Carbenes in Ruthenium Based Olefin Metathesis Catalysts and … · ii Carbenes in Ruthenium Based...
Transcript of Carbenes in Ruthenium Based Olefin Metathesis Catalysts and … · ii Carbenes in Ruthenium Based...
Carbenes in Ruthenium Based Olefin Metathesis Catalysts and Stabilization of Low Coordinate Boron Species
by
Fatme Dahcheh
A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy
Department of Chemistry University of Toronto
© Copyright by Fatme Dahcheh 2014
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Carbenes in Ruthenium Based Olefin Metathesis Catalysts and Stabilization of Low Coordinate Boron Species
Fatme Dahcheh
Doctor of Philosophy
Department of Chemistry
University of Toronto
2014
Abstract
Since their discovery, carbenes have been widely used as organocatalysts and as superb ligands
for transition metal-based catalysts. They have also, more recently, been shown to stabilize
reactive and low-valent main group systems.
Catalytic olefin metathesis has proven to be a powerful tool in various chemical fields. Research
in this area has received considerable attention specifically with the development of new
catalysts. The vast majority of catalysts developed, thus far, have been modifications to the
Grubbs catalyst architecture. The research presented herein focuses on the development of a new
route for the synthesis of new olefin metathesis catalysts and testing their activity.
A new method of preparing ruthenium alkylidene complexes starting with bis-carbene RuHCl
species and alkenyl sulfides is developed. This provides a route to bis-mixed carbene ruthenium
alkylidene complexes with a hemilabile tridentate carbene and conveniently installs both an
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alkylidene fragment and a thiolate in one step. The resulting Ru-alkylidene species are either
inactive or minimally active for the standard metathesis tests. The species generated by the
addition of one equivalent of BCl3, however, show improved activity for ROMP, RCM and CM
either at room temperature or at slightly elevated temperatures. Halide exchange for these
systems results in enhanced metathesis activity for the standard tests where catalytic olefin
metathesis was observed at room temperature.
Cyclic (alkylamino)carbenes are utilized to stabilize iminoboryl moieties which have only been
previously stabilized in the coordination sphere of transition metals. Some of the species are also
shown to undergo [2+2] cycloaddition with CO2. CAACs are also used for the synthesis of a
boron derivative, which is isoelectronic with singlet carbenes, namely a borylene. This species is
shown to react with CO and H2, but in contrast with carbenes, it acts as an electrophile and
therefore mimics the behavior of metals.
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Acknowledgments
I would like to take the opportunity to recognize the many people who have assisted and
supported me during my studies. First and foremost, I would like to thank Professor Doug
Stephan for his continued support, guidance and encouragement throughout this degree. I would
also like to thank him for the support which allowed me to travel to many conferences to present
my work as well as setting up an exchange at UCSD with Professor Guy Bertrand which has
proven fruitful.
I would also like to thank all Stephan group members, both past and present, for helpful
chemistry (and general) discussions and for making the lab a very enjoyable place to work.
Particularly, I would like to thank the individuals who took the time to review and edit this
thesis, specifically Dr. Roman Dobrovetsky, Dr. Adam McKinty, Dr. Michael Boone,
Dr. Michael Sgro, Conor Pranckevicius and Lauren Longobardi. I would also like to thank
Dr. Datong Song and Dr. Robert Morris for serving on my committee for the past four years and
offering helpful advice throughout.
I would also like to thank all of the support staff in the Department of Chemistry at the
University of Toronto, specifically Rose Balazs and Giordana Riccitelli and the staff at
ANALEST for their help in obtaining quality elemental analysis results. I must also thank the
staff in the NMR facility for their assistance in the set up of experiments and general help with
NMR related inquiries. Specifically, I would like to thank Dr. Darcy Burns and Dmitry Pichugin
for helping me run specialized NMR experiments and set up Variable Temperature NMR
experiments. I would like to thank Dr. Alan Lough for his assistance with X-ray crystallography
and Dr. David Martin at UCSD for DFT calculations.
Finally, I would like to thank my family and friends who have supported me for the last four
years.
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Table of Contents
Acknowledgments .......................................................................................................................... iv
Table of Contents ............................................................................................................................ v
List of Schemes .............................................................................................................................. ix
List of Figures .............................................................................................................................. xiii
List of Abbreviations .................................................................................................................. xxii
Chapter 1 Introduction .................................................................................................................... 1
1.1 Carbenes .............................................................................................................................. 1
1.1.1 Synthesis and Isolation of Stable Free Carbenes .................................................... 2
1.1.2 Carbenes as Ligands for Transition Metal-Based Catalysts ................................... 5
1.1.3 Carbenes in Stabilizing Low-Valent and Reactive Species .................................... 6
1.2 Catalytic Olefin Metathesis ................................................................................................. 9
1.2.1 Well-Defined, Homogenous Catalysts .................................................................... 9
1.2.2 Mechanism of Catalytic Olefin Metathesis ........................................................... 10
1.3 Nitrile Butadiene Rubber .................................................................................................. 12
1.4 Lanxess Project ................................................................................................................. 13
1.5 Scope of Thesis ................................................................................................................. 14
References ..................................................................................................................................... 17
Chapter 2 Synthesis and Characterization of Bis-Mixed-Carbene Ruthenium-Alkylidene-
Thiolate Complexes ................................................................................................................. 22
2.1 Introduction ....................................................................................................................... 22
2.1.1 First Isolated Transition Metal Based Alkylidene Complex ................................. 22
2.1.2 Modifications to Grubbs’ Catalyst ........................................................................ 22
2.1.3 Bis-Carbene Olefin Metathesis Catalysts .............................................................. 24
2.1.4 Routes to Ru-Alkylidene Complexes .................................................................... 25
2.2 Results and Discussion ..................................................................................................... 30
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2.2.1 Synthesis of Ru-Hydride Complexes .................................................................... 30
2.2.2 Synthesis of Ru-Alkylidene Complexes Using Aryl-Alkenyl Sulfides ................ 33
2.3 Reactions of Ru-Hydride Species with Ethyl Vinyl Sulfide ............................................. 41
2.4 Conclusion ........................................................................................................................ 48
2.5 Experimental Section ........................................................................................................ 48
2.5.1 General Considerations ......................................................................................... 48
2.5.2 Synthetic Procedures ............................................................................................. 48
2.5.3 X-ray Data Collection and Reduction ................................................................... 61
2.5.4 X-ray Data Solution and Refinement .................................................................... 61
References ..................................................................................................................................... 65
Chapter 3 Catalytic Olefin Metathesis .......................................................................................... 68
3.1 Introduction ....................................................................................................................... 68
3.1.1 Types of Olefin Metathesis Reactions .................................................................. 68
3.1.2 Catalyst Screening ................................................................................................ 69
3.1.3 Acid Assisted Olefin Metathesis ........................................................................... 71
3.1.4 Halide Abstraction for Activation of Metathesis Catalysts .................................. 73
3.1.5 Cross Metathesis of NBR and 1-Hexene .............................................................. 74
3.2 Results and Discussion ..................................................................................................... 74
3.2.1 ROMP of 1,5-Cyclooctadiene ............................................................................... 75
3.2.2 RCM of Diethyl Diallylmalonate .......................................................................... 78
3.2.3 CM of 5-Hexenyl Acetate and Methyl Acrylate ................................................... 80
3.2.4 Trends in Catalytic Olefin Metathesis .................................................................. 83
3.2.5 Cross Metathesis of NBR and 1-Hexene .............................................................. 83
3.3 Conclusion ........................................................................................................................ 86
3.4 Experimental Section ........................................................................................................ 87
3.4.1 General Considerations ......................................................................................... 87
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3.4.2 Synthetic Procedures ............................................................................................. 87
References ................................................................................................................................... 106
Chapter 4 Synthesis of Bis-Mixed-Carbene Ruthenium-Alkylidene Complexes Through
Anion Exchange ..................................................................................................................... 108
4.1 Introduction ..................................................................................................................... 108
4.1.1 Halide Variation in Grubbs Catalyst ................................................................... 108
4.1.2 Pseudo-halides as Ligands in Ruthenium Metathesis Catalysts ......................... 108
4.2 Results and Discussion ................................................................................................... 109
4.2.1 Synthesis of Ru Complexes ................................................................................ 109
4.2.2 Standard Olefin Metathesis Tests ....................................................................... 116
4.2.3 Cross Metathesis of NBR with 1-Hexene ........................................................... 120
4.3 Conclusion ...................................................................................................................... 121
4.4 Experimental Section ...................................................................................................... 122
4.4.1 General Considerations ....................................................................................... 122
4.4.2 Synthetic Procedures ........................................................................................... 122
4.4.3 Standard Metathesis Reaction Tests ................................................................... 128
4.4.4 Cross Metathesis of NBR and 1-hexene ............................................................. 133
4.4.5 X-ray Crystallography ........................................................................................ 135
References ................................................................................................................................... 138
Chapter 5 Carbene Stabilized Iminoboranes ............................................................................... 140
5.1 Introduction ..................................................................................................................... 140
5.1.1 Iminoboranes and Iminoboryl Transition Metal Complexes .............................. 140
5.1.2 Carbenes in Stabilizing Low Valent Boron Species and Boron Centered
Radicals ............................................................................................................... 142
5.2 Results and Discussion ................................................................................................... 145
5.2.1 Synthesis of Iminoborane species ....................................................................... 145
5.2.2 Reactivity of Iminoboranes with CO2 ................................................................. 158
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5.3 Conclusion ...................................................................................................................... 162
5.4 Experimental Section ...................................................................................................... 162
5.4.1 General Considerations ....................................................................................... 162
5.4.2 Synthetic Procedures ........................................................................................... 163
5.4.3 X-ray Crystallography ........................................................................................ 174
References ................................................................................................................................... 179
Chapter 6 A Room Temperature Stable Organoboron Isoelectronic with Singlet Carbenes ..... 182
6.1 Introduction ..................................................................................................................... 182
6.1.1 Borylenes: Group 13 Carbene Analogues ........................................................... 182
6.1.2 Transition Metal Borylene Complexes ............................................................... 184
6.1.3 CO Adducts of Carbenes and of Boranes ........................................................... 185
6.2 Results and Discussion ................................................................................................... 187
6.2.1 Reduction Route to Borylene Synthesis ............................................................. 187
6.2.2 Reactivity of Borylenes ....................................................................................... 194
6.3 Conclusion ...................................................................................................................... 198
6.4 Experimental Section ...................................................................................................... 198
6.4.1 General Considerations ....................................................................................... 198
6.4.2 Synthetic Procedures ........................................................................................... 198
6.4.3 X-ray Crystallography ........................................................................................ 203
References ................................................................................................................................... 205
Chapter 7 Summary .................................................................................................................... 208
7.1 Summary ......................................................................................................................... 208
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List of Schemes
Scheme 1.1.1 The Wanzlick equilibrium. ...................................................................................... 2
Scheme 1.1.2 In situ preparation of imidazol-2-ylidenes. .............................................................. 2
Scheme 1.1.3 Nitrogen atom transfer using a carbene stabilized phosphinonitrene. ..................... 8
Scheme 1.2.1 Depiction of olefin metathesis. ................................................................................ 9
Scheme 1.2.2 Synthesis of the first well-defined Ru olefin metathesis catalyst. ......................... 10
Scheme 1.2.3 Chauvin’s mechanism of olefin metathesis. .......................................................... 11
Scheme 1.2.4 Olefin metathesis mechanism with Grubbs I. ........................................................ 12
Scheme 2.1.1 Synthesis of the first isolated transition metal alkylidene. .................................... 22
Scheme 2.1.2 Synthesis of the first ruthenium alkylidene. .......................................................... 26
Scheme 2.1.3 Synthesis of Grubbs I catalyst using phenyl diazomethane. .................................. 26
Scheme 2.1.4 Synthesis of Grubbs I catalyst using a sulfur ylide. ............................................... 27
Scheme 2.1.5 Synthesis of Grubbs I catalyst via an indenylidene intermediate. ......................... 27
Scheme 2.1.6 Synthesis of Grubbs I catalyst from Ru(0) species. ............................................... 28
Scheme 2.1.7 Synthesis Ru-alkylidenes from dithioacetals and Ru(PPh3)3(H)2. ......................... 28
Scheme 2.1.8 Synthesis of a ruthenium phosphonium alkylidene complex. ............................... 28
Scheme 2.1.9 Synthesis of a vinylalkylidene using propargyl chloride. ...................................... 29
Scheme 2.1.10 Synthesis of a Ru-alkylidene using vinyl chloride. ............................................. 29
Scheme 2.1.11 Synthesis of a Ru-ethylidene using vinyl chloroformate. .................................... 29
Scheme 2.2.1 Synthesis of 2-1 to 2-3. .......................................................................................... 30
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Scheme 2.2.2 Synthesis of 2-4 to 2-8. .......................................................................................... 33
Scheme 2.2.3 Synthesis of 2-10 and 2-11. ................................................................................... 37
Scheme 2.2.4 Synthesis of 2-12 to 2-14. ...................................................................................... 38
Scheme 2.3.1 Synthesis of 2-17 to 2-19. ...................................................................................... 41
Scheme 2.3.2 Synthesis of 2-21. .................................................................................................. 44
Scheme 2.3.3 Synthesis of 2-22 and 2-23. ................................................................................... 46
Scheme 3.1.1 Common olefin metathesis reactions. .................................................................... 69
Scheme 3.1.2 Standard test reaction for ROMP. .......................................................................... 70
Scheme 3.1.3 Standard test reaction for RCM. ............................................................................ 70
Scheme 3.1.4 Standard test reaction for CM. ............................................................................... 70
Scheme 3.1.5 Lewis acid assisted RCM. ...................................................................................... 71
Scheme 3.1.6 Use of acid as a phosphine scavenger. ................................................................... 72
Scheme 3.1.7 Activation of metathesis catalysts with BCl3. ........................................................ 72
Scheme 3.1.8 Activation of metathesis catalyst through halide abstraction. ............................... 73
Scheme 3.1.9 Synthesis of a 4-coordinate olefin metathesis catalyst by halide abstraction. ....... 73
Scheme 3.2.1 List of catalysts used for catalytic olefin metathesis. ............................................ 75
Scheme 4.2.1 Synthesis of 4-1 and 4-2. ..................................................................................... 110
Scheme 4.2.2 Synthesis of 4-3 and 4-4. ..................................................................................... 112
Scheme 4.2.3 Synthesis of 4-5. .................................................................................................. 114
Scheme 4.2.4 Synthesis of 4-6. .................................................................................................. 115
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Scheme 5.1.1 Synthesis of iminoboranes via thermally induced elimination of Me3SiX. ......... 140
Scheme 5.1.2 [2+2] Cycloaddition reactions of amino iminoboranes. ...................................... 141
Scheme 5.1.3 Reaction of an amino iminoborane with CX2. ..................................................... 141
Scheme 5.1.4 Synthesis of iminoboryl transition metal complexes. .......................................... 142
Scheme 5.1.5 Reactions of an iminoboryl complex with various substrates. ............................ 142
Scheme 5.1.6 Synthesis of a neutral borolyl radical. ................................................................. 144
Scheme 5.2.1 Synthesis of 5-1. .................................................................................................. 145
Scheme 5.2.2 Synthesis of 5-2 to 5-4. ........................................................................................ 147
Scheme 5.2.3 Synthesis of 5-5 to 5-7. ........................................................................................ 151
Scheme 5.2.4 Synthesis of 5-8 to 5-11. ...................................................................................... 154
Scheme 5.2.5 Reaction of 5-11 with NaBPh4. ............................................................................ 157
Scheme 5.2.6 Synthesis of 5-12 to 5-14. .................................................................................... 158
Scheme 5.2.7 Reaction of a π-conjugated iminoborane with CO2. ............................................ 161
Scheme 6.1.1 Schematic representation of singlet carbenes A, nitrenes B, borylenes C, and
Lewis base-borylene adducts D. ................................................................................................. 182
Scheme 6.1.2 Synthesis of a stable diborene stabilized by NHCs. ............................................ 183
Scheme 6.1.3 Formation of a borane through C-H activation of a transient borylene. .............. 183
Scheme 6.1.4 Formation of a borirane by trapping of a transient borylene. .............................. 183
Scheme 6.1.5 Synthesis of a bis-CAAC-borylene. ..................................................................... 184
Scheme 6.1.6 Synthesis of the first terminal borylene complexes. ............................................ 185
Scheme 6.1.7 CO fixation to a CAAC and a DAC. ................................................................... 186
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Scheme 6.1.8 Synthesis of tris(trifluoromethyl)borane carbonyl adduct. .................................. 186
Scheme 6.1.9 Synthesis of pentaarylborole-CO adduct. ............................................................ 186
Scheme 6.1.10 Synthesis of Piers borane-CO adduct. ............................................................... 187
Scheme 6.2.1 Synthesis of 6-1 and 6-2. ..................................................................................... 188
Scheme 6.2.2 Synthesis of a neutral boron-containing radical stabilized by a CAAC. ............. 191
Scheme 6.2.3 Synthesis of 6-3. .................................................................................................. 194
Scheme 6.2.4 Synthesis of 6-5. .................................................................................................. 195
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List of Figures
Figure 1.1.1 Singlet and triplet forms of a carbene. ....................................................................... 1
Figure 1.1.2 First isolable and crystalline carbenes. ...................................................................... 3
Figure 1.1.3 Select examples of carbenes. ..................................................................................... 3
Figure 1.1.4 Steric environment differences between phosphines and carbenes. .......................... 4
Figure 1.1.5 Carbene coordination to metals and p-block elements. ............................................. 4
Figure 1.1.6 Fischer and Schrock carbene complexes. .................................................................. 5
Figure 1.1.7 Carbene-stabilized main group species in the (0) oxidation state. ............................ 7
Figure 1.1.8 Carbene stabilized paramagnetic main group species. .............................................. 7
Figure 1.1.9 Stable oxyallyl radical cation. ................................................................................... 8
Figure 1.2.1 Generalized structure of a Mo-based Schrock-type catalyst. .................................... 9
Figure 1.2.2 First and second generation Grubbs catalysts. ......................................................... 10
Figure 1.3.1 Depiction of functional groups found in Nitrile Butadiene Rubber. ....................... 12
Figure 1.3.2 Depiction of hydrogenated Nitrile Butadiene Rubber. ............................................ 13
Figure 2.1.1 Grubbs’ catalysts and a generalized structure of a Ru olefin metathesis catalysts. . 22
Figure 2.1.2 Examples of 4- and 6-coordinate Ru-alkylidene olefin metathesis catalysts. ......... 23
Figure 2.1.3 Generalized structures of NHCs used as ligands for Ru olefin metathesis catalysts.
....................................................................................................................................................... 24
Figure 2.1.4 Examples of bis-carbene Ru-alkylidene complexes. ............................................... 25
Figure 2.2.1 POV-ray depiction of the molecular structure of 2-1. ............................................. 31
Figure 2.2.2 POV-ray depiction of the molecular structure of 2-2. ............................................. 32
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Figure 2.2.3 POV-ray depiction of the molecular structure of 2-3. ............................................. 32
Figure 2.2.4 POV-ray depiction of the molecular structure of 2-4. ............................................. 34
Figure 2.2.5 POV-ray depiction of the molecular structure of 2-5. ............................................. 35
Figure 2.2.6 POV-ray depiction of the molecular structure of 2-6. ............................................. 36
Figure 2.2.7 POV-ray depiction of the molecular structure of 2-8.. ............................................ 37
Figure 2.2.8 POV-ray depiction of the molecular structure of 2-12. ........................................... 39
Figure 2.2.9 POV-ray depiction of the molecular structure of 2-13. ........................................... 40
Figure 2.2.10 POV-ray depiction of the molecular structure of 2-14. ......................................... 40
Figure 2.3.1 POV-ray depiction of the molecular structure of 2-17. ........................................... 42
Figure 2.3.2 POV-ray depiction of the molecular structure of 2-18. ........................................... 43
Figure 2.3.3 POV-ray depiction of the molecular structure of 2-21. ........................................... 45
Figure 2.3.4 POV-ray depiction of the molecular structure of 2-22. ........................................... 47
Figure 3.2.1 ROMP of 1,5-cyclooctadiene using 2-4. ................................................................. 76
Figure 3.2.2 ROMP of 1,5-cyclooctadiene using 2-8. ................................................................. 77
Figure 3.2.3 ROMP of 1,5-cyclooctadiene using 2-12. ............................................................... 77
Figure 3.2.4 RCM of diethyl diallylmalonate using 2-4.. ............................................................ 78
Figure 3.2.5 RCM of diethyl diallylmalonate using 2-8. ............................................................. 79
Figure 3.2.6 RCM of diethyl diallylmalonate using 2-12. ........................................................... 80
Figure 3.2.7 CM of methyl acrylate and 5-hexenyl acetate using 2-4. ........................................ 81
Figure 3.2.8 CM of methyl acrylate and 5-hexenyl acetate using 2-8. ........................................ 82
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Figure 3.2.9 CM of methyl acrylate and 5-hexenyl acetate using 2-12. ...................................... 82
Figure 3.2.10 CM of NBR and 1-hexene using 2-4 and Grubbs II at 45 °C. ............................... 85
Figure 3.2.11 CM of NBR and 1-hexene using 2-4 at 60 °C. ...................................................... 86
Figure 4.1.1 Alkoxide and electron deficient aryloxides as ligands on olefin metathesis catalysts.
..................................................................................................................................................... 109
Figure 4.1.2 Z-selective olefin metathesis catalyst with a thiolate ligand. ................................ 109
Figure 4.2.1 POV-ray depiction of the molecular structure of 4-1. ........................................... 111
Figure 4.2.2 POV-ray depiction of the molecular structure of 4-2. ........................................... 111
Figure 4.2.3 POV-ray depiction of the molecular structure of 4-3. ........................................... 113
Figure 4.2.4 POV-ray depiction of the molecular structure of 4-5. ........................................... 114
Figure 4.2.5 POV-ray depiction of the molecular structure of 4-6. ........................................... 116
Figure 4.2.6 ROMP of 1,5-cyclooctadiene with 4-1. ................................................................. 117
Figure 4.2.7 RCM of diethyl diallylmalonate with 4-1. ............................................................. 118
Figure 4.2.8 RCM of diethyl diallylmalonate with 4-3, 4-4 and 4-6. ........................................ 118
Figure 4.2.9 CM of 5-hexenyl acetate and methyl acrylate with 4-1. ........................................ 119
Figure 4.2.10 CM of 5- hexenyl acetate and methyl acrylate with 4-3, 4-4 and 4-6. ................ 120
Figure 4.2.11 CM of NBR and 1-hexene with 4-1 at 25 °C. ..................................................... 121
Figure 4.4.1 1H NMR spectrum of 4-3 in C6D6. ........................................................................ 126
Figure 4.4.2 13
C{1H} NMR spectrum of 4-3 in C6D6. ............................................................... 127
Figure 4.4.3 1H NMR spectrum of 4-5 in C6D6. ........................................................................ 127
Figure 4.4.4 13
C{1H} NMR spectrum of 4-5 in C6D6. ............................................................... 128
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Figure 5.1.1 Carbene stabilized neutral diborene. ..................................................................... 143
Figure 5.1.2 Carbene stabilized diboryne and diborabutatriene. ............................................... 144
Figure 5.1.3 Examples of carbene stabilized boron centered radicals. ...................................... 144
Figure 5.2.1 POV-ray depiction of the molecular structure of 5-1. ........................................... 146
Figure 5.2.2 POV-ray depiction of the molecular structure of the cation of 5-2. ...................... 148
Figure 5.2.3 POV-ray depiction of the molecular structure of 5-3. ........................................... 149
Figure 5.2.4 POV-ray depiction of the molecular structure of 5-4. ........................................... 150
Figure 5.2.5 POV-ray depiction of the molecular structure of 5-5. ........................................... 152
Figure 5.2.6 POV-ray depiction of the molecular structure of 5-6. ........................................... 152
Figure 5.2.7 POV-ray depiction of the molecular structure of 5-7. ........................................... 153
Figure 5.2.8 POV-ray depiction of the molecular structure of 5-8. ........................................... 155
Figure 5.2.9 POV-ray depiction of the molecular structure of 5-10. ......................................... 156
Figure 5.2.10 POV-ray depiction of the molecular structure of the cation of 5-11a. ................ 157
Figure 5.2.11 POV-ray depiction of the molecular structure of 5-12. ....................................... 159
Figure 5.2.12 POV-ray depiction of the molecular structure of 5-13.. ...................................... 160
Figure 5.2.13 POV-ray depiction of the molecular structure of 5-14. ....................................... 161
Figure 5.4.1 1H NMR spectrum of 5-1 in C6D6. ........................................................................ 170
Figure 5.4.2 11
B{1H} NMR spectrum of 5-1 in C6D6. ............................................................... 171
Figure 5.4.3 13
C{1H} NMR spectrum of 5-1 in C6D6. ............................................................... 171
Figure 5.4.4 29
Si{1H} NMR spectrum of 5-1 in C6D6. .............................................................. 172
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Figure 5.4.5 11
B{1H} NMR spectrum of 5-9 in C6D6. ............................................................... 172
Figure 5.4.6 1H NMR spectrum of 5-9 in C6D6. ........................................................................ 173
Figure 5.4.7 13
C{1H} NMR spectrum of 5-9 in C6D6. ............................................................... 173
Figure 5.4.8 29
Si{1H} NMR spectrum of 5-9 in C6D6. .............................................................. 174
Figure 6.1.1 Orbital interaction between borylenes and metal fragments. ................................ 185
Figure 6.2.1 POV-Ray depiction of the molecular structure of 6-1. .......................................... 189
Figure 6.2.2 Representation of the SOMO of 6-1 with isovalue at 0.06 a.u. ............................. 190
Figure 6.2.3 Experimental X-band EPR spectrum of 6-1 in toluene at 280 K (green) and
simulated EPR spectrum (blue) with the following set of hyperfine coupling constants: a(B) =
4.7, a(N) = 18.4 and a(Cl) = 2.5 MHz......................................................................................... 190
Figure 6.2.4 POV-Ray depiction of the molecular structure of 6-2. .......................................... 192
Figure 6.2.5 a) and a’): highest occupied molecular orbital (HOMO) of 6-2, and of 6-2* with a
frozen C-B-N angle at 155°, respectively. b-d) and b’-d’) lowest unoccupied molecular orbitals
(LUMO) of 6-2 and 6-2*, respectively. ...................................................................................... 193
Figure 6.2.6 POV-Ray depiction of the molecular structure of 6-3. .......................................... 195
Figure 6.2.7 POV-Ray depiction of the molecular structure of 6-5. .......................................... 196
Figure 6.2.8 Calculated transition state for the activation of H2 by 6-2. ................................... 197
Figure 6.2.9 a) Primary interaction between the LUMO of 6-2* and theorbital of H2. b)
Secondary interaction between the HOMO of 6-2* and the * orbital of H2. ........................... 197
Figure 6.4.1 1H NMR spectrum of 6-2 in C6D6. ........................................................................ 201
Figure 6.4.2 11
B{1H} NMR spectrum of 6-2 in C6D6. ............................................................... 201
Figure 6.4.3 13
C{1H} NMR spectrum of 6-2 in C6D6. ............................................................... 202
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Figure 6.4.4 29
Si{1H} NMR spectrum of 6-2 in C6D6. .............................................................. 202
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List of Tables
Table 2.5.1 Select crystallographic parameters for 2-1 to 2-4. .................................................... 62
Table 2.5.2 Select crystallographic parameters for 2-5, 2-6, 2-8 and 2-12. ................................. 63
Table 2.5.3 Select crystallographic parameters for 2-17, 2-18, 2-21 and 2-22. ........................... 64
Table 3.1.1 Standard olefin metathesis reactions using common catalysts. ................................ 71
Table 3.4.1 ROMP of 1,5-cyclooctadiene with 2-4. .................................................................... 88
Table 3.4.2 ROMP of 1,5-cyclooctadiene with 2-5. .................................................................... 88
Table 3.4.3 ROMP of 1,5-cyclooctadiene with 2-6. .................................................................... 89
Table 3.4.4 ROMP of 1,5-cyclooctadiene with 2-7. .................................................................... 89
Table 3.4.5 ROMP of 1,5-cyclooctadiene with 2-8. .................................................................... 90
Table 3.4.6 ROMP of 1,5-cyclooctadiene with 2-12. .................................................................. 90
Table 3.4.7 ROMP of 1,5-cyclooctadiene with 2-13. .................................................................. 91
Table 3.4.8 ROMP of 1,5-cyclooctadiene with 2-14. .................................................................. 91
Table 3.4.9 RCM of diethyl diallylmalonate with 2-4. ................................................................ 92
Table 3.4.10 RCM of diethyl diallylmalonate with 2-5. .............................................................. 93
Table 3.4.11 RCM of diethyl diallylmalonate with 2-6. .............................................................. 94
Table 3.4.12 RCM of diethyl diallylmalonate with 2-7. .............................................................. 94
Table 3.4.13 RCM of diethyl diallylmalonate with 2-8. .............................................................. 95
Table 3.4.14 RCM of diethyl diallylmalonate with 2-12. ............................................................ 95
Table 3.4.15 RCM of diethyl diallylmalonate with 2-13. ............................................................ 96
xx
Table 3.4.16 RCM of diethyl diallylmalonate with 2-14. ............................................................ 97
Table 3.4.17 CM of 5-hexenyl acetate and methyl acrylate with 2-4. ......................................... 98
Table 3.4.18 CM of 5-hexenyl acetate and methyl acrylate with 2-5. ......................................... 98
Table 3.4.19 CM of 5-hexenyl acetate and methyl acrylate with 2-6. ......................................... 99
Table 3.4.20 CM of 5-hexenyl acetate and methyl acrylate with 2-7. ......................................... 99
Table 3.4.21 CM of 5-hexenyl acetate and methyl acrylate with 2-8. ......................................... 99
Table 3.4.22 CM of 5-hexenyl acetate and methyl acrylate with 2-12. ..................................... 100
Table 3.4.23 CM of 5-hexenyl acetate and methyl acrylate with 2-13. ..................................... 100
Table 3.4.24 CM of 5-hexenyl acetate and methyl acrylate with 2-14. ..................................... 101
Table 3.4.25 GPC data for CM of NBR and 1-hexene using 2-4 and Grubbs II at 25 °C. ........ 102
Table 3.4.26 GPC data for CM of NBR and 1-hexene using 2-4 and Grubbs II at 45 °C. ........ 103
Table 3.4.27 GPC data for CM of NBR and 1-hexene using 0.14 phr of 2-4 at 45 °C. ............. 104
Table 3.4.28 GPC data for CM of NBR and 1-hexene using 2-4 at 60 °C. ............................... 105
Table 4.4.1 ROMP of 1,5-cyclooctadiene with 4-1. .................................................................. 129
Table 4.4.2 ROMP of 1,5-cyclooctadiene with 4-2. .................................................................. 129
Table 4.4.3 ROMP of 1,5-cyclooctadiene with 4-3, 4-4, and 4-6. ............................................. 129
Table 4.4.4 RCM of diethyl diallylmalonate with 4-1. .............................................................. 130
Table 4.4.5 RCM of diethyl diallylmalonate with 4-2. .............................................................. 130
Table 4.4.6 RCM of diethyl diallylmalonate with 4-3. .............................................................. 130
Table 4.4.7 RCM of diethyl diallylmalonate with 4-4. .............................................................. 131
xxi
Table 4.4.8 RCM of diethyl diallylmalonate with 4-6. .............................................................. 131
Table 4.4.9 CM of 5- hexenyl acetate and methyl acrylate with 4-1. ........................................ 131
Table 4.4.10 CM of 5- hexenyl acetate and methyl acrylate with 4-2. ...................................... 132
Table 4.4.11 CM of 5- hexenyl acetate and methyl acrylate with 4-3. ...................................... 132
Table 4.4.12 CM of 5- hexenyl acetate and methyl acrylate with 4-4. ...................................... 132
Table 4.4.13 CM of 5- hexenyl acetate and methyl acrylate with 4-6. ...................................... 132
Table 4.4.14 GPC data for CM of NBR and 1-hexene using 4-1 at 25 °C. ............................... 134
Table 4.4.15 Select crystallographic parameters for 4-1 to 4-3. ................................................ 136
Table 4.4.16 Select crystallographic parameters for 4-5 and 4-6. .............................................. 137
Table 5.4.1 Select crystallographic parameters for 5-1 to 5-4. .................................................. 176
Table 5.4.2 Select crystallographic parameters for 5-5 to 5-8. .................................................. 177
Table 5.4.3 Select crystallographic parameters for 5-10, 5-12 to 5-14. ..................................... 178
Table 6.4.1 Select crystallographic parameters for 6-1 to 6-5. .................................................. 204
xxii
List of Abbreviations
° degree
°C degrees Celsius
Å angstrom, 10-10
m
atm atmosphere
Ar Aryl
CAAC Cyclic (alkyl)(amino)carbene
Cy-CAAC 1-(2,6-diisopropylphenyl)-3-cyclohexyl-5,5-dimethylpyrrolidin-2-ylidene
CD2Cl2 deuterated dichloromethane
calc calculated
cat. catalyst
CCD charge coupled device
CM cross metathesis
cm centimeter
cod cyclooctadiene
cot cyclooctatriene
Cp cyclopentadienyl
Cp* pentamethylcyclopentadienyl
Cy cyclohexyl
d doublet
xxiii
DAC diamidocarbene
dd doublet of doublet
DCM dichloromethane
Dipp 2,6-diisopropylphenyl
equiv. equivalent
Et ethyl
Et2O diethyl ether
FTIR Fourier transform infrared
g gram
GC-MS gas chromatography-mass spectrometry
GOF goodness of fit
h hour
Hz Hertz
I nuclear spin
IDipp 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene
IMe 1,3-bis(methyl)imidazol-2-ylidene
IMes 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene
IMes-Cl2 1,3-bis(2,4,6-trimethylphenyl)-4,5-dichloroimidazol-2-ylidene
Im(OMe)2 C3H2(NCH2CH2OMe)2
Ind indenyl
xxiv
iPr iso-propyl
m meta
m multiplet
Me methyl
Me2Im(OMe)2 C5H6(NCH2CH2OMe)2
Mes mesityl, 2,4,6-trimethylphenyl
min minute
mL milliliter
mm millimeter
mmol millimole
NHC N-Heterocyclic carbene
1Jxy n-bond scalar coupling constant between X and Y atoms
NMR nuclear magnetic resonance
o ortho
OTf triflate, trifluoromethanesulfonate
p para
PDI polydispersity index (Ð)
Ph phenyl
phr parts per hundred rubber
POV-Ray Persistence of Vision Raytracer
xxv
ppm parts per million, 10-6
q quartet
RCM ring closing metathesis
ROMP ring opening metathesis polymerization
RT room temperature
SIMes 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene
SIDipp 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene
t triplet
tBu tert-butyl
Tol toluene
THF tetrahydrofuran
TMS trimethylsilyl
1
Chapter 1 Introduction
1.1 Carbenes
A carbene is defined as a divalent carbon atom with only six valence electrons and attempts to
prepare the parent carbene, CH2, by dehydration of methanol were reported as early as 1835 by
Dumas.1 The prospect of a stable, isolable carbene was, at that time, quite reasonable since the
tetravalency of carbon had not yet been established. It was not until the early 1900s that
Staudinger and Kupfer demonstrated that carbenes, generated in situ from diazo compounds or
ketenes, were highly reactive species.2
Free carbenes can exist in two spin states: singlet or triplet (Figure 1.1.1) with the substituents on
the carbon atom dictating which spin state will be more preferred.3 Bulkier substituents force the
carbene to adopt a less bent geometry, which results in a smaller σ-pπ separation and thus
favoring a triplet state. Steric effects determine the ground-state spin multiplicity only as far as
the electronic effects are negligible, which is rarely the case.4,5
Therefore, careful consideration
of the substituents can favor the formation of either a singlet or a triplet carbene.
Figure 1.1.1 Singlet and triplet forms of a carbene.
A significant discovery for carbene chemistry occurred during the 1950s when Breslow6 and
Wanzlick7 realized that the stability of a carbene could be dramatically enhanced by the presence
of amino substituents (Scheme 1.1.1). They were, however, unable to isolate monomeric
carbenes.
2
Scheme 1.1.1 The Wanzlick equilibrium.
Twenty years later, Wanzlick and co-workers showed that imidazolium salts could be
deprotonated to generate the corresponding imidazol-2-ylidenes, but these could only be isolated
when trapped using phenylisothiocyanate or mercury acetate (Scheme 1.1.2).8,9
Scheme 1.1.2 In situ preparation of imidazol-2-ylidenes.
1.1.1 Synthesis and Isolation of Stable Free Carbenes
The first stable and isolable carbene, a phosphino(silyl)carbene, was reported by Bertrand and
co-workers in 1988 but it was not until 2000 that a crystalline analogue was synthesized and
crystallographically characterized (A and B in Figure 1.1.2).10,11
3
Figure 1.1.2 First isolable and crystalline carbenes.
Following the seminal work of Bertrand, in 1991 Arduengo and co-workers reported the
isolation of the first crystalline N-heterocyclic carbene12
(NHC) (C in Figure 1.1.2). Since their
discovery, NHCs have found broad applications as organocatalysts as well as ligands for
transition metal-based catalysis. Over the last 25 years there have been numerous reports
detailing the syntheses of variations on NHCs as well as new classes of carbenes. Shown in
Figure 1.1.3 are select examples of carbenes reported in recent literature.13-27
Figure 1.1.3 Select examples of carbenes.
4
Several variations, including acyclic and cyclic carbenes, variable ring sizes, heteroatoms within
the heterocycle, and an all-carbon cycle, have been reported. In recent years anionic, dianionic,
and mesoionic NHCs have also been reported (Figure 1.1.3). Of particular relevance to this
thesis, however, are cyclic (alkyl)(amino)carbenes (CAAC).28
First reported in 2005 by Bertrand and co-workers, replacing a σ-withdrawing and π-donating
nitrogen center in an NHC by a σ-donating, but not π-donating, carbon center leads to the
formation of stable and isolable cyclic (alkyl)(amino)carbenes. This substitution makes CAACs
more nucleophilic but also more electrophilic than NHCs which is evidenced by the smaller
HOMO-LUMO gap for CAACs compared to NHCs.27
Figure 1.1.4 Steric environment differences between phosphines and carbenes.
In addition, the presence of a quaternary carbon center α to the carbene center results in a steric
environment that is markedly different than those of phosphines and NHCs (Figure 1.1.4). The
quaternary carbon center is also well situated for implementing stereochemical induction.
There are a number of ways to denote carbenes bound to transition metal centers and main group
elements (Figure 1.1.5).29
As such, the following formalism will be used throughout this thesis.
Figure 1.1.5 Carbene coordination to metals and p-block elements.
5
1.1.2 Carbenes as Ligands for Transition Metal-Based Catalysts
While carbenes (especially NHCs) have been widely used as organocatalysts,30-35
their success as
ligands will be further discussed as it is more pertinent to this thesis. It is worth mentioning that,
although the isolation of a free carbene was a synthetic challenge, the first transition
metal-carbene complexes were reported by Chugaev36
as early as 1925. In the 1960s, there was
also the pioneering work by Fischer and Maasböl37
as well as the work by Öfele38
. NHC
transition metal complexes have been known and their organometallic chemistry was also
investigated by Lappert39
.
One can distinguish coordinated carbenes as two extreme types: a Fischer and a Schrock type.3
For a Fischer type carbene, direct donation from the carbene to the metal center predominates
and the carbon tends to have a partial positive charge (electrophilic). Such carbenes typically
have π-donor substituents (R = OMe or NMe2). In contrast, a Schrock type carbene involves
covalent bonding between the carbon and the metal center. Each of these bonds is polarized
toward the carbon making it have a partial negative charge (nucleophilic). These carbenes
typically have non-π-donating R groups (Figure 1.1.6).
Figure 1.1.6 Fischer and Schrock carbene complexes.
Fischer-carbene complexes are usually formed with low oxidation state, late transition metals
that have π-acceptor ligands, whereas Schrock-carbene complexes are formed with higher
oxidation state, early transition metals that have non-π-acceptor ligands. Since NHCs have two
π-donor substituents at the carbene center, NHC complexes may be classified, at a first glance, as
Fischer-type compounds. However, NHCs bind to transition metals only through σ donation,
with negligible π-back-bonding, and are therefore not Fischer-type carbenes.
It has also been shown experimentally that NHCs are not phosphine mimics, as catalysts
employing NHCs rather than phosphines show enhanced activities.40
In general, NHC-ligated
6
transition metal complexes are less air and moisture sensitive than their phosphine analogues,
explained by the fact that NHCs are more strongly bound to the metal center.41,42
Employing NHC ligands in transition metal catalysis has led to several breakthroughs in
enhancing the catalytic activity for a number of valuable organic transformations. These include,
but are not limited to, Heck,43-45
Suzuki,46,47
Sonogashira48,49
, Kumada50,51
and Stille52
couplings,
aryl amination42,53
and amide α-arylation54
, as well as hydrosilylation.55
More recently, through
the synthesis of chiral NHCs, a number of transformations with high levels of enantioinduction
have been reported. A famous example of the impact NHCs have had on catalysis is olefin
metathesis56
where replacing a phosphine with an NHC highly improves the stability and activity
of the catalyst (discussed in detail in Section 1.2).
Over the past few years, CAACs have also garnered attention as viable ligands for transition
metal-based catalysts as they were shown to stabilize low coordinate metal centers.57,58
The first
example of α-arylation of ketones and aldehydes with aryl chlorides under ambient conditions
was demonstrated using a Pd-based catalyst utilizing a CAAC ligand.57
Cyclic
(alkyl)(amino)carbenes were also shown to stabilize a cationic gold complex which catalyzes the
coupling of enamines and terminal alkynes to generate allenes with subsequent loss of imines.58
Treating the same gold complex with NH3 or hydrazine results in the formation of very efficient
catalysts for the hydroamination of a variety of non-activated alkynes and allenes with ammonia
or hydrazine.59,60
CAACs were also shown to be effective ligands for Ru-based catalysts for olefin metathesis61
especially for the ethenolysis of methyl oleate,62
a process that transforms internal olefins
derived from seed oils to terminal olefin feed stocks.
1.1.3 Carbenes in Stabilizing Low-Valent and Reactive Species
Over the past decade, there have been significant advances in the isolation of stable, low
oxidation state main group compounds which has been largely enabled through the use of neutral
donors to stabilize such species. For example, singlet carbenes such as NHCs and CAACs have
been used as ligands to stabilize main group molecules in their zero oxidation state.26,63,64
Select
examples will be discussed here and boron-based systems will be discussed in Chapters 5 and 6.
7
Robinson and co-workers have reported the isolation of diatomic main group molecules in low
oxidation states which are stabilized by two carbenes. The phosphorous,65
arsenic,66
and silicon67
analogues have been stabilized by two NHCs (A and B in Figure 1.1.7). Bertrand and co-workers
have also shown that larger polyatomic molecules with main group elements in the zero
oxidation state can be isolated when capped by carbenes (C in Figure 1.1.7).68
Figure 1.1.7 Carbene-stabilized main group species in the (0) oxidation state.
