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University of Southern California Dissertations and Theses
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On the pursuit of pushing the limits: from the isolation of highly reactive intermediates to the synthesis of energetic materials
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On the pursuit of pushing the limits: from the isolation of highly reactive intermediates to the synthesis of energetic materials
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Content
ON THE PURSUIT OF PUSHING THE LIMITS:
FROM THE ISOLATION OF HIGHLY REACTIVE INTERMEDIATES TO THE
SYNTHESIS OF ENERGETIC MATERIALS
by
Thomas Helmut Saal
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 2022
Copyright 2022 Thomas Helmut Saal
Dedication
To my family
ii
Acknowledgments
Thank You!
iii
ListofAcademicTeachers
Prof. Dr. Roland Beckmann, Prof. Dr. Thomas Bein, Prof. Dr. Hans-Christian Böttcher, Prof. Dr.
Jeroen Buters, Prof. Dr. Thomas Carell, Prof. Dr. Patrick Cramer, Prof. Dr. Hubert Ebert, Dr.
Moritz Ehrl, Dr.Heidi Feldmann, Prof. Dr. Klaus Förstemann, Prof. Dr. Achim Hartschuh, Dr.
Constantin Hoch, Prof. Dr. Anja Hoffmann-Röder, Prof. Dr. Dirk Johrendt, Prof. Dr. Konstantin
Karaghiosoff, Dr. Bernhard Kempf, Prof. Dr. Thomas M. Klapötke, Prof. Dr. Peter Klüfers, Prof.
Dr. Paul Knochel, Prof. Dr. Andreas Kornath, Dr. Burkhard Krumm, Prof. Dr. Don C. Lamb, Prof.
Dr. Heinz Langhals, Prof. Dr. Harald Lesch, Prof. Dr. Tim Liedl, Prof. Dr. Bettina Lotsch, PD Dr.
Albert Lötz, PD Dr. Dietmar Martin, Prof. Dr. Herbert Mayr, Prof. Dr. Jens Michaelis, Prof. Dr.
Christian Ochsenfeld, PD Dr. Armin Ofial, Prof. Dr. Manfred Ogris, Dr. Martina Rüffer, Prof. Dr.
Christina Scheu, Prof. Dr. Wolfgang Schmahl, Prof. Dr. Wolfgang Schnick, Dr. Werner Spahl, Dr.
David Stephenson, Dr. Jörg Stierstorfer, Prof. Dr. Stefan Wuttke, Prof. Dr. Hendrik Zipse. Dr.
Karl O. Christe, Prof. Dr. Ralf Haiges, Prof. Dr. Smaranda Marinescu, Dr. Sri R. Narayan, Prof.
Dr. G. K. Surya Prakash, Prof. Dr. Travis Williams.
iv
Abstract
The research described here has been published in peer-reviewed journals. Each chapter in this
dissertation corresponds to a single published article or manuscript, with experimental details
provided in the”ExperimentalPart” section that follows each chapter. The content of these papers
or manuscripts is given herein with minor changes to fulfill the University of Southern California
Graduate School’s formatting guidelines. This dissertation summarizes my journey of pushing
the limits of chemical research by the isolation of highly reactive intermediates and the synthesis
of novel energetic materials.
Chapter 1 published in Dalton Transactions describes the synthesis and structural characteri-
zation of the Lewis adducts of hydrogen cyanide, butyronitrile, cyclopropanecarbonitrile, pival-
onitrile and benzonitrile with arsenic pentafluoride and antimony pentafluoride. The 2
nd
chap-
ter (published in Chemistry – A European Journal) reports convenient access to the thermally
unstable, primary perfluoro alcohols, CF
3
OH, C
2
F
5
OH, and n-C
3
F
7
OH and their oxonium salts.
Chapter 3 deals with the protonation site in nitramide, the parent molecule of all nitramine ex-
plosives and has been published in Angewandte Chemie. The protonation of azidomethane and
azidotrifluoromethane yielding highly reactive aminodiazonium ions has been reported in Ange-
wandte Chemie and is discussed in Chapter 4. Chapter 5 describes the synthesis and characteri-
zation of novel azido N–donor adducts of the group 4 tetraazide [(bpy)Ti(N
3
)
4
], [(phen)Ti(N
3
)
4
],
v
[(bpy)
2
Zr(N
3
)
4
]
2
· bpy, and [(bpy)
2
Hf(N
3
)
4
]
2
· bpy which was published in the European Journal of
Inorganic Chemistry.
vi
TableofContents
Dedication ii
Acknowledgments iii
ListofAcademicTeachers iv
Abstract v
ListofTables x
ListofFigures xii
Chapter 1: Lewis Adduct Formation of Hydrogen Cyanide and Nitriles With Ar-
senicandAntimonyPentafluoride 1
1.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3.1 Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3.2 X-ray Crystallography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3.3 Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
1.5 Experimental Part . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
1.5.1 Materials and Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
1.5.2 Crystal Structure Determinations . . . . . . . . . . . . . . . . . . . . . . . 21
1.5.3 Experimental Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Chapter 2: α -Fluoroalcohols: Synthesis and Characterization of Perfluorinated
Methanol,Ethanolandn-Propanol,andtheirOxoniumSalts 36
2.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
2.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
2.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
2.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
2.5 Experimental Part . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
2.5.1 Materials and Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
vii
2.5.2 Computational Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
2.5.3 Experimental Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
2.5.3.1 Kinetic NMR Experiments . . . . . . . . . . . . . . . . . . . . . 55
2.5.3.2 Molecular Characterization NMR Experiments . . . . . . . . . . 55
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
Chapter3: ProtonationofNitramines: WhereDoestheProtonGo? 63
3.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
3.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
3.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
3.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
3.5 Experimental Part . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
3.5.1 Materials and Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
3.5.2 Crystal Structure Determinations . . . . . . . . . . . . . . . . . . . . . . . 77
3.5.3 Computational Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
3.5.4 Experimental Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
3.5.4.1 Preparation of [NH
2
NO
2
H][AsF
6
] . . . . . . . . . . . . . . . . . 78
3.5.4.2 Preparation of [NH
3
NO
2
][SbF
6
] . . . . . . . . . . . . . . . . . . 79
3.5.4.3 Preparation of [MeHNNO
2
H][AsF
6
] . . . . . . . . . . . . . . . . 79
3.5.4.4 Preparation of [MeNHNO
2
H][SbF
6
] . . . . . . . . . . . . . . . . 80
3.5.4.5 Preparation of [Me
2
NNO
2
H][AsF
6
] . . . . . . . . . . . . . . . . 81
3.5.4.6 Preparation of [Me
2
NNO
2
H][SbF
6
] . . . . . . . . . . . . . . . . 82
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
Chapter4: ProtonationofCH
3
N
3
andCF
3
N
3
inSuperacids: IsolationandStructural
CharacterizationofLong-LivedMethyl-andTrifluoromethylaminoDi-
azoniumIons 89
4.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
4.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
4.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
4.3.1 Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
4.3.2 X-ray Crystallography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
4.3.3 NMR Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
4.3.4 Raman Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
4.3.5 Electrophilic Aminations of Aromatics . . . . . . . . . . . . . . . . . . . . 108
4.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
4.5 Experimental Part . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
4.5.1 Materials and Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
4.5.2 Crystal Structure Determination . . . . . . . . . . . . . . . . . . . . . . . 112
4.5.3 Computational Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
4.5.4 Experimental Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
4.5.4.1 Preparation of RN
3
· AsF
5
(R = CH
3
, CF
3
) . . . . . . . . . . . . . . 114
4.5.4.2 Preparation of CH
3
N
3
· SbF
5
. . . . . . . . . . . . . . . . . . . . . 115
4.5.4.3 Preparation of [RN(H)N
2
][AsF
6
] (R = CH
3
, CF
3
) . . . . . . . . . 115
4.5.4.4 Preparation of [CH
2
NH
2
][AsF
6
] . . . . . . . . . . . . . . . . . . 117
viii
4.5.4.5 Preparation and protonation of
15
N-enriched CF
3
N
3
. . . . . . . 117
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
Chapter 5: The Binary Group 4 Azide Adducts [(bpy)Ti(N
3
)
4
], [(phen)Ti(N
3
)
4
],
[(bpy)
2
Zr(N
3
)
4
]
2
· bpy,and[(bpy)
2
Hf(N
3
)
4
]
2
· bpy 125
5.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
5.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
5.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
5.3.1 Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
5.3.2 X-ray Crystallography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
5.3.3 Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
5.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
5.5 Experimental Part . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
5.5.1 Materials and Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
5.5.2 Crystal Structure Determination . . . . . . . . . . . . . . . . . . . . . . . 138
5.5.3 Experimental Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
5.5.3.1 Preparation of [(bipy)Ti(N
3
)
4
] . . . . . . . . . . . . . . . . . . . 138
5.5.3.2 Preparation of [(phen)Ti(N
3
)
4
] . . . . . . . . . . . . . . . . . . . 139
5.5.3.3 Preparation of [(bpy)
2
Zr(N
3
)
4
]
2
· bpy . . . . . . . . . . . . . . . . 140
5.5.3.4 Preparation of [(bpy)
2
Hf(N
3
)
4
]
2
· bpy . . . . . . . . . . . . . . . . 141
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
ix
ListofTables
1.1 Crystallographic and structure determination details for the adductsHCN· MF
5
,
C
3
H
7
CN· MF
5
,c-C
3
H
5
CN· AsF
5
, andC
6
H
5
CN· AsF
5
(M = As, Sb) . . . . . . . 8
1.2 Crystallographic and structure determination details for the adducts
(CH
3
)
3
CCN· MF
5
, CH
2
(CN)
2
· MF
5
, and CH
2
(CN)
2
· 2AsF
5
(M = As,
Sb) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.3 Summary of selected bond distances and angles for the structurally characterized
adducts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
1.4 Comparison of the CN stretching frequencies of the Lewis adducts, free nitriles
and selected protonated nitriles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.1 Summary of the thermodynamic quantities derived from the van’t Hoff plots for
theR
f
COF+HF⇄ R
f
CF
2
OH (R
f
−− F, CF
3
, CF
3
CF
2
) equilibria . . . . . . . . . 43
2.2 Summary of the
19
F,
13
C, and
1
H NMR chemical shifts and
1
J
C− F
coupling
constants of CF
3
OH and CF
3
OH
2
+
. . . . . . . . . . . . . . . . . . . . . . . . . . . 44
2.3 The
19
F and
13
C NMR shifts and the corresponding coupling constants of
perfluoroethanol, CF
3
CF
2
OH, recorded at –60°C in HF, and its protonated cation.
The CF
3
CF
2
OH
2
+
spectra were recorded at –50°C for a 1:3 molar mixture of
CF
3
COF and SbF
5
in HF
[a]
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
2.4 The
19
F and
13
C NMR shifts and the corresponding coupling constants of
perfluoro- n-propanol, CF
3
CF
2
CF
2
OH, and its protonated cation, CF
3
CF
2
CF
2
OH
2
+
,
recorded in HF solution at− 55 and− 50
◦ C, respectively . . . . . . . . . . . . . . 46
2.5 Enthalpy values,∆ H
g
, and Gibbs free energies,∆ G
g
, associated with gas-phase
acidities (HA−−→ A
–
+ H
+
) of the perfluoroalcohols, calculated at the G3MP2
level, their solvation energies in aqueous solution∆ G
aq
and pK
a
values in H
2
O
at 298 K. Energy values are expressed in kcal mol
–1
. . . . . . . . . . . . . . . . . . 48
x
2.6 Enthalpies of formation ∆ H
f
expressed in kcal mol
–1
of gaseous perfluo-
romethanol, perfluoroethanol, and perfluoro- n-propanol, calculated at the
G3MP2 level at 0 and 298 K . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
2.7 Reaction energies for the reactions in kcal mol
–1
at the G3MP2 level at 298 K,
in which solvent effects, ∆ G
solv
, were calculated at B3LYP/DZVP2/COSMO in
water as the solvent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
2.8 Proton affinities (PA) at 298 K in kcal mol
–1
calculated at the G3MP2 level . . . . . 51
4.1 Geometries from G3MP2 (MP2(FULL)/6-31G(d)), bond lengths in
˚A, angles in
degrees. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
4.2 Proton affinities at G3MP2 level in kcal mol
–1
. . . . . . . . . . . . . . . . . . . . . 98
4.3 Reaction energies in kcal mol
–1
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
4.4 B3LYP/aug-cc-pVDZ optimized geometries; bond lengths in
˚A, angles in degrees. 100
4.5 Experimental NMR Data for
15
N-labeled CF
3
N
3
and the protonated species
[CF
3
N(H)N
2
]
+
.
[a]
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
4.6 Experimental and calculated NMR data for CF
3
N
3
, the adduct CF
3
N
3
· AsF
5
, and
the protonated species [CF
3
N(H)N
2
]
+
.
[a,b]
. . . . . . . . . . . . . . . . . . . . . . . 105
4.7 Experimental and calculated NMR data for CH
3
N
3
, the adduct CH
3
N
3
· AsF
5
, and
the protonated species [CH
3
N(H)N
2
]
+
.
[a,b]
. . . . . . . . . . . . . . . . . . . . . . . 106
xi
ListofFigures
1.1 The crystal structures of (A)HCN· AsF
5
and (B)HCN· SbF
5
. Thermal ellipsoids
are set at 50% probability. Hydrogen atom positions were determined from the
electron density map and are depicted as spheres of arbitrary radius. Selected
bond distances (
˚A) and angles (°): (A) As1–F1 1.702(2), As1–F2 1.706(2), As1–F3
1.712(2), As1–N1 1.983(3), C1–N1 1.136(5), F1–As1–N1 87.83(10), F3–As1–N1
180, As1–N1–C1 180.0(2); (B) Sb1–F1 1.867(2), Sb1–F2 1.862(2), Sb1–N1 2.132(3),
C1–N1 1.125(4), F1–Sb1–N1 178.82(9), F2–Sb1–F4 173.48(8), C1–N1–Sb1 172.7(3). . 5
1.2 Hydrogen bonding in the crystal structures ofHCN· SbF
5
. Thermal ellipsoids
are set at 50% probability. Hydrogen atom positions were determined from the
electron density map and are depicted as spheres of arbitrary radius. Selected
distances (
˚A) and angles (°): C1–F1 2.947(3), C1–F2 2.827(4), C1–F5 2.880(3),
C1–H1–F1 163(3), C1–H1–F2 103(2), C1–H1–F5 110(3) . . . . . . . . . . . . . . . . 7
1.3 The crystal structures of (A)C
3
H
7
CN· AsF
5
and (B)C
3
H
7
CN· SbF
5
. Thermal
ellipsoids are set at 50% probability, hydrogen atoms are omitted for clarity. Both
arrangements of the disordered propyl group are depicted in dark grey and light
grey. Selected bond distances (
˚A) and angles (°): (A) As1–F1 1.693(3), As1–F2
1.698(2), As1–F3 1.708(2), As1–N1 1.971(5), C1–N1 1.134(7), As1–N1–C1 172.5(5);
(B) Sb1–F1 1.860(2), Sb1–F2 1.871(2), Sb1–F3 1.864(2), Sb1–N1 2.111(2), C1–N1
1.136(4), C1–N1–Sb1 172.1(2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.4 The crystal structure of c-C
3
H
5
CN· AsF
5
. Thermal ellipsoids are set at 50%
probability, hydrogen atoms are omitted for clarity. Selected bond distances (
˚A)
and angles (
◦ ): (A) As1–F1 1.702(2), As1–F2 1.705(2), As1–F3 1.709(2), As1–N1
1.962(3), C1–N1 1.140(4), As1–N1–C1 170.6(2), C2–C1–N1 177.7(3) . . . . . . . . . 11
1.5 The crystal structure of C
6
H
5
CN· AsF
5
. Thermal ellipsoids are set at 50%
probability, hydrogen atoms are omitted for clarity. Selected bond distances
(
˚A) and angles (
◦ ): As1–F1 1.702(2), As1–F2 1.705(2), As1–F3 1.709(2), As1–N1
1.962(3), C1–N1 1.140(4), As1–N1–C1 170.6(2), C2–C1–N1 177.7(3) . . . . . . . . . 12
xii
1.6 The crystal structures of (A) (CH
3
)
3
CCN· AsF
5
and (B) (CH
3
)
3
CCN· SbF
5
.
Thermal ellipsoids are set at 50% probability, hydrogen atoms are omitted for
clarity. Selected bond distances (
˚A) and angles (°): (A) As1–F2 1.7115(9), As1–F3
1.704(1), As1–N1 1.969(2), C1–N1 1.139(2), As1?–N1–C1 179.2(2), C2–C1–N1
179.2(2); (B) Sb1–F2 1.867(3), Sb1–F3 1.871(4), Sb1–N1 2.104(6), C1–N1 1.150(8),
Sb1–N1–C1 176.7(6), C2–C1–N1 179.1(7) . . . . . . . . . . . . . . . . . . . . . . . 13
1.7 The crystal structures of (A) CH
2
(CN)
2
· AsF
5
and (B) CH
2
(CN)
2
· SbF
5
. Thermal
ellipsoids are set at 50% probability, hydrogen atoms are omitted for clarity.
Selected bond distances (
˚A) and angles (°): (A) As1–F1 1.702(1), As1–F2 1.699(1),
As1–N1 2.001(2), C1–N1 1.133(2), C1–C2 1.466(2), As1–N1–C1 171.7(1), C2–
C1–N1 178.8(2); (B) Sb1–F1 1.871(3), Sb1–F2 1.861(4), Sb1–N1 2.126(4), C1–N1
1.141(7), Sb1–N1–C1 168.9(4), C2–C1–N1 178.4(5) . . . . . . . . . . . . . . . . . . 14
1.8 The crystal structure of CH
2
(CN)
2
· 2AsF
5
. Thermal ellipsoids are set at 50%
probability, hydrogen atoms are omitted for clarity. Selected bond distances
(
˚A) and angles (°): As1–F1 1.699(2), As1–F5 1.688(2), As2–F6 1.708(2), As2–F10
1.685(2), As1–N1 2.043(2), As2–N2 2.068(2), C1–N1 1.123(4), C3–N2 1.129(4),
As1–N1–C1 177.4(2), C2–C1–N1 177.7(3), As2–N2–C3 177.0(2), C2–C3–N2 177.7(3) 14
2.1 van’t Hoff plots for the :uilibrium R
f
COF + HF⇄ R
f
CF
2
OH (R
f
–
–
F, CF
3
, CF
3
CF
2
).
Equations for the lines are as follows: (R
–
–
F) y
–
–
1798.3 x-11.237, R
2
–
–
0.9913;
(R
–
–
CF
3
) y
–
–
1634.4 x-11.524, R
2
–
–
0.99284; (R
–
–
CF
3
CF
2
) y
–
–
1087.7 x-11.481,
R
2
–
–
0.98735 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.1 Calculated structures (CBS-QB3[31, 32]) of protonated nitramine [H
2
NNO
2
]H
+
.
Relative enthalpies and Gibbs energies are given in kcal mol
–1
in the gas phase
(1 atm, 298 K) and in polar solvent (1 M, 298 K) in parenthesis . . . . . . . . . . . . 66
3.2 Crystal structures of [H
2
N – NO
2
H][AsF
6
] (A) and [H
3
N – NO
2
]
–
[SbF
6
] (B).
Thermal ellipsoids are set at 50% probability. Selected distances [
˚A] and angles
[
◦ ]: A) N1–N2 1.284(2), N1–O1 1.196(2), N1–O2 1.346(3), O2··· F4 2.627(2); N2–N1–
O1 125.7(2), N2–N1–O 112.1(2), O1–N1–O2 122.1(2); B) N1–N2 1.520(2), N1–O1
1.196(2), N1–O2 1.193(2), N2··· F6 2.767(2); N2–N1–O1 112.9(1), N2–N1–O2
114.6(1), O1–N1–O2 132.5(2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
3.3 Crystal structures of [MeHN – NO
2
H][SbF
6
] (A) and [Me
2
NNO
2
H][SbF
6
] (B).
Thermal ellipsoids are set at 50% probability. Some hydrogen atoms have been
omitted for clarity. Selected distances [
˚A] and angles [
◦ ]: A) N1–N2 1.276(7), N2–
O1 1.199(5), N2–O2 1.347(6), O2··· F1 2.627(5), N1–N2–O1 125.6(4), N1–N2–O2
112.1(4), O1–N2–O2 122.3(4); B) N1–N2 1.28(1), N2–O1 1.33(1), N2–O2 1.22(1),
O1··· F1 2.58(1); N1–N2–O1 121.3(7), N1–N2–O2 124.3(8), O1–N2–O2 121.3(7) . . . 72
xiii
4.1 Asymmetric unit in the crystal structure of [CH
3
N(H)N
2
][AsF
6
]. Hydrogen atom
positions were determined from the difference electron density map. Selected
bond distances [
˚A] and angles [°]: C1–N1 1.479(2), N1–N2 1.278(2), N2–N3
1.102(2), N1–F1 2.65(2), N1–F5 2.18(2), C1–N1–N2 118.7(1), N1–N2–N3 175.9(1). . 97
4.2 Asymmetric unit in the crystal structure of [CF
3
N(H)N
2
][AsF
6
]. Hydrogen atom
positions were determined from the difference electron density map. Selected
bond distances [
˚A] and angles [°]: C1–N1 1.478(5), N1–N2 1.276(5), N2–N3
1.103(5), N1–F4 2.657(4), N1–F7 3.091(5), C1–N1–N2 117.6(3), N1–N2–N3 179.3(4). 99
4.3 Asymmetric unit in the crystal structure of CH
3
N
3
· AsF
5
. Hydrogen atom
positions were determined from the difference electron density map. Selected
bond distances [
˚A] and angles [°]: C1–N1 1.48(3), N1–N2 1.24(2), N2–N3 1.11(2),
As1–N1 1.97(2), As1–F1 1.799(5), As1–F2 1.701(5), C1–N1–N2 117(2), N1–N2–N3
177(2), As1–N1–N2 116(1), F1–As1–N1 173.9(6), F1–As1–F2 92.2(3). . . . . . . . . 101
5.1 Crystal structures of (A) the two independent molecules of [(bpy)Ti(N
3
)
4
] and
(B) [(phen)Ti(N
3
)
4
]. Thermal ellipsoids are drawn at the 50% probability level
and hydrogen atoms have been omitted for clarity. Selected bond lengths
[
˚A] and angles [°] (A): Ti1–N1 1.984(1), Ti1–N6 1.912(1), Ti1–N7 1.954(1),
Ti1–N10 2.005(1); Ti2–N17 1.969(2), Ti2–N20 1.935(1), Ti2–N23 1.975(1), Ti2–N26
1.995(1), Ti1–N4–N5 134.7(1) Ti1–N6–N5 163.3(1); (B): Ti1–N1 2.025(1), Ti1–N4
1.943(1), Ti1–N7 1.943(1), Ti1–N10 1.978(1), Ti1–N13 2.213(1), Ti1–N14 2.221(1),
Ti1–N4–N5 138.5(1), Ti1–N7–N8 137.4(1). . . . . . . . . . . . . . . . . . . . . . . . 131
5.2 Crystal structures of [(bpy)
2
M(N
3
)
4
]
2
· bpy, (A) M = Hf and (B) M = Zr. Thermal
ellipsoids are drawn at the 50% probability level and hydrogen atoms have been
omitted for clarity. Selected bond lengths [
˚A] (A): Hf–N1 2.151(3), Hf1–N4
2.160(2), Hf1–N7 2.129(3), Hf1–N10 2.129(3), Hf1–N13 2.490(2), Hf1–N14 2.487(2),
Hf1–N15 2.459(3), Hf1–N16 2.460(2); (B): Zr1–N1 2.180(5), Zr1–N4 2.168(5),
Zr1–N7 2.182(5), Zr1–N10 2.156(4), Zr1–N13 2.488(7), Zr1–N14 2.485(6), Zr1–N15
2.516(5), Zr1–N16 2.519(5). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
xiv
Chapter1
LewisAdductFormationofHydrogenCyanideandNitriles
WithArsenicandAntimonyPentafluoride
Thischapterisbasedonthefollowingpublication:
Thomas Saal, Karl O. Christe, Ralf Haiges "Lewis adduct formation of hydrogen cyanide
and nitriles with arsenic and antimony pentafluoride", Dalton Trans. 2019, 48 99-106.
10.1039/C8DT03970D.
1
1.1 Abstract
The reactions of hydrogen cyanide, butyronitrile, cyclopropanecarbonitrile, pivalonitrile and ben-
zonitrile with arsenic pentafluoride and antimony pentafluoride result in the formation of 1:1
Lewis adducts, while malononitrile yielded both 1:1 and 1:2 Lewis adducts. All adducts were iso-
lated and characterized by multinuclear NMR and vibrational spectroscopy, and in most cases by
their X-ray crystal structures.
2
1.2 Introduction
The ability of cyanide to form deeply blue coloured coordination compounds with iron (e.g. Prus-
sian blue or Turnbulls blue) originated the terms cyan and cyanogen for CN and (CN)
2
, as well
as cyanide for the CN
–
anion: kyaneos (Greek) for steel blue and cyanus (Latin) for cornflower
blue.[1] Cyanide ligands are omnipresent in the coordination chemistry of d-[2] and f-block[3, 4]
elements.[5] Adducts ofCH
3
CN with Lewis acids likeBCl
3
,[6, 7]BF
3
,[8]AlCl
3
,[6, 7]AlBr
3
,[6,
7] AsF
5
,[9] SbF
5
,[9] SbCl
5
,[6, 7, 10] TiCl
4
,[6, 7] MoF
5
[11] and WF
5
[11] are known and have
been well characterized particularly by vibrational spectroscopy and X-ray studies. However,
the known coordination chemistry of hydrogen cyanide, the nitrile of formic acid, is limited to
mostly Lewis adducts with main group element (B, Sn, Sb)[12–17] or transition metal (Ti, V,
Nb)[14, 18–21] halides.[22] Recently, the coordination chemistry of HCN was extended to the
uranium halides,β -UF
5
and UCl
4
.[23, 24]
In HCN and organic nitriles, the cyano groups are highly nucleophilic. Therefore, these ni-
triles show reactivity even towards very weak Lewis acids, such as dihalogens and interhalo-
gens.[25, 26] Various Lewis adducts of acetonitrile with metal halides have been reported.[27]
Many organic reactions involving nitriles require either Lewis acids (adduct formation) or
Bronsted acids (protonation) to ease a nucleophilic attack on the carbon atom of the nitrile.[27]
The donor abilities of HCN, (CN)
2
, CCl
2
(CN)
2
, CH
2
(CN)
2
towards AsF
5
and SbF
5
have
been investigated previously.[15] The adducts HCN· AsF
5
, HCN· SbF
5
, (CN)
2
· AsF
5
, (CN)
2
· SbF
5
,
CH
2
(CN)
2
· AsF
5
, CH
2
(CN)
2
· 2AsF
5
, and CCl
2
(CN)
2
· AsF
5
had been characterized only by multin-
uclear NMR and vibrational spectroscopy. The only structurally characterized Lewis adduct of a
pnictogen pentafluoride with a nitrile was NCCN · SbF
5
.[15] Very recently, a highly labile adduct
3
of B(C
6
F
5
)
3
with a hydrogen cyanide dimer was characterized.[28] Herein, we report the Lewis
adducts of hydrogen cyanide, primary, secondary and tertiary alkyl nitriles, an aryl nitrile, and
an alkyl dinitrile with AsF
5
and SbF
5
.
1.3 ResultsandDiscussion
1.3.1 Synthesis
The Lewis adducts RCN· MF
5
(R = H, C
3
H
7
, CH
2
CN, c-C
3
H
5
, C(CH
3
)
3
, C
6
H
5
; M = As, Sb) were ob-
tained in quantitative yield through the reaction of stoichiometric amounts of hydrogen cyanide
or the corresponding nitrile with arsenic or antimony pentafluoride, respectively, in sulphur diox-
ide solution according to Scheme 1.1.
R− C
−−− N+MF
5
SO2
− −− →
–64
◦ C
R− C
−−− N· MF
5
NC
⧸⧹ CN+2MF
5
SO2
− −− →
–64
◦ C
MF
5
· NC
⧸⧹ CN· MF
5
R = H, C
3
H
7
, CH
2
CN, cyclo – C
3
H
5
, C(CH
3
)
3
, C
6
H
5
,; M
–
–
As, Sb
Scheme 1.1: Preparation of Lewis adducts of pnictogen pentafluorides with hydrogen cyanide,
alkyl nitriles and benzonitriles
For the preparation of the AsF
5
adducts, SO
2
and a stoichiometric amount of arsenic pentaflu-
oride were condensed in vacuo onto a frozen sample of hydrogen cyanide or the corresponding
nitrile at− 196°C. The mixture was warmed to− 64°C. TheRCN· AsF
5
adducts were obtained
as colourless solids after slow removal of the SO
2
solvent at− 64°C in a dynamic vacuum. For
the preparation of the SbF
5
adducts, hydrogen cyanide or the corresponding nitrile were added
to a frozen solution of a stoichiometric amount of antimony pentafluoride in SO
2
. The mixture
4
was warmed to− 64°C, and theRCN· SbF
5
adducts were obtained as colourless solids when the
SO
2
solvent was removed in a dynamic vacuum. The AsF
5
and SbF
5
Lewis adducts were obtained
as colourless solids that are very moisture-sensitive. Both types of adducts slowly decompose
when allowed to warm to ambient temperature as evidenced by a brown decolourization of the
solids within 30 minutes at 25°C. Single crystals suitable for X-ray structure determination were
grown from SO
2
solutions by slow evaporation of the solvent in a dynamic vacuum at− 45°C to
− 64°C. The resulting adducts were isolated and characterized by multinuclear NMR and vibra-
tional spectroscopy, and in most cases by their X-ray crystal structures.
1.3.2 X-rayCrystallography
The details of the crystallographic data collection and refinement parameters for the structurally
characterized Lewis adducts are given in Tables 1.1 and 1.2. Further crystallographic details and
as packing diagrams are given in the ESI.
†
.
Figure 1.1: The crystal structures of (A) HCN· AsF
5
and (B) HCN· SbF
5
. Thermal ellipsoids
are set at 50% probability. Hydrogen atom positions were determined from the electron density
map and are depicted as spheres of arbitrary radius. Selected bond distances (
˚A) and angles (°):
(A) As1–F1 1.702(2), As1–F2 1.706(2), As1–F3 1.712(2), As1–N1 1.983(3), C1–N1 1.136(5), F1–As1–
N1 87.83(10), F3–As1–N1 180, As1–N1–C1 180.0(2); (B) Sb1–F1 1.867(2), Sb1–F2 1.862(2), Sb1–N1
2.132(3), C1–N1 1.125(4), F1–Sb1–N1 178.82(9), F2–Sb1–F4 173.48(8), C1–N1–Sb1 172.7(3).
5
The adduct,HCN· AsF
5
, crystallizes in the tetragonal space groupP4
2
2
1
2 with four formula
units per unit cell (V = 497.9(3)
˚A
3
). The asymmetric unit of the structure consists of a seesaw-
shapedF
3
AsNCH fragment. The remaining atoms of the fullHCN· AsF
5
fragment (Figure 1.1A)
are generated through the symmetry operation1− y,1− x,1
1
2
− z of the space group. The cor-
respondingHCN· SbF
5
adduct (Figure 1.1B) crystallizes in a monoclinic cell (V = 516.74(19)
˚A
3
)
of space group P2
1
/c with four formula units per unit cell. The C
–
–
–
N distances of 1.136(5)
˚A
and 1.125(4)
˚A in the HCN· AsF
5
and HCN· SbF
5
adducts, respectively, are shorter than the
C
–
–
–
N distance of 1.154
˚A for HCN determined by X-ray diffraction[29] and are comparable to the
C
–
–
–
N distances observed for other HCN Lewis adducts (about 1.12
˚A)[18, 23, 30] or the[DCND]
+
cation (1.127(11)
˚A).[31] The coordination environment of the metal atom in both adducts can be
described as distorted octahedral with one fluorine atom and the HCN nitrogen atom occupying
the axial positions. The remaining four fluorine atoms form the equatorial plane, which is slightly
pushed away from the axial fluorine atom and located on the side of the HCN ligand.
