
22
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%)
11
are on the surface of a nanoparticle and thus ≤ 50% are
available to be poisoned.
Figure 2.3. 1,10-phenanthroline 2.13, a catalyst poison.
In this present case, two quantitative poisoning experiments were conducted
wherein 0.1 and 0.5 equivalents of 1,10-phenanthroline (phen, 2.13) relative to [Ruatom]
were added to two otherwise standard ammonia borane dehydrogenation runs with
catalyst 2.1 (0.25 mol AB, 70 ˚C, diglyme/ benzene-d6). Although these are not first order
reactions, generally we see that phen accelerates the reaction, apparently by prolonging
fast catalysis portion of the reaction (Figure 2.4). By contrast, 0.5 equivalents of phen
(relative to ruthenium atoms) completely quenches catalytic heterogeneous
hydrogenation of benzene based on the [Ru3(
2
-H)3(
6
-C6H6)(
6
-C6Me6)2(
3
-O)]
+
catalyst precursor.
12
Thus, the evidence here argued against the formation of a ruthenium
nanoparticle.
23
Figure 2.4. [AB] Dehydrogenation with 2.1 in the presence of 1,10-phenanthroline, 2.13.
Data calculated from
11
B NMR kinetic studies. 0.25 mol AB and 5 mol% 2.1 are added to
0.6 mL diglyme/benzene-d6 at 70 °C.
Conceptually, we speculate that 1,10-phenanthroline is protecting the reactive site
on ruthenium from hydroboration (Scheme 2.8). This can enable a hydrogen bond from
2.14’s ligand oxygen to ammonia borane, analogous to those well documented for imine
reduction with the Shvo system.
13
We suspect the reaction process is akin to 2.12
catalyzed ammonia borane dehydrogenation, however the equilibrium between 2.6 and
2.14 is slower (versus the equilibrium between 2.6 and 2.12) and thus enables a faster
catalysis mechanism.
24
Scheme 2.8. 1,10-phenanthroline “semi-site protection” mechanism.
2.4 Shvo Pyridine and Amine Derivatives and Their Reactivity with AB
In order to probe the possibility and veracity of the proposed semi-site protection
mechanism shown in Scheme 2.7, 1,10-phenanthroline coordinated Shvo derivatives
were attempted to be synthesized. However, a phen ligated Shvo complex was not
isolable and therefore alternative nitrogen containing aromatic ligands and bidentate
nitrogen homologs were studied instead.
14
Novel, air-stable, and easily prepared pyridine
and amine containing Ru complexes were synthesized, used as catalysts to dehydrogenate
AB, and their mechanisms were studied. This work was published in 2015, and done in
collaboration with Lena K. Foellmer.
2.3.1 Synthesis, Characterization, and Reactivity of Shvo Pyridine Complexes
Pyridine-ligated ruthenium complexes 2.15-2.17 were prepared by drop-wise
addition of the appropriate substituted pyridine to ruthenium dimer 2.8 in benzene solvent
at room temperature (Scheme 2.9A). Treatment of the reaction mixture with hexanes
25
yielded the corresponding products as pale grey powders. Structural data collected on the
products were as expected; the molecular structure of 2.15 is shown in Scheme 2.9B.
Therein, ruthenium adopts the predicted piano stool geometry, which has analogy to the
known molecular structures of 2.12
10
and [(2,5-Ph2-3,4-Tol2-CPD)Ru(CO)(PPh3)(pyr)].
15
The complexes are bench-stable with stability similar to 2.1.
Scheme 2.9. A. Synthesis of pyridine complexes 2.15-2.17. B. ORTEP diagram of 2.15.
Compounds 2.15-2.17 affect significantly faster AB dehydrogenation than 2.1
(Figure 2.5, left). For example, a reaction catalyzed by 2.17 and 2% EtOH has a rate
constant of H2 evolution of 1.40(4) x 10
-3
s
-1
, which is 4-fold faster than an analogous
reaction of 2.1, with a rate constant of 3.7(1) x 10
-4
s
-1
(Figure 2.5, right). Moreover, 2.17
gives a slightly higher extent of H2 release, 2.1 equiv. Although EtOH has known
26
reactivity with AB,
16
we have previously shown that EtOH is a first order co-catalyst that
functions primarily as a proton shuttle in the Shvo system.
6,7
Comparable results are obtained when free pyridines are added to reaction
solutions of 2.1; these reactions differ from those of isolated pyridine adducts by only a
brief initiation delay. If free nitrogen ligands are added to a reaction mixture, they show a
similar protective behavior. For example, if 2.1 is treated with a 4-DMAP, Et3N, or even
tetramethylethylenediamine (TMEDA) in situ, saturation catalysis (the linear region in
the kinetic profile) is extended from ca. 30% conversion to ca. 80% conversion.
2.3.2 Impact of Pyridine Substitution on Reactivity
In the dehydrogenation studies shown in Figure 2.5, there is an increase in
reaction rates (2.16 < 2.15 < 2.17) as the pyridine ligands become less electron rich. For
example, these reactions have AB consumption rate constants of 2.04(4), 4.3(1), and
5.6(2) x 10
-4
M s
-1
respectively for precatalysts 2.16, 2.15, and 2.17 in their saturation
catalysis periods. This is accounted for by considering binding affinity of the pyridine
ligand to ruthenium: the electron rich 4-dimethylaminopyridine (4-DMAP) ligand in 2.16
should bind an electrophilic metal more tightly than an electron neutral pyridine. The
complement is applicable to the least electron-rich 4-trifluoromethylpyridine (4-TFMP).
Accordingly, when 2.17 is treated with 1 equiv. of 4-DMAP, 2.16 is formed in solution at
room temperature immediately with approximately 90% of 2.17 converted to 2.16 in 90
minutes at 70 C. The data suggest that, while these pyridines can increase the rate for
AB dehydrogenation, a less tightly binding pyridine enables a faster reaction by
facilitating ligand/substrate exchange on the ruthenium center. This is consistent with a
27
proposal that pyridine binding to ruthenium is protective of the catalyst, but pyridine
dissociation is necessary for catalytic turnover.
0
0.1
0.2
0.3
0.4
0.5
0 1 2 3 4 5 6 7 8
Shvo's Catalyst 2.1
Shvo-4-DMAP 2.16
Shvo-Pyridine 2.15
Shvo-TMFP 2.17
[AB] (M)
Time (x 1000 sec)
Figure 2.5. Catalytic AB dehydrogenation Left: AB consumption (
11
B NMR) catalyzed
by 2.1, 2.15, 2.16, and 2.17. (10 mol% Ru atom, 70 C, 1:2 C6D6: diglyme). Right: H2
release by eudiometry with 2.17 (10 mol% Ru atom, 70 C, 1:2 C6D6: diglyme with 2%
EtOH).
2.3.3 Interactions of Pyridine Complexes with BN Compounds in Solution
The pyridine ligand has several reactive options in solution. Beyond ruthenium,
free pyridine can ligate BH3; pyridine–BH3 appears in the
11
B NMR in the course of these
reactions. Further evidence of pyridine ligation to boron, nitrogen species comes from the
19
F NMR handle of 2.17: there over 8 4-TFMP species at the end of the reaction,
including free ligand. In the corresponding
11
B NMR spectra, broad signals that are
consistent with polymeric boron byproducts such as polyborazylene are observed. In
contrast, AB dehydrogenation reactions catalyzed by 2.1 have borazine as the sole boron
product. Based on these observations, the pyridines are ligating intermediates in
dehydrogenation in a way that disfavors borazine accumulation.
28
In reactions involving 2.16, a 4-DMAP adduct [Me2N-C5H4N-BH2NH2-BH3],
2.18, which was characterized by Rivard (Figure 2.6) is seen in the
11
B NMR.
17
To
determine whether 2.18 is a necessary part of the fastest mechanistic pathway for AB
dehydrogenation or merely a cul-de-sac, an independently prepared sample of 2.18 was
dehydrogenated. Treatment of 2.18 (0.11 M) with 2.1 (10% Ru atom) was comparable
with 2.18-catalyzed AB dehydrogenation, if not slightly slower under the same conditions,
so the formation of 2.18 is not enabling more rapid AB dehydrogenation. Since 2.18 and
pyridine-BH3 (2.19) are the only major pyridine-boron adducts that were observed during
catalysis or in a ruthenium-free control reaction (vide infra), and because these
dehydrogenate more slowly than AB under catalytic conditions, pyridine-boron ligation is
not the major cause of reaction acceleration by added pyridine.
0.05
0.06
0.07
0.08
0.09
0.1
0.11
0 5 10 15 20 25
2.18
AB
[BH
3
] (M)
Time (x 1000 sec)
Figure 2.6. Left: Metal catalyzed dehydrogenation of 2.18. Right: Dehydrogenation of
AB (blue squares) and 2.18 (red circles) by Shvo’s catalyst 2.1 (10 mol% Ru atom, 70 C,
1:2 C6D6: diglyme).
Based on these observations, monodentate pyridine ligands can play a protective
role in the dehydrogenation of AB, probably by preempting borazine hydroboration. The
29
catalysis rate is increased and catalyst deactivation is avoided in the presence of
pyridines, whether these ligands are pre-complexed to the catalyst or added separately to
the reaction. Pyridine-ruthenium ligation is most likely important to this behavior.
2.3.4 Ligand Displacement by Bidentate Nitrogen Ligands
When AB dehydrogenation reactions involving 2.1 are treated with bidentate
nitrogen ligands (e.g. phen, bipy, TMEDA), the active catalyst for AB dehydrogenation
could involve multiple [RuLn] species. This is due to the lability of the CPD ligand in the
presence of these bidentate dinitrogen compounds at elevated temperatures. For example,
CPD is displaced in the attempted syntheses of a TMEDA-supported complex, 2.20. In
conditions analogous to the syntheses of 2.15-2.17, we observe that treatment of 2.8 with
TMEDA results in the formation of a binuclear species 2.21 where one CPD ligand is
expelled (Scheme 2.10). CPD displacement is confirmed by GCMS. In addition to
offering a structural suggestion for a reactive, CPD-free AB dehydrogenation catalyst
monomer, [(CO)2Ru(TMEDA)], the dimeric structure of 2.21 presents a first-of-class Ru-
Ru bond (2.7735(6) Å) formed to a Shvo-like fragment, here featuring a single CPD
ligand bridging both metal centers.
As expected, free CPD is detected when 2.8 reacts with bidentate ligands phen or
bipy at 70 C. In contrast to TMEDA, we were unable to isolate a single phen-ligated
ruthenium adduct, even when treating 2.8 with phen at room temperature. A control
experiment with pyridine and 2.8 at 70 C yields only 2.15: no CPD liberation is
observed. Therefore, bidentate nitrogen ligands can expel CPD from the Shvo system
under the reaction conditions, while pyridines will not.
30
Scheme 2.10. Synthesis and Structure of 2.21.
2.3.5 Kinetic Effects of CPD Displacement
Formation of a [(phen)Ru(CO)2] species in situ results in a very rapid system for
AB dehydrogenation, although it is unclear what the active catalyst(s) are in this system.
For example, treating [(phen)RuCl2(CO)2]
18
with 2 equiv. TlOTf afforded an excellent
catalyst for AB dehydrogenation, which completed in 1.5 hours under reaction conditions.
Figure 2.7 (left) shows the relative rates of AB consumption: these conditions are
significantly more reactive than 2.1, but less reactive than 2.15. As expected, no
mechanism change is obvious in this time course plot. Presumably, part of the reason
that phen did not poison the Shvo system as expected, but rather accelerated it, was the
generation of a portion of a relatively reactive [(phen)Ru(CO)2] species in situ. In line
with this proposal, the rate of AB consumption with phen-treated 2.1 lies nicely between
the rates of dehydrogenation with 2.1 and the [(phen)Ru(CO)2] species (Figure 2.7, right),
which is consistent with a mechanism that it involves a [(phen)Ru(CO)2] catalyst that is
attenuated by interaction with a CPD-ligated moiety.
31
CPD was not involved in this accelerated reaction based on the data presented.
Ruthenium must have been involved, however, because while phen, TMEDA, or 4-
DMAP will release H2 from AB without ruthenium, these reactions are slow and have
different product selectivities:
11
B NMR shows these to give a branched
cyclotetraborazane
19
and only traces of borazine after 4 hours at 70 C. Based on these
observations, the origins of rate acceleration in the previous quantitative poisoning
experiments include the formation of a reactive, homogeneous [(phen)Ru(CO)2] species.
0
0.1
0.2
0.3
0.4
0 1 2 3 4 5 6 7
2.1
2.15
2.23 + 2 eq. TlOTf
[AB] (M)
Time (x 1000 sec)
0
0.1
0.2
0.3
0.4
0 1 2 3 4 5 6 7
2.1
2.1 + 1 eq. phen
2.23 + 2 eq. TlOTf
[AB] (M)
Time (x 1000 sec)
Figure 2.7. Left: dehydrogenation of AB by 2.1 (green circles), (phen)RuCl2(CO)2 with 2
equiv. TlOTf (blue diamonds), and 2.15 (red squares). Right: dehydrogenation of AB by
2.1 (green circles), 2.1 with 1 eq. of phen (black diamonds), and (phen)RuCl2(CO)2 with
2 equiv. TlOTf (blue diamonds). Conditions are 10 mol% Ru atom, 70 C, 1:2 C6D6:
diglyme.
2.5 Conclusions
In summary, Shvo’s catalyst 2.1 is an efficient system for catalytic
dehydrogenation of AB that liberates two equivalents of hydrogen and gives borazine as
32
the by-product. The mechanism of 2.1 catalyzed AB dehydrogenation was investigated.
This reaction initiates with dissociation of the dimeric precatalyst 2.1, 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 that are not as reactive as their predecessor. These
deactivated ruthenium species are reactivated by ammonia borane itself and proceed to
further ammonia borane dehydrogenation. The homogeneity of the 2.1 catalyzed AB
dehydrogenation was then investigated and the acceleration of the reaction resulted from
the addition of 1,10-phenanthroline. Thus Shvo derivatives with ligands such as pyridine
and amines were synthesized and studied as phen analogs to determine the cause of the
rate acceleration. The amine and pyridine ligand additions accelerate the 2.1-catalyzed
dehydrogenation of AB. Monodentate pyridines enable accelerated dehydrogenation of
AB without CPD displacement. We proposed that the reason for this is that pyridines can
protect the catalyst against hydroboration by borazine. Bidentate (N-N) ligands appear to
accelerate catalysis through an additional mechanism where a relatively more reactive
ruthenium species is formed upon loss of CPD. Through these intricate mechanistic
studies, we would be able to further design and optimize the AB dehydrogenation
catalysts for H2 release for use in fuel cells.
2.6 References
1
(a) Blaquiere, N.; Diallo-Garcia, S.; Gorelsky, S. I.; Black, D. A.; Fagnou, K.
Ruthenium-Catalyzed Dehydrogenation of Ammonia Boranes. J. Am. Chem. Soc. 2008,
130, 14034-14035. (b) Kaβ, M.; Friedrich, A.; Drees, M.; Schneider, S. Ruthenium
Complexes with Cooperative PNP Ligands: Bifunctional Catalysts for the
33
Dehydrogenation of Ammonia–Borane. Angew. Chem., Int. Ed. 2009, 48, 905−907. (c)
Friedrich, A.; Drees, M.; auf der Günne, J. S.; Schneider, S. Highly Stereoselective
Proton/Hydride Exchange: Assistance of Hydrogen Bonding for the Heterolytic Splitting
of H2. J. Am. Chem. Soc. 2009, 131, 17552–17553.
2
(a) Guo, Y.; He, X.; Li, Z.; Zou, Z. Theoretical Study on the Possibility of Using
Frustrated Lewis Pairs as Bifunctional Metal-Free Dehydrogenation Catalysts of
Ammonia−Borane. Inorg. Chem. 2010, 49, 3419–3423. (b) Miller, A. J. M.; Bercaw, J.
E. “Dehydrogenation of Amine-boranes with a Frustrated Lewis Pair.” Chem. Commun.
2010, 46, 1709-1711.
3
Travis J. Williams research group, Loker Hydrocarbon Research Institute, Department
of Chemistry. University of Southern California. Los Angeles, CA 90089.
4
This chapter is reprinted in part with permission from the American Chemical Society
and the authors. Conley conducted the experiments in the AB dehydrogenation work.
Conley, B. L.; Williams, T. J. Dehydrogenation of ammonia-borane by Shvo's catalyst.
Chem. Commun. 2010, 46, 4815-4817.
5
This chapter is reprinted in part with permission from the American Chemical Society
and the authors. Lu and Conley conducted the experiments in the AB dehydrogenation
mechanism work. Lu, Z.; Conley, B.L.; Williams, T. J. A Three-Stage Mechanistic
Model for Ammonia−Borane Dehydrogenation by Shvo’s Catalyst. Organometallics
2012, 31, 6705−6714.
6
Casey, C. P.; Johnson, J. B.; Singer, S. W.; Cui, Q. Hydrogen Elimination from a
Hydroxycyclopentadienyl Ruthenium(II) Hydride: Study of Hydrogen Activation in a
34
Ligand-Metal Bifunctional Hydrogenation Catalyst. J. Am. Chem. Soc. 2005, 127, 3100-
3109.
7
(a) Casey, C. P.; Singer, S. W.; Powell, D. R.; Hayashi, R. K.; Kavana, M. Hydrogen
Transfer to Carbonyls and Imines from a Hydroxycyclopentadienyl Ruthenium Hydride:
Evidence for Concerted Hydride and Proton Transfer. J. Am. Chem. Soc. 2001, 123,
1090-1100; (b) Casey, C. P.; Beetner, S. E. and 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.
8
Koren-Selfridge, L.; Query, I. P.; Hanson, J. A.; Isley, N. A.; Guzei, I. A.; Clark, T. B.
Synthesis of Ruthenium Boryl Analogues of the Shvo Metal-Ligand Bifunctional
Catalyst. Organometallics 2010, 29, 3896-3900.
9
(a) Stephens, F. H.; Pons, V.; Baker, R. T. Ammonia-Borane: The Hydrogen Source Par
Excellence? Dalton Trans. 2007, 2613-2626. (b) Wright, W. R. H.; Berkeley, E. R.;
Alden, L. R.; Baker, R. T.; Sneddon, L. G. Transition Metal Catalysed Ammonia-Borane
Dehydrogenation in Ionic Liquids. Chem. Commun. 2011, 47, 3177-3179
10
Hollmann, D.; Jiao, H.; Spannenberg, A.; Bähn, S.; Tillack, A.; Parton, R.; Altink, R.;
Beller, M. Deactivation of the Shvo Catalyst by Ammonia: Synthesis, Characterization,
and Modeling. Organometallics 2009, 28, 473– 479.
11
(a) Widegren, J. A.; Finke, R. G. A Review of the Problem of Distinguishing True
Homogeneous Catalysis from Soluble or Other Metal-Particle Heterogeneous Catalysis
Under Reducing Conditions. J. Mol. Catal. 2003, 198, 317-341; (b) Kovács, G.; Nádasdi,
35
L.; Joó, F.; Laurenczy, G. H/D Exchange Between H2–D2O and D2–H2O Catalyzed by
Water Soluble Tertiary Phosphine Complexes of Ruthenium(II) and Rhodium(I). C. R.
Acad. Sci. Paris, Chim. 2000, 3, 601.
12
Hagen, C. M.; Vieille-Petit, L.; Laurenczy, G.; Süss-Fink, G.; Finke, R. G.
Supramolecular Triruthenium Cluster-Based Benzene Hydrogenation Catalysis: Fact or
Fiction? Organometallics 2005, 24, 1819– 1831.
13
(a) Samec, J. S.; Éll, A. H.; Åberg, J. B.; Privalov, T.; Eriksson, L.; Bäckvall, J.-E.
Mechanistic Study of Hydrogen Transfer to Imines from a Hydroxycyclopentadienyl
Ruthenium Hydride. Experimental Support for a Mechanism Involving Coordination of
Imine to Ruthenium Prior to Hydrogen Transfer. J. Am. Chem. Soc. 2006, 128, 14293-
14305. (b) Casey, C. P.; Johnson, J. B. Isomerization and Deuterium Scrambling
Evidence for a Change in the Rate-Limiting Step during Imine Hydrogenation by Shvo's
Hydroxycyclopentadienyl Ruthenium Hydride. J. Am. Chem. Soc. 2005, 127, 1883-1894.
(c) Samec, J. S.; Éll, A. H.; Bäckvall, J.-E. Mechanism of Hydrogen Transfer to Imines
from a Hydroxycyclopentadienyl Ruthenium Hydride. Support for a Stepwise
Mechanism. Chem. Commun. 2004, 23, 2748-2749. (d) Éll, A. H.; Johnson, J. B.;
Bäckvall, J.-E. Mechanism of Ruthenium-Catalyzed Hydrogen Transfer Reactions.
Evidence for a Stepwise Transfer of CH and NH Hydrogens from an Amine to a
(Cyclopentadienone)Ruthenium Complex. Chem. Commun. 2003, 1652-1653. (e) Casey,
C. P.; Bikzhanova, G. A.; Cui, Q.; Guzei, I. A. Reduction of Imines by
Hydroxycyclopentadienyl Ruthenium Hydride: Intramolecular Trapping Evidence for
Hydride and Proton Transfer Outside the Coordination Sphere of the Metal. J. Am. Chem.
Soc. 2005, 127, 14062-14071. (f) Åberg, J. B.; Bäckvall, J.-E. Investigation of a Possible
36
Solvent Cage Effect in the Reduction of 4-Aminocyclohexanone by a
Hydroxycyclopentadienyl Ruthenium Hydride. Chem. Eur. J. 2008, 14, 9169-9174.
14
This chapter is reprinted in part with permission from the American Chemical Society
and the authors. Zhang, X.; Lu, Z.; Foellmer, L. K.; Williams, T. J. Nitrogen-Based
Ligands Accelerate Ammonia Borane Dehydrogenation with the Shvo Catalyst.
Organometallics 2015, 34, 3732–3738.
15
Casey, C. P.; Strotman, N. A.; Beetner, S. E.; Johnson, J. B.; Priebe, D. C.; Guzei, I. A.
PPh3-Substituted [2,5-Ph2-3,4-Tol2(η5-C4COH)]Ru(CO)(PPh3)H Exhibits Slower
Stoichiometric Reduction, Faster Catalytic Hydrogenation, and Higher Chemoselectivity
for Hydrogenation of Aldehydes over Ketones than the Dicarbonyl Shvo Catalyst.
Organometallics 2006, 25, 1236-1244.
16
Dong, H.; Berke, H. A Mild and Efficient Rhenium-Catalyzed Transfer Hydrogenation
of Terminal Olefins Using Alcoholysis of Amine Borane Adducts as a Reducing System.
J. Organomet. Chem. 2011, 696, 1803-1808.
17
Malcolm, A. C.; Sabourin, K. J.; McDonald, R.; Ferguson, M. J.; Rivard, E. Donor-
Acceptor Complexation and Dehydrogenation Chemistry of Aminoboranes. Inorg. Chem.
2012, 51, 12905-12916.
18
(a) Colton, R.; Farthing, R. Carbonyl Halides of the Group VIII Transition Metals. I.
Dicarbonyldihalogenoruthenium(II) and Related Compounds. Aust. J. Chem. 1967, 20,
1283−1286. (b) Anderson, P. A.; Deacon, G. B.; Haarmann, K. H.; Keene, F. R.; Meyer,
T. J.; Reitsma, D. A.; Skelton, B. W.; Strouse, G. F.; Thomas, N. C. Designed Synthesis
of Mononuclear Tris(Heteroleptic) Ruthenium Complexes Containing Bidentate
Polypyridyl Ligands. Inorg. Chem. 1995, 34, 6145−6157.
37
19
Kalviri, H. A.; Gartner, F.; Ye, G.; Korobkov, I.; Baker, R. T. Probing the Second
Dehydrogenation Step in Ammonia-borane Dehydrocoupling: Characterization and
Reactivity of the Key Intermediate, B-(cyclotriborazanyl)amine-borane. Chem. Sci. 2015,
6, 618-624.
38
Chapter 3. Breaking the Third Equivalent Barrier in Ammonia Borane
This work was carried out with the aid of Lena Foellmer and Lisa Kam.
1
Lena
conducted eudiometry experiments for Shvo-TMEDA derivative catalyzed AB
dehydrogenation reactions,
2
and Lisa Kam conducted eudiometry experiments for
phenRu(CO)2(OAc)2 catalyzed AB dehydrogenation reactions.
3
3.1 Introduction to the Dehydrogenation of the Third Equivalent of AB
Homogeneous catalyst systems that dehydrogenate ammonia borane reported to
date fit one of two classes:
4
(1) those that release 1 equivalent of hydrogen quickly
5
and
(2) those that release 2 or more equivalents slowly.
6
The latter are known to proceed
through (or stop at) borazine, N3B3H6 (3.1), as an intermediate with its subsequent
conversion to polyborazylene as a slow step in the overall mechanism of hydrogen
evolution shown in Scheme 3.1.
7
This is problematic because (1) slow borazine
derivatization limits H2 productivity, (2) borazine, which boils at 55 °C, is poisonous to
fuel cells, and (3) borazine is known to deactivate some AB dehydrogenation catalysts.
Scheme 3.1. Mechanism of hydrogen evolution from AB.