Carbenes were also shown to stabilize phosphorus radicals where phosphinyl radicals,69
a
phosphinyl radical cation,70
and a phosphonitride radical cation71
were isolated and fully
characterized (Figure 1.1.8).
Figure 1.1.8 Carbene stabilized paramagnetic main group species.
Bertrand and co-workers demonstrated that carbenes could also be used to stabilize and isolate
organic radical cations (Figure 1.1.9).72
Treating a less bulky carbene such as the anti-Bredt
NHC27
with CO results in the attack of two carbene molecules at the carbon center forming an
oxyallyl species, which can be protonated and then oxidized to form an oxyallyl radical cation.
8
Figure 1.1.9 Stable oxyallyl radical cation.
More recently, carbenes have been utilized to stabilize phosphinonitrenes which were then used
as nitrogen atom transfer agents (Scheme 1.1.3).73
This report by Bertrand and co-workers
demonstrated how main group compounds can mimic the chemical behavior of transition metals,
an observation that has been utilized in recent years to use main group systems to effect
transformations that were otherwise limited to transition metals.
Scheme 1.1.3 Nitrogen atom transfer using a carbene stabilized phosphinonitrene.
9
1.2 Catalytic Olefin Metathesis
Olefin metathesis is a C−C bond forming reaction that proceeds through scission, redistribution,
and bond formation of two molecules containing an alkene functionality.74,75
A generic reaction
depicting catalytic olefin metathesis can be seen in Scheme 1.2.1.76
Scheme 1.2.1 Depiction of olefin metathesis.
1.2.1 Well-Defined, Homogenous Catalysts
1.2.1.1 Schrock's Catalyst
With the isolation of the first transition metal alkylidene complex in 1974, which was shown to
effect olefin metathesis,77
Schrock and others reported the synthesis of several other early
transition metal alkylidene complexes.78,79
Most notably are the W and Mo imido, alkylidene
complexes depicted in Figure 1.2.1, with the Mo species being commercially available as
"Schrock's Catalyst".80
While these early transition metal alkylidene species are extremely active
for olefin metathesis, their extreme reactivity renders them quite functional group intolerant.
Figure 1.2.1 Generalized structure of a Mo-based Schrock-type catalyst.
1.2.1.2 Grubbs Catalyst
Due to the laborious preparation and extreme sensitivity and functional group intolerance of
Schrock's W and Mo metathesis catalysts, new catalysts with better functional group
compatibility were sought out. Based on earlier reports that ill-defined Ru species were effective
10
in ring opening metathesis polymerization (ROMP), the synthesis of well-defined Ru catalysts
was investigated by Grubbs and co-workers. The first well-defined Ru-based catalyst was the
alkylidene species A which is formed through the ring opening of diphenylcyclopropene by
Ru(PPh3)3Cl2 (Scheme 1.2.2).81
This complex was found to be active for ROMP, and exchanging
PPh3 ligands for PCy3 results in a complex that is active for cross metathesis.82
Scheme 1.2.2 Synthesis of the first well-defined Ru olefin metathesis catalyst.
This discovery prompted further research in catalytic olefin metathesis chemistry and, while the
new Ru based systems are less active than Schrock’s systems, they are significantly more stable
and functional group tolerant. A large library of Ru-based olefin metathesis catalysts became
available as new methods for the preparation of Ru-alkylidene complexes (discussed in
Section 2.1.4) were developed.83
Of major significance to this field was the report of
(PCy3)2Ru(CHPh)Cl2 and (SIMes)(PCy3)Ru(CHPh)Cl2 known as Grubbs I and Grubbs II,
respectively (Figure 1.2.2),82,84
where the substitution of PCy3 for SIMes in the second
generation system increased the activity of the catalyst dramatically. These catalysts are
currently used for a variety of commercial applications.85,86
Figure 1.2.2 First and second generation Grubbs catalysts.
1.2.2 Mechanism of Catalytic Olefin Metathesis
A number of possible mechanisms for catalytic olefin metathesis were initially reported,75
with
Calderon proposing the formation of a cyclobutane in the coordination sphere of the metal
11
center87
and Grubbs proposing a metallacyclopentane intermediate.88
Around the same time,
Pettit proposed an intermediate in which four carbon atoms form sigma bonds with the metal
center,89
and Chauvin proposed a mechanism involving a metal alkylidene species undergoing a
[2+2] cycloaddition with an olefin to afford a metallacyclobutane which can undergo either
constructive or non-constructive olefin and alkylidene formation (Scheme 1.2.3).90
Through
labeling experiments and analysis of the distribution of metathesis products Chauvin’s
mechanism gained support and is now the accepted mechanism for catalytic olefin
metathesis.91,92
Scheme 1.2.3 Chauvin’s mechanism of olefin metathesis.
With Grubbs-type systems, there are additional considerations when discussing the catalytic
cycle. Based on kinetic data, it was determined that phosphine dissociation is necessary to
generate the 4-coordinate active species (Scheme 1.2.4).93-95
This was considered for the rational
design of the second generation system, as well as a number of other derivatives, where a
stronger donor is introduced trans to the PCy3 to facilitate phosphine dissociation and thus
enhance rate of catalysis and activity. The incoming olefin could then bind to the active
4-coordinate species, either trans or cis to the neutral ligand. This coordination mode influences
how the metallacyclobutane forms with the metal and in the case of the first and second
generation Grubbs catalyst, as well as derivatives, it has been determined that coordination trans
to the neutral ligand (phosphine or NHC) is the favored pathway.96
An example of cis
coordination will be discussed in Section 2.1.2.
12
Scheme 1.2.4 Olefin metathesis mechanism with Grubbs I.
1.3 Nitrile Butadiene Rubber
Modification of Nitrile Butadiene Rubber (NBR) through olefin metathesis is a specific
industrial application of catalytic olefin metathesis. NBR is a co-polymer of butadiene and
acrylonitrile which is formed on an industrial scale by anionic, emulsion polymerization.97
The
resulting polymer contains cis- and trans-alkene functionalities, vinyl groups, and nitrile groups
(Figure 1.3.1). The nitrile groups give NBR useful properties such as stability in oils, fats and
fuels, low permeability, and high temperature resistance.98
NBR is used in a number of machine
parts and belts, for automotive tubing, and even in the soles of running shoes.
Figure 1.3.1 Depiction of functional groups found in Nitrile Butadiene Rubber.
13
Modifications to crude NBR results in the formation of polymers with tailored properties for
specific applications. For example, cross metathesis of NBR with 1-hexene99,100
lowers the
molecular weight and narrows the polydispersity of the resulting polymer. The residual double
bonds of the resulting polymer can then be hydrogenated to form HNBR (Figure 1.3.2).98,101
HNBR is still extremely resistant to oils and fuels, has better thermal stability than NBR, and is
resistant to ozone and oxidative aging. This high strength polymer has numerous oilfield and
automotive applications.
Figure 1.3.2 Depiction of hydrogenated Nitrile Butadiene Rubber.
1.4 Lanxess Project
Laxness is a multinational specialty chemicals and polymers company and is the world's largest
manufacturer of NBR and HNBR. To accomplish the modifications to NBR described in
Section 1.3, the ruthenium based Grubbs II catalyst is used for the cross metathesis with
1-hexene and Wilkinson's Catalyst is used for the hydrogenation to HNBR. The hydrogenation
process is costly as the catalyst is based on the precious metal Rh. A system using a cheaper
technology would be advantageous and economically beneficial to Lanxess. The use of Grubbs II
requires licensing of the technology which adds considerable cost to the process.
The work presented in Chapters 2 through 4 was sponsored by Lanxess. In general, the goals of
the collaboration are twofold; the development of new catalysts based on less expensive metals
that effect the hydrogenation of NBR, and the development of new proprietary olefin metathesis
14
catalysts that effect the cross metathesis of 1-hexene and NBR. The focus of the majority of this
thesis will be on the development of novel olefin metathesis catalysts. While the current patent
literature is extensive and covers a broad range of catalysts, systems involving tridentate and
hemilabile tridentate ligands are absent.
1.5 Scope of Thesis
The goal of this thesis is to use carbenes for two main purposes. The first was developing
proprietary olefin metathesis catalysts for the cross metathesis of NBR and 1-hexene using a
hemilabile tridentate NHC as one of the neutral donors on a Ru-based system. In Chapter 2, the
synthesis and characterization of bis-mixed-carbene ruthenium-alkylidene-thiolate complexes is
explored. These systems are synthesized by a new route to form Ru-alkylidenes from bis-carbene
RuHCl starting materials and alkenyl sulfides. This new method is safe, high yielding, and uses
inexpensive starting materials. It also conveniently installs the alkylidene fragment as well as
transfers a thiolate in one step. Chapter 2 also discusses how the use of ethyl vinyl sulfide results
in the formation of Ru-alkyl and Ru-vinyl species rather than the expected Ru-alkylidene
compounds.
In Chapter 3, the complexes synthesized in Chapter 2 are tested for catalytic olefin metathesis.
They are used in a variety of reactions including ring opening metathesis polymerization, ring
closing metathesis, cross metathesis and the metathesis of NBR and 1-hexene. The use of BCl3
as an additive was proven necessary to activate the previous complexes, where it is believed to
act as a halide-abstracting agent to form a cationic Ru-center, which then effects metathesis.
Chapter 4 describes the synthesis of bis-mixed-carbene ruthenium-alkylidene complexes, which
are obtained from compounds synthesized in Chapter 2 through anion exchange. The complexes
are tested for a variety of metathesis reactions including, ROMP, RCM, CM and cross metathesis
of NBR with 1-hexene. These systems are shown to be more active, and in the bis-halide systems
BCl3 is no longer a necessary additive.
The second goal of this thesis is to employ carbenes as stabilizing ligands for the isolation of
reactive boron species. As such, Chapter 5 describes the use of cyclic (alkyl)(amino)carbenes for
the stabilization and isolation of iminoboranes. These systems are generated under ambient
conditions, in contrast to previously-reported iminoboranes syntheses. CAACs are shown to
15
replace transition metals that have been used for the synthesis and stabilization of iminoboryl
moieties. The intermediates to these iminoboranes are also isolated and are shown to undergo
[2+2] cycloaddition reactions with CO2.
Finally, in Chapter 6, CAACs are used to stabilize a boron (+2) and a boron (+1) system. This is
the first example of a mono-carbene stabilized borylene. This boron (+1) species is shown to be
very electrophilic and is able to activate H2 and strongly bind CO, two reactions that have not
been previously demonstrated with boron Lewis acids.
The entirety of the synthetic work and characterizations described in this thesis were performed
by the author with the exception of elemental analysis and EPR measurements, which were
completed in house by departmental staff. DFT calculations were performed by Dr. David
Martin in Professor Guy Bertrand’s lab at UCSD.
Portions of the work presented herein have been discussed in the following publications:
Chapter 2:
1. Dahcheh, F.; Lund, C.L.; Sgro, M.J. and Stephan, D.W. Multidentate Carbene-Ru-Based
Metathesis Catalysts. US Patent Application 61827152, filed May 24, 2013. Patent
Pending.
2. Reprinted with permission from: “Dahcheh, F. and Stephan, D.W. A New Route to
Ruthenium Thiolate Alkylidene Complexes. Organometallics 2013, 32, 5253-5255”.
Copyright (2013) American Chemical Society.
3. Reproduced from: “Dahcheh, F. and Stephan, D.W. Reactions of Ruthenium Hydrides
with Ethyl Vinyl Sulfides. Dalton Transactions 2014, 43, 3501-3507” with permission
from The Royal Society of Chemistry.
Chapter 3:
1. Dahcheh, F.; Lund, C.L.; Sgro, M.J. and Stephan, D.W. Multidentate Carbene-Ru-Based
Metathesis Catalysts. US Patent Application 61827152, filed May 24, 2013. Patent
Pending.
16
Chapter 4:
1. Dahcheh, F.; Lund, C.L.; Sgro, M.J. and Stephan, D.W. Multidentate Carbene-Ru-Based
Metathesis Catalysts. US Patent Application 61827152, filed May 24, 2013. Patent
Pending.
Chapter 6:
1. Dahcheh, F.; Martin, D.; Stephan, D.W. and Bertrand, G. Synthesis and Reactivity of a
CAAC-Aminoborylene Adduct: A Hetero-Allene or an Organoboron Isoelectronic with
Singlet Carbenes? Angew. Chem. Int. Ed. 2014, Accepted, DOI:10.1002/anie.201408371.
The following work was also completed during the completion of this degree, but has not been
included in the thesis:
1. Sgro, M.J.; Dahcheh, F. and Stephan, D.W. Synthesis and Reactivity of Ruthenium-
Hydride Complexes Containing a Tripodal Aminophosphine Ligand. Organometallics
2014, 33, 578-586.
2. Dahcheh, F. and Stephan, D.W. Ruthenium and Rhodium Complexes of Thioether-
Alkynylborates. Organometallics 2012, 31, 3222-3227.
17
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22
Chapter 2 Synthesis and Characterization of Bis-Mixed-Carbene
Ruthenium-Alkylidene-Thiolate Complexes
2.1 Introduction
2.1.1 First Isolated Transition Metal Based Alkylidene Complex
In seeking avenues to new metathesis catalysts, a key facet involves strategies to install
Ru-alkylidene moieties. The first isolated transition metal alkylidene complex was based on Ta
and reported by Schrock and co-workers in 1974 (Scheme 2.1.1).1-3
This synthetic route opened
up the field of well defined olefin metathesis catalysts.
Scheme 2.1.1 Synthesis of the first isolated transition metal alkylidene.
2.1.2 Modifications to Grubbs’ Catalyst
Since the discovery of Grubbs catalysts (A, B, and C in Figure 2.1.1), numerous modifications
have been undertaken to improve activity and stability,4 and numerous systems capable of olefin
metathesis have been developed with a generalized structure (D in Figure 2.1.1) shown below.
Based on the accepted mechanism of Ru catalyzed olefin metathesis, the only essential ligand on
the metal center is the alkylidene.5-7
This indicates that the other ligands are open for
modification to alter steric and electronic properties which affects catalyst activity, selectivity,
and stability.
Figure 2.1.1 Grubbs’ catalysts and a generalized structure of a Ru olefin metathesis catalysts.
23
Whereas the majority of systems reported thus far are 5-coordinate species, there are examples of
both 4-coordinate and 6-coordinate systems capable of olefin metathesis. In 2004, Piers and
co-workers reported the synthesis of a 4-coordinate Ru-phosphonium alkylidene species (A in
Figure 2.1.2).8 These 14e
- phosphonium alkylidene complexes were found to be rapidly initiating
olefin metathesis catalysts.9 There is also the possibility of introducing a 6th ligand to the
coordination sphere through chelation as was demonstrated by Grubbs and co-workers in
2011.10-12
. Such systems (B in Figure 2.1.2) were shown to be effective Z-selective olefin
metathesis catalysts.
Figure 2.1.2 Examples of 4- and 6-coordinate Ru-alkylidene olefin metathesis catalysts.
The Z-selectivity is due to the chelating anion occupying the coordination site trans to the NHC
during the catalytic cycle. This geometry forces the incoming olefin to bind cis to the NHC
which is also locked in place by the cyclometallated NHC ligand. The resulting
metallacyclobutane intermediate is formed in a side-on fashion and as such the substituents on
the metallacyclobutane are forced to point away from the Mes group and the resulting olefin that
is produced adopts a Z-conformation.13
In further attempts to increase catalytic activity and catalyst stability, over 400 complexes
containing different NHC ligands have been prepared.14
A summary of generalized structures is
depicted in Figure 2.1.3.
24
Figure 2.1.3 Generalized structures of NHCs used as ligands for Ru olefin metathesis catalysts.
Through modifications to the NHC ligand, catalysts with increased activity for specific
applications and catalyst properties, such as aqueous15,16
and asymmetric17-19
catalysis, were
accessible. Nonetheless, the Grubbs II catalyst or Hoveyda-Grubbs catalyst provide reasonable
activity and stability for most olefin metathesis applications and therefore remain the most
widely applied catalysts in the ruthenium based family.14,20,21
2.1.3 Bis-Carbene Olefin Metathesis Catalysts
One of the modifications applied to Grubbs’s catalysts include replacement of both phosphines
with NHCs. Herrmann22
and co-workers, in 1998, and Grubbs23
and co-workers, in 2003,
prepared bis-NHC ruthenium alkylidene complexes (A and B in Figure 2.1.4). While these
species displayed enhanced stability, the activity was modest compared to the second generation
Grubbs catalyst. This is presumably a result of the strong binding of the carbene to the metal
center which disfavors dissociation.
25
Figure 2.1.4 Examples of bis-carbene Ru-alkylidene complexes.
Several other reports have examined the impact of varying the electronic and steric nature of the
carbenes on the catalytic activity. Plenio and co-workers24,25
introduced electron-poor NHCs in
the bis-NHC ruthenium systems which improved the activity in RCM and ROMP reactions (C
and D in Figure 2.1.4). It was shown in these reports that the electron-poor carbene dissociates
while SIMes stays ligated providing access to the active, 4-coordinate species. Nolan and
co-workers26
studied the impact of introducing one smaller NHC, (E in Figure 2.1.4), in mixed
carbene Ru-indenylidene complexes. Compounds with a smaller carbene showed improved
activity in RCM at very low catalyst loading. In all cases, bis-carbene Ru-alkylidenes metathesis
catalysts are active at elevated temperatures (80 - 120 °C) and are thought to proceed via carbene
dissociation.
2.1.4 Routes to Ru-Alkylidene Complexes
The different known methods for synthesis of Ru-alkylidene complexes have been
comprehensively reviewed by Fogg and Foucault.27
The following are select examples of the
most common routes.
26
The first well-defined Ru-based olefin metathesis catalyst was reported by Grubbs and
co-workers4,28
and was synthesized by the ring opening of 2,2-diphenylcyclopropene with
Ru(PPh3)3Cl2 to give the corresponding vinylalkylidene (Scheme 2.1.2). This method requires
the difficult synthesis of the cyclopropene and the corresponding vinylalkylidene initiates more
slowly than the typical benzylidene.20,29
Scheme 2.1.2 Synthesis of the first ruthenium alkylidene.
Following this discovery, a method to generate an alkylidene through the use of diazomethanes
as transfer reagents was reported by Grubbs and co-workers (Scheme 2.1.3).4 This route is high
yielding and provides a direct pathway to the ruthenium benzylidene and consequently first
generation Grubbs’ catalyst (Grubbs I). Diazomethanes are, however, shock sensitive and
explosive and therefore must be handled with extreme caution.
Scheme 2.1.3 Synthesis of Grubbs I catalyst using phenyl diazomethane.
In 2001, Milstein and co-workers developed a method using a sulfur ylide as an alkylidene
transfer reagent.30
This method provides a safer synthetic route over the use of diazomethanes
and offers a direct route to the ruthenium benzylidene. A stoichiometric amount of thioether
waste is, however, generated (Scheme 2.1.4). This method is also not general and low-yielding if
the phenyl group is changed to other alkyl substituents such as methyl.
27
Scheme 2.1.4 Synthesis of Grubbs I catalyst using a sulfur ylide.
The reaction of Ru(PPh3)3Cl2 with 1,1-diphenyl-2-propyn-1-ol occurs with the loss of H2O to
produce a Ru indenylidene species, which undergoes an exchange reaction by subsequent
addition of PCy3 (Scheme 2.1.5).31
This complex is active for olefin metathesis but is slower
initiating than Grubbs I catalyst. This Ru-indenylidene complex, however, can be used to
synthesize Grubbs Catalyst by the addition of an excess of styrene.
Scheme 2.1.5 Synthesis of Grubbs I catalyst via an indenylidene intermediate.
While the previous routes utilized Ru dichloride species, Grubbs and co-workers also developed
a method for preparing Ru-alkylidene complexes by reacting Ru(0) species and
dichloroalkanes.32
For example, heating Ru(cod)(cot) in the presence of PCy3 and Cl2CHPh
results in the formation of Grubbs I catalyst (Scheme 2.1.6). In an analogous route, mixing
RuH2(H2)2(PCy3)2 with cyclohexene results in the in situ formation of a Ru(0) species which
subsequently reacts with Cl2CHPh to also give Grubbs I catalyst.
28
Scheme 2.1.6 Synthesis of Grubbs I catalyst from Ru(0) species.
Recently a new method of preparing ruthenium alkylidenes from Ru(0) starting materials and
dithioacetals has been developed (Scheme 2.1.7). This method conveniently installs the
alkylidene fragment as well as a tridentate dithiolate ligand in one simple step.33
Scheme 2.1.7 Synthesis Ru-alkylidenes from dithioacetals and Ru(PPh3)3(H)2.
A unique example of modification to the Grubbs framework came from the Piers' group where
they found that protonation of a Ru carbide species with Jutzi's acid leads to the formation of the
Ru-phosphonium alkylidene (Scheme 2.1.8).8
Scheme 2.1.8 Synthesis of a ruthenium phosphonium alkylidene complex.
29
In addition to the previous routes, several methods for generating Ru-alkylidene using Ru
hydrides have been reported. Reacting Ru(PPh3)3HCl with 3-chloro-3-methyl-1-butyne followed
by phosphine exchange affords the Ru vinylalkylidene (Scheme 2.1.9).34
Scheme 2.1.9 Synthesis of a vinylalkylidene using propargyl chloride.
Grubbs and co-workers have used vinyl chlorides in combination with RuHCl(PCy3)2(H2) to
yield Ru-alkylidene complexes (Scheme 2.1.10). This route, however, is not synthetically viable
as isolation of the complexes is problematic and is low yielding.35
Scheme 2.1.10 Synthesis of a Ru-alkylidene using vinyl chloride.
Alternatively, starting from [Ru(PiPr3)2HCl]2, this synthon can be converted to an alkylidene
(Scheme 2.1.11) via the addition of vinyl chloroformate.36
Scheme 2.1.11 Synthesis of a Ru-ethylidene using vinyl chloroformate.
This reaction proceeds through the liberation of CO2 and transfer of the chloride to the Ru center
as the ethylidene is formed.
30
2.2 Results and Discussion
2.2.1 Synthesis of Ru-Hydride Complexes
In recent communications37,38
we described a synthetic strategy to the species of the general
formula (Im(OMe)2)(SIMes)(PPh3)RuHCl (Im(OMe)2 = (C3H2(NCH2CH2OMe)2). In a similar
fashion a series of related compounds with different NHC derivatives were synthesized. The
reaction of (Im(OMe)2)(PPh3)2RuHCl37
with IMes in THF at 60 oC proceeds overnight yielding
2-1 as a red solid in 73% yield (Scheme 2.2.1).
Scheme 2.2.1 Synthesis of 2-1 to 2-3.
The 1H NMR spectrum of 2-1 shows a doublet at -28.12 ppm, with a coupling constant of 26 Hz,
indicative of a hydride coupled to a single phosphorus center which was observed at 43.9 ppm in
the 31
P{1H} NMR spectrum. Single crystal X-ray analysis of 2-1 confirmed its formulation as
(IMes)(Im(OMe)2)(PPh3)RuHCl (Figure 2.2.1) with a five-coordinate square-pyramidal Ru
center where the two NHCs, chloride, and phosphine form the base of the pyramid and the
hydride occupies the apex. The Ru-C distances for IMes and Im(OMe)2 are 2.077(2) and
1.969(2) Å, respectively. The trans influence of these carbene ligands is reflected in the
elongated Ru-P and Ru-Cl distances of 2.2880(6) and 2.4509(6) Å, respectively. The hydride
31
was located from the difference map with a Ru-H distance of 1.50(3) Å and the cis disposition of
the carbene ligands in 2-1 results in a C-Ru-C angle of 91.27(9)°.
Figure 2.2.1 POV-ray depiction of the molecular structure of 2-1. Ru: dark green, O: red, Cl:
green, N: aquamarine, P: orange, C: black, H: gray. H-atoms except for Ru-H and IMes-CH
omitted for clarity.
Similarly, reaction of (Im(OMe)2)(PPh3)2RuHCl37
with IMes-Cl2 in THF at 60 °C for 48 hours
resulted in the formation of 2-2 in 65% yield. The 1H NMR spectrum of 2-2 reveals a doublet at
28.11 ppm with 2JPH of 25 Hz and the
31P{
1H} NMR spectrum shows a singlet at 43.2 ppm.
X-ray analysis of single crystals of 2-2 revealed a 5-coordinate square-pyramidal Ru center
where the base of the pyramid is formed by the two NHCs, chloride, and phosphine ligands and
the hydride occupies the apex: thus, the formulation is (IMes-Cl2)(Im(OMe)2)(PPh3)RuHCl
(Figure 2.2.2). The Ru-C distances for IMes-Cl2 and Im(OMe)2 are 2.058(5) and 1.976(5) Å,
respectively. The Ru-P and Ru-Cl distances are 2.314(1) and 2.452(1) Å, respectively, and the
Ru-H distance is 1.51(4) Å. Similar to 2-1, the cis disposition of the carbene ligands in 2-2
results in a C-Ru-C angle of 97.3(2)°.
32
Figure 2.2.2 POV-ray depiction of the molecular structure of 2-2. Ru: dark green, O: red, Cl:
green, N: aquamarine, P: orange, C: black, H: gray. H-atoms except for Ru-H omitted for clarity.
In an analogous reaction, (Me2Im(OMe)2)(PPh3)2RuHCl38
reacted with SIMes in THF at 50 oC
for 24 hours to give 2-3 in 73% yield. The hydride and phosphorus signals in the 1H and
31P{
1H}
NMR spectra for 2-3 are observed at -27.43 ppm and 36.5 ppm, respectively. Single crystal of
compound 2-3 were grown and an X-ray analysis confirmed its formulation as
(SIMes)(Me2Im(OMe)2)(PPh3)RuHCl (Figure 2.2.3).
Figure 2.2.3 POV-ray depiction of the molecular structure of 2-3. Ru: dark green, O: red, Cl:
green, N: aquamarine, P: orange, C: black, H: gray. H-atoms except for Ru-H omitted for clarity.
33
The Ru-C distances for SIMes and Me2Im(OMe)2 are 2.077(3) and 1.985(3) Å, respectively,
similar to those in 2-1. The Ru-P and Ru-Cl distances are 2.3384(8) and 2.4583(9) Å,
respectively, while the Ru-H distance is 1.50(3) Å. Similar to 2-1 and 2-2, the cis disposition of
the carbene ligands in 2-3 results in a C-Ru-C angle of 92.1(1)°.
2.2.2 Synthesis of Ru-Alkylidene Complexes Using Aryl-Alkenyl Sulfides
With a series of bis-carbene Ru-hydride species in hand, reactions with aryl alkenyl sulfides
were explored. Reaction of (Im(OMe)2)(SIMes)(PPh3)RuHCl37,38
with phenyl vinyl sulfide
(Scheme 2.2.2) in CH2Cl2, for four hours at 25 °C, yielded a new red solid 2-4 in 92% yield. The
1H NMR spectrum of 2-4 reveals signals arising from carbene and thiolate ligands as well as a
broad singlet at 18.29 ppm corresponding to one proton which was assigned to the Ru=CH
fragment with the corresponding carbon signal in the 13
C{1H} NMR spectrum at 313.7 ppm.
Scheme 2.2.2 Synthesis of 2-4 to 2-8.
A single crystal X-ray analysis of compound 2-4 confirmed its formulation as
(Im(OMe)2)(SIMes)(PhS)RuCl(=CHCH3) (Figure 2.2.4). The geometry at the metal center is
distorted square pyramidal in nature and similar to related bis-carbene Ru-alkylidene
complexes,22-26
the two carbenes are positioned trans to each other with a C-Ru-C angle of
158.23(6)o. The two anionic groups are also in a trans disposition with the alkylidene fragment
occupying the pseudo-axial position. The Ru-C distances for the NHCs are 2.084(2) and
2.100(2) Å for SIMes and Im(OMe)2, respectively, while the Ru-C distance for the alkylidene is
34
1.820(2) Å. The corresponding Ru-Cl distance is 2.4744(4) Å while the Ru-S distance is
2.3595(5) Å.
Figure 2.2.4 POV-ray depiction of the molecular structure of 2-4. Ru: dark green, O: red, Cl:
green, N: aquamarine, S: yellow, C: black. H-atoms omitted for clarity.
Compound 2-5 was, analogously, prepared by the addition of phenyl vinyl sulfide to a solution
of 2-1 in CH2Cl2, and stirring for five hours at 25 °C, where it was isolated as a red solid in a
modest 59% yield. The 1H NMR spectrum of 2-5 reveals a quartet at 19.09 ppm, with a
3JHH of
6 Hz, which integrates to one proton and is assigned to the Ru=CH fragment. The corresponding
carbon signal for this fragment was derived from a two dimensional NMR experiment (HSQC)
and is present at 313.6 ppm. A single X-ray analysis of compound 2-5 confirmed its formulation
as (Im(OMe)2)(IMes)Ru(=CHCH3)Cl(SPh) where the geometry around the Ru center is best
described as distorted square pyramidal (Figure 2.2.5).
35
Figure 2.2.5 POV-ray depiction of the molecular structure of 2-5. Ru: dark green, O: red, Cl:
green, N: aquamarine, S: yellow, C: black, H: gray. H-atoms except for IMes-CH omitted for
clarity.
The two carbenes are trans with a C-Ru-C angle of 158.15(13)o, the two anionic groups are also
in a trans disposition while the alkylidene fragment occupies the pseudo-axial position. The
Ru-C distances for the NHCs are 2.102(3) and 2.086(3) Å for IMes and Im(OMe)2, respectively.
The Ru-C distance for the alkylidene is 1.818(4) Å and the corresponding Ru-Cl distance is
2.4783(9) Å while the Ru-S distance is 2.3592(9) Å.
The effect of having an electron withdrawing group on the thiolate ligand was probed using
electron poor aryl vinyl sulfides. The addition of p-fluorophenyl vinyl sulfide to a solution of
(Im(OMe)2)(SIMes)(PPh3)RuHCl37
in CH2Cl2, and stirring for four hours at room temperature,
resulted in the isolation of 2-6 as a red solid in 80% yield (Scheme 2.2.2). The 1H and
13C{
1H}
NMR spectra of 2-6 reveal a broad singlet at 18.34 ppm, which integrates to one proton, and a
signal at 313.5 ppm, respectively, which are assigned to the Ru=CH fragment. The 19
F{1H}
NMR spectrum shows a broad singlet at 124.49 which corresponds to the p-fluorophenyl
thiolate moiety.
36
Figure 2.2.6 POV-ray depiction of the molecular structure of 2-6. Ru: dark green, O: red, Cl:
green, N: aquamarine, S: yellow, C: black, F: deep pink. H-atoms omitted for clarity.
A single crystal X-ray analysis of compound 2-6 confirmed its formulation as
Ru(S(pFC6H4))(Cl)=CHCH3(Im(OMe)2)(SIMes) where the geometry around the metal center is
distorted square pyramidal (Figure 2.2.6). The geometry of 2-6 is related to that seen in 2-4 and
2-5 where the two carbenes are trans with a C-Ru-C angle of 158.59(14)o, the two anionic
groups are also in a trans disposition while the alkylidene fragment occupies the pseudo-axial
position. The Ru-C distances for the carbenes are 2.089(3) and 2.101(3) Å for SIMes and
Im(OMe)2, respectively, while the Ru-C distance for the alkylidene is 1.807(4) Å. The
corresponding Ru-Cl distance is 2.5009(9) Å while the Ru-S distance is 2.3494(9) Å.
In a similar fashion, 2-7 was isolated as a purple solid in 75% yield from the addition of
p-nitrophenyl vinyl sulfide to a solution of (Im(OMe)2)(SIMes)(PPh3)RuHCl in CH2Cl2. The 1H
NMR spectrum of 2-7 reveals signals arising from both carbene and thiolate ligands as well as a
quartet at 18.42 ppm, with a 3JHH of 6 Hz, which integrates to one proton and could be assigned
to the Ru=CH fragment. The corresponding carbon signal for this fragment is seen at 314.2 ppm
in the 13
C{1H} NMR spectrum.
The effect of having a more strongly donating carbene was probed by the addition of phenyl
vinyl sulfide to a solution of 2-3 in CH2Cl2, and stirring for one hour at room temperature, to
form 2-8 which was isolated as a red solid in 80% yield. The 1H NMR spectrum of 2-8 reveals
signals arising from carbene and thiolate ligands as well as a broad singlet at 19.05 ppm which
37
integrates to one proton and is assigned to the Ru=CH fragment. The corresponding carbon
signal for this fragment is at 312.0 ppm in the 13
C{1H} NMR spectrum. Single crystals suitable
for an X-ray diffraction study were grown and the formulation of 2-8 was confirmed as
(Me2Im(OMe)2)(SIMes)Ru(=CHCH3)Cl(SPh) where the geometry around the metal center is
distorted square pyramidal (Figure 2.2.7).
Figure 2.2.7 POV-ray depiction of the molecular structure of 2-8. Ru: dark green, O: red, Cl:
green, N: aquamarine, S: yellow, C: black. H-atoms omitted for clarity.
Similar to 2-4, 2-5 and 2-6, the two carbenes are trans with a C-Ru-C angle of 158.15(13)o, the
two anionic groups are also in a trans disposition while the alkylidene fragment occupies the
pseudo-axial position. The Ru-C distances for the NHCs are 2.070(5) and 2.114(5) Å for SIMes
and Me2Im(OMe)2, respectively. The Ru-C distance for the alkylidene is 1.811(5) Å and the
corresponding Ru-Cl distance is 2.4685(14) Å while the Ru-S distance is 2.3663(14) Å.
Analogues incorporating pentafluorophenylthiolate groups are accessible as the E and Z isomers
of pentafluorophenyl alkenyl sulfides are readily prepared employing a modification of literature
procedures described by Peach and co-workers39
(2-9) and by Ranu and co-workers.40
In a
similar method, (C6F5)SCH=CHR (R = n-Pr 2-10, n-Bu 2-11) were prepared (Scheme 2.2.3).
Scheme 2.2.3 Synthesis of 2-10 and 2-11.
38
The addition of 2-9 to a solution of (Im(OMe)2)(SIMes)(PPh3)RuHCl in C6H5Br at 25 °C gave
rise to a brown solution upon stirring overnight. Isolation of compound 2-12 as a pink/red solid
was achieved in 70% yield (Scheme 2.2.4) and the 1H NMR spectrum of 2-12 displays a doublet
of doublets at 15.65 ppm with coupling constants of 8 and 3 Hz. This signal integrates to one
proton and is assigned to the Ru=CH fragment and the corresponding carbon signal was
identified via a 2-D NMR experiment (HSQC) at 309.6 ppm. The 19
F{1H} NMR spectrum of
2-12 shows five signals indicating a dissymmetric environment of the (C6F5)S- moiety.
Scheme 2.2.4 Synthesis of 2-12 to 2-14.
A single X-ray analysis of compound 2-12 confirmed its formulation as
(Im(OMe)2)(SIMes)(F5C6S)RuCl(=CH(CH2Ph)) where the geometry around the metal center is
best described as distorted square pyramidal (Figure 2.2.8). In contrast to 2-4 to 2-8, the two
carbenes adopt a cis-arrangement, similar to that observed in compounds where a tridentate
bis-carbene ligand is used.41
The SIMes ligand is trans to the chloride whereas the Im(OMe)2
carbene is trans to the thiolate moiety. The alkylidene occupies the pseudo-axial position of the
square pyramidal coordination sphere. The Ru-C distances for the NHCs are 2.047(4) and
2.062(4) Å for SIMes and Im(OMe)2, respectively, while the Ru-C distance for the alkylidene is
1.815(4) Å. The corresponding Ru-Cl distance is 2.4660(9) Å while the Ru-S distance is
2.360(1) Å.
39
Figure 2.2.8 POV-ray depiction of the molecular structure of 2-12. Ru: dark green, O: red, Cl:
green, N: aquamarine, S: yellow, F: deep pink, C: black. H-atoms omitted for clarity.
Similarly, the subsequent reactions of 2-10 and 2-11 with Im(OMe)2(SIMes)(PPh3)RuHCl
afforded orange/brown solids Im(OMe)2(SIMes)(F5C6S)RuCl(=CHC4H9) 2-13 and
Im(OMe)2(SIMes)(F5C6S)RuCl(=CHC5H11) 2-14 in 73% and 71% yield, respectively. The 1H
NMR spectra reveal a triplet at 16.37 ppm, with a coupling constant of 5 Hz, for 2-13 and a
triplet at 16.44 ppm, with a coupling constant of 5 Hz, for 2-14 which correspond to the Ru=CH
fragments. The corresponding carbon signals for these fragments were derived from two
dimensional NMR experiments (HSQC) and are present at 315.2 and 315.3 ppm, for 2-13 and
2-14, respectively. Both 19
F{1H} NMR spectra of 2-13 and 2-14 show five signals, each,
indicating dissymmetric environments of the (C6F5)S- moieties.
40
Figure 2.2.9 POV-ray depiction of the molecular structure of 2-13. Ru: dark green, O: red, Cl:
green, N: aquamarine, S: yellow, F: deep pink, C: black. H-atoms omitted for clarity.
Figure 2.2.10 POV-ray depiction of the molecular structure of 2-14. Ru: dark green, O: red, Cl:
green, N: aquamarine, S: yellow, F: deep pink, C: black. H-atoms omitted for clarity.
Repeated crystallization attempts of these compounds yielded crystals of poor quality,
nonetheless, preliminary X-ray studies (Figure 2.2.9 for 2-13 and Figure 2.2.10 for 2-14)
confirmed their formulations. Similar to 2-12, the two carbenes in 2-13 and 2-14 adopt a cis
orientation.
41
2.3 Reactions of Ru-Hydride Species with Ethyl Vinyl Sulfide
The effect of introducing a more electron rich vinyl sulfide was probed by reacting the
corresponding Ru-hydride species with ethyl vinyl sulfide. The Ru-hydride precursors
(Im(OMe)2)(PPh3)2RuHCl (2-15) and (Me2Im(OMe)2)(PPh3)2RuHCl (2-16), were prepared using
previously published methodologies37,38
and subsequently reacted with ethyl vinyl sulfide. In the
case of 2-15, the mixture of ethyl vinyl sulfide with a benzene solution of 2-15 was stirred for six
hours resulting in an orange solution. After workup, 2-17 was isolated as a light orange solid in
80% yield (Scheme 2.3.1).
Scheme 2.3.1 Synthesis of 2-17 to 2-19.
The presence of diethyl sulfide as a by-product was confirmed by GC-MS analysis of the
reaction mixture. The 31
P{1H} NMR spectrum shows two doublets at 41.9 and 35.7 ppm with a
coupling constant of 319 Hz indicative of two phosphines in a trans disposition. A multiplet at
6.29 ppm in the 1H NMR spectrum, which integrates to one proton, is assigned to the
Ru-CHOMe proton. The 13
C{1H} NMR spectrum displays a triplet at 82.5 ppm with a
2JPC of
42
6 Hz which corresponds to the Ru-alkyl carbon. In addition to these NMR spectra, single crystals
of 2-17 afforded the molecular structure determination (Figure 2.3.1) which affirmed its
formulation as ((MeOCH2CH2)C3H2N2(CH2CH(OMe))RuCl(PPh3)2.
Figure 2.3.1 POV-ray depiction of the molecular structure of 2-17. Ru: dark green, O: red, Cl:
green, N: aquamarine, P: orange, C: black. H-atoms omitted for clarity.
The distorted square pyramidal geometry about the Ru center consists of a square plane of two
phosphine donors, the chloride and the carbene carbon with an alkyl carbon fragment occupying
the pseudo-axial position. The corresponding Ru-Cl bond length is 2.4468(8) Å and the Ru-P
bond lengths are 2.3407(8) and 2.3515(8) Å. A Ru-C bond length of 1.961(3) Å is observed for
the NHC carbon which is similar to previously reported bond lengths for Ru complexes
employing similar ligands. The Ru-C bond length for the alkyl fragment of 2.061(7) Å is slightly
shorter than typical Ru-C single bonds.42-45
Chelation of the two carbons to the Ru center leads to
the formation of a 5-member metalla-ring with a C-Ru-C angle of 78.2(2)°. It is interesting to
note that subjecting a solution of 2-17 in C6D6 to 4 atm of H2 at room temperature leads to the
quantitative reformation of 2-15.
The corresponding reaction of 2-16 with ethyl vinyl sulfide in CH2Cl2 at room temperature
results in the isolation of 2-18 as a red solid in 69% yield (Scheme 2.3.1). Similar to 2-17, the
31P{
1H} NMR spectrum of 2-18 reveals two doublets at 40.8 and 33.0 ppm with a coupling
constant of 315 Hz, while the 1H NMR spectrum shows a doublet of doublet of doublets at
43
6.35 ppm (3JHH = 11 Hz,
3JHH = 7 Hz,
3JPH = 4 Hz) and the corresponding carbon shift is a triplet
at 80.6 ppm (2JPC = 6 Hz) in the
13C{
1H} NMR spectrum. A single crystal X-ray analysis (Figure
2.3.2) confirmed 2-18 to be ((MeOCH2CH2)C3Me2N2(CH2CH(OMe))RuCl(PPh3)2, the analogue
of 2-17. The geometry about the ruthenium center in 2-18 is directly analogous to 2-17 with a
Ru-CNHC distance of 1.986(3) Å and the Ru-Calkyl bond length of 2.066(4) Å and the C-Ru-C
angle of 79.2(2)°.
Figure 2.3.2 POV-ray depiction of the molecular structure of 2-18. Ru: dark green, O: red, Cl:
green, N: aquamarine, P: orange, C: black. H-atoms omitted for clarity.
Interestingly, dissolution of 2-18 in C6D6 prompts the formation of a new product, 2-19, which is
identified by a singlet in the 31
P{1H} NMR spectrum at 36.7 ppm. Stirring a solution of 2-18 in
benzene at 25 °C for 24 hours resulted in the formation of 2-19 as a yellow solid which was
isolated in 77% yield. Loss of methanol from 2-18 was confirmed by the 1H NMR spectrum
which also shows a doublet at 7.67 ppm with a coupling constant of 5 Hz and a doublet of triplet
at 5.66 with 3JHH of 5 Hz and
4JPH of 3 Hz. As each of these signals integrated to one proton, they
are assigned to the Ru-vinyl protons (RuCHCHN). The corresponding carbon signals are present
in the 13
C{1H} NMR spectrum as a triplet at 135.6 ppm (
2JPC = 19 Hz) and a broad singlet at
124.6 ppm. These data are consistent with the formulation of 2-19 as
((MeOCH2CH2)C5H6N2(CHCH)RuCl(PPh3)2 (Scheme 2.3.1).