The average M–F distances and F–M–F angles are typical values for theMF
5
moiety inMF
5
X
compounds.[15] The Sb–N distance of 2.132(3)
˚A agrees well with the value of 2.213(5)
˚A reported
for the(CN)
2
· SbF
5
adduct.[15] The Sb–N–C angle in theHCN· SbF
5
adduct is found to 172.7(3)°
while the crystal structure of(CN)
2
· SbF
5
exhibits an angle of 180° due to symmetry requirements
of the crystal packing.[15] Both HCN Lewis adducts are associated in the solid state through
intermolecular hydrogen bonding. Primary hydrogen bonds are formed with the C–H group of
the HCN moiety as donor and the axial F atom of the MF
5
fragment as acceptor, resulting in
one-dimensional chains of iterating HCN andMF
5
fragments. The chains are further associated
through additional hydrogen bonds to equatorial F atoms of theMF
5
fragments (Figure 1.2).
6
Figure1.2: Hydrogen bonding in the crystal structures ofHCN· SbF
5
. Thermal ellipsoids are set
at 50% probability. Hydrogen atom positions were determined from the electron density map and
are depicted as spheres of arbitrary radius. Selected distances (
˚A) and angles (°): C1–F1 2.947(3),
C1–F2 2.827(4), C1–F5 2.880(3), C1–H1–F1 163(3), C1–H1–F2 103(2), C1–H1–F5 110(3)
The butyronitrile adductsC
3
H
7
CN· AsF
5
andC
3
H
7
CN· SbF
5
are isostructural. Both com-
pounds crystallize in the orthorhombic space groupPnma with four formula units per unit cell
(Z =4). As expected due to the increased atomic volume of Sb vs. As, the SbF
5
adduct features a
slightly larger unit cell with a volume of 836.6(5)
˚A
3
compared to 783.7(17)
˚A
3
for the AsF
5
adduct.
7
Table1.1: Crystallographic and structure determination details for the adductsHCN· MF
5
,C
3
H
7
CN· MF
5
,c-C
3
H
5
CN· AsF
5
, and
C
6
H
5
CN· AsF
5
(M = As, Sb)
HCN· AsF
5
HCN· SbF
5
C
3
H
7
CN· AsF
5
C
3
H
7
CN· SbF
5
c-C
3
H
5
CN· AsF
5
C
6
H
5
CN· AsF
5
Empirical formula CHAsF
5
N CHF
5
NSb C
4
H
7
AsF
5
N C
4
H
7
F
5
NSb C
4
H
5
AsF
5
N C
7
H
5
AsF
5
N
Formula weight 196.95 243.78 239.03 285.86 237.01 273.04
Temperature/K 100(2) 100(2) 100(2) 100(2) 100(2) 100(2)
Crystal system Tetragonal Monoclinic Orthorhombic Orthorhombic Monoclinic Monoclinic
Space group P4
1
2
1
2 P2
1
/c Pnma Pnma P2
1
/m C2/c
a/
˚A 5.4973(15) 6.2721(13) 18.521(4) 18.894(7) 5.227(5) 11.705(5)
b/
˚A 5.4973(15) 8.1472(17) 8.217(2) 8.416(3) 8.481(7) 12.542(6)
c/
˚A 16.475(5) 10.157(2) 5.1493(13) 5.261(2) 8.215(7) 8.136(4)
α /° 90 90 90 90 90 90
β /° 90 95.375(4) 90 90 100.370(12) 131.518(7)
γ /° 90 90 90 90 90 90
Volume/
˚A
3
497.9(3) 516.74(19) 783.7(3) 836.6(5) 358.2(5) 894.3(7)
Z 4 4 4 4 2 4
ρ calc
/gcm
− 3
2.628 3.134 2.026 2.270 2.197 2.028
µ /mm
− 1
6.836 5.348 4.363 3.322 4.772 3.838
F(000) 368 440 464 536 228 528
Reflections collected 12048 1543 11247 17084 8928 9247
Independent reflections 762 1543 964 1101 1148 1120
R
int
0.0400 0.0316 0.1164 0.0521 0.0577 0.0598
Data/restraints/parameters 762/1/41 1543/0/76 964/0/71 1101/0/71 1148/0/58 1120/0/67
Goodness-of-fit on F
2
1.195 1.109 1.137 1.058 1.096 1.072
Flack x/BASF parameter 0.49(7) − − − − − R
1
[I≤ 2σ (I)] 0.0246 0.0224 0.0506 0.0167 0.0249 0.0341
wR
2
[I≤ 2σ (I)] 0.0552 0.0571 0.0702 0.0335 0.0478 0.0877
8
Table1.2: Crystallographic and structure determination details for the adducts(CH
3
)
3
CCN· MF
5
,CH
2
(CN)
2
· MF
5
, andCH
2
(CN)
2
· 2AsF
5
(M = As, Sb)
(CH
3
)
3
CCN· AsF
5
(CH
3
)
3
CCN· SbF
5
CH
2
(CN)
2
· AsF
5
CH
2
(CN)
2
· SbF
5
CH
2
(CN)
2
· 2AsF
5
Empirical formula C
5
H
9
AsF
5
N C
5
H
9
F
5
NSb C
3
H
2
AsF
5
N
2
C
3
H
2
F
5
N
2
Sb C
3
H
2
As
2
F
10
N
2
Formula weight 258.05 299.88 235.99 282.82 405.91
Temperature/K 100(2) 100(2) 100(2) 100(2) 100(2)
Crystal system Monoclinic Monoclinic Monoclinic Monoclinic Orthorhombic
Space group P2
1
/m P2
1
/m P2
1
/n P2
1
/n P2
1
2
1
2
1
a/
˚A 5.9896(16) 5.970(3) 5.3379(12) 5.3862(16) 7.5930(13)
b/
˚A 8.660(2) 8.897(5) 15.257(3) 15.562(5) 10.1881(18)
c/
˚A 8.905(2) 9.201(5) 8.3529(18) 8.490(3) 12.803(2)
α /° 90 90 90 90 90
β /° 97.932(4) 97.380(5) 104.096(3) 102.140(5) 90
γ /° 90 90 90 90 90
Volume/
˚A
3
457.5(2) 484.7(4) 659.8(3) 695.7(4) 990.4(3)
Z 2 2 4 4 4
ρ calc
/gcm
− 3
1.837 2.055 2.376 2.700 2.722
µ /mm
− 1
3.742 2.872 5.185 3.997 6.878
F(000) 248 284 448 520 760
Reflections collected 9013 1532 15124 15914 24363
Independent reflections 1482 1532 2017 2112 3002
R
int
0.0442 0.0555 0.0275 0.0647 0.0457
Data/restraints/parameters 1482/0/69 1532/0/70 2017/0/100 2112/0/100 3002/0/155
Goodness-of-fit on F
2
1.064 1.067 1.098 1.107 1.056
Flack x/BASF parameter − − − − 0.438(10)
R
1
[I≤ 2σ (I)] 0.0184 0.0398 0.0186 0.0402 0.0206
wR
2
[I≤ 2σ (I)] 0.0444 0.0715 0.0411 0.0942 0.0367
9
The propyl chain in both adducts shows a 1:1 positional disorder related to the symmetry
operationx,
1
2
− y,z (Figure 1.3). The observed M–N distances and M–N–C angles of 1.971(5)
˚A
and 172.6(5)° for the AsF
5
adduct and 2.114(3)
˚A and 173.6(8)° for the SbF
5
adduct agree well with
the ones found in the HCN adducts. The C
–
–
–
N distances in the butyronitrile adducts of 1.134(7)
˚A
(AsF
5
) and 1.136(4)
˚A (SbF
5
) do not deviate from the average observed C
–
–
–
N bond distance for
alkyl nitriles (1.136(10)
˚A).[32] This is in contrast to protonated nitrile species,[RCNH]
+
. It was
observed that the protonation of alkyl nitriles results in a significant shortening of the C
–
–
–
N triple
bond with bond distances of 1.115(5) and 1.183(3)
˚A for primaryN-alkyl nitrilium ions.[33] With
observed angles of 180(2)° (AsF
5
) and 179.7(6)° (SbF
5
), the C–C–N skeleton on the other hand,
agree well with the ones found in theN–alkyl nitrilium ions (179.4(8)°).[33]
c-C
3
H
5
CN· AsF
5
, the Lewis adduct of the cyclic nitrile, cyclo–propanecarbonitrile, with arsenic
pentafluoride crystallizes in the monoclinic space group P2
1
/m with two formula units (Z =
2) per unit cell (Figure 1.4). The asymmetric unit of the structure consists of F
3
AsNCCHCH
2
.
The remaining atoms of the entire adduct are generated (x,
1
2
− y, z) through a mirror plane
Figure 1.3: The crystal structures of (A) C
3
H
7
CN· AsF
5
and (B) C
3
H
7
CN· SbF
5
. Thermal
ellipsoids are set at 50% probability, hydrogen atoms are omitted for clarity. Both arrangements
of the disordered propyl group are depicted in dark grey and light grey. Selected bond distances
(
˚A) and angles (°): (A) As1–F1 1.693(3), As1–F2 1.698(2), As1–F3 1.708(2), As1–N1 1.971(5), C1–
N1 1.134(7), As1–N1–C1 172.5(5); (B) Sb1–F1 1.860(2), Sb1–F2 1.871(2), Sb1–F3 1.864(2), Sb1–N1
2.111(2), C1–N1 1.136(4), C1–N1–Sb1 172.1(2)
10
Figure 1.4: The crystal structure of c-C
3
H
5
CN· AsF
5
. Thermal ellipsoids are set at 50% prob-
ability, hydrogen atoms are omitted for clarity. Selected bond distances (
˚A) and angles (
◦ ): (A)
As1–F1 1.702(2), As1–F2 1.705(2), As1–F3 1.709(2), As1–N1 1.962(3), C1–N1 1.140(4), As1–N1–C1
170.6(2), C2–C1–N1 177.7(3)
perpendicular to [010] which bisects the 3-membered ring and the AsF
5
fragment. The observed
As – N distance of 1.962(3)
˚A agrees well with the value found for the butyronitrile adduct. With
an angle of 177.7(3)°, the C–C–N skeleton in the cyclo–propanecarbonitrile fragment is notably
less bent than those in both butyronitrile adducts. The observed C
–
–
–
N distance of 1.140(4)
˚A is
typical for cyanocyclopropanes but the C1–C2 distance of 1.422(4)
˚A is significantly shorter than
observed for similar structural motifs such as 1-phenyl-2-cyanocyclopropane (1.447(4)
˚A)[34] or
trans-1,2,3-tricyanocyclopropane (1.446(5)
˚A).[35]
The arylnitrile adductC
6
H
5
CN· AsF
5
(Figure 1.5) crystallizes with four formula units in the
unit cell (Z = 4) of the monoclinic space group C2/c. The asymmetric unit of the structure
consists of F
3
AsNCC(CH)
3
. The remaining atoms of the entire adduct are generated through
a two-fold rotation axis (− x,y,
1
2
− z) located along As1–N1–C1. The observedAs− N distance
of 1.987(3)
˚A is slightly longer than the ones found for the alkyl nitrile AsF
5
adducts. The C
–
–
–
N
distance of 1.148(4)
˚A is longer than the one in the phenylnitrilium cation (1.132(6)
˚A)[33] as
well as in benzonitrile (1.139(6)
˚A)[36] but is in good agreement with coordination complexes
11
Figure1.5: The crystal structure ofC
6
H
5
CN· AsF
5
. Thermal ellipsoids are set at 50% probabil-
ity, hydrogen atoms are omitted for clarity. Selected bond distances (
˚A) and angles (
◦ ): As1–F1
1.702(2), As1–F2 1.705(2), As1–F3 1.709(2), As1–N1 1.962(3), C1–N1 1.140(4), As1–N1–C1 170.6(2),
C2–C1–N1 177.7(3)
of benzonitrile with metals e.g. [(C
6
H
5
CN)
4
Cu][PF
6
] (1.141(3)
˚A–1.144(3)
˚A).[37] The C–C–N
skeleton is linear due to the presence of the two-fold rotation axis.
The pivalonitrile adducts (CH
3
)
3
CCN· MF
5
(M = As, Sb) are again isostructural and crys-
tallize in the unit cell of the monoclinic space group P2
1
/m (Z = 2) (Figure 1.6). The unit cell
of the antimony adduct has again a slightly larger volume than the one of the arsenic adduct
(457.5(2)
˚A
3
and 484.7(4)
˚A
3
). The asymmetric unit consists ofF
4
MNCC(CH
3
)
2
. The remaining
atoms of the entire adduct are generated (x,
1
2
− y,z) through a mirror plane perpendicular to
[010] which bisects the(CH
3
)
3
C and theMF
5
fragments. The M–N distances of 1.969(2)
˚A and
2.104(6)
˚A for the AsF
5
and SbF
5
adduct, respectively, agree well with the values observed for
the other nitrile adducts. The observed C
–
–
–
N distances of 1.139(2)
˚A (AsF
5
) and 1.150(8)
˚A (SbF
5
)
do not deviate significantly for the AsF
5
and SbF
5
adducts and are in good agreement with the
value of 1.135(8)
˚A determined for the pivalonitrileNbCl
5
adduct.[38] The C–C–N skeleton in the
(CH
3
)
3
CCN fragment is virtually linear in bothMF
5
adducts with C2–C1–N1 angles of 179.2(2)°
(AsF
5
) and 179.1(7)° (SbF
5
).
12
Figure 1.6: The crystal structures of (A) (CH
3
)
3
CCN· AsF
5
and (B) (CH
3
)
3
CCN· SbF
5
. Thermal
ellipsoids are set at 50% probability, hydrogen atoms are omitted for clarity. Selected bond dis-
tances (
˚A) and angles (°): (A) As1–F2 1.7115(9), As1–F3 1.704(1), As1–N1 1.969(2), C1–N1 1.139(2),
As1?–N1–C1 179.2(2), C2–C1–N1 179.2(2); (B) Sb1–F2 1.867(3), Sb1–F3 1.871(4), Sb1–N1 2.104(6),
C1–N1 1.150(8), Sb1–N1–C1 176.7(6), C2–C1–N1 179.1(7)
The Lewis adducts of malononitrile with one equivalent of AsF
5
and SbF
5
crystallize in the
monoclinic space group P2
1
/n (Z = 4) and are isostructural (Figure 1.7). With a volume of
695.7(4)
˚A
3
, the unit cell of the antimony adduct is slightly larger than the one of the arsenic
adduct (V = 659.8(3)
˚A
3
).
The M–N distances for the AsF
5
and SbF
5
adducts are found as 2.001(2)
˚A and 2.126(5)
˚A,
respectively, and are slightly larger than the distances found in the adducts of butyronitrile, cyclo–
propanecarbonitrile, pivalonitrile and benzonitrile. The C
–
–
–
N distances of 1.133(2)
˚A (AsF
5
) and
1.141(7)
˚A (SbF
5
) for the coordinated cyano group are essentially identical to the ones for the non-
coordinated cyano group of 1.141(2)
˚A (AsF
5
adduct) and 1.148(7)
˚A (SbF
5
adduct). This agrees
well with the observed structural features for the Lewis adducts of the other nitriles of this work.
The C–C–N angles of 178.8(2)° (AsF
5
) and 178.4(5)° (SbF
5
) are slightly larger than the one found
in free malononitrile (177.5°).[39, 40]
The adduct CH
2
(CN)
2
· 2AsF
5
crystallizes in the orthorhombic space group P2
1
2
1
2
1
with
four formula units per unit cell (Z = 4) (Figure 1.8). The observed As–N distances of 2.068(2)
˚A
13
Figure1.7: The crystal structures of (A) CH
2
(CN)
2
· AsF
5
and (B) CH
2
(CN)
2
· SbF
5
. Thermal ellip-
soids are set at 50% probability, hydrogen atoms are omitted for clarity. Selected bond distances
(
˚A) and angles (°): (A) As1–F1 1.702(1), As1–F2 1.699(1), As1–N1 2.001(2), C1–N1 1.133(2), C1–C2
1.466(2), As1–N1–C1 171.7(1), C2–C1–N1 178.8(2); (B) Sb1–F1 1.871(3), Sb1–F2 1.861(4), Sb1–N1
2.126(4), C1–N1 1.141(7), Sb1–N1–C1 168.9(4), C2–C1–N1 178.4(5)
and 2.042(2)
˚A are significantly longer than the one in the corresponding mono-AsF
5
adduct
CH
2
(CN)
2
· AsF
5
. Both C
–
–
–
N distances of 1.123(4)
˚A and 1.129(4)
˚A are shorter than the C
–
–
–
N
distances in the mono-AsF
5
adduct, as well as in free malononitrile (1.142
˚A) but significantly
longer than in the protonated malononitrile,[CH
2
(CNH)
2
]
2+
(1.108(4)
˚A and 1.109(4)
˚A).[41] The
C–C–N angles are 177.6(3)° and 177.7(3)°.
Figure1.8: The crystal structure ofCH
2
(CN)
2
· 2AsF
5
. Thermal ellipsoids are set at 50% proba-
bility, hydrogen atoms are omitted for clarity. Selected bond distances (
˚A) and angles (°): As1–F1
1.699(2), As1–F5 1.688(2), As2–F6 1.708(2), As2–F10 1.685(2), As1–N1 2.043(2), As2–N2 2.068(2),
C1–N1 1.123(4), C3–N2 1.129(4), As1–N1–C1 177.4(2), C2–C1–N1 177.7(3), As2–N2–C3 177.0(2),
C2–C3–N2 177.7(3)
14
To summarize the structural characterization, the observed C
–
–
–
N distances in the Lewis
adducts are in the range of 1.123(4)–1.150(8)
˚A. Within the margin of error, no significant dif-
ference was found between C
–
–
–
N distances of the AsF
5
and SbF
5
adducts. The C
–
–
–
N distances of
the butyronitrile, cyclopropanecarbonitrile and malononitrile mono-MF
5
adducts do not deviate
from the ones in the free nitriles, a significant shortening of the C
–
–
–
N distance upon adduct for-
mation was found forHCN· SbF
5
andCH
2
(CN)
2
· 2AsF
5
. On the other hand, the C
–
–
–
N distance
of C
6
H
5
CN· AsF
5
is slightly longer than the one in free benzonitrile. The M–N distances in
the AsF
5
adducts are in the range 1.962(3)–2.068(3)
˚A while the distances of the SbF
5
adducts are
longer with 2.104(6)–2.132(3)
˚A. The C–C–N angles range from 177.7(3) 180°. Table 1.3 summa-
rizes selected bond distances and angles for the structurally characterized Lewis adducts.
Table 1.3: Summary of selected bond distances and angles for the structurally characterized
adducts
Compound C
–
–
–
N/
˚A M–N/
˚A C–C–N/°
HCN· AsF
5
1.136(5) 1.983(3) –
HCN· SbF
5
1.125(4) 2.132(3) 2.132(3)
C
3
H
7
CN· AsF
5
1.134(7) 1.971(5) 180(2)
C
3
H
7
CN· SbF
5
1.136(4) 2.111(2) 179.7(6)
c-C
3
H
5
CN· AsF
5
1.140(4) 1.962(3) 177.7(3)
(CH
3
)
3
CCN· AsF
5
1.139(2) 1.969(2) 179.2(2)
(CH
3
)
3
CCN· SbF
5
1.150(8) 2.104(6) 179.1(7)
CH
2
(CN)
2
· AsF
5
1.133(2) 2.001(2) 178.8(2)
CH
2
(CN)
2
· SbF
5
1.141(7) 2.126(4) 178.4(5)
CH
2
(CN)
2
· 2AsF
5
1.123(4) 2.042(3) 177.7(3)
1.129(4) 2.068(3) 177.7(3)
C
6
H
5
CN· AsF
5
1.148(4) 1.987(3) 180.0
15
1.3.3 Spectroscopy
The obtained AsF
5
and SbF
5
Lewis adducts were characterized by multinuclear NMR spectra and
low-temperature Raman spectra. Attempts to obtain IR spectra and elemental analyses of the
adducts were unsuccessful due to their moisture sensitivity and only marginal stability at room
temperature. The multinuclear NMR spectra were recorded at room temperature in SO
2
solution.
The obtained spectra and observed chemical shifts are listed in the ESI.
†
The
14
N resonances of the cyano-groups of the Lewis adducts are significantly more shielded
than those in the uncoordinated nitriles. While the free nitriles exhibit relatively sharp
14
N reso-
nances at− 125 ppm to− 140 ppm, the ones of the Lewis adducts are observed as broad signals in
the range ofδ =− 185 ppm to− 215 ppm, with the resonances of the SbF
5
adducts being about
10 ppm upfield from the ones of the AsF
5
adducts. ForCH
2
(CN)
2
· AsF
5
, only one
14
N resonance
could be observed at ambient temperature. When the sample was cooled to –55°C, two distinct
CN resonances were detected (ESI
†
), indicating a rapid exchange of AsF
5
between the two cyano
groups in SO
2
solution at ambient temperature. In the
13
C NMR spectra, the CN-groups of the
Lewis adducts exhibit broadened singlets at about δ = 100 ppm to 125 ppm. The resonances
of the AsF
5
adducts are shifted slightly upfield from the ones of the SbF
5
adducts. For the AsF
5
adducts, singlet resonances at about δ =− 40 ppm were observed, while two signals which in-
tegrate in a four to one ratio at aboutδ = 100 ppm (equatorial F) and− 135 ppm (axial F) where
observed for the SbF
5
adducts.
The Raman spectra of all Lewis adducts were recorded at –90°C. The observed frequencies
and intensities are listed in the ESI.
†
A comparison of the observed CN stretching frequencies of
16
Table 1.4: Comparison of the CN stretching frequencies of the Lewis adducts, free nitriles and
selected protonated nitriles
Compound ν (C
–
–
–
N)/cm
− 1
HCN· AsF
5
2192
HCN· SbF
5
2177
[HCNH][AsF
6
] 2116[31]
HCN 2098
C
3
H
7
CN· AsF
5
2331
C
3
H
7
CN· SbF
5
2317
[C
3
H
7
NH][AsF
6
] 2257[33]
C
3
H
7
CN 2250
c-C
3
H
5
CN· AsF
5
2318
c-C
3
H
5
CN· SbF
5
2307
c-C
3
H
5
CN 2237
(CH
3
)
3
CCN· AsF
5
2318
(CH
3
)
3
CCN· SbF
5
2306
(CH
3
)
3
CCN 2235
CH
2
(CN)
2
· 2AsF
5
2377
CH
2
(CN)
2
· 2SbF
5
2367
[CH
2
(CNH)
2
][SbF
6
]
2
2351/2312[41]
CH
2
(CN)
2
2266
C
6
H
5
CN· AsF
5
2308
C
6
H
5
CN· SbF
5
2282
C
6
H
5
CN 2230
the Lewis adducts with the ones of the free nitriles and selected protonated nitriles is given in Ta-
ble 1.4. Most prominently, the formation of the adducts is accompanied by a significant blue-shift
of the characteristic C
–
–
–
N stretching frequency over the corresponding free cyano compound.
This reflects a strengthened C
–
–
–
N triple bond as theσ ∗ electrons of the CN group are donated to
the Lewis acids resulting in an increased bond order. Surprisingly but in accord with the observed
C
–
–
–
N bond distances and contrary to the Lewis acidities of AsF
5
and SbF
5
, the C
–
–
–
N stretching
frequencies of the adducts relative to the free parent compounds are more strongly shifted for
17
the AsF
5
complexes than for SbF
5
ones. This implies that the smaller arsenic atom interacts more
strongly with the nitrogen atom of the CN group than the larger antimony. Compared to the
protonated species[HCNH]
+
,[31][C
3
H
7
CNH]
+
,[33] and[CH
2
(CNH)
2
]
+
,[41] the hypsochromic
shift relative to the starting material is more pronounced. This contradicts the obtained crystal-
lographic data which show no or only a minimal contraction of the C
–
–
–
N bond distance upon
adduct formation. The frequencies of the MF
5
stretching and deformation modes as well as for
the M-N stretching modes are in good agreement with previous assignments.[15, 42–46]
1.4 Conclusion
In conclusion, a series of Lewis adducts of arsenic- and antimony pentafluoride with hydro-
gen cyanide, butyronitrile, cyclopropanecarbonitrile, pivalonitrile, malononitrile and benzoni-
trile have been prepared in quantitative yield. The compounds were characterized by multinu-
clear NMR and Raman spectroscopy as well as their X-ray crystal structures. The observed C
–
–
–
N
distances in the AsF
5
and SbF
5
Lewis adducts are in the range of1.123(4)− 1.150(8)
˚A. The C
–
–
–
N
distances of the butyronitrile, cyclo-propanecarbonitrile and malononitrile mono-MF
5
adducts do
not deviate from the ones in the free nitriles, a significant shortening of the C
–
–
–
N distance upon
adduct formation was found for HCN· SbF
5
and CH
2
(CN)
2
· 2AsF
5
. On the other hand, the C
–
–
–
N
distance of C
6
H
5
CN· AsF
5
is slightly longer than the one in free benzonitrile. In the vibrational
spectra, the C
–
–
–
N stretching frequencies of the adducts relative to the free parent compounds are
more strongly shifted for the AsF
5
complexes than for SbF
5
ones. This implies that the smaller
arsenic atom interacts more strongly with the nitrogen atom of the CN group than the larger
antimony.
18
Acknowledgements
We thank the Hydrocarbon Research Foundation for financial support and Prof. G. K. S. Prakash,
Drs. W. Wilson, R. Wagner, P. Deokar, and M. Wiesemann for their help and stimulating discus-
sions.
19
1.5 ExperimentalPart
Caution! HCN and AsF
5
are volatile and highly poisonous. AsF
5
and SbF
5
can cause severe burns
and contact with the skin must be avoided. The compounds should be handled only in a well-
ventilated fume hood. Appropriate safety precautions should be taken whenever working with
these materials.
1.5.1 MaterialsandApparatus
All reactions were carried out in either Teflon-FEP ampules or NMR tubes that were closed by
stainless steel valves. Volatile materials were handled in grease-less Pyrex glass or in stainless
steel/Teflon-FEP vacuum line.[47] Reaction vessels and the stainless steel vacuum line were passi-
vated with ClF
3
prior to use. Non-volatile materials were handled in the dry nitrogen atmosphere
of a glove box. Sulfur dioxide (Matheson Tri-Gas) was dried by storage over CaH
2
. AsF
5
was pre-
pared from AsF
3
and F
2
.[48–51] SbF
5
(Ozark Mahoning) was triple distilled before use. HCN was
prepared by reacting stearic acid with KCN at 120°C. Malononitrile (Sigma-Aldrich) was purified
by recrystallization from hot ethanol. Butyronitrile, cyclopropanecarbonitrile, pivalonitrile and
benzonitrile (all Sigma-Aldrich) were used as received. The NMR spectra were recorded at 298 K
unless otherwise stated on Bruker AMX-500 or Varian Mercury 400 or VNMRS-500 spectrome-
ters. Spectra were externally referenced to neat tetramethylsilane for
1
H and
13
C NMR spectra, to
neat nitromethane for
14
N NMR spectra and to 80% CFCl
3
in chloroform-d for
19
F NMR spectra.
Raman spectra were recorded directly in the Teflon reactors in the range 4000− 80 cm
–1
on a
Vertex 70/RAM II spectrophotometer, using a Nd-YAG laser at 1064 nm.
20
1.5.2 CrystalStructureDeterminations
The single-crystal X-ray diffraction data were collected on a Bruker SMART APEX DUO 3-circle
platform diffractometer, equipped with an APEX II CCD, using Mo K α radiation (TRIUMPH
curved-crystal monochromator) from a fine-focus tube. The frames were integrated using the
SAINT algorithm to give the hkl files corrected for Lp/decay.[52] The absorption correction
was performed using the SADABS program.[53] The structures were solved by intrinsic phas-
ing and refined on F2 using the Bruker SHELXTL Software Package and ShelXle.[54–58] All non-
hydrogen atoms were refined anisotropically. ORTEP drawings were prepared using the Mercury
CSD program.[59] Further crystallographic details can be obtained from the Cambridge Crystal-
lographic Data Centre (CCDC, 12 Union Road, Cambridge CB21EZ, UK (Fax: (+44) 1223-336-033;
e-mail: deposit@ccdc.cam.ac.uk) on quoting the deposition no. 1855712-1855722.
1.5.3 ExperimentalProcedures
PreparationofRCN· AsF
5
(R=H,CH
2
CN,C
3
H
7
,cyclo–C
3
H
5
,C(CH
3
)
3
,C
6
H
5
) Anhydrous
SO
2
(2.0 mL) and AsF
5
(1.65 mmol, 1.1 eq.) were condensed into a Teflon-FEP ampule containing
a frozen sample of hydrogen cyanide or the corresponding nitrile (1.5 mmol, 1.0 eq.) at− 196°C.
The mixture was allowed to warm to− 64°C, kept at this temperature for 10 min and sporadically
agitated. The volatile compounds were removed in vacuo at− 64°C, leaving behind a colorless
solid. Single crystals were grown from SO
2
solution by slow evaporation of the solvent in vacuo
at− 64 to− 45°C.
HCN· AsF
5
(293 mg; weight expected for 1.50 mmol: 295 mg).
1
H-NMR (SO
2
, unlocked, 25
◦ C): δ = 7.13 (s, 1H,HCN· AsF
5
) ppm.
21
13
C-NMR (SO
2
, unlocked, 25
◦ C): δ = 100.39 (s, HCN· AsF
5
) ppm.
14
N-NMR (SO
2
, unlocked, 25
◦ C): δ =−186.7 (s,∆ 1
⁄2 = 86 Hz, HCN· AsF
5
) ppm.
19
F-NMR (SO
2
, unlocked, 25
◦ C): δ =−39.5 (s,∆ 1
⁄2 = 4150 Hz, HCN· AsF5) ppm.
Raman (-90 °C, 350 mW): ˜ ν (rel. intensity) = 3199.7 (0.3), 2191.6 (10.0), 2162.7 (0.2), 1145.8 (0.3),
806.4 (0.3), 707.8 (7.8), 674.0 (2.1), 610.6 (1.4), 414.0 (0.7), 389.5 (1.9), 363.6 (0.6), 342.6 (0.4), 294.2
(0.8), 270.6 (2.4), 175.2 (1.4), 141.0 (1.8) cm
–1
.
NCCH
2
CN· AsF
5
(356 mg; weight expected for 1.50 mmol: 354 mg).
1
H-NMR (SO
2
, unlocked, 25
◦ C): δ = 5.64 (s, 2H, CH
2
) ppm.
13
C-NMR (SO
2
, unlocked, 25
◦ C): δ = 104.99 (s,CN), 10.85 (s,CH
2
) ppm.
14
N-NMR (SO
2
, unlocked, 25
◦ C): δ =−151.7 (s,∆ 1
⁄2 = 303 Hz, CN) ppm.