Maximizing the H2 release efficiency is highly desirable and only a few catalysts
have achieved of a high release of 2.5 equivalents or greater (Figure 3.1). Currently,
Baker’s nickel-based system supported by Ender’s carbenes is the highest extent of H2
39
release from AB reported to date.
6
The Guan Fe-POCOP catalyst uses inexpensive iron
but dehydrogenates fairly slowly and reacts through 2.5 equiv. H2.
8
The Agapie Mo
catalyst has the same productivity, and is limited by air and water sensitivity.
9
Further,
Wegner has recently presented a metal-free catalyst that is capable of releasing ca. 2.5
equiv. of H2.
10
Despite high extent of H2 release observed with these catalytic systems, no
study of direct borazine consumption was reported with any of them, and borazine
remains a significant constituent of the reactive medium at the end of AB
dehydrogenation reaction for each.
Figure 3.1. Catalysts that dehydrogenate AB over 2.5 equiv.
3.2 Borazine Introduction
As previously stated, borazine is the major byproduct formed after two
equivalents of hydrogen gas has been liberated from ammonia borane. In order to access
the third equivalent of hydrogen from AB, borazine must be dehydrogenated further to
40
polyborazylene and other unsaturated BN polymeric materials (Scheme 3.1). Borazine
has a background, thermal dehydrogenation pathway: under reduced pressure at 70 C in
48-60 hours with periodic degassing, it will dehydrogenate slowly to polyborazylene.
11
However, this rate is insufficient to prevent borazine accumulation under catalytic
hydrogen evolution conditions. Thus, there is a desire to find a catalyst system that will
dehydrogenate borazine at a rate faster than (or commensurate with) the rate at which it
reacts with ammonia borane itself. No such system is yet to be reported. The presence of
borazine itself is a problem, because borazine is a moisture sensitive, volatile compound
that will travel with H2 eluent gas stream into the PEM fuel cell and lower the fuel cell’s
efficiency. Also in specific catalytic cases, borazine is a poison to the active catalyst in
AB dehydrogenation. Therefore, it is vital to prevent borazine accumulation if H 2 release
from AB is to reach its maximum efficiency. Additionally, the added benefits of the final
boron-nitrogen byproducts being polyborazylene polymers is the fact that these solid,
organic solvent insoluble materials will stay in the reaction vessel and never travel to the
PEM fuel cell. These materials are also potential starting materials for rehydrogenation
back to AB in order to complete a renewable fuel cycle.
3.3 Effects of TMEDA, Calcium, and Water on the Third Equivalent of Hydrogen in AB
Dehydrogenation Catalyzed by TMEDA-Supported Shvo Analog
3.3.1. Optimization of a Shvo-TMEDA System for AB dehydrogenation
After development of bidentate nitrogen ligands in the acceleration of AB
dehydrogenation, we sought to study the hydrogen release extent of TMEDA coordinated
Shvo derivative 3.3 (Figure 3.2).
12
While 3.3 dehydrogenates AB faster than parent
Shvo’s catalyst, we hypothesized that 3.3 and TMEDA will increase the amount of
41
hydrogen produced from AB, because of TMEDA’s potential to crosslink BN byproducts
into more easily dehydrogenated adducts.
Optimization of reaction conditions resulted in 2.5 equivalents of H2 released in
6-7 hours from 0.21 M (AB), at 2.8% 3.3 loading (5.6% Ru atom), with 1.1 equivalents
of TMEDA relative to AB shown in Figure 3.2. The reaction was conducted under N2
atmosphere in a glove box and all liquid reagents (TMEDA) and solvent (tetraglyme)
were dried with ketyl (Na
0
and benzophenone) prior to use. In addition to the typical AB
dehydrogenation B-N byproducts seen in
11
B spectra, we observe TMEDA BH3 (-8.5
ppm) and TMEDA 2BH3 (-9.2 ppm) that form and then are consumed with time. We
hypothesized that the 1.1 equivalent of TMEDA in respect to AB is crosslinking the AB
and its dehydrogenation byproducts into more readily hydrogenated species and
eventually into a polymeric, TMEDA-containing material analogous to polyborazylene.
11
B NMR is not an accurate representation of the speed of dehydrogenation because of
TMEDA’s rapid ligation with AB into TMEDA-containing intermediates. Therefore,
eudiometry data of H2 release are more accurate representations of the extent of reaction.
42
0
0.5
1
1.5
2
2.5
3
0 5 10 15 20 25
Data 53
B
Equiv. of H
2
Time (x 1000 sec)
Figure 3.2. H2 release from 0.21 M (AB) catalyzed by 2.8% 3.3 (5.6% Ru atom) with 1.1
equivalents of TMEDA relative to AB in ketyl dried tetraglyme.
3.3.2 Effects of Calcium and Hydroxides in 3.3 Catalyzed AB Dehydrogenation
We discovered that AB dehydrogenation conducted in a previous batch of
tetraglyme that was dried and stored over calcium hydride (CaH2) for 3 years released up
to 2.7-8 equivalents in only 30 minutes. We hypothesized that the tetraglyme might
contain small amounts of soluble calcium ions and potentially water that were responsible
for the rate and H2 release increase. Since calcium-ligated aminoboranes are used as
hydrogen storage material, we made a solution of a known amount of soluble Ca
+2
ions to
test calcium’s effect on our AB dehydrogenation system.
13
The soluble Ca
+2
ions may be
complexing with AB byproducts forming species that are easier to dehydrogenate. To
43
make the soluble calcium solution, we stirred approximately 100 mL of bench-top grade
tetraglyme with approximately 1.0 g CaH2 overnight and filtered through a PTFE filter.
Inductive plasma coupling (ICP) analysis revealed this solution contained 4% Ca
+2
ions,
and this solvent will now be referred to as Solvent 1.
14
AB dehydrogenation in this Ca
+2
containing solvent yielded 2.6 equivalents in less than 2 hours (Figure 3.3). AB without
any catalyst also has a small reaction in this medium losing ca. 35% in 4 hours compared
to AB in ketyl dried tetraglyme which loses only 4% in 4 hours (see section 6.3.4).
0
0.5
1
1.5
2
2.5
3
0 1 2 3 4 5 6 7 8
Data 53
D
Equiv. of H
2
Time (x 1000 sec)
Figure 3.3. H2 release from 0.21 M (AB) catalyzed by 2.8% 3.3 (5.6% Ru atom) with 1.1
equivalents of TMEDA relative to AB in tetraglyme containing 4% Ca
+2
(by ICP)
(Solvent 1).
While AB dehydrogenation in solvent 1 was faster than the initial reaction, the
possibility of water hydrolyzing AB to produce borates was not completely ruled out. A
qualitative ketyl test was conducted on solvent 1 and an aliquot of ketyl dried tetraglyme
as control. A ketyl test consists of Na
0
and benzophenone dissolved in THF stirred for 48
hours at room temperature until the mixture becomes dark blue-purple. Then one drop of
the ketyl THF solution is added to the solvents to be tested. If the tested solvent is dry,
44
the overall solution should remain blue or purple, however if the solvent is wet, the color
will quickly turn from the dark blue-purple to green, yellow, and finally clear. The 4%
Ca
+2
tetraglyme (Solvent 1) was tested and immediately turned clear while the ketyl dried
tetraglyme remained blue. However, we did not observe water in the
1
H NMR of this
solution, therefore we assumed the color change is the result of Ca(OH) 2 present in
solution.
To determine the effect of water (if any), AB dehydrogenations at 0.21 M (AB),
2.8% catalyst loading, and 1.1 equivalents of TMEDA were run in tetraglyme with 1.0
equivalents of water to AB (solvent 2), and in commercial, bench-top grade tetraglyme
(solvent 3). The
11
B spectra during the reactions are shown below in Figure 3.4. In the
reactions where water is present (solvent 2), none of the usual AB dehydrogenation
intermediates (aside from TMEDA BH3 at -8.5 ppm) or borazine was seen (-5, -11, -23
ppm). Instead, a small portion of AB hydrolysis byproduct, boric acid was observed at 20
ppm. Most of the boric acid would be insoluble in tetraglyme and had precipitated out of
the solution. Comparatively, AB dehydrogenation conducted in solvent 1 had
polyunsaturated byproducts at 23-28 ppm, no borazine, and no intermediates besides
TMEDA BH3 (-8.5 ppm) and TMEDA 2BH3 (-9.2 ppm). The dehydrogenation reaction
in rigorously, ketyl-dried solvent has the most amount of borazine and the typical
dehydrogenation intermediates. These experiments concluded that the 4% Ca
+2
solution
(Solvent 1) does not contain stoichiometric, or more amounts of water.
45
Figure 3.4.
11
B spectra comparison of AB dehydrogenation with 0.21 M (AB), 2.8 mol%
3.3, and 1.1 equivalents of TMEDA under various solvents.
3.3.3. Synthesis of Soluble Calcium Adducts
We were uncertain if the speed and H2 release increases were the result of soluble
calcium or the presence of hydroxides. And so we decided to synthesize a more
tetraglyme-soluble calcium adduct that does not contain hydroxyls. Using the procedure
adapted from the synthesis of a crown ether supported a calcium triflate complex, a
tetraglyme supported calcium triflate adduct (3.4) was synthesized as the soluble calcium
source.
15
The synthesis of 3.4 is shown in Scheme 3.2 with the product yielded as a
white crystalline solid in 64%.
46
Scheme 3.2. Synthesis of soluble calcium adduct Ca(OTf)2 Tetraglyme 3.4.
AB dehydrogenation in tetraglyme with 4% of 3.4 (Solvent 4) yielded 2.5 equiv.
of H2 in 3 hours, as opposed in ketyl dried tetraglyme where it took 6 hours to reach 2.5
equivalents of H2 release. A comparison graph of AB dehydrogenation in solvents and
additives is shown below in Figure 3.5. These data show that the presence of calcium in
the reaction does indeed speed up the release of the third equivalent of H2 with Shvo
derivative catalyzed AB dehydrogenation.
Solvent Name Conditions
Solvent 1 CaH2 dried tetraglyme with 4% Ca
+2
by ICP
Solvent 2 1.0 equiv. H2O to AB
Solvent 3 Bench-top grade tetraglyme from Alfa Aesar
Solvent 4 Ketyl dried tetraglyme with 4% of Ca(OTf)2 Tetraglyme 3.4
Table 3.1. List of solvent conditions that affect AB dehydrogenation.
47
Figure 3.5. Comparison of H2 release in three solvent systems at 0.21 M AB, 3.8 mol%
3.3 and 1.1 equiv. of TMEDA. Vigorously ketyl-dried tetraglyme (black circles), solvent
1 (blue squares), and solvent 4 (green diamonds).
While we succeeded in achieving a high extent of H2 release with this system of
reagents, we were unsatisfied with the necessary use of the stoichiometric 1.1 equivalents
of TMEDA to help ligate the B-N byproducts. The ideal AB dehydrogenation system
would not need stoichiometric amounts of any additive. Therefore, we continued our
efforts in a different direction, using the data from previous mechanistic studies of the
Shvo platform that informed us about the more active species generated in the presence
of a bidentate nitrogen ligand and lack of the tetraphenylcyclopentadienone ligand.
12
48
3.4 Development and Execution of (phen)Ru(CO)2(OAc)2-Catalyzed AB Dehyrogenation
through the Third Equivalent
This work was published in 2016 and was done in collaboration with Lisa Kam.
3
3.4.1 Catalyst Development
We have previously demonstrated the Shvo catalyst can lose its
tetraphenylcyclopentadienone (CPD) ligand from the metal center in the presence of
bidentate nitrogen ligands such as 1,10-phenanthroline (phen) shown in Scheme 3.3.
12
We suspect that the species produced by the expulsion of the CPD ligand contributes to
the increase in rate of AB dehydrogenation. This species potentially consists of a
ruthenium center supported by two carbonyl ligands and the bidentate bis(nitrogen)
ligand. We therefore investigated ruthenium scaffolds composed of these constituents.
Whereas these square planar ruthenium complexes are not stable to isolation, we generate
analogous compounds in situ. We reasoned that treatment of the known complex
(phen)RuCl2(CO)2 (3.5)
16
with TlOTf and ammonia borane would generate a reactive
(phen)Ru(CO)2-based fragment. While 10% of 3.5 with 2 equivalents of TlOTf will
liberate 2.5-2.7 equivalents of H2 from AB, the high loading of metals (both Ru and Tl) is
undesirable. Further, this reaction is not effective at lower catalyst loadings. We
eliminated the usage of thallium by replacing the chlorine ligands with acetates (3.6,
Scheme 3.4) with the expectation that acetate groups should dissociate easily from the
metal center in the presence of an excess of boron with this dissociation driven by the
strength of boron-acetate bonds.
49
Scheme 3.3. Shvo’s oxidized form’s loss of CPD ligand in the presence of a bidentate
ligand such as phen. CPD was detected by GC-MS.
Scheme 3.4. Synthesis of 3.6.
3.4.2. Reactivity of 3.6 with AB
Catalyst 3.6 dehydrogenates AB efficiently at low catalyst loading, down to 1
mol%, producing 2.4–2.7 equiv. of hydrogen, as shown in Figure 3.6. The catalyst system
is robust, capable of producing a similarly high extent of H 2 release (2.6, 2.5, 2.4) in each
of the multiple AB reloadings. Further, we compared the rates of catalytic reactions
prepared in the glove box under nitrogen atmosphere and an analogous sample of 3.6
suspended in solvent and immersed in an ultrasonic bath in open air for 20 minutes before
AB addition. We find that exposure of the catalyst to air and water in the atmosphere
neither slows nor accelerates dehydrogenation; rather, these runs have analogous rates
(Chapter 6).
50
0
0.5
1
1.5
2
2.5
3
0 5 10 15 20 25 30 35
Equiv. of H
2
Time (Sec x 1000)
k
obs
= 8.9(3) x 10
-5
s
-1
0
0.1
0.2
0.3
0.4
0 10 20 30 40 50 60
[AB] (M)
Time (Sec x 1000)
Figure 3.6. Left: H2 production from 1% of 3.6 at 70 C in diglyme releasing 2.7 equiv.
Right:
11
B kinetic profile of the consumption of AB catalyzed by 1% of 3.6 in 2:1
diglyme/benzene-d6.
3.4.3. Reactivity of 3.6 with Borazine
Treatment of AB with 3.6 results in the formation of a family of AB
dehydrogenation products (Scheme 3.1) that has homology to those formed when the
dehydrogenation is conducted with the Shvo catalyst (3.7), except that the reaction does
not stop at borazine, but goes on to polyborazylene materials (3.2). Borazine (3.1) is the
exclusive boron-nitrogen material formed when Shvo’s catalyst is used as the catalyst,
and it is the principal material that accumulates after two equivalents of H2 are produced.
Unlike Shvo’s catalyst and its derivatives, 3.6 will react with isolated borazine at
70 C resulting in polyborazylene as the major byproduct. We can observe this directly
by
11
B NMR (Figure 3.7). In this experiment, borazine was isolated by a vacuum transfer
of the volatiles in a spent Shvo catalyzed AB dehydrogenation reaction. After the transfer
of borazine and solvents into a J-Young NMR tube preloaded with 3.6, a
11
B NMR
51
spectrum was taken, and the solution was heated to 70 C (see experimental section for
further details). After heating for 24 hours, borazine (31 ppm) was converted, ca. 70%,
relative to the external
11
B standard (0 ppm) to give signals consistent with cross-linked
polyborazylene and other unsaturated B-N byproducts. These are indicated by broad
signals in the
11
B NMR spectra from 24-34 ppm. Visual inspection of the NMR tube after
the reaction shows insoluble, gelatin-like, white material, which is consistent with the
formation of polymeric B-N species and accounts for the loss in integration of
11
B
products observed by NMR. We conducted control reactions to determine the
background thermal decomposition rate for borazine under these conditions, and we find
that the borazine peak is converted through only ca. 15% in 24 hours in the absence of
3.6 (See Experimental Chapter 6.4).
52
Figure 3.7. Reaction of 3.6 with borazine (3.1) in 2:1 diglyme/benzene-d6. Bottom:
initial
11
B spectrum of 3.6 and borazine at room temperature. Top:
11
B spectrum of 3.6
and borazine after 24 hours at 70 C. Boron external standard BF3-OEt2 is at 0 ppm, and
borazine is the only boron species present in the bottom spectrum at 31 ppm. In the top
spectrum, borazine peak height has decreased and other broad boron species that are
indicative of polyborazylene appear after 24 hrs. Areas under the peaks are integrated
relative to the external standard BF3-OEt2 peak set to 1.00.
53
a.
b.
0
5
10
15
0 5 10 15 20 25
Normalised Peak Height Intensity
Time ( x 1000 sec)
0
5
10
15
0 5 10 15 20 25
Normalised Peak Intensity
Time (Sec x 1000)
Figure 3.8. a. Comparison of end-of-reaction
11
B NMR spectra under representative
conditions. Top: AB dehydrogenation catalyzed by 10% Ru atom of 3.6 after 1.75 hours
in 2:1 diglyme/benzene-d6 at 70 C. Bottom: AB dehydrogenation catalyzed by 10% of
3.7 after 1.75 hours in 2:1 diglyme/benzene-d6 at 70 C. Note the larger proportion of
polyborazylene in the top spectra (24-34 ppm) compared to the bottom spectra. Boron
standard (BF3-OEt2 inset tube) is at 0 ppm. Areas under the peaks are integrated relative
to the external standard BF3-OEt2 peak set to 1.00. b. Peak height of borazine over time
catalyzed by 10% Ru atom of 3.6 (left) and 3.7 (right).
54
3.4.4. Reaction Intermediates
We observe by
11
B NMR that reactions catalyzed by 3.6 generate boron
intermediates common in AB dehydrogenation reactions with catalysts outlined in Figure
1, which include μ-aminodiborane, cyclotriborazane, amine borane cyclic tetramer, and
borazine (3.1). However, at 1% loading of 3.6, we observe the appearance of
polyborazylene (3.2) as early as the first 20 minutes into AB consumption, along with the
aforementioned intermediates. This suggests that as borazine is produced, another
mechanism is concurrently dehydrogenating it into polyborazylene. The borazine
concentration builds steadily until approximately 50% of AB is consumed, and then the
growth of the borazine peak tapers and is slowly consumed as well. This additional
derivatization of borazine mechanism concomitant with the AB dehydrogenation
mechanism results in the less accumulation of borazine at the end of the reaction. When
we compare
11
B NMR spectra of 3.6-catalyzed AB dehydrogenation (10 mol% Ru atom
loading) with our previously-reported reaction catalyzed by the same loading of 3.7,
17
we
observe less borazine (31 ppm) build-up and more polyborazylene (24-34 ppm) in the
reaction catalyzed by 3.6 (Figure 3.8a). Plotting normalized peak height of borazine
against time for reactions catalyzed by 10 mol % of 3.6 and 3.7 (Figure 3.8b), we observe
the decrease of borazine over time with 3.6, while the borazine peak rises in the reaction
catalyzed by 3.7.
In order to probe the homogeneity or heterogeneity of the dehydrogenation
catalysis, we conducted several tests. We added ca. 100 µL of elemental mercury to the
reaction conducted at 10% catalyst loading and monitored AB consumption via
11
B
kinetics in a mercury drop test of homogeneity. While there was a small drop in rate, the
55
reaction proceeded similarly to the parent reaction, completing in approximately the same
amount of time (Figure 3.9). This result suggests homogeneous catalysis, even though
there are observable heterogeneous materials in the reaction. We therefore suspect that
we have a homogeneous active species working in a heterogeneous suspension.
0
0.1
0.2
0.3
0.4
0 4 8 12 16
10% 3.6 with Hg
10% 3.6
[AB] (M)
Time (Sec x 1000)
Figure 3.9. Hg addition homogeniety test in 2:1 diglyme/benzene-d6 at 70 C. Black
circles: 10 mol% of 3.6 with Hg. Blue squares: 10% of 3.6.
3.5 Conclusions
Our efforts focused on maximizing the amount of H2 that could be released from
an equivalent of AB. We probed the effects of soluble calcium ions in solution on AB
dehydrogenation, and found an increase in rate of H2 production with the use of catalytic
Ca
+2
. While we succeeded in achieving a high extent of H2 release with a Shvo-TMEDA
derivative and 1.1 equivalents of TMEDA, we were unsatisfied with the necessary use of
the stoichiometric TMEDA. We eventually optimized our Shvo-based system into
phenanthroline-supported, dicarbonyl, ruthenium acetates to produce 2.7 equiv. of H2 at
low catalyst loading without the use of any additives. The optimized system has desirable
kinetic properties, air and water tolerance, and reusability. We also showed that this
56
catalyst has reactivity with borazine itself, thus minimizing the borazine concentration in
the gas eluent stream of the H2 generation reactor.
3.6 References
1
Travis J. Williams research group, Loker Hydrocarbon Research Institute, Department
of Chemistry. University of Southern California. Los Angeles, CA 90089.
2
Zhang, X.; Foellmer, L. K.; Williams, T. J. Unpublished work.
3
This chapter is reprinted in part with permission from the Royal Society of Chemistry
and thhe authors. Zhang, X.; Kam, L.; Williams, T. J. Dehydrogenation of Ammonia
Borane through the Third Equivalent of Hydrogen. Dalton Trans. 2016, 45, 7672 - 7677.
4
Stephens, F. H.; Baker, R. T.; Matus, M. H.; Grant, D. J.; Dixon, D. A. Acid Initiation
of Ammonia–Borane Dehydrogenation for Hydrogen Storage. Angew. Chem. Int. Ed.
2007, 46, 746-749.
5
(a) Denny, M. C.; Pons, V.; Hebden, T. J.; Heinekey, M.; Goldberg, K. I. Efficient
Catalysis of Ammonia Borane Dehydrogenation. J. Am. Chem. Soc. 2006, 128,
12048−12049. (b) Blaquiere, N.; Diallo-Garcia, S.; Gorelsky, I.; Black, A.; Fagnou, K.
Ruthenium-Catalyzed Dehydrogenation of Ammonia Boranes. J. Am. Chem. Soc. 2008,
130, 14034-14035. (c) Kaβ̈, M.; Fridrich, A.; Drees, M.; Schneider, S. Ruthenium
Complexes with Cooperative PNP Ligands: Bifunctional Catalysts for the
Dehydrogenation of Ammonia–Borane. Angew. Chem. Int. Ed. 2009, 48, 905-907.
6
(a) Keaton, R. J.; Blacquiere, J. M.; Baker, R. T. Base Metal Catalysts for
Dehydrogenation of Ammonia-Borane for Chemical Hydrogen Storage. J. Am. Chem.
Soc. 2007, 129, 1844−1845. (b) Chapman, A. M.; Haddow, M. F.; Wass, D. F. Frustrated
Lewis Pairs Beyond the Main Group: Synthesis, Reactivity, and Small Molecule
57
Activation with Cationic Zirconocene-Phosphinoaryloxide Complexes. J. Am. Chem.
Soc. 2011, 133, 8826-8829. (c) Wright, W. R. H.; Berkeley, E. R.; Alden, L. R.; Baker,
R. T.; Sneddon, L. G. Transition Metal Catalysed Ammonia-Borane Dehydrogenation in
Ionic Liquids. Chem. Commun. 2011, 47, 3177-3179. (d) Kim, S.-K.; Han, W.-S.; Kim,
T.-J.; Kim, T.-Y.; Nam, S. W.; Mitoraj, M.; Piecoś, Ł.; Michalak, A.; Hwang, S.-J.;
Kang, S. O. Palladium Catalysts for Dehydrogenation of Ammonia Borane with
Preferential B−H Activation. J. Am. Chem. Soc. 2010, 132, 9954−9955. (e) Lu, Z.;
Schweighauser, L.; Hausmann, H.; Wegner, H. A. Metal-Free Ammonia–Borane
Dehydrogenation Catalyzed by a Bis(borane) Lewis Acid. Angew. Chem., Int. Ed. 2015,
54, 15556–15559.
7
Lu, Z.; Conley, B. L.; Williams, T. J. A Three-Stage Mechanistic Model for Ammonia–
Borane Dehydrogenation by Shvo’s Catalyst. Organometallics 2012, 31, 6705−6714.
8
Bhattacharya, P.; Krause, J. A.; Guan, H. Mechanistic Studies of Ammonia Borane
Dehydrogenation Catalyzed by Iron Pincer Complexes. J. Am. Chem. Soc. 2014, 136,
11153−11161.
9
Buss, J. A.; Edouard, G. A.; Cheng, C.; Shi, J.; Agapie, T. Molybdenum Catalyzed
Ammonia Borane Dehydrogenation: Oxidation State Specific Mechanisms. J. Am.
Chem. Soc. 2014, 136, 11272−11275.
10
Lu, Z.; Schweighauser, L.; Hausmann, H.; Wegner, H. A. Metal-Free Ammonia–
Borane Dehydrogenation Catalyzed by a Bis(borane) Lewis Acid. Angew. Chem., Int. Ed.
2015, 54, 15556–15559.
58
11
Fazen, P. J.; Remsen, E. E.; Beck, J. S.; Carroll, P. J.; McGhie, A. R.; Sneddon, L. G.
Synthesis, Properties, and Ceramic Conversion Reactions of Polyborazylene. A High-
Yield Polymeric Precursor to Boron Nitride. Chem. Mater. 1995, 7, 1942–1956.