44
We were interested in the reactivity differences of the bis-carbene Ru-hydride systems in
comparison to the mono-carbene systems. As such, the addition of ethyl vinyl sulfide to a
benzene solution of 2-2037
and stirring for 16 hours followed by workup resulted in the isolation
of 2-21 as a purple solid in 76% yield (Scheme 2.3.2). The presence of diethyl sulfide as a
by-product was determined through GC-MS analysis of the reaction mixture and the presence of
methanol was observed in the 1H NMR of the reaction mixture. The
31P{
1H} NMR spectrum of
2-21 shows a singlet at 37.0 ppm which indicates the presence of PPh3 bound to the Ru center. A
doublet at 7.70 ppm with a coupling constant of 5 Hz and a doublet of doublet at 6.10 with a 3JHH
of 5 Hz and a 4JPH of 2 Hz in the
1H NMR spectrum are observed which integrate to one proton
each and are assigned to the Ru-vinyl protons (RuCHCHN). The 13
C{1H} NMR spectrum
displays two doublets at 159.6 ppm with a 2JPC of 15 Hz and at 124.2 ppm with a
4JPC of 2 Hz
which correspond to the Ru-vinyl carbons.
Scheme 2.3.2 Synthesis of 2-21.
In addition to NMR spectra, single crystals of 2-21 were obtained and the molecular structure
(Figure 2.3.3) was determined allowing for its formulation as ((MeOCH2CH2)C3H2N2(CHCH)
RuCl(PPh3)(SIMes). There is a distorted square pyramidal geometry about the Ru center with the
square plane made up of PPh3, SIMes, the chloride as well as the vinylic carbon, with the
carbene carbon of the NHC occupying the pseudo-axial position. A Ru-C bond length of
1.940(2) Å is observed for the NHC carbon while the Ru-C bond length of SIMes is 2.113(2) Å.
The Ru-C bond length for the vinyl fragment is 2.031(3) Å, which is in the range of typical
Ru-Cvinyl bonds.46-51
The C-C bond length of the vinyl fragment is consistent with a C=C double
bond with a distance of 1.328(4) Å. Chelation of the two carbons to the Ru center leads to the
45
formation of a 5-member metalla-ring with a C-Ru-C angle of 75.68(11)°. The corresponding
Ru-Cl bond length is 2.4589(7) Å and the Ru-P bond length is 2.3164(6) Å.
Figure 2.3.3 POV-ray depiction of the molecular structure of 2-21. Ru: dark green, O: red, Cl:
green, N: aquamarine, P: orange, C: black. H-atoms except for vinylic protons omitted for
clarity.
Similarly, the reaction of ethyl vinyl sulfide with 2-1 in C6H6 and stirring for 48 hours resulted in
the isolation of 2-22 as red crystals in 79% yield (Scheme 2.3.3). The 31
P{1H} NMR spectrum of
2-22 reveals a singlet at 38.4 ppm. A doublet of doublets at 7.75 ppm, with 3JHH of 5 Hz and
3JPH
of 1 Hz, and a doublet of doublets at 6.09 with a 3JHH of 5 Hz and a
4JPH of 2 Hz are observed in
the 1H NMR spectrum which integrate to one proton each and are assigned to the Ru-vinyl
protons (RuCHCHN). The 13
C{1H} NMR spectrum displays two doublets at 160.2 ppm with a
2JPC of 12 Hz and at 124.4 ppm with a
4JPC of 2 Hz which correspond to the Ru-vinyl carbons.
46
Scheme 2.3.3 Synthesis of 2-22 and 2-23.
In addition to NMR spectra, single-crystal X-ray analysis of 2-22 was used to confirm its
formulation as ((MeOCH2CH2)C3H2N2(CHCH)RuCl(PPh3)(IMes) (Figure 2.3.4). There is a
distorted square pyramidal geometry about the Ru center with the square plane made up of PPh3,
IMes, the chloride as well as the vinylic carbon, with the carbene carbon of the NHC occupying
the pseudo-axial position. A Ru-C bond length of 1.940(3) Å is observed for the NHC carbon
while the Ru-C bond length of IMes is 2.114(2) Å. The Ru-C bond length for the vinyl fragment
is 2.033(3) Å and the C-C bond length of the vinyl fragment is 1.325(4) Å, consistent with a
C=C bond. Similar to 2-21, chelation of the two carbons to the Ru center leads to the formation
of a 5-member metalla-ring with a C-Ru-C angle of 75.72(12)°. The corresponding Ru-Cl bond
length is 2.4528(8) Å and the Ru-P bond length is 2.3104(7) Å.
47
Figure 2.3.4 POV-ray depiction of the molecular structure of 2-22. Ru: dark green, O: red, Cl:
green, N: aquamarine, P: orange, C: black. H-atoms except for vinylic and IMes-CH protons
omitted for clarity.
The addition of ethyl vinyl sulfide to a benzene solution of 2-2 and stirring for 48 hours followed
by workup afforded 2-23 as red crystals in 82% yield. The 31
P{1H} NMR spectrum shows a
singlet at 38.4 ppm which indicates the presence of PPh3. The 1H NMR spectrum reveals a
doublet of doublets at 7.66 ppm, with 3JHH of 5 Hz and
3JPH of 1 Hz, and a doublet of doublets at
6.04 with a 3JHH of 5 Hz and a
4JPH of 2 Hz which integrate to one proton each and are assigned
to the Ru-vinyl protons (RuCHCHN). The 13
C{1H} NMR spectrum displays two doublets at
159.2 ppm with a 2JPC of 12 Hz and at 124.4 ppm with a
4JPC of 2 Hz which correspond to the
Ru-vinyl carbons. The NMR data allowed for the formulation of 2-23 as
((MeOCH2CH2)C3H2N2(CHCH)RuCl(PPh3)(IMes-Cl2).
The formation of compounds 2-17 to 2-23 is thought to be initiated through the initial insertion
of the vinyl-fragment into the Ru-H (Scheme 2.3.1). Donation from the thioether sulfur enhances
electron density at Ru and prompts C-H activation of the pendant ether arm affording loss of
diethyl sulfide. In this fashion compounds 2-17 and 2-18 are generated. Loss of methanol from
2-18 gives the bis-phosphine Ru-vinyl species 2-19. Compounds 2-21 to 2-23 are thought to
form in a similar fashion, although the increased electron density on Ru derived from the
additional carbene ligand facilitates loss of both Et2S and MeOH.
48
2.4 Conclusion
In conclusion, a new method of preparing ruthenium alkylidenes from bis-carbene RuHCl
starting materials and alkenyl sulfides has been developed. This new method is safe, high
yielding, and uses inexpensive starting materials. This provides a route to bis-mixed carbene
ruthenium alkylidene complexes with a hemilabile tridentate carbene and it conveniently installs
the alkylidene fragment as well as transfers a thiolate in one simple step. The use of ethyl vinyl
sulfide, on the other hand, results in the formation of Ru-alkyl and Ru-vinyl species.
2.5 Experimental Section
2.5.1 General Considerations
All manipulations were carried out under an atmosphere of dry, O2-free N2 employing a Vacuum
Atmospheres glove box and a Schlenk vacuum line. Solvents were purified with a Grubbs-type
column system manufactured by Innovative Technology, dispensed into thick-walled Schlenk
glass flasks equipped with Teflonvalve stopcocks (pentane, hexanes, CH2Cl2) and stored over
molecular sieves. Some solvents were dried over the appropriate agents, vacuum-transferred into
storage flasks with Teflon stopcocks and degassed accordingly (C6H6, C6H5Br, C6D5Br, C6D6,
CD2Cl2). 1H,
13C,
19F, and
31P NMR spectra were recorded at 25
oC on a Bruker 400 MHz
spectrometer. Chemical shifts were given relative to SiMe4 and referenced to the residual solvent
signal (1H,
13C) or relative to an external standard (
31P: 85% H3PO4,
19F: CFCl3). In some
instances, signal and/or coupling assignment was derived from two dimensional NMR
experiments (HSQC). Chemical shifts are reported in ppm and coupling constants as scalar
values in Hz. Combustion analyses were performed in house employing a Perkin-Elmer CHN
analyzer. Phenyl vinyl sulfide and ethyl vinyl sulfide were purchased from Sigma Aldrich and
used as received. SIMes,52
IMes,52
IMes-Cl2,53
p-fluorophenyl vinyl sulfide,54
p-nitrophenyl
vinyl sulfide,55
(C6F5)SC8H7,40
(C6F5)SC6H11,40
(C6F5)SC5H9,40
(Im(OMe)2)(PPh3)2RuHCl,37
(Me2Im(OMe)2)(PPh3)2RuHCl,38
and (Im(OMe)2)(SIMes)(PPh3)RuHCl37
were prepared
according to literature procedures.
2.5.2 Synthetic Procedures
Synthesis of 2-1: IMes (0.105 g, 0.354 mmol) in 5 mL THF was added to a solution of
(Im(OMe)2)(PPh3)2RuHCl (0.150 g, 0.177 mmol) in 5 mL of THF and the mixture was heated at
49
60 °C for 24 h. All volatiles were removed in vacuum. The product was extracted with toluene
(10 mL) and filtered through celite. The solution was concentrated to 2 mL and pentane (15 mL)
was added to the red solution to precipitate the product. The red solid was collected on a frit and
dried under vacuum (0.114 g, 73%). X-ray quality crystals were grown from toluene/pentane at
25 oC.
1H NMR (400 MHz, C6D6): δ 7.54 (t,
3JHH = 8 Hz, 6H, PPh3), 7.39 (m, 1H, IMes-CH),
7.04 (m, 2H, Mes-CH), 6.99-6.90 (m, 13H, (9H) PPh3 + (1H) IMes-CH + (2H) Mes-CH + (1H)
Im(OMe)2-CH), 6.66 (d, 3JHH = 2 Hz, 1H, Im(OMe)2-CH), 4.68 (dd,
2JHH = 15 Hz,
3JHH = 3 Hz,
1H, Im(OMe)2-CH2), 3.90 (m, 1H, Im(OMe)2-CH2), 2.92-2.10 (br m, 30 H, Im(OMe)2-CH3 +
Im(OMe)2-CH2 + Mes-CH3), -28.12 (d, 2
JPH = 26 Hz, 1H, Ru-H). 31
P{1H} NMR (161 MHz,
C6D6): δ 43.9 (s, PPh3). 13
C{1H} NMR (101 MHz, C6D6): δ 141.3 (d,
1JPC = 30 Hz, Cipso, PPh3),
137.3 (br, Cipso), 134.9 (d, 2JPC = 11 Hz, o-C, PPh3), 134.3 (IMes-CH), 134.1 (IMes-CH), 130.3
(br, Cipso) 128.9 (d, 4JPC = 2 Hz, p-C, PPh3), 128.8 (Mes-CH), 128.4 (Mes-CH), 127.6 (d,
3JPC =
8 Hz, m-C, PPh3), 119.9 (Im(OMe)2-CH), 118.4 (Im(OMe)2-CH), 72.6 (Im(OMe)2-CH2), 71.4
(Im(OMe)2-CH2), 58.2 (Im(OMe)2-CH3), 57.9 (Im(OMe)2-CH3), 48.0 (Im(OMe)2-CH2), 47.5
(Im(OMe)2-CH2), 21.3 (br s, Mes-CH3), 19.7 (br s, Mes-CH3). Elemental Analysis for
C48H56ClN4O2PRu: C, 64.89; H, 6.35; N, 6.31. Found: C, 65.08; H, 6.59; N, 6.13.
Synthesis of 2-2: IMes-Cl2 (0.174 g, 0.472 mmol) in 5 mL THF was added to a solution of
(Im(OMe)2)(PPh3)2RuHCl (0.200 g, 0.236 mmol) in 5 mL of THF and the mixture was heated at
60 °C for 48 h. All volatiles were removed in vacuum. The product was extracted with toluene
(10 mL) and filtered through celite. The solution was concentrated to 2 mL and pentane (15 mL)
was added to the red solution to precipitate the product. The red solid was collected on a frit and
dried under vacuum (0.147 g, 65%). X-ray quality crystals were grown from toluene/pentane at
25 oC.
1H NMR (500 MHz, C6D6): δ 7.48 (t,
3JHH = 8 Hz, 6H, PPh3), 6.96 (m, 5H, PPh3 +
Mes-CH), 6.90 (m, 8H, PPh3 + Mes-CH), 6.68 (br s, 1H, Im(OMe)2-CH), 6.67 (d, 3JHH = 2 Hz,
1H, Im(OMe)2-CH), 4.61 (ddd, 2JHH = 15 Hz,
3JHH = 4 Hz,
3JHH = 2 Hz, 1H, Im(OMe)2-CH2),
3.88 (m, 1H, Im(OMe)2-CH2), 2.91 (s, 3H, Im(OMe)2-CH3), 2.87 (m, 1H, Im(OMe)2-CH2), 2.81-
2.57 (m, 13H, Im(OMe)2-CH3 + Mes-CH3 + Im(OMe)2-CH2) 2.36-2.15 (m, 10H, Im(OMe)2-CH2
+ Mes-CH3), 2.05 (br s, 3H, Mes-CH3), 28.11(d, 2
JPH = 25 Hz, 1H, Ru-H). 31
P{1H} NMR (161
MHz, C6D6): δ 43.2 (s, PPh3). 13
C{1H} NMR (126 MHz, C6D6, partial): δ 140.7 (d,
1JPC = 31 Hz,
Cipso, PPh3), 134.9 (d, 2JPC = 11 Hz, o-C, PPh3), 129.4 (br s, Cipso)128.3 (Mes-CH), 128.2 (d,
4JPC = 2 Hz, p-C, PPh3), 127.6 (d,
3JPC = 8 Hz, m-C, PPh3), 120.0 (Im(OMe)2-CH), 118.8
50
(Im(OMe)2-CH), 72.4 (Im(OMe)2-CH2), 71.3 (Im(OMe)2-CH2), 58.2 (Im(OMe)2-CH3), 57.9
(Im(OMe)2-CH3), 48.1 (Im(OMe)2-CH2), 47.4 (Im(OMe)2-CH2), 21.3 (br s, Mes-CH3), 18.2
(br s, Mes-CH3). Elemental Analysis for C48H54Cl3N4O2PRu•C6H14: C, 62.15; H, 6.57; N, 5.37.
Found: C, 62.64; H, 6.43; N, 5.45.
Synthesis of 2-3: SIMes (0.070 g, 0.228 mmol) in 5 mL THF was added to a solution of
(Me2Im(OMe)2)(PPh3)2RuHCl (0.100 g, 0.114 mmol) in 5 mL of THF and the mixture was
heated at 50 °C for 24 h. All volatiles were removed in vacuum. The product was extracted with
toluene (10 mL) and filtered through celite. The solution was concentrated to 2 mL and pentane
(15 mL) was added to the red solution to precipitate the product. The red solid was collected on a
frit and dried under vacuum (0.076 g, 73%). 1H NMR (400 MHz, C6D6): δ 7.52 (t,
3JHH = 8 Hz,
6H, PPh3), 6.94 (m, 11H, (9H) PPh3 + (2H) Mes-CH), 6.82 (s, 1H, Mes-CH), 6.51 (s, 1H,
Mes-CH), 4.43 (dt, 2JHH = 16 Hz,
3JHH = 4 Hz, 1H, Me2Im(OMe)2-CH2), 3.60 (m, 1H,
Me2Im(OMe)2-CH2), 3.39-3.16 (m, 8H, (4H) Me2Im(OMe)2-CH2 + (4H) SIMes-CH2), 2.99 (s,
6H, Me2Im(OMe)2-CH3 + Mes-CH3), 2.83 (br s, 5H, Mes-CH3 + Me2Im(OMe)2-CH2), 2.64 (s,
6H, Me2Im(OMe)2-CH3 + Mes-CH3), 2.33 (s, 3H, Mes-CH3), 2.13 (s, 3H, Mes-CH3), 1.92 (s,
3H, Me2Im(OMe)2-4,5-CH3), 1.83 (s, 3H, Me2Im(OMe)2-4,5-CH3), 1.59 (s, 3H, Mes-CH3), -
27.43 (d, 2JPH = 27 Hz, 1H, Ru-H).
31P{
1H} NMR (161 MHz, C6D6): δ 36.5 (s, PPh3).
13C{
1H}
NMR (101 MHz, C6D6, partial): δ 141.2 (d, 1JPC = 29 Hz, Cipso, PPh3), 139.7 (Cipso), 135.0 (d,
2JPC = 11 Hz, o-C, PPh3), 129.4 (Mes-CH), 128.9 (d,
4JPC = 2 Hz, p-C, PPh3), 128.8 (Mes-CH),
127.5 (d, 3JPC = 8 Hz, m-C, PPh3), 125.7 (Cipso), 124.5 (Me2Im(OMe)2-4,5-Cipso), 122.2
(Me2Im(OMe)2-4,5-Cipso), 72.9 (Me2Im(OMe)2-CH2), 71.0 (Me2Im(OMe)2-CH2), 58.4
(Me2Im(OMe)2-CH3), 57.8 (Me2Im(OMe)2-CH3), 51.5 (SIMes-CH2), 50.8 (SIMes-CH2), 46.5
(Me2Im(OMe)2-CH2), 45.9 (Me2Im(OMe)2-CH2), 21.4 (Mes-CH3), 21.2 (Mes-CH3), 21.0
(Mes-CH3), 20.9 (Mes-CH3), 19.6 (Mes-CH3), 17.1 (Mes-CH3), 10.3 (Me2Im(OMe)2-4,5-CH3),
9.8 (Me2Im(OMe)2-4,5-CH3). Elemental Analysis for C50H62ClN4O2PRu•C5H12: C, 66.68; H,
7.53; N, 5.66. Found: C, 66.24; H, 7.46; N, 5.85.
Synthesis of 2-4: Phenyl vinyl sulfide (16.7 μL, 0.128 mmol) was added to a solution of
(Im(OMe)2)(SIMes)(PPh3)RuHCl (0.100 g, 0.112 mmol) in 5 mL CH2Cl2 at room temperature.
The solution was then stirred for 5 hours before the solvent was concentrated to 0.5 mL and
15 mL of pentane was added and the resulting mixture was filtered over a pad of celite. The
pentane was then removed in vacuo and the resulting residue was layered with 10 mL of pentane
51
and left standing overnight. The free triphenylphosphine is taken up into the pentane layer
yielding a red solid (0.079 g, 92%). X-ray quality crystals were grown by slow evaporation of a
hexane solution. 1
H NMR (400 MHz, CD2Cl2): δ 18.29 (br s, 1H, Ru=CH), 7.01(s, 2H, Mes-CH),
6.96 (s, 1H, Mes-CH), 6.94 (s, 1H, Mes-CH), 6.85 (s, 1H, Im(OMe)2-CH), 6.69 (br s, 1H,
Im(OMe)2-CH), 6.60 (m, 3H, S(C6H5)), 6.56 (m, 2H, S(C6H5)), 3.93 (m, 4H, Mes-CH2), 3.32
(br s, 4H, Im(OMe)2-CH2), 3.19 (br s, 4H, Im(OMe)2-CH2 + Im(OMe)2-CH3), 3.16 (s, 3H,
Im(OMe)2-CH3), 3.06 (br s, 2H, Im(OMe)2-CH2), 2.74 (s, 3H, Mes-CH3), 2.62 (s, 3H, Mes-CH3),
2.50 (s, 3H, Mes- CH3), 2.42 (s, 3H, Mes- CH3), 2.35 (s, 3H, Mes- CH3), 2.31 (s, 3H, Mes- CH3),
1.63 (d, 3JHH = 5 Hz, 3H, Ru=CHCH3).
13C{
1H} NMR (101 MHz, CD2Cl2): δ 313.7
(Ru=CHCH3), 223.6 (NCN), 188.8 (NCN), 139.9 (Cipso), 139.03 (Cipso), 138.3 (S(C6H5)), 137.9
(S(C6H5)), 135.5 (Cipso), 130.0 (Mes-CH), 129.9 (Mes-CH), 129.7 (Mes-CH), 129.6 (Mes-CH),
126.9 (S(C6H5)), 121.6 (Im(OMe)2-CH), 121.2 (Im(OMe)2-CH), 72.2 (Im(OMe)2-CH2), 58.7
(Im(OMe)2-CH3), 58.6 (Im(OMe)2-CH3), 51.8 (SIMes-CH2), 51. 7 (SIMes-CH2), 49.1
(OCO-CH2), 48.7 (Ru=CHCH3), 21.2 (Mes-CH3), 20.3 (Mes-CH3), 19.0 (Mes-CH3), 18.8
(Mes-CH3). Elemental Analysis for C38H51ClN4O2RuS: C, 59.69; H, 6.73; N, 7.33. Found: C,
59.89; H, 6.96; N, 7.35.
Synthesis of 2-5: Phenyl vinyl sulfide (16.7 μL, 0.128 mmol) was added to a solution of 2-1
(0.100 g, 0.112 mmol) in 5 mL CH2Cl2 at room temperature. The solution was then stirred for 5
hours before the solvent was concentrated to 0.5 mL and 15 mL of pentane was added and the
resulting mixture was filtered over a pad of celite. The pentane was then removed in vacuo and
the resulting residue was layered with 10 mL of pentane and left standing overnight. The free
triphenylphosphine is taken up into the pentane layer yielding a red solid (0.050 g, 59%). X-ray
quality crystals were grown from benzene/pentane at 25 oC.
1H NMR (500 MHz, C6D6): δ 19.09
(q, 3JHH = 6 Hz, 1H, Ru=CH), 7.03(br m, 1H, S(C6H5)), 7.01 (br m, 1H, S(C6H5)), 6.94 (d,
3JHH = 2 Hz, 1H, Im(OMe)2-CH), 6.85-6.73 (br m, 7H, (3H) S(C6H5), (4H) Mes-CH), 6.65 (s,
1H, d, 3JHH = 2 Hz, 1H, Im(OMe)2-CH), 6.24 (d,
3JHH = 2 Hz, 1H, Mes-CH), 6.23 (d,
3JHH =
2 Hz, 1H, Mes-CH), 3.84 (br s, 2H, Im(OMe)2-CH2), 3.56 (m, 1H, Im(OMe)2-CH2), 3.46 (m, 1H,
Im(OMe)2-CH2), 3.21 (m, 2H, Im(OMe)2-CH2), 3.08 (m, 1H, Im(OMe)2-CH2), 2.96 (s, 3H,
Im(OMe)2-CH3), 2.85 (m, 1H, Im(OMe)2-CH2), 2.77 (s, 3H, Im(OMe)2-CH3), 2.73 (s, 3H,
Mes-CH3), 2.67 (s, 3H, Mes-CH3), 2.48 (s, 6H, Mes-CH3), 2.16 (s, 3H, Mes-CH3), 2.15 (s, 3H,
Mes-CH3), 2.08 (d, 3JHH = 5 Hz, 3H, Ru=CHCH3).
13C{
1H} NMR (126 MHz, C6D6): δ 313.6
52
(Ru=CHCH3), 193.9 (NCN), 189.8 (NCN), 139.4 (Cipso), 139.2 (Cipso), 138.7 (Cipso), 137.8
(Cipso), 137.2 (Cipso), 135.9 (S(C6H5)), 133.0 (S(C6H5)), 129.6 (Mes-CH), 129.4 (Mes-CH), 129.3
(Mes-CH), 129.2 (Mes-CH), 127.0 (S(C6H5)), 124.0 (IMes-CH), 123.6 (IMes-CH), 121.8
(Im(OMe)2-CH), 121.2 (Im(OMe)2-CH), 73.5 (Im(OMe)2-CH2), 72.4 (Im(OMe)2-CH2), 58.3
(Im(OMe)2-CH3), 58.2 (Im(OMe)2-CH3), 49.8 (Im(OMe)2-CH2), 49.0 (Im(OMe)2-CH2), 47.5
(Ru=CHCH3), 21.1 (Mes-CH3), 21.0 (Mes-CH3), 20.4 (Mes-CH3), 20.3 (Mes-CH3), 19.1
(Mes-CH3), 19.0 (Mes-CH3). Elemental Anal.: C38H49ClN4O2RuS: C, 59.86; H, 6.48; N, 7.35.
Found: C, 60.02; H, 6.20; N, 7.22.
Synthesis of 2-6: p-Fluorophenyl vinyl sulfide (0.017 g, 0.224 mmol) was added to a solution of
(Im(OMe)2)(SIMes)(PPh3)RuHCl (0.100 g, 0.112 mmol) in 5 mL CH2Cl2 at room temperature.
The solution was then stirred for 4 h before the solvent was concentrated to 0.5 mL and 15 mL of
pentane was added and the resulting mixture was filtered over a pad of celite. The pentane was
then removed in vacuo and the resulting residue was layered with 10 mL of pentane and left
standing overnight. The free triphenylphosphine is taken up into the pentane layer yielding a red
solid (0.070 g, 80%). X-ray quality crystals were grown from benzene/pentane at 25 oC.
1H NMR
(400 MHz, CD2Cl2): δ 18.34 (br s, 1H, Ru=CH), 7.01(s, 2H, Mes-CH), 6.96 (s, 1H, Mes-CH),
6.93 (s, 1H, Mes-CH), 6.86 (d, 3JHH = 2 Hz, 1H, Im(OMe)2-CH), 6.68 (br s, 1H, Im(OMe)2-CH),
6.51 (m, 2H, p-F-C6H5), 6.34 (app t, 3JHH = 9 Hz, 2H, p-F-C6H5), 3.92 (m, 4H, SIMes-CH2),
3.44-3.26 (br s, 4H, Im(OMe)2-CH2), 3.23 (br s, 3H, Im(OMe)2-CH3), 3.16 (s, 3H,
Im(OMe)2-CH3), 3.13-3.00 (br s, 4H, Im(OMe)2-CH2), 2.74 (s, 3H, Mes-CH3), 2.61 (s, 3H,
Mes-CH3), 2.48 (s, 3H, Mes-CH3), 2.40 (s, 3H, Mes-CH3), 2.36 (s, 3H, Mes-CH3), 2.31 (s, 3H,
Mes-CH3), 1.63 (d, 3JHH = 5 Hz, Ru=CHCH3).
19F{
1H} NMR (178 MHz, CD2Cl2): δ -124.49
(br s). 13
C{1H} NMR (101 MHz, C6D6): δ 313.5 (Ru=CHCH3), 223.9 (NCN), 188.8 (NCN),
159.7 (d, 1JFF = 239 Hz, S(C6H4F)), 147.0 (d,
4JFC = 3 Hz, S(C6H4F)), 140.5 (Cipso), 139.9 (Cipso),
138.6 (Cipso), 138.5 (Cipso), 138.1 (Cipso), 137.9 (Cipso), 137.8 (Cipso), 135.6 (Cipso), 133.9 (br d,
3JFC = 7 Hz, S(C6H4F)), 129.8 (Mes-CH), 129.6 (Mes-CH), 121.7 (Im(OMe)2-CH), 121.1
(Im(OMe)2-CH), 113.7 (d, 2JFC = 21 Hz, S(C6H4F)), 73.6 (Im(OMe)2-CH2), 72.2
(Im(OMe)2-CH2), 58.4 (Im(OMe)2-CH3), 58.2 (Im(OMe)2-CH3), 51.3 (SIMes-CH2), 51.1
(SIMes-CH2), 49.7 (Im(OMe)2-CH2), 48.9 (Im(OMe)2-CH2), 46.8 (Ru=CHCH3), 21.1
(Mes-CH3), 21.0 (Mes-CH3), 20.7 (Mes-CH3), 20.5 (Mes-CH3), 19.3 (Mes-CH3), 19.2
53
(Mes-CH3). Elemental Anal.: C38H50ClFN4O2RuS: C, 58.33; H, 6.44; N, 7.16. Found: C, 58.27;
H, 6.87; N, 7.13.
Synthesis of 2-7: p-Nitrophenyl vinyl sulfide (0.041 g, 0.224 mmol) was added to a solution of
(Im(OMe)2)(SIMes)(PPh3)RuHCl (0.100 g, 0.112 mmol) in 5 mL CH2Cl2 at room temperature.
The solution was then stirred for 4 h before the solvent was concentrated to 0.5 mL and 15 mL of
pentane was added and the resulting mixture was filtered over a pad of celite. The pentane was
then removed in vacuo and the resulting residue was layered with 10 mL of pentane and left
standing overnight. The free triphenylphosphine is taken up into the pentane layer yielding a
purple solid (0.068 g, 75%). 1
H NMR (400 MHz, C6D6): δ 18.42 (q, 3JHH = 6 Hz, 1H, Ru=CH),
7.71 (d, 3JHH = 9 Hz, 2H, p-NO2(C6H4)), 6.75 (m, 7H, p-NO2(C6H4), Mes-CH, Im(OMe)2-CH),
6.49 (s, 1H, Im(OMe)2CH), 3.44 (m, 3H, Im(OMe)2-CH2), 3.32-3.21 (m, 4H, SIMesCH2), 3.13-
2.94 (m, 3H, Im(OMe)2CH2), 2.86 (s, 3H, Im(OMe)2CH3), 2.76 (s, 5H, Im(OMe)2CH2,
Mes-CH3), 2.73 (s, 3H, Im(OMe)2-CH3), 2.64 (s, 6H, Mes-CH3), 2.49 (s, 3H, Mes-CH3), 2.12 (s,
3H, Mes-CH3), 2.09 (s, 3H, Mes-CH3), 1.87 (d, 3JHH = 6 Hz, Ru=CHCH3).
13C{
1H} NMR
(101 MHz, C6D6): δ 314.2, (Ru=CH), 222.0 (NCN), 186.9 (NCN), 141.6 (Cipso), 139.8 (Cipso),
139.0 (Cipso), 138.6 (Cipso), 138.1 (Cipso), 137.7 (Cipso), 137.3 (Cipso), 137.2 (Cipso), 133.9 (Cipso),
133.7 (Cipso), 130.7 (p-NO2-C6H4) 129.7 (Mes-CH), 129.5 (Mes-CH), 129.2 (Mes-CH), 128.8
(Mes-CH), 121.5 (Im(OMe)2-CH), 121.3 (Im(OMe)2-CH), 121.1 (p-NO2-C6H4), 72.7
(Im(OMe)2-CH2), 71.5 (Im(OMe)2-CH2), 58.0 (Im(OMe)2-CH3), 57.9 (Im(OMe)2-CH3), 50.8
(SIMes-CH2), 50.7 (SIMes-CH2), 49.5 (Im(OMe)2-CH2), 46.3 (Ru=CHCH3), 20.6 (Mes-CH3),
20.5 (Mes-CH3), 20.1 (Mes-CH3), 19.5 (Mes-CH3), 18.7 (Mes-CH3), 18.6 (Mes-CH3). Elemental
Analysis for C38H50ClN5O4RuS•C5H12: C, 58.58; H, 7.09; N, 7.94. Found: C, 58.21; H, 6.76; N,
7.72.
Synthesis of 2-8: Phenyl vinyl sulfide (17.0 μL, 0.131 mmol) was added to a solution of 2-3
(0.100 g, 0.109 mmol) in 5 mL CH2Cl2 at room temperature. The solution was then stirred for
1 h before the solvent was concentrated to 0.5 mL and 15 mL of pentane was added and the
resulting mixture was filtered over a pad of celite. The pentane was then removed in vacuo and
the resulting residue was layered with 10 mL of pentane and left standing overnight. The free
triphenylphosphine is taken up into the pentane layer yielding a red solid (0.069 g, 80%). X-ray
quality crystals were grown from benzene/pentane at 25 oC.
1H NMR (400 MHz, C6D6): δ 19.05
(br s, 1H, Ru=CH), 7.05(m, 2H, S(C6H5)), 6.97 (s, 1H, Mes-CH), 6.94 (s, 1H, Mes-CH), 6.82 (s,
54
2H, Mes-CH), 6.67 (m, 3H, S(C6H5)), 3.73-3.03 (br m, 12H, SIMes-CH2 + Me2Im(OMe)2-CH2),
2.99 (s, 3H, Me2Im(OMe)2-CH3), 2.94 (s, 3H, Mes-CH3), 2.91 (s, 3H, Mes-CH3), 2.78 (s, 3H,
Me2Im(OMe)2-CH3), 2.68 (s, 3H, Mes-CH3), 2.66 (s, 3H, Mes-CH3), 2.25 (s, 3H, Mes-CH3),
2.13 (s, 3H, Mes-CH3), 2.07 (d, 3JHH = 6 Hz, 3H, Ru=CHCH3), 1.70 (s, 3H,
Me2Im(OMe)2-4,5-CH3), 1.44 (s, 3H, Me2Im(OMe)2-4,5-CH3). 13
C{1H} NMR (101 MHz, C6D6):
δ 312.0 (Ru=CHCH3), 223.7 (NCN), 186.3 (NCN), 152.1 (Cipso), 140.1 (Cipso), 139.7 (Cipso),
138.6 (Cipso), 138.5 (Cipso), 138.2 (Cipso), 137.9 (Cipso), 137.7 (Cipso), 136.0, 133.4 (S(C6H5)) 130.3
(Mes-CH), 129.9 (Mes-CH), 129.7 (Mes-CH), 129.6 (Mes-CH), 126.3 (S(C6H5)), 126.1
(Me2Im(OMe)2-Cipso), 125.5 (Me2Im(OMe)2-Cipso), 121.1 (S(C6H5)), 74.5 (Me2Im(OMe)2-CH2),
72.7 (Me2Im(OMe)2-CH2), 58.3 (Me2Im(OMe)2-CH3), 58.2 (Me2Im(OMe)2-CH3), 51.3
(SIMes-CH2), 51.1 (SIMes-CH2), 47.7 (Me2Im(OMe)2-CH2) 46.5 (Ru=CHCH3), 46.0
(Me2Im(OMe)2-CH2), 20.9 (Mes-CH3), 20.6 (Mes-CH3), 20.5 (Mes-CH3), 19.2 (Mes-CH3), 19.1
(Mes-CH3), 9.3 (Me2Im(OMe)2-4,5-CH3), 8.9 (Me2Im(OMe)2-4,5-CH3). Elemental Analysis for
C40H55ClN4O2RuS: C, 60.62; H, 7.00; N, 7.07. Found: C, 60.86; H, 7.11; N, 6.78.
Synthesis of 2-10: A mixture of 1-pentyne (0.74 mL, 7.50 mmol) and pentafluorothiophenol
(1.00 mL, 7.50 mmol) was stirred in 6 mL of H2O at room temperature for 4 hours. The reaction
mixture was extracted with Et2O (3 x 20 mL) and the ether extract was dried over MgSO4.
Solvent removal in vacuo gave a mixture of the (E)- and (Z)- isomers as a clear colorless liquid
(1.61 g, 80%). 1H NMR (400 MHz, C6D6): Isomer 1: δ 5.91-5.84 (m, 2H, (C6F5)SCHCH(C3H7)),
2.00 (m, 2H, (C6F5)SCHCH(C3H7)), 1.34 (m, 2H, (C6F5)SCHCH(C3H7)), 0.82 (m, 3H,
(C6F5)SCHCH(C3H7)). Isomer 2: δ 5.84 (d, 3JHH = 9 Hz, 1H, (C6F5)SCHCH(C3H7)), 5.76 (m,
1H, (C6F5)SCHCH(C3H7)), 2.21 (m, 2H, (C6F5)SCHCH(C3H7)), 1.41 (m, 2H,
(C6F5)SCHCH(C3H7)), 0.89 (m, 3H, (C6F5)SCHCH(C3H7)). 19
F{1H} NMR (178 MHz, C6D6): δ -
132.99 (m, 2F, o-F), -153.05 (t, 3JFF
= 21 Hz, 1F, p-F), -161.00 (m, 2F, m-F).
13C{
1H} NMR
(101 MHz, C6D6): δ 147.2 (dm, 1JCF = 247 Hz, C6F5), 141.2 (dm,
1JCF = 252 Hz, C6F5), 137.7
(dm, 1JCF = 252 Hz, C6F5). Isomer 1: 134.2 ((C6F5)SCHCH(C3H7)), 120.8
((C6F5)SCHCH(C3H7)), 34.7 ((C6F5)SCHCH(C3H7)), 22.1 ((C6F5)SCHCH(C3H7)), 13.4
((C6F5)SCHCH(C3H7)). Isomer 2: 137.7 ((C6F5)SCHCH(C3H7)), 118.7 ((C6F5)SCHCH(C3H7)),
35.0 ((C6F5)SCHCH(C3H7)), 21.9 ((C6F5)SCHCH(C3H7)), 13.3 ((C6F5)SCHCH(C3H7)).
HRMS-ESI+ m/z [M+H]
+ calculated for C11H10F5S: 269.04191, found: 269.04179.
55
Synthesis of 2-11: A mixture of 1-hexyne (0.86 mL, 7.48 mmol) and pentafluorothiophenol (1.00
mL, 7.50 mmol) was stirred in 6 mL of H2O at room temperature for 4 hours. The reaction
mixture was extracted with Et2O (3 x 20 mL) and the ether extract was dried over MgSO4.
Solvent removal in vacuo gave a mixture of the (E)- and (Z)- isomers as a clear colorless liquid
(1.92 g, 91%). 1H NMR (400 MHz, C6D6): Isomer 1: δ 5.80-5.74 (m, 2H, (C6F5)SCHCH(C4H9)),
1.81 (m, 2H, (C6F5)SCHCH(C4H9)), 1.13 (m, 4H, (C6F5)SCHCH(C4H9)), 0.79 (m, 3H,
(C6F5)SCHCH(C4H9)). Isomer 2: δ 5.69 (d, 3JHH = 9 Hz, 1H, (C6F5)SCHCH(C4H9)), 5.55 (dt,
3JHH = 9 Hz,
3JHH = 7 Hz, 1H, (C6F5)SCHCH(C4H9)), 2.19 (m, 2H, (C6F5)SCHCH(C4H9)), 1.26
(m, 4H, (C6F5)SCHCH(C4H9)), 0.84 (m, 3H, (C6F5)SCHCH(C4H9)). 19
F{1H} NMR (178 MHz,
C6D6): δ 133.94 (m, 2F, o-F), -154.03 (t, 3JFF
= 21 Hz, 1F, p-F), -161.80 (m, 2F, m-F).
13C{
1H}
NMR (101 MHz, C6D6): δ 146.9 (dm, 1JCF = 247 Hz, C6F5), 141.2 (dm,
1JCF = 252 Hz, C6F5),
137.6 (dm, 1JCF = 252 Hz, C6F5). Isomer 1: 134.1 ((C6F5)SCHCH(C4H9)), 120.5
((C6F5)SCHCH(C4H9)), 31.0 ((C6F5)SCHCH(C4H9)), 28.6 ((C6F5)SCHCH(C4H9)), 22.3
((C6F5)SCHCH(C4H9)), 13.7 ((C6F5)SCHCH(C4H9)). Isomer 2: 138.0 ((C6F5)SCHCH(C4H9)),
118.0 ((C6F5)SCHCH(C4H9)), 32.5 ((C6F5)SCHCH(C4H9)), 30.9 ((C6F5)SCHCH(C4H9)), 22.2
((C6F5)SCHCH(C4H9)), 13.7 ((C6F5)SCHCH(C4H9)). HRMS-ESI+ m/z [M+H]
+ calculated for
C12H12F5S: 283.05847, found: 283.05744.
Synthesis of 2-12: To a solution of (OCO)(SIMes)(PPh3)RuHCl (0.100 g, 0.112 mmol) in 2 mL
C6H5Br was added 2-9 (0.068 g, 0.224 mmol) at room temperature. The solution was then stirred
for 24 hours before the solution was added drop wise to 15 mL of cold pentane, while stirring, to
precipitate the product. The pink/red solid was collected on a frit and dried under vacuum
(0.073 g, 70%). X-ray quality crystals were grown from tetrahydrofuran/pentane at 25 oC.
1H
NMR (400 MHz, C6D5Br): δ 15.65 (dd, 3JHH = 8 Hz,
3JHH = 3 Hz, 1H, Ru=CH), 7.06 (s, 1H,
Mes-CH), 7.05 (s, 1H, Mes-CH), 6.95 (br s, 1H, Im(OMe)2-CH), 6.87 (s, 2H, Mes-CH), 6.84 (d,
3JHH = 2 Hz, 1H, Im(OMe)2-CH), 6.80 (br s, 1H, C6H5), 6.75 (br s, 2H, C6H5), 6.68 (br s, 2H,
C6H5), 4.08 (dd, 2JHH = 15 Hz,
3JHH = 3 Hz, 1H, Im(OMe)2-CH2), 4.00 (dt,
2JHH = 15 Hz,
3JHH =
3 Hz, 1H, Im(OMe)2-CH2), 3.64 (m, 4H, SIMes-CH2), 3.49 (m, 4H, Im(OMe)2-CH2), 3.32 (m,
2H, Im(OMe)2-CH2), 3.04 (s, 3H, Im(OMe)2-CH3), 2.87 (s, 2H, Ru=CHCH2), 2.72 (s, 3H,
Im(OMe)2-CH3), 2.61 (s, 3H, Mes-CH3), 2.23 (s, 6H, 2 x Mes-CH3), 2.15 (s, 9H, 3 x Mes-CH3).
19F{
1H} NMR (376 MHz, C6D5Br): δ 131.72 (br s, 1F, o-S(C6F5)), 132.36 (br s, 1F, o-
S(C6F5)), 162.33 (t, 3JFF = 22 Hz, 1F, p-S(C6F5)), 166.25 (br s, 1F, m-S(C6F5)), 166.68 (br s,
56
1F, m-S(C6F5)). 13
C{1H} NMR (101 MHz, C6D5Br, partial): δ 309.6 (Ru=CH), 138.0 (Cipso),
137.5 (Cipso), 137.1 (Cipso), 130.0 (C6H5), 129.8 (Mes-CH), 129.6 (Mes-CH), 129.4 (C6H5), 123.5
(C6H5), 122.5 (Im(OMe)2-CH), 121.0 (Im(OMe)2-CH), 72.9 (Im(OMe)2-CH2), 72.6
(Im(OMe)2-CH2), 58.2 (Im(OMe)2-CH3), 58.1 (Im(OMe)2-CH3), 58.0 (Ru=CHCH2), 52.1
(SIMes-CH2), 49.7 (Im(OMe)2-CH2), 49.4 (Im(OMe)2-CH2), 21.0 (Mes-CH3), 19.6 (Mes-CH3),
18.7 (Mes-CH3). Elemental Analysis for C44H50ClF5N4O2RuS•C6H5Br: C, 55.22; H, 5.10; N,
5.15. Found: C, 55.18; H, 5.03; N, 5.54.
Synthesis of 2-13: To a solution of (Im(OMe)2)(SIMes)(PPh3)RuHCl (0.100 g, 0.112 mmol) in
2 mL C6H5Br was added 2-10 (0.060 g, 0.224 mmol) at room temperature. The solution was then
stirred for 24 hours before the solution was added drop wise to 15 mL of cold pentane to
precipitate the product. The orange/brown solid was collected on a frit and dried under vacuum
(0.073 g, 73%). X-ray quality crystals were grown from bromobenzene/pentane at 25 oC.