14
N-NMR (SO
2
, unlocked, – 55
◦ C): δ = −121.4 (s, ∆ 1
⁄2 = 668 Hz, CN), −187.4 (s, ∆ 1
⁄2 = 433 Hz,
CN· AsF
5
) ppm.
19
F-NMR (SO
2
, unlocked, 25
◦ C): δ =−45.1 (s,∆ 1
⁄2 = 1403 Hz, AsF5) ppm.
Raman (-90 °C, 350 mW): ˜ ν (rel. intensity) = 2967.4 (2.6), 2918.9 (6.9), 2366.3 (7.5), 2291.2 (2.5),
2282.0 (2.8), 1379.0 (4.6), 1308.5 (1.4), 1206.0 (3.3), 1146.0 (0.4), 990.7 (0.4), 910.3 (1.4), 904.3 (1.2),
716.5 (8.0), 681.3 (10.0), 608.9 (2.3), 422.0 (1.7), 417.7 (1.5), 386.3 (2.2), 363.6 (1.0), 354.7 (3.9), 328.0
(1.1), 312.1 (1.3), 266.4 (1.6), 245.6 (1.0), 140.9 (4.5), 118.2 (2.6), 96.8 (1.3) cm
–1
.
C
3
H
7
CN· AsF
5
(350 mg; weight expected for 1.50 mmol: 359 mg).
1
H-NMR (SO
2
, unlocked, 25
◦ C): δ = 4.28 (t, J = 7.1 Hz, 2H, CH
2
CN), 3.25 (h, J = 7.3 Hz, 2H,
CH
2
CH
3
), 2.43 (t, J = 7.4 Hz, 3H, CH
3
) ppm.
13
C-NMR (SO
2
, unlocked, 25
◦ C): δ = 113.64 (s, CN), 19.38 (s, CH
2
CN), 18.85 (CH
2
), 13.88 (CH
3
)
ppm.
22
14
N-NMR (SO
2
, unlocked, 25
◦ C): δ =−195.0 (s,∆ 1
⁄2 = 74 Hz, CN) ppm.
19
F-NMR (SO
2
, unlocked, 25
◦ C): δ = −37.6 (s, 4F,∆ 1
⁄2 = 255 Hz, AsF
4
F), −80.0 (s, 1F,∆ 1
⁄2 = 410 Hz,
AsF
4
F) ppm.
Raman (-90 °C, 350 mW): ˜ ν (rel. intensity) = 3000.6 (2.1), 2972.9 (2.9), 2964.4 (4.0), 2945.0 (2.6),
2931.5 (4.8), 2916.2 (1.0), 2886.7 (2.0), 2765.5 (0.7), 2331.2 (6.6), 2466.2 (1.2), 1454.1 (2.0), 1412.2
(1.7), 1346.3 (0.6), 1323.3 (2.4), 1258.4 (0.9), 1230.4 (0.8), 1107.7 (0.7), 1082.3 (0.6), 1046.2 (1.7), 934.6
(0.7), 873.1 (0.7), 841.1 (2.5), 718.7 (5.4), 693.8 (0.5), 670.1 (10.0), 599.8 (1.9), 574.6 (1.0), 434.7 (0.9),
400.5 (1.0), 363.8 (0.7), 342.4 (0.7), 326.5 (1.0), 250.3 (0.9), 153.8 (1.2), 114.6 (2.3), 83.9 (2.2) cm
–1
.
c-C
3
H
5
CN· AsF
5
(354 mg; weight expected for 1.50 mmol: 356 mg).
1
H-NMR (SO
2
, unlocked, 25
◦ C): δ = 3.16-3.25 (m, 1H, CH), 2.92-2.86 (m, 2H, CHH), 2.84-2.78 (m,
2H, CHH) ppm.
13
C-NMR (SO
2
, unlocked, 25
◦ C): δ = 115.41 (s,CN), 11.75 (s,CH),−4.30 (CHH) ppm.
14
N-NMR (SO
2
, unlocked, 25
◦ C): δ =−206.3 (s,∆ 1
⁄2 = 80 Hz, CN) ppm.
19
F-NMR (SO
2
, unlocked, 25
◦ C): δ = −37.8 (s, 4F,∆ 1
⁄2 = 260 Hz, AsF
4
F), −78.7 (s, 1F,∆ 1
⁄2 = 400 Hz,
AsF
4
F) ppm.
Raman (-90 °C, 350 mW): ˜ ν (rel. intensity) = 3117.9 (0.6), 3082.4 (0.7), 3065.6 (0.3), 3036.8 (1.7),
2318.1 (10.0), 2286.8 (1.2), 1608.5 (0.3), 1459.0 (2.6), 1436.1 (0.9), 1342.7 (3.9), 1192.4 (4.5), 1149.0
(0.1), 1134.7 (0.4), 1064.0 (1.5), 952.8 (2.6), 851.5 (2.6), 820.8 (0.5), 814.7 (1.8), 773.9 (1.1), 707.3 (7.7),
663.0 (3.6), 602.4 (0.9), 558.2 (1.4), 395.8 (0.5), 360.0 (1.8), 259.9 (0.9), 241.5 (0.2), 162.5 (0.8), 85.8
(0.2) cm
–1
.
(CH
3
)
3
CCN· AsF
5
(367 mg; weight expected for 1.50 mmol: 380 mg).
1
H-NMR (SO
2
, unlocked, 25
◦ C): δ = 2.88 (m, 9H,(C(CH
3
)
3
) ppm.
23
13
C-NMR (SO
2
, unlocked, 25
◦ C): δ = 117.28 (s,CN), 30.44 (s,C(CH
3
)
3
), 27.27 (CH
3
) ppm.
14
N-NMR (SO
2
, unlocked, 25
◦ C): δ =−196.8 (s,∆ 1
⁄2 = 87 Hz, CN) ppm.
19
F-NMR (SO
2
, unlocked, 25
◦ C): δ = −37.6 (s, 4F,∆ 1
⁄2 = 225 Hz, AsF
4
F), −80.2 (s, 1F,∆ 1
⁄2 = 410 Hz,
AsF
4
F) ppm.
Raman (-90 °C, 350 mW): ˜ ν (rel. intensity) = 3010.7 (3.1), 3000.2 (2.8), 2970.1 (1.8), 2946.0 (3.9),
2928.5 (1.7), 2912.7 (1.3), 2880.3 (1.1), 2795.5 (0.5), 2729.0 (0.7), 2403.0 (0.2), 2317.6 (10.0), 1484.8
(0.4), 1467.3 (5.1), 1449.5 (1.1), 1402.0 (0.6), 1206.9 (2.7), 1148.0 (0.2), 1038.7 (1.6), 940.9 (2.5), 865.6
(1.7), 708.4 (9.0), 666.0 (9.0), 607.4 (1.5), 589.8 (0.7), 435.2 (0.4), 382.5 (2.4), 369.6 (0.8), 362.7 (0.7),
352.6 (0.8), 266.7 (2.0), 192.7 (1.6), 157.5 (2.0) cm
–1
.
C
6
H
5
CN· AsF
5
(406 mg; weight expected for 1.50 mmol: 409 mg).
1
H-NMR (SO
2
, unlocked, 25
◦ C): δ = 9.18 (d, J = 8.2 Hz, 2H), 9.07 (t, J = 7.5 Hz, 1H), 8.82 (t, J = 7.8
Hz, 2H) ppm.
13
C-NMR (SO
2
, unlocked, 25
◦ C): δ = 139.40 (s, CH), 136.09 (s, 2 x CH), 131.32 (s 2 x CH), 110.29
(s,CN), 104.37 (s,CCN) ppm.
14
N-NMR (SO
2
, unlocked, 25
◦ C): δ =−185.4 (s,∆ 1
⁄2 = 120 Hz, CN) ppm.
19
F-NMR (SO
2
, unlocked, 25
◦ C): δ = −37.0 (s, 4F,∆ 1
⁄2 = 250 Hz, AsF
4
F), −80.0 (s, 1F,∆ 1
⁄2 = 410 Hz,
AsF
4
F) ppm.
Raman (-90 °C, 350 mW): ˜ ν (rel. intensity) = 3092.9 (1.1), 3081.3 (0.8), 2969.3 (0.1), 2597.4 (0.2),
2413.3 (0.3), 2391.5 (0.3), 2307.6 (10.0), 1597.1 (7.0), 1577.3 (0.4), 1488.3 (0.4), 1453.3 (0.3), 1329.3
(0.3), 1207.9 (3.4), 1186.5 (1.8), 1171.8 (0.7), 1128.0 (0.2), 1030.0 (0.7), 1010.0 (0.4), 1000.6 (6.1), 990.1
(0.3), 968.4 (0.3), 782.3 (0.8), 722.3 (1.0), 711.3 (2.3), 671.0 (2.8), 626.9 (1.1), 603.1 (0.5), 564.9 (0.8),
24
522.1 (0.4), 410.7 (0.4), 382.5 (1.0), 358.0 (1.0), 340.7 (0.4), 294.4 (0.7), 263.8 (0.6), 182.3 (0.6), 147.6
(2.8), 111.9 (4.3), 74.8 (3.9) cm
–1
.
Preparation of RCN· SbF
5
(R = H, CH
2
CN, C
3
H
7
, c-C
3
H
5
,C(CH
3
)
3
, C
6
H
5
) Anhydrous
SO
2
(2.0 mL) and was condensed into a Teflon-FEP ampule containing a frozen sample of SbF
5
(1.5 mmol, 1.0 eq.) at− 196°C. The mixture was allowed to warm to− 64°C forming a clear solu-
tion. A stochiometric amount of hydrogen cyanide was condensed into the ampule at− 196°C.
In case of the nitrile adducts, theSbF
5
/SO
2
mixture at− 64°C was transferred under a stream of
dry into a second Teflon-FEP ampule containing a sample of the corresponding nitrile (1.50 mmol)
at− 78°C. The mixture was allowed to warm to− 64°C, kept at this temperature for 10 min and
sporadically agitated. The volatile compounds were removedinvacuo at− 64°C, leaving behind a
colorless solid. Single crystals were grown from SO
2
solution by slow evaporation of the solvent
in vacuo at− 64 to− 45°C.
HCN· SbF
5
(370 mg; weight expected for 1.50 mmol: 366 mg).
1
H-NMR (SO
2
, unlocked, 25
◦ C): δ = 7.33 (m, 1H,HC) ppm.
13
C-NMR (SO
2
, unlocked, 25
◦ C): δ = 106.88 (s,CN) ppm.
14
N-NMR (SO
2
, unlocked, 25
◦ C): δ =−194.6 (s,∆ 1
⁄2 = 104 Hz, CN· Sb) ppm.
19
F-NMR (SO
2
, unlocked, 25
◦ C): δ =−100.6 (s, 4F,∆ 1
⁄2 = 1190 Hz, SbF
4
F),−133.2 (s, 1F,∆ 1
⁄2 = 1150
Hz, SbF
4
F) ppm.
Raman (-90 °C, 350 mW): ˜ ν (rel. intensity) = 3151.3 (0.2), 2176.7 (10.0), 2148.4 (0.3), 1145.5 (0.3),
686.1 (0.5), 663.2 (9.5), 639.3 (2.6), 600.6 (1.3), 381.4 (0.8), 327.5 (0.7), 294.4 (2.0), 288.9 (1.3), 278.3
(0.7), 260.1 (0.6), 220.6 (1.4), 207.0 (1.5, 154.1 (1.1) cm
–1
.
NCCH
2
CN· SbF
5
(428 mg; weight expected for 1.50 mmol: 424 mg).
25
1
H-NMR (SO
2
, unlocked, 25
◦ C): δ = 5.59 (s, 2H, CH
2
) ppm.
13
C-NMR (SO
2
, unlocked, 25
◦ C): δ = 110.38 (s,CN), 106.75 (s,CN· SbF
5
), 11.23 (s, CH2) ppm.
14
N-NMR (SO
2
, unlocked, 25
◦ C):δ =−123.1 (s,∆ 1
⁄2 = 443 Hz, CN),−192.3 (s,∆ 1
⁄2 = 270Hz, CN· SbF
5
)
ppm.
19
F-NMR (SO
2
, unlocked, 25
◦ C):δ =−100.1 (s, 4F,∆ 1
⁄2 = 564 Hz, SbF
4
F),−133.5 (s, 1F,∆ 1
⁄2 = 503 Hz,
SbF
4
F) ppm.
Raman (-90 °C, 350 mW): ˜ ν (rel. intensity) = 2960.8 (1.9), 2951.3 (0.9), 2919.2 (4.3), 2908.9 (2.1),
2366.9 (2.6), 2357.6 (6.5), 2290.8 (2.0), 1373.8 (3.2), 1360.9 (1.3), 1325.7 (1.1), 1313.3 (1.0), 1204.2
(1.7), 1151.1 (5.9), 909.7 (1.3), 903.2 (0.8), 701.9 (0.9), 679.6 (2.6), 666.7 (10.0), 654.5 (2.9), 648.2 (4.5),
605.2 (1.9), 600.7 (2.3), 528.3 (1.1), 387.0 (2.2), 353.7 (1.9), 290.4 (3.0), 280.5 (2.3), 263.7 (1.7), 247.6
(1.6), 228.5 (1.3), 197.6 (2.0), 139.2 (2.9), 114.2 (2.7), 92.2 (2.3) cm
–1
.
C
3
H
7
CN· SbF
5
(429 mg; weight expected for 1.50 mmol: 429 mg).
1
H-NMR (SO
2
, unlocked, 25
◦ C): δ = 4.12 (t, J = 7.1 Hz, 2H, CH
2
CN), 3.05 (h, J = 7.3 Hz, 2H,
CH
2
CH
3
), 2.21 (t, J = 7.4 Hz, 3H, CH
3
) ppm.
13
C-NMR (SO
2
, unlocked, 25
◦ C): δ = 121.51 (s, CN), 19.55 (s, CH
2
CN), 18.68 (CH
2
), 13.64 (CH
3
)
ppm.
14
N-NMR (SO
2
, unlocked, 25
◦ C):δ =−202.5 (s,∆ 1
⁄2 = 94 Hz, CN).
19
F-NMR (SO
2
, unlocked, 25
◦ C):
δ =−101.1 (s, 4F,∆ 1
⁄2 = 780 Hz, SbF
4
F),−130.4 (s, 1F,∆ 1
⁄2 = 800 Hz, SbF
4
F) ppm.
Raman (-90 °C, 350 mW): ˜ ν (rel. intensity) = 2944.6 (2.8), 2928.6 (4.9), 2885.1 (2.3), 2764.7 (0.8),
2317.2 (8.9), 1465.1 (1.9), 1454.6 (2.6), 1411.1 (2.4), 1321.1 (2.8), 1255.9 (1.6), 1105.7 (1.5), 1080.7
(1.4), 1044.7 (2.1), 933.9 (1.3), 871.6 (1.5), 841.9 (2.8), 686.3 (3.7), 672.0 (6.4), 652.3 (2.2), 640.4 (10.0),
576.3 (3.0), 407.5 (2.1), 199.6 (2.3), 145.7 (2.2), 108.9 (3.4) cm
–1
.
26
c-C
3
H
5
CN· SbF
5
(415 mg; weight expected for 1.50 mmol: 426 mg).
1
H-NMR (SO
2
, unlocked, 25
◦ C): δ = 3.12-3.01 (m, 1H, CH), 2.80-2.72 (m, 2H, CHH), 2.72-2.64 (m,
2H, CHH) ppm.
13
C-NMR (SO
2
, unlocked, 25
◦ C): δ = 123.56 (s,CN), 12.80 (s,CH),−3.87 (CHH) ppm.
14
N-NMR (SO
2
, unlocked, 25
◦ C): δ =−213 (s,∆ 1
⁄2 = 75 Hz, CN) ppm.
19
F-NMR (SO
2
, unlocked, 25
◦ C):δ =−101.0 (s, 4F,∆ 1
⁄2 = 757 Hz, SbF
4
F),−128.8 (s, 1F,∆ 1
⁄2 = 890 Hz,
SbF
4
F) ppm.
Raman (-90 °C, 350 mW): ˜ ν (rel. intensity) = 3120.1 (0.9), 3063.1 (0.6), 3035.9 (2.4), 2306.7 (10.0),
2281.3 (3.3), 1457.7 (3.0), 1435.5 (1.1), 1359.7 (0.4), 1342.4 (4.3), 1190.1 (7.3), 1056.9 (2.3), 951.2 (4.2),
844.5 (1.8), 822.7 (1.5), 811.3 (2.9), 768.9 (2.2), 683.1 (1.0), 664.1 (9.2), 652.7 (1.4), 639.1 (4.8), 559.0
(2.5), 278.9 (0.5), 229.9 (0.5), 207.7 (1.4), 153.8 (1.0), 115.2 (0.5), 241.5 (0.2), 162.5 (0.7), 85.8 (0.2)
cm
–1
.
(CH
3
)
3
CCN· SbF
5
(438 mg; weight expected for 1.50 mmol: 450 mg).
1
H-NMR (SO
2
, unlocked, 25
◦ C): δ = 2.81 (m, 9H, (CH
3
)
3
) ppm.
13
C-NMR (SO
2
, unlocked, 25
◦ C): δ = 125.08 (s,CN), 30.70 (s,C(CH
3
)
3
), 27.15 (s,CH
3
). ppm.
14
N-NMR (SO
2
, unlocked, 25
◦ C): δ =−204.0 (s,∆ 1
⁄2 = 118 Hz, CN) ppm.
19
F-NMR (SO
2
, unlocked, 25
◦ C):δ =−100.9 (s, 4F,∆ 1
⁄2 = 620 Hz, SbF
4
F),−130.2 (s, 1F,∆ 1
⁄2 = 740 Hz,
SbF
4
F) ppm.
Raman (-90 °C, 350 mW): ˜ ν (rel. intensity) = 3011.0 (4.1), 3000.6 (3.1), 2969.0 (2.1), 2944.9 (4.8),
2928.1 (1.7), 2912.8 (1.7), 2879.2 (1.3), 2792.6 (0.6), 2727.9 (0.8), 2401.1 (0.5), 2305.8 (9.6), 1483.7
(1.0), 1466.7 (5.1), 1449.2 (2.0), 1400.9 (1.2), 1232.6 (0.9), 1205.1 (3.2), 1147.5 (0.9), 1037.8 (2.0), 940.9
(2.9), 864.3 (2.3), 718.5 (2.2), 697.7 (1.8), 689.9 (1.8), 678.5 (1.6), 663.5 (10.0), 649.4 (2.2), 637.9 (6.1),
27
600.1 (1.6), 590.6 (1.4), 436.6 (1.3), 365.0 (1.7), 295.9 (2.2), 285.9 (2.8), 278.0 (2.0), 265.2 (1.5), 231.6
(1.5), 207.5 (4.1), 192.1 (2.2), 143.7 (1.6) cm
–1
.
C
6
H
5
CN· SbF
5
(481 mg; weight expected for 1.50 mmol: 480 mg).
1
H-NMR (SO
2
, unlocked, 25
◦ C): δ = 9.37 (d, J = 7.4 Hz, 2H), 9.24 (t, J = 7.4 Hz, 1H), 8.97 (t, J = 8.1
Hz, 2H) ppm.
13
C-NMR (SO
2
, unlocked, 25
◦ C): δ = 140.19 (s, CH), 136.77 (s, 2 x CH), 131.55 (s 2 x CH), 117.92
(s,CN), 103.90 (s,CCN) ppm.
14
N-NMR (SO
2
, unlocked, 25
◦ C): δ =−192.6 (s,∆ 1
⁄2 = 145 Hz, CN) ppm.
19
F-NMR (SO
2
, unlocked, 25
◦ C):δ =−100.8 (s, 4F,∆ 1
⁄2 = 490 Hz, SbF
4
F),−130.3 (s, 1F,∆ 1
⁄2 = 580 Hz,
SbF
4
F) ppm.
Raman (-90 °C, 350 mW): ˜ ν (rel. intensity) = 3092.4 (1.3), 2281.8 (9.6), 2263.6 (1.1), 1620.8 (0.5),
1592.9 (10.0), 1564.4 (0.7), 1519.3 (0.3), 1485.9 (0.5), 1452.9 (0.4), 1328.2 (0.6), 1303.4 (0.8), 1282.6
(1.1), 1209.5 (3.4), 1182.4 (2.4), 1149.0 (0.3), 1125.8 (0.4), 1024.7 (1.8), 1010.0 (0.5), 1000.0 (7.0), 989.2
(0.4), 958.5 (0.3), 834.0 (0.3), 779.0 (1.1), 673.8 (2.4), 641.3 (3.7), 626.9 (1.3), 612.0 (0.3), 591.5 (0.6),
565.4 (0.9), 525.3 (0.7), 2416.4 (0.2), 2390.9 (0.3), 271.3 (0.9), 201.7 (1.0), 138.9 (4.4), 289.7 (1.4) cm
–1
.
Preparation of CH
2
(CN)
2
· 2AsF
5
Anhydrous SO
2
(3.0 mL) and AsF
5
(3.3 mmol, 2.2 eq.) were
condensed into a Teflon-FEP ampule containing a frozen sample of malononitrile (1.5 mmol,
1.0 eq.) at− 196°C. The mixture was allowed to warm to− 64°C, kept at this temperature for 10
min and sporadically agitated. The volatile compounds were removedinvacuo at− 64°C, leaving
behind a colorless solid. Single crystals were grown from SO
2
solution by slow evaporation of
the solvent in vacuo at− 64 to− 45°C. (602 mg; weight expected for 1.50 mmol: 608 mg).
1
H-NMR (SO
2
, unlocked, 25
◦ C): δ = 5.95 (s, 2H, CH
2
) ppm.
28
13
C-NMR (SO
2
, unlocked, 25
◦ C): δ = 100.83 (s,CN), 11.76 (s,CH
2
) ppm.
14
N-NMR (SO
2
, unlocked, 25
◦ C): δ =−175.8 (s,∆ 1
⁄2 = 414 Hz, CN· AsF
5
) ppm.
19
F-NMR (SO
2
, unlocked, 25
◦ C): δ =−45.3 (s,∆ 1
⁄2 = 487 Hz, AsF5) ppm.
Raman (-90 °C, 350 mW): ˜ ν (rel. intensity) = 2963.9 (0.9), 2926.5 (3.0), 2376.9 (4.1), 2369.0 (2.2),
1370.9 (1.3), 1331.9 (1.2), 1327.7 (1.6), 1311.9 (0.3), 1213.6 (0.4), 1154.4 (10.0), 1147.2 (0.7), 914.5
(0.4), 724.1 (3.6), 719.7 (1.8), 692.2 (4.9), 682.5 (2.1), 618.2 (0.5), 606.9 (1.5), 524.5 (0.8), 421.1 (0.8),
388.6 (1.1), 355.8 (0.4), 317.3 (1.2), 309.2 (0.6), 270.1 (0.9), 222.0 (0.5), 101.3 (1.4), 81.2 (1.8) cm
–1
.
Preparation of CH
2
(CN)
2
· 2SbF
5
Anhydrous SO
2
(2.0 mL) and was condensed into a Teflon-
FEP ampule containing a frozen sample of SbF
5
(3 mmol, 2.0 eq.) at− 196°C. The mixture was
allowed to warm to− 64°C forming a clear solution. The cold mixture was transferred under a
stream of dry into a second into a second Teflon-FEP ampule containing a sample of the corre-
sponding nitrile (1.50 mmol, 1.0 eq.) at− 78°C. The mixture was allowed to warm to− 64°C, kept
at this temperature for 10 min and sporadically agitated. The volatile compounds were removed
in vacuo at− 64°C, leaving behind a colorless solid. (756 mg; weight expected for 1.50 mmol:
749 mg).
1
H-NMR (SO
2
, unlocked, 25
◦ C): δ = 6.36 (s, 2H, CH
2
) ppm.
13
C-NMR (SO
2
, unlocked, 25
◦ C): δ = 106.25 (s,CN), 13.23 (s,CH
2
) ppm.
14
N-NMR (SO
2
, unlocked, 25
◦ C): δ =−186.1 (s,∆ 1
⁄2 = 325 Hz, CN· SbF
5
) ppm.
19
F-NMR (SO
2
, unlocked, 25
◦ C): δ =−99.5 (s, 4F,∆ 1
⁄2 = 538 Hz, SbF
4
F),−134.7 (s, 1F,∆ 1
⁄2 = 420 Hz,
SbF
4
F) ppm.
Raman (-90 °C, 350 mW): ˜ ν (rel. intensity) = 2951.3 (1.1), 2909.0 (2.8), 2367.3 (3.4), 2354.8 (2.1),
2329.8 (0.3), 1382.6 (1.0), 1361.0 (1.3), 1325.8 (1.5), 1316.7 (2.2), 1151.1 (10.0), 1107.6 (0.5), 1085.8
29
(0.7), 909.1 (0.3), 701.8 (0.9), 679.7 (3.6), 668.1 (7.4), 663.5 (4.6), 654.7 (4.1), 645.2 (3.2), 619.8 (1.0),
605.5 (1.6), 578.8 (0.8), 540.7 (0.9), 528.4 (1.5), 406.1 (1.8), 387.1 (1.7), 295.8 (3.4), 263.3 (1.5), 230.7
(2.1), 199.6 (1.6), 135.5 (1.8), 92.1 (2.9) cm
–1
.
Additional experimental details are given in the electronic supporting information
30
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35
Chapter2
α -Fluoroalcohols: SynthesisandCharacterizationof
PerfluorinatedMethanol,Ethanoland n-Propanol,and
theirOxoniumSalts
Thischapterisbasedonthefollowingpublication:
Amanda F. Baxter, Jonas Schaab, Joachim Hegge, Thomas Saal, Monica Vasiliu, David A. Dixon,
Haiges, Karl O. Christe "α -Fluoroalcohols: Synthesis and Characterization of Perfluorinated
Methanol, Ethanol and n-Propanol, and their Oxonium Salts", Chem. Eur. J. 2018 24, 16737–
16742 10.1002/chem.201804306.
36
2.1 Abstract
The thermally unstable, primary perfluoroalcohols, CF
3
OH, C
2
F
5
OH, andn-C
3
F
7
OH, were conve-
niently prepared from the corresponding carbonyl compounds in anhydrous HF solution. Experi-
mental values for the reaction enthalpies and entropies were derived from the temperature depen-
dence of theR
f
COF+HF⇄ R
f
CF
2
OH (R
f
−− F, CF
3
, CF
3
CF
2
) equilibria by NMR spectroscopy.
Electronic structure calculations of the gas-phase and solution reaction energies, gas-phase acidi-
ties and heats of formation were carried out at the G3MP2 level, showing that these compounds
are strong acids. Protonation of these alcohols in HF/SbF
5
produced the perfluoroalkyl oxonium
salts R
f
CF
2
OH
2
+
SbF
6
–
.
37
2.2 Introduction
Under normal conditions, primary perfluorinated alcohols are unstable due to facile HF elimina-
tion.[1] Despite this instability, the simplest primary perfluorinated alcohol, CF
3
OH, is generated
in the atmosphere as a byproduct of the degradation of hydrofluorocarbons,[2] and the mech-
anism of the atmospheric formation of CF
3
OH has been established.[3–14] In the laboratory,
perfluorinated alcohols can be generated by the addition of HF across the C
–
–
O double bond of
a perfluorinated carbonyl compound, as demonstrated in 1961 by Andreades and England, who
added HF to perfluorocyclobutanone to give perfluorocyclobutanol [Eq. (2.1)].[15] Recently, we
reproduced this synthesis and reported the first crystal structure of an α -fluoroalcohol.[16]
c-C
4
F
6
O+HF−−→ c-C
4
F
7
OH (2.1)
In 1977, trifluoromethanol was synthesized by Seppelt.[17] He used a low-temperature dichlo-
rine elimination from two starting materials containing a positively and a negatively polarized
chlorine atom. The reaction was performed in three steps and involved the formation of shock-
sensitive CF
3
OCl as an intermediate. However, this synthesis is inefficient and complicated. In
2007, we published a more convenient procedure to access CF
3
OH,[18] exploiting the equilibrium
between HF and COF
2
[Eq. (2.2)].
COF
2
+HF
−− ⇀
↽−− CF
3
OH (2.2)
Due to facile HF elimination,[19] primary and secondaryα -perfluoroalcohols are thermally
unstable at room temperature. Only tertiaryβ -perfluoroalcohols, such as perfluoro- tert-butanol,
38
are significantly more stable due to the absence of α -fluorine atoms.[20] As a consequence, per-
fluoroethanol and perfluoro- n-propanol were until now unknown and only one article on a low-
temperature
13
C NMR study of perfluoroisopropanol was published.[21] There was one addi-
tional work published in 2002 by Cheburkov and Lillquist.[22] Although the title of their pa-
per, "Perfluoroalcohols", implies the synthesis of these alcohols, evidence for the presence of the
free alcohols was not firmly established. Due to their nature of being strong acids, as shown
below, perfluoroalcohols are well-known to form stable R
f
CF
2
O
–
salts.[23–25] This behavior
was cleverly exploited by Cheburkov and Lillquist.[22] They reacted a [HN(CH
3
)
3
]
+
F
–
equivalent
with neat or solutions of acylfluorides and obtained COF
2
, CF
3
CF
2
CFO and (CF
3
)
2
CFCFO prod-
ucts, which were formulated as adducts of the corresponding perfluoroalcohols with N(CH
3
)
3
and characterized according to the
19
F and
1
H NMR spectra. Considering that perfluoroalco-
hols are strong acids and triethylamine is a strong base, one might expect the products to be
ionic [HN(CH
3
)
3
]
+
[R
f
CF
2
O]
–
salts. Unfortunately, the
19
F NMR evidence for this was not clear-
cut. CF
3
OH resonated at –54.5 ppm and CF
3
O
–
at –20 ppm,[25] whereas Cheburkov’s product
showed a signal at –33.18 ppm. The possibility for the existence of an equilibrium between a
donor-acceptor system and an ionic salt has already been suggested,[22] but more work remains
to be done to establish the exact nature of the products. Nevertheless, an interesting alternative
approach consists of stabilizing the perfluoroalcohols by forming their anions and shifting their
equilibria with acylfluorides towards the products by conversion to the corresponding ethers.
In the present study, we report the synthesis and properties of perfluoroethanol and perfluoro-
n-propanol, as well as a re-investigation of the anomalous temperature dependence previously
reported[18] for the equilibrium in Equation (2.2), which could not be explained satisfactorily.
39
2.3 ResultsandDiscussion
Carbonyl fluoride was added to liquid anhydrous HF in a fluorinated ethylene propylene (FEP)
NMR tube, and the
19
F NMR spectra were recorded at various temperatures after equilibrium had
been achieved. The equilibrium concentrations of carbonyl fluoride and trifluoromethanol were
measured by integrating the areas of the
19
F NMR signals and by using the temperature-sensitive
shift of the
1
H signal of methanol to determine the temperature.[26]
It was found that the molar ratio of CF
3
OH/COF
2
increased with decreasing temperature
from 0.37 at 10°C to 2.52 at –62°C, and that the previously reported anomaly[18] was due to
the long time required to reach equilibrium at low temperatures. To ascertain that the system
had truly reached equilibrium, each temperature point was approached from above and below
the desired temperature, and only when the points had converged within an acceptable margin
of experimental error were these data points accepted. At higher temperatures, equilibration
was fast, and at temperatures≥ 0°C, it occurred in less than ten minutes. At temperatures ,≤ − 50°C, however, reaching equilibrium required 24 h or longer. Even at the lowest measured
temperatures, we still observed signals due to COF
2
and it was not possible to shift the equilibrium
completely towards the alcohol. Higher conversion to CF
3
OH could be achieved by continuously
removing the alcohol from the reaction mixture either by conversion into CF
3
OH
2
+
salts through
addition of strong Lewis acids or by trapping it as an ether through a Lewis acid-catalyzed reaction
with an alkyl fluoride.[18]
Due to the large excess of HF used in our study, the change from a 75- to a 116-fold excess
of HF had little impact on the amount of CF
3
OH formed in the reaction. Also, solubilities do not
influence the equilibrium if HF is used in a large enough excess as a solvent. Furthermore, no
40
evidence was found in the present study to support the previously suggested hypothesis that the
increase in the self-association of HF at lower temperatures plays a significant role in the slow
formation of CF
3
OH.[18]
The successful synthesis of CF
3
OH from COF
2
and HF prompted us to extend this approach
to the longer-chain primary perfluoroalcohols, pentafluoroethanol and heptafluoro-npropanol.