12
Zhang, X.; Lu, Z.; Foellmer, L. K.; Williams, T. J. Nitrogen-Based Ligands Accelerate
Ammonia Borane Dehydrogenation with the Shvo Catalyst. Organometallics 2015, 34,
3732–3738.
13
Diyabalanage, H. V. K.; Shrestha, R. P.; Semelsberger, T. A.; Scott, B. L.; Bowden, M.
E.; Davis, B. L.; Burrell, A. K. Calcium Amidotrihydroborate: A Hydrogen Storage
Material. Angew. Chem. Int. Ed. 2007, 46 (47), 8995–8997.
14
ICP analysis was conducted by Rudiger Laufhutte at the University of Illinois.
15
Park, Y. J.; Ziller, J. W.; Borovik, A. S. The Effects of Redox-Inactive Metal Ions on
the Activation of Dioxygen: Isolation and Characterization of a Heterobimetallic
Complex Containing a Mn
III
–(μ-OH)–Ca
II
Core. J. Am. Chem. Soc. 2011, 133, 9258–
9261.
16
(a) Krishnamurthy, G. N.; Shashikala, N. Synthesis of Ruthenium(II) Carbonyl
Complexes with 2-Monosubstituted and 1,2-Disubstituted Benzimidazoles. J. Serb.
Chem. Soc. 2009, 74, 1085–1096. (b) Black, D.; Deacon, G. N. Thomas, Ruthenium
Carbonyl Complexes. I. Synthesis of [Ru(CO)2(bidentate)2]2+ Complexes. Aust. J. Chem.
1982, 35, 2445–2453. (c) Colton, R.; Farthing, R. Carbonyl Halides of the Group VIII
Transition Metals. I. Dicarbonyldihalogenoruthenium(II) and Related Compounds. Aust.
J. Chem. 1967, 20, 1283. (d) Anderson, P. A.; Deacon, G. B.; Haarmann, K. H.; Keene,
F. R.; Meyer, T. J.; Reitsma, D. A.; Skelton, B. W.; Strouse, G. F.; Thomas, N. C.
Designed Synthesis of Mononuclear Tris(heteroleptic) Ruthenium Complexes Containing
59
Bidentate Polypyridyl Ligands. Inorg. Chem. 1995, 34, 6145–6157. (e) Frediani, P.;
Bianchi, M.; Salvini, A.; Guarducci, R.; Carluccio, L. C.; Piacenti, F.; Ruthenium
Carbonyl Carboxylates with Nitrogen-Containing Ligands: II. Synthesis and
Characterization of Mononuclear Compounds. J. Organomet. Chem. 1994, 476, 7-11.
17
Conley, B. L.; Williams, T. J. Dehydrogenation of Ammonia-Borane by Shvo's
Catalyst. Chem. Commun. 2010, 46, 4815−4817.
60
Chapter 4. Transition Metal Catalyzed Decarboxylation of Formic Acid and
Ammonium Boroformate
4.1 Introduction to Formic Acid: A Method for Spent Fuel Regeneration
Once the issue of maximum hydrogen production has been addressed, the next
major hurdle in the practicability of ammonia borane in alternative energy is the reuse
and recycling of the spent fuel. To create a renewable source of energy for the hydrogen
economy, the byproducts of the AB dehydrogenation reaction should be repurposed into
AB to be dehydrogenated again.
1
Direct hydrogenation of borazine and polyborazylene
will be too energetically unfavorable to be efficient,
2
and therefore transition metal
catalyzed regeneration of AB was studied as a more promising route. A compound that
may digest borazine or polyborazylene and then participate in rehydrogenating the spent
fuel via transition metal catalysis is needed.
3
Formic acid (FA, H2CO2, 4.1) is an
inexpensive, mass produced chemical that has been investigated as a potential H2 source.
4
Its decarboxylation or dehydrogenation forms H2 and CO2 as the only products as shown
in Scheme 4.1. The CO2 can be reduced to produce more formic acid resulting in a
carbon neutral cycle. The goal is to use FA as a multipurpose compound that may
facilitate various steps in the overall cycle. Dr. Brian Conley began this work, and he
generated the data in Figure 4.1.
5
Scheme 4.1. Dehydrogenation/Decarboxylation of formic acid into H2 and CO2.
61
4.2 AB Spent Fuel Regeneration Cycle Using Formic Acid
Scheme 4.2 shows the proposed the dehydrogenation of AB and then its
regeneration facilitated by FA. First step of the cycle is to obtain the spent fuel
byproducts from an AB dehydrogenation reaction. Most likely, they will consist of
borazine and polyborazylene. When reacted with FA, step 1 shows the borazine and
polyborazylene will form ammonium boroformate (ABF, NH4B(COOH)4, 4.2). Next in
step 2, the formate from the boroformate will be transferred onto a ruthenium metal
center with the aid of a boron Lewis acid on an appended ligand. In step 3, the transition
metal formate will then lose CO2 resulting in a metal hydride. This metal hydride now
back-transfers onto the boron creating a borohydride (B-H) bond in step 4. Now, the
process will repeat three more times until all the formates on the boron are transformed
into borohydrides resulting eventually in ammonium borohydride (NH4BH4, 4.5), which
can be dehydrogenated back to AB in step 5 to complete the cycle.
6
Scheme 4.2. Proposed spent fuel regeneration cycle using FA.
62
Some key features of this proposed regeneration cycle include the use of a cheap,
abundant reducing agent (FA), clean waste stream of only H2 ad CO2 where the CO2 can
be reduced to form more FA, and the use of B-O bonds. This counterintuitive strategy to
incorporate B-O bonds is to encompass any potential B-O bonds formed during AB
dehydrogenation, and therefore this technology can also use the waste stream produced
during AB hydrolysis.
4.3 Derivatization of Borazine by Formic Acid
The first step to the regeneration cycle is the formic acid digestion of the spent
fuel products of borazine and polyborazylene.
7
Scheme 4.3 shows the reaction we hope
to observe upon the treatment of borazine (4.6) and polyborazylene (4.7) with FA (4.1) to
yield ammonium boroformate (4.2).
Scheme 4.3. Goal of treating borazine and polyborazylene with FA to form ABF.
Borazine was synthesized via the method by Sneddon and Wideman
8
and treated
with FA in various solvents including CDCl3, CD3CN, DMF, and diglyme. Figure 4.1
below shows the
1
H NMR of the treatment of borazine with FA in CD3CN at room
temperature.
9
After the addition of FA, the broad NH and BH peaks of borazine
disappeared and three new formate peaks appeared downfield the formate peak of formic
acid (8.06 ppm). However, a majority of the material in NMR tube precipitated upon
63
addition of FA, which suggests that the ammonium boroformate, if formed, has poor
solubility in organic NMR solvents.
Figure 4.1.
1
H NMR of borazine reaction with FA in CD3CN at room temperature
immediately following addition. Top: Borazine. Middle: Solution after the addition of
FA. Bottom: Zoomed formate region of middle spectra.
While FA derivatized borazine, the identities of the three resulting formate peaks
are not confirmed. One of the peaks could belong to ABF and therefore ABF must be
64
synthesized separately in order for it to be compared to the digested material. Synthesis
of ABF is discussed in section 4.5.
4.4 Catalytic Decarboxylation of Formic Acid
In the second and third steps of the cycle, a formate moiety needs to coordinate to
the catalyst in order for it to be dehydrogenated or decarboxylated. For convenience and
availability, FA was used as the test compound for certain catalysts when screening
formate decarboxylation. Scheme 4.4 below shows a modified ruthenium mediated FA
decarboxylation. The formate from FA will coordinate to the Ru and B and
decarboxylation will leave a metal-boron hydride. Once catalyst and conditions were set
for FA, a fresh batch of ABF would be synthesized for further optimization.
Scheme 4.4. Proposed ruthenium catalyst appended boron Lewis acid assisted FA
decarboxylation.
The first catalyst investigated was a chlororuthenium cymene complex with a
dipyridylborate ligand attached previously reported by our laboratory.
10
4.8 (Scheme 4.5)
was synthesized from di-μ-chlorobis[(p-cymene)chlororuthenium(II)] and
[Na(CH3)2B(py)2]. Scheme 4.4 shows the modified scheme specifically using 4.8 to
decarboxylate FA. In order for FA to coordinate to the metal, the cymene fragment of the
catalyst must dissociate from the metal center opening the necessary active sites for
catalysis, and coordinating solvent molecules can stabilize the resulting Ru complex. 4.8
must also lose one of the boron methyl groups as methane with the additional proton
coming from FA. The formate can then coordinate to the resulting complex, and the
65
coordinated formate will decarboxylate producing CO2 and a Ru-H-B complex. The Ru-
formate should be regenerated when the proton from a second FA molecule forms H 2
with the hydride from the Ru-hydride complex.
Scheme 4.5. Hypothesized 4.8-catalyzed FA decarboxylation.
Firstly, catalyst 4.8’s ability to decarboxylate FA had to be determined before
optimization. At a high catalyst loading of 20%, 4.8 and FA were loaded into a J-Young
tube, and the reaction was monitored by
1
H NMR arrays. Figure 4.2 shows the stacked
1
H
NMR spectra of the reaction hypothesized in Scheme 4.4 monitored over 3 hours and 50
minutes at 70 °C in deuterated acetonitrile (d-ACN, CD3CN). Acetonitrile (ACN) was
chosen for its ligation properties to Ru, and its presence can help stabilize the metal
complex formed after the cymene ligand dissociates. In d-ACN, FA’s formate peak
appears at 8.1 ppm and decreases over time indicating consumption. CH4 gas appears at
0.23 ppm and increases for the first third of the reaction indicating loss of one of the
boron methyl’s from 4.8. H2 gas is generated at 4.6 ppm, but the peak grows until
approximately halfway through the reaction where it is likely consumed by a substrate in
the reaction due the reaction being a closed system where H2 could not escape. Even
though H2 and CH4 were observed in the NMR, the NMR could not quantify the amount
of gas produced. Therefore, eudiometry experiments were conducted in which the gas
bubbles were passed through an inverted burette of oil. The oil used in the burette
66
ensured that if CO and water were produced as the decarbonylation products of FA, the
water produced would be immiscible with the oil and form an observable layer. There
was no such water/oil layer observed in the eudiometry experiments. So, coupled with
the H2 peak observed by NMR, the conclusion was that FA was decarboxylating into CO2
and H2.
Figure 4.2.
1
H NMR of 4.8 (20 mol %) catalyzed FA decarboxylation in CD3CN for 3
hours and 50 min at 70 °C.
After showing that 4.8 can decarboxylate FA at mild temperatures in ACN, the
catalyst system had to be optimized to use lower catalyst loadings and find the optimal
FA concentration in ACN. Screening of solvent amounts was done to find the ideal
amount of ACN needed to release the most amount of gas from FA. Figure 4.3
summarizes the eudiometry results obtained from the ACN screening using 4.8 as the
catalyst in 1% loading in respect to FA.
67
0
10
20
30
40
50
60
0 200 400 600 800 1 10
3
1.2 10
3
1.4 10
3
Neat FA
Catalytic amount of MeCN
0.25 mL of MeCN
0.5 mL of MeCN
1 mL of MeCN
5 mL of MeCN
% Conversion to H
2
Time (min)
Figure 4.3. Graph summarizing optimal amount of ACN for the decarboxylation of FA
by 1.0 mol % of catalyst 4.8 at 70 °C.
1.0 mL or 20 equivalents of ACN to formic acid was the optimal amount of
solvent for catalyst 4.8 to dehydrogenate FA. However, the best conversion of FA to H2
was a low 53%, and therefore catalyst 4.8 wasn’t the most efficient catalyst for FA
decarboxylation. Therefore, other catalyst scaffolds were investigated in search of higher
FA conversion. In addition to 4.8, transition metal compounds such as [RuCymCl2]2
(4.9), Ru3(CO)12 (4.10), [RuCl2(CO)3]2 (4.11), [Rh(COD)Cl]2 (4.12), [Ir(COD)Cl]2
(4.13), and Os3(CO)12 (4.14) shown in Figure 4.4, were screened, and their results are
summarized in Figure 4.5. At 1% catalyst loading, 1 mL of ACN at 70 °C, ruthenium
68
dimer [RuCymCl2]2 indicated by the white circles in Figure 4.4 was the fastest catalyst
and produced the best conversion up to almost 100%.
Figure 4.4. Catalysts screened for the decarboxylation of FA in addition to 4.8.
69
0
20
40
60
80
100
0 100 200 300 400 500 600
[RuCl
2
(CO)
3
]
2
Ru
3
(CO)
12
[Rh(COD)Cl
2
]
2
[Ir(COD)Cl
2
]
2
Os
3
(CO)
12
[RuCymCl
2
]
2
LRuClCym (4.8)
% Conversion to H
2
Time (min)
Figure 4.5. Catalyst screening for best conversion of FA at 1% catalyst loading, 1 mL of
ACN at 70 °C.
4.5 Catalytic Decarboxylation of Ammonia Boroformate
After finding a catalyst system that was capable of decarboxylating FA, the next
step was to test that catalyst system on the decarboxylation of ABF. ABF was
synthesized from AB and FA in the presence of THF and recrystallized in MeOH
(Scheme 4.6).
11
Scheme 4.6. Synthesis of ABF (4.2).
70
Figure 4.6. NMR spectra of ABF in CD3OH. Top:
1
H NMR. Middle:
13
C NMR. Bottom:
11
B NMR.
The white solid that resulted is very hygroscopic and insoluble in most organic
solvents, which is consistent with our previous finding. The
1
H NMR and
11
B are shown
below in Figure 4.6. The half-height width of the formate peak in ABF is twice as wide
as the formate peak in ammonium formate, and the
11
B peak corroborates the presence of
boron in the recrystallized compound. Unfortunately, the compound’s poor solubility
means that ACN is not a good solvent to test ABF’s reactivity. Therefore, the solvent
system used for decarboxylating FA could not be applied to ABF, and we had to choose
another aprotic solvent that could solubilize ABF with a high enough boiling point to run
decarboxylation reactions.
71
Scheme 4.7. Hypothesized decarboxylation and decomplexation of ABF, and eventual
dehydrogenation of AB.
Deuterated dimethyl sulfoxide (DMSO-d6) was chosen for its high boiling point
and the ability to solvate ABF, and it is a solvent from which water and air could be
removed. Scheme 4.7 illustrates the planned pathway of ABF decomposition. At 5%
catalyst loading in DMSO-d6 at 70 °C, a series of transition metals such as, [RuCymCl2]2
(4.9), Ru3(CO)12 (4.10), [RuCl2(CO)3]2 (4.11), Os3(CO)12 (4.14), and [FeCp(CO)2]2 (4.15)
were screened (Figure 4.7). Reactions were conducted in J-Young NMR tubes and
submerged in a 70 °C oil bath and monitored by
1
H NMR every few hours. The formate
peak of ABF appears at 8.4 ppm in DMSO-d6, and Figure 4.8 (a-e) summarizes the
results of ABF decarboxylation by
1
H NMR.
Figure 4.7. Catalysts screened to decarboxylate ABF.
72
a. Top: [FeCp(CO)2]2 catalyzed ABF Derivatization. Bottom: increased intensity
spectrum.
73
b. Top: [RuCymCl2]2 catalyzed ABF Derivatization. Bottom: increased intensity
spectrum.
74
c. Top: Ru3(CO)12 catalyzed ABF Derivatization. Bottom: increased intensity spectrum.
75
d. Top: [RuCl2(CO)3]2 catalyzed ABF Derivatization. Bottom: increased intensity
spectrum.
76
e. Top: Os3(CO)12 catalyzed ABF Derivatization. Bottom: increased intensity spectrum.
Figure 4.8. ABF decarboxylation by various catalysts (a-e) at 5% loading, 70 °C in
DMSO-d6.
77
The most exciting result was [FeCp(CO)2]2 (4.15) catalyzed consumption of ABF
in under 23 hours forming a new formate species that is eventually decomposed into
another species (Figure 4.8a). A small H2 gas peak is observed at 4.6 ppm after 23 hours
but is consumed after further heating. The wide ammonium peak at 6.5 ppm persists
throughout the reaction, but a new wide peak at 9-10 ppm emerges suggesting the
presence of a new N-H or O-H containing compound. Iron is very desirable as a catalyst
platform because of its abundance and availability. If confirmed that [FeCp(CO)2]2 is
indeed decarboxylating ABF into CO2 and borohydrides, a large potential venue for
cheap, reactive iron-based catalysts could be developed for the overall fuel cycle.
The same catalyst that decarboxylated FA in ACN, [RuCymCl2]2 (4.9), was also
effective at decomposing ABF in under 48 hours (Figure 4.8b). The reaction is almost
complete after 27 hours with both the ammonium peak and the formate peaks
disappearing. H2 gas peak is also apparent after 3.5 hours but is also subsequently
consumed later in the reaction. Triruthenium dodecacarbonyl (4.10) also consumes ABF
albeit slowly in Figure 4.8c. After 96 hours, the resulting H2 peak persists unlike the
other catalysts, and the formate and ammonium peaks are consumed. This catalyst may
have potential benefits by taking advantage of its slow reaction time and prevent side
reactions such as the consumption of the generated H2. Figure 4.8d shows [RuCl2(CO)3]2
(4.11) catalyzed ABF Derivatization with full consumption of the formate peak in only
24 hours. H2 gas is again observed, along with the emergence of a new N-H containing
compound at 6.1 ppm. With all Ru catalysts, we observe additional alkyl peaks at 2.8
and 3.2 ppm. With no other alkyl proton sources in the reaction, these might be the result
of solvento DMSO molecules binding via sulfur and oxygen to the metal.
12
78
Lastly, we tried the triosmium analog of the dodecacarbonyl complex (4.14), and
Figure 4.8e shows similarly slow consumption of the formate peak, and H2 peak that
persists throughout the reaction. With preliminary NMR data in hand, the next step is to
identify the species formed after ABF is consumed. Eudiometry experiments with
analysis of the gaseous components will show if ABF is being decarboxylated by the
catalysts.
4.6 Conclusions
In conclusion, a spent fuel regeneration cycle was proposed, and its necessary
pieces were broken down into separate goals to achieve. Firstly, formic acid was shown
to derivatize borazine. Various catalysts and solvent conditions were screened to
determine their efficiency at the decarboxylation of formic acid. Ammonium boroformate
was synthesized, and a catalyst screening to study the consumption of ABF was
conducted via NMR kinetics. The [RuCymCl2]2 is the leading catalyst candidate for the
derivatization of both FA and ABF. Inexpensive iron complex [FeCp(CO)2]2 will also be
developed as a promising candidate to both digest spent fuel byproducts and
decarboxylate ABF.
4.7 References
1
Fazen, P. J.; Remsen, E. E.; Beck, J. S.; Carroll, P. J.; McGhie, A. R.; Sneddon, L. G.
Synthesis, Properties, and Ceramic Conversion Reactions of Polyborazylene. A High-
Yield Polymeric Precursor to Boron Nitride. Chem. Mater. 1995, 7, 1942–1956.
79
2
(a) Stephens, F. H.; Pons, V.; Baker, R. T. Ammonia-Borane: The Hydrogen Source Par
Excellence? Dalton Trans. 2007, 2613-2626. (b) Marder, T. B. Will We Soon Be Fueling
our Automobiles with Ammonia–Borane? Angew. Chem. Int. Ed. 2007, 46, 8116-8118.
3
Sutton, A. D.; Burrell, A. K.; Dixon, D. A.; Garner, E. B.; Gordon, J. C.; Nakagawa, T.;
Ott, K. C.; Robinson, J. P.; Vasiliu, M. Regeneration of Ammonia Borane Spent Fuel by
Direct Reaction with Hydrazine and Liquid Ammonia. Science 2011, 331, 1426–1429.
4
Joo, F. Breakthroughs in Hydrogen Storage—Formic Acid as a Sustainable Storage
Material for Hydrogen. ChemSusChem. 2008, 1, 805–808.
5
Zhang, X.; Conley, B. L.; Williams, T. J. Unpublished work.
6
Hausdorf, S.; Baitalow, F.; Wolf, G.; Mertens, F. O. R. L. A Procedure for the
Regeneration of Ammonia Borane from BNH-Waste Products. Int. J. Hydrogen Energy
2008, 33, 608-614.
7
Borazine synthesis was conducted by Dr. Brian Conley. Travis J. Williams research
group, Loker Hydrocarbon Research Institute, Department of Chemistry. University of
Southern California. Los Angeles, CA 90089.
8
Wideman, T.; Sneddon, L. G. Convenient Procedures for the Laboratory Preparation of
Borazine. Inorg. Chem. 1995, 34, 1002–1003.
9
Initial treatment of borazine by formic acid was conducted by Dr. Brian Conley.
10
(a) Conley, B. L.; Williams, T. J. Thermochemistry and Molecular Structure of a
Remarkable Agostic Interaction in a Heterobifunctional Ruthenium-Boron Complex. J.
Am. Chem. Soc. 2010, 132, 1764–1765. (b) Conley, B. L.; Williams, T. J. Dual Site
Catalysts for Hydride Manipulation. Comments Inorg. Chem. 2011, 32, 195–218. (c)
80
Conley, B. L.; Williams, T. J. A Robust, Air-Stable, Reusable Ruthenium Catalyst for
Dehydrogenation of Ammonia Borane. J. Am. Chem. Soc. 2011, 133, 14212−14215.
11
First synthesis of ABF was done by Dr. Brian Conley.
12
Hazarika, P.; Deka, J.; Bhola, S.; Bhola, R.K.; Medhi, C.; Medhi, O. K. DNA binding
Properties and Biological Studies of cis-dichloro-tetrakis(dimethylsulphoxide)-
ruthenium(II) Complex. Int. J. Drug Discov. 2012, 3, 907-913.
81
Chapter 5. Acceptorless Dehydrogenation and Dehydrative Coupling by 2-((di-tert-
butylphosphino)methyl)pyridine-Supported Ru and Ir Catalysts
This work is done in collaboration with Jeff Celaje who synthesized the catalysts,
performed the 1-phenylthanol coupling reactions, and initial screening reactions.
1
Forrest
Zhang and Lisa Kam developed HPLC methods for separation of product mixtures.
2
5.1 Introduction to Acceptorless Dehydrogenation and Dehydrative Coupling Reactions
As our group as an overall focus on developing chemistry that is environmentally
friendly, we have become interested in providing green alternatives to current synthetic
practices, hoping to avoid harsh chemicals, reduce total reagents used, and minimize the
amount of waste generated. Keeping with the theme of utilizing hydrogen, we studied
acceptorless dehydrogenations (AD), which are reactions where hydrogen gas is liberated
and potentially repurposed. Typically, dehydrogenation of organic compounds often
requires stoichiometric amounts of hydrogen acceptors or harsh oxidants. Thus, AD
reactions are a type of green chemistry because it produces clean hydrogen as a
byproduct, is atom economical (no H2 acceptor needed), and uses milder conditions.
3
We
began with alcohol AD reactions, because AD is a good route to install a versatile
aldehyde or ketone without the use of harsh reagents that may compromise the rest of a
complex molecule with sensitive functionality. From an alternative fuel standpoint,
simple alcohols could be a cheap source of hydrogen gas, and esterification is an essential
reaction in the synthetic chemistry of pharmaceutical drugs and fragrances. Scheme 5.1
compares traditional transfer hydrogenation that requires a stoichiometric hydrogen
acceptor in the form of benzophenone vs AD where H2 is liberated. Current examples of
alcohol AD reactions include the oxidation to aldehydes
4
and ketones,
5
coupling to esters,
82
acetals, carboxylic acids,
3
and coupling of alcohols with amines to produce amines,
6
imines,
7
and amides.
8
Scheme 5.1. AD versus traditional dehydrogenation or oxidations.
Often, the hydrogen gas produced from the initial dehydrogenation can be used in
the same reaction pot to affect further transformations. The “borrowing hydrogen”
mechanism shown in Scheme 5.2 is an example of this repurposing of the generated
hydrogen. The H2 produced from the dehydrogenation of an alcohol into an aldehyde
later hydrogenates the imine formed after a condensation reaction to yield an alkylated
amine as the final product. The overall coupling reaction between the alcohol and amine
is called “dehydrative coupling” from the net loss of water.
Scheme 5.2. “Borrowing Hydrogen” mechanism of an alcohol-amine couple reaction.
5.2 Milstein Pincer Catalysts for AD and Coupling Reactions
A leader in the AD and coupling chemistry is Israeli chemist, David Milstein. He
and his group utilize a class of “pincer” ligand supported metal complexes that are
capable of splitting hydrogen via homogeneous catalysis. The “pincer” designation refers
83
to the mer tridentate coordination of donor atoms. Figure 5.1 gives a two examples of
pincer-bound ruthenium catalysts known to split H2.
3, 9
The pincer ligands below are
colloquially called PNP and PNN pincers after the coordinating atoms.
a.
b.
Figure 5.1. a. Pincer scaffold on a metallic species where M = metal. b. Examples of
Milstein’s ruthenium pincer catalysts.