1H
NMR (400 MHz, C6D5Br): δ 16.37 (t, 3JHH = 5 Hz, 1H, Ru=CH), 7.04 (d,
3JHH = 2 Hz, 1H,
Im(OMe)2-CH), 6.85 (s, 2H, Mes-CH), 6.83 (d, 3JHH = 2 Hz, 1H, Im(OMe)2-CH), 6.71 (s, 2H,
Mes-CH), 4.16 (m, 1H, Im(OMe)2-CH2), 3.69 (m, 3H, Im(OMe)2-CH2), 3.59 (m, 1H,
Im(OMe)2-CH2), 3.55 (m, 4H, SIMes-CH2), 3.37 (m, 1H, Im(OMe)2-CH2), 3.15 (m, 2H,
Im(OMe)2-CH2), 2.92 (s, 3H, Im(OMe)2-CH3), 2.90 (s, 3H, Im(OMe)2-CH3), 2.66 (s, 6H, 2 x
Mes-CH3), 2.23 (s, 6H, 2 x Mes-CH3), 2.16 (s, 6H, 2 x Mes-CH3), 1.31 (m, 2H,
pentylidene-CH2), 1.13 (m, 2H, pentylidene-CH2), 1.05 (m, 2H, pentylidene-CH2), 0.83 (t,
3JHH = 7 Hz, 3H, pentylidene-CH3) .
19F{
1H} NMR (376 MHz, C6D5Br): δ -131.87 (br s, 1F, o-
S(C6F5)), -132.41 (br s, 1F, o-S(C6F5)), -162.70 (t, 3JFF = 22 Hz, 1F, p-S(C6F5)), -166.45 (br s,
1F, m-S(C6F5)), -166.98 (br s, 1F, m-S(C6F5)). 13
C{1H} NMR (101 MHz, C6D5Br, partial): δ
315.2 (Ru=CH), 212.6 (NCN), 181.8 (NCN), 137.9 (Cipso), 137.4 (Cipso), 129.9 (Mes-CH), 129.6
(Mes-CH), 122.6 (Im(OMe)2-CH), 121.3 (Im(OMe)2-CH), 73.0 (Im(OMe)2-CH2), 71.4
(Im(OMe)2-CH2), 58.5 (Im(OMe)2-CH3), 58.0 (Im(OMe)2-CH3), 52.2 (SIMes-CH2), 49.4
(Im(OMe)2-CH2), 48.3 (Im(OMe)2-CH2), 29.3 (pentylidene-CH2), 22.9 (pentylidene-CH2), 21.0
(Mes-CH3), 19.6 (Mes-CH3), 18.7 (Mes-CH3), 14.3 (pentylidene-CH3). Elemental Analysis for
C41H52ClF5N4O2RuS•(C6H5Cl): C, 55.47; H, 5.77; N, 5.88. Found: C, 55.78; H, 5.87; N, 6.06.
Synthesis of 2-14: To a solution of (Im(OMe)2)(SIMes)(PPh3)RuHCl (0.100 g, 0.112 mmol) in
2 mL C6H5Br was added 2-11 (0.063 g, 0.224 mmol) at room temperature. The solution was then
stirred for 24 hours before the solution was added drop wise to 15 mL of cold pentane, while
57
stirring, to precipitate the product. The orange/brown solid was collected on a frit and dried
under vacuum (0.072 g, 71%). X-ray quality crystals were grown from bromobenzene/pentane at
25 oC.
1H NMR (400 MHz, C6D5Br): δ 16.44 (t,
3JHH = 5 Hz, 1H, Ru=CH), 7.00 (s, 1H,
Im(OMe)2-CH), 6.85 (s, 2H, Mes-CH), 6.82 (d, 3JHH = 2 Hz, 1H, Im(OMe)2-CH), 6.71 (s, 2H,
Mes-CH), 4.15 (dd, 2JHH = 14 Hz,
3JHH = 4 Hz, 1H, Im(OMe)2-CH2), 3.67 (m, 2H,
Im(OMe)2-CH2), 3.59 (m, 1H, Im(OMe)2-CH2), 3.50 (m, 4H, SIMes-CH2), 3.33 (m, 1H,
Im(OMe)2-CH2), 3.12 (m, 1H, Im(OMe)2-CH2), 2.92 (s, 4H, Im(OMe)2-CH2 + Im(OMe)2-CH3),
2.89 (s, 4H, Im(OMe)2-CH2 + Im(OMe)2-CH3), 2.66 (s, 7H, hexylidene-CH2 + 2 x Mes-CH3),
2.22 (s, 6H, 2 x Mes-CH3), 2.15 (s, 7H, hexylidene-CH2 + 2 x Mes-CH3), 1.21 (m, 3H,
hexylidene-CH2), 1.07 (m, 3H, hexylidene-CH2), 0.85 (t, 3JHH = 7 Hz, 3H, hexylidene-CH3).
19F{
1H} NMR (376 MHz, C6D5Br): δ -131.83 (br s, 1F, o-S(C6F5)), -132.44 (br s, 1F,
o-S(C6F5)), -162.69 (t, 3JFF = 22 Hz, 1F, p-S(C6F5)), -166.42 (br s, 1F, m-S(C6F5)), -166.96 (br s,
1F, m-S(C6F5)). 13
C{1H} NMR (101 MHz, C6D5Br, partial): δ 315.3 (Ru=CH), 212.3 (NCN),
181.8 (NCN), 137.7 (Cipso), 137.2 (Cipso), 129.3 (Mes-CH), 129.0 (Mes-CH), 122.0
(Im(OMe)2-CH), 120.7 (Im(OMe)2-CH), 72.9 (Im(OMe)2-CH2), 71.4 (Im(OMe)2-CH2), 58.5
(Im(OMe)2-CH3), 58.0 (Im(OMe)2-CH3), 52.3 (SIMes-CH2), 49.4 (Im(OMe)2-CH2), 48.3
(Im(OMe)2-CH2), 32.0 (hexylidene-CH2), 26.7 (hexylidene-CH2), 22.8 (hexylidene-CH2), 21.05
(hexylidene-CH2), 21.0 (Mes-CH3), 19.5 (Mes-CH3), 18.7 (Mes-CH3), 14.2 (hexylidene-CH3).
Elemental Analysis for C42H54ClF5N4O2RuS•(C6H5Cl): C, 55.91; H, 5.89; N, 5.80. Found: C,
56.27; H, 5.83; N, 6.19.
Synthesis of 2-17: Ethyl vinyl sulfide (24 μL, 0.236 mmol) was added to a solution of 2-15
(0.100 g, 0.118 mmol) in 5 mL C6H6 at room temperature. The solution was then stirred for 6 h
before the solvent was concentrated to 1 mL and 15 mL of pentane was added which caused a
light orange precipitate to form. The solid was collected by filtration, washed with pentane and
dried under high vacuum (0.080 g, 80%). X-ray quality crystals were grown from
benzene/pentane at 25 oC.
1H NMR (400 MHz, C6D6): δ 7.71(br s, 10H, PPh3), 7.39 (m, 1H,
PPh3), 7.03 (br s, 19H, PPh3), 6.33 (d, 3
JHH = 2 Hz, 1H, Im-CH), 6.29 (m, 1H, Ru-CHOMe), 5.83
(d, 3
JHH = 2 Hz, 1H, Im-CH), 3.54 (br s, 4H, NCH2CH2OMe + O(CH3)), 3.45 (m, 1H,
NCH2CH2OMe), 2.77 (s, 3H, O(CH3)), 2.55 (dd, 2JHH = 12 Hz,
3JHH = 5 Hz, 1H, NCH2CHOMe),
2.48 (m, 2H, NCH2CH2OMe), 2.37 (dd, 2JHH = 12 Hz,
3JHH = 5 Hz, 1H, NCH2CHOMe).
31P{
1H}
NMR (161 MHz, C6D6): δ 41.9 (d, 2JPP = 319 Hz, PPh3), 35.7 (d,
2JPP = 319 Hz, PPh3).
13C{
1H}
58
NMR (101 MHz, C6D6): δ 192.8 (t, 2JPC = 14 Hz, NCN), 138.0 (d,
2JPC = 12 Hz, PPh3), 134.7 (br
s, PPh3), 134.1 (d, 1JPC = 20 Hz, PPh3), 129.0 (PPh3), 128.8 (d,
3JPC = 7 Hz, PPh3), 128.5 (PPh3),
120.4 (Im-CH), 116.7 (Im-CH), 82.5 (t, 2JPC = 6 Hz, NCH2CHOMe), 71.1 (NCH2CH2OMe), 58.8
(O(CH3)), 58.1 (O(CH3)), 57.9 (NCH2CHOMe), 49.2 (NCH2CH2OMe). Elemental Analysis for
C45H45ClN2O2P2Ru: C, 64.01; H, 5.37; N, 3.32. Found: C, 63.90; H, 5.71; N, 3.45.
Synthesis of 2-18: Ethyl vinyl sulfide (24 μL, 0.234 mmol) was added to a solution of 2-16
(0.140 g, 0.161 mmol) in 5 mL CH2Cl2 at room temperature. The solution was then stirred for
6 h before the solution was filtered over celite and the solvent was concentrated to 1 mL. Pentane
(15 mL) was added while stirring to precipitate a red solid which was collected on a frit and
dried under high vacuum (0.071 g, 51%). X-ray quality crystals were grown from
benzene/hexane at 25 oC.
1H NMR (400 MHz, C6D6): δ 7.90(br s, 6H, PPh3), 7.11-6.88 (br m,
24H, PPh3), 6.35 (ddd, 3JHH = 11 Hz,
3JHH = 7 Hz,
3JPH = 4 Hz, 1H, Ru-CHOMe), 3.59 (m, 2H,
NCH2CH2OMe), 3.42 (s, 3H, O(CH3)), 2.76 (s, 3H, O(CH3)), 2.73-2.64 (m, 2H, NCH2CH2OMe
+ NCH2CHOMe), 2.61 (dt, 2JHH = 10 Hz,
3JHH = 5 Hz, 1H, NCH2CHOMe), 2.22 (dd,
2JHH = 12 Hz,
3JHH = 7 Hz, 1H, NCH2CHOMe), 1.47 (s, 3H, Me2Im(OMe)2-4,5-CH3), 1.27 (s,
3H, Me2Im(OMe)2-4,5-CH3). 31
P{1H} NMR (161 MHz, C6D6): δ 40.8 (d,
2JPP = 315 Hz, PPh3),
33.0 (d, 2JPP = 315 Hz, PPh3).
13C{
1H} NMR (101 MHz, CD2Cl2): δ 134.6 (t,
2JPC = 6 Hz, PPh3),
134.2 (d, 1JPC = 18 Hz, PPh3), 129.2 (PPh3), 127.7 (t,
3JPC = 4 Hz, PPh3), 124.6 (Im-4,5-Cipso),
122.6 (Im-4,5-Cipso), 80.6 (t, 2JPC = 6 Hz, NCH2CHOMe), 71.3 (NCH2CH2OMe), 58.5 (O(CH3)),
58.1 (O(CH3)), 56.1 (NCH2CHOMe), 46.4 (NCH2CH2OMe), 9.2 (Me2Im-4,5-CH3), 8.9
(Me2Im-4,5-CH3), NCN peak not observed. Elemental Analysis for
C47H49ClN2O2P2Ru•(CH2Cl2)0.5: C, 62.36; H, 5.51; N, 3.06. Found: C, 62.14; H, 5.76; N, 3.28.
Synthesis of 2-19: A solution of 2-18 (0.060 g, 0.068 mmol) in 3 mL C6H6 was left stirring for
24 h before the solvent was concentrated to 1 mL and pentane was added while stirring to
precipitate a yellow solid which was collected on a frit and dried under high vacuum (0.045 g,
77%).1H NMR (400 MHz, CD2Cl2): δ 7.67 (d,
3JHH = 5 Hz, 1H, RuCHCHN),7.44 (br s, 12H,
PPh3), 7.29 (br m, 6H, PPh3), 7.23 (br m, 12H, PPh3), 5.66 (dt, 3JHH = 5 Hz,
4JPH = 3 Hz, 1H,
RuCHCHN), 3.63 (t, 3JHH = 6 Hz, 2H, NCH2CH2OMe), 3.01 (t,
3JHH = 6 Hz, 2H,
NCH2CH2OMe) 2.93 (s, 3H, O(CH3)), 1.71 (s, 3H, Me2Im(OMe)2-4,5-CH3), 1.42 (s, 3H,
Me2Im(OMe)2-4,5-CH3). 31
P{1H} NMR (161 MHz, CD2Cl2): δ 36.7 (s, PPh3).
13C{
1H} NMR
(101 MHz, CD2Cl2): δ 135.6 (t, 2JPC = 19 Hz, RuCHCHN), 134.6 (t,
2JPC = 6 Hz, PPh3), 134.2 (d,
59
1JPC = 18 Hz, PPh3), 129.2 (PPh3), 127.6 (t,
3JPC = 4 Hz, PPh3), 125.2 (Im-4,5-Cipso), 124.6 (br s,
RuCHCHN), 122.6 (Im-4,5-Cipso), 71.8 (NCH2CH2OMe), 58.7 (O(CH3)), 46.4 (NCH2CH2OMe),
9.2 (Me2Im-4,5-CH3), 8.9 (Me2Im-4,5-CH3), NCN peak not observed. Elemental Analysis for
C46H45ClN2OP2Ru•(C5H12)0.5: C, 66.47; H, 5.87; N, 3.20. Found: C, 66.49; H, 6.18; N, 2.98.
Synthesis of 2-21: Ethyl vinyl sulfide (13.0 μL, 0.128 mmol) was added to a solution of 2-20
(0.100 g, 0.112 mmol) in 5 mL C6H6 at room temperature. The solution was then stirred for 16 h
before the solvent was concentrated to 1 mL and 15 mL of pentane was added to which caused a
purple precipitate to form. The purple solid was collected by filtration, washed with pentane and
dried under high vacuum (0.073 g, 76%). X-ray quality crystals were grown from
benzene/pentane at 25 oC.
1H NMR (400 MHz, C6D6): δ 7.70 (d,
3JHH = 5 Hz, 1H, RuCHCHN),
7.27 (t, 3
JHH = 8 Hz, 6H, C6H5, PPh3 ), 7.04-6.90 (m, 13H, C6H5, PPh3 + Mes-CH), 6.61 (d,
3JHH = 2 Hz, 1H, Im-CH), 6.42 (d,
3JHH = 2 Hz, 1H, Im-CH), 6.10 (dd,
3JHH = 5 Hz,
4JPH = 2 Hz,
1H, RuCHCHN), 4.95 (dt, 2JHH = 14 Hz,
3JHH = 3 Hz, 1H, NCH2CH2OMe), 3.44 (br m, 1H,
SIMes-CH2), 3.29 (br m, 1H, SIMes-CH2), 3.09-2.96 (br m, 4H, SIMes-CH2 + NCH2CH2OMe),
2.90 (dt, 2JHH = 11 Hz,
3JHH = 3 Hz, 1H, NCH2CH2OMe), 2.83 (s, 3H, O(CH3)), 2.74 (s, 3H,
Mes-CH3), 2.43 (s, 6H, Mes-CH3), 2.25 (s, 3H, Mes-CH3), 2.14 (s, 3H, Mes-CH3), 1.74 (s, 3H,
Mes-CH3). 31
P{1H} NMR (161 MHz, C6D6): δ 37.0 (s, PPh3).
13C{
1H} NMR (101 MHz, C6D6): δ
202.6 (NCN), 184.9 (d, 2JPC = 15 Hz, NCN), 159.6 (d,
2JPC = 15 Hz, RuCHCHN), 137.6 (Cipso),
137.3 (Cipso), 136.3 (br, Cipso), 134.6 (d, 2JPC = 11 Hz, PPh3), 130.4 (br, Cipso), 129.7 (Mes-CH),
128.6 (Mes-CH), 127.6 (d, 3JPC = 9 Hz, PPh3), 124.2 (d,
4JPC = 2 Hz, RuCHCHN), 118.9
(Im-CH), 114.5 (Im-CH), 72.9 (NCH2CH2OMe), 58.0 (O(CH3)), 51.3 (br s, NCH2CH2N), 48.6
(NCH2CH2OMe), 21.3 (Mes-CH3), 21.0 (Mes-CH3), 20.4 (Mes-CH3), 20.1 (Mes-CH3), 19.9
(Mes-CH3), 17.9 (Mes-CH3). Elemental Analysis for C47H52ClN4OPRu•(C6H6)0.5: C, 67.06; H,
6.19; N, 6.26. Found: C, 66.69; H, 6.65; N, 6.25.
Synthesis of 2-22: Ethyl vinyl sulfide (14.0 μL, 0.138 mmol) was added to a solution of 2-1
(0.100 g, 0.112 mmol) in 5 mL C6H6 at room temperature. The solution was then stirred for 48 h
before the solvent was concentrated to 1 mL. Pentane (15 mL) was layered and left overnight at
room temperature yielding red crystals. The pentane was decanted and the crystals were dried
under high vacuum (0.076 g, 79%). X-ray quality crystals were grown from benzene/hexane at
25 oC.
1H NMR (400 MHz, C6D6): δ 7.75 (dd,
3JHH = 5 Hz,
3JPH = 1 Hz, 1H, RuCHCHN), 7.32
(ddd, 3JPH = 10 Hz,
3JHH = 8 Hz,
4JHH = 2 Hz, 6H, C6H5, PPh3 ), 7.06-6.92 (m, 11H, (9H) PPh3 +
60
(2H) Mes-CH), 6.86 (br s, 2H, Mes-CH), 6.69 (d, 3
JHH = 2 Hz, 1H, Im-CH), 6.45 (d, 3
JHH = 2 Hz,
1H, Im-CH), 6.12 (br s, 2H, IMes-CH), 6.09 (dd, 3JHH = 5 Hz,
4JPH = 2 Hz, 1H, RuCHCHN),
4.97 (m, 1H, NCH2CH2OMe), 2.97 (m, 1H, NCH2CH2OMe), 2.91 (m, 1H, NCH2CH2OMe), 2.84
(s, 3H, O(CH3)), 2.47 (app dt, 2JHH = 10 Hz,
3JHH = 2 Hz, 1H, NCH2CH2OMe), 2.79-2.05 (br s,
18H, Mes-CH3). 31
P{1H} NMR (161 MHz, C6D6): δ 38.4 (s, PPh3).
13C{
1H} NMR (101 MHz,
C6D6): δ 185.9 (d, 2JPC = 16 Hz, NCN), 160.2 (d,
2JPC = 12 Hz, RuCHCHN), 137.8 (Cipso), 137.5
(Cipso), 134.6 (d, 2JPC = 11 Hz, PPh3), 134.2 (d,
1JPC = 20 Hz, PPh3), 129.3 (Mes-CH), 128.6
(Mes-CH), 127.6 (d, 3JPC = 9 Hz, PPh3), 124.4 (d,
4JPC = 2 Hz, RuCHCHN), 122.9 (IMes-CH),
122.8 (IMes-CH), 118.7 (Im-CH), 114.5 (Im-CH), 73.0 (NCH2CH2OMe), 58.0 (O(CH3)), 48.5
(NCH2CH2OMe), 21.2 (br s, Mes-CH3), 19.8 (br s, Mes-CH3). Elemental Analysis for
C47H50ClN4OPRu•(C6H6)0.5: C, 67.21; H, 5.98; N, 6.27. Found: C, 66.90; H, 6.28; N, 6.22.
Synthesis of 2-23: Ethyl vinyl sulfide (13.0 μL, 0.128 mmol) was added to a solution of 2-2
(0.100 g, 0.104 mmol) in 5 mL C6H6 at room temperature. The solution was then stirred for 48 h
before the solvent was concentrated to 1 mL. Pentane (15 mL) was layered and left overnight at
room temperature yielding red crystals. The pentane was decanted and the crystals were dried
under high vacuum (0.079 g, 82%). X-ray quality crystals were grown from benzene/hexane at
25 oC.
1H NMR (400 MHz, C6D6): δ 7.66 (dd,
3JHH = 5 Hz,
3JPH = 1 Hz, 1H, RuCHCHN), 7.27
(m, 6H, C6H5, PPh3 ), 6.99 (m, 3H, C6H5, PPh3), 6.94 (m, 6H, C6H5, PPh3), 6.83 (br s, 2H,
Mes-CH), 6.75 (br s, 2H, Mes-CH), 6.63 (d, 3
JHH = 2 Hz, 1H, Im-CH), 6.40 (d, 3
JHH = 2 Hz, 1H,
Im-CH), 6.04 (dd, 3JHH = 5 Hz,
4JPH = 2 Hz, 1H, RuCHCHN), 4.89 (ddd,
2JHH = 10 Hz,
3JHH =
4 Hz, 3JHH = 3 Hz, 1H, NCH2CH2OMe), 2.99 (m, 1H, NCH2CH2OMe), 2.86 (ddd,
2JHH = 14 Hz,
3JHH = 4 Hz,
3JHH = 2 Hz, 1H, NCH2CH2OMe), 2.82 (s, 3H, O(CH3)), 2.44 (app dt,
2JHH = 10 Hz,
3JHH = 3 Hz, 1H, NCH2CH2OMe), 2.16 (br s, 9H, Mes-CH3), 2.11 (br s, 9H, Mes-CH3).
31P{
1H}
NMR (161 MHz, C6D6): δ 38.4 (s, PPh3). 13
C{1H} NMR (101 MHz, C6D6): δ 184.8 (d,
2JPC =
16 Hz, NCN), 159.2 (d, 2JPC = 12 Hz, RuCHCHN), 137.2 (Cipso), 136.9 (Cipso), 134.4 (d,
2JPC =
11 Hz, PPh3), 128.9 (Mes-CH), 128.8 (Mes-CH), 127.5 (d, 3JPC = 9 Hz, PPh3), 124.4 (d,
4JPC =
2 Hz, RuCHCHN), 118.8 (Im-CH), 114.5 (Im-CH), 72.6 (NCH2CH2OMe), 57.9 (O(CH3)), 48.5
(NCH2CH2OMe), 21.1 (Mes-CH3), 20.9 (Mes-CH3), 17.8 (Mes-CH3). Elemental Analysis for
C47H48Cl3N4OPRu: C, 61.14; H, 5.24; N, 6.07. Found: C, 56.84; H, 5.74; N, 6.08.
61
2.5.3 X-ray Data Collection and Reduction
Crystals were coated in Paratone-N oil in the glove-box, mounted on a MiTegen Micromount
and placed under an N2 stream, thus maintaining a dry, O2-free environment for each crystal. The
data for crystals were collected on a Bruker Apex II diffractometer employing Mo Kα radiation
(λ = 0.71073 Å). The data were collected at 150(±2) K for all crystals. The frames were
integrated with the Bruker SAINT software package using a narrow-frame algorithm. Data were
corrected for absorption effects using the empirical multi-scan method (SADABS).56
2.5.4 X-ray Data Solution and Refinement
Non-hydrogen atomic scattering factors were taken from the literature tabulations.57
The heavy
atom positions were determined using direct methods employing the SHELXTL direct methods
routine. The remaining non-hydrogen atoms were located from successive difference Fourier
map calculations. The refinements were carried out by using full-matrix least squares techniques
on F, minimizing the function (Fo–Fc)2 where the weight is defined as 4Fo2/2 (Fo
2) and Fo
and Fc are the observed and calculated structure factor amplitudes, respectively. In the final
cycles of each refinement, all non-hydrogen atoms were assigned anisotropic temperature factors
in the absence of disorder or insufficient data. In the latter cases atoms were treated isotropically.
C-H atom positions were calculated and allowed to ride on the carbon to which they are bonded
assuming a C-H bond length of 0.95 Å. H-atom temperature factors were fixed at 1.10 times the
isotropic temperature factor of the C-atom to which they are bonded. The H-atom contributions
were calculated, but not refined. The locations of the largest peaks in the final difference Fourier
map calculation as well as the magnitude of the residual electron densities in each case were of
no chemical significance.
62
Table 2.5.1 Select crystallographic parameters for 2-1 to 2-4.
2-1 2-2 2-3•1.5(C7H8)•0.5(C5H12) 2-4
Formula C48H56ClN4O2PRu C48H54Cl3N4O2PRu C63H80ClN4O2PRu C38H51ClN4O2RuS
Wt 888.46 957.34 1092.80 764.42
Cryst. syst. Monoclinic Monoclinic Monoclinic Triclinic
Space group P21/n P21 P21/n P-1
a(Å) 13.9645(12) 9.9860(6) 14.687(2) 8.7915(3)
b(Å) 17.4923(14) 19.2471(12) 16.729(2) 11.7773(4)
c(Å) 18.6694(18) 24.3594(16) 24.285(3) 19.4743(7)
(deg) 90.00 90.00 90.00 79.623(2)
(deg) 107.017(3) 90.793(2) 101.749(8) 88.313(2)
(deg) 90.00 90.00 90.00 68.932(2)
V(Å3) 4360.7(7) 4681.5(5) 5841.9(14) 1849.44(11)
Z 4 4 4 2
d(calc) gcm–3
1.353 1.358 1.243 1.373
R(int) 0.0431 0.0831 0.0870 0.0303
, mm–1
0.501 0.582 0.387 0.591
Total data 10004 19065 13365 12822
>2(FO2) 7963 10494 8756 10894
Variables 518 1085 704 421
R (>2) 0.0338 0.0460 0.0479 0.0342
Rw 0.0836 0.0656 0.1244 0.0930
GOF 1.019 0.662 1.010 1.032
63
Table 2.5.2 Select crystallographic parameters for 2-5, 2-6, 2-8 and 2-12.
2-5 2-6 2-8 2-12•(C4H8O)
Formula C38H49ClN4O2RuS C38H50ClFN4O2RuS C40H55ClN4O2RuS C48H58ClF5N4O3RuS
wt 762.39 782.41 792.46 1002.56
Cryst. syst. Triclinic Triclinic Orthorhombic Orthorhombic
Space group P-1 P-1 Pbca Pbca
a(Å) 8.7968(3) 8.8552(11) 25.5160(18) 20.6520(7)
b(Å) 11.7375(4) 11.8620(15) 8.8783(6) 21.0587(7)
c(Å) 19.4493(7) 19.312(3) 38.627(3) 21.6212(7)
(deg) 79.242(2) 80.115(7) 90.00 90.00
(deg) 88.387(1) 89.229(6) 90.00 90.00
(deg) 68.968(1) 69.025(6) 90.00 90.00
V(Å3) 1839.81(11) 1863.5(4) 8750.6(11) 9403.2(5)
Z 2 2 8 8
d(calc) gcm–3
1.376 1.394 1.203 1.416
R(int) 0.0608 0.0646 0.1370 0.1100
, mm–1
0.594 0.592 0.502 0.500
Total data 8388 8517 7699 10790
>2(FO2) 6011 6283 5084 6457
Variables 421 430 439 568
R (>2) 0.0480 0.0486 0.0655 0.0491
Rw 0.1107 0.1172 0.1399 0.1116
GOF 1.024 1.032 1.082 1.003
64
Table 2.5.3 Select crystallographic parameters for 2-17, 2-18, 2-21 and 2-22.
2-17 2-18•(C6H6)•(C5H12) 2-21•(C6H6) 2-22•0.5(C6H6)
Formula C45H45ClN2O2P2Ru C50.5H58ClN2O2P2Ru C50H55ClN4OPRu C50H53ClN4OPRu
wt 844.29 947.47 895.47 893.45
Cryst. syst. Triclinic Monoclinic Monoclinic Monoclinic
Space group P-1 P21/n I2/a C2/c
a(Å) 10.2192(4) 13.009(3) 21.7335(12) 34.768(5)
b(Å) 13.8275(6) 18.484(4) 14.9240(6) 14.8467(17)
c(Å) 14.7190(6) 20.315(5) 26.9039(13) 21.575(4)
(deg) 105.817(2) 90.00 90.00 90.00
(deg) 93.474(2) 104.490(12) 91.324(4) 128.344(9)
(deg) 101.154(2) 90.00 90.00 90.00
V(Å3) 1949.13(14) 4729.5(19) 8726.2(7) 8735(2)
Z 2 4 8 8
d(calc) gcm–3
1.439 1.331 1.363 1.359
R(int) 0.0453 0.0407 0.0443 0.0699
, mm–1
0.594 0.497 0.499 0.499
Total data 8958 10871 10073 10076
>2(FO2) 6406 8829 7770 7422
Variables 475 528 523 523
R (>2) 0.0434 0.0505 0.0363 0.0384
Rw 0.0966 0.1463 0.0906 0.0903
GOF 1.006 1.068 1.015 1.013
65
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68
Chapter 3 Catalytic Olefin Metathesis
3.1 Introduction
3.1.1 Types of Olefin Metathesis Reactions
Olefin metathesis has become a synthetic tool for the modification of organic substrates that is
exploited across the discipline in natural product synthesis, polymer, pharmaceutical and
industrial chemistry.1-7
The most common olefin metathesis reactions are typically grouped into
three specific types (Scheme 3.1.1). These types include ring opening metathesis polymerization
(ROMP) which is used to synthesize polymers.8-10
This is an example of a living polymerization
where the continuous opening of cyclic olefinic monomers results in a growing polymer attached
to the metal center. The driving force for this reaction is typically from the relief of ring strain in
the monomer. A number of interesting polymers can be obtained through ROMP including
polydicyclopentadiene11
and polynorbornene12,13
which are used for body panels of vehicles and
anti-vibration, anti-impact and grip improvement, respectively. The second reaction type is ring
closing metathesis (RCM) where two terminal olefinic functionalities that are contained within
the same molecule are linked together creating a cyclic product with the elimination of
ethylene.14
This transformation is applied for the synthesis of heterocycles, bicycles and
cycloalkenes15-17
where the equilibrium can be driven to the cyclic product by the removal of
ethylene from the system. Pressurizing the system with ethylene drives the equilibrium to the
ring opened product in a process called ring opening metathesis (ROM). Cross metathesis (CM)
of two olefinic substrates results in the formation of a coupled product with the liberation of
ethylene as a byproduct.18-20
Similar to RCM, this equilibrium is driven to the coupled product
by the liberation of ethylene. The reverse reaction can be accomplished by applying an ethylene
pressure which results in cleavage of the carbon-carbon double bond resulting in two terminal
olefinic products.21,22
This process is called ethenolysis and is used for the production of
biodiesel from unsaturated fatty acids.23
69
Scheme 3.1.1 Common olefin metathesis reactions.
3.1.2 Catalyst Screening
3.1.2.1 Standard Test Reactions
Due to the lack of standard test reactions for catalytic olefin metathesis, assessing the activity of
new catalytic systems and comparing them to existing catalysts was highly inconsistent. To this
end, Grubbs and co-workers developed a series of standard transformations to serve as an easily
applicable platform for catalyst comparison.24
The three standard tests provide insight into a
catalyst’s ability to perform the three most common reactions described above (ROMP, RCM,
and CM).
To investigate the effectiveness of a catalyst to accomplish ROMP, 1,5-cyclooctadiene is
polymerized using the catalyst of choice (Scheme 3.1.2), typically as a solution in CD2Cl2.
70
Scheme 3.1.2 Standard test reaction for ROMP.
The standard test for RCM is the ring closing of diethyl diallylmalonate (Scheme 3.1.3) again as
a CD2Cl2 solution with the catalyst of choice.
Scheme 3.1.3 Standard test reaction for RCM.
The standard metathesis test for CM involves the coupling of 5-hexenyl acetate and methyl
acrylate in a CD2Cl2 solution (Scheme 3.1.4). This reaction can give the desired heterocoupled
product as shown below and also the homocoupled 5-hexenyl acetate product.
Scheme 3.1.4 Standard test reaction for CM.
3.1.2.2 Activity of Common Catalysts
The activity of some common olefin metathesis catalysts is presented in Table 3.1.124
to provide
a comparison with activities presented in this chapter. For ROMP, Grubbs II and 2nd Generation
Hoveyda-Grubbs Catalyst (HG II) (see Figure 2.1.1 for structures) are the most active achieving
99% conversion in 6 and 5 min respectively. After 90 min Grubbs I achieves 40% conversion
and after 100 min 1st Generation Hoveyda-Grubbs Catalyst (HG I) achieves 4% conversion.
Grubbs I converts diethyl diallylmalonate to the ring closed product in 66% yield after 30 min
whereas Grubbs II, HG I, and HG II can accomplish RCM of diethyl diallylmalonate to over
90% in 30 min. For CM Grubbs II and HG II achieve 90% conversion after 70 min with 97 and
99% consumption of 5-hexenyl acetate respectively. Grubbs I achieves 8% conversion to the
71
heterocoupled product with 87% consumption of 5-hexenyl acetate and HG I achieves 3%
conversion with 73% consumption of 5-hexenyl acetate. The consumption of 5-hexenyl acetate
in this reaction is due to both the formation of the heterocoupled product and the 5-hexenyl
acetate homocoupling product.
Table 3.1.1 Standard olefin metathesis reactions using common catalysts.
Catalyst ROMPa
RCMb CM
c
Grubbs I 40% (90 min) 66% 8% (87%)
Grubbs II 99% (6 min) 96% 90% (97%)
HG I 4% (100 min) 90% 3% (73%)
HG II 99% (5 min) 99.5% 90% (99%)
aMaximum conversions at the respective reaction times. Conditions: 0.1 mol% cat., 0.5 M, 30 °C.
bConversions at 30 min reaction time under standard conditions. Conditions: 1 mol% cat., 0.1 M, 30 °C.
cConversion to heterocoupled product at 70 min. In brackets, consumption of 5-hexenyl acetate. Conditions: 2.5
mol% cat., 0.4 M, 30 °C.
3.1.3 Acid Assisted Olefin Metathesis
There are a number of reports on the use of Brønsted or Lewis acids as additives in olefin
metathesis reactions. Their role is different depending on the catalyst and/or metathesis
transformation being studied. Lewis acids could be used to increase yields for transformations
that utilize substrates with a reactive or Lewis basic functional group. For example, it has been
shown that the use of Lewis acids, such as Ti(OiPr)4, for RCM reactions involving substrates
with a nucleophilic nitrogen results in increased yield of the ring closed product (Scheme
3.1.5).25
It is believed that the Lewis acid binds to the nucleophilic N atom to prevent it from
binding to the Ru center and thus shutting off metathesis.
Scheme 3.1.5 Lewis acid assisted RCM.
72
Alternatively, and more relevant to this thesis, acids could activate the catalysts by protonation of
a ligand, ligand abstraction via coordination of a ligand to the Lewis acid or via halide
abstraction. In these cases, the acid activation of olefin metathesis catalysts creates a vacant
coordination site which results in a coordinatively unsaturated metal center for substrate to bind.
Grubbs and co-workers have reported the use of HCl as an additive to enhance the activity of
Grubbs II in CM reactions (Scheme 3.1.6).26
The acid acts as a phosphine scavenger to generate
the active 4-coordinate catalyst and prevents the phosphine from competing with substrate for
coordination to the ruthenium center.
Scheme 3.1.6 Use of acid as a phosphine scavenger.
BCl3 has recently been shown to activate Ru alkylidene complexes with tridentate, dianionic
thiolate ligands for olefin metathesis (Scheme 3.1.7).27
Scheme 3.1.7 Activation of metathesis catalysts with BCl3.
The first equivalent results in a chloride being transferred to the metal center and the remaining
BCl2 fragment bridges the two anionic donors. The second equivalent of BCl3 abstracts the
73
chloride from the metal center resulting in a cationic ruthenium species which is active in
standard metathesis tests.
3.1.4 Halide Abstraction for Activation of Metathesis Catalysts
Hofmann and co-workers showed that Ru-based alkylidene species could be activated for olefin
metathesis through halide abstraction (Scheme 3.1.8).28
The addition of TMS-OTf abstracts a
halide and results in the formation of a chloride bridged Ru dimer.
Scheme 3.1.8 Activation of metathesis catalyst through halide abstraction.
This species exists in equilibrium with the monomeric form which is metathesis active. This is an
example of a metathesis active Ru-alkylidene species that is attained through halide abstraction
and not phosphine dissociation. More recently, Cazin and co-workers reported a 4-coordinate
cationic olefin metathesis catalyst29
that is formed through halide abstraction using AgSbF6
(Scheme 3.1.9).
Scheme 3.1.9 Synthesis of a 4-coordinate olefin metathesis catalyst by halide abstraction.
74
Both the neutral parent species and the cationic species are active for ring closing metathesis and
cross metathesis at 140 °C. Interestingly, even though the cationic species is more active for
olefin metathesis than the parent neutral complex, it is slower initiating.
3.1.5 Cross Metathesis of NBR and 1-Hexene
As mentioned in Section 1.3, a lower molecular weight polymer can be obtained by performing
cross metathesis of NBR with 1-hexene.5,30
This process is conceptually similar to ethenolysis
where the polymer is cut into smaller chains through cross metathesis of the internal olefins in
the polymer structure with a small olefinic substrate (1-hexene). Industrially this is accomplished
by employing Grubbs II where, depending on the catalyst loading and reaction times, polymers
of varying molecular weights and viscosities can be obtained.
3.2 Results and Discussion
With a number of ruthenium alkylidene species prepared, their activity for catalytic olefin
metathesis was probed. To do this, the standard system of characterization for olefin metathesis
catalysts was followed using the three specific reactions outlined above. As for the CM of NBR
with 1-hexene, only one example is shown below. Shown in Scheme 3.2.1 is a list of compounds
tested for catalytic olefin metathesis in this chapter.
75
Scheme 3.2.1 List of catalysts used for catalytic olefin metathesis.
3.2.1 ROMP of 1,5-Cyclooctadiene
Compound 2-4 was ineffective as a catalyst for ROMP at room temperature, but upon heating to
45 °C conversion to the product was achieved in 93% after 24 hours. The addition of one
equivalent of BCl3 to 2-4 resulted in increased activity in ROMP affording 100% product yield
after 6 hours at room temperature (Figure 3.2.1). This reaction was accelerated at 45 °C affording
98% conversion after 2 hours.
76
Figure 3.2.1 ROMP of 1,5-cyclooctadiene using 2-4. Conditions: 1 mol% cat., 0.1 M in CD2Cl2.
Similarly, using 2-5, ROMP activity was only observed at 45 °C with product yields of 69% after
24 hours. In the presence of one equivalent of BCl3, conversion of 92% was observed after 24
hours at room temperature and complete conversion after 8 hours at 45 °C.
Compound 2-6 was more effective than 2-5 with complete conversion after 24 hours at 45 °C
and upon the addition of BCl3, at room temperature, product yields of 100% were observed after
8 hours. At 45 °C the same conversion was achieved after 2 hours. Compound 2-7 effected
ROMP catalysis with complete conversion after 2 hours both at room temperature and at 45 °C
upon using one equivalent of BCl3. Minimal activity is shown in the absence of BCl3 (58% after
24 hours at 45 °C).
Better rates are observed using 2-8 where, at 45 °C in the absence of BCl3, ROMP was achieved
in 100% yield after 4 hours. Upon the addition of BCl3, the same conversion was obtained after 6
hours at room temperature or 30 min at 45 °C (Figure 3.2.2).
77
Figure 3.2.2 ROMP of 1,5-cyclooctadiene using 2-8. Conditions: 1 mol% cat., 0.1 M in CD2Cl2.
ROMP activity at room temperature, in the absence of a Lewis acid, was obtained using 2-12
with conversions of 79% after 24 hours. This was enhanced to 93% after 6 hours at 45 °C. Upon
the addition of BCl3, complete conversion was obtained after 8 hours at room temperature or
after 30 minutes at 45 °C (Figure 3.2.3).
Figure 3.2.3 ROMP of 1,5-cyclooctadiene using 2-12. Conditions: 1 mol% cat., 0.1 M in
CD2Cl2.
78
Using 2-13, product yields for ROMP of 95% were achieved at room temperature without the
addition of BCl3. Complete conversion was obtained at room temperature upon the addition of
BCl3 after 2 hours and similar conversions were obtained with 2-14.
3.2.2 RCM of Diethyl Diallylmalonate
In the case of RCM of diethyl diallylmalonate, 2-4 was ineffective both at room temperature and
at 45 °C in the absence of a Lewis acid. Upon the addition of BCl3, moderate conversion to the
ring-closed product (14%) was achieved after 24 hours at room temperature. Conversion was
enhanced at 45 °C to 88% when BCl3 was added (Figure 3.2.4). Adding two equivalents of BCl3
did not enhance catalysis and at elevated temperatures lower conversions were obtained.
Figure 3.2.4 RCM of diethyl diallylmalonate using 2-4. Conditions: 5 mol% cat., 0.16 M in
CD2Cl2.
Compound 2-5 was only effective as a catalyst for RCM at 45 °C upon the addition of BCl3
(100% after 24 hours). Slightly enhanced conversions were obtained with 2-6 where upon the
addition of BCl3, at room temperature, product yields of 33% were observed after 24 hours. At
45 °C complete conversion was obtained after 6 hours for 2-6 while compound 2-7 showed
moderate activities with 52% product yield after 24 hours, under similar conditions.
79
Better rates were observed using 2-8 as the catalyst where upon the addition of BCl3 at room
temperature product yields of 79% were observed after 24 hours. This is further enhanced at
45 °C where complete conversion was obtained after 2 hours (Figure 3.2.5).
Figure 3.2.5 RCM of diethyl diallylmalonate using 2-8. Conditions: 5 mol% cat., 0.16 M in
CD2Cl2.
Minimal RCM activity at room temperature, in the absence of a Lewis acid, was obtained using
2-12 with conversions of 17% after 24 hours. Upon the addition of BCl3, enhanced activity was
observed with conversions of 93% after 24 hours at room temperature and 100% after 2 hours at
45 °C (Figure 3.2.6).
80
Figure 3.2.6 RCM of diethyl diallylmalonate using 2-12. Conditions: 5 mol% cat., 0.16 M in
CD2Cl2.
Using 2-13, product yields for RCM of 60% were achieved at 45 °C without the addition of BCl3
and 88% upon the addition of BCl3 after 24 hours at 25 °C. At 45 °C, in the presence of BCl3,
complete conversion was obtained after 2 hours. Under similar conditions, using 2-14,
conversions of 56, 93 and 100% were obtained, respectively.
3.2.3 CM of 5-Hexenyl Acetate and Methyl Acrylate
For the CM of 5-hexenyl acetate and methyl acrylate, 2-4 was ineffective both at room
temperature and at 45 °C in the absence of a Lewis acid. Upon the addition of BCl3, minimal
conversion to the heterocoupled product (26%) was achieved after 24 hours at 25 °C. Conversion
was enhanced at 45 °C to 51% when BCl3 was added (Figure 3.2.7). Adding two equivalents of
BCl3 enhanced the conversions slightly at room temperature (41% after 24 hours) and at 45 °C
(61% after 8 hours). It is worth noting that although conversions to the heterocoupled product
were not very high, the homocoupled product was also observed.
81
Figure 3.2.7 CM of methyl acrylate and 5-hexenyl acetate using 2-4. Conditions: 2 mol% cat.,
0.4 M in CD2Cl2.
Compound 2-5 was only minimally effective as a catalyst for CM at 45 °C upon the addition of
BCl3 (28% after 2 hours). Slightly enhanced conversions were obtained with 2-6 with the
addition of BCl3, at 45 °C, where 48% of the heterocoupled product was obtained after 2 hours
while compound 2-7 showed conversions of 32% after 24 hours, under similar conditions.