Both compounds were obtained in the same manner as trifluoromethanol, albeit with lower yields.
As for CF
3
OH, the concentrations of C
2
F
5
OH and n – C
3
F
7
OH increased with both decreasing
temperature and chain length (Figures S1, S2, Supporting Information). The maximum observed
R
f
CF
2
OH/R
f
COF molar ratios were 0.993 and 0.0809 for R
f
–
–
CF
3
and CF
3
CF
2
, respectively, as
compared to 2.52 for CF
3
OH. All three alcohols were characterized in anhydrous HF solution by
19
F and
13
C NMR spectroscopy. The proton resonances for the hydroxyl groups of the primary
perfluoroalcohols are expected to occur in the range of 7–10 ppm, based on the value of 8.65 ppm
observed for neat CF
3
OH.[17] However, the resonance of HF also falls in this range and the alcohol
undergoes a fast exchange with HF on the NMR timescale, preventing direct observation of the
hydroxyl proton.
Using the known initial concentrations of R
f
COF (R
f
–
–
F, CF
3
, CF
3
CF
2
), the known amounts
of HF used and the relative equilibrium concentrations from the integrated
19
F NMR signals,
the equilibrium constantK
eq
was calculated for each temperature. These equilibrium constants
were then used to construct van’t Hoff plots to determine the enthalpies ∆ H and entropies∆ S
of reaction[27–31] according to Equation (2.3).
lnk
eq
=− ∆ H
RT
∆ S
R
(2.3)
41
The van’t Hoff plots for the formation of the three alcohols are shown in Figure 1, and the
thermochemical quantities derived from these are listed in Table 2.1. The reaction enthalpies
from these plots are slightly exothermic, and the entropy values are negative, as expected from
the formation of one molecule of alcohol from two molecules of reactants, making the− T∆ S
term positive. Therefore, the free energy values∆ G=∆ H− T∆ S pass through zero and become
increasingly more positive with increasing temperature, consistent with the favored formation of
the alcohols at low temperature and their decomposition at higher temperatures. The decomposi-
tion of gaseous CF
3
OH is slow and has a high activation energy barrier of about 43–45 kcal mol
–1
,
but is substantially lowered in the presence of HF, other CF
3
OH molecules, or H
2
O.[2, 19]
Figure 2.1: van’t Hoff plots for the :uilibrium R
f
COF + HF ⇄ R
f
CF
2
OH (R
f
–
–
F, CF
3
,
CF
3
CF
2
). Equations for the lines are as follows: (R
–
–
F) y
–
–
1798.3 x-11.237, R
2
–
–
0.9913; (R
–
–
CF
3
)
y
–
–
1634.4 x-11.524, R
2
–
–
0.99284; (R
–
–
CF
3
CF
2
) y
–
–
1087.7 x-11.481, R
2
–
–
0.98735
42
Table 2.1: Summary of the thermodynamic quantities derived from the van’t Hoff plots for the
R
f
COF+HF⇄ R
f
CF
2
OH (R
f
−− F, CF
3
, CF
3
CF
2
) equilibria
R
f
−− F R
f
−− CF
3
R
f
−− CF
3
CF
2
∆ H
[a]
–3.6 –3.2 –2.2
∆ S
[b]
–22.3 –22.9 –22.8
∆ G
[c]
3.1 3.6 4.6
[a] Enthalpies of reaction (kcalmol
− 1
).
[b] Entropies of reaction (kcalmol
− 1
).
[c] Gibbs free energies at 298 K
(kcalmol
− 1
).
The slopes of the van’t Hoff plots depend only on the temperature dependence of the equilib-
rium constants. Considering that the equilibrium concentrations were measured for each com-
pound on the same sample, most systematic errors canceled out. Therefore, we expect the values
of ∆ H to be quite accurate. The ∆ S values, obtained from the extrapolated intercepts of the
curves with the ordinate, are much less accurate because any systematic errors do not necessar-
ily cancel out.
The observed ∆ H value of− 3.6kcalmol
− 1
for the formation of CF
3
OH from COF
2
in HF
solution is in good agreement with the most reliable calculated gas-phase values of –5.7[32] and
–6.5 kcal mol
–1
.[19] These experimental and computational values are consistent with the experi-
mental value of –(2.8
+1.1
− 1.7
) kcal mol
–1
from a photoionization study.[33] The enthalpies of reaction
become less negative with increasing chain length, whereas the entropy term is very similar in
all three cases. Therefore,∆ G becomes more positive explaining our observed decrease in alco-
hol formation with increasing chain length at all temperatures. When the HF solutions of these
perfluoroalcohols were acidified by the addition of strong Lewis acids, such as SbF
5
, the alcohols
were protonated affording the corresponding oxonium salts [Eq. (2.4) and Tables 2.2-2.4]
43
Table2.2: Summary of the
19
F,
13
C, and
1
H NMR chemical shifts and
1
J
C− F
coupling constants
of CF
3
OH and CF
3
OH
2
+
CF
3
OH CF
3
OH CF
3
OH
2
+
CF
3
OH/ CF
3
OH
2
+
(neat)
a
(in HF)
b
(in HF)
c
(in HF)
d
δ 19
F [ppm] –54.5 (s) –57.8 (s) –59.37 (s) –58.70
δ 13
C [ppm] 118 (q) 120.92 (q) 118.40 (q) 119.84 (q)
1
J
C− F
[Hz] 256 254.9 270.0 262.8
δ 1
H [ppm] 8.65 (s) (9.39)
e
(10.29)
f
(10.98)
g
[a] Data from Ref. [17]. [b] Data recorded at –60
◦ C. [c] Data for the [CF
3
OH
2
+
SbF
6
–
]· n SbF
5
salt, recorded at –60
◦ C in the presence of a large excess of SbF
5
. [d] 0.5 equivalents of SbF
5
were used. [e] Concentration-dependent exchange of HF and CF
3
OH; the chemical shift
of pure HF was found to be strongly temperature-dependent and was observed at 8.77 and
9.46 ppm at 25 and –60
◦ C, respectively. [f] Concentration- and temperature-dependent
exchange of HF and CF
3
OH
2
+
; the addition of SbF
5
to neat HF resulted in a downfield shift
of its proton resonance from 8.77 to 9.23 ppm and from 9.46 to 10.03 ppm at 25 and –60
◦ C,
respectively. [g] Concentration- and temperature-dependent exchange of HF, CF
3
OH and
CF
3
OH
2
+
. Signal multiplicity indicated in parentheses as singlet (s) or quartet (q).
R
f
CF
2
OH+HF+SbF
5
−−→ [R
f
CF
2
OH
2
]
+
SbF
6
− (2.4)
Considering that the identification and characterization of the alcohols and their correspond-
ing oxonium salts relied on NMR spectroscopy and that the differences in their NMR parameters
are quite small, it is important to measure these spectra with high accuracy, that is, with the
same instrument under identical conditions and using the same methods for referencing.[34] In
the present study, we measured the
19
F and
13
C NMR spectra of trifluoromethanol, pentafluo-
roethanol, heptafluoron-propanol and their corresponding oxonium cations in anhydrous HF so-
lution, utilizing the substitution method with 80% CFCl
3
in 20% CDCl
3
as the external standard.
The results showed that protonation of the – OH group influences the signals of the fluorocarbon
backbones only very weakly. The most pronounced changes were slightly increased shielding of
44
Table 2.3: The
19
F and
13
C NMR shifts and the corresponding coupling constants of perflu-
oroethanol, CF
3
CF
2
OH, recorded at –60°C in HF, and its protonated cation. The CF
3
CF
2
OH
2
+
spectra were recorded at –50°C for a 1:3 molar mixture of CF
3
COF and SbF
5
in HF
[a]
CF
3
CF
2
OH CF
3
CF
2
OH
2
+
δ 19
F obs [ppm] -CF
2
- 86.00 (q) –87.50 (q)
-CF
3
–89.89 (t) –89.49 (t)
δ 13
C obs [ppm] -CF
2
- 114.39 (tq) 114.04 (tq)
-CF
3
117.25 (qt) 116.63 (qt)
1
J
C− F
[Hz] -CF
2
- 268.3 273.6
-CF
3
282.7 283.5
2
J
C− F
[Hz] -CF
2
- 43.0 43.8
-CF
3
46.6 45.1
4
J
F− F
[Hz] -CF
2
- 2.5 2.5
-CF
3
2.5 2.5
[a] Signal multiplicity indicated in parentheses
as triplet (t), quartet (q), triplet of quartets (qt) or
quartet of triplets (tq).
the
19
F and
13
C signals of theα – CF
n
groups and the increase of their
1
J
C− F
coupling constants
upon protonation. This increase in the coupling constants agrees with previous experimental
and computational results for CF
3
OH/CF
3
OH
2
+
.[18] Observation of the proton chemical shift was
complicated by the alcohols undergoing rapid exchange with the HF solvent and by the oxonium
salts exchanging with both the alcohols and HF. Furthermore, the chemical shift of the proton
resonance of HF depends strongly on the temperature and the acidification with SbF
5
. Therefore,
no conclusions can be drawn concerning the proton shifts of these alcohols and their oxonium
salts in HF solution.
In the protonation reactions of the alcohols, only one set of signals was always observed. This
fact might be explained by the alcohol and its oxonium salt undergoing a fast exchange on the
45
Table2.4: The
19
F and
13
C NMR shifts and the corresponding coupling constants of perfluoro-
n-propanol, CF
3
CF
2
CF
2
OH, and its protonated cation, CF
3
CF
2
CF
2
OH
2
+
, recorded in HF solution
at− 55 and− 50
◦ C, respectively
CF
3
CF
2
CF
2
OH CF
3
CF
2
CF
2
OH
2
+
δ 19
F obs [ppm] -CF
2
OH –80.64 (q) –81.82 (q)
-CF
3
–84.43 (t) –84.51 (t)
-CF
2
- –132.84 (s) –132.78 (s)
δ 13
C obs [ppm] -CF
2
OH 115.73 (t of t) 115.54 (t of t)
-CF
3
117.72 (q of t) 117.40 (q of t)
-CF
2
- 109.22 (t) 108.90 (t)
1
J
C− F
1
J
F− C
[Hz] -CF
2
OH 269.4 (270.1) 272.8 (275)
-CF
3
284.5 (286.7) 284.3 (286)
-CF
2
- (263.1) (264.7)
2
J
C− F
[Hz] -CF
2
OH 30.7 31.3
-CF
3
32.4 32.5
-CF
2
- 40.6 39.1
3
J
F− F
[Hz] -CF
2
OH 8.0 8.0
-CF
3
8.0 8.0
NMR timescale. This was verified experimentally by using only half a molar equivalent of SbF
5
in the protonation reaction. In the absence of a fast exchange, two sets of signals, one for the
alcohol and one for the oxonium salt, should be observed, whereas, for a fast exchange, only one
set of signals should be seen with chemical shift and coupling constant values between those of
the pure alcohol and the pure oxonium salt. This second case is shown in the experimental data in
the last column of Table 2.2, which are proof of a rapid exchange between CF
3
OH and CF
3
OH
2
+
.
We also investigated to what extent the protonation of CF
3
OH shifts theCOF
2
+HF⇄ CF
3
OH
equilibrium towards the products. Although a complete conversion of COF
2
to CF
3
OH
2
+
at –
60°C was not observed under our reaction conditions, the concentration of unreacted COF
2
was
reduced from 27.6 to 5.7% upon addition of a five-fold molar excess of SbF
5
to the alcohol solution.
The difficulty to observe a complete shift of the equilibrium to the right might be caused by the
46
slow conversion of COF
2
to CF
3
OH at low temperature, compared to the fast protonation reaction
of the alcohol.
The
19
F spectra of CF
3
OH, C
2
F
5
OH, and C
3
F
7
OH were very similar to those of CF
4
, C
2
F
6
, and
C
3
F
8
, respectively,[35] demonstrating that the OH group behaves as a pseudo-fluorine atom.[36]
Furthermore, the signals of – CF
2
OH groups did not appear in the typical CF
2
region of 120–
130 ppm but closer to the CF
3
region between 80–90 ppm. The
19
F NMR signals of perfluoro-
n-propanol showed an interesting through-space coupling[37] between the terminal CF
3
–
and
the CF
2
OH-groups with
4
J
F− F
= 8.0 Hz. By analogy with closely related CF
3
– CF
2
– CF
3
,[38]
the internal CF
2
group coupled only weakly with the fluorine atoms of the terminal CF
3
–
and
CF
2
OH-groups, and therefore, no additional fine structure was observed. In C
3
F
8
, the
3
J
F− F
is
only 0.7 Hz, whereas
4
J
F− F
is 7.31 Hz.[38]
The above NMR data show the following general trends. (1) The
19
F chemical shifts of theα -
perfluoro-alcohols mirror closely those of the corresponding perfluorocarbons,[35–38] showing
that the OH group can be treated as a pseudo-fluorine atom. (2) As expected, the influence of the
– OH or – OH
2
+
group is the strongest on theα -fluorine atoms; however, the observed changes
are rather subtle. Protonation of the alcohol increases the one-bond coupling constant,
1
J
C− F
,
only by 3–15 Hz. (3) The
1
H NMR spectra are not useful for distinguishing the free alcohols from
the oxonium salts due to exchange reactions with each other and with HF as well as to a strong
temperature and concentration dependence.
In addition to the experimental study described above, electronic structure computations were
carried out to gain a better understanding of the nature of these perfluoroalcohols. Although
numerous theoretical studies have been previously carried out on free gaseous CF
3
OH,[2–14, 24,
47
25, 39–50] very little is known on its condensed-phase chemistry and, particularly, on the higher
perfluoroalcohols. Therefore, extensive calculations were carried out at the G3MP2[51] level of
theory for all three fluoroalcohols both in the gas phase and in aqueous solutions. We calculated
enthalpies and free energies of formation, reaction and decomposition, entropies, decomposition
barriers, and gas-phase acidities. These results, together with a description of the computational
methods used, have been compiled in the Supporting Information, and only the most important
findings are summarized below.
The calculated gas-phase acidities (Table 2.5) show that these perfluoroalcohols are reasonably
strong acids in the gas phase, with perfluoro- n-propanol having a gas-phase acidity comparable
to that of
H
3
PO
4
.[52, 53] The alcohols become more acidic in the gas phase with increasing chain length,
although differential solvation effects can change this trend slightly in aqueous solutions, as ob-
served many years ago for the proton affinities of methyl-substituted amines. With p K
a
values
ranging from 1.1 to 1.5 in aqueous solution, these perfluoroalcohols are predicted to be surpris-
ingly strong acids, stronger than HF (pK
a
=3.17), HN
3
(4.72), H
3
PO
4
(2.12), acetic acid (4.76), and
HCN (9.4), although not quite as strong as trifluoroacetic acid (–0.25).
Table2.5: Enthalpy values,∆ H
g
, and Gibbs free energies,∆ G
g
, associated with gas-phase acidi-
ties (HA−−→ A
–
+ H
+
) of the perfluoroalcohols, calculated at the G3MP2 level, their solvation
energies in aqueous solution∆ G
aq
and pK
a
values in H
2
O at 298 K. Energy values are expressed
in kcal mol
–1
CF
3
OH CF
3
CF
2
OH CF
3
CF
2
CF
2
OH
∆ H
g
329.2 325.2 322.9
∆ G
g
321.9 317.9 315.2
∆ G
aq
1.9 2.0 1.5
pK
a
1.4 1.5 1.1
48
Table 2.6: Enthalpies of formation ∆ H
f
expressed in kcal mol
–1
of gaseous perfluoromethanol,
perfluoroethanol, and perfluoro- n-propanol, calculated at the G3MP2 level at 0 and 298 K
CF
3
OH CF
3
CF
2
OH CF
3
CF
2
CF
2
OH
∆ H
f
(0 K) –215.1 –315.4 –414.7
∆ H
f
(298 K) –217.1 –318.0 –417.9
The heats of formation∆ H
f
of the free alcohols, calculated at the G3MP2 level are summarized
in Table 2.6. The value of 217.1 kcal mol
–1
for ∆ H
f
(298K) of CF
3
OH is in very good agreement
with the experimental values of –220.7± 3.2 kcal mol
–1
[43] and –217.2± 0.9 kcal mol
–1
,[33] as well
as a calculated value of –217.8 kcal mol
–1
at the CCSD(T)/CBS level.[19]
The reaction energies for the formation of the three alcohols from the corresponding acyl
fluoride and HF were calculated at the G3MP2 level with solvent effects in H
2
O (H
2
O and HF have
comparable dielectric coefficients) obtained at the B3LYP/DZVP2/COSMO level (Table 2.7) and
are in good agreement with the experimental values derived from the van’t Hoff plots (Table 2.1).
The order of the gas-phase reaction enthalpy∆ H
g
values differs from the experimental values in
HF solution because they do not show a decrease in the reaction enthalpy with increasing chain
length and they are all more negative than the experimental values by 3–4 kcal mol
–1
. There
is only a small effect of solvation on the reactions making the free energy values slightly more
positive at 298 K. The calculated∆ H values confirm that the reaction enthalpies for the formation
of these alcohols are slightly exothermic; however, at 298 K theT∆ S term dominates and the free
energies become endothermic. This is in line with the fact these species are unstable at ambient
temperature, as observed by us and others. Furthermore, the data also show that even small
changes in the Gibbs free energy of reaction in the gas phase∆ G
aq
are sufficient to significantly
shift the equilibrium.
49
The decomposition barriers of the perfluoroalcohols are strongly influenced by self-
association, solvation effects, and adduct formation. For example, the calculated free energy
barrier for the unimolecular decomposition of gaseous CF
3
OH of 45.9 kcal mol
–1
was lowered to
32.1 and 28.2 kcal mol
–1
by one and two molecules of HF, respectively. If H
2
O and OH are also
present, the decomposition barrier is reduced to 11.0 kcal mol
–1
.[32] Our barrier calculations at
the G3MP2 level for CF
3
OH, C
2
F
5
OH, and C
3
F
7
OH confirmed that the free energy barriers of
44–47 kcal mol
–1
for unimolecular decomposition can be lowered to 27–28 kcal mol
–1
by adduct
formation with two molecules of HF. These barriers are still too high when compared to the
experimentally observed thermal stabilities of these compounds; the true barriers in HF solution
are expected to be around 10 kcal mol
–1
. The use of an implicit solvent model based on aqueous
solutions led to a decrease in the barrier over the gas-phase values. Hence, the HF solvent plays
an additional role in lowering the reaction barrier, perhaps by serving as a proton transfer agent
and further catalyzing the decomposition reaction.
The proton affinities of the three perfluoroalcohols increase slightly from trifluoromethanol to
hepta-fluoro- n-propanol and, as expected, all are weaker bases than methanol (Table 2.8). Again,
the agreement between the calculated and the available experimental values[43, 54] is very good.
Table2.7: Reaction energies for the reactions in kcal mol
–1
at the G3MP2 level at 298 K, in which
solvent effects, ∆ G
solv
, were calculated at B3LYP/DZVP2/COSMO in water as the solvent
Reaction ∆ H
g
[a]
∆ G
g
[a]
∆ G
solv
[b]
∆ G
aq
[b]
∆ H
HF
[c]
COF
2
+ HF⇄ CF
3
OH –6.6 3.5 2.6 6.1 –3.6
CF
3
COF + HF⇄ C
2
F
5
OH –7.7 3.3 2.9 6.2 –3.2
C
2
F
5
COF + HF⇄ C
3
F
7
OH –6.7 4.2 2.4 6.6 –2.0
[a] Values calculated in the gas phase. [b] Values calculated in aqueous solution. [c]
Experimental values for the formation enthalpies in HF solution from Table 2.1
50
Table2.8: Proton affinities (PA) at 298 K in kcal mol
–1
calculated at the G3MP2 level
PA(∆ H) PA(∆ H) Expt PA(∆ H)
[a]
CH
3
OH 180.5 173.3 180.2
[b]
CF
3
OH 147.7 140.7 151.1± 1.7
[c]
CF
3
CF
2
OH 148.1 141.3
CF
3
CF
2
CF
2
OH 149.1 142.3
[a] Experimental values obtained from the litera-
ture. [b] Ref. [54]. [c] Ref. [43].
2.4 Conclusion
In conclusion, the previously reported convenient access to CF
3
OH from carbonyl fluoride and
HF was extended to the syntheses of pentafluoroethanol and heptafluoro- n-propanol. Although
CF
3
OH had previously been studied in much detail due to its presence in the atmosphere and
stratosphere, no data were available in the literature on its higher analogues. It has now been
shown that the corresponding pentafluoroethanol and heptafluoro- n-propanol can also be pre-
pared in the same manner, albeit with lower yields, and exhibit similar characteristics. They are
thermally unstable molecules that decompose under ambient conditions back to the acyl fluorides
and HF. The equilibrium can be shifted towards the alcohol by protonation, forming the perfluori-
nated oxonium saltsR
f
OH
2
+
SbF
6
− . All the new compounds were characterized by multinuclear
NMR spectroscopy and electronic structure calculations, and the previously reported anomalous
temperature dependence of the COF
2
/HF equilibrium[18] was found to be caused by its slow rate
at low temperature. From the temperature dependence of the equilibrium and van’t Hoff plots,
the reaction enthalpies were experimentally determined and were in good agreement with the
electronic structure calculations in the gas phase. The ready access to perfluoroalcohols could
51
transform them from exotic laboratory curiosities to useful compounds of significant scientific
and industrial interest.
Acknowledgements
The work at USC was funded by the Office of Naval Research and the Hydrocarbon Research
Foundation. The work at UA was supported by the Chemical Sciences, Geosciences and Bio-
sciences Division, Office of Basic Energy Sciences, U.S. Department of Energy (DOE) under
the Catalysis Center Program by a subcontract from the Pacific Northwest National Laboratory
(KC0301050-47319). D.A.D. also thanks the Robert Ramsay Chair Fund of The University of Al-
abama for support. A.F.B. thanks NSF for a graduate research fellowship.
52
2.5 ExperimentalPart
Caution! Anhydrous HF can cause severe burns and contact with the skin must be avoided. COF
2
is a highly toxic gas. A well-ventilated fume hood and use of appropriate personal protective
equipment is required.
2.5.1 MaterialsandApparatus
All reactions were carried out in Teflon-FEP ampules that were closed by stainless steel valves.
Volatile materials were handled in a grease-free Pyrex glass vacuum line equipped with Kontes©
HI-VAC© valves or stainless steel/Teflon-FEP vacuum lines.[55] All gaseous chemicals were han-
dled on a stainless-steel/Teflon-FEP vacuum line.[56] Anhydrous HF (Galaxy Chemicals) was
dried by storage over BiF
5
.[57] SbF
5
(Ozark Mahoning) was handled in the dry nitrogen atmo-
sphere of a glovebox. Prior to use, the stainless-steel vacuum line and reaction vessels were
passivated with ClF
3
. NMR spectra were recorded either on Varian VNMRS 500 MHz, Varian VN-
MRS 600 MHz or Bruker AMX-500 spectrometer, using 5 mm variable-temperature broad-band
probes. NMR spectra were externally referenced to 25% tetramethylsilane in dichloromethane-d2
for
1
H and
13
C NMR spectra, to 80% CFCl
3
in chloroform-d for
19
F NMR spectra.
2.5.2 ComputationalMethods
The geometries were optimized at the density functional theory (DFT)3 level with the hybrid
B3LYP exchange-correlation functional[58, 59] and DZVP2 as basis set.[60] Vibrational frequen-
cies were calculated to show that the structures were minima. The B3LYP/DZVP2 optimized
geometries were used as starting points for G3MP2 calculations.[51] All calculations were done
53
with Gaussian09.[61] Using the optimized DFT gas phase geometries, the solvation free energies
in water at 298 K were calculated using the self-consistent reaction field (SCRF) approach[62] with
the COSMO parameters[63, 64] as implemented in Gaussian 03[61] at the same B3LYP/DZVP2
level of theory and in ADF[65–67] with the BLYP[59, 68] exchange-correlation functional and
the TZ2P basis set in ADF. The radii developed by Klamt and co-workers were used to define
the cavity.[63, 64] The aqueous Gibbs free energy (free energy in aqueous solution),∆ G
aq
, was
calculated from Equation 2.5
∆ G
aq
=∆ G
gas
+∆∆ G
solv
(2.5)
where∆ G
gas
is the gas phase free energy and∆∆ G
solv
is the aqueous solvation free energy
calculated as differences between the conjugate base and the acid (HA −−→ H
+
+ A
–
).
A dielectric constant of 78.39 corresponding to that of bulk water was used in the COSMO
calculations. TheoreticalpK
a
values in aqueous solution were calculated from Equation 2.6 and
compared to the available experimental (estimated)pK
a
values:
pK
a
=
∆ G
aq
2.303· RT
(2.6)
whereR is the gas constant and T = 298 K is the temperature.
Proton affinities were calculated at G3MP2 level as HA + H
+
−−→ H
2
A
+
.
The G3MP2 method was used to predict the heats of formation (∆ H
gas
) in the gas phase using
the experimental heats of formation of the atoms at 0 K (169.98 kcal mol
–1
for C, 18.47 kcal mol
–1
for F, 58.99 kcal mol
–1
for F, and 51.63 kcal mol
–1
for H).
54
2.5.3 ExperimentalProcedures
2.5.3.1 KineticNMRExperiments
Generalprocedure: In a typical experiment, anhydrous HF (0.5–0.8 mL) and the carbonyl com-
pound (0.2–1 mmol) were condensed on the vacuum line at –196°C into a passivated 4 mm O.D.
TeflonFEP tube that was heat-sealed. The sealed tube was placed in a cooling bath cryostat for 0.5
to 72 h depending on the desired temperature (lower temperatures required longer times). Then,
the sample was placed in a precooled NMR spectrometer and its NMR spectra were recorded.
The instrument temperature was calibrated with a pure methanol sample.[69] The relative con-
centrations of theα -fluoroalcohol and carbonyl compound were established by integrating the
area of their
19
F signals. The protonated perfluoroalcohols were prepared by adding SbF
5
to the
sample tubes and mixing it with the HF, before adding the carbonyl compound.
2.5.3.2 MolecularCharacterizationNMRExperiments
Perfluorinated alcohols: CF
3
OH, CF
3
CF
2
OH and CF
3
CF
2
CF
2
OH Anhydrous HF (0.5 mL)
was condensed on a vacuum line at –196°C into a passivated and tared 4 mm Teflon-FEP ampule,
which was closed by a stainless-steel valve. The HF in the ampule was warmed to ambient tem-
perature and the exact amount of HF was determined by weight. The ampule was connected to
the vacuum line, cooled to –196°C and RfCOF (R
f
= F, CF
3
, CF
3
CF
2
) (1–2 mmol) was condensed.
The Teflon-FEP ampule containing a frozen mixture of HF and R
f
COF was heat-sealed, warmed
to ambient temperature and once again weight to determine the exact amount of R
f
COF. The
sealed tube was placed in a cooling bath at –65°C for 2 days for COF
2
, 14 days for CF
3
COF and
16 days for CF
3
CF
2
COF.
55
CF
3
OH: HF (31.4 mmol); COF
2
(2.76 mmol)
CF
3
CF
2
OH: HF (29.0 mmol); CF
3
COF (1.41 mmol)
CF
3
CF
2
CF
2
OH: HF (30.1 mmol); CF
3
CF
2
COF (2.65 mmol)
Perfluorinated alcohol oxoniums salts: CF
3
OH
2
+
, CF
3
CF
2
OH
2
+
and CF
3
CF
2
CF
2
OH
2
+
Anhydrous HF (0.75 mL) was condensed into a tared 9 mm Teflon-FEP ampule containing a frozen
sample of SbF5 (1–2 mmol) at –196°C. The mixture was warmed to ambient temperature to form
a clear colorless solution and the exact amount of HF added was determined by weight. The
solution was cooled to –64°C and, under a stream of dry nitrogen using an 18-gauge TeflonFEP
tubing, transferred into a passivated 4 mm Teflon-FEP ampule cooled to –78 °C. R
f
COF (Rf = F,
CF
3
, CF
3
CF
2
) (1 mmol) were condensed on a vacuum line at –196°C into the 4 mm Teflon-FEP
ampule containing the cooled HF/SbF
5
mixture. The Teflon-FEP ampule was heatsealed, warmed
to room-temperature and once again weighed to determine the exact amount of RfCOF (Rf = F,
CF3, CF3CF2). The sealed tube was placed in a cooling bath at –65°C for 2 days for COF
2
, for
3 days for CF
3
COF and 5 days for CF
3
CF
2
COF.
CF3OH2+; HF (43.2 mmol); SbF
5
(1.61 mmol); COF
2
(0.29 mmol)
CF3OH/CF3OH2+; HF (25.3 mmol); SbF
5
(0.55 mmol); COF
2
(1.08 mmol)
CF3CF2OH2+; HF (44.1 mmol); SbF
5
(1.90 mmol) CF
3
COF (0.72 mmol)
CF3CF2CF2OH2+; HF (40.8 mmol); SbF
5
(2.33 mmol); CF
3
CF
2
COF (0.71 mmol)
Additional experimental details are given in the electronic supporting information
56
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62
Chapter3
ProtonationofNitramines: WhereDoestheProtonGo?
Thischapterisbasedonthefollowingpublication:
Thomas Saal, Martin Rahm, Karl O. Christe, and Ralf Haiges "Protonation of Nitramines: Where
Does the Proton Go?", Angew. Chem. Int. Ed. 2017 56, 9587-9591. 10.1002/anie.201705397.
63
3.1 Abstract
The reactions of nitramine, N-methyl nitramine, and N,N-dimethyl nitramine with anhydrous
HF and the superacids HF/MF
5
(M=As, Sb) were investigated at temperatures below –40°C. In
solution, exclusiveO-protonation was observed by multinuclear NMR spectroscopy. Whereas no
solid product could be isolated from the neat HF solutions even at –78°C, in the HF/MF
5
systems,
protonated nitramine MF
6
–
salts were isolated for the first time as moisture-sensitive solids that
decompose at temperatures above –40°C. In the solid state, depending on the counterion, O-
protonated orN-protonated cations can be formed, in accord with theoretical calculations which
show that the energy differences between O-protonation and N-protonation are very small.
The salts [H
2
N – NO
2
H][AsF
6
], [H
3
N – NO
2
][SbF
6
], [MeHNNO
2
H][SbF
6
], and [Me
2
NNO
2
H][SbF
6
]
were characterized by their X-ray crystal structures.