These catalysts split hydrogen in the presence of base where the methylene
sidearm of the pincer ligand is deprotonated resulting in the concomitant dearomatization
of the pyridine ring in a ruthenium complex (5.1) shown in Scheme 5.3. The generated
complex (5.2) can now perform hydrogen splitting via metal-ligand cooperation into a re-
protonated sidearm and metal hydride. The resulting metal hydride (5.3) may now
transfer the proton and/or hydride to a receiving substrate. The reversibility of species
5.2 and 5.3 demonstrate the flexibility these compounds in both hydrogenation and
dehydrogenation reactions. Milstein and coworkers have explored their ruthenium-pincer
catalyzed AD and coupling of alcohols to esters,
10
and the reaction of alcohol and
ammonia to primary amines.
3, 6, 9a
The primary amine synthesis did not further yield
alkylated amines, and all of these AD or coupling reactions require the use of base and
solvent.
84
Scheme 5.3. Hydrogen splitting of pincer type catalysts in the presence of base.
5.3 Bidentate Phosphorous-Nitrogen Ligand Design and Catalyst Syntheses
Examining the Milstein catalysts carefully, we determined that the active portion
of the catalyst is centered upon the methylene arm and the pyridine system, and the rest
of the ligands often consist of a metal hydride, carbonyl, and chloride. The chloride is
extruded upon addition of base but the carbonyl and initial metal hydride only function as
spectator ligands stabilizing the metal complexes. Thus, we hoped to use only the active
pyridylphosphine (PN) portion of the catalyst and in turn, have the option of changing the
spectator ligands in order to tune catalysts in accordance to reactions of interest. Jeff
Celaje synthesized an array of a 2-((di-tert-butylphosphino)methyl)pyridine (PN ligand)
supported ruthenium, iridium, and rhodium complexes, and a select few are featured in
Figure 5.2. He has shown recently that catalyst 5.4 has extraordinary reactivity in
dehydrogenation of formic acid to H2 and CO2, and that the methylene arm on the PN
ligand can be selectively deprotonated generating complex 5.7.
11
85
Figure 5.2. Iridium (5.4 and 5.5) and ruthenium (5.6) complexes supported with PN
ligand.
Of those, the iridium 5.5 and ruthenium 5.6 showed reactivity in AD of alcohols
during initial screening reactions. The iridium catalyst 5.5 was synthesized from the
reaction of [IrCl2(Cp*)]2 with 2-((di-tert-butylphosphino)methyl)pyridine in excess
NaOTf (Scheme 5.4). The ruthenium complex 5.6 was synthesized in a similar fashion
from the reaction of [RuCl2(Cym)]2 and the PN ligand in the presence of excess NaOTf
(Figure 5.3). Ruthenium 5.6 was characterized by single X-ray crystallography.
Scheme 5.4. Synthesis of PN ligand supported iridium Cp* complex 5.5.
86
Figure 5.3. Synthesis of ruthenium 5.6.
5.4 AD of Alcohols to Ketones and Esters, and Dehydrative Coupling of Alcohols to
Ethers
We initially studied the reactivity of complex 5.6 in AD reactions with benzyl 1-
phenylthanol (5.8), benzyl alcohol (5.9), and 1-octanol (5.10). These alcohols were
chosen initially for their high boiling points, low cost, and availability. Iridium complex
5.5 also had AD reactivity with these alcohols, but 5.5 catalyzed reactions had overall
lower conversions than 5.6 catalyzed AD reactions. Typically, AD reactions are
conducted in the presence of base analogous to the Milstein examples in order to generate
the deprotonated active catalyst. Thus, we heated 1-phenylethanol 5.8 with 1 mol% of
ruthenium 5.6 and 10 mol% of KOH to 130 °C for 40 hours (Scheme 5.5) in mesitylene.
Under these conditions, 5.8 is oxidized into acetophenone in 87% conversion by NMR.
Interestingly, when alcohol 5.8 is heated with catalyst 5.6 in the absence of base,
dehydrative coupling to yield a diastereomeric mixture of ethers occurs. We optimized
the reaction to 0.15 mol% catalyst loading and heating a neat solution 5.8 to 130 °C for
40 hours. This results in 86% conversion by NMR to give a mixture of ethers as well as
acetophenone (16%) (Scheme 5.5). Thus, the preferred mechanism differs depending on
the presence or absence of base. However, acceptorless dehydrogenation is occurring
whether base is present or not because the ketone is formed in both reactions. The
87
coupled ether product was also observed with the benzyl alcohol (5.9) coupling reaction
without base (Scheme 5.6).
Scheme 5.5. Reactions of Catalyst 5.6 with 1-Phenylethanol (5.8).
Scheme 5.6. Coupling reaction of benzyl alcohol (5.9) without base.
However, when we attempted the coupling of 1-octanol 5.10 for an aliphatic
substrate, we observed different results. In the presence of 0.2% 5.6 and 10 mol% KOH,
coupling to the ester occurs (Scheme 5.7). In the absence of base, no coupled product
was observed. Reaction optimization of the reaction of 1-octanol with 10% KOH yielded
30% conversion of octanol to the ester in 53 hours at 130 °C at 3 mol% 5.6 loading.
Scheme 5.7. Reactions of Catalyst 5.6 with 1-Octanol (5.10).
88
5.5 Iridium Catalyzed Guerbet-like Coupling of Alcohols to Longer Chain Alcohols
While screening reactivity of 1-octanol with various catalysts in base and absence
of base conditions, we tried iridium catalyst 5.5 and KOH to catalyze the ester formation.
However, we did not observe the usual NMR triplet at 4.05 ppm (in CDCl3) that is
usually indicative of an octyl octanoate ester product. Instead, there was only one product
indicated by a doublet at 3.6 ppm after 48 hours at 130 °C in neat 1-octanol. This
selective product was the result of a C-C coupling reaction between two molecules of 1-
octanol in an aldol condensation, then the unsaturated product is re-hydrogenated to give
final product 5.11 shown in Scheme 5.8.
Scheme 5.8. C-C coupling of 1-octanol catalyzed by 5.5.
This set of transformations is called the Guerbet reaction (Scheme 5.9) after
1800’s chemist, Marcel Guerbet, and is of interest today for its potential uses in the
conversion of cheap, light alcohols such as ethanol into usable longer chain alcohols such
as butanol for alternative fuel purposes.
12
Butanol is considered a great substitute for
gasoline because of its similar energy profile and non-corrosive nature.
13
89
Scheme 5.9. Guerbet reaction. Two equivalents of alcohols are dehydrogenated to
aldehydes. The aldehydes undergo an aldol condensation and the enone is hydrogenated
to form a saturated alcohol as the final product.
The logical next step was to use 5.5 to catalyze ethanol to form butanol. 17%
conversion to butanol was observed at 150 °C catalyzed by 0.2 mol% 5.5 and 10 mol%
KOH (Scheme 5.10). Although 34% conversion seems low, the leading result from
literature for this transformation is only 37% yield.
12
With more reaction optimization,
this 5.5 catalyzed reaction could make major contributions to the butanol biofuel
industry.
Scheme 5.10. Guerbet reaction of ethanol to butanol catalyzed by 5.5.
5.6 Dehydrative Coupling of Alcohols and Amines to Alkylated Amines
While the alcohol homocoupling was interesting, and the iridium catalyzed
Guerbet reaction had potential, we decided to focus on the most promising chemistry
observed thus far, which was the dehydrative coupling of alcohol and amines into
alkylated amines. Alkylation of amines is of great importance in organic synthesis
because alkylated amines are intermediates in a wide range of useful compounds such as
polymers, dyes, agrochemicals, and pharmaceuticals. The alcohol-amine coupling
90
reaction was discovered from an initial nucleophile screening between 1-phenylethanol
(5.8) and hexadecylamine (5.12a). We sought to optimize the coupling of benzyl alcohols
with amines because coupling of benzyl alcohols with other alcohols were complicated
by competing homo-coupling and hetero-coupling, as well as by isolation problems. We
found that coupling of benzyl type alcohols with various amines generally proceeded
with complete conversion under 24 hours at 110 °C or 130 °C in the presence of 1-5
mol% of complex 5.6. Benzyl alcohol (5.9) and amine dehydrative coupling generally
proceeded at 1 mol% of 5.6, at 110 °C, in neat alcohol and amine, and completed in
under 24 hours with a few exceptions. The reaction tends to over-alkylate at 130 °C in 24
hours producing a mixture of products with no observable ester product(s) (Scheme
5.11). 1-phenylethanol (5.8) reactions required more catalyst (5 mol%) and higher
temperatures of 130 °C for 24 hours and was less likely to contain over-alkylated
products than benzyl alcohol reactions.
Scheme 5.11. Over-alkylation of benzyl alcohol at higher temperatures.
Table 5.1 shows the different amines coupled with 1-phenylethanol (5.8), and
Table 5.2 shows the substrate scope for benzyl alcohol (5.9) and various amines. Note
that in both of these substrate tables, the coupling reaction tolerates sensitive
functionality on the amine. Tryptamine’s (5.13a) sensitive indole functionality is
unaffected by our reaction conditions. The reaction selectively couples tyramine (5.15a)
91
at the amine and not the phenol. The reaction also couples secondary amines such as 4-
methylpiperidine (5.18a) to form tertiary amines. Complex 5.6 catalyzes the clean
coupling between benzyl alcohol and 3,5-dimethylaniline (5.17a) despite the reaction
between benzyl alcohol and aniline did not result in a single product. Similar amine
substrates are presented with 1-phenylethanol and substituted 1-phenylethanol electron-
rich or electron-poor aryl substituents are also shown to couple.
14
All alcohol-amine
reactions are conducted neat, using the alcohol as the only solvent for the solid catalyst,
thus generating less chemical waste.
92
Table 5.1. Secondary benzyl alcohol (1-phenylethanol, 5.8) and amine (5.12a-16a)
coupling. Yields are isolated via column chromatography unless stated otherwise.
93
Table 5.2. Primary benzyl alcohol (5.9) and amine (5.12a-18a) coupling. Yields are
isolated via column chromatography unless stated otherwise.
94
5.7 Mechanistic Insight into Dehydrative Coupling of Alcohols and Amines
We propose a Ru-benzyl mechanism (Scheme 5.12) based on the data collected
such as the lack of reaction expected from 1-octanol, and the observation that 3,5-
dimethylaniline (5.17a) could be coupled to benzyl alcohol while the parent aniline
cannot. Initially, the catalyst loses the cymene ligand, freeing up the necessary
coordination sites for the benzyl alcohol and amine to bind. The ruthenium metal is
oxidized to ruthenium(IV) either before or after the condensation step, and the coupled
product is reductively eliminated from the metal center. Based on our observations, the
increased bulk of 3,5-dimethylaniline, compared to the parent aniline, supports a
reductive elimination step that relieves steric strain from the metal center. Visually, we
have observed that the reaction turns from the usual orange-brown to green when left out
open in air. The green color is consistent with a Ru
IV
species. While we cannot observe a
ruthenium-benzyl species in the
1
H NMR, however, we can observe the cymene ligand of
5.6 dissociating from the metal center, and that the PN ligand is unchanged. Without
base, the deprotonation of the methylene arm is unlikely.
Scheme 5.12. Proposed Ru-benzyl mechanism.
95
The “borrowing hydrogen” mechanism is still a likely candidate, and we can
begin to test its validity by using tertiary benzyl alcohols as the substrate, as tertiary
alcohols cannot be oxidized in this way. We can also gain more mechanistic insight by
probing if a benzyl group is necessary for this transformation. The aryl group may serve
only as a directing group by binding to the metal, and if this is true, substrates such as 2-
phenyl-1-ethanol would also couple with amines.
5.8 Conclusions
In conclusion, acceptorless dehydrogenation and coupling reactions are useful,
environmentally benign synthetic routes to useful transformations. We discovered the AD
reactions of alcohols to ketones and esters catalyzed by ruthenium complex 5.6 in the
presence of base. Without base, we observed a dehydrative coupling reaction to yield
ethers instead. In the case of iridium catalyst 5.5 and KOH, 1-octanol couples via C-C
coupling to yield a Guerbet-like product that could have potential uses in the
investigation of ethanol to butanol transformations for biofuels. 5.6 also catalyzed amine
coupling reactions with benzyl alcohol and 1-phenylethanol that boast high conversions
and isolated yields, and mechanistic studies are underway.
5.9 References
1
Celaje, J.; Zhang, F.; Kam, L.; Williams, T.J. Unpublished work.
2
Travis J. Williams research group, Loker Hydrocarbon Research Institute, Department
of Chemistry. University of Southern California. Los Angeles, CA 90089.
3
Gunanathan, C.; Milstein, D. Applications of Acceptorless Dehydrogenation and
Related Transformations in Chemical Synthesis. Science 2013, 341, 1229712.
96
4
Kawahara, R.; Fujita, K.; Yamaguchi, R. Dehydrogenative Oxidation of Alcohols in
Aqueous Media Using Water-Soluble and Reusable Cp*Ir Catalysts Bearing a Functional
Bipyridine Ligand. J. Am. Chem. Soc. 2012, 134, 3643–3646.
5
Chakraborty, S.; Lagaditis, P. O.; Förster, M.; Bielinski, E. A.; Hazari, N.; Holthausen,
M. C.; Jones, W. D.; Schneider, S. Well-Defined Iron Catalysts for the Acceptorless
Reversible Dehydrogenation-Hydrogenation of Alcohols and Ketones. ACS Catal. 2014,
4, 3994–4003.
6
Gunanathan, C.; Milstein, D. Selective Synthesis of Primary Amines Directly from
Alcohols and Ammonia. Angew. Chem. Int. Ed. 2008, 47, 8661–8664.
7
(a) Zhang, G.; Vasudevan, K. V.; Scott, B. L.; Hanson, S. K. Understanding the
Mechanisms of Cobalt-Catalyzed Hydrogenation and Dehydrogenation Reactions. J. Am.
Chem. Soc. 2013, 135, 8668–8681. (b) Gnanaprakasam, B.; Zhang, J.; Milstein, D. Direct
Synthesis of Imines from Alcohols and Amines with Liberation of H2. Angew. Chem. Int.
Ed. 2010, 49, 1468–1471.
8
(a) Gunanathan, C.; Ben-David, Y.; Milstein, D. Direct Synthesis of Amides from
Alcohols and Amines with Liberation of H2. Science 2007, 317, 790–792. (b)
Gnanaprakasam, B.; Milstein, D. Synthesis of Amides from Esters and Amines with
Liberation of H2 under Neutral Conditions. J. Am. Chem. Soc. 2011, 133, 1682–1685.
9
(a) Gunanathan, C.; Milstein, D. Bond Activation and Catalysis by Ruthenium Pincer
Complexes Chem. Rev. 2014, 114, 12024–12087. (b) Choi, J.; MacArthur, A. H. R.;
Brookhart, M.; Goldman, A. S. Dehydrogenation and Related Reactions Catalyzed by
Iridium Pincer Complexes. Chem. Rev. 2011, 111, 1761–1779. (c) Morales, D. M. Pincer
Complexes. Applications in Catalysis. Rev. Soc. Quím. Méx. 2004, 48, 338-346.
97
10
Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D. Facile Conversion of Alcohols into
Esters and Dihydrogen Catalyzed by New Ruthenium Complexes. J. Am. Chem. Soc.
2005, 127, 10840–10841.
11
Celaje, J. J. A.; Lu, Z.; Kedzie, E. A.; Terrile, N. J.; Lo, J. N.; Williams, T. J. A Prolific
Catalyst for Dehydrogenation of Neat Formic Acid. Nat. Commun. 2016, 7, 11308.
12
Chakraborty, S.; Piszel, P. E.; Hayes, C. E.; Baker, R. T.; Jones, W. D. Highly
Selective Formation of n-Butanol from Ethanol through the Guerbet Process: A Tandem
Catalytic Approach. J. Am. Chem. Soc. 2015, 137, 14264–14267.
13
(a) Dürre, P. Biobutanol: An Attractive Biofuel. Biotechnol. J. 2007, 2, 1525– 1534.
(b) Harvey, B. G.; Meylemans, H. A. The Role of Butanol in the Development of
Sustainable Fuel Technologies. J. Chem. Technol. Biotechnol. 2011, 86, 2– 9.
14
Celaje, J. J. A. Transition Metal Catalysts of Pyridylphosphine and Dipyridylborate
Ligands in Dehydrogenation Reactions. Ph.D. Dissertation, University of Southern
California, Los Angeles, CA, 2016.
98
Chapter 6. Experimental Procedures and Spectral Data
6.1 General Procedures
6.1.1 Chemical Reagents
All air and water sensitive procedures were carried out either in a Vacuum
Atmospheres glove box under nitrogen (2-10 ppm O2 for all manipulations) or using
standard Schlenk techniques under nitrogen. Deuterated NMR solvents were purchased
from Cambridge Isotopes Laboratories. Dry hexane, diethyl ether, dichloromethane and
tetrahydrofuran were obtained from a J. C. Meyer solvent purification system with
alumina/copper(II) oxide columns and used without further purification. Silica gel (230-
400 mesh) was purchased as pre-packed columns from Teledyne, and silica gel (free
flowing) was purchased from VWR. Organic reagents were purchased from Sigma-
Aldrich Co., Alfa-Aesar, J. T. Baker, Lancaster Chemicals, EMD Millipore and TCI
America and used as received. Organometallic precursors were purchased from Strem
Chemicals, except RuCl3 3H2O which was purchased from Pressure Chemical Co., and
were used as received. Ammonia borane (NH3BH3, AB) was purchased from Sigma
Aldrich and used under N2 atmosphere without further purifications. Integrity of this
material was checked regularly by
1
H and
11
B NMR.
6.1.2 Prepared Reagents
When indicated, benzene, benzene-d6, diethylene glycol dimethyl ether (diglyme),
tetraethylene glycol dimethyl ether (tetraglyme,), tetramethylethylenediamine (TMEDA),
and triethylamine (Et3N) were dried over sodium benzophenone ketyl and distilled prior
to use. Acetonitrile and deuterated acetonitrile were dried and distilled over CaH2.
Methanol was distilled from sodium methoxide and stored in the glovebox. Ethanol was
distilled from sodium ethoxide and stored over molecular sieves. Dimer 2.8, Shvo.-
Ammonia adduct 2.12,
1
2.18,
2
(phen)RuCl2(CO)2 (3.5) & (phen)Ru(OAc)2(CO)2 (3.6),
3
and 4.8
4
were prepared according to literature procedures.
6.1.3 Instrumentation
NMR spectra were recorded on a Varian Mercury 400, 400MR (outfitted with an
AS7600 autosampler), 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 in the solvent 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. All
1
H chemical
shifts were referenced to the residual
1
H solvent (relative to TMS). All
11
B chemical
shifts were referenced to a BF3-OEt2 in diglyme in a co-axial external standard (0 ppm).
All
19
F chemical shifts were referenced to a CFCl 3 external standard. Air-sensitive NMR
spectra were taken in 8” J-Young tubes (Wilmad or Norell) with Teflon valve plugs. The
NMR tubes were shaken vigorously for several minutes with chlorotrimethylsilane then
99
dried in vacuo on a Schlenk line prior to use. All spectra were processed using MesRe
Nova (v. 9.0.0-12821). Melting points were obtained on a melt-temp apparatus and are
uncorrected. Infrared spectra were acquired on a Bruker OPUS FTIR spectrometer on
either a KBr salt plate or KBr pellet. High-resolution ESI mass spectra were recorded at
the University of California, Riverside. CHN elemental analyses were collected at the
University of Illinois at Urbana Champaign at the School of Chemical Sciences
Microanalysis Laboratory and at the University of Southern California. Some column
chromatography was done in automation using the Teledyne CombiFlash Rf 200 system.
Sonication procedures were done in a VWR desktop sonic cleaner bath. GCMS spectra
were acquired with a Thermo Focus GC equipped with a DSQII mass-selective detector.
Lyophilization was carried out on a Millirock BT85A lyophilizer. UV-Vis measurements
were carried out either on a Shimadzu UV spectrophotometer (UV-1800) or Cary 14 UV-
Vis-NIR spectrophotometer using a quartz cuvette (10 mm, 3.5 mL, Science Outlet).
6.1.4 General Procedure for
11
B NMR kinetics
In a typical reaction, AB was combined with catalyst (and additives) in a J-Young
NMR tube while in a glovebox under nitrogen. The AB concentration and catalyst
concentrations may be varied. Diglyme or tetraglyme (0.4 mL) and benzene-d6 (0.2 mL)
were added to the tube. The sample tube was immediately inserted into a preheated, pre-
shimmed, and pre-locked NMR (usually 70 ˚C) and the kinetic monitoring commenced.
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 BF3-OEt2 standard. The
acquisition involved a 1.84 sec pulse sequence in which 16,384 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 backward linear
prediction. Safety note: caution should be used when carrying out these reactions as the
release of hydrogen can lead to sudden pressurization of reaction vessels.
6.1.5 General Procedure for AB Dehydrogenation Eudiometry
In a typical reaction, AB was combined with catalyst (and additives) in a 2 mL
Schlenk bomb equipped with a Teflon stir bar while in a glovebox under nitrogen.
Benzene (0.2 mL) and diglyme containing (0.4 mL) was added to the flask. A eudiometer
was constructed as follows: the side arm of the valve of the Schlenk flask was connected
to a piece of Tygon tubing, which was adapted to 20 gauge (0.03”) Teflon tubing with a
needle. The tubing was threaded through open end of a burette that was sealed with a
Teflon stopcock on the other end. The burette was filled with water or pump oil. The
entire apparatus was then inverted into a 500 mL Erlenmeyer filled with water or pump
oil 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. The volume of liberated
gas was recorded periodically until gas evolution ceased. Liberated hydrogen was
quantified by recording its volume displacement in the eudiometer. (See Section 6.3 for
more details for more rigorous air-free methods).
100
6.2 Chapter 2 Experimental and Spectral Data
For preparative procedures and spectroscopic data of the Shvo mechanism described in
the beginning of Chapter 2, please see A Three-Stage Mechanistic Model for
Ammonia−Borane Dehydrogenation by Shvo’s Catalyst by Lu et.al. and New
Bifunctional Catalysts for Ammonia-Borane Dehydrogenation, Nitrile Reduction, Formic
Acid Dehydrogenation, Lactic Acid Synthesis, and Carbon Dioxide Reaction by Zhiyao
Lu.
5
6.2.1. Preparative and Spectroscopic Details of Ruthenium Catalysts.
101
Preparation of 2.15:
Ru-Pyridine adduct 2.15 was prepared under air by dissolving 2.8 (50 mg, 0.046 mmol, 1
equiv.) in benzene (5 mL). Pyridine (15 L, 0.18 mmol, 4 equiv.) was dissolved
separately in benzene (0.1 mL) and added dropwise to the 2.8/benzene solution. The
reaction mixture color lightened from an orange to a lighter yellow-orange. The reaction
was stirred at room temperature overnight to ensure completion. The reaction was then
passed through a pipette filter filled with cotton and Celite and benzene was removed by
rotary evaporation. The residue was dried under reduced pressure overnight. Hexanes (5
mL) was added to the residue and immersed in a sonication bath for 15 minutes. A pale
yellow-white precipitate was filtered out and washed with hexanes. The solid was then
lyophilized from a benzene solution to give a pale yellow-white powder in 84% yield (46
mg).
1
H NMR (600 MHz, Benzene-d6) δ 8.19 (d, J = 7.7 Hz, 4H, Ph), 8.10 (d, J = 4.8 Hz, 2H,
C2,6 of Pyr), 7.31 (d, J = 7.0 Hz, 4H, Ph), 7.06 (t, J = 7.6 Hz, 4H, Ph), 6.94 (t, J = 7.4 Hz,
2H, Ph), 6.86 (t, J = 7.5 Hz, 4H, Ph), 6.80 (t, J = 7.4 Hz, 2H, Ph), 6.44 (t, J = 7.7 Hz, 1H,
C4 of Pyr), 6.05 (t, J = 6.7 Hz, 2H, C3,5 of Pyr).
13
C{
1
H} NMR (150 MHz, Benzene-d6) δ 201.46 (CO), 170.05 (C1 of Cp), 155.68 (C2,6 of
Pyr), 137.07 (C4 of Pyr), 134.69 (Ph), 132.79 (Ph), 132.57 (Ph), 130.34 (Ph), 128.35 (Ph),
128.18 (Ph), 127.95 (Ph), 126.43 (Ph), 125.51 (C3,5 of Pyr), 104.71 (C2,5 of Cp), 80.81
(C3,4 of Cp).
FTIR ( , cm
-1
): 2008.2, 1956.4 (M-CO’s), 1615.8 (C=O).
ESI MS for [M-H]
+
: calc’d 622.0951 g/mol, found 622.0950 g/mol.
MP: 181-187 °C, decomposed, black crust.
102
1
H NMR
13
C NMR
103
Preparation of 2.16:
Ru-4-DMAP adduct 2.16 was prepared under air by dissolving 2.8 (50.0 mg, 0.046
mmol, 1 equiv.) in benzene (5 mL). 4-Dimethylaminopyridine (22.6 mg, 0.18 mmol, 4
eq) was separately dissolved in benzene (0.1 mL), immersed in a sonication bath briefly
until homogeneous, and added drop-wise to the 2.8/benzene solution. The reaction
mixture color lightened from an orange to a lighter yellow-orange. The reaction was
stirred at room temperature overnight to ensure completion. The reaction was then
passed through a pipette filter filled with cotton and Celite and benzene was removed by
rotary evaporation. The residue was dried under reduced pressure overnight. Diethyl ether
(5 mL) was added to the residue and the resulting suspension was immersed in a
sonication bath for 15 minutes. A pale yellow-white precipitate was filtered out and
washed with hexanes. The solid was then lyophilized from a benzene solution to give a
pale yellow-white powder in 60% yield (37 mg).