Better rates were observed using 2-8, with the addition of BCl3 at room temperature, where
product yields of 50% were obtained after 6 hours. This is further enhanced at 45 °C where
conversion of 72% was obtained after 4 hours (Figure 3.2.8).
82
Figure 3.2.8 CM of methyl acrylate and 5-hexenyl acetate using 2-8. Conditions: 2 mol% cat.,
0.4 M in CD2Cl2.
No CM activity at 25 °C, in the absence of a Lewis acid, was observed using 2-12. Upon the
addition of BCl3, enhanced activity was observed with conversions to the heterocoupled product
of 40% after 6 hours at 25 °C and 50% after 4 hours at 45 °C (Figure 3.2.9).
Figure 3.2.9 CM of methyl acrylate and 5-hexenyl acetate using 2-12. Conditions: 2 mol% cat.,
0.4 M in CD2Cl2.
83
Using 2-13, product yields for CM of 21%, at room temperature upon the addition of BCl3 after 6
hours, were obtained while at 45 °C, in the presence of BCl3, 60% conversion was obtained after
4 hours. The best CM conversions were obtained with 2-14 where, at room temperature with one
equivalent of BCl3, conversions of 79% were observed after 6 hours. Similar conversions were
obtained after 2 hours at 45 °C.
3.2.4 Trends in Catalytic Olefin Metathesis
Pentafluorophenyl thiolate-containing compounds (2-12, 2-13, and 2-14) were shown to be more
active for ROMP, RCM, and CM than phenyl thiolate analogues (2-4, 2-5, 2-6, 2-7, and 2-8).
Pseudo-halides have been studied by Fogg and co-workers who reported substitution of chlorides
with catecholate31
or phenoxide32,33
based anionic ligands in Grubbs type systems. Most of the
systems reported showed slow initiation, but very good activities, where metathesis was done at
elevated temperatures. It is also observed that compounds with more electron donating carbenes
(i.e. Me2Im(OMe)2 > Im(OMe)2 and SIMes > IMes) show better activity with 2-8 being the most
active. It has been demonstrated that Ru-alkylidene complexes with more electron donating
NHCs tend to be more active for olefin metathesis. This is presumably because they enhance the
rate of oxidative addition needed for metallacyclobutane formation during catalysis.34
The
addition of BCl3 as an additive enhanced the activity of all the pre-catalysts tested both at room
temperature and at 45 oC. The role of the BCl3 as an activator was probed by monitoring
reactions of several of the compounds discussed above with BCl3 by 11
B{1H} NMR
spectroscopy. The products showed resonances at 6.9 ppm attributable to the formation of the
[BCl4] anion.35
Nonetheless, efforts to either isolate the corresponding cation or its complex with
a series of donor molecules were unsuccessful. In spite of this, these data suggest that the borane
abstracts the halide to generate a site of unsaturation on ruthenium, presumably accounting for
the enhanced catalytic activity.
3.2.5 Cross Metathesis of NBR and 1-Hexene
The systems described above were developed with the ultimate goal of catalyzing the cross
metathesis of NBR and 1-hexene to decrease to molecular weight and PDI (Ð) of the polymer.
The NBR tested had an initial Mw of 275 000 Da and a Mn of 82 000 Da with a PDI of 3.4. Only
catalysts that displayed activity for CM of 5-hexenyl acetate and methyl acrylate were tested for
CM of NBR and 1-hexene and only one example is provided in this chapter. Several parameters
84
were probed including changing the catalyst loading, using either no BCl3 or one or two
equivalents of the Lewis acid, as well as running the reactions at room temperature, 45 °C and
60 °C. For comparison purposes, Grubbs II was tested under similar conditions.
After one hour at 25 °C, using Grubbs II with a catalyst loading of 0.07 phr, a decrease in Mw
and Mn to 80 000 and 38 000 Da, respectively, was observed. The Ð was also reduced to 2.1
After 24 hours, Mw, Mn and Ð are further lowered to 33 000 Da, 20 000 Da, and 1.6,
respectively. Using 0.07 phr catalyst loading with 2-4 at room temperatures resulted in no
considerable CM where after 24 hours the Mw, Mn and Ð were essentially unchanged at 231 000
Da, 76 000 Da, and 3.0, respectively. Upon the addition of one or two equivalents of BCl3, at
room temperature, minimal CM is observed. In the case of one equivalent of BCl3, the Mw, Mn
and Ð are 207 000 Da, 68 000 Da, and 3.0 respectively, after 24 hours. In the case of two
equivalents of BCl3, the Mw, Mn and Ð are 191 000 Da, 70 000 Da, and 2.7 respectively, after 24
hours.
Using Grubbs II at a catalyst loading of 0.07 phr at 45 °C resulted in very effective CM of NBR
with 1-hexene where, after one hour, the Mw, Mn and Ð are 23 000 Da, 16 000 Da, and 1.5
respectively. These numbers remain essentially unchanged after 24 hours. On the other hand,
using 2-4 under the same reaction conditions resulted in no tangible metathesis where, after 24
hours, the Mw, Mn and Ð are 235 000 Da, 76 000 Da, and 3.1 respectively. Slightly enhanced
metathesis was observed upon the addition of one or two equivalents of BCl3. In the case of one
equivalent of BCl3, after 24 hours at 45 °C, the Mw, Mn and Ð are 170 000 Da, 64 000 Da, and
2.6 respectively. In the case of two equivalents of BCl3, after 24 hours at 45 °C, the Mw, Mn and
Ð are 176 000 Da, 66 000 Da, and 2.7 respectively (Figure 3.2.10).
85
Figure 3.2.10 CM of NBR and 1-hexene using 2-4 and Grubbs II at 45 °C.
Increasing the catalyst loading of 2-4 to 0.14 phr at 45 °C significantly enhanced the CM of NBR
and 1-hexene. With no BCl3 added, after 24 hours, the Mw, Mn and Ð are 221 000 Da,
75 000 Da, and 2.9 respectively. Upon the addition of one equivalent of BCl3, after 24 hours at
45 °C, the Mw, Mn and Ð are 157 000 Da, 63 000 Da, and 2.5 respectively. In the case of two
equivalents of BCl3, after 24 hours at 45 °C, the Mw, Mn and Ð are 76 000 Da, 39 000 Da, and
2.0 respectively (Fig. 3.2.10).
Increasing the temperature to 60 °C enhanced catalysis slightly compared to runs at 45 °C with
2-4 (Figure 3.2.11). At a catalyst loading of 0.07 phr with one equivalent of BCl3, after 24 hours,
the Mw, Mn and Ð are 147 000 Da, 59 000 Da, and 2.5 respectively. With two equivalents of
BCl3, after 24 hours at 60 °C, the Mw, Mn and Ð are 119 000 Da, 52 000 Da, and 2.3
respectively. At a catalyst loading of 0.14 phr with one equivalent of BCl3, after 24 hours, the
Mw, Mn and Ð arere 126 000 Da, 54 000 Da, and 2.3 respectively. With two equivalents of BCl3,
after 24 hours at 60 °C, the Mw, Mn and Ð are 88 000 Da, 43 000 Da, and 2.1 respectively.
86
Figure 3.2.11 CM of NBR and 1-hexene using 2-4 at 60 °C.
There is no enhancement to catalysis at 60 °C both with one and two equivalents of BCl3. This is
presumably because the generated species upon adding two equivalents of BCl3 is less stable
than the one generated upon addition of just one equivalent of BCl3.
3.3 Conclusion
Ruthenium alkylidene complexes bearing the hemilabile tridentate NHC 2-4 to 2-14 are either
inactive or minimally active for RCM, ROMP and CM. The species generated by the addition of
one equivalent of BCl3 show improved activity for RCM, ROMP and CM either at room
temperature or at 45 °C. In general, the catalysts which contain more electron donating carbenes
are more active than those containing less donating NHCs. The catalysts with S(C6F5), as one of
the anionic ligands, are most active compared to the catalysts with the PhS- ligand. Complex 2-4
is chosen as an example to demonstrate the activity in the cross metathesis of NBR and 1-hexene
at different conditions. This system was active but higher catalyst loadings and elevated
temperatures were required to achieve similar conversions as Grubbs II catalyst at room
temperature.
87
3.4 Experimental Section
3.4.1 General Considerations
All synthetic manipulations were carried out under an atmosphere of dry, O2-free N2 employing
a VAC Atmospheres glove box. Dichloromethane-d2 was dried over CaH2 and vacuum
transferred into a Young bomb. All solvents were thoroughly degassed after purification (three
freeze-pump-thaw cycles). NMR spectra were recorded at 25 °C on a Bruker Avance 400 MHz
spectrometer. Commercially available substrates were obtained from Sigma-Aldrich and used
without further purification. NBR was obtained from Lanxess and stored at -40 ºC. GPC data
was collected using Styragel HR 5E-THF columns at 45 °C using a Waters 2414 RI Detector.
Data was processed using Empower Pro software and Mw and Mn data were determined against a
polystyrene calibration curve.
3.4.2 Synthetic Procedures
3.4.2.1 Standard Metathesis Reaction Tests
All standard metathesis reaction tests were performed employing a modified procedure of
Grubbs and co-workers.24
The standard procedure for the ring opening metathesis polymerization of 1,5-cyclooctadiene is
as follows: The required amount of the catalyst (1 mol%), was weighed out and dissolved in
0.5 mL CD2Cl2. For the tests that involved the use of an additive (i.e. BCl3, 1M in hexane) the
required volume was added and the mixture was allowed to stand for 5 min. The solutions were
placed in an NMR tube, 1,5-cyclooctadiene (60 μL, 0.50 mmol) was added, the NMR tube was
capped and the solution was mixed at the desired temperature. Reaction progress was monitored
by 1H NMR every 2 hours (unless otherwise noted). Reaction progress was determined by
integration of the peaks corresponding to the starting material versus the product.
88
Table 3.4.1 ROMP of 1,5-cyclooctadiene with 2-4.
Compound 2-4
Additive Temperature (oC) Time (h) Conversion (%)
None 25 24 0
None 45 2 4
4 8
6 14
24 93
1 mol% BCl3 25 2 54
4 96
6 100
1 mol% BCl3 45 2 98
4 100
2 mol% BCl3 25 2 53
3 71
2 mol% BCl3 45 2 100
Table 3.4.2 ROMP of 1,5-cyclooctadiene with 2-5.
Compound 2-5
Additive Temperature (oC) Time (h) Conversion (%)
None 25 24 0
None 45 2 6
4 11
6 16
8 21
24 69
1 mol% BCl3 25 2 11
4 16
6 21
8 30
89
24 92
1 mol% BCl3 45 2 26
4 64
6 90
8 100
2 mol% BCl3 25 2 70
4 92
6 100
2 mol% BCl3 45 2 100
Table 3.4.3 ROMP of 1,5-cyclooctadiene with 2-6.
Compound 2-6
Additive Temperature (oC) Time (h) Conversion (%)
None 25 24 0
None 45 2 5
4 13
6 23
8 36
24 100
1 mol% BCl3 25 2 30
4 56
6 83
8 100
1 mol% BCl3 45 2 100
Table 3.4.4 ROMP of 1,5-cyclooctadiene with 2-7.
Compound 2-7
Additive Temperature (oC) Time (h) Conversion (%)
None 25 24 0
None 45 2 3
90
4 9
6 14
24 58
1 mol% BCl3 25 2 100
1 mol% BCl3 45 2 100
Table 3.4.5 ROMP of 1,5-cyclooctadiene with 2-8.
Compound 2-8
Additive Temperature (oC) Time (h) Conversion (%)
None 25 24 0
None 45 2 54
4 100
1 mol% BCl3 25 0.5 21
2 61
4 92
6 100
1 mol% BCl3 45 0.5 100
Table 3.4.6 ROMP of 1,5-cyclooctadiene with 2-12.
Compound 2-12
Additive Temperature (oC) Time (h) Conversion (%)
None 25 2 14
4 23
6 30
8 37
24 79
None 45 2 79
4 89
6 93
91
1 mol% BCl3 25 2 64
4 83
6 95
8 100
1 mol% BCl3 45 0.5 100
2 mol% BCl3 25 2 42
4 73
6 89
8 97
Table 3.4.7 ROMP of 1,5-cyclooctadiene with 2-13.
Compound 2-13
Additive Temperature (oC) Time (h) Conversion (%)
None 25 2 38
4 58
6 71
8 80
24 95
1 mol% BCl3 25 2 100
Table 3.4.8 ROMP of 1,5-cyclooctadiene with 2-14.
Compound 2-14
Additive Temperature (oC) Time (h) Conversion (%)
None 25 2 71
4 62
6 84
8 90
24 93
1 mol% BCl3 25 2 100
92
2 mol% BCl3 25 2 77
4 93
6 100
A standard procedure for the ring closing metathesis of diethyl diallylmalonate is as follows. The
required amount of catalyst (5 mol%) was weighed out and dissolved in 0.5 mL CD2Cl2. For the
tests that involved the use of an additive (i.e. BCl3, 1M in hexane) the required volume was
added and the mixture was allowed to stand for 5 min. The solution was placed in an NMR tube,
diethyl diallylmalonate (20 μL, 0.50 mmol) was added, the NMR tube was capped and the
solution was mixed at the desired temperature. Reaction progress was monitored by 1H NMR
every 2 hours (unless otherwise noted). Reaction progress was determined by integration of the
olefinic peaks of the starting material versus the product.
Table 3.4.9 RCM of diethyl diallylmalonate with 2-4.
Compound 2-4
Additive Temperature (oC) Time (h) Conversion (%)
None 25 24 0
None 45 24 0
5 mol% BCl3 25 24 14
5 mol% BCl3 45 2 5
4 16
6 34
8 47
24 88
10 mol% BCl3 25 2 4
4 6
6 10
8 12
24 28
10 mol% BCl3 45 2 17
4 41
6 54
93
8 59
24 67
Table 3.4.10 RCM of diethyl diallylmalonate with 2-5.
Compound 2-5
Additive Temperature (oC) Time (h) Conversion (%)
None 25 24 0
None 45 24 0
5 mol% BCl3 25 24 0
5 mol% BCl3 45 2 5
4 11
6 14
8 21
24 100
10 mol% BCl3 25 2 3
4 7
6 12
8 17
24 46
10 mol% BCl3 45 2 19
4 44
6 49
8 51
24 75
94
Table 3.4.11 RCM of diethyl diallylmalonate with 2-6.
Compound 2-6
Additive Temperature (oC) Time (h) Conversion (%)
None 25 24 0
None 45 2 6
4 10
6 13
8 16
5 mol% BCl3 25 2 0
4 3
6 7
8 15
24 33
5 mol% BCl3 45 2 63
4 91
6 100
Table 3.4.12 RCM of diethyl diallylmalonate with 2-7.
Compound 2-7
Additive Temperature (oC) Time (h) Conversion (%)
None 25 24 0
None 45 24 0
5 mol% BCl3 25 8 3
24 10
5 mol% BCl3 45 2 16
4 24
6 28
8 29
24 52
.
95
Table 3.4.13 RCM of diethyl diallylmalonate with 2-8.
Compound 2-8
Additive Temperature (oC) Time (h) Conversion (%)
None 25 24 0
None 45 2 7
4 12
5 mol% BCl3 25 2 5
4 13
6 28
8 47
24 79
5 mol% BCl3 45 0.5 42
2 100
Table 3.4.14 RCM of diethyl diallylmalonate with 2-12.
Compound 2-12
Additive Temperature (oC) Time (h) Conversion (%)
None 25 2 0
24 4
None 45 2 2
4 4
6 6
8 8
24 17
5 mol% BCl3 25 2 15
4 51
6 72
8 81
24 93
5 mol% BCl3 45 2 100
10 mol% BCl3 25 2 3
96
4 7
6 13
8 22
24 57
10 mol% BCl3 45 2 67
4 87
6 92
Table 3.4.15 RCM of diethyl diallylmalonate with 2-13.
Compound 2-13
Additive Temperature (oC) Time (h) Conversion (%)
None 25 24 7
None 45 2 10
4 16
6 24
8 30
24 60
5 mol% BCl3 25 2 12
4 20
6 32
8 52
24 88
5 mol% BCl3 45 2 100
97
Table 3.4.16 RCM of diethyl diallylmalonate with 2-14.
Compound 2-14
Additive Temperature (oC) Time (h) Conversion (%)
None 25 2 1
24 15
None 45 2 9
4 14
6 19
8 25
24 56
5 mol% BCl3 25 2 28
4 62
6 85
8 90
24 93
5 mol% BCl3 45 2 100
10 mol% BCl3 25 2 6
4 17
6 28
8 41
24 86
10 mol% BCl3 45 2 83
4 100
A standard procedure for cross metathesis of 5-hexenyl acetate and methyl acrylate is as follows.
The required amount of catalyst (2 mol%) was weighed out and dissolved in 0.5 mL CD2Cl2. For
the tests that involved the use of an additive (i.e. BCl3, 1M in hexane) the required volume was
added and the mixture was allowed to stand for 5 min. The solution was placed in an NMR tube
and a mixture of 5-hexenyl acetate (20 μL, 0.12 mmol) and methyl acrylate (10 μL, 0.11 mmol)
was added and the solution was mixed at the desired temperature. Reaction progress was
monitored by 1H NMR every 2 hours (unless otherwise noted). Reaction progress was
determined by integration of the olefinic peaks of the starting material versus the product.
98
Table 3.4.17 CM of 5-hexenyl acetate and methyl acrylate with 2-4.
Compound 2-4
Additive Temperature (oC) Time (h) Conversion (%)
None 25 24 0
None 45 24 0
2 mol% BCl3 25 2 0
6 13
8 24
24 26
2 mol% BCl3 45 2 24
4 43
6 49
8 51
4 mol% BCl3 25 2 15
4 20
6 24
8 31
24 41
4 mol% BCl3 45 2 35
4 47
6 56
8 61
Table 3.4.18 CM of 5-hexenyl acetate and methyl acrylate with 2-5.
Compound 2-5
Additive Temperature (oC) Time (h) Conversion (%)
None 25 24 0
None 45 24 0
2 mol% BCl3 25 24 0
2 mol% BCl3 45 2 28
99
4 mol% BCl3 25 24 0
4 mol% BCl3 45 2 20
Table 3.4.19 CM of 5-hexenyl acetate and methyl acrylate with 2-6.
Compound 2-6
Additive Temperature (oC) Time (h) Conversion (%)
None 25 24 0
None 45 24 0
2 mol% BCl3 25 2 48
2 mol% BCl3 45 2 42
Table 3.4.20 CM of 5-hexenyl acetate and methyl acrylate with 2-7.
Compound 2-7
Additive Temperature (oC) Time (h) Conversion (%)
None 25 24 0
None 45 24 0
2 mol% BCl3 25 24 0
2 mol% BCl3 45 4 21
6 23
8 28
24 32
Table 3.4.21 CM of 5-hexenyl acetate and methyl acrylate with 2-8.
Compound 2-8
Additive Temperature (oC) Time (h) Conversion (%)
None 25 24 0
None 45 24 0
2 mol% BCl3 25 2 38
100
4 46
6 50
2 mol% BCl3 45 2 65
4 72
Table 3.4.22 CM of 5-hexenyl acetate and methyl acrylate with 2-12.
Compound 2-12
Additive Temperature (oC) Time (h) Conversion (%)
None 25 24 0
None 45 24 0
2 mol% BCl3 25 2 32
4 37
6 40
2 mol% BCl3 45 2 47
4 50
4 mol% BCl3 25 2 0
4 28
6 33
4 mol% BCl3 45 2 65
Table 3.4.23 CM of 5-hexenyl acetate and methyl acrylate with 2-13.
Compound 2-13
Additive Temperature (oC) Time (h) Conversion (%)
None 25 24 0
None 45 24 0
2 mol% BCl3 25 2 15
4 18
6 21
2 mol% BCl3 45 2 55
4 60
101
Table 3.4.24 CM of 5-hexenyl acetate and methyl acrylate with 2-14.
Compound 2-14
Additive Temperature (oC) Time (h) Conversion (%)
None 25 24 0
None 45 24 0
2 mol% BCl3 25 2 20
4 50
6 79
2 mol% BCl3 45 2 80
4 mol% BCl3 25 2 18
4 28
6 32
8 36
24 53
4 mol% BCl3 45 2 61
4 75
3.4.2.2 Cross Metathesis of NBR and 1-hexene
A standard procedure for the cross metathesis of nitrile butadiene rubber (NBR) and 1-hexene is
as follows. NBR (1.5 g) were placed in 13.5 g of chlorobenzene and placed on a shaker for 48 h
to give a 10 wt% NBR solution. 1-Hexene (60 mg, 90 μL) was added to the solution and shaken
for 1 h. The catalysts were prepared by dissolving the required mass of precatalyst in CH2Cl2
(2 mL) in a glove box and the appropriate amount of BCl3 was then added and the solutions were
stirred for 5 min before being taken out of the glove box and added to the NBR solutions. The
solutions were then stirred at the desired temperature for a total of 24 h. Samples were taken at 1,
2, 3, 4, and 24 h. The catalysts were poisoned with ethyl vinyl ether (0.1 mL) to stop the
metathesis. All volatiles were removed from the samples. GPC samples were made by preparing
a 1 mg/mL THF solution of the resulting NBR. The samples were passed through a microporous
filter and the Mn, Mw, and Ð were determined by GPC using a polystyrene calibration curve. The
102
Mw and Mn for NBR used with 2-4 and Grubbs II are 275 000 and 82 000 Da, respectively, and
the Ð is 3.3.
Table 3.4.25 GPC data for CM of NBR and 1-hexene using 2-4 and Grubbs II at 25 °C.
Catalyst 2-4 2-4 2-4 Grubbs II
Catalyst loading (phr) 0.07 0.07 0.07 0.07
BCl3 (equiv.) None 1 2 None
1 h
Mw 251 000 240 000 257 000 80 000
Mn 82 000 88 000 92 000 38 000
Ð 3.0 2.7 2.8 2.1
2 h
Mw 255 000 242 000 235 000 58 000
Mn 81 000 80 000 78 000 30 000
Ð 3.1 3.0 3.0 1.9
3 h
Mw 242 000 236 000 221 000 48 000
Mn 79 000 79 000 77 000 27 000
Ð 3.0 3.0 2.9 1.8
4 h
Mw 248 000 233 000 - 42 000
Mn 84 000 79 000 - 24 000
Ð 3.0 3.0 - 1.7
24 h
Mw 231 000 207 000 191 000 33 000
Mn 76 000 68 000 70 000 20 000
Ð 3.0 3.0 2.7 1.6
103
Table 3.4.26 GPC data for CM of NBR and 1-hexene using 2-4 and Grubbs II at 45 °C.
Catalyst 2-4 2-4 2-4 Grubbs II
Catalyst loading (phr) 0.07 0.07 0.07 0.07
BCl3 (equiv.) None 1 2 None
1 h
Mw 240 000 204 000 220 000 24 000
Mn 87 000 80 000 80 000 15 000
Ð 2.8 2.6 2.7 1.5
2 h
Mw 236 000 209 000 231 000 23 000
Mn 90 000 76 000 81 000 15 000
Ð 2.6 2.7 2.9 1.5
3 h
Mw 261 000 205 000 231 000 24 000
Mn 82 000 69 000 77 000 16 000
Ð 3.2 3.0 3.0 1.5
4 h
Mw 251 000 207 000 217 000 23 000
Mn 85 000 73 000 81 000 15 000
Ð 2.9 2.8 2.7 1.5
24 h
Mw 235 000 170 000 176 000 23 000
Mn 76 000 64 000 66 000 15 000
Ð 3.1 2.6 2.7 1.5
104
Table 3.4.27 GPC data for CM of NBR and 1-hexene using 0.14 phr of 2-4 at 45 °C.
Catalyst 2-4 2-4 2-4
Catalyst loading (phr) 0.14 0.14 0.14
BCl3 (equiv.) None 1 2
1 h
Mw 232 000 175 000 103 000
Mn 79 000 66 000 48 000
Ð 2.9 2.6 2.1
2 h
Mw 239 000 170 000 88 000
Mn 89 000 67 000 43 000
Ð 2.6 2.6 2.1
3 h
Mw 230 000 173 000 85 000
Mn 77 000 64 000 42 000
Ð 3.0 2.7 2.0
4 h
Mw 238 000 169 000 82 000
Mn 76 000 66 000 41 000
Ð 3.1 2.6 2.0
24 h
Mw 221 000 157 000 76 000
Mn 75 000 63 000 39 000
Ð 3.0 2.5 2.0
105
Table 3.4.28 GPC data for CM of NBR and 1-hexene using 2-4 at 60 °C.
Catalyst 2-4 2-4 2-4 2-4
Catalyst loading (phr) 0.07 0.07 0.14 0.14
BCl3 (equiv.) 1 2 1 2
1 h
Mw 161 000 159 000 144 000 97 000
Mn 72 000 61 000 58 000 45 000
Ð 2.3 2.6 2.5 2.1
2 h
Mw 168 000 146 000 141 000 94 000
Mn 64 000 58 000 56 000 45 000
Ð 2.7 2.5 2.5 2.1
3 h
Mw 162353 145277 134695 93700
Mn 62 000 58 000 54 000 44 000
Ð 2.6 2.5 2.5 2.1
4 h
Mw 164 000 140 000 137 000 95 000
Mn 63 000 57 000 56 000 45 000
Ð 2.6 2.4 2.4 2.0
24 h
Mw 147 000 119 000 126 000 88 000
Mn 59 000 52 000 54 000 43 000
Ð 2.5 2.3 2.3 2.0
106
References
(1) Astruc, D. New J. Chem. 2005, 29, 42.
(2) Dragutan, I.; Dragutan, V.; Demonceau, A. Rsc. Adv. 2012, 2, 719.
(3) Grubbs, R. H.; Chang, S. Tetrahedron 1998, 54, 4413.
(4) Meek, S. J.; O'Brien, R. V.; Llaveria, J.; Schrock, R. R.; Hoveyda, A. H. Nature 2011,
471, 461.
(5) Ong, C.; Mueller, J. M.; Soddemann, M.; Koenig, T. Metathesis of nitrile rubbers in the
presence of transition metal catalysts. WO2011023763A1, 2011.
(6) Pederson, R. L.; Fellows, I. M.; Ung, T. A.; Ishihara, H.; Hajela, S. P. Adv. Synth. Catal.
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(7) Grubbs, R. H. Handbook of Metathesis; WILEY-VCH Verlag GmbH and Co.: Germany,
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108
Chapter 4 Synthesis of Bis-Mixed-Carbene Ruthenium-Alkylidene
Complexes Through Anion Exchange
4.1 Introduction
4.1.1 Halide Variation in Grubbs Catalyst
Modifications to the NHC ligand in Grubbs Catalyst have received considerable attention while,
by comparison, modification of the anionic ligands has received very little.1 To study the effect
of replacing the chloride ligands in Grubbs I and II for other halides on catalytic activity, Grubbs
and co-workers synthesized (PCy3)2RuX2(CHPh) and SIMes(PCy3)RuX2(CHPh) where X=Cl,
Br, I.2 In terms of initiation it was found that systems with X = I were the fastest initiating
followed by X = Br and X = Cl. Increasing the size of the halide from Cl- to I
- lowers the barrier
to phosphine dissociation due to steric congestion at the metal center which results in faster
initiation rates. The activity of the catalysts, however, decreased from X=Cl to X=Br to X=I.
This is, again, due to the size of the halide preventing the coordination of the incoming olefin.
4.1.2 Pseudo-halides as Ligands in Ruthenium Metathesis Catalysts
In addition to halides, a number of other X-type ligands have been investigated and the effect of
the resulting complexes on metathesis activity studied. Typical ligands have included alkoxides,3
aryloxides,4-8
carboxylates,9-11
and more recently, thiolates12-14
and nitrates.15
A common
decomposition pathway for most Grubbs-type systems is through the formation of inactive halide
bridged dimers.16-18
This was successfully avoided by changing the halides for different anionic
groups when Grubbs and co-workers reported the synthesis of the 4-coordinate species
(PCy3)Ru(OtBu)2(CHPh) (A in Figure 4.1.1).3 Although these complexes are 4-coordinate, the
ability of the alkoxide to act as an XL-type ligand and donate 3 electrons to the metal center
results in low activity for olefin metathesis even at elevated temperatures. Fogg and co-workers
successfully overcame this problem through the use of electron deficient, perhalogenated
aryloxides as ligands (B and C in Figure 4.1.1).4,6
These catalysts showed slower initiation rates
but had activities similar to or better than Grubbs II.
109
Figure 4.1.1 Alkoxide and electron deficient aryloxides as ligands on olefin metathesis catalysts.
Changing the anionic groups not only affects activity and stability of catalysts, it also induces
unprecedented reactivity. More recently, Jensen and co-workers have prepared an olefin
metathesis catalyst with a bulky arylthiolate ligand (Figure 4.1.2).13
The large aryl group on the
thiolate induces Z-selectivity on the resulting product olefin with up to 96% selectivity.
Figure 4.1.2 Z-selective olefin metathesis catalyst with a thiolate ligand.
This compound is formed through chloride exchange in Grubbs-Hoveyda II with the sterically
demanding 2,4,6-triphenylbenzenethiolate ligand.
4.2 Results and Discussion
4.2.1 Synthesis of Ru Complexes
Based on the results presented in Chapters 2 and 3, we were interested in probing the effect of
the anionic ligands on metathesis activity, especially on initiation. Therefore, a series of
complexes featuring other halides were synthesized. The addition of trimethylsilyl iodide to a
solution of 2-4 in C6H6 and stirring for one hour at room temperature followed by workup
afforded the isolation of compound 4-1 as a red solid in 87% yield (Scheme 4.2.1). The 1H NMR
110
spectrum of 4-1 reveals a broad singlet at 18.82 ppm which integrates to one proton and is
assigned to the Ru=CH fragment. The corresponding carbon signal was derived from two
dimensional NMR experiments (HSQC) and is observed at 314.2 ppm.
Scheme 4.2.1 Synthesis of 4-1 and 4-2.
Single crystal X-ray analysis of 4-1 confirmed its formulation as
(Im(OMe)2)(SIMes)Ru(=CHCH3)I(SPh). The geometry around the Ru center is distorted square
pyramid (Figure 4.2.1) where the two carbenes are trans disposed with a C-Ru-C angle of
158.22(14)o. The two anionic groups in 4-1 are also mutually trans while the alkylidene fragment
occupies the pseudo-apical position. The Ru-C distances for the carbenes are 2.082(3) Å and
2.108(3) Å for SIMes and Im(OMe)2, respectively, while the Ru-C distance for the alkylidene is
1.826(4) Å. The corresponding Ru-I distance is 2.7781(4) Å and the Ru-S distance is
2.3860(10) Å.
111
Figure 4.2.1 POV-ray depiction of the molecular structure of 4-1. Ru: dark green, O: red, I:
magenta, N: aquamarine, S: yellow, C: black. H-atoms omitted for clarity.
A similar reaction utilizing 2-8 resulted in the isolation of 4-2 as a red solid in a modest 53%
yield. The 1H NMR spectrum of 4-2 reveals a diagnostic broad singlet at 19.04 ppm which
integrates to one proton and is assigned to the Ru=CH fragment. Single crystal X-ray analysis of
4-2 confirmed the formulation as (Me2Im(OMe)2)(SIMes)Ru(=CHCH3)I(SPh) where the
geometry around the metal center is best described as distorted square pyramid (Figure 4.2.2).
Figure 4.2.2 POV-ray depiction of the molecular structure of 4-2. Ru: dark green, O: red, I:
magenta, N: aquamarine, S: yellow, C: black. H-atoms omitted for clarity.
Similar to 4-1, the two carbenes are trans disposed with a C-Ru-C angle of 160.20(15)o. The two
anionic groups in 4-2 are also mutually trans while the alkylidene fragment occupies the
pseudo-apical position. The Ru-C distances for the carbenes are 2.087(4) Å and 2.120(4) Å for
112
SIMes and Me2Im(OMe)2, respectively, while the Ru-C distance for the alkylidene is 1.827(4) Å.
The Ru-I distance is 2.7599(7) Å and the Ru-S distance is 2.3783(11) Å.
When trimethylsilyl iodide was added to a solution of 2-13 in C6D6, the 19
F{1H} NMR displayed
signals corresponding to 2-13 and another set of signals consistent with Me3SiS(C6F5) in a 1:1
ratio. Thus, when the reaction was repeated using two equivalents of trimethylsilyl iodide, 4-3
was isolated as a red solid in 88% yield (Scheme 4.2.2). The 1H NMR spectrum of 4-3 reveals a
triplet at 18.81 ppm, with 3JHH of 4 Hz, which integrates to one proton and is assigned to the
Ru=CH fragment with the corresponding carbon signal being present at 323.5 ppm in the
13C{
1H} NMR spectrum. The
19F{
1H} NMR shows an absence of signals which indicates the
loss of the S(C6F5) moiety.
Scheme 4.2.2 Synthesis of 4-3 and 4-4.
Single crystals of 4-3 suitable for X-ray analysis were grown and the study confirmed its
formulation as (Im(OMe)2)(SIMes)Ru(=CHC4H9)I2 where the geometry around the ruthenium
center is distorted square pyramid (Figure 4.2.3) and the two carbenes are trans disposed with a
C-Ru-C angle of 160.7(3)o. The two iodide ligands in 4-3 are mutually trans while the alkylidene
fragment occupies the pseudo-apical position. The Ru-C distances for the carbenes are
2.089(6) Å and 2.100(7) Å for SIMes and Im(OMe)2, respectively, while the Ru-C distance for
the alkylidene is 1.826(7) Å. The corresponding Ru-I distances are 2.7156(7) Å and 2.7224(7) Å.
113
Figure 4.2.3 POV-ray depiction of the molecular structure of 4-3. Ru: dark green, O: red, I:
magenta, N: aquamarine, C: black. H-atoms omitted for clarity.
Similarly, adding two equivalents of trimethylsilyl iodide to a solution of 2-14 in C6H6 results in
the formation of 4-4 in 90% yield. The 1H NMR spectrum of 4-4 reveals a triplet at 18.81 ppm,
with 3JHH of 4 Hz, which is assigned to the Ru=CH fragment and the corresponding carbon
signal was derived from two dimensional NMR experiments (HSQC) and is observed at
324.0 ppm. The 19
F{1H} NMR displays an absence of signals which indicates the loss of the
S(C6F5) moiety and these NMR data lead to the formulation of 4-4 as
(Im(OMe)2)(SIMes)Ru(=CHC5H11)I2.
Upon observing the facile exchange of the S(C6F5) ligand for an iodide, we targeted the synthesis
of the bis-chloride analogue of 4-4. As such, the addition of trimethylsilyl chloride to a solution
of 2-14 in CH2Cl2 and stirring for two days at room temperature followed by workup afforded
the isolation of compound 4-5 as a red solid in 85% yield (Scheme 4.2.3). The 1H NMR
spectrum of 4-5 reveals a broad singlet at 19.11 ppm which integrates to one proton and is
assigned to the Ru=CH fragment and the corresponding carbon signal is observed at 321.7 ppm
in the 13
C{1H} NMR spectrum. The
19F{
1H} NMR shows an absence of signals which again
indicates the loss of the S(C6F5) moiety.
114
Scheme 4.2.3 Synthesis of 4-5.
Single crystal X-ray analysis of 4-5 confirmed its formulation as
(Im(OMe)2)(SIMes)Ru(=CHC5H11)Cl2. The geometry around the metal center is distorted square
pyramid (Figure 4.2.4) and the two carbenes are cis disposed with a C-Ru-C angle of 95.7(3)o.
The two chloride ligands groups in 4-5 are also mutually cis while the alkylidene fragment
occupies the pseudo-apical position.
Figure 4.2.4 POV-ray depiction of the molecular structure of 4-5. Ru: dark green, O: red, Cl:
green, N: aquamarine, C: black. H-atoms omitted for clarity.
The Ru-C distances for the carbenes are 2.051(7) Å and 2.064(7) Å for SIMes and Im(OMe)2,
respectively, while the Ru-C distance for the alkylidene is 1.809(7) Å. The corresponding Ru-Cl
distances are 2.3681(17) and 2.4983(18) Å. This is a rare example of bis-carbene bis-halide Ru-
alkylidene complexes where the halides are in a cis disposition.19
We were interested in comparing the metathesis activity of 4-5 to a system where the chlorides
would be trans. Such a system is accessible through the reaction of (Im(OMe)2)AgCl with
115
Grubbs II where a green solid was isolated in 78% yield (Scheme 4.2.4). The 1H NMR spectrum
of 4-6 reveals a broad singlet at 19.10 ppm which integrates to one proton and is assigned to the
Ru=CH fragment with the corresponding carbon signal being observed at 299.0 ppm in the
13C{
1H} NMR spectrum.
Scheme 4.2.4 Synthesis of 4-6.
Single crystals suitable for an X-ray diffraction study were grown and the formulation of 4-6 was
confirmed as (Im(OMe)2)(SIMes)Ru(=CHPh)Cl2. The geometry around the metal center is
square pyramid (Figure 4.2.5) and the two carbenes are trans disposed with a C-Ru-C angle of
164.51(10)o. The two chloride ligands groups in 4-6 are also mutually trans while the alkylidene
fragment occupies the pseudo-apical position. The Ru-C distances for the carbenes are 2.089(2)
and 2.114(2) Å for SIMes and Im(OMe)2, respectively, while the Ru-C distance for the
alkylidene is 1.830(3) Å. The corresponding Ru-Cl distances are 2.4000(8) and 2.4118(7) Å.
116
Figure 4.2.5 POV-ray depiction of the molecular structure of 4-6. Ru: dark green, O: red, Cl:
green, N: aquamarine, C: black. H-atoms omitted for clarity.
4.2.2 Standard Olefin Metathesis Tests
Similar to previous examples (Chapter 3) these alkylidene complexes were also subjected to the
standard olefin metathesis standard tests to gauge their catalytic activity for this process.
Compound 4-1 was effective as a catalyst for ROMP of 1,5-cyclooctadiene at room temperature,
at 1 mol% catalyst loading, where conversion to the product was achieved in 100% after 24
hours. The addition of one equivalent of BCl3 to 4-1 resulted in increased activity for the ROMP
of this substrate, affording 100% product yield after 20 minutes at room temperature (Figure
4.2.6). Using 4-2, ROMP of 1,5-COD was accomplished with product yields of 68% after 24
hours at room temperature and 100% after 2 hours upon the addition of BCl3.
117
Figure 4.2.6 ROMP of 1,5-cyclooctadiene with 4-1.
The bis-halide systems showed increased activity in the absence of BCl3 where 4-3, 4-4 and 4-6
showed complete conversion to the product, at room temperature, after 30, 45, and 20 minutes,
respectively. Reducing the catalyst loading of 4-4 to 0.5 mol% still effects complete conversion
after 45 minutes.
Compound 4-1 was ineffective for the RCM of diethyl diallylmalonate in the absence of a Lewis
acid at room temperature, with 5 mol% catalyst loading, but complete conversion to the
ring-closed product was achieved, after 6 hours, upon the addition of one equivalent of BCl3
(Figure 4.2.7). Similarly, using 4-2 as the catalyst resulted in 100% conversion to the ring closed
product of diethyl diallylmalonate at room temperature, with one equivalent of BCl3, after 2
hours. Similar to 4-1, compound 4-2 was ineffective in the absence of BCl3 at room temperature.
118
Figure 4.2.7 RCM of diethyl diallylmalonate with 4-1.
The bis-halide systems showed increased activity in the absence of BCl3 where 4-3, at room
temperature, achieved RCM of diethyl diallylmalonate in 97% conversion after 2 hours.
Similarly, compound 4-4 resulted in 92% conversion of the same substrate after 2 hours while
4-6 showed complete conversion to the product, at room temperature, after 2.25 hours (Figure
4.2.8).
Figure 4.2.8 RCM of diethyl diallylmalonate with 4-3, 4-4 and 4-6.
119
For the CM of 5-hexenyl acetate and methyl acrylate, 4-1 was ineffective at room temperature in
the absence of a Lewis acid. Upon the addition of BCl3, at room temperature, conversion to the
heterocoupled product in a 72% yield was achieved after 6 hours (Figure 4.2.9). Using 4-2 with
one equivalent of BCl3 as the catalyst resulted in 70% conversion to the heterocoupled product at
25 °C after 2 hours. Just like 4-1, 4-2 was ineffective for CM in the absence of BCl3 at room
temperature.
Figure 4.2.9 CM of 5-hexenyl acetate and methyl acrylate with 4-1.
Similar to ROMP and RCM, the bis-halide systems showed increased activity in the absence of
BCl3 for CM. Compound 4-3, at 25 °C, showed CM of 5-hexenyl acetate and methyl acrylate to
the heterocoupled product in 54% conversion after 4 hours. Similarly, compound 4-4 resulted in
56% conversion after 4 hours while 4-6 showed conversion of 66% to the product, at room
temperature, after 2.25 hours (Figure 4.2.10).
120
Figure 4.2.10 CM of 5- hexenyl acetate and methyl acrylate with 4-3, 4-4 and 4-6.
It is worth noting that although conversions to the heterocoupled were not very high, the
homocoupled product was also observed.
4.2.3 Cross Metathesis of NBR with 1-Hexene
The systems described above were developed with the ultimate goal of effecting the cross
metathesis of NBR and 1-hexene at room temperature to make them industrially viable. The
NBR used in the following tests has an initial Mw of 274 000 Da and a Mn of 76 000 Da with a Ð
of 3.6. Only one example using 4-1 as a catalyst for CM of NBR and 1-hexene is provided in this
chapter. Several parameters were probed including changing the catalyst loading and using either
one or two equivalents of the Lewis acid, BCl3.
Using 0.07 phr catalyst loading of 4-1, at room temperature with one equivalent of BCl3, the
resulting Mw, Mn and Ð, after 4 hours, are 106 000 Da, 41 000 Da, and 2.6 respectively. After 24
hours the polymer was further metathesized to give Mw, Mn and Ð values of 51 000 Da, 23 000,
and 2.2, respectively. In the case of two equivalents of BCl3, after 24 hours, the Mw, Mn and Ð
are 1156 000 Da, 45 000 Da, and 2.6 respectively (Figure 4.2.11).
121
Figure 4.2.11 CM of NBR and 1-hexene with 4-1 at 25 °C.
Using 0.14 phr catalyst loading with 4-1, at 25 °C with one equivalent of BCl3, the resulting Mw,
Mn and Ð, after 4 hours, are 51 000 Da, 23 000 Da, and 2.2 respectively. After 24 hours the
polymer was further metathesized to give Mw, Mn and Ð values of 20 000 Da, 11 000, and 1.8
respectively. In the case of two equivalents of BCl3, after 24 hours, the Mw, Mn and Ð are 35 000
Da, 17 000 Da, and 2.0 respectively.