64
3.2 Introduction
The mechanisms used to rationalize many well-known reactions involve proton transfers and
the formation of protonated species, which are often short-lived and difficult to observe in the
condensed phase. It is well known that the superacids HF/AsF
5
and HF/SbF
5
can protonate a wide
variety of substrates,[1–3] resulting in the formation of protonated salts of sufficient stability for
structural characterization. In molecules containing multiple competing proton-acceptor sites,
such as the nitrogen and oxygen atoms in nitramines and imides, the question arises as to where
the molecules will be protonated.[4, 5]
H
2
NNO
2
, the parent compound of all nitramines, was first synthesized more than 120 years
ago.[6] Despite its pronounced instability with respect to decomposition to N
2
O and H
2
O,[7] the
structure of nitramine has been well-established by microwave,[8] IR and Raman spectroscopy,[9,
10] and X-ray crystallography.[11, 12] The base-induced decomposition of nitramine was one of
the principal reactions used to test theories and mechanisms of catalysis in homogeneous solution
and was investigated quantitatively in pioneering work from the early twentieth century.[13, 14]
Although H
2
NNO
2
is not an organic compound, its base-induced decomposition is a key reaction
of physical organic chemistry. Many experimental studies concerning this reaction have been
carried out, leading to significant advances in our understanding of acid-base catalysis, as well as
the discovery of the Bronsted acid relationship.[13, 15] While the acid-catalyzed decomposition of
nitramine has also been known for a long time, the reaction has received only scant attention.[7,
16] It was experimentally investigated by spectroscopic methods only much later than the base-
catalyzed reaction,[17] and it is interesting to note that it is only significant in strongly acidic
media. In addition, it was found that under acidic conditions the N-alkyl nitramines, RHNNO
2
,
65
undergo the same type of decomposition reaction as the parent nitramine, but not under basic
conditions. Also, nitramines and their catalyzed decomposition reactions have been the subjects
of several computational studies.[12, 17–30]
3.3 ResultsandDiscussion
Nitramine possesses two different potential proton-acceptor sites. Protonation on the nitro-
gen atom of the amino group results in the formation of the nitroammonium ion [H
3
N – NO
2
]
+
(1), while protonation on an oxygen atom of the nitro group results in the formation of the
[H
2
N – NO
2
H]
+
cation (2) (Figure 3.1).
Figure3.1: Calculated structures (CBS-QB3[31, 32]) of protonated nitramine [H
2
NNO
2
]H
+
. Rel-
ative enthalpies and Gibbs energies are given in kcal mol
–1
in the gas phase (1 atm, 298 K) and in
polar solvent (1 M, 298 K) in parenthesis
Examples of both protonated species had been elusive, and could not be isolated and struc-
turally characterized previously. While a mass spectroscopic study concluded that the protona-
tion of nitramine in the gas phase results predominantly in the formation of the O-protonated
species [H
2
N – NO
2
H]
+
,[17] there were no experimental data available for the protonation of
66
nitramine in the condensed phase. Our own quantum chemical calculations with the CBS-
QB3[31, 32] composite method show that both protomers are true minimum structures that are
vibrationally stable, but that the O-protonated isomer 2 is slightly lower in energy (∆ G
0
= –
3.5 kcal mol
–1
, ∆ H
0
= –5.7 kcal mol
–1
) than the ammonium isomer 1. These values are in good
agreement with earlier calculations at the G1 level of theory, favoring2 by 5.2 kcal mol
–1
.[17] In
solution, the difference in free energy between the two protomers is even smaller (Figure 3.1).
Even at temperatures as low as –60°C, nitramine is immediately protonated in anhydrous HF
(aHF) solutions, as evidenced by their
14
N NMR spectra. The
14
N NMR resonance of the nitro
group is shifted fromδ = –27 ppm for unprotonated nitramine in CDCl
3
toδ = –45 ppm in aHF
at –60°C, while the
14
N NMR resonance of the amino group remains virtually unshifted atδ =
–220 ppm. This chemical shift pattern is conclusive proof for O-protonation of H
2
NNO
2
in HF
solution [Eq. (3.1)].
H
2
N− NO
2
+(n+2)HF
− 60
◦ C
− −−− → [H
2
N− NO
2
H]
+
[HF
2
· nHF]
− (3.1)
In agreement with the well-known acid-catalyzed decomposition,[7, 16] the nitramine solu-
tions in aHF slowly decomposed, as evidenced by the observation of appreciable amounts of N
2
O
in the
14
N NMR spectra of samples that had been kept sealed at –40°C for 12 h [Eq. (5.2)].
H
2
N− NO
2
H
+
−−→ N
2
O+H
2
O (3.2)
It was not possible to isolate the protonated nitramine as a polybifluoride salt because the
removal of the HF solvent in vacuo, even at temperatures as low as –78°C, resulted in complete
67
decomposition. However, when the polybifluoride anion was replaced by [AsF
6
]
–
through the
addition of 1.05 equivalents of AsF
5
, [H
2
N – NO
2
H][AsF
6
] was isolated as a colorless, moisture-
and air-sensitive solid after removal of the HF solvent in a dynamic vacuum at –40°C [Eq. (5.3)].
[H
2
N− NO
2
H][HF
2
· nHF]+AsF
5
− (n+1)HF
− −−−−− → [H
2
N− NO
2
H][AsF
6
] (3.3)
The resulting salt was identified and characterized by its low-temperature Raman spectrum,
solid state structures as well as
1
H and
14
N NMR spectra in SO
2
solution. Crystals of the compound
were grown from an aHF solution by slow evaporation of the solvent in a vacuum at –40°C.
The obtained X-ray crystal structure of the [AsF
6
]
–
salt (Figure 3.2 A) proves unequivocally the
presence of the O-protonated cation.
It is interesting to note that when 1.5 or more equivalents of AsF
5
were used instead of a
stoichiometric amount, the low-temperature
14
N NMR spectrum of the resulting solid in SO
2
solution showed a second set of resonance signals atδ = –82 ppm andδ = –236 ppm in addition
to the resonances of the [H
2
N – NO
2
H]
+
cation atδ = –63 ppm (NO
2
H) andδ = –219 ppm (NH
2
).
This indicates the likely formation of a doublyO-protonated nitramine and will be the subject of
future work.
When a stoichiometric amount of SbF
5
was added to a solution of nitramine in aHF, again,
a colorless, moisture- and air-sensitive solid was obtained after removal of the HF solvent in a
dynamic vacuum at –40°C [Eq. (3.4)].
[H
2
N− NO
2
H][HF
2
· nHF]+SbF
5
− (n1
+
)HF
− −−−−− → [H
2
N− NO
2
]H[SbF
6
] (3.4)
68
Figure3.2: Crystal structures of [H
2
N – NO
2
H][AsF
6
] (A) and [H
3
N – NO
2
]
–
[SbF
6
] (B). Thermal
ellipsoids are set at 50% probability. Selected distances [
˚A] and angles [
◦ ]: A) N1–N2 1.284(2),
N1–O1 1.196(2), N1–O2 1.346(3), O2··· F4 2.627(2); N2–N1–O1 125.7(2), N2–N1–O 112.1(2), O1–
N1–O2 122.1(2); B) N1–N2 1.520(2), N1–O1 1.196(2), N1–O2 1.193(2), N2··· F6 2.767(2); N2–N1–O1
112.9(1), N2–N1–O2 114.6(1), O1–N1–O2 132.5(2)
The
14
N NMR spectrum of this [SbF
6
]
–
salt in SO
2
solution at –40°C exhibited a sharp signal at
δ = –65 ppm and a broad feature atδ = –220 ppm, similar to the
14
N NMR spectrum of the [AsF
6
]
–
salt. However, we were puzzled by the fact that the obtained Raman spectra of the [SbF
6
]
–
and
[AsF
6
]
–
salts did not show any resemblance, indicating that the two salts contain the same cations
in solution but different cations in the solid state. This conundrum was resolved by growing single
crystals of the [SbF
6
]
–
salt. The obtained crystal structure of the salt (Figure 3.2 B) revealed the
presence of the N-protonated ammonium ion in the solid state in contrast to the O-protonated
ion in solution. These experimental results are supported by our quantum chemical calculations
69
which show an energy difference of ∆ G
0
= 3.5kcal mol
–1
(∆ H
0
= 5.7kcal mol
–1
) between the two
protomers in the gas phase and ∆ G
0
= 2.0kcal mol
–1
(∆ H
0
= 0.9kcal mol
–1
) in solution. These
small energy differences might be overcome in the solid state by cation-anion interactions or
other solid-state effects, resulting in the formation of the [H
2
N – NO
2
H]
+
isomer for the [AsF
6
]
–
but the [H
3
N – NO
2
]
+
isomer for the [SbF
6
]
–
salt in the solid state. We estimate the gas-phase
proton affinity of H
2
NNO
2
to be∆ H
0
= 178.8kcal mol
–1
at the CBS-QB3 level.
Since methyl substitution of amines increases the basicity relative to the parent amine (e.g.
Me
2
NH pK
b
=3.27; MeNH
2
pK
b
=3.34, NH
3
pK
b
=4.75),[33] we were curious to see if the site of pro-
tonation changes for the nitramines MeHNNO
2
and Me
2
NNO
2
. The low-temperature
14
N NMR
spectra of solutions of bothN-methylated nitramines in aHF confirmed, in analogy to H
2
NNO
2
,
their immediate O-protonation. This result is supported by our quantum chemical calculations
which show that for the protonation of Me
2
NNO
2
, the O-protonated isomer is clearly favored
over the ammonium isomer by 4.0kcal mol
–1
(∆ G
0
), and 5.0kcal mol
–1
(∆ H
0
). The proton affinity
of Me
2
NNO
2
in the gas phase is estimated to be∆ H
0
= 196.7kcal mol
–1
at the CBS-QB3 level.
Colorless, moisture- and air-sensitive [AsF
6
]
–
and [SbF
6
]
–
salts of the protonated species were
obtained after addition of stoichiometric amounts of AsF
5
or SbF
5
to the HF solutions [Eq. (3.5)]
and removal of the solvent in a dynamic vacuum at –40°C.
RR
′
N− NO
2
+HF+MF
5
HF
− −−− →
− 60
◦ C
[RR
′
N− NO
2
H]
+
[MF
6
]
− (3.5)
R
−− Me;R
′
−− H,Me;M
−− As,Sb
70
The Raman spectra of both protonated species show close resemblance between the [AsF
6
]
–
and [SbF
6
]
–
salts, indicating that the salts contain similar cations. Crystals of the two [SbF
6
]
–
salts were grown from aHF solution by slow evaporation of the solvent in a vacuum at –40°C. The
obtained X-ray crystal structures (Figure 3.3) prove again the presence of O-protonated cations
in both salts.
The low-temperature
14
N NMR spectra of the [AsF
6
]
–
and [SbF
6
]
–
salts of the protonated
methylnitramines in SO
2
solution are very similar and are consistent with O-protonation. For
[MeHN – NO
2
H] [MF
6
], the
14
N NMR spectrum displays sharp resonances atδ = –57 ppm (M=As)
andδ = –56 ppm (M=Sb) for the NO
2
H group, and relatively broad resonances atδ = –201 ppm
(M=As) and δ = –208 ppm (M=Sb) for the MeHN group. For [Me
2
N – NO
2
H][MF
6
], resonances
are observed at δ = –60 ppm (M=As) and δ = –56 ppm (M=Sb) for the NO
2
H group, and at δ = –196 ppm (M=As) and δ = –195 ppm (M=Sb) for the Me
2
N group. As for the unsubstituted
nitramine, the low-temperature
14
N NMR spectra exhibited additional sets of signals, when 1.5
equivalents of Lewis acid MF
5
(M=As, Sb) were used, indicating a probable diprotonation of the
methylnitramines. For MeHNNO
2
, these signal sets were observed at δ = –73 ppm and δ = –
228 ppm, and for Me
2
NNO
2
atδ = –61 ppm andδ = –216 ppm.
Details of the crystallographic data collection and refinement parameters for the struc-
turally characterized compounds of this work are given in the Supporting Information.
[H
2
N – NO
2
H][AsF
6
] crystallizes in the triclinic space groupP
¯1 (Z=2), while the ammonium salt
[H
3
N – NO
2
][SbF
6
] crystallizes in the orthorhombic space group Pbca (Z=8). Both solid state
structures consist of cations and anions that are associated through X–H··· F (X=O, N) hydrogen
bonds (Figure 3.2). As expected for an O-protonated nitro group, two very different N–O bond
71
Figure3.3: Crystal structures of [MeHN – NO
2
H][SbF
6
] (A) and [Me
2
NNO
2
H][SbF
6
] (B). Thermal
ellipsoids are set at 50% probability. Some hydrogen atoms have been omitted for clarity. Selected
distances [
˚A] and angles [
◦ ]: A) N1–N2 1.276(7), N2–O1 1.199(5), N2–O2 1.347(6), O2··· F1 2.627(5),
N1–N2–O1 125.6(4), N1–N2–O2 112.1(4), O1–N2–O2 122.3(4); B) N1–N2 1.28(1), N2–O1 1.33(1),
N2–O2 1.22(1), O1··· F1 2.58(1); N1–N2–O1 121.3(7), N1–N2–O2 124.3(8), O1–N2–O2 121.3(7)
lengths were found for the [H
2
N – NO
2
H]
+
cation. The longer distance of 1.346(2)
˚A for the pro-
tonated oxygen atom and the shorter distance of 1.195(2)
˚A for the unprotonated one are typical
distances for N–O single and N=O double bonds, respectively. The distances of 1.239(1)
˚A for
the two equivalent N–O bonds in nitramine lie right between these two distances.[34] The N–N
bond in theO-protonated cation of 1.284(2)
˚A is significantly shorter than that in nitramine with a
length of 1.322(2)
˚A,[34] and agrees well with the length of 1.291
˚A, calculated at the M06-2X/cc-
pVTZ level of theory. For the ammonium ion [H
3
N – NO
2
]
+
, the observed bond length pattern
is different, featuring a symmetric NO
2
group with an N–O distance of 1.195(2)
˚A. The observed
72
N–N bond length of 1.521(2)
˚A in the [NH
3
NO
2
]
+
cation is longer than twice the Pyykkö radius of
nitrogen (1.42
˚A),[35, 36] but shorter than that predicted at the M06-2X/cc-pVTZ level (1.715
˚A).
[MeHN – NO
2
H][SbF
6
] crystallizes with two formula units in space group P
¯1, while
[Me
2
N – NO
2
H] [SbF
6
] crystallizes in the monoclinic space group P2
1
/c (Z=4). Both solid-state
structures consist of cations and anions that are associated through O–H··· F hydrogen bonds (Fig-
ure 3.3). Both methyl-substitutedO-protonated cations exhibit a longer N–O single bond of about
1.33–1.35(1)
˚A and a shorter N
–
–
O double bond of about 1.20–1.22(1)
˚A. As already observed for
nitramine, the N–N bond contracts uponO-protonation and is about 0.04
˚A shorter in the cation
than in freeN-methyl- orN,N-dimethyl nitramine, respectively.[37, 38]
The low-temperature Raman spectra of the isolated products from the protonation of H
2
NNO
2
with HF/AsF
5
, as well as MeHNNO
2
and Me
2
NNO
2
with HF/MF
5
(M=As, Sb) show close re-
semblance, and are fully consistent with O-protonated nitramines. Besides the bands of the
[MF
6
]
–
anions, the most characteristic features of the Raman spectra are the strong bands of
theν (HO – N) stretching vibration in the range of 940–987 cm
–1
. In addition, less intense bands
are observed forν (N
–
–
O) at 1670–1721 cm
–1
, ν as
(HO – N – N) at 1550–1580 cm
–1
, andν (N – N) at
1182–1350 cm
–1
. Theν (H – O) andν (H – N) vibrations are observed at 3020–3210 cm
–1
, while the
δ (NO
2
) deformation modes exhibit bands at 670–770 cm
–1
. The most prominent difference in the
low-temperature Raman spectra of [H
3
N – NO
2
][SbF
6
] and the aforementionedO-protonated salts
is the absence of the strong ν (HO – N) vibration band at around 950 cm
–1
for the N-protonated
salt. However, a closer analysis of the vibrational spectra of the compounds and full assignment
of the vibrations is difficult because in all cations most of the vibrations are strongly coupled. A
full list of Raman frequencies and intensities is given in the Supporting Information.
73
3.4 Conclusions
In summary, the reactions of nitramine, N-methyl nitramine, and N,N-dimethyl nitramine with
HF and the HF/MF
5
(M=As, Sb) superacids were studied. The first examples of protonated
nitramine salts were obtained as moisture-sensitive, colorless solids that are stable only at tem-
peratures well below –20°C. While protonation can potentially occur either on the N atom of the
amino group or an O atom of the nitro group, the formation of theO-protonated cation is favored
over the nitroammonium ion by∆ G
0
= 3.5kcal mol
–1
in the gas phase and by∆ G
0
= 2.0kcal mol
–1
in solution, and is observed in all cases for these systems in solution. Only for the protonated
SbF
6
–
salt of nitramine, N-protonation was observed in the solid state, implying that solid-state
effects, such as cation-anion interactions, can overcompensate for these relatively small energy
differences. The X-ray crystal structure determinations revealed that in the solid state the pro-
tonation products of H
2
NNO
2
with HF/AsF
5
, as well as those of MeHNNO
2
and Me
2
NNO
2
with
HF/MF
5
(M=As, Sb), contain O-protonated ions, while that from the reaction of H
2
NNO
2
with
HF/SbF
5
containsN-protonated ammonium ions. The fact that the energy differences between O-
protonation and N-protonation are very small and both O-protonation and N-protonation were
experimentally observed is in accord with our finding that in nitramido-substituted dihydrobo-
rate anions the dinitramido ligand can be attached to boron either through the nitrogen or the
oxygen atom.[39]
74
Acknowledgements
The Office of Naval Research (ONR) funded this work. We thank the Hydrocarbon Research
Foundation for financial support and Prof. G. K. S. Prakash, Drs. W. Wilson, R. Wagner, and P.
Deokar, and also A. Baxter for their help and stimulating discussions.
75
3.5 ExperimentalPart
Caution! Anhydrous HF, AsF
5
, and SbF
5
can cause severe burns and contact with the skin must
be avoided and the compounds should only be handled in a well-ventilated fume hood. Nitramine
is potentially hazardous, owing to its recognized explosive properties. Although its detonation
has not been observed, deflagration occurs when nitramine is heated above its melting point.
Appropriate safety precautions should be taken when working with these materials.
3.5.1 MaterialsandApparatus
All reactions were carried out in either Teflon-FEP ampules or NMR tubes that were closed by
stainless steel valves. Volatile materials were handled in a stainless steel / TeflonFEP[40] or
Pyrex glass vacuum line. Reaction vessels and the stainless-steel vacuum line were passivated
with ClF
3
prior to use. Non-volatile materials were handled in the dry nitrogen atmosphere of
a glove box. aHF (Galaxy Chemicals) was dried by storage over BiF
5
.[41] AsF
5
was prepared
from AsF
3
and F
2
.[42–44] SbF
5
(Ozark Mahoning) was freshly distilled before use. Nitramine,
N-methylnitramine, and N,N-dimethylnitramine were prepared according to literature proce-
dures.[38, 45, 46] The NMR spectra were recorded on Bruker AMX-500, Varian VNMRS-400, Var-
ian VNMRS-500, or Varian VNMRS-600 spectrometer. The spectra were externally referenced
to neat nitromethane for
14
N-NMR spectra, neat tetramethylsilane for
1
H and
13
C NMR spectra,
and to 80% CFCl
3
in chloroform-d for
19
F NMR spectra. Raman spectra were recorded directly in
9 mm Teflon-FEP reactors or FEP inliners in the range 4000—80 cm
-1
on a Bruker Vertex 70/RAM
II spectrophotometer, using a Nd-YAG laser at 1064 nm.
76
3.5.2 CrystalStructureDeterminations
Diffraction-quality crystals were grown from aHF solution inside Teflon-FEP ampules by slow
evaporation of the HF solvent in a dynamic vacuum at –40°C. The FEP reactors were cooled
to –78°C and opened under a stream of N
2
gas, and the crystalline content was dropped into
the trough of a low-temperature crystal-mounting apparatus at –110°C. A glass fiber that was
attached to a magnetic CrystalCap and dipped into PFPE (perfluoropolyether) oil was used to
mount the crystals on the goniometer with a magnetic base. The single-crystal X-ray diffraction
data were collected on a Bruker SMART APEX DUO diffractometer using Mo K α radiation. The
diffractometer was equipped with an Oxford Cryosystems Cryostream 700 apparatus for low-
temperature data collection. The structures were solved by intrinsic phasing and refined on F
2
using the Bruker SHELXTL Software Package and ShelXle.[47–51] Structure drawings were pre-
pared using Mercury CSD 3.9.[52] CCDC 1549714, 1549715, 1549716, and 1549717 contain the sup-
plementary crystallographic data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centr (CCDC, 12 Union Road, Cambridge CB21EZ, UK
(Fax: (+44) 1223-336-033; e-mail: deposit@ccdc.cam.ac.uk) on quoting the deposition no. CCDC
1549714-1549717.
3.5.3 ComputationalMethods
The CBS-QB[31, 32] composite method was employed for accurate gas-phase energies. CBS-QB3
is based on CCSD(T) energies extrapolated to the basis set limit using MP2 and MP4 calculations
together with empirical corrections, and is expected to be highly reliable for thermochemistry.[31,
32, 53, 54] Water was chosen as a model for HF in our SMD-PCM implicit treatment of solvation.
77
The SMD-PCM[55] model has a mean unsigned error of <1kcal mol
–1
for neutral molecules. The
free energy solvent corrections utilized density functional theory (M06-2X/cc-pVTZ)[56] geome-
tries and thermal corrections, which were evaluated in both gas phase and in solution. Structural
relaxation upon solvation is therefore included. All calculations were performed using Gaus-
sian09, revision A.02.[57] Please see the Supporting Information for further experimental details.
3.5.4 ExperimentalProcedures
3.5.4.1 Preparationof[NH
2
NO
2
H][AsF
6
]
Anhydrous HF (3.0 mL) and AsF
5
(1.50 mmol) were condensed into a TeflonFEP ampule containing
a frozen sample of nitramine (1.00 mmol) at –196°C. The mixture was warmed to –64°C, kept at
this temperature for 10 min and sporadically agitated. All volatile compounds were removed from
the clear, colorless solution in in vacuo at –78°C, leaving behind a colorless crystalline solid of
[H
2
NNO
2
H][AsF
6
].
Raman (–80
◦ C, 350 mW):˜ ν (rel. intensity) = 3211 (0.2), 3124 (0.2), 1721 (2.0), 1579 (1.1), 1492 (0.8),
1477 (2.1), 1341 (0.8), 1183 (1.7), 939 (10.0), 699 (9.9), 668 (5.7), 628 (3.7), 572 (3.5), 550 (1.7), 402
(0.7), 373 (5.4), 367 (3.1), 151 (6.4) cm
-1
.
1
H-NMR (SO
2
, unlocked, – 40
◦ C): δ = 11.4 (s,H2NNO
2
H
+
) ppm.
14
N-NMR (SO
2
, unlocked, – 40
◦ C): δ =−63 (s,∆ 1
⁄2 = 60 Hz,NO
2
H
+
),−219 (s,∆ 1
⁄2 = 890 Hz,H
2
N)
ppm.
19
F-NMR (SO
2
, unlocked, – 40
◦ C): δ =−56 (s,∆ 1
⁄2 = 2300 Hz,AsF6− ) ppm.
78
3.5.4.2 Preparationof[NH
3
NO
2
][SbF
6
]
Anhydrous HF (3.0 mL) was condensed into a Teflon-FEP ampule containing a frozen sample
of SbF
5
(1.00 mmol) at –196°C. The mixture was warmed to ambient temperature to form a clear
colorless solution. The solution was cooled to –64°C and, under a stream of dry nitrogen using 18
gauge FEB tubing, transferred into a second Teflon-FEP ampule containing a sample of nitramine
(1.00 mmol) dissolved in anhydrous HF (1.0 mL) cooled to –78°C. The mixture was warmed to
–64°C, kept at this temperature for 10 min and sporadically agitated. All volatile compounds
were removed from the clear, colorless solution in in vacuo at –78 °C, leaving behind a colorless
solid of [H
3
NNO
2
][SbF
6
]. Single crystals were grown from HF solution by slow evaporation of
the solvent in in vacuo at –64°C.
Raman (-80°C, 350 mW): ˜ ν (rel. intensity) = 3089 (0.2), 3057 (0.2), 3024 (0.3), 1733 (0.3), 1723 (0.4),
1568 (0.2), 1543 (0.6), 1534 (0.6), 1430 (0.3), 1303 (1.9), 1271 (1.6), 1113 (0.6), 1016 (0.4), 819 (2.1),
665 (10.0), 646 (2.7), 573 (4.2), 563 (3.4), 497 (1.0), 285 (2.9), 202 (0.6), 184 (0.7), 169 (0.6), 129 (2.5)
cm
-1
.
1
H-NMR (SO
2
, unlocked, – 40
◦ C): δ = 11.5 (s,H2NNO
2
H
+
) ppm.
14
N-NMR (SO
2
, unlocked, – 40
◦ C): δ = −65 (s,∆ 1
⁄2 = 60 Hz, NO
2
H
+
), −219 (s,∆ 1
⁄2 = 850 Hz, H
2
N)
ppm.
19
F-NMR (SO
2
, unlocked, – 40
◦ C): δ ==−113 (s,∆ 1
⁄2 = 5500 Hz, SbF6-) ppm.
3.5.4.3 Preparationof[MeHNNO
2
H][AsF
6
]
Anhydrous HF (3.0 mL) and AsF
5
(1.50 mmol) were condensed into a Teflon-FEP ampule con-
taining a frozen sample of N-methylnitramine (1.00 mmol) at –196°C. The mixture was warmed
79
to 64 °C, kept at this temperature for 10 min and sporadically agitated. All volatile compounds
were removed from the clear, colorless solution in in vacuo at –78°C, leaving behind a colorless
crystalline solid of [MeHNNO
2
H][AsF
6
]. Single crystals were grown from HF solution by slow
evaporation of the solvent in in vacuo at –64°C.
Raman (-80°C, 350 mW): ˜ ν (rel. intensity) = 3060 (0.8), 3026 (0.8), 2965 (4.4), 1702 (1.4), 1556 (1.5),
1456 (1.7), 1441 (1.6), 1422 (1.5), 1251 (3.7), 1192 (3.0), 1130 (1.1), 1062 (3.2), 924 (6.1), 697 (5.0), 675
(10.0), 598 (2.3), 580 (3.5), 556 (3.8), 540 (2.3), 386 (7.7), 373 (5.7), 364 (3.6), 231 (2.1), 205 (1.8) cm
-1
.
1
H-NMR (SO
2
, unlocked, – 40
◦ C): δ = 13.7 (s, 1H MeHNNO
2
H
+
), 12.1 (s, 1H, MeHNNO
2
H), 4.0
(s, 3H, CH3) ppm.
13
C-NMR (SO
2
, unlocked, – 40
◦ C): δ = 35.8 (s, CH
3
) ppm.
14
N-NMR (SO
2
, unlocked, – 40
◦ C):δ =−57 (s,∆ 1
⁄2 = 70 Hz,NO
2
H
+
),−201 (s,∆ 1
⁄2 = 900 Hz, MeHN)
ppm.
19
F-NMR (SO
2
, unlocked, – 40
◦ C): δ =−62 (s,∆ 1
⁄2 = 6100 Hz, AsF6
–
) ppm.
3.5.4.4 Preparationof[MeNHNO
2
H][SbF
6
]
Anhydrous HF (3.0 mL) was condensed into a Teflon-FEP ampule containing a frozen sample of
SbF
5
(1.00 mmol) at –196°C. The mixture was warmed to ambient temperature to form a clear
colorless solution. The solution was cooled to –64°C and, under a stream of dry nitrogen using
18 gauge FEB tubing, transferred into a second TeflonFEP ampule containing a sample of N-
methylnitramine (1.00 mmol) dissolved in anhydrous HF (1.0 mL) cooled to –78°C. The mixture
was warmed to –64°C, kept at this temperature for 10 min and sporadically agitated. All volatile
compounds were removed from the clear, colorless solution ininvacuo at –78°C, leaving behind
80
[MeHNNO
2
H][SbF
6
] as a colorless solid. Single crystals were grown from HF solution by slow
evaporation of the solvent in in vacuo at –64°C.
Raman (-80°C, 350 mW): ˜ ν (rel. intensity) = 3211 (0.2), 3124 (0.2), 1721 (2.0), 1579 (1.1), 1492 (0.8),
1477 (2.1), 1183 (1.7), 939 (10.0), 699 (9.9), 668 (5.7), 628 (3.7), 572 (3.5), 550 (1.7), 373 (5.4), 367 (3.1),
151 (6.4) cm
-1
.
1
H-NMR (SO
2
, unlocked, – 40
◦ C): δ = 11.6 (s, 2H, MeHNNO
2
H
+
), 4.5 (s, 3H, CH3) ppm.
13
C-NMR (SO
2
, unlocked, – 40
◦ C): δ = 35.9 (s,CH
3
) ppm.
14
N-NMR (SO
2
, unlocked, – 40
◦ C):δ =−56 (s,∆ 1
⁄2 = 80 Hz,NO
2
H
+
),−208 (s,∆ 1
⁄2 = 1250 Hz, MeHN)
ppm.
19
F-NMR (SO
2
, unlocked, – 40
◦ C): δ =−115 (s,∆ 1
⁄2 = 5300 Hz, SbF6-) ppm.
3.5.4.5 Preparationof[Me
2
NNO
2
H][AsF
6
]
Anhydrous HF (3.0 mL) and AsF
5
(1.50 mmol) was condensed into a TeflonFEP ampule containing
a frozen sample of N,N-dimethylnitramine (1.00 mmol) at –196°C. The mixture was warmed to
–64°C, kept at this temperature for 10 min and sporadically agitated. All volatile compounds
were removed from the clear, colorless solution in vacuo at –78°C, leaving behind crystalline
[Me
2
NNO
2
H][AsF
6
] as a colorless crystalline solid. Single crystals were grown from HF solution
by slow evaporation of the solvent in vacuo at –64°C.
Raman (-80°C, 350 mW): ˜ ν (rel. intensity) = 3070 (1.4), 3035 (1.5), 2971 (7.4), 2859 (1.6), 2816 (0.8),
1678 (1.0), 1475 (1.4), 1456 (1.7), 1429 (2.9), 1413 (3.1), 1346 (2.2), 1272 (3.0), 1164 (1.4), 983 (2.9),
762 (10.0), 686 (7.0), 611 (2.6), 577 (1.8), 548 (0.7), 539 (0.6), 427 (2.8), 382 (1.7), 370 (4.0), 290 (1.8)
cm
-1
.
1
H-NMR (SO
2
, unlocked, – 40
◦ C): δ = 12.2 (s, 1H,Me
2
NNO
2
H
+
), 4.4 (s, 6H, CH3) ppm.
81
13
C-NMR (SO
2
, unlocked, – 40
◦ C): δ = 35.9 (s,CH
3
) ppm.
14
N-NMR (SO
2
, unlocked, – 40
◦ C):δ =−60 (s,∆ 1
⁄2 = 220 Hz,NO
2
H
+
),−196 (s,∆ 1
⁄2 = 650 Hz, Me
2
N)
ppm.
14
N-NMR (SO
2
, unlocked, – 40
◦ C): δ =−56 (s,∆ 1
⁄2 = 2500 Hz, AsF6-) ppm.