1
H NMR (400 MHz, Benzene-d6) δ 8.33 – 8.30 (m, 4H, Ph), 7.80 – 7.77 (m, 2H, C2,6 of
DMAP), 7.39 – 7.36 (m, 4H, Ph), 7.12 (t, J = 7.8 Hz, 4H, Ph), 6.97 (t, J = 7.4 Hz, 2H,
Ph), 6.88 (t, J = 7.4 Hz, 4H, Ph), 6.84 – 6.79 (m, 2H, Ph), 5.34 (d, J = 7.3 Hz, 2H, C3,5 of
DMAP), 1.77 (s, 6H, methyl’s of DMAP).
13
C NMR (101 MHz, CDCl3) δ 200.94 (CO), 169.46 (C1 of Cp), 154.51 (C2,6 of DMAP),
154.26 (C4 of DMAP), 134.04 (Ph), 132.54 (Ph), 132.26 (Ph), 130.05 (Ph), 127.69 (Ph),
127.61 (Ph), 127.46 (Ph), 125.80 (Ph), 108.63 (C3,5 of DMAP), 103.18 (C2,5 of Cp), 80.42
(C3,4 of Cp), 39.11 (methyl’s of DMAP).
FTIR ( , cm
-1
): 2009.0, 1939.3 (M-CO’s), 1627.1 (C=O).
ESI MS for [M-H]
+
: calc’d 665.1373 g/mol, found 665.1376 g/mol.
MP: 199-209 °C, decomposed, black crust.
104
1
H NMR
13
C NMR
105
Preparation of 2.17:
Ru-4-TFMP adduct 2.17 was prepared by dissolving 2.17 (50 mg, 0.046 mmol, 1 equiv.)
in benzene (5 mL) under air. 4-Trifluoromethylpyridine (21 L, 0.18 mmol, 4 equiv.) was
dissolved in benzene (0.1 mL) and added drop-wise to the 2.8/benzene solution. The
reaction mixture color lightened from an orange to a lighter yellow-orange. The reaction
was stirred at room temperature overnight to ensure completion. The reaction was then
passed through a pipette filter filled with cotton and Celite and benzene was removed by
rotary evaporation. The residue was dried under reduced pressure overnight. Hexanes (5
mL) was added to the residue and immersed in a sonication bath for 15 minutes. A pale
yellow-white precipitate was filtered out and washed with hexanes. The solid was then
lyophilized from a benzene solution to give a dull yellow-orange powder in 82% yield
(52 mg).
1
H NMR (400 MHz, Benzene-d6) δ 8.18 – 8.14 (m, 4H, Ph), 8.08 (d, J = 5.9 Hz, 2H, C2,6
of TFMP), 7.31 – 7.28 (m, 4H, Ph), 7.06 (t, J = 7.7 Hz, 4H, Ph), 6.96 – 6.91 (m, 2H, Ph),
6.87 (dd, J = 8.2, 6.4 Hz, 4H, Ph), 6.84 – 6.78 (m, 2H, Ph), 6.08 (d, J = 5.8 Hz, 2H, C3,5
of TFMP).
13
C NMR (101 MHz, CDCl3) δ 199.98 (CO), 168.46 (C1 of Cp), 156.78 (C2,6 of TFMP),
139.72 (q, J = 35.4 Hz, C4 of TFMP), 133.04 (Ph), 132.09 (Ph), 131.74 (Ph), 130.06 (Ph),
128.02 (Ph), 127.87 (Ph), 127.79 (Ph), 126.49, (Ph), 122.09 (q, J =273.7 Hz, CF3),
121.69 (q, J = 3.5 Hz, C3,5 of TFMP), 103.89 (C2,5 of Cp), 81.42(C3,4 of Cp).
19
F NMR (564 MHz, Benzene-d6) δ -65.69. FTIR ( , cm
-1
): 2015.8, 1960.1 (M-CO’s),
1606.1 (C=O).
ESI MS for [M-H]
+
: calc’d 690.0825 g/mol, found 690.0831 g/mol.
MP: 200-207 °C, Decomposed, black liquid.
106
1
H NMR
13
C NMR
107
19
F NMR
108
Preparation of Dimer 2.20:
Ru-TMEDA dimer 2.20 was prepared under air by dissolving 2.8 (50 mg, 0.046 mmol, 1
equiv.) in benzene (5 mL). Tetramethylethylenediamine (14 L, 0.18 mmol, 2 equiv.) was
separately dissolved in hexanes (20 mL). The 2.8/benzene solution was added drop-wise
into the hexanes solution. The reaction was stirred vigorously at room temperature
overnight. The color of the heterogeneous reaction mixture lightened from an orange to a
pale yellow-white upon completion. Solvent was then removed by rotary evaporation.
The residue was dried under reduced pressure overnight. Hexanes (5 mL) was added to
the residue and immersed in a sonication bath for 15 minutes. A pale yellow-white
precipitate was filtered out and washed with hexanes. The solid was then lyophilized
from a benzene solution to give the dull white-yellow powder in 89% yield (49 mg).
1
H NMR (600 MHz, Methylene Chloride-d2) δ 7.59 (dd, J = 8.3, 1.3 Hz, 8H, Ph), 7.12 (t,
J = 7.5 Hz, 8H, Ph), 7.10 – 7.02 (m, 24H, Ph), 2.51 (s, 4H, ethylene), 2.05 (s, 12H,
tetramethyls).
13
C NMR (150 MHz, Benzene-d6) δ 201.79 (CO), 167.62 (C1 of Cp), 134.34 (Ph), 132.24
(Ph), 132.18 (Ph), 130.46 (Ph), 128.14 (Ph), 127.98 (Ph), 127.81 (Ph), 126.39 (Ph),
103.68 (C2,5 of Cp), 82.46 (C3,4 of Cp), 57.02 (ethylene), 53.27 (tetramethyls).
FTIR ( , cm
-1
): 2012.3, 1950.6 (M-CO’s), 1648.4 (C=O).
Elemental analysis: calc’d C: 68.39, H: 5.08, N: 2.28; found C: 68.1, H: 4.72, N: 2.15.
MP: 182-191 °C, decomposed, black crust.
109
1
H NMR
13
C NMR
110
Unsymmetrical Dimer 2.21: 2.21 is not readily separated from 2.20 and 2.22. Two
separate synthetic routes are described.
1. Preparation of 2.20, 2.21, and 2.22 from 2.8 and TMEDA was conducted by dissolving
2.8 (50 mg, 0.046 mmol, 1 equiv.) in benzene (5 mL) under air.
Tetramethylethylenediamine (7 L, 0.09 mmol, 1 equiv.) was added to the solution while
stirring vigorously. The reaction was then stirred in a 35 C oil bath overnight. The
resulting dark red/purple solution was then passed through a filter filled with cotton and
Celite, solvent was removed by rotary evaporation, and the residue was dried under
reduced pressure. Hexanes (5 mL) was added to the residue, and the resulting suspension
was immersed in a sonication bath for 5 minutes to produce a dark red powder. The
powder was isolated via filtration and washed with hexanes. Benzene was added to the
powder, and the resulting solution was lyophilized to dryness. NMR analysis of the
powder showed 2.22 as the major product with 2.20 and 2.21 as minor products.
Minimum fresh benzene was added to dissolve a portion of the powder (ca. 15 mg) and
vapor diffusion was conducted with hexanes selectively to yield crystals of 2.21 for X-ray
analysis.
2. In a separate synthesis of 2.21, 2.8 (7.62 mg, 0.014 mmol of Ru atom, 1 equiv. Ru
atom) and Ru3(CO)12 (3.0 mg, 0.014 mmol of Ru atom, 1 equiv. Ru atom) were dissolved
in benzene-d6 (0.5 mL) in a J-Young NMR tube in a nitrogen-filled glovebox.
Tetramethylethylenediamine (2.1 L, 0.014 mmol, 1 equiv.) was added, and the tube was
swirled to allow solvation of all reactants. The tube was put in a 45 C oil bath and after
10 minutes, the orange solution had darkened to a darker orange, then the tube was left to
react for 36 hours before NMR analysis. Although the major product (2.21) could not be
separated from other Ru-containing compounds in the reaction, it matched
1
H NMR
shifts of the minor product observed above.
1
H NMR (400 MHz, Benzene-d6) δ 7.91, 6.99, 6.94, 6.86, 6.73, 2.18 (s, 6H), 1.65-1.68
(m, 8H, ethylene and methyl), 1.47 (m, 2H).
111
Table 6.1. Synthetic optimization for 2.20.
Entry Conditions Compounds formed (NMR taken in C6D6)
1
Benzene
35C, overnight
5% 10% 85%
2
Benzene
RT, overnight
7% 93%
3
Benzene/Hexanes
RT, overnight
112
Figure 6.1.
1
H NMR spectra of syntheses of 2.21 taken in C6D6. Top: first synthesis of
2.22 yielding a mixture of products from which the crystal for 2.21 was isolated for X-ray
diffraction studies. Bottom: second synthesis of 2.21 where the aliphatic peaks match the
mixture of products. 2.21 is the major product of this reaction, ca. 75%.
113
6.2.2. Kinetic Profiles of Shvo Analogs Utilizing
11
B and
1
H NMR Spectroscopy
All
11
B and
1
H NMR kinetics were conducted under air-free, water-free, EtOH-free
conditions.
A. Kinetics for AB dehydrogenation by pyridine-ligated species 2.15
2.15-catalyzed AB dehydrogenation run at 70 °C were determined using
11
B NMR with
7.7 mg AB (0.25 mmol) and 2.15 (15.5 mg, 25 µmol, 10 mol%) in diglyme (0.4 mL) and
benzene-d6 (0.2 mL).
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
10% Ru atom
Shvo-Pyridine 11
[AB] (M)
Time (x 1000 sec)
y = m1 + m2 * M0
Error Value
0.0051471 0.42001 m1
0.011669 -0.42614 m2
NA 0.00027394 Chisq
NA 0.99813 R
B. Kinetics for AB dehydrogenation by 4-dimethylaminopyridine-ligated species 2.16
2.16-catalyzed AB dehydrogenation run at 70 °C were determined using
11
B NMR with
7.7 mg AB (0.25 mmol) and 2.16 (16.6 mg, 25 μmol, 10 mol%) in tetraglyme (0.4 mL)
and benzene-d6 (0.2 mL).
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 0.5 1 1.5
10% ru atom
Shvo-4-DMAP 12
[AB] (M)
TIme ( x 1000 sec)
y = m1 + m2 * M0
Error Value
0.0037156 0.43 m1
0.004234 -0.20384 m2
NA 0.00051246 Chisq
NA 0.99764 R
114
C. Kinetics for AB dehydrogenation by 4-trifluoromethylpyridine-ligated species 2.17
2.17-catalyzed AB dehydrogenation run at 70 °C were determined using
11
B NMR with
7.7 mg AB (0.25 mmol) and 2.17 (17.2 mg, 25μmol, 10 mol%) in diglyme (0.4 mL) and
benzene-d6 (0.2 mL).
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 0.1 0.2 0.3 0.4 0.5 0.6
10% Ru atom
Shvo-TMFP 13
[AB] (M)
Time (x 1000 sec)
y = m1 + m2 * M0
Error Value
0.0065996 0.42 m1
0.017356 -0.55591 m2
NA 0.00018074 Chisq
NA 0.99854 R
When comparing this slope with an analogous reaction measured by eudiometry (10% Ru
atom, 1:2 benzene:diglyme, 70 °C), the two rates agree within a 26±3% error through
75% of conversion. The error can be attributed to usage of two different modes of
measurement including two different thermometers.
115
D. Kinetics for AB dehydrogenation by Shvo’s Catalyst 2.1 and free 4-
dimethylaminopyridine
AB dehydrogenation run at 70 °C were determined using
11
B NMR with 7.7 mg AB (0.25
mmol), 2.1 (13.6 mg, 12.5 μmol, 5 mol%), and 4-DMAP (3.1 mg, 25 μmol, 10 mol%) in
tetraglyme (0.4 mL) and benzene-d6 (0.2 mL).
0
0.1
0.2
0.3
0.4
0.5
0 2 4 6 8 10
[AB] (M)
Time (x 1000 sec)
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 0.5 1 1.5 2 2.5 3 3.5
[AB] (M)
Time (x 1000 sec)
y = m1 + m2 * M0
Error Value
0.0040433 0.47433 m1
0.0018882 -0.11945 m2
NA 0.0020225 Chisq
NA 0.99689 R
116
E. Kinetics for AB dehydrogenation by Shvo’s Catalyst 2.1 and free triethylamine
AB dehydrogenation run at 70 °C were determined using
11
B NMR with 7.7 mg AB (0.25
mmol), 2.1 (13.6 mg, 12.5 μmol, 5 mol%), and TEA (3.5 μL, 25 μmol, 10 mol%) in
tetraglyme (0.4 mL) and benzene-d6 (0.2 mL).
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0 2 4 6 8 10 12
[AB] (M)
Time (x 1000 sec)
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0 0.5 1 1.5 2 2.5 3 3.5
[AB] (M)
Time (x 1000 sec)
y = m1 + m2 * M0
Error Value
0.0018254 0.42216 m1
0.00091 -0.10562 m2
NA 0.00034301 Chisq
NA 0.99915 R
117
F. Kinetics for AB dehydrogenation by Shvo’s Catalyst 2.1 and free TMEDA
AB dehydrogenation run at 70 °C were determined using
11
B NMR with 7.7 mg AB (0.25
mmol), 2.1 (13.6 mg, 12.5 μmol, 5 mol%), and TMEDA (1.9 μL, 12.5 μmol, 5 mol%) in
tetraglyme (0.4 mL) and benzene-d6 (0.2 mL).
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0 2 4 6 8 10 12
[AB] (M)
Time (x 1000 sec)
0
0.1
0.2
0.3
0.4
0.5
0 2 4 6 8 10 12
2.1 + TMEDA
2.1 + NEt
3
2.1 + 4-DMAP
[AB] (M)
Time (x 1000 sec)
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0 0.5 1 1.5 2 2.5 3 3.5
[AB] (M)
Time (x 1000 sec)
y = m1 + m2 * M0
Error Value
0.0019111 0.42771 m1
0.0010349 -0.10762 m2
NA 0.00044361 Chisq
NA 0.99894 R
118
G. Kinetics for AB dehydrogenation by 1,10-phenanthroline
AB dehydrogenation run at 70 °C were determined using
11
B NMR with 7.7 mg AB (0.25
mmol) and 1,10-phenanthroline (2.2 mg, 12.5 μmol, 5 mol%) in tetraglyme (0.4 mL) and
benzene-d6 (0.2 mL).
0.34
0.36
0.38
0.4
0.42
0.44
0 2 4 6 8 10 12 14
Data 10
B
[AB] (M)
Time (x1000 sec)
y = m1 + m2 * M0
Error Value
0.00038674 0.41997 m1
5.018e-5 -0.0053648 m2
NA 0.00044921 Chisq
NA 0.99522 R
H. Kinetics for AB dehydrogenation by 4-DMAP
AB dehydrogenation run at 70 °C were determined using
11
B NMR with 7.7 mg AB (0.25
mmol) and 4-dimethylaminopyridine (3.1 mg, 25 μmol, 10 mol%) in diglyme (0.4 mL)
and benzene-d6 (0.2 mL).
0.2
0.25
0.3
0.35
0.4
0.45
0 2 4 6 8 10 12
Data 10
B
[AB] (M)
Time (x1000 sec)
119
I. Kinetics for AB dehydrogenation by TMEDA
AB dehydrogenation run at 70 °C were determined using
11
B NMR with 7.7 mg AB (0.25
mmol) and tetramethylethylenediamine (1.9 μL, 12.5 μmol, 5 mol%) in diglyme (0.4 mL)
and benzene-d6 (0.2 mL).
0.28
0.3
0.32
0.34
0.36
0.38
0.4
0.42
0.44
0 2 4 6 8 10
Data 10
B
[AB] (M)
Time (x1000 sec)
J. Kinetics for AB dehydrogenation by phenRuCl2(CO)2 (2.23) and TlOTf
AB dehydrogenation run at 70 °C were determined using
11
B NMR. 2.23 (10.2 mg, 25
μmol, 10 mol%) and TlOTf (17.7 mg, 50 μmol, 20 mol%) were mixed in diglyme (0.4
mL) and benzene-d6 (0.2 mL). Then AB (7.7 mg, 0.25 mmol) was added to the mixture.
0
0.1
0.2
0.3
0.4
0 1 2 3 4 5 6
Data 10
B
[AB] (M)
Time (x1000 sec)
120
6.2.3. GCMS Data for Tetraphenylcyclopentadienone Dissociation
In a typical reaction, 2.8 and a bidentate nitrogen ligand (1,10-phenanthroline, TMEDA,
or 2,2’-bipyridine) were added into a vial at room temperature. The reaction was placed
in a 70 C oil bath for 12-16 hrs. The solvent was then removed under reduced pressure
and the residue was immersed in a sonication bath in hexanes for 1 minute. The
supernatant was then passed through a Celite filter and analyzed by GCMS. CPD eludes
at ca. 16.8 min with this method with the major mass peaks being 384 m/z (CPD) and 178
m/z (portion of acetylene).
Figure 6.2. Reaction between 2.8 and 2,2’-bipyridine.
121
Figure 6.3. Reaction between 2.8 and 1,10-phenanthroline.
122
Figure 6.4. Reaction between 2.8 and TMEDA.
123
6.2.4. Representative
11
B and
19
F NMR Spectra for AB Dehydrogenations
A. End of Reaction
11
B and
19
F Spectra for AB Dehydrogenation Catalyzed by 2.17
Figure 6.5.
19
F spectra of end of AB dehydrogenation reaction catalyzed by 2.17 (10
mol% Ru atom, 70 C, 1:2 C6D6: diglyme). Zoomed in window from -65 ppm to -75
ppm. Reaction time = 5.5 hrs.
Figure 6.6.
11
B spectra of end of AB dehydrogenation reaction catalyzed by 2.17 (10
mol% Ru atom, 1:2 C6D6: diglyme). Reaction time = 1 hr. Note presence of broad
polyunsaturated peaks at 25-33 ppm. Other intermediates include borazine (32 ppm),
BF3-OEt2 internal standard (0 ppm), amine borane cyclic tetramer (-5, -11, -23 ppm),
cyclotriborazane (-11 ppm), 4-DMAP-BH3 (-13 ppm) and residual AB (-22 ppm).
124
B. Metal-Free AB Dehydrogenation Reactions
Figure 6.7. AB dehydrogenation by catalytic amount of 4-DMAP (10%, 1:2 C6D6:
diglyme) at 70 C. Bottom: Initial
11
B spectra of reaction. Top: Last spectra at 3 hours,
15 minutes. Note emergence of -3 and -21 ppm peaks characterized by Rivard et. al., and
borazine peak at 31 ppm, and B-N byproduct peaks at -5, -11, and -23 ppm (amine
borane cyclic tetramer and cyclotriborazane), indications that AB has been
dehydrogenated. Other intermediates include standard (0 ppm), 4-DMAP-BH3 (-13 ppm),
and AB (-22 ppm).
Figure 6.8. AB dehydrogenation by catalytic amount of TMEDA (5%, 1:2 C6D6:
diglyme) at 70 C. Bottom: Initial
11
B spectra of reaction. Top: Last spectra at 2 hours,
40 minutes. Note emergence of borazine peak at 31 ppm, and B-N byproduct peaks at -5,
-11, and -23 ppm (amine borane, cyclic tetramer, and cyclotriborazane), indications that
AB has been dehydrogenated. Other intermediates include standard (0 ppm), TMEDA-
BH3 (-9 ppm), H3B-TMEDA-BH3 (-10 ppm), and AB (-22 ppm).
125
Figure 6.9. AB dehydrogenation by catalytic amount of 1,10-phenanthroline (5%, 1:2
C6D6: diglyme) at 70 C. Bottom: Initial
11
B spectra of reaction. Top: Last spectra at 3
hours, 40 minutes. Note emergence of borazine peak at 31 ppm, and B-N byproduct
peaks at -5, -11, -23 ppm (amine borane cyclic tetramer and cyclotriborazane) indication
that AB has been dehydrogenated.
126
6.2.5. Crystal Structural Data
A. Crystal Structure Data for 2.15
A colorless plate-like specimen of C40H33NO4Ru was grown from slowly diffusing
pentane into a tetrahydrofuran solution. A specimen with approximate
dimensions 0.218 mm x 0.219 mm x 0.461 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). 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 77236 reflections to a maximum θ angle
of 30.61° (0.70 Å resolution), of which 9784 were independent (average
redundancy 7.894, completeness = 99.1%, Rint = 2.42%, Rsig = 1.35%)
and 8858 (90.54%) were greater than 2σ(F
2
). The final cell constants of a = 37.751(3) Å,
b = 10.6602(7) Å, c = 17.1777(12)Å, β = 112.0060(10)°, volume = 6409.2(8) Å
3
, are
based upon the refinement of the XYZ-centroids of 198 reflections above 20 σ(I)
with 5.193° < 2θ < 58.49°. Data were corrected for absorption effects using the multi-
scan method (SADABS). The ratio of minimum to maximum apparent transmission
was 0.887. The calculated minimum and maximum transmission coefficients (based on
crystal size) are 0.7910 and 0.8930.
The structure was solved and refined using the Bruker SHELXTL Software Package,
using the space group C 1 2/c 1, with Z = 8 for the formula unit, C40H33NO4Ru. The final
anisotropic full-matrix least-squares refinement on F
2
with 431 variables converged at R1
= 2.57%, for the observed data and wR2 = 7.36% for all data. The goodness-of-fit
was 1.098. The largest peak in the final difference electron density synthesis was 1.103 e
-
/Å
3
and the largest hole was -0.709 e
-
/Å
3
with an RMS deviation of 0.099 e
-
/Å
3
. On the
basis of the final model, the calculated density was 1.436 g/cm
3
and F(000), 2848 e
-
.
127
a. b.
c.
d.
Figure 6.10. a. X-Ray ORTEP of 2.15 (50% probability). b. Ellipsoid plot including a
THF is disordered between two orientations. c. Labeled ORTEP including hydrogens. d.
ORTEP with all atoms labeled including disordered THF.
128
Table 6.2. Crystal data and structure refinement for 2.15.
Chemical formula C40H33NO4Ru
Formula weight 692.74
Temperature 100(2) K
Wavelength 0.71073 Å
Crystal size 0.218 x 0.219 x 0.461 mm
Crystal habit colorless plate
Crystal system monoclinic
Space group C 1 2/c 1
Unit cell dimensions a = 37.751(3) Å α = 90°
b = 10.6602(7) Å β = 112.0060(10)°
c = 17.1777(12) Å γ = 90°
Volume 6409.2(8) Å
3
Z 8
Density (calculated) 1.436 g/cm
3
Absorption coefficient 0.533 mm
-1
F(000) 2848
Theta range for data collection 1.16 to 30.61°
Index ranges -52<=h<=53, -15<=k<=15, -24<=l<=24
Reflections collected 77236
Independent reflections 9784 [R(int) = 0.0242]
Coverage of independent reflections 99.1%
Absorption correction multi-scan
Max. and min. transmission 0.8930 and 0.7910
Structure solution technique direct methods
Structure solution program SHELXTL XT 2013/6 (Sheldrick, 2013)
Refinement method Full-matrix least-squares on F
2
Refinement program SHELXTL XLMP 2014/1 (Bruker AXS, 2013)
Function minimized Σ w(Fo
2
- Fc
2
)
2
Data / restraints / parameters 9784 / 70 / 431
129
Goodness-of-fit on F
2
1.098
Δ/σmax 0.002
Final R indices 8858 data; I>2σ(I) R1 = 0.0257, wR2 = 0.0663
all data R1 = 0.0301, wR2 = 0.0736
Weighting scheme
w=1/[σ
2
(Fo
2
)+(0.0347P)
2
+10.3475P]
where P=(Fo
2
+2Fc
2
)/3
Largest diff. peak and hole 1.103 and -0.709 eÅ
-3
R.M.S. deviation from mean 0.099 eÅ
-3
Table 6.3. Bond lengths (Å) for 2.15.