4.3 Conclusion
In conclusion, it was demonstrated that exchanging a chloride for an iodide to form 4-1 and 4-2
resulted in enhanced metathesis activity for the standard tests as well as for the CM of NBR with
1-hexene. Catalytic olefin metathesis was observed for both systems at room temperature and
elevated temperatures were not necessary. The bis-halide containing complexes (4-3, 4-4 and
4-6) showed the highest activity for all standard metathesis tests without the need for a Lewis
acid to initiate. This is presumably due to the presence of the bulky iodides on the metal center
which results in faster initiation. These systems, however, too closely resemble Grubbs’ catalysts
and were therefore not patented and not tested further for the CM of NBR with 1-hexene.
122
4.4 Experimental Section
4.4.1 General Considerations
All manipulations were carried out under an atmosphere of dry, O2-free N2 employing a Vacuum
Atmospheres glove box and a Schlenk vacuum line. Solvents were purified with a Grubbs-type
column system manufactured by Innovative Technology, dispensed into thick-walled Schlenk
glass flasks equipped with Teflonvalve stopcocks (pentane, hexanes, CH2Cl2) and stored over
molecular sieves. Some solvents were dried over the appropriate agents, vacuum-transferred into
storage flasks with Teflon stopcocks and degassed accordingly (C6H6, C6D6, CD2Cl2). 1H,
13C,
and 19
F spectra were recorded at 25 oC on a Bruker 400 MHz spectrometer. Chemical shifts were
given relative to SiMe4 and referenced to the residual solvent signal (1H,
13C) or relative to an
external standard (19
F: CFCl3). In some instances, signal assignment was derived from two
dimensional NMR experiments (HSQC). Chemical shifts are reported in ppm and coupling
constants as scalar values in Hz. Combustion analyses were performed in house employing a
Perkin-Elmer CHN analyzer. Trimethylsilyl iodide, trimethylsilyl chloride and Grubbs II were
purchased from Sigma Aldrich and used as received. 1,5-Cyclooctadiene, diethyl
diallylmalonate, 5-hexenyl acetate, and methyl acrylate were purchased from Sigma Aldrich or
Alfa Aesar and used as received.
4.4.2 Synthetic Procedures
Synthesis of 4-1: Trimethylsilyl iodide (10.0 µL, 0.071 mmol) was added to a solution of 2-4
(0.050 g, 0.065 mmol) in 2 mL C6H6 at room temperature. The solution was then stirred for one
hour before the solvent was removed and the residue washed with pentane. The pentane was then
decanted to yield a red solid (0.048 g, 87%). X-ray quality crystals were grown from
benzene/pentane at 25 oC.
1H NMR (400 MHz, C6D6): δ 18.82 (br s, 1H, Ru=CH), 7.09 (m, 2H,
S(C6H5)), 6.90 (d, 3JHH = 2 Hz, 1H, Im(OMe)2-CH), 6.84 (s, 1H, Mes-CH), 6.82 (s, 1H, Mes-
CH), 6.78 (m, 6H, Mes-CH + S(C6H5) + Im(OMe)2-CH), 3.44 (m, 4H, SIMes-CH2 + Im(OMe)2-
CH2), 3.30 (m, 4H, SIMes-CH2 + Im(OMe)2-CH2), 3.19-2.99 (m, 4H, Im(OMe)2-CH2), 2.95 (s,
3H, Im(OMe)2-CH3), 2.89 (s, 3H, Mes-CH3), 2.85 (s, 3H, Mes-CH3), 2.81 (s, 3H, Im(OMe)2-
CH3), 2.75 (s, 3H, Mes-CH3), 2.71 (s, 3H, Mes-CH3), 2.14 (s, 3H, Mes-CH3), 2.13 (s, 3H, Mes-
CH3), 2.00 (d, 3JHH = 6 Hz, 3H, Ru=CHCH3).
13C{
1H} NMR (101 MHz, C6D6): δ 314.2
(Ru=CHCH3), 223.7 (NCN), 188.5 (NCN), 139.8 (Cipso), 139.6 (Cipso), 138.4 (Cipso), 138.3
123
(Cipso), 138.3 (Cipso), 137.9 (S(C6H5)), 133.5 (S(C6H5)), 130.2 (Mes-CH), 129.9 (Mes-CH), 129.7
(Mes-CH), 127.2 (S(C6H5)), 122.1 (Im(OMe)2-CH), 121.0 (Im(OMe)2-CH), 72.5 (Im(OMe)2-
CH2), 71.7 (Im(OMe)2-CH2), 58.3 (Im(OMe)2-CH3), 58.2 (Im(OMe)2-CH3), 51.6 (SIMes-CH2),
51.5 (SIMes-CH2), 49.3 (Ru=CHCH3), 49.0 (Im(OMe)2-CH2), 23.2 (Mes-CH3), 21.3 (Mes-CH3),
21.1 (Mes-CH3), 20.9 (Mes-CH3), 20.7 (Mes-CH3), 19.6 (Mes-CH3). Elemental Analysis for
C38H51IN4O2RuS: C, 53.33; H, 6.01; N, 6.55. Found: C, 52.59; H, 5.66; N, 6.51.
Synthesis of 4-2: Trimethylsilyl iodide (14.0 µL, 0.104 mmol) was added to a solution of 2-8
(0.065 g, 0.095 mmol) in 2 mL C6H6 at room temperature. The solution was then stirred for one
hour before the solvent was removed and the residue washed with pentane. The pentane was then
decanted to yield a red solid (0.038 g, 53%). X-ray quality crystals were grown from
benzene/pentane at 25 oC.
1H NMR (400 MHz, C6D6): δ 19.04 (br s, 1H, Ru=CH), 7.13(br s, 2H,
S(C6H5)), 6.94 (s, 1H, Mes-CH), 6.91 (s, 1H, Mes-CH), 6.84 (s, 1H, Mes-CH), 6.77 (s, 1H, Mes-
CH), 6.72 (m, 1H, S(C6H5)), 6.64 (br m, 2H, S(C6H5)), 3.70-3.17 (br m, 12H, SIMes-CH2 +
Me2Im(OMe)2-CH2), 2.98 (s, 3H, Me2Im(OMe)2-CH3), 2.95 (br s, 6H, Mes-CH3), 2.88 (s, 3H,
Me2Im(OMe)2-CH3), 2.74 (s, 6H, Mes-CH3), 2.23 (s, 3H, Mes-CH3), 2.13 (s, 3H, Mes-CH3),
2.09 (d, 3JHH = 6 Hz, 3H, Ru=CHCH3),1.69 (s, 3H, Me2Im(OMe)2-4,5-CH3), 1.47 (s, 3H,
Me2Im(OMe)2-4,5-CH3). 13
C{1H} NMR partial (101 MHz, C6D6): δ 185.2 (NCN), 139.6 (Cipso),
139.3 (Cipso), 138.8 (Cipso), 138.6 (Cipso), 138.3 (Cipso), 136.7 (S(C6H5)), 134.3 (br s, S(C6H5)),
130.6 (Mes-CH), 130.1 (Mes-CH), 129.9 (Mes-CH), 129.8 (Mes-CH), 126.6 (S(C6H5)), 126.4
(Me2Im(OMe)2-Cipso), 126.1 (Me2Im(OMe)2-Cipso), 122.6 (S(C6H5)), 73.5 (Me2Im(OMe)2-CH2),
71.7 (Me2Im(OMe)2-CH2), 58.6 (Me2Im(OMe)2-CH3), 58.3 (Me2Im(OMe)2-CH3), 51.8 (SIMes-
CH2), 47.7 (Me2Im(OMe)2-CH2), 46.1 (Ru=CHCH3), 23.0 (Mes-CH3), 21.4 (Mes-CH3), 21.1
(Mes-CH3), 20.6 (Mes-CH3), 19.6 (Mes-CH3), 9.55 (Me2Im(OMe)2-4,5-CH3), 9.05
(Me2Im(OMe)2-4,5-CH3).
Synthesis of 4-3: Trimethylsilyl iodide (26.0 µL, 0.184 mmol) was added to a solution of 2-13
(0.075 g, 0.084 mmol) in 2 mL C6H6 at room temperature. The solution was then stirred
overnight before the solvent was removed and the residue washed with pentane. The pentane was
then decanted to yield a red solid (0.068 g, 88%). X-ray quality crystals were grown from
benzene/pentane at 25 oC.
1H NMR (400 MHz, C6D6): δ 18.81 (t,
3JHH = 4 Hz, 1H, Ru=CH),
7.14 (d, 3JHH = 2 Hz, 1H, Im(OMe)2-CH), 6.91 (d,
3JHH = 2 Hz, 1H, Im(OMe)2-CH), 6.78 (s, 2H,
Mes-CH), 6.76 (s, 2H, Mes-CH), 3.50 (m, 4H, Im(OMe)2-CH2), 3.40 (br s, 2H, Im(OMe)2-CH2),
124
3.31 (br s, 6H, (2H) Im(OMe)2-CH2, (4H) SIMes-CH2), 2.99 (s, 3H, Im(OMe)2-CH3), 2.83 (br s,
6H, 2 x Mes-CH3), 2.82 (s, 3H, Im(OMe)2-CH3), 2.71 (br s, 6H, 2 x Mes-CH3), 2.16 (s, 3H, Mes-
CH3), 2.10 (s, 3H, Mes-CH3), 1.26-1.12 (br m, 6H, pentylidene-CH2), 0.91 (app t, 3JHH = 7 Hz,
3H, pentylidene-CH3). 13
C{1H} NMR (101 MHz, C6D6): δ 323.5 (Ru=CH), 226.4 (NCN), 187.3
(NCN), 139.4 (Cipso), 138.5 (Cipso), 138.04 (Cipso), 138.02 (Cipso), 137.9 (Cipso), 135.9 (Cipso),
130.1 (Mes-CH), 129.9 (Mes-CH), 122.7 (Im(OMe)2-CH), 121.7 (Im(OMe)2-CH), 72.7
(Im(OMe)2-CH2), 72.1 (Im(OMe)2-CH2), 58.3 (Im(OMe)2-CH3), 58.2 (Im(OMe)2-CH3), 51.9
(SIMes-CH2), 51.5 (SIMes-CH2), 49.9 (Im(OMe)2-CH2), 48.5 (Im(OMe)2-CH2), 33.6
(pentylidene-CH2), 23.1 (pentylidene-CH2), 22.9 (Mes-CH3), 21.0 (Mes-CH3), 20.9 (Mes-CH3),
20.6 (Mes-CH3), 14.5 (pentylidene-CH3). Elemental Analysis for C35H52I2N4O2Ru: C, 45.91; H,
5.72; N, 6.12. Found: C, 45.65; H, 5.64; N, 6.03. Repeated attempts to obtain EA were
unsuccessful and as such the NMR spectra of 4-3 are attached at the end of this section.
Synthesis of 4-4: Trimethylsilyl iodide (25.6 µL, 0.181 mmol) was added to a solution of 2-14
(0.075 g, 0.082 mmol) in 2 mL C6H6 at room temperature. The solution was then stirred
overnight before the solvent was removed and the residue washed with pentane. The pentane was
then decanted to yield a red solid (0.069 g, 90%). 1H NMR (400 MHz, C6D6): δ 18.81 (t,
3JHH = 4
Hz, 1H, Ru=CH), 7.14 (d, 3JHH = 2 Hz, 1H, Im(OMe)2-CH), 6.91 (d,
3JHH = 2 Hz, 1H,
Im(OMe)2-CH), 6.78 (s, 2H, Mes-CH), 6.75 (s, 2H, Mes-CH), 3.52 (m, 4H, Im(OMe)2-CH2),
3.40 (br s, 2H, Im(OMe)2-CH2), 3.31 (br s, 6H, (2H) Im(OMe)2-CH2, (4H) SIMes-CH2), 2.98 (s,
3H, Im(OMe)2-CH3), 2.83 (br s, 9 H, (6H) Mes-CH3, (3H) Im(OMe)2-CH3), 2.71 (br s, 6H, 2 x
Mes-CH3), 2.17 (s, 3H, Mes-CH3), 2.10 (s, 3H, Mes-CH3), 1.34-1.10 (br m, 8H, hexylidene-
CH2), 0.92 (app t, 3JHH = 7 Hz, 3H, hexylidene-CH3).
13C{
1H} NMR (101 MHz, C6D6): δ 324.0
(Ru=CH), 226.4 (NCN), 187.4 (NCN), 139.4 (Cipso), 138.5 (Cipso), 138.1 (Cipso), 137.9 (Cipso),
135.9 (Cipso), 130.1 (Mes-CH), 129.9 (Mes-CH), 122.7 (Im(OMe)2-CH), 121.7 (Im(OMe)2-CH),
72.7 (Im(OMe)2-CH2), 72.1 (Im(OMe)2-CH2), 58.3 (Im(OMe)2-CH3), 58.2 (Im(OMe)2-CH3),
52.0 (SIMes-CH2), 51.5 (SIMes-CH2), 49.9 (Im(OMe)2-CH2), 48.5 (Im(OMe)2-CH2), 32.2
(hexylidene-CH2), 31.3 (hexylidene-CH2), 23.4 (hexylidene-CH2), 23.1 (Mes-CH3), 21.0 (Mes-
CH3), 20.9 (Mes-CH3), 20.6 (Mes-CH3), 14.5 (hexylidene-CH3). Elemental Analysis for
C36H54I2N4O2Ru: C, 46.51; H, 5.85; N, 6.03. Found: C, 46.56; H, 5.86; N, 6.01.
Synthesis of 4-5: Trimethylsilyl chloride (139.0 µL, 1.099 mmol) was added to a solution of
2-14 (0.100 g, 0.109 mmol) in 2 mL CH2Cl2 at room temperature. The solution was then stirred
125
for 48 hours before the solvent was removed and the residue was layered with 10 mL of pentane
and was left at room temperature for 16 hours. The pentane was then decanted to yield a
red/brown solid which was dried under high vacuum (0.070 g, 85%). X-ray quality crystals were
grown from benzene/pentane at 25 oC.
1H NMR (400 MHz, C6D6): δ 19.11 (br s, 1H, Ru=CH),
6.97 (s, 1H, Mes-CH), 6.94 (s, 1H, Im(OMe)2-CH), 6.88 (s, 1H, Mes-CH), 6.83 (s, 1H, Mes-
CH), 6.77 (s, 1H, Mes-CH), 6.35 (d, 3JHH = 2 Hz, 1H, Im(OMe)2-CH), 3.77 (m, 2H, Im(OMe)2-
CH2), 3.45-3.10 (m, 10H, (4H) SIMes-CH2 + (6H) Im(OMe)2-CH2), 3.01 (s, 3H, Mes-CH3), 2.98
(s, 3H, Im(OMe)2-CH3), 2.73 (s, 3H, Mes-CH3), 2.71 (s, 3H, Im(OMe)2-CH3), 2.69 (s, 3H, Mes-
CH3), 2.43 (s, 3H, Mes-CH3), 2.19 (s, 3H, Mes-CH3), 2.18 (s, 3H, Mes-CH3), 1.37-1.08 (br m,
8H, hexylidene CH2), 0.92 (t, 3JHH = 7 Hz, 3H, hexylidene-CH3).
13C{
1H} NMR (101 MHz,
C6D6): δ 321.7 (Ru=CH), 222.2 (NCN), 188.2 (NCN), 139.9 (Cipso), 139.7 (Cipso), 138.6 (Cipso),
137.7 (Cipso), 137.6 (Cipso), 137.5 (Cipso), 135.4 (Cipso), 129.9 (Mes-CH), 129.8 (Mes-CH), 129.6
(Mes-CH), 121.5 (Im(OMe)2-CH), 121.1 (Im(OMe)2-CH), 73.6 (Im(OMe)2-CH2), 71.6
(Im(OMe)2-CH2), 58.2 (Im(OMe)2-CH3), 58.0 (Im(OMe)2-CH3), 51.6 (SIMes-CH2), 51.1
(SIMes-CH2), 49.4 (Im(OMe)2-CH2), 48.4 (Im(OMe)2-CH2), 32.3 (hexylidene-CH2), 30.3
(hexylidene-CH2), 23.2 (hexylidene-CH2), 22.6 (Mes-CH3), 21.0 (Mes-CH3), 20.8 (Mes-CH3),
20.4 (Mes-CH3), 19.1 (Mes-CH3), 18.9 (Mes-CH3), 14.5 (hexylidene-CH3). Repeated attempts to
obtain EA were unsuccessful and as such the NMR spectra of 4-5 are attached at the end of this
section.
Synthesis of 4-6: A solution of (PCy3)(SIMes)Ru(=CHPh)Cl2 (0.274 g, 0.323 mmol) in 5 mL
toluene was added to a suspension of AgCl(Im(OMe)2) (0.117 g, 0.357 mmol) in 5 mL toluene.
The mixture was stirred at 25 °C for 72 hours to give a green solution. The AgCl precipitate was
filtered through celite and the solution was concentrated to 2 mL and 15 mL of pentane was
added to precipitate a green solid. The solution was decanted yielding a green solid
(0.190 g, 78%). 1H NMR (400 MHz, CD2Cl2): δ 19.10 (s, 1H, Ru=CH), 7.68 (d,
3JHH = 8 Hz, 2H,
Ru=CH(C6H5)), 7.45 (t, 3
JHH = 7 Hz, 1H, Ru=CH(C6H5)), 7.11-7.02 (m, 7H, Ru=CH(C6H5) +
Mes-CH + Im(OMe)2-CH), 6.89 (d, 3JHH = 2 Hz, 1H, Im(OMe)2-CH), 4.05 (br m, 2H, SIMes-
CH2), 3.93 (br m, 2H, SIMes-CH2), 3.69 (br m, 2H, Im(OMe)2-CH2), 3.49 (br m, 2H, Im(OMe)2-
CH2), 3.38 (s, 3H, Im(OMe)2-CH3), 3.09 (br s, 2H, Im(OMe)2-CH2), 2.96 (s, 3H, Im(OMe)2-
CH3), 2.70 (br s, 11H, Mes-CH3 + Im(OMe)2-CH2), 2.38 (s, 3H, Mes-CH3), 2.23 (s, 3H, Mes-
CH3), 1.97 (br s, 3H, Mes-CH3). 13
C NMR (400 MHz, CD2Cl2): δ 299.0 (Ru=CH), 223.3 (NCN),
126
186.4 (NCN), 140.2 (Cipso), 139.0 (Cipso), 138.0 (Cipso), 137.5 (Cipso), 135.4 (Cipso), 122.2
(Im(OMe)2-CH), 121.6 (Im(OMe)2-CH), 73.8 (Im(OMe)2-CH2), 72.2 (Im(OMe)2-CH2), 58.8
(Im(OMe)2-CH3), 58.4 (Im(OMe)2-CH3), 52.1 (SIMes-CH2), 51.7 (SIMes-CH2), 49.5
(Im(OMe)2-CH2), 49.1 (Im(OMe)2-CH2), 21.2 (Mes-CH3), 21.1 (Mes-CH3), 20.3 (Mes-CH3),
18.5 (br s, Mes-CH3). Elemental Analysis for C37H48Cl2RuN4O2: C, 59.03; H, 6.43; N, 7.44.
Found: C, 59.58; H,6.73; N, 7.90.
Figure 4.4.1 1H NMR spectrum of 4-3 in C6D6.
127
Figure 4.4.2 13
C{1H} NMR spectrum of 4-3 in C6D6.
Figure 4.4.3 1H NMR spectrum of 4-5 in C6D6.
128
Figure 4.4.4 13
C{1H} NMR spectrum of 4-5 in C6D6.
4.4.3 Standard Metathesis Reaction Tests
All standard metathesis reaction tests were performed employing a modified procedure of
Grubbs and co-workers.20
The standard procedure for the ring opening metathesis polymerization of 1,5-cyclooctadiene is
as follows: The required amount of the catalyst (1 mol%), was weighed out and dissolved in 0.5
mL CD2Cl2. For the tests that involved the use of an additive (i.e. BCl3, 1M in hexane) the
required volume was added and the mixture was allowed to stand for 5 min. The solutions were
placed in an NMR tube, 1,5-cyclooctadiene (60 μL, 0.50 mmol) was added, the NMR tube was
capped and the solution was mixed at the desired temperature. Reaction progress was monitored
by 1H NMR every 2 hours (unless otherwise noted). Reaction progress was determined by
integration of the peaks corresponding to the starting material versus the product.
129
Table 4.4.1 ROMP of 1,5-cyclooctadiene with 4-1.
Compound 4-1
Additive Temperature (oC) Time (h) Conversion (%)
None 25 2 6
4 8
6 39
8 55
24 100
1 mol% BCl3 25 0.33 100
Table 4.4.2 ROMP of 1,5-cyclooctadiene with 4-2.
Compound 4-2
Additive Temperature (oC) Time (h) Conversion (%)
None 25 2 0
4 3
6 9
8 15
24 68
1 mol% BCl3 25 0.25 85
2 100
Table 4.4.3 ROMP of 1,5-cyclooctadiene with 4-3, 4-4, and 4-6.
Catalyst Time (h) Conversion (%)
4-3 0.5 96
2 100
4-4 0.75 100
4-4 (0.5 mol%) 0.75 100
4-6 0.3 100
130
A standard procedure for the ring closing metathesis of diethyl diallylmalonate is as follows. The
required amount of catalyst (5 mol%) was weighed out and dissolved in 0.5 mL CD2Cl2. For the
tests that involved the use of an additive (i.e. BCl3, 1M in hexane) the required volume was
added and the mixture was allowed to stand for 5 min. The solution was placed in an NMR tube,
diethyl diallylmalonate (20 μL, 0.50 mmol) was added, the NMR tube was capped and the
solution was mixed at the desired temperature. Reaction progress was monitored by 1H NMR
every 2 hours (unless otherwise noted). Reaction progress was determined by integration of the
olefinic peaks of the starting material versus the product.
Table 4.4.4 RCM of diethyl diallylmalonate with 4-1.
Compound 4-1
Additive Temperature (oC) Time (h) Conversion (%)
None 25 24 0
5 mol% BCl3 25 2 81
4 95
6 100
Table 4.4.5 RCM of diethyl diallylmalonate with 4-2.
Compound 4-2
Additive Temperature (oC) Time (h) Conversion (%)
None 25 24 0
5 mol% BCl3 25 2 100
Table 4.4.6 RCM of diethyl diallylmalonate with 4-3.
Compound 4-3
Additive Temperature (oC) Time (h) Conversion (%)
None 25 0.5 87
2 97
4 100
131
Table 4.4.7 RCM of diethyl diallylmalonate with 4-4.
Compound 4-4
Additive Temperature (oC) Time (h) Conversion (%)
None 25 0.5 80
2 92
4 100
Table 4.4.8 RCM of diethyl diallylmalonate with 4-6.
Compound 4-6
Additive Temperature (oC) Time (h) Conversion (%)
None 25 0.3 39
0.6 54
2.25 100
A standard procedure for cross metathesis of 5-hexenyl acetate and methyl acrylate is as follows.
The required amount of catalyst (2 mol%) was weighed out and dissolved in 0.5 mL CD2Cl2. For
the tests that involved the use of an additive (i.e. BCl3, 1M in hexane) the required volume was
added and the mixture was allowed to stand for 5 min. The solution was placed in an NMR tube
and a mixture of 5-hexenyl acetate (20 μL, 0.12 mmol) and methyl acrylate (10 μL, 0.11 mmol)
was added and the solution was mixed at the desired temperature. Reaction progress was
monitored by 1H NMR every 2 hours (unless otherwise noted). Reaction progress was
determined by integration of the olefinic peaks of the starting material versus the product.
Table 4.4.9 CM of 5- hexenyl acetate and methyl acrylate with 4-1.
Compound 4-1
Additive Temperature (oC) Time (h) Conversion (%)
None 25 24 0
2 mol% BCl3 25 2 47
4 63
6 72
132
Table 4.4.10 CM of 5- hexenyl acetate and methyl acrylate with 4-2.
Compound 4-2
Additive Temperature (oC) Time (h) Conversion (%)
None 25 24 0
2 mol% BCl3 25 2 70
Table 4.4.11 CM of 5- hexenyl acetate and methyl acrylate with 4-3.
Compound 4-3
Additive Temperature (oC) Time (h) Conversion (%)
None 25 0.5 37
2 47
4 54
Table 4.4.12 CM of 5- hexenyl acetate and methyl acrylate with 4-4.
Compound 4-4
Additive Temperature (oC) Time (h) Conversion (%)
None 25 0.5 37
2 46
4 52
6 56
Table 4.4.13 CM of 5- hexenyl acetate and methyl acrylate with 4-6.
Compound 4-6
Additive Temperature (oC) Time (h) Conversion (%)
None 25 0.4 56
2.25 66
133
4.4.4 Cross Metathesis of NBR and 1-hexene
A standard procedure for the cross metathesis of nitrile butadiene rubber (NBR) and 1-hexene is
as follows. NBR (1.5 g) were placed in 13.5 g of chlorobenzene and placed on a shaker for 48 h
to give a 10 wt% NBR solution. 1-Hexene (60 mg, 90 μL) was added to the solution and shaken
for 1 h. The catalysts were prepared by dissolving the required mass of precatalyst in CH2Cl2
(2 mL) in a glove box and the appropriate amount of BCl3 was then added and the solutions were
stirred for 5 min before being taken out of the glove box and added to the NBR solutions. The
solutions were then stirred at the desired temperature for a total of 24 h. Samples were taken at 1,
2, 3, 4, and 24 h. The catalysts were poisoned with ethyl vinyl ether (0.1 mL) to stop the
metathesis. All volatiles were removed from the samples. GPC samples were made by preparing
a 1 mg/mL THF solution of the resulting NBR. The samples were passed through a microporous
filter and the Mn, Mw, and Ð were determined by GPC using a polystyrene calibration curve. The
Mw and Mn for NBR used with 4-1 for are 274 000 and 76 000 Da, respectively, and the Ð is 3.6.
134
Table 4.4.14 GPC data for CM of NBR and 1-hexene using 4-1 at 25 °C.
Catalyst loading (phr) 0.07 0.07 0.14 0.14 0.28 0.28 0.28
BCl3 (equiv.) 1 2 1 2 1 2 10
1 hour
Mw 193 000 - 131 000 148 000 94 000 91 000 50 000
Mn 56 000 - 46 000 57 000 36 000 38 000 24 000
Ð 3.4 - 2.9 2.6 2.6 2.4 2.1
2 hours
Mw 144 000 196 000 89 000 121 000 75 000 59 000 40 000
Mn 53 000 64 000 37 000 50 000 30 000 27 000 19 000
Ð 2.7 3.1 2.4 2.4 2.5 2.3 2.1
3 hours
Mw 131 000 171 000 62 000 112 000 65 000 48 000 32 000
Mn 46 000 58 000 29 000 44 000 26 000 22 000 16 000
Ð 2.8 2.9 2.1 2.5 2.5 2.2 2.0
4 hours
Mw 106 000 160 000 51 000 93 000 45 000 37 000 35 000
Mn 41 000 58 000 23 000 39 000 21 000 18 000 17 000
Ð 2.6 2.8 2.2 2.4 2.1 2.0 2.0
24 hours
Mw 51 000 116 000 20 000 35 000 10 000 19 000 23 000
Mn 23 000 45 000 11 000 17 000 6 000 10 000 12 000
Ð 2.2 2.6 1.8 2.0 1.6 1.9 1.9
135
4.4.5 X-ray Crystallography
4.4.5.1 X-ray Data Collection and Reduction
Crystals were coated in Paratone-N oil in the glove-box, mounted on a MiTegen Micromount
and placed under an N2 stream, thus maintaining a dry, O2-free environment for each crystal. The
data for crystals were collected on a Bruker Apex II diffractometer employing Mo Kα radiation
(λ = 0.71073 Å). The data were collected at 150(±2) K for all crystals. The frames were
integrated with the Bruker SAINT software package using a narrow-frame algorithm. Data were
corrected for absorption effects using the empirical multi-scan method (SADABS).21
4.4.5.2 X-ray Data Solution and Refinement
Non-hydrogen atomic scattering factors were taken from the literature tabulations.22
The heavy
atom positions were determined using direct methods employing the SHELXTL direct methods
routine. The remaining non-hydrogen atoms were located from successive difference Fourier
map calculations. The refinements were carried out by using full-matrix least squares techniques
on F, minimizing the function (Fo–Fc)2 where the weight is defined as 4Fo2/2 (Fo
2) and Fo
and Fc are the observed and calculated structure factor amplitudes, respectively. In the final
cycles of each refinement, all non-hydrogen atoms were assigned anisotropic temperature factors
in the absence of disorder or insufficient data. In the latter cases atoms were treated isotropically.
C-H atom positions were calculated and allowed to ride on the carbon to which they are bonded
assuming a C-H bond length of 0.95 Å. H-atom temperature factors were fixed at 1.10 times the
isotropic temperature factor of the C-atom to which they are bonded. The H-atom contributions
were calculated, but not refined. The locations of the largest peaks in the final difference Fourier
map calculation as well as the magnitude of the residual electron densities in each case were of
no chemical significance.
136
Table 4.4.15 Select crystallographic parameters for 4-1 to 4-3.
4-1 4-2 4-3
Formula C38H51IN4O2RuS C40H55IN4O2RuS C35H52I2N4O2Ru
wt 855.86 883.91 915.68
Cryst. syst. Triclinic Triclinic Triclinic
Space group P-1 P-1 P-1
a(Å) 9.1565(4) 8.747(2) 9.3246(4)
b(Å) 11.7895(5) 11.097(3) 11.9304(5)
c(Å) 19.3652(8) 21.731(6) 17.6952(8)
(deg) 97.521(2) 95.845(12) 74.445(2)
(deg) 93.603(2) 95.823(11) 77.912(2)
(deg) 112.499(2) 105.936(10) 89.472(2)
V(Å3) 1900.14(14) 1999.3(9) 1852.07(14)
Z 2 2 2
d(calc) gcm-3
1.496 1.468 1.642
R(int) 0.0403 0.0503 0.0357
, mm–1
1.317 1.254 2.125
Total data 8624 6720 6287
>2(FO2) 6652 5308 5147
Variables 461 442 397
R (>2) 0.0380 0.0358 0.0461
Rw 0.0841 0.0810 0.1079
GOF 1.023 1.046 1.105
137
Table 4.4.16 Select crystallographic parameters for 4-5 and 4-6.
4-5 4-6
Formula C36H54Cl2N4O2Ru C37H48Cl2N4O2Ru
wt 746.80 752.76
Cryst. syst. Orthorhombic Monoclinic
Space group Pca2(1) P21/c
a(Å) 17.2005(13) 12.7136(17)
b(Å) 13.0632(10) 14.8473(18)
c(Å) 16.3886(12) 21.807(3)
(deg) 90.00 90.00
(deg) 90.00 118.462(8)
(deg) 90.00 90.00
V(Å3) 3682.4(5) 3618.8(8)
Z 4 4
d(calc) gcm-3
1.347 1.382
R(int) 0.1451 0.0876
, mm–1
0.607 0.618
Total data 7186 8277
>2(FO2) 4570 6082
Variables 406 415
R (>2) 0.0622 0.0377
Rw 0.1371 0.0828
GOF 1.010 1.004
138
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(17) Leitao, E. M.; Dubberley, S. R.; Piers, W. E.; Wu, Q.; McDonald, R. Chem. Eur. J. 2008,
14, 11565.
(18) Amoroso, D.; Yap, G. P. A.; Fogg, D. E. Organometallics 2002, 21, 3335.
(19) Lund, C. L.; Sgro, M. J.; Stephan, D. W. Organometallics 2012, 31, 580.
(20) Ritter, T.; Hejl, A.; Wenzel, A. G.; Funk, T. W.; Grubbs, R. H. Organometallics 2006,
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139
(21) Bruker AXS Inc. : 2013.
(22) D. T. Cromer, J. T. W. Int. Tables X-Ray Crystallography, 1974; Vol. 4.
140
Chapter 5 Carbene Stabilized Iminoboranes
5.1 Introduction
5.1.1 Iminoboranes and Iminoboryl Transition Metal Complexes
Iminoboranes, X-B≡N-R, are isoelectronic to alkynes; a relationship that has inspired significant
experimental1-6
and theoretical7,8
studies, particularly by the research groups of Paetzold1-4
and
Nöth5,6
. Such systems are commonly generated by the vacuum gas-phase pyrolysis of
(trimethylsilylamino)boron halides, a forcing procedure that has subsequently been adopted for
the synthesis of amino iminoboranes (Scheme 5.1.1).1
Scheme 5.1.1 Synthesis of iminoboranes via thermally induced elimination of Me3SiX.
The polarity and relative weakness of the B≡N moiety leads to increased reactivity of
iminoboranes compared to alkynes. This, however, also results in their thermodynamic
instability towards cyclooligomerization and therefore low temperature, high dilution, and the
presence of sterically demanding substituents are typically needed for the isolation of monomeric
species. The reactivity of such systems was heavily studied and they were shown to undergo
several transformations. Some of these reactions include 1,2-additions of polar reagents across
the B≡N multiple bond where, for example, hydrometallations with [Cp2Zr(H)Cl] afforded
examples of N-metallated aminoboranes.9,10
Amino iminoboranes were also shown to undergo
[2+2] cycloaddition reactions, across the B≡N linkage, with substrates (Scheme 5.1.2) such as
formaldehyde or tetracyanoethylene.5,11
141
Scheme 5.1.2 [2+2] Cycloaddition reactions of amino iminoboranes.
Nöth and co-workers also reported the [2+2] cycloaddition reactions between molecules of the
form CX2 (X = O, S, Se) and an amino iminoborane (Scheme 5.1.3).12
The corresponding
products decompose under thermal duress to form (CH3)3CN=C=X (X = O, S, Se) and the
dimers of the (tetramethylpiperidino)boron chalcogenides.
Scheme 5.1.3 Reaction of an amino iminoborane with CX2.
Over the last decade, transition metal iminoboryl complexes have garnered more attention as
metal centers can be used to stabilize the iminoboryl moiety. Braunschweig and co-workers have
reported the synthesis and characterization of (trimethylsilyl)iminoboryl complexes
trans-[(PCy3)2M(B≡N-SiMe3)(Br)] (M = Pd, Pt) obtained through the oxidative addition of the
B-Br bond in (Me3Si)2NBBr2 to a metal precursor followed by a facile intramolecular
elimination of Me3SiBr (Scheme 5.1.4).13
These reactions occur at room temperature and provide
a much gentler route to iminoboryl fragments. However, while the products of these reactions
were isolated and characterized, the intermediates in these transformations were not.
142
Scheme 5.1.4 Synthesis of iminoboryl transition metal complexes.
Notably, the transition metal iminoboryl systems were found to be reactive towards a variety of
substrates14
(Scheme 5.1.5). For example, when in the presence of a Lewis acid such as AlCl3, a
classical Lewis acid-base adduct forms leading to a shortening of the Pt-B bond and lengthening
of the B-N bond. Furthermore, these systems undergo 1,2-dipolar addition reactions with protic
reagents such as CH3OH, as well as the regiospecific addition of B-H bonds to the B≡N moiety.
Scheme 5.1.5 Reactions of an iminoboryl complex with various substrates.
These systems can also undergo substitution reactions through the bromide on the metal center.15
5.1.2 Carbenes in Stabilizing Low Valent Boron Species and Boron Centered Radicals
Since their discovery in the late 1980s, stable singlet carbenes have been utilized as ligands for
transition metal based catalysts16
, in addition to being organocatalysts17
on their own. They have
also been shown to coordinate and stabilize main group elements in low oxidation states, and
even activate small molecules.18
143
Carbenes have been traditionally viewed as strong σ-donors with negligible π-accepting
properties.16,19-21
This view has changed over the last few years as recent reports demonstrated
that modifications to the carbene influence both the donor ability and the electron accepting
properties.22-30
It was shown that the empty p-orbital on the carbene carbon can engage in π-back
donation and thus delocalize electron density. This observation has been used to stabilize
reactive species and has allowed for their detection spectroscopically and in some cases their
isolation.31,32
A few examples of main group systems stabilized by carbenes were discussed in
Chapter 1 and only select boron based systems will be discussed here.
Robinson and co-workers employed NHCs to stabilize and isolate a neutral diborene containing
a B=B double bond which was evidenced by the short distance in the molecular structure (Figure
5.1.1).33
Figure 5.1.1 Carbene stabilized neutral diborene.
More recently, Braunschweig and co-workers have reported the synthesis of the first example of
a compound with a triple bond between two boron atoms that is stable at room temperature,
using an IDipp as the stabilizing ligand (A in Figure 5.1.2).34
The importance of the NHC in the
stabilization and isolation of A indicated that the central diboron moiety is sensitive to the
electronic structure of the carbene. More recently, the same research group has isolated an
analogous complex utilizing a CAAC as the stabilizing ligand. Interestingly, the diboron species
that was isolated showed an elongated B-B bond distance that falls between a B≡B triple and
double bond and B-C bonds that fall in the range of B-C single and double bonds (B in Figure
5.1.2).35
The resulting compound is an example of an organic/inorganic analogue of butatriene
which is formed because CAACs are superior π acceptors compared to NHCs.
144
Figure 5.1.2 Carbene stabilized diboryne and diborabutatriene.
Carbenes have also been used to stabilize and allow for the isolation of boron centered radicals.
Curran and Lacôte have shown that tert-butoxy radicals abstract hydrogen atoms from
NHC-boranes to form NHC-boryl radicals (A-C in Figure 5.1.3).36,37
Gabbaï and co-workers
were also able to synthesize and spectroscopically characterize a carbene-BR2 radical
(D in Figure 5.1.3).38
Figure 5.1.3 Examples of carbene stabilized boron centered radicals.
More recently, Braunschweig and co-workers reported the synthesis of the first neutral borolyl
radical which is stabilized by an NHC (Scheme 5.1.6).39
The radical is formed through
single-electron-transfer between a borolyl anion, based on the borole framework, with
triorganotetrel halides.
Scheme 5.1.6 Synthesis of a neutral borolyl radical.
145
Since stable carbenes have been shown to activate small molecules as well as stabilize highly
reactive intermediates18
, two tasks previously exclusive to transition metals, we wanted to study
their ability to replace transition metals and stabilize iminoboranes.
5.2 Results and Discussion
5.2.1 Synthesis of Iminoborane species
To study the use of CAACs as a means of stabilizing iminoboranes, a series of boranes of the
general formula (Me3Si)2NBX2, where X = Cl, Br, I, were synthesized. The boranes
(Me3Si)2NBCl2 and (Me3Si)2NBBr2 were prepared according to literature procedures.13,40
Compound 5-1 was prepared according to a modified literature procedure and was isolated in
78% yield (Scheme 5.2.1). The NMR data indicate the formation of 5-1 where the 11
B{1H} NMR
spectrum shows a singlet at 7.9 ppm and a signal at 7.1 ppm is observed in the 29
Si{1H} NMR
spectrum corresponding to the Me3Si groups which have a signal at 0.28 ppm in the 1H NMR
spectrum. Complementary to NMR data, a single crystal X-ray diffraction study was performed
confirming the formulation of 5-1 as (Me3Si)2NBI2 (Figure 5.2.1).
Scheme 5.2.1 Synthesis of 5-1.
The geometry around boron is perfectly planar with the sum of angles at B being 360°. The B-I
bond lengths are each 2.1809(19) Å and the B-N distance is 1.385(4) Å which is typical of a BN
double bond indicating the donation of the N lone pair into the empty p orbital on boron.
146
Figure 5.2.1 POV-ray depiction of the molecular structure of 5-1. C: black, N: aquamarine, Si:
blue, B: yellow-green, I: magenta. H-atoms omitted for clarity.
Adding one equivalent of Cy-CAAC to a solution of 5-1 in benzene results in the formation of a
new product in less than 50% yield along with unreacted 5-1. When the same reaction was
repeated using two equivalents of Cy-CAAC and 5-1, compound 5-2 was obtained as an orange
solid in 95% yield (Scheme 5.2.2). The 11
B{1H} NMR spectrum of 5-2 shows a singlet at 6.5
ppm, similar to 5-1, and the 29
Si{1H} NMR spectrum shows a signal at -8.9 ppm. The
1H NMR
spectrum reveals a peak at 0.33 ppm corresponding to the trimethylsilyl group and it integrates to
9H which indicates a loss of Me3SiI. The second equivalent of Cy-CAAC traps the released
trimethylsilyl iodide and forms [Cy-CAAC-TMS][I] which was identified in the 1H NMR
spectrum.
147
Scheme 5.2.2 Synthesis of 5-2 to 5-4.
A single crystal X-ray study was performed which further confirmed the formulation of 5-2 as
[(Cy-CAAC)BN(SiMe3)][I] (Figure 5.2.2). The geometry around boron is slightly bent where the
C-B-N angle is 170.6(3)° and the iodide counter anion is outersphere. The B-C distance is
1.543(3) Å which lies between typical B-C single (1.59 Å) and double (1.44 Å) bonds.41,42
The
B-N distance is 1.229(3) Å which is indicative of a B-N triple bond and is similar to reported
XB≡NR, (Me3Si)3SiB≡NtBu, and (Me3Si)N≡BM(PCy3)2Br systems.1,2,4-6,13,43
The B-N-Si bond
angle is found to be 164.4(2)° which deviates from linearity.
148
Figure 5.2.2 POV-ray depiction of the molecular structure of the cation of 5-2. C: black, N:
aquamarine, Si: blue, B: yellow-green. H-atoms omitted for clarity.
Similarly, the addition of two equivalents of Cy-CAAC to a benzene solution of (Me3Si)2NBBr2
resulted in the isolation of compound 5-3 as a yellow solid in 95% yield. The 11
B{1H} NMR
spectrum of 5-3 shows a singlet at 12.6 ppm, which is more downfield compared to 5-2,
suggesting a different environment around the boron center and the 29
Si{1H} NMR shows a
signal at -10.5 ppm. A single crystal X-ray study was performed confirming the formulation of
5-3 as (Cy-CAAC)BBrN(SiMe3) (Figure 5.2.3). The geometry around boron is trigonal planar
(sum of angles at B is 360°). The B-C distance is 1.606(4) Å which is typical of a B-C single
bond and the B-Br and B-N distances are 2.078(3) and 1.304(3) Å, respectively. The shortened
B-N bond length is indicative of double bond character and the B-N-Si is 133.3(2) ° which
indicates that the lone pair on nitrogen is accessible and not involved in bonding with the boron
center.
149
Figure 5.2.3 POV-ray depiction of the molecular structure of 5-3. C: black, N: aquamarine, Si:
blue, B: yellow-green, Br: maroon. H-atoms omitted for clarity.