3.5.4.6 Preparationof[Me
2
NNO
2
H][SbF
6
]
Anhydrous HF (3.0 mL) was condensed into a Teflon-FEP ampule containing a frozen sample of
SbF
5
(1.00 mmol) at –196°C. The mixture was warmed to ambient temperature to form a clear
colorless solution. The solution was cooled to –64°C and, under a stream of dry nitrogen using
18 gauge FEB tubing, transferred into a second TeflonFEP ampule containing a sample of N,N-
dimethylnitramine (1.00 mmol) dissolved in anhydrous HF (1.0 mL) cooled to –78°C. The mixture
was warmed to –64°C, kept at this temperature for 10 min and sporadically agitated. All volatile
compounds were removed from the clear, colorless solution in vacuo at –78°C, leaving behind
[Me
2
NNO
2
H][SbF
6
] as a colorless solid. Single crystals were grown from HF solution by slow
evaporation of the solvent in vacuo at –64°C
Raman (-80°C, 350 mW): ˜ ν (rel. intensity) = 3065 (0.7), 3047 (0.8), 3018 (0.7), 2979 (3.8), 2967 (3.4),
2824 (0.5), 1671 (0.7), 1476 (1.4), 1450 (1.3), 1431 (1.5), 1338 (1.7), 1285 (1.0), 1274 (2.2), 1180 (0.7),
1165 (0.6), 1094 (0.4), 1058 (0.3), 1017 (1.0), 987 (2.0), 820 (1.4), 765 (7.7), 659 (10.0), 647 (2.3), 641
(4.0), 626 (0.8), 611 (1.8), 586 (1.5), 557 (1.0), 549 (0.9), 427 (1.4), 294 (2.1), 284 (4.3) cm
-1
.
1
H-NMR (SO
2
, unlocked, – 40
◦ C): δ = 14.2 (s, 1H,Me
2
NNO
2
H
+
), 5.0 (s, 6H, CH3) ppm.
13
C-NMR (SO
2
, unlocked, – 40
◦ C): δ = 45.5 (s, CH
3
) ppm.
14
N-NMR (SO
2
, unlocked, – 40
◦ C):δ =−56 (s,∆ 1
⁄2 = 80 Hz,NO
2
H
+
),−196 (s,∆ 1
⁄2 = 1100 Hz, Me
2
N)
ppm.
82
19
F-NMR (SO
2
, unlocked, – 40
◦ C): δ =−116 (s,∆ 1
⁄2 = 5800 Hz, SbF6-) ppm.
Additional experimental details are given in the electronic supporting information
83
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88
Chapter4
ProtonationofCH
3
N
3
andCF
3
N
3
inSuperacids: Isolation
andStructuralCharacterizationofLong-LivedMethyl-and
TrifluoromethylaminoDiazoniumIons
Thischapterisbasedonthefollowingpublication:
Thomas Saal, Zsófia E. Blastik, Ralf Haiges, Archith Nirmalchandar, Amanda F. Baxter, Karl
O. Christe, Monica Vasiliu, David A. Dixon, Petr Beier, G. K. Surya Prakash "Protonation of
CH
3
N
3
and CF
3
N
3
in Superacids: Isolation and Structural Characterization of Long-Lived Methyl-
and Trifluoromethylamino Diazonium Ions", Angew. Chem. Int. Ed. 2020 59, 12520-12526.
10.1002/anie.202002750.
89
4.1 Abstract
The methylamino diazonium cations [CH
3
N(H)N
2
]
+
and [CF
3
N(H)N
2
]
+
were prepared as their
low-temperature stable [AsF
6
]
− salts by protonation of azidomethane and azidotrifluoromethane
in superacidic systems. They were characterized by NMR and Raman spectroscopy. Unequivocal
proof of the protonation site was obtained by the crystal structures of both salts, confirming
the formation of alkylamino diazonium ions. The Lewis adducts CH
3
N
3
· AsF
5
and CF
3
N
3
· AsF
5
were also prepared and characterized by low-temperature NMR and Raman spectroscopy, and
also by X-ray structure determination for CH
3
N
3
· AsF
5
. Electronic structure calculations were
performed to provide additional insights. Attempted electrophilic amination of aromatics such as
benzene and toluene with methyl- and trifluoromethylamino diazonium ions were unsuccessful.
90
4.2 Introduction
Reactive intermediates play a crucial role in synthetic organic chemistry.[1, 2] Experimental ob-
servation of these often short-lived, high-energy species is key to gaining insight into the mecha-
nism of well-known and industrially relevant reactions.[3–5] Protonation of fundamental classes
of compounds, such as organic azides, leads to the formation of important electrophilic species,
including aminodiazonium ions and nitrenium ions.[6] Although the intermediacy of the amin-
odiazonium ion has been proposed for certain reaction mechanisms, the direct isolation and struc-
tural characterization of these highly reactive species, especially in the condensed phase, have
been elusive.[7–15]
The formation of intermediate aminodiazonium ions in the gas-phase has been postu-
lated,[16–18] and the first aminodiazonium salt, [MeN(H)N
2
][SbCl
6
], was isolated and charac-
terized by vibrational spectroscopy by Schmidt in 1966.[19] The preparation and IR spectra of
the parent species, [H
2
N
3
]
+
, were also described by Schmidt.[19] Olah and co-workers studied
the protonation of HN
3
, CH
3
N
3
, and C
2
H
5
N
3
by NMR spectroscopy under superacidic conditions,
using mixtures of either FSO
3
H/SbF
5
, HF/SbF
5
, or HF/BF
3
,[20] but there were no reports on the
isolation of any alkyl- or arylamino diazonium salts and their structural characterization. Further-
more, the synthesis of [H
2
N
3
]
+
from either sodium azide or trimethylsilyl azide using AlCl
3/
HCl or
triflic acid was investigated by the group of Olah.[20, 21] Also reported was the use of insitu pre-
pared aminodiazonium ions as reagents for electrophilic amination reactions of aromatics.[20–
23] However, again the aminodiazonium ion intermediates could not be isolated under the reac-
tion conditions.[22] Christeetal. reported the isolation of the [H
2
N
3
]
+
ion as its [AsF
6
]
–
, [SbF
6
]
–
,
and [BF
4
]
–
salts from anhydrous HF (aHF) solutions of HN
3
with the corresponding Lewis acids
91
and the crystal structure of [H
2
N
3
]
+
[SbF
6
]
–
.[24] Several computational studies concerning the
[H
2
N
3
]
+
cation have been published, reflecting the general importance of protonated azides.[20,
24–32] The formation of Lewis adducts between alkyl azides and SbCl
5
has previously been de-
scribed and their IR spectra have been reported.[16, 18, 19] The complexes decompose under N
2
elimination and formation of methylene iminium ion salts when reacted with anhydrous HCl at
room temperature.[17, 18]
Herein, we report the protonation of azidomethane and azidotrifluoromethane by superacids,
and the isolation and full characterization of the protonated species. In addition to the first struc-
tural characterization of these alkylamino diazonium ion salts, we report the formation and char-
acterization of the Lewis adducts of arsenic pentafluoride with azidomethane and azidotrifluo-
romethane. We also explored the electrophilic amination reactions of benzene and toluene with
the respective amino diazonium ions.
4.3 ResultsandDiscussion
4.3.1 Synthesis
Our initial attempts to protonate CF
3
N
3
using a large excess of either trifluoromethanesulfonic
acid (TfOH) or fluorosulfonic acid (FSO
3
H) in dichloromethane (DCM) solution at ambient and
sub-ambient temperatures failed to provide any evidence for the formation of the trifluoromethyl
aminodiazonium species. Even Magic Acid (FSO
3
H/SbF
5
), whose acidity is several magnitudes
higher than that of either triflic acid or fluorosulfonic acid,[4] failed to protonate CF
3
N
3
in DCM.
92
Suspecting that the protonation reaction might be hampered by the low solubility of the su-
peracids in DCM, we tested SO
2
ClF, a more suitable, low-nucleophilic solvent for superacid
chemistry.[4] However, even in SO
2
ClF at− 78°C, no evidence for the protonation of CF3N3
by fluorosulfonic acid could be found. The formation of the [CF
3
N(H)N
2
]
+
cation was observed
by multinuclear NMR spectroscopy when a mixture of FSO
3
H/SbF
5
(1:1) was added slowly to a
solution of CF
3
N
3
in SO
2
ClF at− 78°C [Eq. 4.1].
[CF
3
N
3
]+FSO
3
H+SbF
5
SO2ClF,− 78
◦ C
− −−−−−−−− →
− SO3
[CF
3
N(H)N
2
][SbF
6
] (4.1)
Since AsF
5
is easier to handle than SbF
5
, and HF/AsF
5
is also a strong superacid,[4, 33–
37] the FSO
3
H/SbF
5
in Equation (4.1) was replaced by HF/AsF
5
, resulting in the formation of
[CF
3
N(H)N
2
][AsF
6
] at− 78°C [Eq.4.2].
[CF
3
N
3
]+HF+AsF
5
aHF,− 78
◦ C
− −−−−−− → [CF
3
N(H)N
2
][AsF
6
] (4.2)
Single crystals suitable for X-ray structure determination were grown from aHF solution by
slow evaporation of the solvent under a dynamic vacuum at− 78°C. [CF
3
N(H)N
2
][AsF
6
] was
identified and characterized by multinuclear NMR and vibrational spectra, and by its X-ray crystal
structure. The [CF
3
N(H)N
2
][AsF
6
] salt and solutions containing the [CF
3
NHN
2
]
+
cation were
found to be stable when stored at− 78°C, and no signs of decomposition were observed even
after three weeks. Variable-temperature
19
F NMR experiments revealed that the cation possesses
good thermal stability up to –32°C.
Whereas no evidence for the protonation of CF
3
N
3
by neataHF was found by low-temperature
multinuclear NMR spectroscopy, the NMR spectra of a solution of CH
3
N
3
inaHF showed complete
93
protonation of this more basic azide, and exclusive formation of the methylamino diazonium
cation [Eq. 4.3] was observed.
[CH
3
N
3
]+(n+2)HF
aHF,− 78
◦ C
− −−−−−− → [CH
3
N(H)N
2
][HNF
2
· nHF] (4.3)
It was not possible to isolate the protonated azidomethane cation as its poly-bifluoride salt be-
cause the removal of the HF solvent in a dynamic vacuum, even at temperatures as low as− 78°C,
resulted in complete decomposition. However, when the poly-bifluoride anion was replaced by
[AsF
6
]
–
through the addition of a stoichiometric amount of AsF
5
, [CH
3
N(H)N
2
][AsF
6
] was iso-
lated after removal of the HF solvent in a dynamic vacuum at− 78°C as a colorless, moisture-
and air-sensitive solid [Eq. 4.4].
[CH
3
N(H)N
2
][HNF
2
· nHF]+AsF
5
aHF,− 78
◦ C
− −−−−−− →
− (n+1)HF
[CH
3
N(H)N
2
][AsF
6
] (4.4)
Single crystals suitable for X-ray structure determination were grown fromaHF solutions by
slow evaporation of the solvent under a dynamic vacuum at− 78°C. The [CH
3
N(H)N
2
][AsF
6
] salt
was identified and characterized by its multinuclear NMR and vibrational spectra, and by its X-
ray crystal structure. The [CH
3
N(H)N
2
]
+
cation is only stable at low temperature and decomposes
when warmed to ambient temperature.[16, 17] For this reason, solutions of CH
3
N
3
inaHF are only
stable below− 45°C and decompose under dinitrogen evolution and formation of the methylene
iminium cation, [CH
2
NH
2
]
+
, as evidenced by NMR spectroscopy [Eq. 4.5].
[CH
3
N(H)N
2
][AsF
6
]
aHF,− 25
◦ C
− −−−−−− → [CH
2
NH
2
][AsF
6
]+N
2
(4.5)
94
Caution! Attempts to prepare [CH
3
N(H)N
2
][AsF
6
] directly by condensing CH
3
N
3
, together
with a stoichiometric amount of AsF
5
, onto frozen aHF at− 196°C consistently resulted in ex-
plosions, when the mixtures were warmed to− 78°C. Direct protonation of CH
3
N
3
with HF/AsF
5
should therefore be avoided, and the addition of HF and AsF
5
should be carried out stepwise!
To further investigate the interactions of azidomethane and azidotrifluoromethane with AsF
5
,
stoichiometric amounts of AsF
5
were condensed at− 196°C onto frozen solutions of the azides in
SO
2
, and the mixtures warmed to− 64°C. After removal of the solvent under a dynamic vacuum
at− 64°C, colorless solids of the corresponding Lewis adduct, RN
3
· AsF
5
(R = CH
3
, CF
3
), were
obtained [Eq. 4.6].
RN
3
+AsF
5
SO2,− 64
◦ C
− −−−−−− → RN
3
·AsF
5
(R=CH
3
,CF
3
) (4.6)
Neither violent reactions or explosions were observed under these reaction conditions. Single
crystals of CH
3
N
3
· AsF
5
were grown from the SO
2
solution by slow evaporation of the solvent
under a dynamic vacuum at− 64°C, while only either microcrystalline or amorphous material
was obtained for CF
3
N
3
· AsF
5
. Both adducts, CH
3
N
3
· AsF
5
and CF
3
N
3
· AsF
5
, are only marginally
stable in SO
2
solution at ambient temperature and slowly decompose over the course of several
hours. Attempts to prepare the [AsF
6
]
–
salts of the [CH
3
N(H)N
2
]
+
and [CF
3
N(H)N
2
]
+
cations by
dissolving the Lewis adducts, CH
3
N
3
· AsF
5
and CF
3
N
3
· AsF
5
, respectively, in aHF were not suc-
cessful because of the slow reaction between the adducts and HF even at temperatures as high as
–45°C. As a result, only mixtures of RN
3
· AsF
5
, [RN(H)N
2
][AsF
6
] (R = CH
3
, CF
3
), and decomposi-
tion products were obtained.
95
4.3.2 X-rayCrystallography
The amino diazonium salt [CH
3
N(H)N
2
][AsF
6
] crystallizes in the monoclinic space groupP2
1
/c
with four formula units in the unit cell (Z =4). The asymmetric unit of the structure consists of
one [CH
3
N(H)N
2
]
+
cation and one [AsF
6
]
–
anion associated through hydrogen bonding (Figure
4.1). As already observed for [H
2
N
3
]
+
,[24] the methylamino diazonium ion is protonated at theα -
nitrogen atom of the azido group rather than theβ - orγ -nitrogen atoms. Whereas the free azide
anion is symmetric and linear with an N–N bond distance of about 1.186(4)
˚A,[38] covalent azides
are more polarized, exhibiting a shorter internal (N1–N2) and longer terminal (N2–N3) bond with
typical bond distances of 1.13
˚A and 1.25
˚A, respectively.[38] As a result of the protonation, the
observed N–N bond distances become more asymmetric with 1.278(2)
˚A for the internal N–N
bond and 1.102(2)
˚A for the terminal bond. Typical for a covalent azide,[38] the azido moiety
is slightly bent with a N–N–N angle of 175.9(1)°. The observed C,N distance of 1.479(2)
˚A is in
the usual range expected for a C–N single bond distance.[39] The observed geometry is also in
good agreement with the results from our calculation at the G3MP2 level of theory (Table 4.1).
The C–N bond shortens significantly when CF
3
is substituted for CH
3
. The N1–N2 bond shows
a much smaller change upon substitution of H by F. The N2–N3 bond is much shorter than the
N1–2 bond. The N3 group deviates from linearity by up to about 10°. The addition of a proton
slightly lengthens the N–C bond by 0.02
˚A for the CH
3
derivative but the same bond in the CF
3
derivative increases by about 0.08
˚A. The N1–N2 bond distances increase by almost 0.05
˚A in
both cases. At the same time the terminal N2–N3 bond distances decrease upon protonation. The
complete crystallographic details are given in the ESI.
96
Figure 4.1: Asymmetric unit in the crystal structure of [CH
3
N(H)N
2
][AsF
6
]. Hydrogen atom
positions were determined from the difference electron density map. Selected bond distances [
˚A]
and angles [°]: C1–N1 1.479(2), N1–N2 1.278(2), N2–N3 1.102(2), N1–F1 2.65(2), N1–F5 2.18(2),
C1–N1–N2 118.7(1), N1–N2–N3 175.9(1).
[CF
3
N(H)N
2
][AsF
6
] also crystallizes with four formula units per monoclinic unit cell in space
group P21/c with the [CF
3
N(H)N
2
]
+
cations and [AsF
6
]
–
anions being associated through hy-
drogen bonds (Figure 4.2). The trifluoromethylamino diazonium ion is again protonated at the
α -nitrogen atom, and the internal and terminal N–N and C–N bond distances are similar to those
in [CH
3
N(H)N
2
][AsF
6
]. However, the azido group is virtually linear with a N–N–N bond angle of
179.3(4)°.
We have also calculated the proton affinities of these methyl azides at the G3MP2 level (see
Table 4.2). Protonation at theα -position relative to protonation at theγ -position is favored by
18 kcal mol
–1
for CH
3
N
3
and by 21 kcal mol
–1
for CF
3
N
3
. The value for the CF
3
-substituted com-
pound is like that for H
2
O whereas the value for the CH
3
-substituted compound is 14 kcal mol
–1
less than that of NH
3
.
97
Table4.1: Geometries from G3MP2 (MP2(FULL)/6-31G(d)), bond lengths in
˚A, angles in degrees.
Geometry parameter MeN
3
[MeN(H)N
2
]
+
CF
3
N
3
[CF
3
N(H)N
2
]
+
N–C 1.474 1.494 1.425 1.509
C–H
avg
1.092 1.090 ;
C–F
avg
1.343 1.315
N–H 1.026 1.030
N1–N 1.243 1.290 1.252 1.299
N–N 1.163 1.141 1.156 1.138
N1–N–N 172.5 174.7 171.6 174.5
C–N1–N 115.5 117.6 113.8 115.2
H–N1–C 120.9 118.6
Table4.2: Proton affinities at G3MP2 level in kcal mol
–1
.
Protonated azide PA (298 K)
[MeN(H)N
2
]
+
190.1
[MeN
2
N(H)]
+
172.4
[CF
3
N(H)N
2
]
+
167.9
[CF
3
N
2
N(H)]
+
146.8
Table4.3: Reaction energies in kcal mol
–1
.
B3LYP/aug-cc-pVDZ G4MP2-6X
Reaction ∆ H
298 K
∆ G
298 K
∆ H
298 K
∆ G
298 K
MeN
3
+ AsF
5
(D
3
h)−−→ MeN
3
· AsF
5
-15.0 -3.9 -18.0 -7.2
CF
3
N
3
+ AsF
5
(D
3
h)−−→ CF
3
N
3
· AsF
5
-3.0 7.5 -7.0 3.2
MeN
3
H
+
+ AsF
6
–
−−→ [MeN
3
H][AsF
6
] -99.1 -90.5 -100.0 -93.0
CF
3
N
3
H
+
+ AsF
6
–
−−→ [CF
3
N
3
H][AsF
6
] -112.6 -105.6 -112.6 -105.8
A set of reaction energies is given in Table 4.3. Addition of AsF
5
to both azides is exothermic
for∆ H
298 K
but is endothermic for∆ G
298 K
for R = CF
3
. The density-functional theory (DFT) B3LYP
values are in semi-quantitative agreement with the more accurate G4MP2-6X values.
98
Figure 4.2: Asymmetric unit in the crystal structure of [CF
3
N(H)N
2
][AsF
6
]. Hydrogen atom
positions were determined from the difference electron density map. Selected bond distances [
˚A]
and angles [°]: C1–N1 1.478(5), N1–N2 1.276(5), N2–N3 1.103(5), N1–F4 2.657(4), N1–F7 3.091(5),
C1–N1–N2 117.6(3), N1–N2–N3 179.3(4).
The cation-anion interaction between the protonated azide and [AsF
6
]
–
is highly exothermic,
as expected. The geometries for the isolated ion pair differ from those in the crystal. In the crystal,
two F atoms from [AsF
6
]
–
bind to the proton on the azide (Figure ). In the isolated ion pair for R
= CH
3
, one fluoride from [AsF
6
]
–
pulls the proton towards it, elongating both the N–H and As–F
bonds to generate a strong hydrogen bond. For R = CF
3
, a fluoride is essentially eliminated from
[AsF
6
]
–
and abstracts the proton from N1 to form an HF bridging AsF
5
and CF
3
N
3
. This likely
occurs because of the low proton affinity of CF
3
N
3
and does not occur for the methyl derivative
because of its much higher proton affinity. The crystal packing clearly is important in determining
the solid-state structure.
The CH
3
N
3
· AsF
5
adduct was refined as a three-component non-merohedral twin with ap-
proximate twin ratios of 0.4:0.4:0.2 in the triclinic space group P1. The twin domains 2 and 3
99
are related to domain 1 by 180° rotations around the rotation vectors –1,1,0 and 0,–1,0, respec-
tively. The resulting structure shows a 1:1 positional disorder in which one of the azidomethane
ligands is rotated by 180° around the As–N bond. The solid-state structure consists of isolated
MeN
3
· AsF
5
molecules (Figure 4.3). The closest intermolecular N–N, N–F, and C–F distances are
2.82(2)
˚A, 2.98(1)
˚A, and 3.15(2)
˚A, respectively. AsF
5
is coordinated through theα -nitrogen atom,
the most basic atom of the azido group. The azido group is slightly bent with a N–N–N angle of
177(2)°. The observed As–N bond distance of 1.97(2)
˚A is within the range of typical As–N single
bonds.[39, 40]
A comparison of the experimental and calculated geometries for CH
3
N
3
· AsF
5
(Table 4.4)
shows good agreement for the C,N,N,N-part of the molecule, but the N–As bond is predicted
to be too long and the axial As–F is predicted to be too short. Therefore, the calculations predict
the equatorial As–F bonds to be longer than the As–F axial bond, in contrast to experiment.
Table4.4: B3LYP/aug-cc-pVDZ optimized geometries; bond lengths in
˚A, angles in degrees.
MeN
3
· AsF
5
calc. MeN
3
· AsF
5
exp. CF
3
N
3
· AsF
5
calc.
N–As 2.114 1.97(2) 2.322
N–C 1.484 1.48(3) 1.457
As–F
eq (avg)
1.754 1.701(5) 1.743
As–F
ax
1.725 1.799(5) 1.715
N–N1 1.252 1.24(2) 1.262
N–N 1.130 1.11(2) 1.125
N–N–N 176.0 177(2) 173.7
N–N–As 117.5 116(1) 114.8
N–N–C 117.4 117(2) 113.8
100
Figure 4.3: Asymmetric unit in the crystal structure of CH
3
N
3
· AsF
5
. Hydrogen atom posi-
tions were determined from the difference electron density map. Selected bond distances [
˚A]
and angles [°]: C1–N1 1.48(3), N1–N2 1.24(2), N2–N3 1.11(2), As1–N1 1.97(2), As1–F1 1.799(5),
As1–F2 1.701(5), C1–N1–N2 117(2), N1–N2–N3 177(2), As1–N1–N2 116(1), F1–As1–N1 173.9(6),
F1–As1–F2 92.2(3).
4.3.3 NMRSpectroscopy
The NMR spectroscopic data for RN
3
, RN
3
· AsF
5
, and the protonated species [RN(H)N
2
]
+
(R = CH
3
,
CF
3
) are summarized in Tables table 4.6,table 4.5,??.
The
14
N NMR spectra of CF
3
N
3
displayed resonances typical for covalent azides at around
− 287 ppm (broad) for N1,− 150 ppm (sharp) for N2, and –145 ppm (broad) for N3, in agreement
with our expectations and the previously reported experimental and computational values.[41]
Compared to the azide anion, the N1 and N3 atoms are deshielded and the N2 atoms become
101
more shielded in the AsF
5
adducts. Except for the extreme case of FN
3
,[41] N1 is usually the
most shielded and gives rise to a very broad
14
N resonance, making its assignment unequivocal.
However, in the absence of reliable theoretical predictions and experimental
15
N-NMR data, the
assignment for the N1 and N3 nitrogen atoms can become more ambiguous when their chem-
ical shifts are similar, as in CF
3
N
3
· AsF
5
. In such cases, the
14
N line widths allow a distinction
because N
2
always has a considerably smaller line width than N3. In the
19
F-NMR spectra of the
CF
3
N
3
compounds, the fluorine atom resonances of the CF
3
group are observed as sharp singlets
in the range of− 60 to− 55 ppm and match the chemical shift reported for CF
3
N
3
.[42] The
13
C-
NMR spectra show the expected quartet with a chemical shift of about 120 ppm. The strongest
NMR spectroscopic evidence for the formation of the protonated species is found in the
1
J
C–F
coupling constant, measured either in the
13
C-NMR spectrum or by the
13
C satellites in the
19
F-
NMR spectrum, which changes from 266 Hz for CF
3
N
3
to 282 Hz for the [CF
3
N(H)N
2
]
+
cation.
The amino hydrogen atom on N1 was observed as a broad signal in the
1
H-NMR spectrum at
11.5 ppm in SO
2
solution. The protonation site of CF
3
N
3
in solution was confirmed using singly
15
N-labeled azidotrifluoromethane. After protonation of a mixture consisting of equal amounts
of CF
3
15
NNN and CF
3
NN
15
N with FSO
3
H/SbF
5
(1:1) at− 78°C, two NH group signals were ob-
served in the
1
H-NMR spectrum (Table 4.5). The NH resonance of [CF
3
15
N(H)NN]
+
is split into
a doublet by the coupling to the
15
N at the N1-position (
1
J
H–N
=106.9 Hz) whereas the NH reso-
nance of [CF
3
N(H)N
15
N]
+
remains a singlet. The two protonated species were also easily distin-
guishable by their splitting patterns in the
13
C and
19
F-NMR spectra as [CF
3
15
N(H)NN]
+
exhibits
additional doublet splittings. In the
15
N-NMR spectrum, the resonance of [CF
3
15
N(H)NN]
+
shows
a characteristic doublet of quartets, whereas the resonance of [CF
3
N(H)N
15
N]
+
remains a sin-
glet. These observations are unequivocal proof that also in solution CF
3
N
3
is protonated on N1.
102
This finding is in agreement with the observed protonation site from the solid-state structure of
[CF
3
N(H)N
2
][AsF
6
]. The observed
1
H,
13
C, and
14
N chemical shifts of CH
3
N
3
, CH
3
N
3
· AsF
5
and
[CH
3
N(H)N
2
]
+
are again similar and in good agreement with previously reported values (Table
4.7).[21] As in CF
3
N
3
, protonation results again in a pronounced shielding of N
2
from –132 ppm
to –155 ppm. Based on the observed
14
N-NMR chemical shifts of CH
3
N
3
in HF solution relative
to SO
2
solution and the absence of comparable shifts for CF
3
N
3
under the same conditions, we
conclude that CH
3
N
3
is protonated in aHF solution while the perfluorinated azide CF
3
N
3
remains
unprotonated.
We calculated the
1
H,
13
C,
14
N, and
19
F-NMR chemical shifts in the gas phase and in acetone
for RN
3
, their protonated species, and the AsF
5
complexes. For CH
3
N
3
containing molecules there
is a very good agreement between experimental and calculated chemical shifts. The protons of
the CH3 group are slightly deshielded by about 1 ppm from CH
3
N
3
to CH
3
N
3
· AsF
5
and protonated
[CH
3
N(H)N
2
]
+
. For CF
3
N
3
containing species there are larger differences in the chemical shifts
between the experimental and the calculated values, but overall there are similar trends. Thus,
the
13
C-NMR chemical shifts are calculated to be downfield by up to 15 ppm as compared to the
reported experimental values. The
19
F-NMR chemical shifts are calculated to be upfield by about
15 ppm for the CF
3
fluorine atoms and by 35 ppm for AsF
5
relative to the experimental values.
4.3.4 RamanSpectroscopy
In addition to multinuclear NMR spectroscopy, the RN
3
· AsF
5
adducts and [RN(H)N
2
][AsF
6
] salts
(R=CH
3
, CF
3
) were characterized by their low-temperature Raman spectra. The frequencies and
intensities of the observed vibrational bands are listed in the Supporting Information. The Raman
103
Table 4.5: Experimental NMR Data for
15
N-labeled CF
3
N
3
and the protonated species
[CF
3
N(H)N
2
]
+
.
[a]
CF
3
NN
15
N CF
3
15
NNN [CF
3
N(H)N
15
N]
+
[CF
3
15
N(H)NN]
+
δ (
1
H) 10.4 (s) 10.4 (d)
δ (
13
C) 121.9 (q) 121.9 (qd) 116.4 (q) 116.4 (qd)
δ (
15
N) –56.1 (s) –56.1 (d) –54.8 (s) –54.8 (d)
δ (
19
F) –145.4 –287.1(q) –93.2 (s) –272.2 (dq)
1
J
N–H
107
1
J
C–F
265.8 265.8 283.5 283.5
1
J
C–N
5.4 22.5
2
J
N–F
22.3 25.4
[a] NMR chemical shifts in ppm, coupling constants in Hz; measured
at –50
˙
°C.
104
Table 4.6: Experimental and calculated NMR data for CF
3
N
3
, the adduct CF
3
N
3
· AsF
5
, and the
protonated species [CF
3
N(H)N
2
]
+
.
[a,b]
CF
3
N
3
Solvent Gas calc [D6]acetone exp/calc SO
2
aHF
δ (
13
C) 135.8 121.9/137.3 123.6 122.3
δ (
14
N) N1 –300 –287/–314 –286 –288
N2 –157 –150/–171 –149 –151
N3 –144 –146/–171 –144 –143
δ (
19
F) –72.2 –55.6/–69.5 –55.9 –59.8
1
J
C–F
266 / – 266 266
CF
3
N
3
· AsF
5
Solvent Gas calc [D6]acetone calc SO
2
δ (
13
C) 133.5 134.9 122.5 (q)
δ (
14
N) N1 –270 –279 –279
N2 –162 –179 –154
N3 –100 –112 –131
δ (
19
F) –71.8 (CF
3
) –68.8 (CF
3
) –54.1 (CF
3
)
–12.2 (AsF
5
) –13.9 (AsF
5
) –48.4 (AsF
5
)
1
J
C–F
270
[CF
3
N(H)N
2
]
+
Solvent Gas calc [D6]acetone calc SO
2
ClF SO
2
δ (
1
H) 7.8 8.8 10.4 11.5
δ (
13
C) 130.2 131.0 116.3 (q) 118.6 (q)
δ (
14
N) N1 –275 –286 n. o. n. o.
N2 –169 –181 –174 –172
N3 –53 –78 n. o. n. o.
δ (
19
F) –68.4 –73.7 –54.8 –55.2
1
J
C–F
283 282
[a] NMR chemical shifts in ppm, coupling constants in Hz; n.o.
= not observed; measured at− 70°C. [b] Average values for
19
F-
NMR spectroscopy.
105
Table 4.7: Experimental and calculated NMR data for CH
3
N
3
, the adduct CH
3
N
3
· AsF
5
, and the
protonated species [CH
3
N(H)N
2
]
+
.
[a,b]
CH
3
N
3
Solvent Gas calc [D6]acetone exp/calc SO
2
aHF
δ (
1
H) CH
3
2.8 2.4/3.1 4.2 3.9
δ (
13
C) 38.5 35.6/39.2 37.9 36.6
δ (
14
N) N1 –331 –321/–344 –317 –308
N2 –138 –132/–150 –132 –155
N3 –171 –173/–193 –169 –108
CH
3
N
3
· AsF
5
Solvent Gas calc [D6]acetone calc SO
2
δ (
1
H) CH
3
3.3 3.5 4.7
δ (
13
C) 40.8 43.0 41.5
δ (
14
N) N1 –290 –293 –281
N2 –149 –165 –149
N3 –101 –113 –109
(
19
F) –30.9 –36.0 –50.9/-72.4
[CH
3
N(H)N
2
]
+
Solvent Gas calc [D6]acetone calc SO
2
ClF SO
2
δ (
1
H) CH
3
4.0 3.8 2.7 4.4
NH 6.3 7.2 n.o. 9.7
δ (
13
C) 43.5 43.8 36.4 39.0
δ (
14
N) N1 –340 –331 n. o. -308
N2 –163 –171 –156 –155
N3 –80 –107 n. o. -105
[a] NMR chemical shifts in ppm, coupling constants in Hz; n.o.