C1-O1 1.2429(16) C1-C5 1.4812(18)
C1-C2 1.4831(18) C1-Ru1 2.4913(14)
C2-C3 1.4551(17) C2-C6 1.4806(18)
C2-Ru1 2.2515(13) C3-C4 1.4444(17)
C3-C12 1.4879(17) C3-Ru1 2.1765(13)
C4-C5 1.4474(17) C4-C18 1.4887(18)
C4-Ru1 2.1842(12) C5-C24 1.4815(18)
C5-Ru1 2.2549(13) C6-C7 1.4015(18)
C6-C11 1.4027(18) C7-C8 1.3918(19)
C7-H7 0.95 C8-C9 1.390(2)
C8-H8 0.95 C9-C10 1.387(2)
C9-H9 0.95 C10-C11 1.395(2)
C10-H10 0.95 C11-H11 0.95
C12-C13 1.3970(18) C12-C17 1.4004(18)
C13-C14 1.3958(19) C13-H13 0.95
C14-C15 1.386(2) C14-H14 0.95
C15-C16 1.392(2) C15-H15 0.95
C16-C17 1.3929(19) C16-H16 0.95
C17-H17 0.95 C18-C19 1.3953(18)
C18-C23 1.3991(18) C19-C20 1.3953(19)
C19-H19 0.95 C20-C21 1.385(2)
C20-H20 0.95 C21-C22 1.391(2)
C21-H21 0.95 C22-C23 1.3939(19)
C22-H22 0.95 C23-H23 0.95
C24-C25 1.3949(19) C24-C29 1.4011(19)
C25-C26 1.3922(19) C25-H25 0.95
130
C26-C27 1.388(2) C26-H26 0.95
C27-C28 1.389(2) C27-H27 0.95
C28-C29 1.395(2) C28-H28 0.95
C29-H29 0.95 C30-O2 1.138(2)
C30-Ru1 1.9110(16) C31-O3 1.1478(19)
C31-Ru1 1.8906(15) C32-N1 1.342(2)
C32-C33 1.389(3) C32-H32 0.95
C33-C34 1.361(3) C33-H33 0.95
C34-C35 1.378(3) C34-H34 0.95
C36-N1 1.340(2) C36-C35 1.387(3)
C36-H36 0.95 C35-H35 0.95
N1-Ru1 2.1695(12) C37_a-C38_a 1.482(5)
C37_a-O4_a 1.639(5) C37_a-H37A_a 0.99
C37_a-H37B_a 0.99 C38_a-C39_a 1.466(5)
C38_a-H38A_a 0.99 C38_a-H38B_a 0.99
C39_a-C40_a 1.515(4) C39_a-H39A_a 0.99
C39_a-H39B_a 0.99 C40_a-O4_a 1.447(4)
C40_a-H40A_a 0.99 C40_a-H40B_a 0.99
C37A_b-O4A_b 1.180(7) C37A_b-C38A_b 1.524(8)
C37A_b-H37A_b 0.99 C37A_b-H37B_b 0.99
C38A_b-C39A_b 1.724(8) C38A_b-H38A_b 0.99
C38A_b-H38B_b 0.99 C39A_b-C40A_b 1.506(7)
C39A_b-H39A_b 0.99 C39A_b-H39B_b 0.99
C40A_b-O4A_b 1.427(7) C40A_b-H40A_b 0.99
C40A_b-H40B_b 0.99
Table 6.4. Bond angles (°) for 2.15.
O1-C1-C5 126.92(12) O1-C1-C2 127.90(12)
C5-C1-C2 104.58(11) O1-C1-Ru1 130.78(10)
C5-C1-Ru1 63.30(7) C2-C1-Ru1 63.16(7)
C3-C2-C6 126.14(11) C3-C2-C1 108.52(11)
C6-C2-C1 123.71(11) C3-C2-Ru1 68.05(7)
C6-C2-Ru1 128.62(9) C1-C2-Ru1 80.85(8)
C4-C3-C2 107.92(11) C4-C3-C12 124.82(11)
C2-C3-C12 125.89(11) C4-C3-Ru1 70.95(7)
C2-C3-Ru1 73.63(7) C12-C3-Ru1 131.53(9)
131
C3-C4-C5 108.23(11) C3-C4-C18 126.91(11)
C5-C4-C18 124.10(11) C3-C4-Ru1 70.37(7)
C5-C4-Ru1 73.64(7) C18-C4-Ru1 129.55(9)
C4-C5-C1 108.69(11) C4-C5-C24 125.86(11)
C1-C5-C24 123.11(11) C4-C5-Ru1 68.35(7)
C1-C5-Ru1 80.77(8) C24-C5-Ru1 130.81(9)
C7-C6-C11 118.16(12) C7-C6-C2 121.98(12)
C11-C6-C2 119.85(12) C8-C7-C6 120.81(13)
C8-C7-H7 119.6 C6-C7-H7 119.6
C9-C8-C7 120.47(13) C9-C8-H8 119.8
C7-C8-H8 119.8 C10-C9-C8 119.36(13)
C10-C9-H9 120.3 C8-C9-H9 120.3
C9-C10-C11 120.50(14) C9-C10-H10 119.8
C11-C10-H10 119.8 C10-C11-C6 120.67(13)
C10-C11-H11 119.7 C6-C11-H11 119.7
C13-C12-C17 118.76(12) C13-C12-C3 123.22(12)
C17-C12-C3 117.97(12) C14-C13-C12 120.38(13)
C14-C13-H13 119.8 C12-C13-H13 119.8
C15-C14-C13 120.52(13) C15-C14-H14 119.7
C13-C14-H14 119.7 C14-C15-C16 119.49(13)
C14-C15-H15 120.3 C16-C15-H15 120.3
C15-C16-C17 120.30(14) C15-C16-H16 119.9
C17-C16-H16 119.9 C16-C17-C12 120.52(13)
C16-C17-H17 119.7 C12-C17-H17 119.7
C19-C18-C23 118.73(12) C19-C18-C4 123.75(12)
C23-C18-C4 117.52(11) C20-C19-C18 120.46(13)
C20-C19-H19 119.8 C18-C19-H19 119.8
C21-C20-C19 120.42(14) C21-C20-H20 119.8
C19-C20-H20 119.8 C20-C21-C22 119.67(13)
C20-C21-H21 120.2 C22-C21-H21 120.2
C21-C22-C23 120.11(14) C21-C22-H22 119.9
C23-C22-H22 119.9 C22-C23-C18 120.60(13)
C22-C23-H23 119.7 C18-C23-H23 119.7
C25-C24-C29 118.52(13) C25-C24-C5 122.58(12)
C29-C24-C5 118.89(12) C26-C25-C24 120.69(14)
C26-C25-H25 119.7 C24-C25-H25 119.7
C27-C26-C25 120.43(14) C27-C26-H26 119.8
C25-C26-H26 119.8 C26-C27-C28 119.56(14)
132
C26-C27-H27 120.2 C28-C27-H27 120.2
C27-C28-C29 120.18(14) C27-C28-H28 119.9
C29-C28-H28 119.9 C28-C29-C24 120.62(14)
C28-C29-H29 119.7 C24-C29-H29 119.7
O2-C30-Ru1 178.74(15) O3-C31-Ru1 174.88(14)
N1-C32-C33 123.0(2) N1-C32-H32 118.5
C33-C32-H32 118.5 C34-C33-C32 119.34(19)
C34-C33-H33 120.3 C32-C33-H33 120.3
C33-C34-C35 118.43(18) C33-C34-H34 120.8
C35-C34-H34 120.8 N1-C36-C35 122.53(19)
N1-C36-H36 118.7 C35-C36-H36 118.7
C34-C35-C36 119.5(2) C34-C35-H35 120.2
C36-C35-H35 120.2 C36-N1-C32 117.14(15)
C36-N1-Ru1 119.23(11) C32-N1-Ru1 123.56(12)
C31-Ru1-C30 91.53(6) C31-Ru1-N1 95.75(6)
C30-Ru1-N1 90.12(6) C31-Ru1-C3 121.70(6)
C30-Ru1-C3 97.34(6) N1-Ru1-C3 141.38(5)
C31-Ru1-C4 93.85(6) C30-Ru1-C4 127.96(6)
N1-Ru1-C4 140.39(5) C3-Ru1-C4 38.69(5)
C31-Ru1-C2 157.54(6) C30-Ru1-C2 100.48(6)
N1-Ru1-C2 103.07(5) C3-Ru1-C2 38.32(4)
C4-Ru1-C2 63.80(5) C31-Ru1-C5 101.54(6)
C30-Ru1-C5 160.78(6) N1-Ru1-C5 102.37(5)
C3-Ru1-C5 63.81(5) C4-Ru1-C5 38.02(4)
C2-Ru1-C5 62.72(5) C31-Ru1-C1 135.66(5)
C30-Ru1-C1 132.80(5) N1-Ru1-C1 86.36(5)
C3-Ru1-C1 61.01(5) C4-Ru1-C1 60.77(5)
C2-Ru1-C1 35.99(4) C5-Ru1-C1 35.93(4)
C38_a-C37_a-O4_a 96.5(3) C38_a-C37_a-H37A_a 112.5
O4_a-C37_a-H37A_a 112.4 C38_a-C37_a-H37B_a 112.4
O4_a-C37_a-H37B_a 112.4 H37A_a-C37_a-H37B_a 110.0
C39_a-C38_a-C37_a 104.5(3) C39_a-C38_a-H38A_a 110.9
C37_a-C38_a-H38A_a 110.9 C39_a-C38_a-H38B_a 110.9
C37_a-C38_a-H38B_a 110.9 H38A_a-C38_a-H38B_a 108.9
C38_a-C39_a-C40_a 100.0(3) C38_a-C39_a-H39A_a 111.8
C40_a-C39_a-H39A_a 111.8 C38_a-C39_a-H39B_a 111.8
C40_a-C39_a-H39B_a 111.8 H39A_a-C39_a-H39B_a 109.5
O4_a-C40_a-C39_a 105.7(2) O4_a-C40_a-H40A_a 110.6
133
C39_a-C40_a-H40A_a 110.6 O4_a-C40_a-H40B_a 110.6
C39_a-C40_a-H40B_a 110.6 H40A_a-C40_a-H40B_a 108.7
C40_a-O4_a-C37_a 106.4(2)
O4A_b-C37A_b-
C38A_b
104.5(6)
O4A_b-C37A_b-
H37A_b
110.9
C38A_b-C37A_b-
H37A_b
110.9
O4A_b-C37A_b-
H37B_b
110.9
C38A_b-C37A_b-
H37B_b
110.9
H37A_b-C37A_b-
H37B_b
108.9
C37A_b-C38A_b-
C39A_b
85.0(4)
C37A_b-C38A_b-
H38A_b
114.5
C39A_b-C38A_b-
H38A_b
114.5
C37A_b-C38A_b-
H38B_b
114.5
C39A_b-C38A_b-
H38B_b
114.5
H38A_b-C38A_b-
H38B_b
111.6
C40A_b-C39A_b-
C38A_b
97.0(4)
C40A_b-C39A_b-
H39A_b
112.4
C38A_b-C39A_b-
H39A_b
112.4
C40A_b-C39A_b-
H39B_b
112.4
C38A_b-C39A_b-
H39B_b
112.4
H39A_b-C39A_b-
H39B_b
109.9
O4A_b-C40A_b-
C39A_b
105.4(4)
O4A_b-C40A_b-
H40A_b
110.7
C39A_b-C40A_b-
H40A_b
110.7
O4A_b-C40A_b-
H40B_b
110.7
C39A_b-C40A_b-
H40B_b
110.7
H40A_b-C40A_b-
H40B_b
108.8
C37A_b-O4A_b-
C40A_b
90.9(5)
134
B. Crystal Structure Data for 2.21
Clear orange prism-like crystals of C39H36N2O5Ru2 were grown from slowly diffusing
hexanes into a benzene solution. A specimen with approximate dimensions 0.16 × 0.11 ×
0.07, was used for the X-ray crystallographic analysis. The X-ray intensity data were
measured on a Bruker APEX II CCD system equipped with a TRIUMPH curved-crystal
monochromator and a Mo fine-focus tube (λ = 0.71073 Å). A total of 1592 frames were
collected. The total exposure time was 18.08 hours. The frames were integrated with the
Bruker SAINT software package using a SAINT V8.32B (Bruker AXS, 2013) algorithm.
The integration of the data using an orthorhombic unit cell yielded a total
of 59658 reflections to a maximum θ angle of 30.52° (0.70 Å resolution), of
which 10566 were independent (average redundancy 5.646, completeness = 99.8%,
Rint = 6.39%) and 9004 (85.22%) were greater than 2σ(F
2
). The final cell constants
of a = 9.3629(17) Å, b = 17.807(3) Å, c = 20.791(3) Å, volume = 3466.4(10) Å
3
, are
based upon the refinement of the XYZ-centroids of 9964 reflections above 20 σ(I)
with 4.542° < 2θ < 61.07°. Data were corrected for absorption effects using the multi-
scan method (SADABS). The ratio of minimum to maximum apparent transmission
was 0.895. The calculated minimum and maximum transmission coefficients (based on
crystal size) are 0.8650 and 0.9380.
The structure was solved and refined using the Bruker SHELXTL Software Package,
using the space group P 21 21 21, with Z = 4 for the formula unit, C39H36N2O5Ru2. The
final anisotropic full-matrix least-squares refinement on F
2
with 437 variables converged
at R1 = 3.30%, for the observed data and wR2 = 7.11% for all data. The goodness-of-fit
was 1.026. The largest peak in the final difference electron density synthesis was 0.834 e
-
/Å
3
and the largest hole was -0.850 e
-
/Å
3
with an RMS deviation of 0.095 e
-
/Å
3
. On the
basis of the final model, the calculated density was 1.561g/cm
3
and F(000), 1648 e
-
,
density was 1.487 g/cm
3
and F(000), 1336 e
-
.
135
a.
b.
Figure 6.11. a. X-Ray ORTEP of 2.21 (50% probability). Hydrogens omitted for clarity.
b. ORTEP including hydrogens, and all atoms labeled.
136
Table 6.5. Crystal data and structure refinement for 2.21.
Chemical formula C39H36N2O5Ru2
Formula weight 814.84
Temperature 100(2) K
Wavelength 0.71073 Å
Crystal size 0.071 x 0.113 x 0.163 mm
Crystal habit clear orange prism
Crystal system orthorhombic
Space group P 21 21 21
Unit cell dimensions
a = 9.3629(17)
Å
α = 90°
b = 17.807(3) Å β = 90°
c = 20.791(3) Å γ = 90°
Volume 3466.4(10) Å
3
Z 4
Density (calculated) 1.561 g/cm
3
Absorption coefficient 0.918 mm
-1
F(000) 1648
Theta range for data collection 1.96 to 30.52°
Index ranges -13<=h<=13, -25<=k<=25, -29<=l<=29
Reflections collected 59658
Independent reflections 10566 [R(int) = 0.0639]
Coverage of independent
reflections
99.8%
Absorption correction multi-scan
Max. and min. transmission 0.9380 and 0.8650
Structure solution technique direct methods
Structure solution program SHELXTL XS 2013/1 (Bruker AXS)
Refinement method Full-matrix least-squares on F2
Refinement program SHELXL-2013 (Sheldrick, 2013)
Function minimized Σ w(Fo2 - Fc2)2
Data / restraints / parameters 10566 / 0 / 437
Goodness-of-fit on F2 1.026
Δ/σmax 0.002
Final R indices
9004
data;
I>2σ(I)
R1 = 0.0330, wR2 = 0.0666
all data R1 = 0.0473, wR2 = 0.0711
Weighting scheme w=1/[σ
2
(Fo
2
)+(0.0336P)
2
+0.5760P]
137
where P=(Fo
2
+2Fc
2
)/3
Absolute structure parameter -0.0(0)
Largest diff. peak and hole 0.834 and -0.850 eÅ
-3
R.M.S. deviation from mean 0.095 eÅ
-3
Table 6.6. Bond lengths (Å) for 2.21.
C1-O1 1.143(5) C1-Ru1 1.874(4)
C2-O2 1.154(5) C2-Ru1 1.866(5)
C3-O5 1.325(4) C3-C7 1.452(5)
C3-C4 1.455(5) C3-Ru1 2.276(4)
C4-C5 1.440(5) C4-C8 1.488(5)
C4-Ru1 2.247(4) C5-C6 1.435(5)
C5-C14 1.495(5) C5-Ru1 2.235(4)
C6-C7 1.437(6) C6-C20 1.494(5)
C6-Ru1 2.267(4) C7-C26 1.486(5)
C7-Ru1 2.280(4) C8-C13 1.390(5)
C8-C9 1.404(6) C9-C10 1.401(6)
C9-H9 0.95 C10-C11 1.378(6)
C10-H10 0.95 C11-C12 1.386(6)
C11-H11 0.95 C12-C13 1.392(5)
C12-H12 0.95 C13-H13 0.95
C14-C15 1.392(6) C14-C19 1.394(6)
C15-C16 1.383(6) C15-H15 0.95
C16-C17 1.379(7) C16-H16 0.95
C17-C18 1.368(7) C17-H17 0.95
C18-C19 1.394(7) C18-H18 0.95
C19-H19 0.95 C20-C25 1.391(6)
C20-C21 1.394(6) C21-C22 1.392(6)
C21-H21 0.95 C22-C23 1.367(6)
C22-H22 0.95 C23-C24 1.382(6)
C23-H23 0.95 C24-C25 1.387(6)
C24-H24 0.95 C25-H25 0.95
C26-C27 1.386(6) C26-C31 1.401(5)
C27-C28 1.395(5) C27-H27 0.95
C28-C29 1.380(6) C28-H28 0.95
C29-C30 1.394(6) C29-H29 0.95
C30-C31 1.386(5) C30-H30 0.95
138
C31-H31 0.95 C32-O3 1.153(5)
C32-Ru2 1.848(4) C33-O4 1.151(5)
C33-Ru2 1.843(4) C34-N1 1.487(6)
C34-C35 1.500(7) C34-H34A 0.99
C34-H34B 0.99 C35-N2 1.505(6)
C35-H35A 0.99 C35-H35B 0.99
C36-N1 1.478(6) C36-H36A 0.98
C36-H36B 0.98 C36-H36C 0.98
C37-N1 1.478(6) C37-H37A 0.98
C37-H37B 0.98 C37-H37C 0.98
C38-N2 1.491(6) C38-H38A 0.98
C38-H38B 0.98 C38-H38C 0.98
C39-N2 1.481(6) C39-H39A 0.98
C39-H39B 0.98 C39-H39C 0.98
N1-Ru2 2.272(4) N2-Ru2 2.292(4)
O5-Ru2 2.163(3) Ru1-Ru2 2.7735(6)
Table 6.7. Bond angles (°) for 2.21.
O1-C1-Ru1 177.2(4) O2-C2-Ru1 178.0(4)
O5-C3-C7 126.2(3) O5-C3-C4 126.9(3)
C7-C3-C4 106.6(3) O5-C3-Ru1 117.8(2)
C7-C3-Ru1 71.6(2) C4-C3-Ru1 70.1(2)
C5-C4-C3 108.7(3) C5-C4-C8 124.8(4)
C3-C4-C8 124.9(4) C5-C4-Ru1 70.8(2)
C3-C4-Ru1 72.3(2) C8-C4-Ru1 134.4(3)
C6-C5-C4 107.8(3) C6-C5-C14 124.0(3)
C4-C5-C14 127.2(3) C6-C5-Ru1 72.6(2)
C4-C5-Ru1 71.7(2) C14-C5-Ru1 130.2(3)
C5-C6-C7 108.5(3) C5-C6-C20 123.7(3)
C7-C6-C20 127.3(3) C5-C6-Ru1 70.2(2)
C7-C6-Ru1 72.1(2) C20-C6-Ru1 129.7(3)
C6-C7-C3 108.5(3) C6-C7-C26 126.4(3)
C3-C7-C26 125.0(4) C6-C7-Ru1 71.1(2)
C3-C7-Ru1 71.3(2) C26-C7-Ru1 126.7(3)
C13-C8-C9 118.8(4) C13-C8-C4 118.5(4)
C9-C8-C4 122.7(4) C10-C9-C8 119.8(4)
C10-C9-H9 120.1 C8-C9-H9 120.1
139
C11-C10-C9 120.7(4) C11-C10-H10 119.7
C9-C10-H10 119.7 C10-C11-C12 119.6(4)
C10-C11-H11 120.2 C12-C11-H11 120.2
C11-C12-C13 120.4(4) C11-C12-H12 119.8
C13-C12-H12 119.8 C8-C13-C12 120.7(4)
C8-C13-H13 119.7 C12-C13-H13 119.7
C15-C14-C19 117.5(4) C15-C14-C5 119.6(4)
C19-C14-C5 122.8(4) C16-C15-C14 121.5(4)
C16-C15-H15 119.3 C14-C15-H15 119.3
C17-C16-C15 120.3(5) C17-C16-H16 119.8
C15-C16-H16 119.8 C18-C17-C16 119.0(4)
C18-C17-H17 120.5 C16-C17-H17 120.5
C17-C18-C19 121.2(5) C17-C18-H18 119.4
C19-C18-H18 119.4 C14-C19-C18 120.3(5)
C14-C19-H19 119.9 C18-C19-H19 119.9
C25-C20-C21 119.4(4) C25-C20-C6 122.0(4)
C21-C20-C6 118.5(4) C22-C21-C20 119.8(4)
C22-C21-H21 120.1 C20-C21-H21 120.1
C23-C22-C21 120.8(4) C23-C22-H22 119.6
C21-C22-H22 119.6 C22-C23-C24 119.4(4)
C22-C23-H23 120.3 C24-C23-H23 120.3
C23-C24-C25 121.0(5) C23-C24-H24 119.5
C25-C24-H24 119.5 C24-C25-C20 119.5(4)
C24-C25-H25 120.3 C20-C25-H25 120.3
C27-C26-C31 118.7(4) C27-C26-C7 119.8(4)
C31-C26-C7 121.5(4) C26-C27-C28 120.4(4)
C26-C27-H27 119.8 C28-C27-H27 119.8
C29-C28-C27 120.7(4) C29-C28-H28 119.6
C27-C28-H28 119.6 C28-C29-C30 119.2(4)
C28-C29-H29 120.4 C30-C29-H29 120.4
C31-C30-C29 120.3(4) C31-C30-H30 119.8
C29-C30-H30 119.8 C30-C31-C26 120.6(4)
C30-C31-H31 119.7 C26-C31-H31 119.7
O3-C32-Ru2 169.6(4) O4-C33-Ru2 177.2(4)
N1-C34-C35 110.3(4) N1-C34-H34A 109.6
C35-C34-H34A 109.6 N1-C34-H34B 109.6
C35-C34-H34B 109.6 H34A-C34-H34B 108.1
C34-C35-N2 110.6(4) C34-C35-H35A 109.5
140
N2-C35-H35A 109.5 C34-C35-H35B 109.5
N2-C35-H35B 109.5 H35A-C35-H35B 108.1
N1-C36-H36A 109.5 N1-C36-H36B 109.5
H36A-C36-
H36B
109.5 N1-C36-H36C 109.5
H36A-C36-
H36C
109.5 H36B-C36-H36C 109.5
N1-C37-H37A 109.5 N1-C37-H37B 109.5
H37A-C37-
H37B
109.5 N1-C37-H37C 109.5
H37A-C37-
H37C
109.5 H37B-C37-H37C 109.5
N2-C38-H38A 109.5 N2-C38-H38B 109.5
H38A-C38-
H38B
109.5 N2-C38-H38C 109.5
H38A-C38-
H38C
109.5 H38B-C38-H38C 109.5
N2-C39-H39A 109.5 N2-C39-H39B 109.5
H39A-C39-
H39B
109.5 N2-C39-H39C 109.5
H39A-C39-
H39C
109.5 H39B-C39-H39C 109.5
C36-N1-C37 108.3(4) C36-N1-C34 109.6(4)
C37-N1-C34 108.7(3) C36-N1-Ru2 111.9(3)
C37-N1-Ru2 109.6(3) C34-N1-Ru2 108.7(3)
C39-N2-C38 107.8(4) C39-N2-C35 108.1(3)
C38-N2-C35 107.6(3) C39-N2-Ru2 115.0(3)
C38-N2-Ru2 116.3(3) C35-N2-Ru2 101.3(2)
C3-O5-Ru2 99.9(2) C2-Ru1-C1 89.23(17)
C2-Ru1-C5 106.82(17) C1-Ru1-C5 130.98(17)
C2-Ru1-C4 104.96(18) C1-Ru1-C4 163.72(16)
C5-Ru1-C4 37.49(14) C2-Ru1-C6 137.45(17)
C1-Ru1-C6 102.22(16) C5-Ru1-C6 37.15(13)
C4-Ru1-C6 61.92(14) C2-Ru1-C3 132.87(16)
C1-Ru1-C3 133.82(16) C5-Ru1-C3 62.84(13)
C4-Ru1-C3 37.52(13) C6-Ru1-C3 62.11(13)
C2-Ru1-C7 166.80(17) C1-Ru1-C7 103.50(16)
C5-Ru1-C7 62.13(13) C4-Ru1-C7 61.96(13)
C6-Ru1-C7 36.83(14) C3-Ru1-C7 37.16(13)
C2-Ru1-Ru2 98.46(14) C1-Ru1-Ru2 96.11(14)
141
C5-Ru1-Ru2 125.36(10) C4-Ru1-Ru2 89.75(10)
C6-Ru1-Ru2 120.36(10) C3-Ru1-Ru2 64.39(9)
C7-Ru1-Ru2 83.85(10) C33-Ru2-C32 84.18(19)
C33-Ru2-O5 166.01(15) C32-Ru2-O5 98.85(14)
C33-Ru2-N1 102.59(16) C32-Ru2-N1 90.65(17)
O5-Ru2-N1 91.07(11) C33-Ru2-N2 89.19(16)
C32-Ru2-N2 166.46(16) O5-Ru2-N2 90.42(11)
N1-Ru2-N2 79.26(13) C33-Ru2-Ru1 89.30(13)
C32-Ru2-Ru1 85.31(14) O5-Ru2-Ru1 77.40(7)
N1-Ru2-Ru1 167.01(9) N2-Ru2-Ru1 106.45(9)
142
6.3 Chapter 3 Experimental and Spectral Data
6.3.1 Eudiometry
Hydrogen Quantification for 3.3 catalyzed reactions:
In a typical reaction, 3.85 mg AB (0.125 mmol) was combined with 4.2 mg catalyst
(0.007 mmol), and 41 µL TMEDA (0.128 mmol) (sometimes with 2.89 mg (0.005 mmol)
Ca(OTf)2 tetraglyme) in a 2 mL Schlenk bomb equipped with a Teflon stir bar while in a
glovebox under nitrogen. Benzene (0.2 mL) and diglyme containing (0.4 mL) was added
to the flask. A eudiometer was constructed as follows: the side arm of the valve of the
Schlenk flask was connected to a piece of Tygon tubing, which was adapted to 20 gauge
(0.03”) Teflon tubing with a needle. The tubing was threaded through open end of a
burette that was sealed with a Teflon stopcock on the other end. The burette was filled
with water or pump oil. The entire apparatus was then inverted into a 500 mL Erlenmeyer
filled with water or pump oil 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. The volume of liberated gas was recorded periodically until gas evolution ceased.