An iminoborane similar to 5-2 could conceivably be accessed through halide abstraction from
5-3. As such, the addition of NaBPh4 to a solution of 5-3 in C6H5Br was undertaken, and
following workup, resulted in the isolation of 5-4 in 75% yield (Scheme 5.2.2). The
11B{
1H} NMR of 5-4 shows a singlet at 7.7 ppm, which is upfield from 5-3 and similar to 5-2,
and a second singlet at -6.5 ppm corresponding to the BPh4 counter anion. The 29
Si{1H} NMR
spectrum shows a signal at 1.7 ppm, while the signal corresponding to the trimethylsilyl group
shifts upfield to 0.00 ppm in the 1H NMR spectrum. An X-ray analysis of single crystals of 5-4
was performed which showed an analogous geometry to that observed for 5-2 and confirmed the
formulation as [(Cy-CAAC)BN(SiMe3)][BPh4] (Figure 5.2.4). The geometry around boron is
slightly bent as the C-B-N angle is 173.4(4)°. The B-C distance of 1.545(5) Å is similar to that in
5-2 and the B-N distance is 1.218(4) Å, indicative of a B-N triple bond and similar to 5-2 as well
as reported systems containing a BN triple bond.1,2,4-6,13,40,43
The B-N-Si bond angle is 175.8(3)°
which deviates slightly from linearity.
150
Figure 5.2.4 POV-ray depiction of the molecular structure of 5-4. C: black, N: aquamarine, Si:
blue, B: yellow-green. H-atoms omitted for clarity.
While the addition of one equivalent of Cy-CAAC to either 5-1 or (Me3Si)2NBBr2 resulted in a
50:50 mixture of 5-2 or 5-3, respectively, and the starting borane, the addition of one equivalent
of Cy-CAAC in pentane to a solution of (Me3Si)2NBCl2 afforded 5-5 as a white solid in 65%
yield (Scheme 5.2.3). The 11
B{1H} NMR spectrum of 5-5 shows a singlet at 4.3 ppm, which is
indicative of a 4-coordinate boron center, and the 29
Si{1H} NMR shows a signal at -0.49 ppm.
The 1H and
13C{
1H} NMR spectra show signals belonging to the carbene and the
bis-trimethylsilyl amide moieties with a peak at 0.56 ppm in the 1H NMR integrating to 18H
corresponding to the Me3Si groups.
151
Scheme 5.2.3 Synthesis of 5-5 to 5-7.
X-ray analysis of single crystals of 5-5 confirmed its formulation as (Cy-CAAC)BCl2N(SiMe3)2
(Figure 5.2.5). The geometry around the boron center is tetrahedral but further discussion of
metric parameters is prevented due to severe disorder. Interestingly, over time in solution, 5-5
loses trimethylsilyl chloride and forms a new product, 5-6. The same result is observed when a
solid sample is left at room temperature for prolonged periods of time (over one month) or when
a solid sample is heated under vacuum.
152
Figure 5.2.5 POV-ray depiction of the molecular structure of 5-5. C: black, N: aquamarine, Si:
blue, B: yellow-green, Cl: green. H-atoms omitted for clarity.
Compound 5-6 was also obtained as a yellow solid in 70% yield when two equivalents of
Cy-CAAC were added to a benzene solution of (Me3Si)2NBCl2. The 11
B{1H} NMR spectrum of
5-6 shows a singlet at 17.4 ppm, which is indicative of a three coordinate boron center, and the
29Si{
1H} NMR shows a signal at -11.9 ppm. The
1H NMR spectrum reveals a peak at 0.45 ppm
corresponding to the trimethylsilyl group which integrates to 9H indicating the loss of Me3SiCl.
An X-ray analysis of single crystals of 5-6 showed an analogous geometry to that observed for
5-3 and confirmed its formulation as (Cy-CAAC)BClN(Me3Si) (Figure 5.2.6).
Figure 5.2.6 POV-ray depiction of the molecular structure of 5-6. C: black, N: aquamarine, Si:
blue, B: yellow-green, Cl: green. H-atoms omitted for clarity.
153
The geometry around boron is trigonal planar (sum of angles at B is 360°) and the B-C distance
is 1.612(3) Å which is similar to that observed for 5-3. The B-Cl and B-N distances are 1.881(2)
and 1.300(3) Å, respectively. The shortened B-N bond length is indicative of a double bond and
the B-N-Si angle is 139.21(16)°.
Similar to 5-3, the addition of KB(C6F5)4 to a solution of 5-6 in C6H5Br resulted in the isolation
of 5-7 as pale yellow crystals in 74% yield (Scheme 5.2.3). The 11
B{1H} NMR spectrum of 5-7
shows a singlet at 7.4 ppm, and a second singlet at -16.7 ppm corresponding to the B(C6F5)4
counter anion and the 29
Si{1H} NMR shows a signal at 0.96 ppm. The
19F{
1H} NMR spectrum
of 5-7 shows three peaks at -133.09, -163.80, and -167.62 ppm corresponding to the B(C6F5)4
moiety and the signal corresponding to the trimethylsilyl group shifts upfield to -0.02 ppm in the
1H NMR spectrum. Single crystals suitable for X-ray diffraction were obtained and the study
performed which confirmed the formulation of 5-7 as [(Cy-CAAC)BN(SiMe3)][B(C6F5)4]
(Figure 5.2.7).
Figure 5.2.7 POV-ray depiction of the molecular structure of 5-7. C: black, N: aquamarine, Si:
blue, B: yellow-green, F: deep pink. H-atoms omitted for clarity.
The geometry around boron is approximately linear with a C-B-N angle of 175.0(4)° and the
B-C distance is 1.550(5) Å which is similar to that in 5-2 and 5-4. The B-N distance is
1.192(5) Å, indicative of a BN triple bond, and is similar to 5-2 and 5-4. The B-N-Si bond angle
is 169.8(3)° which deviates from linearity.
As cyclic(alkylamino)carbenes often show reactivity that is not attained with NHCs we were
interested in probing whether this reactivity could be extended to NHCs. No reaction was
154
observed when the carbene SIMes was used, but the addition of two equivalents of IDipp to 5-1
resulted in the isolation of 5-8 as a pale yellow solid in 76% yield (Scheme 5.2.4). The second
equivalent of IDipp traps the released trimethylsilyl iodide and forms [IDipp-SiMe3][I] which
was identified in the 1H NMR spectrum. The
11B{
1H} NMR of 5-8 shows a singlet at 2.9 ppm
and the 29
Si{1H} NMR shows a signal at -9.1 ppm. The signal corresponding to the trimethylsilyl
group is found at 0.19 ppm in the 1H NMR spectrum and it integrates to 9H which indicates a
loss of Me3SiI.
Scheme 5.2.4 Synthesis of 5-8 to 5-11.
In addition to the NMR data, a single crystal X-ray study was performed which further
confirmed the formulation of 5-8 as (IDipp)B(I)N(SiMe3) (Figure 5.2.8). The geometry around
boron is trigonal planar (sum of angles at B is 360°), the B-C distance is 1.581(8) Å which is
typical of a B-C single bond,33,44
and the B-I and B-N distances are 2.361(6) and 1.282(7) Å,
respectively. The shortened B-N bond length is indicative of a double bond and the B-N-Si angle
is 145.5(4)°.
155
Figure 5.2.8 POV-ray depiction of the molecular structure of 5-8. C: black, N: aquamarine, Si:
blue, B: yellow-green, I: magenta. H-atoms omitted for clarity.
In a similar fashion, 5-9 was isolated as a pale yellow solid in 87% yield when two equivalents
of IDipp were added to a benzene solution of (Me3Si)2NBBr2. The 11
B{1H} NMR of 5-9 shows a
singlet at 10.9 ppm, and the 29
Si{1H} NMR spectrum shows a signal at -10.8 ppm. Along with
peaks corresponding to IDipp in the 1H NMR spectrum, a singlet at 0.20 ppm which integrates to
9H is present corresponding to the trimethylsilyl group. Based on the NMR data, the formulation
of 5-9 is (IDipp)BBrN(SiMe3).
Similar to the reactivity observed with Cy-CAAC, the addition of one equivalent of IDipp in
pentane to a solution of (Me3Si)2NBCl2 resulted in the formation of 5-10 as a white solid in 65%
yield (Scheme 5.2.4). The 11
B{1H} NMR spectrum of 5-10 shows a singlet at 3.7 ppm and the
29Si{
1H} NMR shows a signal at 0.04 ppm. The
1H and
13C{
1H} NMR spectra show signals
corresponding to the carbene and the bis-trimethylsilyl amide moieties. Single crystal X-ray
analysis of 5-10 confirmed its formulation as (IDipp)B(Cl)2N(SiMe3)2 (Figure 5.2.9). The
geometry around boron is tetrahedral with a B-C distance of 1.665(2) Å which is in the range of
B-C single bonds. The B-N distance is 1.507(2) Å which is in line with a typical B-N single bond
and the B-Cl bond lengths are 1.9080(17) and 1.9026(16) Å. Similar to 5-5, over time in
solution, 5-10 loses trimethylsilyl chloride and forms a new product, 5-11.
156
Figure 5.2.9 POV-ray depiction of the molecular structure of 5-10. C: black, N: aquamarine, Si:
blue, B: yellow-green, Cl: green. H-atoms omitted for clarity.
Compound 5-11 was also obtained as colorless crystals in 71% yield when one equivalent of
IDipp was added to a benzene solution of (Me3Si)2NBCl2 (Scheme 5.2.4). The 11
B{1H} NMR of
5-11 shows a singlet at 15.8 ppm and the 29
Si{1H} NMR spectrum shows a signal at -12.2 ppm.
The 1H NMR spectrum reveals a peak at 0.21 ppm corresponding to the trimethylsilyl group
integrating to 9H which indicates the loss of Me3SiCl. Based on the NMR data, the formulation
of 5-11 is (IDipp)B(Cl)N(SiMe3). In an effort to generate an iminoborane stabilized by IDipp,
NaBPh4 was added to a solution of 5-11 in C6H5Br and NMR data of the reaction mixture
showed several products. Upon workup, however, poor quality single crystals were formed and a
preliminary crystal structure indicated the formation of 5-11a (Figure 5.2.10).
157
Figure 5.2.10 POV-ray depiction of the molecular structure of the cation of 5-11a. C: black, N:
aquamarine, Si: blue, B: yellow-green, Cl: green. H-atoms and iPr groups omitted for clarity.
The molecular structure indicates the formation of the IDipp stabilized iminoborane but the
compound undergoes further reactivity including the generation of a bridging normal-abnormal
carbene, loss of [IDipp][HCl], and migration of one Me3Si group to another N atom resulting in
the formation of (Me3Si)2N and a bridging N moiety (Scheme 5.2.5).
Scheme 5.2.5 Reaction of 5-11 with NaBPh4.
158
At this time, it is unclear how this transformation proceeds and further studies are needed to
elucidate the reactivity including attempting to make the saturated version of IDipp (SIDipp) to
prevent the formation of an abnormal carbene.
5.2.2 Reactivity of Iminoboranes with CO2
We were interested in testing the reactivity of the iminoborane systems synthesized with CO2. To
that end, solutions of 5-2, 5-3, 5-4, 5-6, and 5-7 were exposed to 1 atm of CO2. While there was
no observed reactivity between 5-4 or 5-7 and CO2, both at room temperature and upon heating,
the reaction of 5-2, 5-3, and 5-6 with CO2 in C6H6 led to the isolation of 5-12, 5-13, and 5-14 as
white solids in 86, 83 and 91% yields, respectively (Scheme 5.2.6). The lack of reactivity
observed with 5-4 and 5-7 is attributed to the unavailability of the lone pair on N to react with
CO2. This is evidenced in the shortened B-N distances in 5-4 and 5-7 (BN triple bonds) which
indicates donation of the lone pair on N into the empty p orbital on the boron center, preventing
further reactivity.
Scheme 5.2.6 Synthesis of 5-12 to 5-14.
159
The 11
B{1H} NMR spectrum of 5-12 shows a singlet at -1.9 ppm, which is indicative of a
4-coordinate boron center, and the 29
Si{1H} NMR shows a signal at 2.6 ppm. A signal at
157.1 ppm in the 13
C{1H} NMR spectrum, which falls in the carbamate region, is assigned to the
CO2 carbon. A single crystal X-ray study was performed and confirmed the formulation of 5-12
as (Cy-CAAC)B(CO2)N(SiMe3)I (Figure 5.2.11) which is a result of [2+2] cycloaddition
between CO2 and the BN fragment in 5-2.
Figure 5.2.11 POV-ray depiction of the molecular structure of 5-12. C: black, N: aquamarine,
Si: blue, B: yellow-green, O: red, I: magenta. H-atoms omitted for clarity.
The geometry around boron is distorted tetrahedral where the N-B-O angle is 88.6 (4)° which is
significantly smaller than typically observed angles. This is presumably due to the chelation of
the CO2 molecule and formation of a tight 4-member ring. The B-C distance is 1.619(9) Å which
is similar to 5-3 and the B-I distance is 2.331(6) Å. The B-N and B-O bond distances are
1.524(8) and 1.480(7) Å, respectively, which are in line with typical B-N and B-O single
bonds.45-47
The C-OB and C-O bond lengths are 1.371(7) and 1.200(8) Å, which are typical of
CO single and double bonds, respectively.
The 11
B{1H} NMR of 5-13 shows a singlet at 0.49 ppm, which is indicative of a 4-coordinate
boron center, and the 29
Si{1H} NMR spectrum shows a signal at 2.2 ppm. Similar to 5-12, the
13C{
1H} NMR shows a signal at 158.9 ppm which is assigned to the CO2 carbon. X-ray analysis
of single crystals of 5-13 showed an analogous geometry to that observed for 5-12 and confirmed
160
the formulation as (Cy-CAAC)B(CO2)N(SiMe3)Br (Figure 5.2.12). The geometry around B is
distorted tetrahedral where the N-B-O angle is 88.6(3)° due to the formation of a tight 4-member
ring. The B-C distance is 1.633(6) Å which is similar to 5-12 and the B-Br distance is
2.082(5) Å. The B-N and B-O bond distances are 1.525(6) and 1.483(6) Å, respectively, which
are in line with typical B-N and B-O single bonds and the C-OB and C-O bond lengths are
1.353(6) and 1.211(5) Å.
Figure 5.2.12 POV-ray depiction of the molecular structure of 5-13. C: black, N: aquamarine,
Si: blue, B: yellow-green, O: red, Br: maroon. H-atoms omitted for clarity.
Similar to 5-12 and 5-13, the 11
B{1H} NMR spectrum of 5-14 shows a singlet at 1.6 ppm and the
29Si{
1H} NMR shows a signal at 0.8 ppm. Similar to 5-12 and 5-13, the
13C{
1H} NMR shows a
signal at 159.1 ppm which is assigned to the CO2 carbon. Single crystals suitable for an X-ray
study were obtained and confirmed the formulation of 5-14 as (Cy-CAAC)B(CO2)N(Me3Si)Cl
(Figure 5.2.13). The geometry around boron is distorted tetrahedral and the N-B-O angle is
87.81(18)°. The B-C distance is 1.635(4) Å which is similar to 5-12 and 5-13 and the B-Cl
distance is 1.885(3) Å. The B-N and B-O bond distances are 1.534(3) and 1.507(3) Å,
respectively and the C-OB and C-O bond lengths are 1.357(3) and 1.204(3) Å, respectively.
161
Figure 5.2.13 POV-ray depiction of the molecular structure of 5-14. C: black, N: aquamarine,
Si: blue, B: yellow-green, O: red, Cl: green. H-atoms omitted for clarity.
These results are consistent with a recent report by Cui and co-workers where they described the
intermolecular [2+2] cycloaddition of CO2 with a π-conjugated iminoborane stabilized by an
intramolecular imine group (Scheme 5.2.7).48
This system, however, loses CO2, regenerating the
iminoborane, upon heating under vacuum at 180 °C.
Scheme 5.2.7 Reaction of a π-conjugated iminoborane with CO2.
Efforts to further functionalize the CO2 moiety either with triaryl- or trialkylsilane or H2 were
unsuccessful. While compounds 5-2, 5-3, and 5-6 reacted with CO2, they were unreactive
towards other small molecules such as H2, CO, and small olefins.
162
5.3 Conclusion
Cyclic (alkylamino)carbenes have been shown to stabilize iminoboryl moieties which have only
been previously stabilized in the coordination sphere of transition metals. These species were
characterized crystallographically and, depending on the halide size, can be directly formed
through elimination of TMS-X (X = I), or through halide exchange for a non-coordinating anion
after TMS-X elimination (X = Br, Cl). The analogous species using IDipp instead of a CAAC
could not be isolated as further reactivity with the carbene backbone is observed. Some of the
species were shown to undergo [2+2] cycloaddition with CO2.
5.4 Experimental Section
5.4.1 General Considerations
All synthetic manipulations were carried out under an atmosphere of dry, O2-free N2 employing
a VAC Atmospheres glove box and a Schlenk vacuum-line. Hexanes and pentane were purified
with a Grubbs-type column system manufactured by Innovative Technology and dispensed into
thick-walled glass Schlenk bombs equipped with Young-type Teflon valve stopcocks.
Anhydrous benzene was purchased from Sigma Aldrich and stored over molecular sieves.
Dichloromethane-d2 and toluene-d8 were dried over CaH2 and benzene-d6 was dried over Na
metal and vacuum-transferred into a Young bomb. All solvents were thoroughly degassed after
purification (three freeze-pump-thaw cycles). NMR spectra were recorded at 25 °C on a Bruker
Avance 400 MHz spectrometer or Agilent 500 MHz. BI3, BBr3, BCl3 (1 M in hexane),
K(N(SiMe3)2), and CO2, were obtained from Sigma-Aldrich and used without further
purification. IDipp49
, Cy-CAAC50
, Cl2B(N(SiMe3)2)40
and Br2B(N(SiMe3)2)13
were prepared
according to literature procedures. Chemical shifts are given relative to SiMe4 and referenced to
the residual solvent signal (1H,
13C) or relative to an external standard (
29Si: Me4Si,
11B: BF3
.Et2O,
19F: CFCl3). In some instances, signal assignment was derived from two-
dimensional NMR experiments. Chemical shifts are reported in ppm and coupling constants as
scalar values in Hz. Combustion analyses were performed in house employing a Perkin-Elmer
CHN Analyzer.
163
5.4.2 Synthetic Procedures
Synthesis of 5-1: A solution of KHMDS (0.112 g, 0.561 mmol) in 5 mL toluene was added at
room temperature to a stirring solution of BI3 (0.200 g, 0.511 mmol) in 5 mL toluene. A white
solid instantaneously starts to precipitate. The reaction mixture was left stirring overnight and
then filtered to remove the white solid. The solvent was then removed in vacuo to yield an oily
solid (0.170 g, 0.400 mmol, 78%). X-ray quality crystals were grown from toluene and melt at
room temperature. 1H NMR (400 MHz, C6D6): δ 0.28 (s, 18H, (CH3)3Si).
13C{
1H} NMR (101
MHz, C6D6): δ 4.2 (s, (CH3)3Si). 11
B{1H} NMR (96 MHz, C6D6): δ 7.9 (s).
29Si{
1H} NMR (80
MHz, C6D6): δ 7.1 (s, (CH3)3Si). Elemental Analysis for C6H18BI2NSi2: C, 16.96; H, 4.27; N,
3.30. Found: C, 17.81; H, 4.65; N, 3.58. NMR spectra are attached at the end of this section.
Synthesis of 5-2: A solution of Cy-CAAC (0.300 g, 0.921 mmol) in 10 mL of C6H6 was added at
room temperature to a stirring solution of 5-1 (0.196 g, 0.461 mmol) in 5 mL of C6H6. The
solution turns bright orange and a white solid instantaneously starts to precipitate. The reaction
mixture was left stirring overnight and then filtered to remove the white solid. The solvent was
then removed in vacuo to yield an orange solid (0.205 g, 0.372 mmol, 95%). X-ray quality
crystals were grown from pentane at -35 oC.
1H NMR (400 MHz, C6D6): δ 7.08 (m, 1H, C6H3),
7.00 (m, 2H, C6H3), 2.83 (septet, 3JHH = 7 Hz, 2H, CH(CH3)2), 2.74 (dt,
2JHH = 13 Hz,
3JHH =
4 Hz, 2H, C6H10), 1.83 (s, 1H, Dipp-NCCH2), 1.80 (s, 1H, Dipp-NCCH2), 1.66 (d, 3JHH = 7 Hz,
6H, CH(CH3)2), 1.60 (s, 2H, C6H10), 1.49 (m, 2H, C6H10), 1.36 (m, 2H, C6H10), 1.07 (d, 3JHH =
7 Hz, 6H, CH(CH3)2), 0.98 (br s, 2H, C6H10), 0.94 (s, 6H, Dipp-NC(CH3)2), 0.33 (s, 9H,
(CH3)3Si). 11
B{1H} NMR (96 MHz, C6D6): δ 6.5 (s).
13C{
1H} NMR (101 MHz, C6D6, Dipp-
NCC-Cy not observed): δ 146.1 (C6H3), 130.5 (C6H3), 128.2 (C6H3), 127.9 (C6H3), 125.8 (C6H3),
78.7 (Dipp-NC(CH3)2), 56.6 (C6H10), 46.4 (C6H10), 39.5 (C6H10 + Dipp-NCCH2), 29.5
(CH(CH3)2), 27.8 (Dipp-NC(CH3)2), 27.4 (CH(CH3)2), 25.4 (CH(CH3)2), 24.7 (C6H10), 22.8
(C6H10), 2.5 (s, (CH3)3Si). 29
Si{1H} NMR (99 MHz, C6D6): δ -8.9 (s, (CH3)3Si). Elemental
Analysis for C26H44BIN2Si: C, 56.73; H, 8.06; N, 5.09. Found: C, 56.69; H, 8.05; N, 5.28.
Synthesis of 5-3: A solution of Cy-CAAC (0.300 g, 0.921 mmol) in 10 mL of C6H6 was added at
room temperature to a stirring solution of (Me3Si)2NBBr2 (0.152 g, 0.461 mmol) in 5 mL of
C6H6. The solution turns bright yellow and a white solid instantaneously starts to precipitate. The
reaction mixture was left stirring overnight and then filtered to remove the white solid. The
164
solvent was then removed in vacuo to yield a yellow solid (0.190 g, 0.377 mmol, 95%). X-ray
quality crystals were grown from pentane at -35 oC.
1H NMR (400 MHz, C6D6): δ 7.09 (m, 1H,
C6H3), 7.01 (m, 2H, C6H3), 2.77 (m, 4H, CH(CH3)2 + C6H10), 1.68 (br s, 2H, Dipp-NCCH2), 1.65
(d, 3JHH = 7 Hz, 6H, CH(CH3)2), 1.55 (s, 2H, C6H10), 1.51 (m, 2H, C6H10), 1.40 (m, 2H, C6H10),
1.10 (d, 3JHH = 7 Hz, 6H, CH(CH3)2), 0.99 (m, 2H, C6H10), 0.88 (s, 6H, Dipp-NC(CH3)2), 0.42 (s,
9H, (CH3)3Si). 11
B{1H} NMR (128 MHz, C6D6): δ 12.6 (s).
13C{
1H} NMR (101 MHz, C6D6,
Dipp-NCC-Cy not observed): δ 146.0 (C6H3), 130.3 (C6H3), 128.2 (C6H3), 127.9 (C6H3), 125.6
(C6H3), 79.3 (Dipp-NC(CH3)2), 56.9 (C6H10), 45.6 (C6H10), 37.6 (C6H10 + Dipp-NCCH2), 29.6
(CH(CH3)2), 28.7 (Dipp-NC(CH3)2), 27.5 (CH(CH3)2), 25.2 (CH(CH3)2), 25.0 (C6H10), 22.5
(C6H10), 3.7 (s, (CH3)3Si). 29
Si{1H} NMR (99 MHz, C6D6): δ -10.5 (s, (CH3)3Si). Elemental
Analysis for C26H44BBrN2Si: C, 62.03; H, 8.81; N, 5.56. Found: C, 61.71; H, 9.19; N, 5.62.
Synthesis of 5-4: Solid NaBPh4 (0.075 g, 0.219 mmol) was added to a stirring solution of 5-3
(0.100 g, 0.199 mmol) in 5 mL of C6H5Cl. The mixture was left stirring at room temperature
overnight and was then filtered over a pad of celite and the filtrate concentrated to 3 mL. The
solution was layered with 15 mL of pentane and left standing overnight. The solvent was then
decanted and the product was isolated as pale yellow crystals which were dried under high
vacuum (0.110 g, 75%). X-ray quality crystals were grown from C6H5Cl/pentane. 1H NMR
(400 MHz, CD2Cl2): δ 7.66 (m, 1H, C6H3), 7.46 (s, 1H, C6H3), 7.44 (s, 1H, C6H3), 7.34 (m, 8H,
o-H, BPh4), 7.05 (m, 8H, m-H, BPh4), 6.90 (m, 4H, p-H, BPh4), 2.50 (septet, 3JHH = 7 Hz, 2H,
CH(CH3)2), 2.33 (s, 2H, C6H10), 1.94 (m, 4H, C6H10 + Dipp-NCCH2), 1.82 (m, 4H, C6H10), 1.48
(br s, 8H, C6H10 + Dipp-NC(CH3)2), 1.39 (d, 3JHH = 7 Hz, 6H, CH(CH3)2), 1.25 (d,
3JHH = 7 Hz,
6H, CH(CH3)2), 0.00 (s, 9H, (CH3)3Si). 11
B{1H} NMR (128 MHz, CD2Cl2): δ 7.7 (br, CBN), -6.5
(sharp s, BPh4). 13
C{1H} NMR (101 MHz, CD2Cl2): δ 164.6 (q,
1JCB = 50 Hz, Cipso BPh4), 144.8
(C6H3), 136.5 (q, 2JCB = 1 Hz, o-C BPh4), 133.4 (C6H3), 130.3 (C6H3), 129.0 (C6H3), 126.9
(C6H3), 126.1 (q, 3JCB = 3 Hz, m-C BPh4), 122.2 (s, p-C BPh4), 87.1 (Dipp-NC(CH3)2), 57.9
(C6H10), 45.4 (C6H10), 35.8 (C6H10 + Dipp-NCCH2), 30.4 (CH(CH3)2), 29.6 (Dipp-NC(CH3)2),
26.6 (CH(CH3)2), 24.6 (CH(CH3)2), 23.4 (C6H10), 21.6 (C6H10), 0.8 (s, (CH3)3Si). 29
Si{1H} NMR
(99 MHz, CD2Cl2): δ 1.7 (s, (CH3)3Si). Elemental Analysis51
for C50H64B2N2Si: C, 80.85; H,
8.68; N, 3.77. Found: C, 79.6; H, 9.10; N, 3.87.
Synthesis of 5-5: A solution of Cy-CAAC (0.300 g, 0.921 mmol) in 10 mL pentane was added to
a solution of (Me3Si)2NBCl2 (0.223 g, 0.921 mmol) in 5 mL hexane. A white solid precipitates
165
instantaneously. The reaction mixture was stirred for 3 more hours before it was filtered through
a frit. The white solid was washed with 10 mL pentane and dried under high vacuum (0.348 g,
65%). X-ray quality crystals were grown from layering a hexane solution of Cy-CAAC over a
benzene solution of (Me3Si)2NBCl2 at room temperature. Single crystals were grown at the
interface. 1H NMR (400 MHz, Tol-d8): δ 7.08-6.99 (m, 3H, C6H3), 2.99 (m, 2H, CH(CH3)2), 2.94
(m, 2H, C6H10), 1.75 (br s, 1H, Dipp-NCCH2), 1.72 (br s, 1H, Dipp-NCCH2), 1.68-1.58 (m, 6H,
C6H10), 1.44 (d, 3JHH = 7 Hz, 6H, CH(CH3)2), 1.17 (d,
3JHH = 7 Hz, 6H, CH(CH3)2), 1.13 (m, 2H,
C6H10), 0.88 (s, 6H, Dipp-NC(CH3)2), 0.56 (s, 18H, (CH3)3Si). 11
B{1H} NMR (96 MHz, Tol-d8):
δ 4.3 (s). 13
C{1H} NMR (101 MHz, Tol-d8, Dipp-NCC-Cy not observed): δ 145.6 (C6H3), 135.6
(C6H3), 129.1 (C6H3), 128.2 (C6H3), 125.8 (C6H3), 80.4(Dipp-NC(CH3)2), 59.9 (C6H10), 45.4
(C6H10), 37.8 (C6H10 + Dipp-NCCH2), 29.9 (CH(CH3)2), 29.7 (Dipp-NC(CH3)2), 29.3
(CH(CH3)2), 25.5 (CH(CH3)2), 25.3 (C6H10), 22.8 (C6H10), 7.7 (s, (CH3)3Si). 29
Si{1H} NMR
(99 MHz, Tol-d8): δ -0.49 (s, (CH3)3Si). Elemental Analysis for C30H55BCl2N2Si2: C, 61.95; H,
9.53; N, 4.82. Found: C, 61.66; H, 9.65; N, 5.11.
Synthesis of 5-6: A solution of Cy-CAAC (0.300 g, 0.921 mmol) in 10 mL of C6H6 was added at
room temperature to a stirring solution of (Me3Si)2NBCl2 (0.111 g, 0.461 mmol) in 5 mL of
C6H6. The solution turns orange/yellow. The reaction mixture was left stirring overnight and the
solvent was then removed in vacuo. The product was extracted into 15 mL of pentane and
filtered over a pad of celite. The yellow pentane solution was cooled to -35 oC, overnight,
yielding the product as yellow crystals (0.148 g, 70%). X-ray quality crystals were grown from
pentane at -35 oC.
1H NMR (400 MHz, C6D6): δ 7.10 (m, 1H, C6H3), 7.02 (br s, 1H, C6H3), 7.00
(m, 1H, C6H3), 2.75 (m, 4H, C6H10 + CH(CH3)2), 1.61 (d, 3JHH = 7 Hz, 6H, CH(CH3)2), 1.55-1.44
(br m, 8H, C6H10 + NCCH2), 1.12 (d, 3JHH = 7 Hz, 6H, CH(CH3)2), 1.01 (m, 2H, C6H10), 0.87 (s,
6H, Dipp-NC(CH3)2), 0.45 (s, 9H, (CH3)3Si). 11
B{1H} NMR (96 MHz, C6D6): δ 17.4 (s).
13C{
1H} NMR (101 MHz, C6D6, Dipp-NCC-Cy not observed): δ 145.8 (C6H3), 132.1 (C6H3),
130.2 (C6H3), 128.3 (C6H3), 125.5 (C6H3), 79.4 (Dipp-NC(CH3)2), 57.2 (C6H10), 45.3 (C6H10),
36.8 (C6H10 + Dipp-NCCH2), 29.6 (CH(CH3)2), 29.0 (Dipp-NC(CH3)2), 27.4 (CH(CH3)2), 25.2
(CH(CH3)2), 24.9 (C6H10), 22.4 (C6H10), 4.2 (s, (CH3)3Si). 29
Si{1H} NMR (99 MHz, C6D6): δ
-11.9 (s, (CH3)3Si). Elemental Analysis for C26H44BClN2Si: C, 68.04; H, 9.66; N, 6.10. Found:
C, 67.46; H, 9.63; N, 6.14.
166
Synthesis of 5-7: Solid KB(C6F5)4 (0.156 g, 0.218 mmol) was added to a stirring solution of 5-5
(0.100 g, 0.218 mmol) in 10 mL of C6H5Cl. The mixture was left stirring at room temperature
overnight and was then filtered over a pad of celite and the filtrate concentrated to 5 mL. The
solution was layered with 15 mL of pentane and left standing overnight. The solvent was then
decanted and the product was isolated as pale yellow crystals which were dried under vacuum
(0.178 g, 74%). X-ray quality crystals were grown from C6H5Cl/pentane. 1H NMR (400 MHz,
CD2Cl2): δ 7.65 (apparent t, 3JHH = 8 Hz, 1H, C6H3), 7.46 (s, 1H, C6H3), 7.44 (s, 1H, C6H3), 2.53
(septet, 3JHH = 7 Hz, 2H, CH(CH3)2), 2.49 (s, 2H, C6H10), 1.95 (m, 4H, C6H10 + Dipp-NCCH2),
1.85 (m, 4H, C6H10), 1.58 (s, 6H, Dipp-NC(CH3)2), 1.49 (m, 2H, C6H10), 1.37 (d, 3JHH = 7 Hz,
6H, CH(CH3)2), 1.24 (d, 3JHH = 7 Hz, 6H, CH(CH3)2), -0.02 (s, 9H, (CH3)3Si).
19F NMR (376
MHz, CD2Cl2): δ -133.09 (m, 8F, o-C6F5), -163.80 (t, 3JFF = 20 Hz, 4F, p-C6F5),
-167.62 (apparent t, 3JFF = 20 Hz, 8F, m-C6F5).
11B{
1H} NMR (128 MHz, CD2Cl2): δ 7.4 (br,
CBN), -16.7 (sharp s, B(C6F5)4). 13
C{1H} NMR (101 MHz, CD2Cl2): δ 148.7 (dm,
1JCF = 241 Hz,
o-C6F5), 144.9 (C6H3), 138.8 (dm, 1JCF = 244 Hz, p-C6F5), 136.9 (dm,
1JCF = 245 Hz, m-C6F5),
133.5 (C6H3), 130.4 (C6H3), 129.1 (C6H3), 126.9 (C6H3), 86.9 (Dipp-NC(CH3)2), 58.0 (C6H10),
45.6 (C6H10), 35.9 (C6H10 + Dipp-NCCH2), 30.4 (CH(CH3)2), 29.7 (Dipp-NC(CH3)2), 26.6
(CH(CH3)2), 24.6 (CH(CH3)2), 23.3 (C6H10), 21.5 (C6H10), 0.7 (s, (CH3)3Si). 29
Si{1H} NMR (99
MHz, CD2Cl2): δ 0.96. Elemental Analysis for C50H44B2F20N2Si: C, 54.47; H, 4.02; N, 2.54.
Found: C, 54.14; H, 3.73; N, 2.88.
Synthesis of 5-8: A solution of IDipp (0.150 g, 0.386 mmol) in 5 mL of C6H6 was added at room
temperature to a stirring solution of 5-1 (0.082 g, 0.193 mmol) in 5 mL of C6H6. The solution
turns pale orange and a white solid instantaneously starts to precipitate. The reaction mixture was
left stirring overnight and then filtered to remove the white solid. The solvent was then removed
in vacuo and the residue was washed with 3 mL of cold pentane. The pentane was decanted and
the product dried under high vacuum to yield a pale yellow solid (0.090 g, 76%). X-ray quality
crystals were grown from pentane at -35 oC.
1H NMR (400 MHz, C6D6): δ 7.21 (m, 2H, C6H3),
7.09 (br s, 2H, C6H3), 7.07 (br s, 2H, C6H3), 6.37 (s, 2H, IDipp-4,5-CH), 2.89 (septet, 3JHH = 7
Hz, 4H, CH(CH3)2), 1.49 (d, 3JHH = 7 Hz, 12H, CH(CH3)2), 1.00 (d,
3JHH = 7 Hz, 12H,
CH(CH3)2), 0.19 (s, 9H, (CH3)3Si). 11
B{1H} NMR (96 MHz, C6D6): δ 2.9 (s).
13C{
1H} NMR
(101 MHz, C6D6, NCN not observed ): δ 145.7 (C6H3), 133.0 (C6H3), 131.2 (C6H3), 128.6
(C6H3), 128.3 (C6H3), 124.5 (C6H3), 122.6 (IDipp-4,5-CH), 29.2 (CH(CH3)2), 25.7 (CH(CH3)2),
167
23.4 (CH(CH3)2), 2.5 (s, (CH3)3Si). 29
Si{1H} NMR (99 MHz, C6D6): δ -9.1 (s, (CH3)3Si).
Elemental Analysis for C30H45BIN3Si: C, 58.73; H, 7.39; N, 6.85. Found: C, 58.29; H, 7.87; N,
6.75.
Synthesis of 5-9: A solution of IDipp (0.150 g, 0.386 mmol) in 5 mL of C6H6 was added at room
temperature to a stirring solution of (Me3Si)2NBBr2 (0.064 g, 0.193 mmol) in 5 mL of C6H6. The
solution turns pale yellow and a white solid instantaneously starts to precipitate. The reaction
mixture was left stirring overnight and then filtered to remove the white solid. The solvent was
then removed in vacuo and the residue was washed with 3 mL of cold pentane. The pentane was
decanted and the product dried under high vacuum to yield a pale yellow solid (0.095 g, 87%).
1H NMR (400 MHz, C6D6): δ 7.21 (m, 2H, C6H3), 7.09 (br s, 2H, C6H3), 7.07 (br s, 2H, C6H3),
6.33 (s, 2H, IDipp-4,5-CH), 2.80 (septet, 3JHH = 7 Hz, 4H, CH(CH3)2), 1.47 (d,
3JHH = 7 Hz, 12H,
CH(CH3)2), 1.02 (d, 3JHH = 7 Hz, 12H, CH(CH3)2), 0.20 (s, 9H, (CH3)3Si).
11B{
1H} NMR (96
MHz, C6D6): δ 10.9 (s). 13
C{1H} NMR (101 MHz, C6D6, NCN not observed ): δ 145.6 (C6H3),
133.4 (C6H3), 130.9 (C6H3), 129.0 (C6H3), 128.3 (C6H3), 124.4 (C6H3), 122.4 (IDipp-4,5-CH),
29.2 (CH(CH3)2), 25.4 (CH(CH3)2), 23.3 (CH(CH3)2), 3.3 (s, (CH3)3Si). 29
Si{1H} NMR (99
MHz, C6D6): δ -10.8 (s, (CH3)3Si). Repeated attempts to obtain EA were unsuccessful and as
such the NMR spectra of 5-9 are attached at the end of this section.
Synthesis of 5-10: To a solution of (Me3Si)2NBCl2 (0.124 g, 0.512 mmol) in 15 mL pentane was
added a solution of IDipp (0.200 g, 0.515 mmol) in 0.5 mL benzene. A white solid precipitates
within minutes. The reaction mixture was stirred for 6 more hours before it was filtered through
a frit. The white solid was washed with 10 mL pentane and dried under high vacuum (0.210 g,
65%). X-ray quality crystals were grown from C6H5Cl/pentane at -35 °C. 1H NMR (400 MHz,
C6D6): δ 7.20-7.10 (m, 6H, C6H3), 6.34 (s, 2H, IDipp-4,5-CH), 3.00 (br s, 4H, CH(CH3)2), 1.43
(d, 3JHH = 7 Hz, 12H, CH(CH3)2), 0.91 (d,
3JHH = 7 Hz, 12H, CH(CH3)2), 0.35 (s, 18H,
(CH3)3Si).11
B{1H} NMR (96 MHz, C6D6): δ 3.7 (s).
13C{
1H} NMR (101 MHz, C6D6, NCN not
observed ): δ 145.4 (C6H3), 133.9 (C6H3), 130.7 (C6H3), 128.3 (C6H3), 124.6 (C6H3), 122.5
(IDipp-4,5-CH), 29.1 (CH(CH3)2), 25.2 (CH(CH3)2), 23.4 (CH(CH3)2), 7.3 (s, (CH3)3Si).
29Si{
1H} NMR (99 MHz, C6D6): δ 0.04 (s, (CH3)3Si). Elemental Analysis for C33H54BCl2N3Si2:
C, 62.84; H, 8.63; N, 6.66. Found: C, 62.96; H, 9.07; N, 6.86.
168
Synthesis of 5-11: A solution of IDipp (0.185 g, 0.386 mmol) in 5 mL of C6H6 was added at
room temperature to a stirring solution of (Me3Si)2NBCl2 (0.114 g, 0.386 mmol) in 5 mL of
C6H6. The solution was left stirring overnight and then filtered over a pad of celite. The solvent
was then removed in vacuo and the residue extracted with 5 mL pentane which was left at -35 °C
for 24 hours to yield colorless crystals (0.174 g, 71%). 1H NMR (400 MHz, C6D6): δ 7.21 (m,
2H, C6H3), 7.08 (br s, 2H, C6H3), 7.06 (br s, 2H, C6H3), 6.24 (s, 2H, IDipp-4,5-CH), 2.72 (septet,
3JHH = 7 Hz, 4H, CH(CH3)2), 1.45 (d,
3JHH = 7 Hz, 12H, CH(CH3)2), 1.03 (d,
3JHH = 7 Hz, 12H,
CH(CH3)2), 0.21 (s, 9H, (CH3)3Si). 11
B{1H} NMR (96 MHz, C6D6): δ 15.8 (s).
13C{
1H} NMR
(101 MHz, C6D6, NCN not observed ): δ 145.4 (C6H3), 133.9 (C6H3), 130.7 (C6H3), 129.0
(C6H3), 124.3 (C6H3), 122.5 (IDipp-4,5-CH), 29.2 (CH(CH3)2), 25.2 (CH(CH3)2), 23.4
(CH(CH3)2), 3.7 (s, (CH3)3Si). 29
Si{1H} NMR (99 MHz, C6D6): δ -12.2 (s, (CH3)3Si). Elemental
Analysis for C30H45BClN3Si: C, 69.02; H, 8.69; N, 8.05. Found: C, 68.84; H, 8.75; N, 8.18.
Synthesis of 5-12: A solution of 5-2 (0.100 g, 0.182 mmol) in 5 mL C6H6 was degassed and put
under 1 atm of CO2. The orange solution turns pale yellow within two minutes. The solution was
left stirring under an atmosphere of CO2 for 2 hours before the solvent was concentrated to 1 mL
and 15 mL of pentane were added to precipitate the product as an off-white solid which was
dried under high vacuum (0.093 g, 86%). X-ray quality crystals were grown from C6H6/pentane.