= not observed; measured at− 70
˙
°C. [b] Average values for
19
F-
NMR spectroscopy.
106
spectra of the Lewis adducts are dominated by bands because of the AsF
5
-moiety at about 730
and 680 cm
–1
, while the ones of the methylamino diazonium salts are dominated by bands due to
the [AsF
6
]
–
anion at about 710, 680, 570, and 370 cm
–1
.
The intenseν as
(N
3
) modes at about 2250-2230 cm
–1
are characteristic for covalent azido com-
pounds. As already observed for the IR spectra of CH
3
N
3
and CH
3
N
3
· SbCl
5
,[16] adduct formation
with a Lewis acid shifts theν as
(N
3
) mode to higher frequencies. For CF
3
N
3
, the adduct forma-
tion with AsF
5
shifts theν as
(N
3
) mode from 2198 cm
–1
to 2229 cm
–1
while for CH
3
N
3
, the mode is
shifted from 2098 cm
–1
to 2215 and 2185 cm
–1
. The splitting of some of the modes in CH
3
N
3
· AsF
5
into two bands might be due to the positional disorder of the azido group, shown in its X-ray
crystal structure. The protonation of RN
3
(R = CH
3
, CF
3
) and cation formation results in an even
stronger decrease in the electron density in the azido group than the adduct formation and further
increases the frequency of theν as
(N
3
) mode to 2251 cm
–1
in [CH
3
N(H)N
2
][AsF
6
] and 2278 cm
–1
in
[CF
3
N(H)N
2
][AsF
6
]. Also, the more electronegative CF
3
group results in higherν as
(N
3
) frequen-
cies than the CH
3
group.
The vibrational assignments are supported by DFT/B3LYP calculations. The most intense
bands in the Raman spectra for the CH
3
N
3
compounds are due to the C – H stretches above
3000 cm
–1
. The azido moiety gives intense Raman modes at 2233 cm
–1
and 2287 cm
–1
for CH
3
N
3
and CF
3
N
3
, respectively. Formation of the AsF
5
adduct shifts these modes to 2291 cm
–1
and
2317 cm
–1
for CH
3
N
3
· AsF
5
and CF
3
N
3
· AsF
5
, respectively. For the protonated species, the cal-
culated azido group mode is shifted even more to 2348 cm
–1
and 2371 cm
–1
for both cations,
[CH
3
N(H)N
2
]
+
and [CF
3
N(H)N
2
]
+
, respectively. The Raman spectra of the Lewis adducts
RN
3
· AsF
5
(R=CH
3
, CF
3
) show intense bands for AsF
5
, thus, the As-F stretching modes are assigned
to 635 and 652 cm
–1
for CH
3
N
3
· AsF
5
and CF
3
N
3
· AsF
5
, respectively. Lower intensity Raman bands
107
resulting from the antisymmetric As-F stretching modes are calculated to be at 599 and 585 cm
–1
for CH
3
N
3
· AsF
5
and CF
3
N
3
· AsF
5
, respectively.
4.3.5 ElectrophilicAminationsofAromatics
As mentioned earlier, both amino and phenylamino diazonium ions were capable of effecting
Fridel—Crafts-type electrophilic amination of benzene and toluene, resulting in their respec-
tive protonated anilines in preparatively useful yields.[22, 23] We attempted a similar reaction
with the trifluoromethylamino diazonium ion with large excess of benzene as well as toluene (ca.
50 equivalents) at 0°C over a 48 hour period. After aqueous work-up and neutralization no re-
spective aniline reaction products were observed and unreacted CF
3
N
3
was detected by
19
F NMR
spectroscopy. It appears that the trifluoromethylamino diazonium is a very poor nitrogen elec-
trophile and is not capable of reacting with aromatics. We also reacted methylamino diazonium
ion with large excess of benzene (ca. 50 equivalents around 0°C). Analysis of the reaction mixture
did not indicate any protonated aniline product. However, the methylamino diazonium ion was
found to transform cleanly into the methylene immonium ion
∗
by a 1,2-hydrogen shift and loss
of nitrogen (characterized by the
13
C shift of 177.6 ppm, see in the Supporting Information for
the characterization of the decomposition product of methylamino diazonium ion). Under the
reaction conditions, the methylene immonium ion appears to be a poor carbon electrophile to
react with benzene.
∗
Protonated formaldhydeimine is also known as the Mannich salt. It is a poor carbon electrophile capable of
undergoing condensation reactions with electron-rich nucleophiles such as amines, enamines and enolates.
108
4.4 Conclusion
In conclusion, the aminodiazonium ions [CX
3
N(H)N
2
]
+
(X=H, F) have been isolated and struc-
turally identified for the first time as their [AsF
6
]
− salts. In SO
2
solution, CH
3
N
3
and CF
3
N
3
form
donor-acceptor adducts with AsF
5
. The crystal structure of the CH
3
N
3
· AsF
5
adduct has been ob-
tained, showing it to beα -nitrogen bridged. In aHF solution, the more basic CH
3
N
3
undergoes
protonation, forming an unstable [CH
3
N(H)N
2
]
+
[HF
2
· n HF]
− salt, which could not be isolated
because of rapid decomposition upon HF removal. By addition of the stronger Lewis acid AsF
5
,
this compound was converted into the more stable [AsF
6
]
− salt which could be isolated and
structurally characterized below –35°C. Direct protonation of CH
3
N
3
by HF/AsF
5
failed, consis-
tently resulting in explosions. However, forming the [CH
3
N(H)N
2
]
+
[HF
2
· nHF]
− salt first and
then adding AsF
5
to its aHF solution at low-temperatures allowed the synthesis of the [AsF
6
]
− salt. In the solid state, the [CX
3
N(H)N
2
]
+
[AsF
6
]
− salts (X = H, F) are stabilized by strong hydrogen
bonds between the hydrogen atom of the cation and two fluorine atoms of the anion. The salts
were further characterized by vibrational and multinuclear NMR spectroscopy. The electron-
withdrawing effects of AsF
5
and fluorine substitution in the methyl group result in an increased
triple-single bond polarity of the azido group in the azidomethanes, as evidenced by a shift of the
N
–
–
–
N stretching mode to higher frequencies. Protonation of the azidomethanes causes an even
stronger electron-withdrawing effect and polarization of the nitrogen-nitrogen bonds. Finally,
attempted Friedel-Crafts-type electrophilic amination of aromatics such as benzene and toluene
with trifluoromethyl and methylamino diazonium ions were, however, not successful.
109
SafetyDisclaimer
Caution! Low-molecular-weight organic azides are hazardous and potentially explosive sub-
stances that can decompose with the slightest input of external energy (e.g., pressure, impact,
friction, or heat).[43] Serious explosions involving methyl azide have been reported.[43–45]
Azidomethane, azidotrifluoromethane as well as the Lewis-adducts and protonated species de-
scribed in this work are energetic and potentially explosive. These compounds should be handled
on a small scale while taking appropriate safety measures, such as wearing face shields, leather
gloves and protective clothing, and working in a well-ventilated environment. Attempts to di-
rectly protonate CH
3
N
3
with MF
5
(M = As, Sb) and anhydrous HF (aHF) resulted in explosions
on several occasions. aHF, AsF
5
, and SbF
5
can cause severe burns and contact with the skin must
be avoided. These compounds need to be handled in a well-ventilated fume hood while taking
appropriate safety measures.
Acknowledgments
Z.B. and P.B. acknowledge financial support from the Academy of Sciences of the Czech Republic
(RVO: 61388963) and the Ministry of Education, Youth and Sports of the Czech Republic in the
program INTER-EXCELLENCE (LTAUSA18037). The computational work at UA was supported
by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences,
U.S. Department of Energy (DOE) under the DOE BES Catalysis Center Program by a subcontract
from Pacific Northwest National Laboratory. D. A. Dixon thanks the Robert Ramsay fund of The
University of Alabama for partial support. Support by the Loker Hydrocarbon Research Institute
is also gratefully acknowledged.
110
4.5 ExperimentalPart
Caution! Anhydrous HF (aHF), CF
3
SO
3
H, and FSO
3
H can cause severe burns and contact with
the skin must be avoided. Lewis-adducts and protonated species obtained from the studied azides
should be handled on a small scale while taking appropriate safety measures, such as wearing face
shields, leather gloves and protective clothing, and working in a well-ventilated environment.
4.5.1 MaterialsandApparatus
All reactions were carried out in Teflon-FEP ampules that were closed by stainless steel
valves. Volatile materials were handled in a grease-free Pyrex glass vacuum line equipped with
Kontes© HI-VAC© valves or stainless steel/Teflon-FEP vacuum lines.[46] Reaction vessels and
the stainless-steel vacuum line were passivated with ClF
3
prior to use. Non-volatile materials
were handled in the dry nitrogen atmosphere of a glove box. aHF (Galaxy Chemicals) was dried
by storage over BiF
5
.[47] AsF
5
was prepared from AsF
3
and F
2
.[48–50] SbF
5
(Ozark Mahoning)
was freshly distilled before use. Neat azidomethane and azidotrifluoromethane were prepared
according to literature procedures and purified by fractional condensation.[42, 51] Sulfur dioxide
(Matheson Tri-Gas) was dried by storage over CaH
2
.
15
N labeled azidotrifluoromethane was pre-
pared from (CH
3
)
3
SiCF
3
and singly labeled NaN
3
(Cambridge Isotope Laboratories Inc.)[52] All
solvents were dried over activated 3
˚A molecular sieves and stored under nitrogen. FSO
3
H was
freshly distilled before use. The NMR spectra were recorded on Bruker AM-500, Varian VNMRS-
400, Varian VNMRS-500, or Varian VNMRS-600 spectrometer. Spectra were externally referenced
to 25% tetramethylsilane in dichloromethane-d2 for
1
H and
13
C NMR spectra, to 80% CFCl
3
in
111
chloroform-d for
19
F NMR spectra and to neat nitromethane for
14
N and
15
N NMR spectra. Ra-
man spectra were recorded either directly in 9 mm Teflon-FEP ampules or J. Young NMR tubes in
the range 4000–80 cm
–1
on a Bruker Vertex 70/RAM II spectrophotometer, using a Nd-YAG laser
at 1064 nm.
4.5.2 CrystalStructureDetermination
Diffraction quality crystals were grown from aHF (CH
3
N(H)N
2
AsF
6
and CF
3
N(H)N
2
AsF
6
) or SO
2
(CH
3
N
3
· AsF
5
) solutions inside TeflonFEP ampules by slow evaporation of the solvent in a dy-
namic vacuum at− 78°C. The cold (− 78°C) FEP reactors were opened under a stream of dry
N
2
gas and the crystalline content was dropped into the trough of a low-temperature crystal-
mounting apparatus at –110°C. A glass fiber that was attached to a magnetic CrystalCapTM
and dipped into PFPE (perfluoropolyether) oil was used to mount the crystals on the goniome-
ter with a magnetic base. The single-crystal X-ray diffraction data were collected on a Bruker
SMART APEX DUO 3-circle platform diffractometer, equipped with an APEX II CCD detector,
using Mo Kα radiation (TRIUMPH curved-crystal monochromator) from a fine-focus tube. The
diffractometer was equipped with an Oxford Cryosystems Cryostream 700 apparatus for low-
temperature data collection. The frames were integrated using the SAINT algorithm to give the
hkl files corrected for Lp/decay.[53] The absorption correction was performed using the SAD-
ABS or TWINABS program.[54] The structures were solved by intrinsic phasing and refined on
F2 using the Bruker SHELXTL Software Package and ShelXle.[4Sheldrick_201, 55–57] All non-
hydrogen atoms were refined anisotropically. Hydrogen atom positions were determined from
112
the difference electron density map. ORTEP drawings were prepared using the Mercury CSD pro-
gram.[58] Further crystallographic details can be obtained from the Cambridge Crystallographic
Data Centre (CCDC, 12 Union Road, Cambridge CB21EZ, UK (Fax: (+44) 1223-336-033; e-mail:
deposit@ccdc.cam.ac.uk) on quoting the deposition no. CCDC 1949452-1949453, and 1949759.
4.5.3 ComputationalMethods
The geometries were optimized at the density functional theory (DFT)[59] level with the hybrid
B3LYP exchange-correlation functional[60, 61] and aug-cc-pVDZ as basis.[62–64] Vibrational fre-
quencies were calculated to show that the structures were minima and for comparison to the
experimental Raman spectra. The B3LYP optimized geometries were used as starting points for
composite correlated molecular orbital theory G3(MP2)[65] and G4(MP2)-6X[66] calculations.
Proton affinities of MeN
3
and CF
3
N
3
were calculated at G3MP2 level. The AsF
5
binding ener-
gies to MeN
3
and CF
3
N
3
were calculated at the G4(MP2)-6X level. All calculations were done
with Gaussian16.[67] The optimized B3LYP geometries were used to predict the
1
H-,
13
C-,
19
F,
and
14
N-NMR chemical shifts at the B3LYP level with a valence triple-ζ basis set with polariza-
tion functions (VTZP) from Ahlrichs and coworkers.[68] The nuclear magnetic shielding tensors
were calculated using the gauge-independent atomic orbital (GIAO) approach[69] implemented
in Gaussian16. The
1
H-,
13
C-,
19
F-, and
14
N-NMR chemical shifts are reported relative to their spe-
cific standard calculated at the same level: TMS, CFCl
3
, and MeNO
2
, respectively.
113
4.5.4 ExperimentalProcedures
4.5.4.1 PreparationofRN
3
· AsF
5
(R=CH
3
,CF
3
)
Anhydrous SO
2
(3.0 mL) was condensed at− 196°C into a FEP ampule containing a frozen sample
of RN
3
(1.0 mmol, 1.0 eq.). The mixture was warmed to− 64°C forming a clear solution. A sto-
ichiometric amount of AsF
5
(1.0 mmol, 1.0 eq.) was condensed into the ampule at− 196°C. The
mixture was re-warmed to− 64°C, kept at this temperature for 15 min and sporadically agitated
yielding a clear colorless solution. The volatile compounds were pumped off at − 64°C, leaving
behind a colorless solid.
CH
3
N
3
· AsF
5
1
H-NMR (SO
2
, unlocked,− 70
◦ C): δ = 4.69 (s, CH
3
) ppm.
13
C-NMR (SO
2
, unlocked,− 70
◦ C): δ = 41.5 (s,CH
3
) ppm.
14
N-NMR (SO
2
, unlocked,− 60
◦ C): δ = −109.0 (s, ∆ 1
⁄2 = 880 Hz, N
γ ), −149.2 (s, ∆ 1
⁄2 = 20 Hz, N
β ),
−281.1 (s,∆ 1
⁄2 = 1400 Hz, N
α ) ppm.
19
F-NMR (SO
2
, unlocked,− 60
◦ C): δ =−50.9 (s, 4F∆ 1
⁄2 = 575 Hz, AsF
4
F),−74.2 (s, 1F∆ 1
⁄2 = 540 Hz,
AsF
4
F) ppm.
Raman (− 90°C, 150 mW): ˜ ν (rel. intensity) = 3063.9 (0.8), 2984.4 (4.8), 2839.4 (0.6), 2214.8 (2.6),
2185.0 (0.9), 1443.8 (0.7), 1255.1 (0.3), 1149.6 (0.9), 708.5 (3.2), 686.4 (1.1), 655.3 (10.0), 639.0 (3.7),
464.8 (0.5), 379.1 (4.0), 369.7 (1.1), 352.2 (2.4), 338.9 (0.7), 325.3 (2.6), 274.2 (0.6), 227.8 (0.6), 211.6
(2.2), 148.4 (2.3) cm
–1
.
CF
3
N
3
· AsF
5
13
C-NMR (SO
2
, unlocked,− 60
◦ C): δ = 122.5 (q,
1
J
CF
= 270 Hz,CF
3
) ppm.
114
14
N-NMR (SO
2
, unlocked,− 70
◦ C): δ = −130.8 (s, ∆ 1
⁄2 = 230 Hz, N
γ ), −154.2 (s, ∆ 1
⁄2 = 22 Hz, N
β ),
−278.7 (s,∆ 1
⁄2 = 450 Hz, N
α ) ppm.
19
F-NMR (SO
2
, unlocked,− 60
◦ C): δ =−48.4 (s, 5F∆ 1
⁄2 = 475 Hz, AsF
5
),−54.1 (s, 3F, CF
3
) ppm.
Raman (− 90°C, 150 mW):˜ ν (rel. intensity) = 2228.7 (6.8), 1513.8 (0.7), 933.2 (0.1), 926.8 (0.8), 677.5
(10.0), 666.1 (0.4), 609.0 (1.6), 583.6 (0.8), 489.4 (1.0), 421.9 (0.9), 344.9 (0.8), 313.7 (1.4), 303.9 (0.8),
206.6 (1.5), 189.8 (3.0), 165.3 (2.3), 92.2 (7.5), 60.1 (2.5) cm
–1
.
4.5.4.2 PreparationofCH
3
N
3
· SbF
5
Anhydrous SO
2
(2.0 mL) and was condensed into a Teflon-FEP ampule containing a frozen sample
of SbF
5
(1.0 mmol, 1.0 eq.) at− 196°C. The mixture was warmed to− 64°C forming a clear solu-
tion. A stoichiometric amount of methyl azide (1.0 mmol, 1.0 eq.) was condensed into the ampule
at− 196°C. The mixture was re- warmed to− 64°C, kept at this temperature for 15 min and spo-
radically agitated yielding a clear colorless solution. The volatile compounds were pumped off at
− 64°C, leaving behind a colorless solid.
Raman (− 90 °C, 150 mW): ˜ ν (rel. intensity) = 3053.9 (0.6), 2981.5 (2.6), 2837.5 (0.4), 2207.0 (2.4),
1447.4 (0.6), 1423.8 (0.4), 1317.4 (1.1), 1149.8 (2.2), 1108.4 (0.5), 1083.3 (0.4), 937.2 (0.5), 665.2 (5.2),
645.3 (0.7), 627.7 (10.0), 591.2 (1.3), 538.8 (0.3), 381.2 (0.2), 357.9 (2.7), 287.0 (1.4), 267.3 (2.2), 231.5
(0.8), 182.7 (1.7), 128.0 (0.6) cm
–1
.
4.5.4.3 Preparationof[RN(H)N
2
][AsF
6
](R=CH
3
,CF
3
)
Anhydrous HF (2.0 mL) was condensed to a Teflon-FEP ampule containing frozen sample of RN
3
(1.0 mmol, 1.0 eq.) at− 196°C. The mixture was warmed to− 78°C to form a clear colorless solu-
tion. The solution was cooled to− 196°C and a stoichiometric amount of AsF
5
(1.0 mmol, 1.0 eq.)
115
was addedinvacuo. The mixture was warmed to− 78°C, kept at this temperature for 15 min and
carefully agitated. All volatile compounds were pumped off at − 78°C, leaving behind a colorless
solid.
[CH
3
N(H)N
2
][AsF
6
]
1
H-NMR (SO
2
, unlocked,− 70
◦ C): δ = 9.69 (s, 1H, NH) 4.37 (s, 3H, CH
3
) ppm.
13
C-NMR (SO
2
, unlocked,− 70
◦ C): δ = 39.0 (s,CH
3
) ppm.
14
N-NMR (SO
2
, unlocked,− 70
◦ C): δ = −105.1 (s, ∆ 1
⁄2 = 475 Hz, N
γ ), −154.7 (s, ∆ 1
⁄2 = 10 Hz, N
β ),
−308.0 (s,∆ 1
⁄2 = 1500 Hz, N
α ) ppm.
19
F-NMR (SO
2
, unlocked,− 70
◦ C): δ =−74.2 (s,∆ 1
⁄2 = 4700 Hz, AsF
6
) ppm.
Raman (− 90 °C, 150 mW): ˜ ν (rel. intensity) = 2987.6 (2.5), 2251.0 (3.0), 1439.8 (0.8), 1309.7 (0.9),
894.9 (1.4), 700.3 (2.0), 684.9 (10.0), 578.5 (2.1), 371.6 (4.3), 268.2 (1.8), 246.6 (0.3), 158.4 (0.5), 108.9
(7.8), 76.0 (1.0) cm
–1
.
[CF
3
N(H)N
2
][AsF
6
]
1
H-NMR (SO
2
, unlocked,− 70
◦ C): δ = 11.5 (s, 1H, NH) ppm.
13
C-NMR (SO
2
, unlocked,− 70
◦ C): δ = 118.6 (q,
1
J
CF
= 282 Hz,CF
3
) ppm.
14
N-NMR (SO
2
, unlocked,− 70
◦ C): δ = −93.2 (s, very broad, N
γ ), −172.5 (s,∆ 1
⁄2 = 25 Hz, N
β ) (N
α not observed) ppm.
19
F-NMR (SO
2
, unlocked,− 70
◦ C): δ =−55.2 (s, 3F, CF3)−71.4 (s, 6F∆ 1
⁄2 = 4900 Hz, AsF6), ppm.
Raman (− 90 °C, 150 mW): ˜ ν (rel. intensity) = 2277.5 (6.0), 1480.3 (0.6), 1213.7 (0.5), 1185.9 (0.8),
952.8 (0.6), 877.2 (4.4), 717.4 (0.8), 695.4 (10.0), 674.2 (4.6), 592.6 (1.3), 563.3 (0.6), 473.5 (2.3), 421.7
(0.3), 376.8 (5.2), 315.2 (0.5), 202.0 (3.7), 107.3 (7.2), 77.5 (2.4) cm
–1
.
116
4.5.4.4 Preparationof[CH
2
NH
2
][AsF
6
]
Anhydrous HF (2.0 mL) was condensed to a Teflon-FEP ampule containing frozen sample of MeN
3
(1.0 mmol, 1.0 eq.) at− 196°C. The mixture was warmed to 0°C to form a clear colorless solution.
The solution was cooled to− 196°C and a stoichiometric amount of AsF
5
(1.0 mmol, 1.0 eq.) was
added in vacuo. The mixture was warmed to 25°C, kept at this temperature for 60 min and care-
fully agitated. All volatile compounds were pumped off at − 78°C to 25°C, leaving behind a
colorless solid.
1
H-NMR (SO
2
, unlocked,− 60
◦ C): δ = 11.50 (tt,
1
J
NH
= 68 Hz,
3
J
HH
= 15 Hz, 2H, NH
2
) 9.39 (t,
3
J
HH
= 15 Hz, 2H, NH
2
) ppm.
13
C-NMR (SO
2
, unlocked,− 60
◦ C): δ = 177.6 (t,
1
J
CN
= 12 Hz,CN) ppm.
14
N-NMR (SO
2
, unlocked,− 60
◦ C): δ =−181.6 (t,
1
J
NH
= 68 Hz,NH
2
), ppm.
19
F-NMR (SO
2
, unlocked,− 60
◦ C): δ =−57.5 (s,∆ 1
⁄2 = 2710 Hz, AsF
6
), ppm.
Raman (− 90 °C, 100 mW): ˜ ν (rel. intensity) = 3049.4 (0.5), 2858.0 (0.5), 1709.1 (1.4), 1689.7 (0.5),
1547.8 (1.1), 1433.3 (3.4), 1332.1 (0.7), 687.0 (10.0), 580.0 (1.7), 569.1 (1.6), 371.8 (4.3), 293.9 (1.1)
cm
–1
.
4.5.4.5 Preparationandprotonationof
15
N-enrichedCF
3
N
3
A solution of
15
N enriched sodium azide (NaNN
15
N, 500 mg, 7.58 mmol, 1.03 eq.) in water (4 mL)
was added dropwise over 15 min to a solution of tosyl chloride (1402 mg, 7.36 mmol, 1.0 eq.) in
acetone (14.4 mL) at 0°C. The reaction was allowed to warm up to room temperature and was
stirred for 18 h. The acetone was removed under reduced pressure and the reaction mixture was
extracted with diethyl ether (2 x 7 mL). The combined organic layers were washed with water (2
117
x 7 mL), 5% NaHCO
3
(2 x 7 mL), water (2 x 7 mL), dried over MgSO
4
, filtered and concentrated
in vacuo.
15
N enriched tosyl azide (a 1:1 mixture of Tos
15
NNN and TosNN
15
N, 1.36 g) was ob-
tained as a colorless oil in 93% yield. In a glovebox, CsF (647 mg, 4.26 mmol) was weighed into
a 25 mL round bottom flask. Dry DMF (7 mL) was added under nitrogen and the mixture was
cooled to− 60°C while being stirred. A cold solution of TMSCF
3
(525 µL, 3.55 mmol) and
15
N
enriched TosN
3
(544µL, 3.55 mmol) in dry DMF (2 mL) was added dropwise over 10 min, and then
the reaction mixture was stirred at− 60°C to –30°C for 4 hours. The resulting mixture contained
15
N enriched azidotrifluoromethane (a 1:1 mixture of CF
3
15
NNN and CF
3
NN
15
N), fluoroform and
fluorotrimethylsilane which were condensed into an NMR tube in a liquid N
2
bath. Cold SO
2
ClF
(0.4 mL) was added at− 78°C and the NMR sample was measured on a Varian NMRS-600 spec-
trometer at –50°C. To the cold SO
2
ClF solution of the
15
N enriched CF
3
N
3
(0.4 mL), freshly pre-
pared magic acid (FSO
3
H/SbF
5
, 1:1, 0.1 mL) was added at− 78°C. The mixture was then shaken
gently to obtain a homogeneous solution which was analyzed on a Varian NMRS-400 spectrom-
eter at –75°C.
CF
3
NN
15
N
13
C-NMR (151 MHz, acetone – d
6
): δ = 121.9 (q,
1
J
CF
= 265.8 Hz,CF
3
) ppm.
15
N-NMR (61 MHz, CH
3
NO
2
): δ =−145.4 (s, N
γ ) ppm.
19
F-NMR (564 MHz, SO
2
ClF): δ =−56.1 (s, CF
3
) ppm.
CF
3
15
NNN
13
C-NMR (151 MHz, acetone – d
6
): δ = 121.9 (q,
1
J
CF
= 265.8 Hz,CF
3
) ppm.
15
N-NMR (61 MHz, CH
3
NO
2
): δ =−287.1 (q,
2
J
NF
= 22.3 Hz, N
α ) ppm.
19
F-NMR (564 MHz, SO
2
ClF): δ =−56.1 (d,
2
J
NF
= 21.3 Hz,CF3) ppm.
118
[CF
3
NHN
15
N]
+
1
H-NMR (400 MHz, acetone – d
6
): δ = 10.37 (s, NH) ppm.
13
C-NMR (100 MHz, acetone – d
6
): δ = 116.4 (q,
1
J
CF
= 283.5 Hz,CF
3
) ppm.
15
N-NMR (40 MHz, CH
3
NO
2
): δ =−93.2 (s, N
γ ) ppm.
19
F-NMR (376 MHz, SO
2
ClF): δ =−54.8 (s, CF3) ppm.
1
H-NMR (400 MHz, acetone – d
6
): δ = 10.37 (s, NH) ppm.
13
C-NMR (151 MHz, acetone – d
6
): δ = 116.4 (qd,
1
J
CF
= 283.5 Hz,
1
J
CN
= 22.7 Hz,CF
3
) ppm.
15
N-NMR (61 MHz, CH
3
NO
2
): δ =−272.2 (dq,
1
J
NH
= 106.2 Hz,
2
J
NF
= 25.4 Hz, N
α ) ppm.
19
F-NMR (564 MHz, SO
2
ClF): δ =−54.8 (d,
2
J
NF
= 25.6 Hz,CF
3
) ppm.
Additional experimental details are given in the electronic supporting information
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124
Chapter5
TheBinaryGroup4AzideAdducts[(bpy)Ti(N
3
)
4
],
[(phen)Ti(N
3
)
4
],[(bpy)
2
Zr(N
3
)
4
]
2
· bpy,and
[(bpy)
2
Hf(N
3
)
4
]
2
· bpy
Thischapterisbasedonthefollowingpublication:
Thomas Saal, Piyush Deokar, Karl O. Christe, Ralf Haiges “The Binary Group 4 Azide Adducts
[(bpy)Ti(N
3
)
4
], [(phen)Ti(N
3
)
4
], [(bpy)
2
Zr(N
3
)
4
]
2
· bpy, and [(bpy)
2
Hf(N
3
)
4
]
2
· bpy” , Eur. J. Inorg.
Chem. 2019, 2388-2391. 10.1002/ejic.201900334.
125
5.1 Abstract
The N-donor adducts of the group 4 tetraazide [(bpy)Ti(N
3
)
4
], [(phen)Ti(N
3
)
4
],
[(bpy)
2
Zr(N
3
)
4
]
2
· bpy and [(bpy)
2
Hf(N
3
)
4
]
2
· bpy were prepared from the corresponding metal fluo-
rides MF
4
and stoichiometric amounts of theN-donor by fluoride–azide exchange with Me
3
SiN
3
.
The novel azido adducts were characterized by their X-ray crystal structures, vibrational and
NMR spectra, and decomposition temperatures.
126
5.2 Introduction
Polyazides are highly energetic materials because each azidogroup adds about 70 kcal mol
–1
to the
energy content of amolecule. Azides and polyazides are interesting molecules that are consid-
ered viable candidates for high-energy-density materials (HEDM).[1–3] The amount of research
into azide containing compounds has increased rapidly and inorganic polyazido compounds have
been the focus of numerous experimental studies in recent years.[2, 4–12] Polyazido compounds
are typically difficult to synthesize as their highly endothermic nature renders them explosive
and very shock sensitive. Neutral metal polyazides can be stabilized through adduct formation
with N-donor ligands such as 2,2’-bipyridine (bpy) or 1,10-phenanthroline (phen).[13] This in-
creases the ionicity of the azido ligands andstrengthens the weak N
α –N
β bond, thereby raising
the activation energy barrier towards fatal N
2
elimination.[14]
Partially azido-substituted titanium and zirconium compounds have been reported more
than 30 years ago,[15–18] while the first binary titanium polyazides [Ti(N
3
)
4
], [Ti(N
3
)
5
]
–
and
[Ti(N
3
)
6
]
2 –
were prepared in 2004.[19, 20] More recently, the syntheses and crystal structures
of the binary zirconium and hafnium azides [PPh
4
]
2
[Zr(N
3
)
6
] and [PPh
4
]
2
[Hf(N
3
)
6
] were re-
ported.[21, 22] Based on quantum chemical calculations,[23] the metal tetraazides [M(N
3
)
4
] (M =
Ti, Zr, Hf) were predicted to exhibit tetrahedral structures with unique linear M–N–N bonds. Lin-
ear M–N–N coordination had previously been predicted also for [Nb(N
3
)
5
] and [Ta(N
3
)
5
],[24, 25]
and the first experimental evidence for a near-linear M–N–N coordination was provided by the
structural characterization of the acetonitrile adduct [(CH
3
CN)Nb(N
3
)
5
].[24, 25] Herein, we report
the synthesis of N-donor adducts with the binary titanium, zirconium and hafnium tetraazides,
[M(N
3
)
4
] (M = Ti, Zr, Hf), which were isolated and characterized by their X-ray crystal structures.