Liberated hydrogen was quantified by recording its volume displacement in the
eudiometer.
Air-Free Hydrogen Quantification (generally for 3.6 catalyzed reactions):
In a typical reaction, 7.7 mg AB (0.25 mmol) was combined with catalyst (1-10 mol%) in
a 2 mL Schlenk bomb equipped with a Teflon stir bar while in a glove box under
nitrogen. Diglyme (0.6 mL) was added to the flask to create a light yellow mixture. The
eudiometer was constructed as follows: The side arm of the valve was connected to a
piece of Tygon tubing, which was connected to a 3-way valve. Center of the 3-way valve
was connected to the Schlenk line to allow vacuum and nitrogen purge of the apparatus.
The last valve was connected to Tygon tubing that was adapted to 20 gauge (0.03”)
Teflon tubing with a needle. The tubing was threaded through open end of a burette that
was sealed with a Teflon stopcock on the other end. The burette was filled with water.
The entire apparatus was then inverted into a 500 mL Erlenmeyer filled with water and
clamped onto a metal ring stand. Opening the 3-way-valve to the Schlenk line and the
bomb (reaction still closed and wrapped in foil at room temperature), the space in the 3-
way valve and tubing was vacuumed and refilled with N2 for 5 minutes twice and then
opening all 3 valves to the line, N2 purge was conducted throughout the burette for 10
minutes. Then the 3-way valve was turned so that only the bomb and the side connected
to the burette were open, and N2 was turned off. The water in the burette was pulled
upwards by a pipette filler bulb and the initial volume was recorded. The bomb was then
submerged into a 70 C oil bath for 2 minutes to allow the temperature of the system to
equilibrate. The reactor’s valve was opened to release gas from the reactor headspace
while heating in a regulated oil bath. The volume of liberated gas was recorded
periodically until gas evolution ceased. Liberated hydrogen was quantified by recording
its volume displacement in the eudiometer. Reaction proceeds from initial yellow to
gray, to black suspension.
143
6.3.2 Synthetic Procedures
See Chapter 6.2 for Synthesis of Shvo-TMEDA dimer (2.20 and 3.3)
Solvent 1:
100 mL of benchtop grade tetraglyme was stirred over 1.0g of CaH2 for 16 hours and then
filtered through a PTFE syringe filter and stored in the glove box until use.
144
Ca(OTf)2 tetraglyme 3.4
Following a modified procedure by Park et.al. calcium triflate (100 mg, 0.295 mmol),
acetonitrile (1 mL, 19 mmol), and tetraglyme (65 µL, 0.295mmol) were added to a
schlenk bomb in a nitrogen filled glove box.
6
The reaction was run at 50 °C in an oil bath
for 2 hours then let cool to room temperature overnight. ACN was removed by vacuum
and ether was added to the residue and sonicated. The resulting white powder was then
filtered in 64% yield (0.106 g).
1
H NMR (400 MHz, C6D6) δ 3.18 – 3.14 (m, 4H), 3.11 – 3.00 (m, 18H).
13
C NMR (151 MHz, C6D6) δ 123.93-117.58 (J = 320 Hz), 70.88, 68.77, 68.41, 67.97,
59.19.
19
F NMR (470 MHz, C6D6) δ -78.07.
Accurate mass could not obtained due to the hygroscopic nature of 3.4, and similar
compounds have shown the similar hygroscopicity.
6, 7
145
1
H NMR
13
C NMR
19
F NMR
146
[RuCl2(CO)2]n
Following a modified Krishnamurthy and Shashikala’s synthetic procedure,
rutheniumdichlorodicarbonyl polymer was synthesized by adding RuCl3 3H2O (500 mg,
1.91 mmol, 1 equiv.) to a round bottom flask with a stir bar. Formic acid 97% (4 mL, 103
mmol, 53 equiv.) was added to the flask and a reflux condenser was added to the
apparatus with a nitrogen line fitted to the top of the condenser. Ice water was recycled
through the condenser from a bucket with a water pump. The initially black/red reaction
mixture was refluxed in an oil bath for 5-6 hours when the reaction turns a light yellow.
The reaction mixture was allowed to cool to room temperature then sealed and
refrigerated overnight to allow the polymerization to approach completion. The formic
acid was removed via a vacuum line and oil bath set at 70 C. The resulting light yellow
residue was dried on the Schlenk line overnight to remove residual solvent. The residue
was then washed with hexanes (5 mL) and filtered to obtain a light yellow powder in
85% yield (371 mg). The polymer is used without further purification through the
subsequent reactions.
FTIR (ν, cm
−1
): 2098.03, 2033.35 (M − CO’s). 2140.08 is an often seen side product of
[Ru(CO)3Cl2]2 as observed with this case.
Data are consistent with a known compound.
8
147
[(phen)RuCl2(CO)2] (3.5)
Following a modified Thomas et. al.’s synthetic procedure,
phenanthrolinerutheniumdichlorodicarbonyl was synthesized by adding [RuCl2(CO)2]n
(250 mg, 1.1 mmol, 1 equiv.) to a round bottom flask with a stir bar. 1,10-Phenanthroline
(212 mg, 1.1 mmol, 1 equiv.) was added to the flask. MeOH (25 mL) was added to the
flask and a reflux condenser was added to the apparatus with a nitrogen line fitted to the
top of the condenser. Ice water was recycled through the condenser from a bucket with a
water pump. The initially light yellow reaction mixture was refluxed in an oil bath for 1
hour until the reaction turns into a lemon yellow. The reaction was allowed to cool to
room temperature then the mixture was filtered. The yellow powder was dried on a
vacuum line overnight to remove residual solvent and yielded 66% yield (295 mg). Note:
This compound is light sensitive in solution.
1
H NMR (400 MHz, Chloroform-d) δ 9.53 (dd, J = 5.1, 1.4 Hz, 2H), 8.60 (dd, J = 8.3, 1.4
Hz, 2H), 8.07 (s, 2H), 7.99 (dd, J = 8.2, 5.1 Hz, 2H).
FTIR (ν, cm
−1
): 2062.03, 2011.93 (M − CO’s).
Data are consistent with a known compound.
9
1
H NMR
148
[(phen)Ru(OAc)2(CO)2] (3.6)
Following a modified Thomas et. al.’s synthetic procedure,
phenrutheniumdiacetatedicarbonyl was synthesized by adding [(phen)RuCl2(CO)2] (3.5)
(295 mg, 0.72 mmol, 1 equiv.) to a round bottom flask with a stir bar. Silver acetate (242
mg, 1.45 mmol, 2 equiv.) was added to the flask. Acetic acid (8 mL) was added to the
flask and the round bottom flask was wrapped in aluminum foil. A reflux condenser was
added to the apparatus with a nitrogen line fitted to the top of the condenser. Ice water
was recycled through the condenser from a bucket with a water pump. The initially
grey/yellow reaction mixture was refluxed in an oil bath for 1 hour. The reaction was
allowed to cool to room temperature then the mixture was filtered. The filtrate was
collected and all solvent was removed in vacuo. The yellow residue was dried on a
vacuum line overnight to remove residual solvent. A minimum volume of hot MeOH was
added to the residue and the product was recrystallized in MeOH overnight yielding
yellow crystals in 62% yield (203 mg). Note: This compound is light sensitive in
solution.
1
H NMR (400 MHz, Chloroform-d) δ 9.76 (dd, J = 5.1, 1.4 Hz, 2H), 8.57 (dd, J = 8.3, 1.4
Hz, 2H), 8.05 (s, 2H), 7.94 (dd, J = 8.3, 5.1 Hz, 2H), 1.58 (s, 6H).
Data are consistent with a known compound.
9
1
H NMR
149
6.3.4. Kinetic Profiles of 3.3 Catalyzed AB Dehydrogenation via
11
B NMR
A. 3.3 Catalyzed AB dehydrogenation in ketyl dried tetraglyme
AB dehydrogenation run at 70 °C were determined using
11
B NMR with 3.85 mg AB
(0.125 mmol) was combined with 4.2 mg 3.3 (0.007 mmol), 41 µL TMEDA (0.128
mmol, previously dried benzene-d6 (0.2 mL) and previously ketyl dried tetraglyme (0.4
mL) and added to a J-Young tube.
B. 3.3 Catalyzed AB dehydrogenation in Solvent 1
AB dehydrogenation run at 70 °C were determined using
11
B NMR with 3.85 mg AB
(0.125 mmol) was combined with 4.2 mg 3.3 (0.007 mmol), 41 µL TMEDA (0.128
mmol, previously dried benzene-d6 (0.2 mL) and previously prepared Solvent 1 (0.4 mL)
and added to a J-Young tube.
C. 3.3 Catalyzed AB dehydrogenation in ketyl dried solvent with Ca(OTf)2 tetraglyme
addition
AB dehydrogenation run at 70 °C were determined using
11
B NMR with 3.85 mg AB
(0.125 mmol) was combined with 4.2 mg 3.3 (0.007 mmol), 41 µL TMEDA (0.128
mmol) and 2.89 mg Ca(OTf)2 Tetraglyme (0.005 mmol), previously dried benzene-d6
(0.2 mL) and previously prepared ketyl dried tetraglyme (0.4 mL) and added to a J-
Young tube.
D. 3.3 Catalyzed AB dehydrogenation in Solvent 2 (1 equiv. water addition)
AB dehydrogenation run at 70 °C were determined using
11
B NMR with 3.85 mg AB
(0.125 mmol) was combined with 4.2 mg 3.3 (0.007 mmol), 41 µL TMEDA (0.128
mmol), 2.2 µL water (0.125 mmol) previously dried benzene-d6 (0.2 mL) and previously
prepared ketyl dried tetraglyme (0.4 mL) and added to a J-Young tube.
E. 3.3 Catalyzed AB dehydrogenation in Solvent 3 (bench top tetraglyme)
AB dehydrogenation run at 70 °C were determined using
11
B NMR with 3.85 mg AB
(0.125 mmol) was combined with 4.2 mg 3.3 (0.007 mmol), 41 µL TMEDA (0.128
mmol, previously dried benzene-d6 (0.2 mL) and benchtop grade tetraglyme (not dried)
(0.4 mL) and added to a J-Young tube.
11
B plots are summarized below.
150
11
B NMR Comparison of AB Consumption in Various Solvent Systems
0
0.05
0.1
0.15
0.2
0 0.5 1 1.5 2 2.5 3 3.5 4
Ketyl dried tetraglyme
Solvent 1
Ca(OTf)
2
-Tetraglyme added
to ketyl dried tetraglyme
Solvent 2
Solvent 3
AB (M)
Time ( x 1000 sec)
151
6.3.4. Kinetic Profiles of AB Dehydrogenation in Solvent via
11
B NMR
A. AB Dehydrogenation in Solvent 1
AB dehydrogenation run at 70 °C were determined using
11
B NMR with 3.85 mg AB in
previously dried benzene-d6 (0.2 mL) and previously prepared Solvent 1 (0.4 mL) and
added to a J-Young tube.
B. AB Dehydrogenation in ketyl dried tetraglyme
AB dehydrogenation run at 70 °C were determined using
11
B NMR with 3.85 mg AB in
previously dried benzene-d6 (0.2 mL) and previously ketyl dried tetraglyme (0.4 mL) and
added to a J-Young tube.
0
0.05
0.1
0.15
0.2
0.25
0 2 4 6 8 10
Solvent 1
Ketyl dried Tetraglyme
Time ( x 1000 sec)
AB (M)
152
6.3.5. Kinetic Profiles of 3.6 Catalyzed AB Dehydrogenation via
11
B NMR
A. Kinetics for AB dehydrogenation by 3.6 at 1 mol% catalyst loading
3.6-catalyzed AB dehydrogenation run at 70 °C was determined using
11
B NMR with 7.7
mg AB (0.25 mmol) and 3.6 (1.2 mg, 2.5 mol, 1 mol%) in diglyme (0.4 mL) and
benzene-d6 (0.2 mL).
a.
0
0.1
0.2
0.3
0.4
0 10 20 30 40 50 60
[AB] (M)
Time (sec x 1000)
b.
0.3
0.35
0.4
0.45
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
parent 1
I
[AB] (M)
Time ( x 1000 sec)
y = m1 + m2 * M0
Error Value
0.0054903 0.4276 m1
0.0080568 -0.12598 m2
NA 0.00012589 Chisq
NA 0.98993 R
153
c.
0
0.05
0.1
0.15
0.2
0.25
0 10 20 30 40 50 60
parent 2
K
[AB] (M)
Time ( x 1000 sec)
y = m1 + m2*exp(-m3*x)
Error Value
0.00070633 -0.0040804 m1
0.00052146 0.25675 m2
0.00019562 0.03052 m3
NA 0.0011417 Chisq
NA 0.99959 R
Figure 6.12. Kinetics for AB dehydrogenation catalyzed by 3.6. (a). Entirety of graph.
(b) Fast portion of dehydrogenation fitted linearly. (c) Slow portion of reaction fitted to
exponential decay equation. The entirely of the kinetic profile does not fit exponential
decay, thus it is split into a linear and an exponential portion for easier rate comparisons
with air and water exposure experiments.
154
B. Kinetics for AB dehydrogenation catalyzed by 3.6 after 3.6’s exposure to air and
water
3.6-catalyzed AB dehydrogenation run at 70 °C was determined using
11
B NMR after 3.6
(1.2 mg, 2.5 mol, 1 mol%) in diglyme (0.4 mL) and benzene-d6 (0.2 mL) was
submerged in an ultrasonic cleaning bath for 20 minutes open to air, and then the addition
of 7.7 mg of AB (0.25mmol).
a.
0
0.1
0.2
0.3
0.4
0.5
0 10 20 30 40 50 60 70
air/water full
F
[AB] (M)
Time ( x 1000 sec)
b.
0.26
0.28
0.3
0.32
0.34
0.36
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
air 1
Q
[AB] (M)
Time ( x 1000 sec)
y = m1 + m2 * M0
Error Value
0.007446 0.37797 m1
0.012423 -0.12159 m2
NA 0.00014966 Chisq
NA 0.97975 R
155
c.
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0 10 20 30 40 50 60 70
air 2
S
[AB] (M)
Time ( x 1000 Sec)
y = m1 + m2*exp(-m3*x)
Error Value
0.00061356 0.027142 m1
0.00040705 0.16404 m2
0.00030075 0.030593 m3
NA 0.0011906 Chisq
NA 0.99887 R
Figure 6.13. Kinetics for AB dehydrogenation catalyzed by 3.6 after catalyst/solvent
system was exposed to air. Note similarity to reaction with no air exposure in Figure
6.12. (a). Entirety of graph. (b). Fast portion of dehydrogenation fitted linearly. (c) Slow
portion of reaction fitted to exponential decay equation.
156
6.3.6. Reactions of 3.6 with Borazine
Borazine was generated in a J. Young NMR tube by the reaction of Shvo’s catalyst (6.7
mg, 2.5%, 0.0062 mmol) with AB (7.7 mg, 0.25 mmol) in 2:1 diglyme/benzene-d6 (0.4
mL and 0.2 mL respectively) in a 70 C oil bath for 16-20 hours.
12a,16
The NMR tube was
connected via a flame-dried U-tube to another J. Young NMR tube containing 3.6 (1.2
mg, 0.0025 mmol), diglyme (0.4 mL), and
11
B external standard. The volatiles (H2,
borazine, benzene-d6) were transferred under static vacuum while the borazine-containing
tube was heated gently with a heat gun while the receiving tube was submerged in liquid
N2 until all of the ca. 0.2 mL of benzene was transferred to the receiving tube. The J.
Young valves were closed and the U-tube system was flushed with N2. While the N2 line
was closed and the U-tube is under N2, the receiving tube was opened briefly to return it
to atmospheric pressure.
1
H and
11
B NMR spectra were taken, and then the borazine-
containing tube was submerged in a 70 C oil bath for 24 hours. Peak heights relative to
the external standard (BF3-OEt2 in diglyme (0 ppm)) were compared since
polyborazylene and borazine peaks overlap in the
11
B spectrum and integrations could not
be done accurately. The procedure for the borazine background reaction was the same,
except for the absence of 3.6 in the receiving tube.
Figure 6.14.
11
B spectra borazine in diglyme/benzene-d6, control reaction of no catalyst
3.6. Bottom: initial. Top: 24 hours in a 70 C oil bath. 15% decrease based on peak
height (31 ppm) relative to
11
B standard. Boron external standard BF3-OEt2 is at 0 ppm.
157
6.3.7 Representative
11
B Spectra of AB Dehydrogenations
A. End of Reaction
11
B and
1
H Spectra for AB Dehydrogenation Catalyzed by 3.6
Figure 6.15.
11
B spectra of end of AB dehydrogenation reaction catalyzed by 3.6 (10
mol% Ru, 1:2 benzene-d6:diglyme). Note presence of broad polyunsaturated peaks at 23-
33 ppm. Other intermediates include borazine (31 ppm), BF3-OEt2 external standard (0
ppm), amine borane cyclic tetramer (-5, -11, -23 ppm), cyclotriborazane (-11 ppm),
residual AB (-22 ppm), and aminodiborane (-27 ppm).
Figure 6.16.
1
H spectra of end of AB dehydrogenation reaction catalyzed by 3.6. Note H2
gas formation at 4.42 ppm.
158
B. End of Reaction
11
B and
1
H Spectra for AB Dehydrogenation Catalyzed by 3.6 After
Catalyst Exposure to Air/Water
Figure 6.17. Top:
11
B spectra of end of AB dehydrogenation reaction catalyzed by 3.6 (1
mol% Ru, 1:2 benzene-d6:diglyme) after 3.6 and solvents were exposed to air and
sonicated for 20 min. Note presence of broad polyunsaturated peaks at 23-33 ppm. Other
intermediates include borazine (31 ppm), BF3-OEt2 external standard (0 ppm), amine
borane cyclic tetramer (-5, -11, -23 ppm), cyclotriborazane (-11 ppm), residual AB (-22
ppm), and aminodiborane (-27 ppm). Bottom:
11
B spectra of end of AB dehydrogenation
reaction catalyzed by 3.6 (1 mol% Ru, 1:2 C6D6: diglyme). The boron intermediates of
the two spectra are comparable, no evidence of B(OH)3, the hydrolysis of boron
byproducts at 20 ppm.
159
Figure 6.18.
1
H spectra of end of AB dehydrogenation reaction catalyzed by 3.6 after 3.6
and solvents (diglyme/benzene-d6) were exposed to air and submerged in an ultrasonic
cleaning bath for 20 min. Note H2 gas formation at 4.42 ppm.
160
6.3.8 3.6-Catalyzed Hydrogen Quantification
A. Catalyst 3.6 Reuse Reactions.
To test the reusability of catalyst 1, we studied the production of H2 gas by eudiometry
for successive runs with 1.0 mol% catalyst. For run 1, we added 7.7 mg AB, 1.2 mg 1,
and 0.6 mL diglyme to a 2 mL Schlenk flask equipped with a small stir bar. The reaction
was heated at 70 °C in a regulated oil bath and reaction progress monitored by
displacement of water by H2 gas in an inverted 50 mL burette. For successive runs, we
added 7.7 mg AB and 0.2 mL diglyme and repeated the reaction at 70 °C. Pseudo-first
order rate constants for H2 productions over these runs are 12.1, 10.8, and 7.0 x 10
-5
s
-1
.
0
0.5
1
1.5
2
2.5
3
0 5 10 15 20 25 30
Run 1
Equiv. of H
2
Time (x 1000 sec)
y = m1 + m2*(1-exp(-m3*x))
Error Value
0.022808 0.077955 m1
0.048197 2.7185 m2
0.0056594 0.12109 m3
NA 0.062058 Chisq
NA 0.99825 R
0
0.5
1
1.5
2
2.5
3
0 10 20 30 40 50
Run 3
Equiv. of H
2
Time ( x 1000 sec)
y = m1 + m2*(1-exp(-m3*x))
Error Value
0.024299 0.10654 m1
0.052479 2.3692 m2
0.0042312 0.070436 m3
NA 0.082741 Chisq
NA 0.99739 R
161
B. Representative H2 quantification of [(phen)RuCl2(CO)2] (3.5) and 2 equivalents of
TlOTf
AB dehydrogenation eudiometry run at 70 °C with 7.7 mg AB (0.25 mmol), 3.5 (10.2
mg, 25 μmol, 10 mol%), and TlOTf (17.7 mg, 50 μmol, 20 mol%) in diglyme (0.6 mL).
0
0.5
1
1.5
2
2.5
3
0 10 20 30 40 50 60 70 80
Equiv. of H
2
Time (x 1000 sec)
162
6.4 Chapter 4 Experimental and Spectral Data
6.4.1. Borazine Disproportionation by Formic Acid
Borazine was synthesized by Dr. Brian Conley via the method reported by Sneddon and
Wideman.
10
50 µL of borazine was mixed with 35 µL of formic acid in 0.6 mL of
CD3CN at room temperature in the glove box.
1
H NMR was taken immediately after.
See Figure 4.1 for resulting spectra.
163
6.4.2. Catalytic FA dehydrogenation
A. Proof of concept
4.8 (20 mg, 0.046 mmol) and formic acid (8 µL, 0.212 mmol) were dissolved in CD3CN
(0.6 mL) in a J-Young tube in the glove box at room temperature, and monitored over 3
hours and 50 min via
1
H NMR at 70 °C.
B. Optimizing Solvent Amount
In a typical experiment, 1 mol% of 4.8 (4 mg, 0.0092 mmol) was loaded into a 2 mL
Schlenk bomb with a side arm with a Teflon stir bar while in a glove box under nitrogen.
32 µL of FA (0.855 mmol) and a varied amount of CD3CN (none to 5 µL to 5 mL) was
added to the bomb. A eudiometer was constructed as follows: the side arm of the valve
of the Schlenk flask was connected to a piece of Tygon tubing, which was adapted to 20
gauge (0.03”) Teflon tubing with a needle. The tubing was threaded through open end of
a burette that was sealed with a Teflon stopcock on the other end. The burette was filled
with pump oil. The entire apparatus was then inverted into a 500 mL Erlenmeyer filled
with pump oil 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. The volume
of liberated gas was recorded periodically until gas evolution ceased. Liberated hydrogen
and presumably carbon dioxide were quantified by recording its volume displacement in
the eudiometer.
C. Catalyst Screening for FA dehydrogenation
In a typical experiment, 1 mol% of catalyst was loaded into a 2 mL Schlenk bomb with a
side arm with a Teflon stir bar while in a glove box under nitrogen. A varied amount of
FA and a varied amount of CD3CN was added to the bomb (See Table 6.8 for amounts
added). A eudiometer was constructed as follows: the side arm of the valve of the
Schlenk flask was connected to a piece of Tygon tubing, which was adapted to 20 gauge
(0.03”) Teflon tubing with a needle. The tubing was threaded through open end of a
burette that was sealed with a Teflon stopcock on the other end. The burette was filled
with pump oil. The entire apparatus was then inverted into a 500 mL Erlenmeyer filled
with pump oil 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. The volume
of liberated gas was recorded periodically until gas evolution ceased. Liberated hydrogen
and presumably carbon dioxide were quantified by recording its volume displacement in
the eudiometer.
164
Table 6.8. Amount of Catalyst, FA, and ACN for FA dehydrogenation.
See Figure 4.5 for summary of data.
165
6.4.3. Ammonium Boroformate (4.2) Synthesis
In a nitrogen filled glove box, ammonia borane (0.25g, 8.1 mmol) was added to a
Schlenk flask. AB was dissolved in 50 mL of dried THF. FA (1.2 mL, 32.4 mmol) was
added to the mixture and stirred at 50 °C for 4 hours. The heat was then turned off and
the reaction was allowed to stir overnight at room temperature under a constant stream of
N2. The solvent was then removed in vacuo and the remaining residue was dissolved in
minimum methanol and added to a large excess of diethyl ether. The resulting white
precipitate was filtered and let dry on vacuum line overnight to result in 0.177 mg
(10.5%).
1
H NMR (500 MHz, CD3OD) δ 8.53 (s, formate), 4.91(s, NH).
13
C NMR (126 MHz, CD3OD) δ 170.24.
11
B NMR (160 MHz, CD3OD) δ 18.23.
Accurate mass could not be obtained due to the hygroscopic nature of 4.2 as
demonstrated by decomposition when stored outside the glovebox and literature
description of similar compounds.
11
166
Figure 6.19. NMR spectra of ABF in CD3OH. Top:
1
H NMR. Middle:
13
C NMR.
Bottom:
11
B NMR.
167
6.4.4. ABF (4.2) Dehydrogenation by Various Catalysts
In a typical experiment, 2.0 mg of catalyst at 5 mol% was loaded into a J-Young tube in a
glove box under nitrogen See Table 6.9 for specific amounts of various catalysts. A
varied amount of ABF and 0.6 mL of DMSO-d6 were added to the tube. The reactions
were submerged into a 70 °C oil bath and regularly monitored by
1
H NMR.
Table 6.9. Amount of Catalyst, FA, and DMSO for ABF dehydrogenation.
See Figure 4.8 for summary of data.
168
6.5 Chapter 5 Experimental and Spectral Data
6.5.1 Catalyst Synthetic Procedures
Iridium complex 5.5 was graciously provided by Jeff Celaje.
12
In the glove box under nitrogen, 2-((di-t-butylphosphino)methyl)pyridine (18.3 mg, 77.1
µmol) was dissolved in a dry vial in 10 mL of dry dichloromethane. In another vial
containing a Teflon stir bar, chloro(1,5-cyclooctadiene)iridium(I) dimer (30.3 mg, 38.0
µmol) and sodium triflate (35.0 mg, 203 µmol) were suspended in 15 mL of dry
dichloromethane. The suspension was stirred vigorously and then the phosphinopyridine
solution was added slowly dropwise. The phosphinopyridine vial was rinsed with 5 mL
of dichloromethane and added to the stirred suspension. After stirring for 30 min, the
solution was filtered to remove the sodium chloride byproduct and the excess sodium
triflate. The solvent was evaporated under reduced pressure to yield a yellow glassy
solid. Hexanes (15 mL) was added to the residue and then triturated by sonication. The
hexane was decanted and the residue washed with an additional 5 mL of hexanes. The
pure iridium complex was dried under reduced pressure to give a yellow solid (40 mg,
69%).
1
H NMR (600 MHz, Methylene Chloride-d2) δ 8.45 (d, J = 6.0 Hz, 1H), 7.91 (t, J = 7.5
Hz, 1H), 7.70 (d, J = 7.9 Hz, 1H), 7.33 (t, J = 6.7 Hz, 1H), 4.15 (dd, J = 16.4, 9.1 Hz,
1H), 3.92 (dd, J = 16.4, 11.4 Hz, 1H), 1.77 (d, J = 1.8 Hz, 15H), 1.50 (d, J = 14.6 Hz,
9H), 1.14 (d, J = 13.8 Hz, 9H).
13
C NMR (151 MHz, Methylene Chloride-d2) δ 175.43, 164.47, 152.86, 140.65, 125.85,
124.98 (d, J = 7.4 Hz), 95.33 (d, J = 2.3 Hz), 38.24 (d, J = 20.9 Hz), 37.92 (d, J = 19.7
Hz), 36.10 (d, J = 28.5 Hz), 31.05 (d, J = 2.4 Hz), 29.63 (d, J = 3.0 Hz).
31
P NMR (243 MHz, Methylene Chloride-d2) δ 59.86.
19
F NMR (564 MHz, Methylene Chloride-d2) δ -78.91.
169
Ruthenium complex 5.6 was graciously provided by Jeff Celaje.
In the glove box under nitrogen, 2-((di-t-butylphosphino)methyl)pyridine
13
(173.5 mg,
0.73 mmol) was dissolved in a dry vial in 5 mL of dry dichloromethane. In another vial
containing a Teflon stir bar, dichloro(p-cymene)ruthenium(II) dimer (218 mg, 0.36
mmol) and sodium triflate (230 mg, 1.34 mmol) were suspended in 10 mL of dry
dichloromethane. The suspension was stirred vigorously and then the phosphinopyridine
solution was added slowly dropwise. The phosphinopyridine vial was rinsed with 5 mL
of dichloromethane and added to the stirred suspension. After stirring for 1 hour, the
solution was filtered to remove the sodium chloride byproduct and the excess sodium
triflate. The solvent was evaporated under reduced pressure to yield an orange glassy
solid. A 5:1 mixture of dry hexanes/ethyl ether (10 mL) was added to the residue and
then triturated by sonication. The hexane was decanted and the residue washed with an
additional 10 mL of hexanes/ethyl ether. The pure ruthenium complex was dried under
reduced pressure to give an orange solid (350 mg, 73%).
1
H NMR (600 MHz, Methylene Chloride-d2) δ 9.21 (d, J = 5.7 Hz, 1H), 7.82 (t, J = 7.6
Hz, 1H), 7.46 – 7.41 (m, 2H), 6.30 (d, J = 6.6 Hz, 1H), 6.20 (d, J = 6.6 Hz, 1H), 6.09 (d,
J = 6.0 Hz, 1H), 5.80 (d, J = 6.0 Hz, 1H), 3.76 (dd, J = 16.4, 8.5 Hz, 1H), 3.33 (dd, J =
16.4, 12.8 Hz, 1H), 2.80 (dt, J = 13.7, 6.8 Hz, 1H), 2.17 (s, 3H), 1.53 (d, J = 14.2 Hz,
9H), 1.34 (d, J = 6.9 Hz, 3H), 1.30 (d, J = 6.9 Hz, 3H), 1.22 (d, J = 13.2 Hz, 9H).
13
C NMR (151 MHz, Methylene Chloride-d2) δ 163.68, 157.80, 140.05, 125.30, 124.76
(d, J = 8.5 Hz), 100.08, 94.77 (d, J = 5.4 Hz), 93.72 (d, J = 6.1 Hz), 87.26, 84.46, 39.36
(d, J = 15.7 Hz), 38.71 (d, J = 15.9 Hz), 33.72 (d, J = 22.6 Hz), 31.55, 31.23, 30.27,
23.81, 21.75, 18.31.
31
P NMR (243 MHz, Methylene Chloride-d2) δ 87.31.
19
F NMR (564 MHz, Methylene Chloride-d2) δ -78.84.
170
6.5.2 Alcohol Homocoupling and AD Procedures
A. Homocoupling Procedures of Alcohols Catalyzed by 5.6
Oxidation of 5.8 to acetophenone in the presence of base:
1.0 mol% of 5.6 (2.0 mg, 3 µmol), 1-phenylethanol (5.8, 37 µL, 0.3 mmol), and KOH
(1.7 mg, 30 µmol) were added to a 2 mL Schlenk bomb in a glove box. Mesitylene (0.2
mL) was then added as a solvent to the bomb. The reaction was sealed and removed from
the glove box and into a 130 °C oil bath for 48 hours under positive N2 pressure. After
48 hours, an aliquot from the solution was dissolved in CDCl3 (0.6 mL), and a
1
H NMR
spectra was taken to determine conversion.
171
Coupling of 5.8 to ethers and acetophenone in the absence of base:
0.15 mol% of 5.6 (2.0 mg, 3 µmol) and 1-phenylethanol (5.8, 250 µL, 20 mmol) were
added to a 2 mL Schlenk bomb in a glove box. The reaction was sealed and removed
from the glove box and into a 130 °C oil bath for 48 hours under positive N2 pressure.
After 48 hours, an aliquot from the solution was dissolved in CDCl3 (0.6 mL), and a
1
H
NMR spectra was taken to determine conversion.
172
Coupling of 5.9 to benzylether in the absence of base:
0.33 mol% of 5.6 (2.1 mg, 3.2 µmol) and benzyl alcohol (5.9, 0.1 mL, 0.96 mmol) were
added to a 5 mL Schlenk bomb in a glove box. The reaction was sealed and removed
from the glove box and into a 130 °C oil bath for 20 hours. After 20 hours, an aliquot
from the solution was dissolved in CDCl3 (0.6 mL), and a
1
H NMR spectra was taken to
determine conversion.
173
Coupling of 5.10 to octyl octanoate ester in the presence of base
Initial:
0.20 mol% of 5.6 (2.0 mg, 3.0 µmol), 1-octanol (5.10, 250 µL, 1.6 mmol), and 10 mol%
KOH (8.9 mg, 0.16 mmol) were added to a 5 mL Schlenk bomb in a glove box. The
reaction was sealed and removed from the glove box and into a 130 °C oil bath for 48
hours under positive N2 pressure. After 48 hours, an aliquot from the solution was
dissolved in CDCl3 (0.6 mL), and a
1
H NMR spectra was taken to determine 30%
conversion.
Optimized:
3.0 mol% of 5.6 (12 mg, 18 µmol), 1-octanol (5.10, 100 µL, 0.63 mmol), and 10 mol%
KOH (3.6 mg, 0.06 mmol) were added to a 5 mL Schlenk bomb in a glove box. The
reaction was sealed and removed from the glove box and into a 130 °C oil bath for 48
hours under positive N2 pressure. After 48 hours, an aliquot from the solution was
dissolved in CDCl3 (0.6 mL), and a
1
H NMR spectra was taken to determine 70%
conversion.
174
Coupling of 5.10 to dioctyl ether in the absence of base
0.20 mol% of 5.6 (2.0 mg, 3.0 µmol) and 1-octanol (5.11, 236 µL, 1.5 mmol) were added
to a 5 mL Schlenk bomb in a glove box. The reaction was sealed and removed from the
glove box and into a 130 °C oil bath for 40 hours. After 40 hours, an aliquot from the
solution was dissolved in CDCl3 (0.6 mL), and a
1
H NMR spectra was taken to determine
conversion.
175
B. Homocoupling Procedures of Alcohols Catalyzed by 5.5 (Guerbet Reactions)
5.5 catalyzed Guerbet Coupling of 1-butanol
1.0 mol% of 5.5 (5.8 mg, 7.7 µmol), 10 mol% KOH (4.4 mg, 77 µmol), and 1-octanol
(5.5, 0.125 mL, 0.79 mmol) were added to a 2 mL Schlenk bomb in a glove box. The
reaction was sealed and removed from the glove box and into a 130 °C oil bath for 48
hours under positive N2 pressure. After 48 hours, an aliquot from the solution was
dissolved in CDCl3 (0.6 mL), and a
1
H NMR spectra was taken to determine conversion.
The reaction mixture was extracted with hexanes and run through the GC-MS to confirm
identity of 5.11.
176
5.5 catalyzed Guerbet Coupling of EtOH to 1-butanol
0.20 mol% of 5.5 (2.6 mg, 3.5 µmol), 10 mol% KOH (9.6 mg, 35 µmol), and ethanol (0.1
mL, 1.7 mmol) were added to a 2 mL Schlenk bomb in a glove box. The reaction was
sealed and removed from the glove box and into a 150 °C oil bath for 90 hours. After 90
hours, an aliquot from the solution was dissolved in CDCl3 (0.6 mL), and a
1
H NMR
spectra was taken to determine conversion.
177
6.5.3. Alcohol-Amine Dehydrative Coupling Procedures
A. 5.6-Catalyzed 1-phenylethanol (5.8) and Amine Coupling
Amine 5.12b:
5.0 mol% of 5.6 (6.6 mg, 5.5 µmol), 5.12a (26.5 mg, 0.11 mmol, 1 equiv.), and 5.8 (100
µL, 0.83 mmol, 7.5 equiv.) were added to a 5 mL Schlenk bomb in a glove box. The
reaction was sealed and removed from the glove box and into a 130 °C oil bath for 24
hours. After 24 hours, an aliquot from the solution was dissolved in CDCl3 (0.6 mL), and
a
1
H NMR spectra was taken to determine conversion. When complete, the reaction was
dissolved in minimal DCM and loaded into a silica column. The reaction was purified
with 4:1 hexanes:ethyl acetate and dried on the vacuum to yield 24.4 mg (71%). The
5.12a starting material was only 90% pure.
Data are consistent with a known compound.
14
178
Amine 5.13b:
5.0 mol% of 5.6 (6.6 mg, 5.5 µmol), 5.13a (17.6 mg, 0.11 mmol, 1 equiv.), and 5.8 (100
µL, 0.83 mmol, 7.5 equiv.) were added to a 5 mL Schlenk bomb in a glove box. The
reaction was sealed and removed from the glove box and into a 130 °C oil bath for 24
hours. After 24 hours, an aliquot from the solution was dissolved in CDCl3 (0.6 mL), and
a
1
H NMR spectra was taken to determine conversion. When complete, the reaction was
dissolved in minimal DCM and loaded into a silica column. The reaction was purified
with 100% ethyl acetate and dried on the vacuum to yield 21 mg (72%).
Data are consistent with a known compound.
15
179
Amine 5.14b:
5.0 mol% of 5.6 (6.6 mg, 5.5 µmol), 5.14a (19 µL, 0.11 mmol, 1 equiv.), and 5.8 (100
µL, 0.83 mmol, 7.5 equiv.) were added to a 5 mL Schlenk bomb in a glove box. The
reaction was sealed and removed from the glove box and into a 130 °C oil bath for 24
hours. After 24 hours, an aliquot from the solution was dissolved in CDCl3 (0.6 mL), and
a
1
H NMR spectra was taken to determine conversion. When complete, the reaction was
dissolved in minimal DCM and loaded into a silica column. The reaction was purified
with 1:1 hexanes:ethyl acetate and dried on the vacuum to yield 21 mg (67%).
Data are consistent with a known compound.
16
180
Amine 5.15b:
5.0 mol% of 5.6 (6.6 mg, 5.5 µmol), 5.15a (15 mg, 0.11 mmol, 1 equiv.), and 5.8 (100
µL, 0.83 mmol, 7.5 equiv.) were added to a 5 mL Schlenk bomb in a glove box. The
reaction was sealed and removed from the glove box and into a 130 °C oil bath for 24
hours. After 24 hours, an aliquot from the solution was dissolved in CDCl3 (0.6 mL), and
a
1
H NMR spectra was taken to determine conversion. When complete, the reaction was
dissolved in minimal DCM and loaded into a silica column. The reaction was purified
with 1:1 hexanes:ethyl acetate and dried on the vacuum to yield 23 mg (85%).
5.15b is a known compound,
17
however
1
H and
13
C NMR peaks are not reported,
therefore they are included here and are analogous to the benzyl alcohol coupling product
5.15c.
18
1
H NMR (600 MHz, Methylene Chloride-d2) δ 7.44 – 7.17 (m, 5H), 6.94 (d, J = 8.5 Hz,
2H), 6.68 (d, J = 8.4 Hz, 2H), 4.39 (bs, 1H), 3.86 (q, J = 6.7 Hz, 1H), 2.79 – 2.62 (m,
4H), 1.37 (d, J = 6.6 Hz, 3H).
13
C NMR (151 MHz, Methylene Chloride-d2) δ 155.26, 144.22, 131.09, 130.08, 128.94,
127.71, 127.15, 115.83, 58.59, 48.92, 34.87, 23.42.
181
Amine 5.16b:
5.0 mol% of 5.6 (6.6 mg, 5.5 µmol), 5.16a (13 µL, 0.11 mmol, 1 equiv.), and 5.8 (100
µL, 0.83 mmol, 7.5 equiv.) were added to a 5 mL Schlenk bomb in a glove box. The
reaction was sealed and removed from the glove box and into a 130 °C oil bath for 19
hours. After 19 hours, an aliquot from the solution was dissolved in CDCl3 (0.6 mL), and
a
1
H NMR spectra was taken to determine conversion. When complete, the reaction was
dissolved in minimal DCM and loaded into a silica column. The reaction was purified
with 1:1 hexanes:ethyl acetate and dried on the vacuum to yield 15 mg (66%).
Data are consistent with a known compound.
19
182
B. 5.6-Catalyzed benzyl alcohol (5.9) and Amine Coupling
Amine 5.12c:
1.0 mol% of 5.6 (1.1 mg, 1.6 µmol), 5.12a (38.63 µL, 0.16 mmol, 1 equiv.), and 5.9 (25
µL, 0.24 mmol, 1.5 equiv.) were added to a 5 mL Schlenk bomb in a glove box. The
reaction was sealed and removed from the glove box and into a 110 °C oil bath for 20
hours. After 20 hours, an aliquot from the solution was dissolved in CDCl3 (0.6 mL), and
a
1
H NMR spectra was taken to determine conversion. When complete, the reaction was
dissolved in minimal DCM and loaded into a silica column. The reaction was purified
with 2:1 hexanes:ethyl acetate and dried on the vacuum to yield 38 mg (79%). The 5.12a
starting material was only 90% pure.
Data are consistent with a known compound.
20
183
Amine 5.13c:
1.0 mol% of 5.6 (1.1 mg, 1.6 µmol), 5.13a (25.6, 0.16 mmol, 1 equiv.), and 5.9 (25 µL,
0.24 mmol, 1.5 equiv.) were added to a 5 mL Schlenk bomb in a glove box. The reaction
was sealed and removed from the glove box and into a 110 °C oil bath for 20 hours.
After 20 hours, an aliquot from the solution was dissolved in CDCl3 (0.6 mL), and a
1
H
NMR spectra was taken to determine conversion. When complete, the reaction was
dissolved in minimal DCM and loaded into a silica column. The column was first flushed
with 1% trimethylamine, then the reaction was loaded and purified with 1:1
hexanes:ethyl acetate and dried on the vacuum to yield 28 mg (72%).
Data are consistent with a known compound.
21
184
Amine 5.14c:
1.0 mol% of 5.6 (1.1 mg, 1.6 µmol), 5.14a (27 µL, 0.16 mmol, 1 equiv.), and 5.9 (25 µL,
0.24 mmol, 1.5 equiv.) were added to a 5 mL Schlenk bomb in a glove box. The reaction
was sealed and removed from the glove box and into a 110 °C oil bath for 72 hours.
After 72 hours, an aliquot from the solution was dissolved in CDCl3 (0.6 mL), and a
1
H
NMR spectra was taken to determine conversion. When complete, the reaction was
dissolved in minimal DCM and loaded into a silica column. The column was first flushed
with 1% trimethylamine, then the reaction was loaded excess benzyl alcohol was eluded
with 1:1 hexanes:ethyl acetate, then the product was eluded with 1:1 hexanes:ethyl
acetate and dried on the vacuum to yield 38.4 mg (90%).
Data are consistent with a known compound.
22
185
Amine 5.15c:
1.0 mol% of 5.6 (1.1 mg, 1.6 µmol), 5.15a (22.4 mg, 0.16 mmol, 1 equiv.), and 5.9 (25
µL, 0.24 mmol, 1.5 equiv.) were added to a 5 mL Schlenk bomb in a glove box. The
reaction was sealed and removed from the glove box and into a 110 °C oil bath for 5
hours. After 5 hours, an aliquot from the solution was dissolved in CDCl3 (0.6 mL), and
a
1
H NMR spectra was taken to determine conversion. When complete, the reaction was
dissolved in minimal DCM and loaded into an HPLC. The product was eluded with a
hexanes and ethyl acetate gradient, then dried on the vacuum to yield 25.8 mg (72%).
Data are consistent with a known compound.
18
186
Amine 5.16c:
1.0 mol% of 5.6 (1.1 mg, 1.6 µmol), 5.16a (19 µL, 0.16 mmol, 1 equiv.), and 5.9 (25 µL,
0.24 mmol, 1.5 equiv.) were added to a 5 mL Schlenk bomb in a glove box. The reaction
was sealed and removed from the glove box and into a 110 °C oil bath for 24 hours.
After 24 hours, an aliquot from the solution was dissolved in CDCl 3 (0.6 mL), and a
1
H
NMR spectra was taken to determine conversion. When complete, the reaction was
dissolved in minimal DCM and loaded into an HPLC. The product was eluded with a
hexanes and ethyl acetate gradient, then dried on the vacuum to yield 18 mg (58%).
Data are consistent with a known compound.
23
187
Amine 5.17c:
1.0 mol% of 5.6 (1.1 mg, 1.6 µmol), 5.17a (20 µL, 0.16 mmol, 1 equiv.), and 5.9 (25 µL,
0.24 mmol, 1.5 equiv.) were added to a 5 mL Schlenk bomb in a glove box. The reaction
was sealed and removed from the glove box and into a 110 °C oil bath for 24 hours.
After 24 hours, an aliquot from the solution was dissolved in CDCl3 (0.6 mL), and a
1
H
NMR spectra was taken to determine conversion. When complete, the reaction was
dissolved in minimal DCM and loaded into an HPLC. The product was eluded with a
hexanes and ethyl acetate gradient, then dried on the vacuum to yield 18.6 mg (58%).
Data are consistent with a known compound.
24
188
Amine 5.18c:
1.0 mol% of 5.6 (1.1 mg, 1.6 µmol), 5.18a (19 µL, 0.16 mmol, 1 equiv.), and 5.9 (25 µL,
0.24 mmol, 1.5 equiv.) were added to a 5 mL Schlenk bomb in a glove box. The reaction
was sealed and removed from the glove box and into a 110 °C oil bath for 24 hours.
After 24 hours, an aliquot from the solution was dissolved in CDCl 3 (0.6 mL), and a
1
H
NMR spectra was taken to determine 87% conversion.
Data are consistent with a known compound.
25
189
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Wiley & Sons, Ltd, 2001.
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Krishnamurthy, G. N.; Shashikala, N. Synthesis of Ruthenium(II) Carbonyl Complexes
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9
Black, D.; Deacon, G. N. Thomas, Ruthenium Carbonyl Complexes. I. Synthesis of
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Wideman, T.; Sneddon, L. G. Convenient Procedures for the Laboratory Preparation of
Borazine. Inorg. Chem. 1995, 34, 1002–1003.
11
Knopf, I.; Cummins, C. C. Revisiting CO2 Reduction with NaBH4 under Aprotic
Conditions: Synthesis and Characterization of Sodium Triformatoborohydride.
Organometallics 2015, 34, 1601–1603.
12
Celaje, J.J. Transition Metal Catalysts of Pyridylphosphine and Dipyridylborate
Ligands in Dehydrogenation Reactions. Ph.D. Dissertation, University of Southern
California, Los Angeles, CA, 2016.
13
Beddie, C.; Wei, P.; Douglas, S. Titanium Pyridyl-Phosphinimide Complexes
Synthesis, Structure, and Ethylene Polymerization Catalysis. Can. J. Chem. 2006, 84,
755-761.
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Carboxylic Acid and an Amine in Benzene Studied by Induced Circular Dichroism. J.
Chem. Soc. Perkin Trans. 2 1976, 5, 555-559.
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Heureux, N.; Wouters, J.; Norberg, B.; Markó, I. E. Short, Asymmetric Synthesis of
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Saitoh, T.; Sano, T. A Convenient Synthesis of 1,1-Disubstituted 1,2,3,4-
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Saidi, O.; Blacker, A. J.; Farah, M. M.; Marsden, S. P.; Williams, J. M. J. Iridium-
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Abstract (if available)
Abstract
In the field of alternative energy, hydrogen gas has become a popular fuel source because of its clean waste stream and abundance. However, one of the greatest challenges in using H₂ as a fuel source is finding a safe, efficient, and inexpensive method for its storage. Ammonia borane (AB) is a solid hydrogen storage material that has garnered attention for its high hydrogen weight density of 19.6%. Shvo’s catalyst is a ruthenium dimer capable of hydrogenation and dehydrogenation of substrates via metal-ligand cooperation. AB dehydrogenation with Shvo’s catalyst releases three possible equivalents of H₂, and the mechanism of this reaction has been extensively studied, leading to a detailed understanding of the role of borazine in the dehydrogenation. Borazine is a poison not only for Shvo’s catalyst, but also for fuel cells. ❧ Through the close study of independent syntheses of Shvo derivatives, a protective mechanism was presented wherein catalyst deactivation by borazine was prevented by coordination of a potentially inhibiting ligand. These studies showed how bidentate nitrogen ligands can transform Shvo’s catalyst into more reactive species for dehydrogenation of AB. Further studies with a tetramethylethylenediamine supported Shvo derivative and optimized ruthenium catalyst phenRu(OAc)₂(CO)₂ provided highly efficient H₂ release while avoiding the accumulation of borazine. ❧ While hydrogen release from AB was optimized, the issue of fuel release was also of interest. After an AB dehydrogenation cycle, the most environmentally friendly solution to would be to reuse the spent fuel generated by the reaction. A means to regenerate spent fuel was to use formic acid and a transition metal catalyst to digest the B-N byproducts produced at the end of AB dehydrogenation into formates. Therefore, the dehydrogenation of ammonium boroformate by various catalysts was studied by ¹H NMR. ❧ In the development of other green, catalytic reactions, acceptorless dehydrogenations (AD) and its dehydrative coupling reactions were studied. An environmentally friendly reaction involving hydrogen transfer catalyzed by a pyridylphosphine-supported ruthenium complex has rapidly developed into a powerful method for coupling of alcohols and amines to alkylated amines. Alkylation of amines is of great importance in synthetic routes because they are often intermediates in a wide range of useful compounds in the pharmaceutical and agriculture industries. Moreover, this reaction is atom economical and environmentally friendly because it can be accomplished in the absence of solvent and produces only water as a byproduct.
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Zhang, Xingyue
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Ruthenium catalysis for ammonia borane dehydrogenation and dehydrative coupling
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Chemistry
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07/05/2018
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