127
5.3 ResultsandDiscussion
5.3.1 Synthesis
By analogy with our synthesis of other binary metal azides,[26–34] TiF
4
was reacted at ambient
temperature with an excess of Me
3
SiN
3
and one equivalent of theN-donor ligands 2,2’-bipyridine
(bpy) and 1,10-phenanthroline (phen) in acetonitrile solution. This resulted in a rapid and quan-
titative fluoride–azide exchange and formation of yellow solutions of the corresponding titanium
tetraazide adducts [(bipy)Ti(N
3
)
4
] and [(phen)Ti(N
3
)
4
], respectively, [Equation 5.1].
TiF
4
+4Me
3
SiN
3
CH3CN
− −−−−− →
− 4Me3SiF
[Ti(N
3
)
4
] (5.1)
When TiF
4
was treated with Me
3
SiN
3
and two equivalents of 2,2’-bipyridine, mixtures of
[(bpy)Ti(N
3
)
4
] and uncoordinated 2,2’-bipyridine were obtained, see Equation 5.2. The adducts
[(bipy)Ti(N
3
)
4
] and [(phen)Ti(N
3
)
4
] were isolated from the reaction mixtures in quantitative yields
after the removal of all volatile compounds (Me
3
SiF, CH
3
CN and excess Me
3
SiN
3
) in vacuo. Sin-
gle crystals were obtained from acetonitrile solution through slow evaporation of the solvent.
[(bpy)Ti(N
3
)
4
] and [(phen)Ti(N
3
)
4
] are yellow to orange solids that show smooth decompositions
at 197°C and 165°C, respectively, as determined by DTA (5°/min heating rate).
TiF
4
+4Me
3
SiN
3
+
N
N
CH3CN
− −−−−− →
− 4Me3SiF
[Ti(N
3
)
4
]
N
N
=
N
N
bpy
N
N
phen
(5.2)
128
It is interesting to note that while the reaction of TiF
4
with an excess of Me
3
SiN
3
in CH
3
CN
without an N-donor resulted in the isolation of neat titanium tetraazide (Equation 5.2), no metal
azido compound was obtained when ZrF
4
and HfF
4
were treated with an excess of Me
3
SiN
3
in
acetonitrile with or without 2,2’-bipyridine. Instead, only unreacted starting material could be
recovered. This can be attributed to the insolubility of zirconium and hafnium fluoride in acetoni-
trile. The solubility problem was overcome by using the acetonitrile soluble hydrates, MF
4
· H
2
O
(M = Zr, Hf), which were obtained by dissolving MF
4
in 48% hydrofluoric acid, followed by the
removal of all volatiles in vacuo.[35, 36]
When ZrF
4
· H
2
O and HfF
4
· H
2
O were reacted in acetonitrile with an excess of Me
3
SiN
3
and 2.5
equivalents of 2,2’-bipyridine, clear colorless solutions of the zirconium and hafnium tetraazide
adducts, respectively, were obtained (Equation 5.3).
2MF
4
·H
2
O+12Me
3
SiN
3
+5bpy
CH3CN
− −−−−−−− →
− 8Me3SiF
− 4HN3
− 2(Me3Si)2O
[(bpy)
2
M(N
3
)
4
]
2
·bpy (M = Zr, Hf) (5.3)
Removal of the volatile compounds Me
3
SiF, CH
3
CN, HN
3
, (Me
3
Si)
2
O and excess Me
3
SiN
3
in
vacuo resulted in the isolation of colorless crystals of [(bpy)
2
M(N
3
)
4
]
2
· bpy (M = Zr, Hf). When
ZrF
4
· H
2
O and [HfF
4
]· H
2
O were treated with an excess of Me
3
SiN
3
and less than 2.5 equivalents
of 2,2’-bipyridine, only mixtures of [(bpy)
2
M(N
3
)
4
]
2
· bpy (M = Zr, Hf) and unreacted MF
4
(M = Zr,
Hf) were isolated. The adducts [(bpy)
2
Zr(N
3
)
4
]
2
· bpy and [(bpy)
2
Hf(N
3
)
4
]
2
· bpy are room tempera-
ture stable, colorless solids that show smooth decompositions with onset temperatures of 170°C
and 241°C, respectively. The metal tetraazide adducts were identified and characterized by their
129
crystal structures, vibrational and
14
N NMR spectra as well as the observed material balances. The
experimental vibrational frequencies and intensities are given in the experimental section. Ele-
mental analyses were not established for the azides due to the danger of their potential explosive
nature. Instead, the purity of the compounds was established by a material balance.
5.3.2 X-rayCrystallography
The X-ray crystal structures of [(bpy)Ti(N
3
)
4
] and [(phen)Ti(N
3
)
4
] reveal the presence of isolated
molecules (Figure 5.1). The closest intermolecular N···N distances are 2.970(2)
˚A and 3.116(1)
˚A,
respectively. [(bpy)Ti(N
3
)
4
] crystallizes with two symmetry independent molecules as asymmet-
ric unit in the triclinic unit cell of space groupP1 (Z = 4,Z’ = 2). The 1,10-phenanthroline adduct
[(phen)Ti(N
3
)
4
] crystallizes in space groupP1 as well but with just one molecule per asymmetric
unit (Z = 2).
The structures of both symmetry independent [(bpy)Ti(N
3
)
4
] molecules are similar and de-
rived from a pseudo octahedral geometry with the bipyridine ligand and two azido groups in the
equatorial positions and the remaining two azido groups occupying the axial positions (Figure
5.1A). The major difference between both structures and most interesting feature of the structure
is the orientation of one equatorial azido ligand. In molecule 1 (Ti1) only one of the equatorial
ligands features the typical bent coordination geometry of azido ligands with an Ti–N–N angle
of 134.7(1). The second equatorial azido group shows a more linear geometry with a consider-
ably larger Ti–N–N angle of 163.3(1). Both equatorial azido ligands of molecule 2 (Ti2) show
the typical bent coordination geometry with Ti–N–N angles of 129.2(1) and 141.3(1). The axial
azido groups of both molecules are bent with Ti–N–N angles between 131.3(1) and 138.7(1). The
130
Figure 5.1: Crystal structures of (A) the two independent molecules of [(bpy)Ti(N
3
)
4
] and (B)
[(phen)Ti(N
3
)
4
]. Thermal ellipsoids are drawn at the 50% probability level and hydrogen atoms
have been omitted for clarity. Selected bond lengths [
˚A] and angles [°] (A): Ti1–N1 1.984(1),
Ti1–N6 1.912(1), Ti1–N7 1.954(1), Ti1–N10 2.005(1); Ti2–N17 1.969(2), Ti2–N20 1.935(1), Ti2–N23
1.975(1), Ti2–N26 1.995(1), Ti1–N4–N5 134.7(1) Ti1–N6–N5 163.3(1); (B): Ti1–N1 2.025(1), Ti1–N4
1.943(1), Ti1–N7 1.943(1), Ti1–N10 1.978(1), Ti1–N13 2.213(1), Ti1–N14 2.221(1), Ti1–N4–N5
138.5(1), Ti1–N7–N8 137.4(1).
131
Ti–Nazide distances show slight variations between axial and equatorial ligands. In molecule 1,
the average Ti–Nazide distance is found to 1.933(2)
˚A for the equatorial ligands and 1.995(2)
˚A
for the axial ligands. In molecule 2, the average Ti–Nazide distances for the equatorial and ax-
ial ligands are 1.955(2)
˚A and 1.982(2)
˚A, respectively. The average Ti–Nbpy distances for both
molecules are 2.217(1)
˚A and 2.203(1)
˚A, respectively. The average internal and terminal N–N
distances of the azido ligands for both molecules of 1.212(3)
˚A and 1.141(3)
˚A are typical values
for covalent azido groups. The structure of [(phen)Ti(N
3
)
4
] is derived from a pseudo octahedral
ligand arrangement around the Ti atom and closely resembles that of the second [(bpy)Ti(N
3
)
4
]
molecule (Figure 5.1B). The average equatorial and axial Ti–N–N angles are 138.0(1) and 130.2(1),
respectively. The average Ti–Nazide distance is 1.972(2)
˚A and the average Ti–Nphen distance is
2.217(1)
˚A. The average internal and terminal N–N distances in the azido ligands are 1.206(2)
˚A
and 1.143(2)
˚A.
Both [(bpy)
2
M(N
3
)
4
]
2
· bpy adducts crystallize with a triclinic unit cell of space groupP1 (Fig-
ure 5.2). The solid-state structures consist of isolated and well separated [(bpy)
2
M(N
3
)
4
] units
and uncoordinated 2,2’-bipyridine molecules in a 2:1 molar ratio. The shortest intermolecular
N
azide
–N
azide
distance is 3.238(6)
˚A in the zirconium compound and 3.223(4)
˚A in the hafnium
compound. The asymmetric unit of both structures consists of a full [(bpy)
2
M(N
3
)
4
] molecule
and half a 2,2’-bipyridine molecule. The full bipyridine molecule is generated through sym-
metry (operation –x, –y, –z). The structures of the [(bpy)
2
M(N
3
)
4
] (M = Zr, Hf) molecules
in the crystal structures closely resemble those of the cations in [(bpy)
2
Nb(N
3
)
4
][Nb(N
3
)
6
] and
[(bpy)
2
Ta(N
3
)
4
][Ta(N
3
)
6
].[26–34] The central metal atom is coordinated by a total of four azido
ligands as well as four N-atoms from the two two-coordinated 2,2’-bipyridine ligands. This results
in a pseudo square anti-prismatic coordination environment around the metal atom in which each
132
Figure 5.2: Crystal structures of [(bpy)
2
M(N
3
)
4
]
2
· bpy, (A) M = Hf and (B) M = Zr. Thermal
ellipsoids are drawn at the 50% probability level and hydrogen atoms have been omitted for clar-
ity. Selected bond lengths [
˚A] (A): Hf–N1 2.151(3), Hf1–N4 2.160(2), Hf1–N7 2.129(3), Hf1–N10
2.129(3), Hf1–N13 2.490(2), Hf1–N14 2.487(2), Hf1–N15 2.459(3), Hf1–N16 2.460(2); (B): Zr1–N1
2.180(5), Zr1–N4 2.168(5), Zr1–N7 2.182(5), Zr1–N10 2.156(4), Zr1–N13 2.488(7), Zr1–N14 2.485(6),
Zr1–N15 2.516(5), Zr1–N16 2.519(5).
133
of the squares is composed of two azido ligands and two N-atoms from two different bpy ligands.
Both squares are elongated along the N
bpy
–N
bpy
diagonal. In the [(bpy)
2
M(N
3
)
4
] molecule, the
average Zr–N
azide
distance of 2.172(5)
˚A is larger than in [Zr(N
3
)
6
]
2–
2.150(3)
˚A.[21, 22] The av-
erage Zr–N
bpy
distance of 2.502(8)
˚A is in good agreement with typical Zr–N distances reported
in the literature.[37–39] The average Zr–N–N angle of 135.9(2) and average N–N distances of
the azido ligands of 1.211(6)
˚A (internal) and 1.127(6)
˚A (terminal) are typical values for covalent
azido compounds. The average metal-nitrogen distance for the azido ligands in [(bpy)
2
Hf(N
3
)
4
] is
0.024(5)
˚A shorter than in [(bpy)
2
Zr(N
3
)
4
] but in good agreement with the Hf–N
azide
distances in
the [Hf(N
3
)
6
]
2–
anion[21, 22] and with typical Hf–N distances reported in the literature.[40–43]
A similar trend is observed for the average Hf–N
bpy
distance which is 0.028(5)
˚A shorter than
the average Zr–N
bpy
distance in [(bpy)
2
Zr(N
3
)
4
]. A slight decrease of M–N distances of hafnium
over zirconium has also been observed for the related hexaazidometallates.[21, 22] The average
Hf–N–N angle of 136.3(2) and average N–N distances in the azido ligands of 1.205(4)
˚A (internal)
and 1.148(4)
˚A (terminal) are again typical values for covalent azido compounds.
5.3.3 Spectroscopy
The observed vibrational spectra of the tetraazide donor–acceptor complexes of titanium, zirco-
nium and hafnium are listed in the supplementary information and support the presence of co-
valent azides. The mid–IR spectra of the compounds are dominated by bands due to theν as
(N
3
)
vibration modes at about 2000–2200 cm
–1
. The remaining large number of observed IR absorption
bands are primarily due to vibration modes of the organic ligand moieties. The strong bands of
134
theν as
(N
3
) modes in the region 2000–2200 cm
–1
are the dominating features in the Raman spec-
tra and are characteristic for the presence of covalently bound azido groups. The much weaker
bands of theν s
(N
3
) modes are observed at 1200–1300 cm
–1
and overlap with Raman bands of the
organic donor molecules. The M–N azide stretching modes are observed at about 470–350 cm
–1
.
The presence of covalent azides was also confirmed by the
14
N NMR spectra. Solutions of all com-
pounds in DMSO – d
6
exhibited broad resonances at aboutδ = —135 to —150 ppm (∆ 1/2 = 100 Hz)
andδ = —250 to —270 ppm (∆ 1/2 = 280 Hz) for N
β and N
α of the azido ligands. Resonances for N
textsubscripttextgamma were not observed due to signal broadening.
5.4 Conclusion
Novel adducts of titanium, zirconium, and hafnium tetraazides with N-donor ligands were
prepared from the corresponding metal fluorides and stoichiometric amounts of the donor by
fluoride–azide exchange with Me
3
SiN
3
. The interesting feature of the 2,2’-bipyridine adduct
[(bpy)Ti(N
3
)
4
] is that it crystallizes with two symmetry independent molecules in the asym-
metric unit. One of the molecules features azido ligands with the typical bent Ti–N–N bond
angles of 130–142° while the second molecule exhibits one azido group with a rare more lin-
ear Ti–N–N angle of 161°. The coordination geometry around the central Ti atom of both
molecules of [(bpy)Ti(N
3
)
4
] and the 1,10-phenanthroline adduct [(phen)Ti(N
3
)
4
] is pseudo octa-
hedral. With excess 2,2’-bipyridine zirconium and hafnium tetraazide form the octacoordinated
adducts [(bpy)
2
M(N
3
)
4
] which crystallize with half a molecule of uncoordinated 2,2’-bipyridine
per azido molecule. Attempts to prepare the 1:1 Zr and Hf adducts resulted in mixtures of the
corresponding 2:1 adduct with unreacted metal fluoride.
135
Acknowledgements
We thank the Hydrocarbon Research Foundation for financial support and Prof. G. K. S. Prakash,
Drs W. Wilson, R. Wagner, P. Deokar, and M. Wiesemann for their help and stimulating discus-
sions.
136
5.5 ExperimentalPart
Caution! Polyazides are extremely shock-sensitive and can explode violently upon the slightest
provocation. Because of the high energy content and the high detonation velocity of these azides,
their explosions are particularly violent and can cause, even on a one mmol scale, significant
damage. The use of appropriate safety precautions (safety shields, face shields, leather gloves,
protective clothing, such as heavy leather welding suits and ear plugs) is mandatory. Care should
be taken during handling of such compounds.Ignoringsafetyprecautionscanleadtoserious
injuries![44, 45]
5.5.1 MaterialsandApparatus
All reactions were carried out in Teflon-FEP ampules that were closed by stainless steel valves.
Volatile materials were handled in a grease-free Pyrex glass vacuum line equipped with Kontes©
HI-VAC© valves. Non-volatile materials were handled in the dry nitrogen atmosphere of a glove
box. Me
3
SiN
3
(95%, Sigma Aldrich) was purified by fractional condensation. Acetonitrile (Spec-
trum) was distilled from P
2
O
5
, stored over molecular sieves and freshly distilled prior to each use.
TiF
4
, ZrF
4
, HfF
4
, 2,2’-bipyridine, and 1,10-phenanthroline (all Sigma Aldrich) were used with-
out further purification. The NMR spectra were recorded at 298 K on Bruker AMX–500 or Var-
ian VNMRS–500 spectrometer. Spectra were externally referenced to neat nitromethane for the
14
N NMR spectra. Raman spectra were recorded either directly in 9 mm Teflon-FEP ampules or J.
Young NMR tubes in the range 3600–80 cm
–1
on a Bruker Vertex 70/RAM II spectrophotometer,
using a Nd-YAG laser at 1064 nm. Infrared spectra were recorded in the range 4000–400 cm
–1
on
Bruker Alpha or Bruker Tensor FT-IR spectrometers using KBr pellets. The pellets were prepared
137
inside the glove box using an Econo Press (Thermo Scientific) and transferred in a closed con-
tainer to the spectrometer before placing them quickly into the sample compartment, which was
purged with dry nitrogen to minimize exposure to atmospheric moisture and potential hydroly-
sis of the sample. Grinding of the friction sensitive neat polyazides must be avoided. The azides
were added to the finely powdered KBr and blended into the KBr using a non-metallic spatula.
DTA curves were recorded with a purge of dry nitrogen gas on an OZM Research DTA552-Ex
instrument with the Meavy 2.2.0 software. The heating rate was 5°C/min and the sample size
was 5–25 mg.
5.5.2 CrystalStructureDetermination
The single-crystal X-ray diffraction data were collected on a Bruker SMART APEX DUO diffrac-
tometer, equipped with an APEX II CCD detector, using Mo-Kα radiation. The frames were inte-
grated using the SAINT algorithm,[46] a semi-empirical absorption correction was applied,[47]
and the structures solved by intrinsic phasing[48] and refined on F2.[48–50]. All non-hydrogen
atoms were refined anisotropically. Drawings were prepared using the CSD Mercury pro-
gram.[51]
5.5.3 ExperimentalProcedures
5.5.3.1 Preparationof[(bipy)Ti(N
3
)
4
]
A sample of TiF
4
(50 mg; 0.40 mmol) and 2,2’-bipyridine (62 mg, 0.40 mmol) was loaded into a
Teflon-FEP ampule, followed by the addition of Me
3
SiN
3
(144 mg, 1.60 mmol) and CH
3
CN (2 mL)
invacuo at –196°C. The mixture was allowed to warm to ambient temperature and agitated. After
138
8 h, a yellow solution with some yellow precipitate was obtained. The solution was cooled to –
20°C and the solvent slowly pumped off, leaving behind a crystalline yellow solid, [(bipy)Ti(N
3
)
4
]
(144 mg; weight expected for 0.40 mmol [(bipy)Ti(N
3
)
4
]: 149 mg). Material balance: Me
3
SiN
3
:
138 mg (1.20 mmol); Me
3
SiF: 37 mg (0.40 mmol).
DTA: 197°C (onset, exotherm, decomposition).
14
N-NMR (DMSO – d
6
): δ =−133 (s,∆ 1
⁄2 = 100 Hz, N
β ),−262 (s,∆ 1
⁄2 = 292 Hz, N
α ).
IR (KBr): ˜ ν = 3374 (s), 3283 (vw), 3205 (vw), 3110 (w), 3077 (w), 3034 (vw), 2982 (vw), 2954 (vw),
2924 (w), 2854 (vw), 2650 (w), 2113 (vs), 2053 (vs), 2002 (vs), 1871 (vw), 1637 (m), 1600 (vs), 1561
(m), 1543 (vw), 1526 (vw), 1492 (m), 1472 (s), 1440 (vs), 1388 (w), 1331 (vs), 1313 (vs), 1243 (w),
1157 (m), 1106 (s), 1060 (m), 1024 (vs), 981 (vw), 901 (vw), 844 (vw), 806 (w), 768 (vs), 731 (s), 693
(vw), 657 (s), 636 (w), 612 (vs), 586 (m), 528 (m), 444 (vs) cm
–1
.
Raman (25°C, 35 mW): ˜ ν (rel. intensity) = 3089.8 (0.4), 3077.3 (0.7), 2111.6 (10.0), 2082.5 (1.0),
2060.0 (1.8), 1609.1 (1.1), 1600.8 (6.2), 1564.4 (2.8), 1495.8 (2.2), 1435.4 (0.3), 1426.0 (0.3), 1337.3
(0.7), 1318.5 (5.2), 1268.9 (1.2), 1244.6 (0.3), 1161.3 (0.7), 1062.7 (1.0), 1045.1 (0.4), 1025.2 (6.1), 768.1
(1.1), 614.5 (1.2), 477.3 (0.8), 441.9 (8.7), 378.9 (3.0), 284.2 (1.0), 252.3 (1.4), 235.9 (1.7), 196.7 (2.5),
172.6 (2.9) cm
–1
.
5.5.3.2 Preparationof[(phen)Ti(N
3
)
4
]
A sample of TiF
4
(123 mg; 1.00 mmol) and 1,10-phenanthroline (180 mg, 1.00 mmol) was loaded
into a Teflon-FEP ampule, followed by the addition of Me
3
SiN
3
(461 mg, 4 mmol) and CH
3
CN
(2 mL) in vacuo at –196°C. The mixture was allowed to warm to ambient temperature and agi-
tated. After 8 h, a yellow solution with some yellow precipitate was obtained. The solution was
139
cooled to –20°C and the solvent slowly pumped off, leaving behind a crystalline yellow solid,
[(phen)Ti(N
3
)
4
] (365 mg; weight expected for 1 mmol [(phen)Ti(N
3
)
4
]: 396 mg). Material balance:
Me
3
SiN
3
: 339 mg (2.95 mmol); Me
3
SiF: 90 mg (0.98 mmol).
DTA: 165°C (onset, exotherm, decomposition).
14
N-NMR (DMSO – d
6
): δ =−134 (s,∆ 1
⁄2 = 40 Hz, N
β ),−164 (s,∆ 1
⁄2 = 100 Hz, N
γ ),−255 (s,∆ 1
⁄2 = 295
Hz, N
α ).
IR (KBr): ˜ ν = 3375 (w), 3086 (vw), 3068 (vw), 2659 (m), 2109 (vw), 2051 (vs), 1651 (vw), 1628 (vw),
1603 (vw), 1579 (w), 1519 (m), 1492 (w), 1422 (s), 1343 (vs), 1303 (vw), 1251 (vw), 1221 (w), 1142
(w), 1105 (vw), 1057 (vw), 998 (vw), 964 (w), 906 (vw), 870 (w), 845 (s), 805 (vw), 769 (vw), 736 (w),
723 (s), 674 (vw), 648 (m), 619 (s), 597 (w), 583 (w), 557 (vw), 508 (vw), 444 (vs) cm
–1
.
Raman (25°C, 35 mW):˜ ν (rel. intensity) = 3088.4 (0.4), 3072.4 (0.8), 2111.4 (10), 2068.9 (1.6), 2061.2
(1.3), 1629.4 (1.7), 1605.5 (1.8), 1587.1 (1.2), 1580.3 (1.2), 1519.7 (0.9), 1456.7 (7.2), 1429.5 (5.3), 1338.6
(0.7), 1302.8 (2.6), 1253.6 (0.9), 1198.9 (0.4), 1145.7 (0.3), 1107.9 (0.4), 1058.4 (2), 956.3 (0.3), 871.7
(0.7), 737 (2.9), 619 (0.8), 598.7 (1.1), 558 (0.5), 487.9 (0.7), 470.3 (2.2), 457.3 (5.8), 427.8 (5.9), 366.8
(1.9), 295.4 (2), 256.4 (2.3), 241 (2.4), 191.2 (3.7), 165.2 (6.4), 132.1 (9.2) cm
–1
.
5.5.3.3 Preparationof[(bpy)
2
Zr(N
3
)
4
]
2
· bpy
A sample of ZrF
4
(62 mg; 0.37 mmol) was loaded into a Teflon-FEP ampule and 5 mL of hydroflu-
oric acid (48%) were added. After stirring for 48 hours at ambient temperature a clear solution
was obtained. The reaction mixture was pumped to dryness leaving behind a pale-yellow powder
(110 mg, weight expected for 0.37 mmol if ZrF
4
· HF· 6 H
2
O: 109 mg). A solution of 2,2’-bpyridine
(172 mg, 1.1 mmol) in CH
3
CN (2.0 mL) was added, followed by the addition of Me
3
SiN
3
(1.382 g,
140
12 mmol) in vacuo at –196°C. The mixture was allowed to warm to ambient temperature and
agitated. After 48 h, a solution with a pale pink precipitate was obtained. All volatile materi-
als were removed in vacuo, first at –20 °C and then later at ambient temperature, leaving be-
hind a crystalline pale pink solid, [(bpy)
2
Zr(N
3
)
4
· bpy] (253 mg; weight expected for 0.19 mmol
[(bpy)
2
Zr(N
3
)
4
]
2
· bpy: 241 mg). Material balance: Me
3
SiF: 166 mg (1.80 mmol).
DTA: 65–70°C (endotherm, melting of bpy), 170°C (onset, exotherm, decomposition).
14
N-NMR (DMSO – d
6
): δ =−135 (s,∆ 1
⁄2 = 92 Hz, N
β ),−255 (s,∆ 1
⁄2 = 260 Hz, N
α ).
IR (KBr): ˜ ν = 3425 (vs), 3119 (s), 3098 (s), 3055 (s), 3008 (m), 2695 (vw), 2322 (vw), 2293 (vw), 2088
(vs), 2000 (m), 1966 (w), 1894 (vw), 1870 (vw), 1799 (vw), 1643 (m), 1600 (m), 1580 (vs), 1558 (s),
1495 (vw), 1473 (w), 1455 (s), 1438 (vs), 1421 (s), 1363 (vs), 1313 (m), 1280 (w), 1250 (m), 1224 (vw),
1179 (m), 1160 (s), 1140 (vw), 1089 (m), 1067 (m), 1040 (m), 1013 (s), 993 (w), 894 (w), 810 (vw), 769
(vs), 738 (m), 648 (m), 628 (s), 602 (vw), 523 (m) cm
–1
.
Raman (25°C, 35 mW):˜ ν (rel. intensity) = 3088.8 (1.0), 3077.2 (1.2), 3064.2 (1.3), 3010.2 (0.6), 2106.4
(1.5), 1599.4 (7.2), 1590.6 (8.4), 1573.2 (8.8), 1567.3 (4.9), 1493.5 (2.9), 1483.3 (3.0), 1447.6 (4.9), 1372.5
(2.6), 1364.3 (1.8), 1356.6 (1.3), 1314.1 (7.7), 1302.6 (3.4), 1260.0 (0.9), 1237.4 (3.7), 1159.7 (0.8), 1147.1
(1.2), 1093.9 (0.7), 1069.1 (2.4), 1045.6 (1.6), 1013.0 (6.1), 995.3 (10.0), 814.7 (0.7), 765.4 (3.0), 647.3
(0.7), 625.6 (0.8), 614.9 (1.2), 437.8 (0.7), 374.6 (2.0), 350.3 (1.3), 334.6 (0.9), 223.7 (1.9), 171.9 (2.1)
cm
–1
.
5.5.3.4 Preparationof[(bpy)
2
Hf(N
3
)
4
]
2
· bpy
A sample of HfF
4
(333 mg; 1.31 mmol) was loaded into a Teflon-FEP ampule and 5 mL of hydroflu-
oric acid (48%) were added. After stirring for 48 hours at ambient temperature a clear solution
141
was obtained. The reaction mixture was pumped to dryness leaving behind a pale-yellow powder
(368 mg, weight expected for 1.31 mmol if HfF
4
· HF· 6 H
2
O: 357 mg). A solution of 2,2’-bpyridine
(609 mg, 3.9 mmol) in CH
3
CN (4.0 mL) was added, followed by the addition of Me
3
SiN
3
(1.843 g,
16 mmol) in vacuo at –196°C. The mixture was allowed to warm to ambient temperature and
agitated. After 48 h, a solution with a pale pink precipitate was obtained. All volatile materi-
als were removed in vacuo, first at –20 °C and then later at ambient temperature, leaving be-
hind a crystalline pale pink solid, [(bpy)
2
Hf(N
3
)
4
· bpy] (1028 mg; weight expected for 0.65 mmol
[(bpy)
2
Hf(N
3
)
4
]
2
· bpy: 966 mg). Material balance: Me
3
SiF: 614 mg (6.67 mmol).
DTA: 64–71°CC (endotherm, melting of bpy), 241°C (onset, exotherm, decomposition).
14
N-NMR (DMSO – d
6
): δ =−133 (s,∆ 1
⁄2 = 360 Hz, N
β ),−262 (s,∆ 1
⁄2 = 480 Hz, N
α ).
IR (KBr): ˜ ν = 3438 (m), 3086 (w), 3055 (m), 3007 (w), 2925 (vw), 2087 (vs), 1967 (s), 1871 (m), 1602
(m), 1581 (s), 1559 (m), 1444 (s), 1420 (w), 1369 (vs), 1313 (w), 1269 (vw), 1250 (m), 1159 (m), 1089
(m), 1066 (w), 1040 (m), 1014 (w), 993 (m), 894 (w), 760 (vs), 735 (vw), 702 (vw), 653 (m), 620 (s),
604 (vw) cm
–1
.
Raman (25°C, 35 mW):˜ ν (rel. intensity) = 3088.3 (0.9), 3064.5 (1.9), 3008.3 (0.6), 2148.6 (0.8), 2115.8
(0.5), 1604.5 (7.6), 1590.4 (9.9), 1572.8 (10.0), 1489.5 (7.8), 1447.1 (5.6), 1377.8 (1.9), 1320.2 (9.5),
1302.1 (4.2), 1274.6 (3.2), 1237.2 (4.3), 1217.9 (1.1), 1167.2 (1.5), 1146.9 (1.4), 1107.5 (1.3), 1095.7
(0.7), 1065.7 (1.6), 1036.1 (5.9), 1025.4 (4.4), 1014.2 (3.1), 995.3 (9.7), 814.7 (0.7), 766.7 (4.5), 661.3
(1.0), 641.9 (2.3), 614.9 (1.3), 550.0 (0.3), 494.8 (0.4), 475.0 (0.3), 457.2 (0.5), 438.2 (0.5), 384.1 (1.2),
371.4 (1.1), 287.9 (1.1), 224.2 (1.5) cm
–1
.
Additional experimental details are given in the electronic supporting information
142
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Abstract (if available)
Abstract
The research described here has been published in peer-reviewed journals. Each chapter in this dissertation corresponds to a single published article or manuscript, with experimental details provided in the "Experimental Part" section that follows each chapter. The content of these papers or manuscripts is given herein with minor changes to fulfill the University of Southern California Graduate School's formatting guidelines. This dissertation summarizes my journey of pushing the limits of chemical research by the isolation of highly reactive intermediates and the synthesis of novel energetic materials.
Chapter 1 published in Dalton Transactions describes the synthesis and structural characterization of the Lewis adducts of hydrogen cyanide, butyronitrile, cyclopropanecarbonitrile, pivalonitrile and benzonitrile with arsenic pentafluoride and antimony pentafluoride. The 2nd chapter (published in Chemistry – A European Journal) reports convenient access to the thermally unstable, primary perfluoro alcohols, CF3OH, C2F5OH, and n-C3F7OH and their oxonium salts. Chapter 3 deals with the protonation site in nitramide, the parent molecule of all nitramine explosives and has been published in Angewandte Chemie. The protonation of azidomethane and azidotrifluoromethane yielding highly reactive aminodiazonium ions has been reported in Angewandte Chemie and is discussed in Chapter 4. Chapter 7 describes the synthesis and characterization of novel azido N-donor adducts of the group 4 tetraazide [(bpy)Ti(N3)4], [(phen)Ti(N3)4], [(bpy)2Zr(N3)4] and [(bpy)2Zr(N3)4].
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Saal, Thomas Helmut
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On the pursuit of pushing the limits: from the isolation of highly reactive intermediates to the synthesis of energetic materials
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College of Letters, Arts and Sciences
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Doctor of Philosophy
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Chemistry
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2022-08
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energetic materials
fluorine chemistry
main-group chemistry