1H NMR (400 MHz, C6D6): δ 7.03 (m, 2H, C6H3), 6.87 (dd,
3JHH = 8 Hz,
4JHH = 1 Hz, 1H, C6H3),
3.19 (septet, 3JHH = 7 Hz, 1H, CH(CH3)2), 2.84 (dt,
2JHH = 14 Hz,
3JHH = 4 Hz, 1H, C6H10), 2.43
(dt, 2JHH = 14 Hz,
3JHH = 4 Hz, 1H, C6H10), 2.34 (septet,
3JHH = 7 Hz, 1H, CH(CH3)2), 2.12 ( br
m, 1H, Dipp-NCCH2), 1.62 (d, 3JHH = 7 Hz, 3H, CH(CH3)2), 1.54 (m, 2H, C6H10), 1.43 (m, 2H,
C6H10), 1.36 (m, 2H, C6H10), 1.20 (d, 3JHH = 7 Hz, 3H, CH(CH3)2), 1.16 (d,
3JHH = 7 Hz, 3H,
CH(CH3)2), 1.13 (s, 3H, Dipp-NC(CH3)2), 1.07 (m, 2H, C6H10), 0.99 (d, 3JHH = 7 Hz, 3H,
CH(CH3)2), 0.64 (s, 3H, Dipp-NC(CH3)2), 0.58 (s, 9H, (CH3)3Si). 11
B{1H} NMR (128 MHz,
C6D6): δ -1.9 (s). 13
C{1H} NMR (101 MHz, C6D6): δ 157.1 (BOCN), 145.0 (C6H3), 144.6
(C6H3), 134.0 (C6H3), 130.3 (C6H3), 126.4 (C6H3), 124.6 (C6H3), 80.0 (Dipp-NC(CH3)2), 57.9
(C6H10), 46.0 (Dipp-NCCH2), 37.1 (C6H10), 29.8 (Dipp-NC(CH3)2), 29.2 (CH(CH3)2), 29.1
(CH(CH3)2), 28.7 (CH(CH3)2), 25.7 (Dipp-NC(CH3)2), 25.4 (C6H10), 24.6 (CH(CH3)2), 24.1
(C6H10), 24.0 (CH(CH3)2), 23.7 (CH(CH3)2), 21.5 (C6H10), 1.2 ((CH3)3Si). 29
Si{1H} NMR (99
MHz, C6D6): δ 2.6 (s, (CH3)3Si). Elemental Analysis for C27H44BIN2O2Si: C, 54.55; H, 7.46; N,
4.71. Found: C, 54.40; H, 7.95; N, 4.93.
169
Synthesis of 5-13: A solution of 5-3 (0.100 g, 0.199 mmol) in 5 mL C6H6 was degassed and put
under 1 atm of CO2. The yellow solution turns colorless within two minutes. The solution was
left stirring under an atmosphere of CO2 for 1 hour before the solvent was concentrated to 1 mL
and 15 mL of pentane were added to precipitate the product as a white solid which was dried
under high vacuum (0.091 g, 83%). X-ray quality crystals were grown from C6H6/pentane. 1H
NMR (400 MHz, CD2Cl2): δ 7.45 (t, 3JHH = 8 Hz, 1H, C6H3), 7.32 (dd,
3JHH = 8 Hz,
4JHH = 1 Hz,
1H, C6H3), 7.24 (dd, 3JHH = 8 Hz,
4JHH = 1 Hz, 1H, C6H3), 2.99 (septet,
3JHH = 7 Hz, 1H,
CH(CH3)2), 2.55 (m, 1H, C6H10), 2.48 (septet, 3JHH = 7 Hz, 1H, CH(CH3)2), 2.32 (d,
2JHH = 13
Hz, 1H, Dipp-NCCH2), 2.29 (m, 1H, C6H10), 2.15 (d, 2JHH = 13 Hz, 1H, Dipp-NCCH2), 2.06 (m,
1H, C6H10), 1.91 (m, 1H, C6H10), 1.85 (m, 1H, C6H10), 1.74 (m, 1H, C6H10), 1.55 (s, 3H, Dipp-
NC(CH3)2), 1.49 (m, 4H, C6H10), 1.31 (m, 9H, CH(CH3)2), 1.21 (s, 3H, Dipp-NC(CH3)2), 1.03
(d, 3JHH = 7 Hz, 3H, CH(CH3)2), 0.29 (s, 9H, (CH3)3Si).
11B{
1H} NMR (128 MHz, CD2Cl2): δ
0.49 (s). 13
C{1H} NMR (101 MHz, CD2Cl2): δ 158.9 (BOCN), 145.1 (C6H3), 145.0 (C6H3),
134.0 (C6H3), 130.3 (C6H3), 126.3 (C6H3), 125.0 (C6H3), 81.1 (Dipp-NC(CH3)2), 58.7 (C6H10),
45.9 (Dipp-NCCH2), 37.9(C6H10), 37.3 (C6H10), 31.5 (Dipp-NC(CH3)2), 29.9 (CH(CH3)2), 29.6
(CH(CH3)2), 27.7 (CH(CH3)2), 26.7 (Dipp-NC(CH3)2), 25.5 (C6H10), 24.7 (CH(CH3)2), 24.4
(C6H10), 24.0 (CH(CH3)2), 23.6 (CH(CH3)2), 21.8 (C6H10), 1.1 ((CH3)3Si). 29
Si{1H} NMR (99
MHz, CD2Cl2): δ 2.2 (s, (CH3)3Si). Elemental Analysis for C27H44BBrN2O2Si•(C6H6)0.25: C,
60.37; H, 8.09; N, 4.94. Found: C, 60.37; H, 8.57; N, 5.34.
Synthesis of 5-14: A solution of 5-5 (0.100 g, 0.218 mmol) in 5 mL C6H6 was degassed and put
under 1 atm of CO2. The yellow solution turns colorless instantaneously. The solution was left
stirring under an atmosphere of CO2 for 1 hour before the solvent was concentrated to 1 mL and
15 mL of pentane were added to precipitate the product as a white solid which was dried under
high vacuum (0.100 g, 91%). X-ray quality crystals were grown from C6H6/pentane. 1H NMR
(400 MHz, C6D6): δ 7.08 (t, 3JHH = 8 Hz, 1H, C6H3), 7.03 (dd,
3JHH = 8 Hz,
4JHH = 2 Hz, 1H,
C6H3), 6.95 (dd, 3JHH = 8 Hz,
4JHH = 2 Hz, 1H, C6H3), 2.86 (septet,
3JHH = 7 Hz, 1H, CH(CH3)2),
2.67 (dt, 2JHH = 14 Hz,
3JHH = 4 Hz, 1H, C6H10), 2.49 (m, 2H, Dipp-NCCH2 + CH(CH3)2), 1.76
(m, 1H, C6H10), 1.62 (m, 7H, Dipp-NCCH2 + C6H10), 1.50 (d, 3JHH = 7 Hz, 3H, CH(CH3)2), 1.44
(m, 2H, C6H10), 1.34 (d, 3JHH = 7 Hz, 3H, CH(CH3)2), 1.15 (d,
3JHH = 7 Hz, 3H, CH(CH3)2), 1.06
(d, 3JHH = 7 Hz, 3H, CH(CH3)2), 0.97 (s, 3H, Dipp-NC(CH3)2), 0.79 (s, 3H, Dipp-NC(CH3)2),
0.54 (s, 9H, (CH3)3Si). 11
B{1H} NMR (128 MHz, C6D6): δ 1.6 (s).
13C{
1H} NMR (101 MHz,
170
C6D6): δ 159.1 (BOCN), 144.6 (C6H3), 144.5 (C6H3), 130.2 (C6H3), 125.6 (C6H3), 125.0 (C6H3),
124.8 (C6H3), 80.4 (Dipp-NC(CH3)2), 58.4 (C6H10), 45.2 (Dipp-NCCH2), 36.3 (C6H10), 35.9
(C6H10), 30.2 (Dipp-NC(CH3)2), 29.7 (CH(CH3)2), 29.4 (CH(CH3)2), 27.3 (CH(CH3)2), 27.2
(Dipp-NC(CH3)2), 25.3 (C6H10), 24.9 (CH(CH3)2), 24.5 (C6H10), 24.2 (CH(CH3)2), 22.6
(CH(CH3)2), 21.7 (C6H10), 1.3 ((CH3)3Si). 29
Si{1H} NMR (99 MHz, C6D6): δ 0.8 (s, (CH3)3Si).
Elemental Analysis for C27H44BClN2O2Si: C, 64.47; H, 8.82; N, 5.57. Found: C, 63.98; H, 9.32;
N, 5.49.
Figure 5.4.1 1H NMR spectrum of 5-1 in C6D6.
171
Figure 5.4.2 11
B{1H} NMR spectrum of 5-1 in C6D6.
Figure 5.4.3 13
C{1H} NMR spectrum of 5-1 in C6D6.
172
Figure 5.4.4 29
Si{1H} NMR spectrum of 5-1 in C6D6.
Figure 5.4.5 11
B{1H} NMR spectrum of 5-9 in C6D6.
173
Figure 5.4.6 1H NMR spectrum of 5-9 in C6D6.
Figure 5.4.7 13
C{1H} NMR spectrum of 5-9 in C6D6.
174
Figure 5.4.8 29
Si{1H} NMR spectrum of 5-9 in C6D6.
5.4.3 X-ray Crystallography
5.4.3.1 X-ray Data Collection and Reduction
Crystals were coated in Paratone-N oil in the glove-box, mounted on a MiTegen Micromount
and placed under an N2 stream, thus maintaining a dry, O2-free environment for each crystal. The
data for crystals were collected on a Bruker Apex II diffractometer employing Mo Kα radiation
(λ = 0.71073 Å). The data were collected at 150(±2) K for all crystals. The frames were
integrated with the Bruker SAINT software package using a narrow-frame algorithm. Data were
corrected for absorption effects using the empirical multi-scan method (SADABS).52
5.4.3.2 X-ray Data Solution and Refinement
Non-hydrogen atomic scattering factors were taken from the literature tabulations.53
The heavy
atom positions were determined using direct methods employing the SHELXTL direct methods
routine. The remaining non-hydrogen atoms were located from successive difference Fourier
175
map calculations. The refinements were carried out by using full-matrix least squares techniques
on F, minimizing the function (Fo–Fc)2 where the weight is defined as 4Fo2/2 (Fo
2) and Fo
and Fc are the observed and calculated structure factor amplitudes, respectively. In the final
cycles of each refinement, all non-hydrogen atoms were assigned anisotropic temperature factors
in the absence of disorder or insufficient data. In the latter cases atoms were treated isotropically.
C-H atom positions were calculated and allowed to ride on the carbon to which they are bonded
assuming a C-H bond length of 0.95 Å. H-atom temperature factors were fixed at 1.10 times the
isotropic temperature factor of the C-atom to which they are bonded. The H-atom contributions
were calculated, but not refined. The locations of the largest peaks in the final difference Fourier
map calculation as well as the magnitude of the residual electron densities in each case were of
no chemical significance.
176
Table 5.4.1 Select crystallographic parameters for 5-1 to 5-4.
5-1 5-2 5-3 5-4
Formula C6H18BI2NSi2 C26H44BIN2Si C26H44BBrN2Si C50H64B2N2Si
wt 425.00 550.43 503.44 742.74
Cryst. syst. Monoclinic Monoclinic Monoclinic Triclinic
Space group C2/c P2/c P21/n P-1
a(Å) 15.6776(10) 14.3428(5) 9.8668(6) 12.4924(11)
b(Å) 8.5777(5) 10.6121(4) 16.8993(11) 13.0727(11)
c(Å) 12.6262(13) 21.2246(7) 16.6423(12) 15.2439(14)
(deg) 90.00 90.00 90.00 92.753(5)
(deg) 118.583(2) 94.641(1) 91.432(2) 94.776(4)
(deg) 90.00 90.00 90.00 113.285(4)
V(Å3) 1491.0(2) 3219.9(2) 2774.1(3) 2269.8(3)
Z 4 4 4 2
d(calc) gcm
–3
1.893 1.135 1.205 1.087
R(int) 0.0177 0.0379 0.0634 0.0589
, mm–1
4.342 1.045 1.540 0.086
Total data 1712 7377 6377 7891
>2(FO2) 1584 5527 3915 4441
Variables 59 289 280 506
R (>2) 0.0183 0.0339 0.0449 0.0649
Rw 0.0408 0.0821 0.1008 0.1977
GOF 1.126 1.047 1.015 1.031
177
Table 5.4.2 Select crystallographic parameters for 5-5 to 5-8.
5-5 5-6 5-7 5-8
Formula C70H118B2Cl4N4Si4 C26H44BClN2Si C50H44B2F20N2Si C60H90B2I2N6Si2
wt 1291.46 458.98 1102.58 1226.98
Cryst. syst. Monoclinic Monoclinic Monoclinic Triclinic
Space group P21/c P21/n P2/c P-1
a(Å) 19.4594(16) 11.1637(7) 19.594(2) 12.6172(12)
b(Å) 18.5314(13) 10.1237(6) 15.6615(18) 16.7229(17)
c(Å) 22.2381(19) 23.8515(14) 17.5583(17) 18.938(2)
(deg) 90.00 90.00 90.00 90.760(3)
(deg) 114.504(2) 97.853(3) 110.206(3) 106.310(3)
(deg) 90.00 90.00 90.00 111.827(3)
V(Å3) 7297.0(10) 2670.4(3) 5056.5(9) 3528.9(6)
Z 4 4 4 2
d(calc) gcm–3
1.176 1.142 1.448 1.155
R(int) 0.0928 0.0462 0.0420 0.0775
, mm–1
0.270 0.204 0.158 0.961
Total data 12845 6153 8904 12384
>2(FO2) 7651 4405 6096 7519
Variables 837 280 707 649
R (>2) 0.0667 0.0497 0.0543 0.0506
Rw 0.1836 0.1270 0.1583 0.1178
GOF 1.007 1.027 1.017 0.910
178
Table 5.4.3 Select crystallographic parameters for 5-10, 5-12 to 5-14.
5-10 5-12 5-13 5-14
Formula C33H54BCl2N3Si2 C27H44BIN2O2Si C27H44BBrN2O2Si C27H44BClN2O2Si
wt 630.68 594.44 547.45 502.99
Cryst. syst. Orthorhombic Monoclinic Monoclinic Monoclinic
Space group P212121 C2/c P21/n P21/n
a(Å) 12.7630(7) 40.906(4) 10.7506(9) 10.8576(10)
b(Å) 14.5858(8) 11.0189(8) 15.3661(12) 15.1405(14)
c(Å) 22.6290(11) 16.0649(14) 17.8097(14) 17.7482(17)
(deg) 90.00 90.00 90.00 90.00
(deg) 90.00 100.112(7) 104.526(3) 104.260(4)
(deg) 90.00 90.00 90.00 90.00
V(Å3) 4212.6(4) 7128.6(11) 2848.0(4) 2827.7(5)
Z 4 8 4 4
d(calc) gcm–3
0.994 1.108 1.277 1.182
R(int) 0.0215 0.0690 0.0652 0.0540
, mm–1
0.233 0.953 1.511 0.203
Total data 9595 6176 5014 4973
>2(FO2) 8844 4280 3830 3452
Variables 385 307 307 307
R (>2) 0.0329 0.0618 0.0593 0.0457
Rw 0.0821 0.1496 0.1295 0.1128
GOF 1.042 1.063 1.167 1.016
179
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182
Chapter 6 A Room Temperature Stable Organoboron Isoelectronic with
Singlet Carbenes
6.1 Introduction
6.1.1 Borylenes: Group 13 Carbene Analogues
Low-valent main-group element derivatives have been a subject of considerable interest. Among
them, singlet carbenes A have arguably been the most widely studied (Scheme 6.1.1). These
species feature a carbon center with a lone pair and a vacant orbital, and are therefore both Lewis
acids and bases. Despite their expected high reactivity, many types of carbenes1-5
, as well as their
higher group-14 counterparts6, are stable at room temperature. On the other hand, there is only
one nitrene B7, which has been isolated, and no borylenes C.
Scheme 6.1.1 Schematic representation of singlet carbenes A, nitrenes B, borylenes C, and
Lewis base-borylene adducts D.
Borylenes C have a lone pair, but also two vacant orbitals, which make them extremely unstable.
They are accessible under drastic reaction conditions such as the preparation of the
fluoroborylene BF at high temperatures as was reported by Timms and co-workers in 19678, and
no free borylene has been isolated under synthetically useful conditions. Although borylenes are
often considered as the group 13 element analogues of carbenes, they are not isoelectronic with
carbenes since they possess one lone pair but two vacant orbitals. To fulfill the isoelectronic
criteria, a Lewis base should be appended to borylenes C, as shown in D. All attempts to isolate
Lewis base stabilized borylenes D have so far failed. For example, Robinson and co-workers9
reported that the reduction of the IDipp-BBr3 adduct led to the formation of the neutral diborene
183
in very low yields, a compound that possesses a boron-boron double bond (Scheme 6.1.2). This
species could be regarded as the dimer of the IDipp-BH borylene.
Scheme 6.1.2 Synthesis of a stable diborene stabilized by NHCs.
Curran and co-workers10
have shown that the reduction of (IDipp)BHCl2 led to the formation of
the borane shown in Scheme 6.1.3, which formally results from the intramolecular insertion of a
transient borylene into the CH bond of the iPr group of the Dipp substituent.
Scheme 6.1.3 Formation of a borane through C-H activation of a transient borylene.
In an analogous reaction, Braunschweig and co-workers11
observed that the reduction of
(IMe)BHCl2, in the presence of naphthalene, afforded a borirane (Scheme 6.1.4) that is described
as the trapping product of the transient derivative borylene of type D. These reactions represent
the expected high reactivity of these electron deficient species.
Scheme 6.1.4 Formation of a borirane by trapping of a transient borylene.
184
More recently, Bertrand and co-workers12
have reported the synthesis of a bis-CAAC stabilized
borylene through the reduction of the (CAAC)BBr3 adduct (Scheme 6.1.5) and developed a more
general route to synthesize bis-carbene-BH species13
which are isoelectronic with amines.
Scheme 6.1.5 Synthesis of a bis-CAAC-borylene.
We believe that the borylene-Lewis base adducts of type D are accessible and could be isolated.
These compounds are isoelectronic with carbenes and therefore most of the stabilization modes
known for carbenes should be applicable for the borylene adducts (L)BR.
6.1.2 Transition Metal Borylene Complexes
While free borylenes have yet to be isolated, borylene fragments have been isolated in the
coordination sphere of transition metals where they are generated.14-16
While there are different
types of borylene metal complexes14
only terminal borylenes, which feature a 2-coordinate boron
center will be discussed here.
Borylenes bind to metal centers through σ as well as π-back donation (Figure 6.1.1) where, for
terminal transition metal borylene complexes, the sigma donation exceeds π acceptance.14
This
leads to a buildup of positive charge on boron making it more susceptible to nucleophilic attack
which results in its kinetic instability. This was lessened and in some cases eliminated through
the introduction of sterically encumbered substituents on the boron center.17,18
185
Figure 6.1.1 Orbital interaction between borylenes and metal fragments.
The same effect is achieved through the introduction of strong π-donor substituents (such as
amino groups) on the boron center.19,20
The first borylene complexes were prepared via double
salt elimination reactions using dihaloboranes (Scheme 6.1.6).17,18
Scheme 6.1.6 Synthesis of the first terminal borylene complexes.
The potential of using these complexes as borylene sources for organic synthesis under standard
conditions was shown in the reaction where the borylene fragment transfers to alkynes forming
borirenes.21,22
6.1.3 CO Adducts of Carbenes and of Boranes
Carbenes over the last ten years have been shown to mimic transition metals and activate small
molecules such as H2 and CO.23
In contrast with NHCs24
, more electrophilic stable singlet
carbenes, such as CAACs25
and diamidocarbenes (DACs)26
, readily react with CO to give the
corresponding adducts (Scheme 6.1.7).
186
Scheme 6.1.7 CO fixation to a CAAC and a DAC.
Boranes on the other hand do not form stable adduct with CO and have not been experimentally
shown to activate H2 on their own. Willner and co-workers reported in 2002 the formation of a
tris(trifluoromethyl)borane carbonyl adduct (Scheme 6.1.8) by the solvolysis of K[B(C6F5)4] in
concentrated sulfuric acid.27,28
This compound has a melting point of 9 °C and decomposes
rapidly above that temperature with CO dissociation being the first step. Poor quality single
crystals can only be grown near -70 °C.
Scheme 6.1.8 Synthesis of tris(trifluoromethyl)borane carbonyl adduct.
Another example was reported by the Piers group in 2012 where they isolated a CO adduct of
pentafluoropentaphenylborole (Scheme 6.1.9).29
This reaction is reversible and under vacuum
results in the rapid reformation of the free borole.
Scheme 6.1.9 Synthesis of pentaarylborole-CO adduct.
187
The most recent example was reported by Erker and co-workers where they isolate the CO
adduct of Piers borane, HB(C6F5)2, (Scheme 6.1.10).30
Single crystals of the adduct were
obtained under an atmosphere of CO at -40 °C and this system loses CO at room temperature
regenerating Piers borane.
Scheme 6.1.10 Synthesis of Piers borane-CO adduct.
To demonstrate the carbene-like and electrophilic character of borylenes, preliminary reactivity
studies with small molecules, like H2 and CO are of interest.
6.2 Results and Discussion
6.2.1 Reduction Route to Borylene Synthesis
The first successful isolation of a carbene31,32
followed the prediction in 1980 by Pauling33
that
substituents of opposing electronic properties (push-pull effect) should stabilize singlet carbenes
by preserving the electroneutrality of the carbon center. The same strategy utilized to prepare the
first carbene is used to synthesize the boron analogue. As the push-substituent, a
bis(trimethylsilyl)amino group was chosen because an amino group is arguably a better π-donor
than a phosphino moiety, and silyl groups were chosen to further stabilize the electron-deficiency
at boron via the well-known -effect. As a Lewis base, and the pull-substituent, a CAAC was
chosen as it is a strong -acceptor23
and therefore can withdraw excessive electron-density from
boron center. Having already made the CAAC adduct of the
bis(trimethylsilylamino)dichloroborane, namely 5-5, its reduction chemistry was probed.
Reactions with KC8 produced intractable mixtures and as such Co(Cp*)2 was used as the
reducing agent. The choice of solvent for the reduction is important since 5-5 in solution, over
time, converts to the CAAC stabilized iminoborane (5-6) as was discussed in the previous
chapter. This reaction is much faster in polar organic solvents and as such benzene was chosen as
the solvent for reduction. Thus, adding one equivalent of Co(Cp*)2 to a C6H6 suspension of 5-5
188
and after stirring for four hours, followed by workup, the radical 6-1 was isolated as a yellow
solid in 74% yield (Scheme 6.2.1).
Scheme 6.2.1 Synthesis of 6-1 and 6-2.
Single crystals suitable for X-ray diffraction studies were grown and the formulation of 6-1 was
confirmed as [(Cy-CAAC)BClN(SiMe3)2]• (Figure 6.2.1). The geometry around boron is
perfectly planar where the sum of angles at B is 359.94°. The B-N distance is 1.460(3) Å which
falls within the range of B-N single bond lengths. The B-C distance is 1.527(3) Å which is
shortened and falls in the range of typical B-C single and double bonds (1.59-1.48 Å
respectively).34
The CAAC C-N distance is 1.378(3) Å which is slightly elongated compared to
typical CAAC adducts; similarly, the B-Cl distance is 1.831(3) Å which is shorter than that
typically observed.35
189
Figure 6.2.1 POV-Ray depiction of the molecular structure of 6-1. C: black, N: aquamarine, Si:
blue, B: yellow-green, Cl: green. H-atoms omitted for clarity.
This indicates that the unpaired electron is delocalized on the Cy-CAAC ligand and the planar
geometry around the boron center indicates delocalization over a p orbital which makes this
species a π-type radical. The radical 6-1 was analyzed by EPR spectroscopy in toluene at 280 K
as well as by DFT calculations.
The SOMO of 6-1 (B3LYP/6-311+g** level of theory) formally results from the bonding
interaction of the LUMO of the Cy-CAAC moiety and the * orbital of the B-Cl fragment
(Figure 6.2.2). This indicates a spin delocalization across the -system, which was also
supported by a Mulliken analysis of the spin density (N: 26%; C: 42%; B: 28%; Cl: 2%). These
theoretical results are in line with the simulation of the pattern of the experimental EPR spectra
of 6-1 in solution, which requires the introduction of isotropic hyperfine coupling constants with
boron, as well as nitrogen and chlorine (Figure 6.2.3).
190
Figure 6.2.2 Representation of the SOMO of 6-1 with isovalue at 0.06 a.u.; hydrogen atoms are
omitted for clarity.
Figure 6.2.3 Experimental X-band EPR spectrum of 6-1 in toluene at 280 K (green) and
simulated EPR spectrum (blue) with the following set of hyperfine coupling constants: a(B) =
4.7, a(N) = 18.4 and a(Cl) = 2.5 MHz.
This neutral CAAC-stabilized boron-centered radical is reminiscent to the arylboryl radical
recently prepared by Braunschweig and co-workers (Scheme 6.2.2).35
191
Scheme 6.2.2 Synthesis of a neutral boron-containing radical stabilized by a CAAC.
Thanks to the stability of 6-1 in solution the second reduction with one equivalent of Co(Cp*)2
was performed in benzene at 25 °C for 8 h where, upon workup, 6-2 was isolated as a red solid
in 86% yield. The 11
B{1H} NMR of 6-2 shows a broad singlet at 83.7 ppm and the
29Si{
1H}
NMR spectrum shows a signal at 7.2 ppm. The 1H NMR spectrum reveals a peak at 0.13 ppm
corresponding to the Me3Si group and it integrates to 18H which indicates that the N(SiMe3)2
moiety remains intact. The chemical shift observed in the 11
B{1H} NMR spectrum is more
downfield compared to aminoboraalkenes (R2C=BNR’2) (+ 59 to +71 ppm)36-39
and in the range
of transition metal stabilized terminal aminoborylenes (LnM=BNR2) (+ 67 to + 92 ppm).14
A
single crystal X-ray study was performed which confirmed the formulation of 6-2 as
(Cy-CAAC)BN(SiMe3) (Figure 6.2.4).
192
Figure 6.2.4 POV-Ray depiction of the molecular structure of 6-2. C: black, N: aquamarine, Si:
blue, B: yellow-green. H-atoms omitted for clarity.
The geometry around boron is slightly bent with a C-B-N angle of 174.8(3)°. The B-C distance
is 1.401(5) Å which is significantly shorter than those reported for compounds 5-6 and 6-1 and is
indicative of a B-C double bond. The B-N distance is 1.382(5) Å which is shorter than the B-N
bond in 6-1 and similar to that in 5-6 which indicates a double bond character.
DFT calculations at the 6-311g(d,p) level of theory well reproduce the solid state geometry of
6-2 [B-N: 1.383 Å; B-C: 1.412 Å; C-B-N angle 175.4°]. The highest occupied molecular orbital
(HOMO) results from a bonding interaction of the vacant * molecular orbital of the Cy-CAAC
moiety with the occupied p orbital of the boron atom, as expected from the stabilization of the
formal lone pair at the boron center by the -accepting Cy-CAAC ligand (Figure 6.2.5).
Similarly, the interaction of the lone pair at nitrogen with the formal empty p orbital at boron
results in a high-energy * molecular orbital (+0.16 eV). The latter is not even the lowest
unoccupied molecular orbital (LUMO), but the LUMO+2. However, a vibrational analysis
indicates that the C-B-N bending mode corresponds to an abnormally low energy frequency
(389 cm-1
). This suggests significant flexibility of the molecule along this coordinate and, indeed,
bending the C-B-N angle up to 155° has nearly no energetic cost (5.7 kcal.mol-1
). In marked
contrast with 6-2, the resulting bent structure 6-2* has pronounced electrophilicity, since the
formal empty p orbital at boron becomes sp2 hybridized, and is lower in energy. As a
193
consequence, the * orbital of the B-N moieties becomes the LUMO (Figure 6.2.5), its energy
being dramatically decreased by more than 0.6 eV. Consequently, although the minimum on the
energy hyper-surface of 6-2 corresponds to an apparently non-electrophilic molecule with a
nearly linear C-B-N alignment, organoboron 6-2 is expected to be highly electrophilic due to the
flexibility at the boron center.
Figure 6.2.5 a) and a’): highest occupied molecular orbital (HOMO) of 6-2, and of 6-2* with a
frozen C-B-N angle at 155°, respectively. b-d) and b’-d’) lowest unoccupied molecular orbitals
(LUMO) of 6-2 and 6-2*, respectively.
Based on these results small molecule activation with 6-2 which can easily bend to form 6-2*
was attempted.
194
6.2.2 Reactivity of Borylenes
Exposing a solution of 6-2 in pentane to 1 atm. of 13
CO, followed by workup, resulted in the
formation of 6-3 as a pink solid in 80% yield (Scheme 6.2.3). The 11
B{1H} NMR of 6-3 shows a
resonance with a significant upfield shift from 83.7 ppm for 6-2 to -3.4 ppm which is a doublet
with 1JBC of 87 Hz indicating a change to the coordination around the boron center. The
corresponding carbon of the 13
CO fragment is observed as a broad multiplet at 236.3 ppm in the
13C{
1H} NMR spectrum. A signal is observed at 3.01 ppm in the
29Si{
1H} NMR spectrum
indicating the presence of the (Me3Si)2N moiety and a single crystal X-ray diffraction study
confirmed the formulation of 6-3 as (Cy-CAAC)B(CO)[N(SiMe3)2] (Figure 6.2.6).
Scheme 6.2.3 Synthesis of 6-3.
The geometry around boron is perfectly planar where the sum of angles at B is 360°. The
B-CCAAC distance is 1.506(4) Å which is lengthened compared to 6-2 but still shorter than a
typical B-C single bond. The linear B-C≡O unit (172.7(3)°) showed a B-CCO distance of
1.529(5) Å, which is significantly shorter than previously reported B-CO distances (1.609(3) to
1.601(2) Å),29,30
indicative of a B-C bond halfway between typical BC single and double bonds.
The C-O distance is 1.091(3) Å which is slightly shorter than previously reported BC≡O
distances (1.115(3) to 1.107(2) Å) and is shorter than that of free CO (1.1281 Å as determined by
microwave spectroscopy)40
. The IR spectrum of 6-3 in the solid state shows a 12
CO stretching
frequency of 1956 cm-1
which is much lower than free 12
CO (2143 cm-1
) and is consistent with
the IR stretching frequency predicted by DFT calculations. The B-N distance is 1.526(4) Å
which is longer than that observed in 6-2.
195
Figure 6.2.6 POV-Ray depiction of the molecular structure of 6-3. C: black, N: aquamarine, Si:
blue, B: yellow-green, O: red. H-atoms omitted for clarity.
Compound 6-3 is a rare example of a stable, isolable CO adduct of a borane with very few
reported examples of CO adducts of organoboranes.28-30,41
To our knowledge, 6-3 is the first
example of a CO adduct of a formally B(+1) species that is also stable both in the solid state and
in solution where this reaction is not reversible either under vacuum or upon heating to 100 °C.
Based on the observed reactivity with CO we were also interested in the reactivity of 6-2 with
H2. Exposing a solution of 6-2 in toluene to 4 atm of H2 followed by workup resulted in the
isolation of 6-5 as pale yellow crystals in 85% yield (Scheme 6.2.4).
Scheme 6.2.4 Synthesis of 6-5.
196
The 11
B{1H} NMR of 6-5 shows a broad singlet at 51.2 ppm which is indicative of a three- and
not a four-coordinate boron center. The 1H NMR spectrum shows peaks corresponding to the
Cy-CAAC and N(SiMe3)2 moieties along with a singlet at 5.55 ppm and a doublet at 3.62 ppm
with 3JHH = 6 Hz. These signals were assigned to the BH and Dipp-NCHCCy, respectively, and
the corresponding carbon signal is observed at 67.3 ppm in the 13
C{1H} NMR spectrum.
A single crystal X-ray diffraction study confirmed that the isolated compound was not the boron
dihydride 6-4, but the monohydride 6-5 (Figure 6.2.7). The geometry around boron is perfectly
planar where the sum of angles at B is 360°. The B-CCAAC-H distance is 1.582(2) Å which is
typical of a B-C single bond and the B-N distance is 1.4243(19) Å, similar to that observed in
6-1. The hydride was located from the difference density map and the B-H distance is
1.118(16) Å.
Figure 6.2.7 POV-Ray depiction of the molecular structure of 6-5. C: black, N: aquamarine, Si:
blue, B: yellow-green. H-atoms other than B-H and Cy-CAAC-CH omitted for clarity.
The formation of 6-5 results from a formal 1,2-addition through the B-C bond of 6-2 but it has
been shown42,43
that CAAC-borohydride adducts, such as 6-4, can rearrange via 1,2-hydride
migration, and therefore it is quite likely that the first step of the reaction is an oxidative addition
of hydrogen at the boron center. Such a two-step process leading to 6-5 has been confirmed by
DFT calculations where both steps are strongly exergonic (G = -18 and -12 kcal.mol-1
,
197
respectively). Not surprisingly, the rate-limiting step is the oxidative addition
(Gǂ = +25 kcal.mol-1
) with hydride migration having a very low activation barrier
(Gǂ = +4.6 kcal.mol-1
). The first step corresponds to an early transition state (Figure 6.2.8)
which is reminiscent of the homolytic bond cleavage of H2 by electrophilic metals.44,45
Figure 6.2.8 Calculated transition state for the activation of H2 by 6-2.
Indeed, the approach of dihydrogen results in the primary interaction of the low-lying LUMO of
the bent 6-2* with the bonding σ orbital of H2 (Figure 6.2.9 a). The concomitant secondary back-
donation from the HOMO of 6-2* to the anti-bonding σ* orbital of H2 finally triggers the
cleavage of the activated H-H bond (Figure 6.2.9 b). Importantly, despite multiple attempts, a
transition state leading directly to 6-5 from 6-2 by direct addition of dihydrogen on the B-C bond
was not found.
Figure 6.2.9 a) Primary interaction between the LUMO of 6-2* and theorbital of H2. b)
Secondary interaction between the HOMO of 6-2* and the * orbital of H2.
Compound 6-2 shows reactivity unprecedented before with boron species where it forms a strong
adduct with CO and is capable of H2 activation.
198
6.3 Conclusion
The synthesis of a group-13 derivative, which is isoelectronic with singlet carbenes, namely a
borylene was described. This compound, which is stabilized by a push-pull effect, is formed by
the double reduction of a CAAC adduct of bis(trimethylsilylamino)dichloroborane, going
through a boron based radical. Similarly to singlet carbenes, it reacts with carbon monoxide and
hydrogen, but in contrast with the former, the latter acts as an electrophile and therefore mimics
the behavior of metals.
6.4 Experimental Section
6.4.1 General Considerations
All synthetic manipulations were carried out under an atmosphere of dry, O2-free N2 employing
a VAC Atmospheres glove box and a Schlenk vacuum-line. Hexanes and pentane were purified
with a Grubbs-type column system manufactured by Innovative Technology and dispensed into
thick-walled glass Schlenk bombs equipped with Young-type Teflon valve stopcocks.
Anhydrous benzene was purchased from Sigma Aldrich and stored over molecular sieves.
Toluene-d8 was dried over CaH2 and benzene-d6 was dried over Na metal and vacuum-
transferred into a Young bomb. All solvents were thoroughly degassed after purification (three
freeze-pump-thaw cycles). NMR spectra were recorded at 25 °C on a Bruker Avance 400 MHz
spectrometer or Agilent 500 MHz. 13
CO was purchased from Sigma-Aldrich and used without
further purification. Chemical shifts are given relative to SiMe4 and referenced to the residual
solvent signal (1H,
13C) or relative to an external standard (
29Si: Me4Si,
11B: BF3
.Et2O). In some
instances, signal assignment was derived from two-dimensional NMR experiments. Chemical
shifts are reported in ppm and coupling constants as scalar values in Hz. Combustion analyses
were performed in house employing a Perkin-Elmer CHN Analyzer.
6.4.2 Synthetic Procedures
Synthesis of 6-1: To a mixture of 5-5 (0.100 g, 0.176 mmol) and Co(Cp*)2 (0.058 g,
0.176 mmol) was added 10 mL of C6H6. The reaction mixture was left stirring at room
temperature for 6 hours before the volatiles were removed. The product was extracted into
pentane (15 mL) and filtered over a pad of celite. The solvent was then removed in vacuo
yielding a pale yellow solid (0.070 g, 74%). X-ray quality crystals were grown by slow
199
evaporation of a pentane solution at room temperature. Elemental Analysis for C29H53BClN2Si2:
C, 65.45; H, 10.04; N, 5.26. Found: C, 64.92; H, 10.21; N, 5.41.
Synthesis of 6-2: To a mixture of 6-1 (0.200 g, 0.376 mmol) and Co(Cp*)2 (0.124 g, 0.376
mmol) was added 15 mL of C6H6. The reaction mixture was left stirring at 25 °C for 8 hours it
was filtered over celite. The filtrate was concentrated to 1 mL and 15 mL of pentane was added
drop wise to precipitate residual [Co(Cp*)2][Cl]. The mixture was then filtered over a pad of
celite and the solvent was then removed in vacuo yielding a dark orange/red solid (0.150 g,
86%). X-ray quality crystals were grown by slow evaporation of a benzene solution at room
temperature. 1H NMR (400 MHz, C6D6): δ 7.14 (s, 1H, C6H3), 7.12 (s, 1H, C6H3), 7.10 (s, 1H,
C6H3), 3.72 (d, 3JHH = 7 Hz, 2H, CH(CH3)2), 2.28 (d,
2JHH = 12 Hz, 2H, C6H10), 1.94 (s, 2H,
Dipp-NCCH2), 1.72-1.63 (m, 4H, C6H10), 1.58-1.47 (m, 4H, C6H10), 1.36 (s, 6H, Dipp-
NC(CH3)2), 1.33 (d, 3JHH = 7 Hz, 6H, CH(CH3)2), 1.30 (d,
3JHH = 7 Hz, 6H, CH(CH3)2), 0.13 (s,
18H, (CH3)3Si). 11
B{1H} NMR (96 MHz, C6D6): δ 83.7 (br s).
13C{
1H} NMR (101 MHz, C6D6,
Dipp-NCC-Cy not observed ): δ 151.8 (C6H3), 142.3 (C6H3), 128.6 (C6H3), 126.7 (C6H3), 124.1
(C6H3), 64.0 (Dipp-NC(CH3)2), 53.9 (C6H10), 47.0 (C6H10), 43.7 (C6H10 + Dipp-NCCH2), 28.0
(CH(CH3)2), 27.6 (Dipp-NC(CH3)2), 26.8 (CH(CH3)2), 26.5 (CH(CH3)2), 25.5 (C6H10), 25.4
(C6H10), 2.7 (s, (CH3)3Si). 29
Si{1H} NMR (99 MHz, C6D6): δ 7.2 (s, (CH3)3Si). Repeated
attempts to obtain EA were unseccussful. NMR spectra are included at the end of this section.
Synthesis of 6-3: A solution of 6-2 (0.100 g, 0.201 mmol) was dissolved in 2 mL pentane and
transferred to a tube bomb and sealed. The solution was degassed using three freeze-pump-thaw
cycles before being warmed to room temperature and charged with 1 atm. 13
CO. The mixture
was stirred at room temperature for 18 h yielding a pale red solution and pink solid. The volatiles
were removed in vacuo and a pink solid was obtained (0.085 g, 80%). X-ray quality crystals
were grown from pentane at -35 °C. 1H NMR (400 MHz, C6D6): δ 7.23 (m, 1H, C6H3), 7.12
(br s, 1H, C6H3), 7.10 (m, 1H, C6H3), 2.94 (d, 3JHH = 7 Hz, 2H, CH(CH3)2), 2.52 (m, 2H, C6H10),
1.77 (s, 2H, NCCH2), 1.69 (m, 5H, C6H10), 1.45 (d, 3JHH = 7 Hz, 6H, CH(CH3)2), 1.31 (m, 3H,
C6H10), 1.18 (d, 3JHH = 7 Hz, 6H, CH(CH3)2), 1.00 (s, 6H, Dipp-NC(CH3)2), 0.37 (s, 18H,
(CH3)3Si). 11
B{1H} NMR (96 MHz, C6D6): δ -3.4 (d,
1JBC = 87 Hz).
13C{
1H} NMR (101 MHz,
C6D6, Dipp-NCC-Cy not observed ): δ 236.3 (br m, B-CO), 148.9 (C6H3), 133.6 (C6H3), 129.8
(C6H3), 127.9 (C6H3), 125.6 (C6H3), 68.1 (Dipp-NC(CH3)2), 48.4 (C6H10), 37.4 (C6H10 +
Dipp-NCCH2), 30.2 (CH(CH3)2), 27.9 (Dipp-NC(CH3)2), 27.2 (CH(CH3)2), 25.4 (CH(CH3)2),
200
25.0 (C6H10), 22.9 (C6H10), 4.2 (s, (CH3)3Si). 29
Si{1H} NMR (99 MHz, C6D6): δ 3.01 (s,
(CH3)3Si). IR (solid) 1904 cm-1
(13
CO). Elemental Analysis for C30H53BN2OSi2: C, 68.67; H,
10.18; N, 5.34. Found: C, 68.25; H, 10.74; N, 5.45.
Synthesis of 6-5: A solution of 6-2 (0.075 g, 0.151 mmol) was dissolved in 5 mL toluene and
transferred to a tube bomb and sealed. The solution was degassed using three freeze-pump-thaw
cycles before being put under 4 atm. of hydrogen. The mixture was stirred at room temperature
for 24 h yielding a pale orange solution. The mixture was filtered over a plug of silica yielding a
yellow solution. The volatiles were removed in vacuo and the residue was dissolved in 2 mL of
pentane and left at -35 °C for 24 h. Pale yellow crystals were formed, the solvent was decanted
and crystals dried under high vacuum (0.064 g, 0.85%). X-ray quality crystals were grown from
pentane at -35 °C. 1H NMR (400 MHz, C6D6): δ 7.18 (m, 2H, C6H3), 7.11 (dd,
3JHH = 6 Hz,
4JHH = 3 Hz, 1H, C6H3), 5.55 (br s, 1H, Dipp-NCH-BH), 4.27 (d,
3JHH = 7 Hz, 1H, CH(CH3)2),
3.62 (d, 3JHH = 6 Hz, 1H, Dipp-NCH-BH) 3.48 (d,
3JHH = 7 Hz, 1H, CH(CH3)2), 2.41 (d,
2JHH =
12 Hz, 1H, C6H10), 2.09 (d, 2JHH = 13 Hz, 1H, Dipp-NCCH2), 1.91 (d,
2JHH = 12 Hz, 1H, C6H10),
1.82 (d, 2JHH = 13 Hz, 1H, Dipp-NCCH2), 1.76-1.65 (m, 4H, C6H10), 1.50-1.41 (m, 4H, C6H10),
1.39 (s, 3H, Dipp-NC(CH3)2), 1.36 (d, 3JHH = 7 Hz, 3H, CH(CH3)2), 1.35 (d,
3JHH = 7 Hz, 3H,
CH(CH3)2), 1.30 (d, 3JHH = 7 Hz, 3H, CH(CH3)2), 1.29 (d,
3JHH = 7 Hz, 3H, CH(CH3)2), 1.19 (s,
3H, Dipp-NC(CH3)2), 0.23 (s, 9H, (CH3)3Si), 0.20 (s, 9H, (CH3)3Si). 11
B{1H} NMR (96 MHz,
C6D6): δ 51.2 (br s). 13
C{1H} NMR (101 MHz, C6D6, Dipp-NCC-Cy not observed): δ 153.2
(C6H3), 150.8 (C6H3), 141.2 (C6H3), 126.7 (C6H3), 124.6 (C6H3), 124.4 (C6H3), 67.3 (Dipp-NCH-
BH), 63.7 (Dipp-NC(CH3)2), 52.7 (C6H10), 48.2 (C6H10), 41.1 (Dipp-NCCH2), 37.4 (C6H10), 30.7
(Dipp-NC(CH3)2), 28.4 (CH(CH3)2), 27.8 (CH(CH3)2), 27.0 (CH(CH3)2), 26.9 (C6H10), 26.7
(Dipp-NC(CH3)2), 26.2 (CH(CH3)2), 25.2 (CH(CH3)2), 25.1 (CH(CH3)2), 24.8 (C6H10), 23.7
(C6H10), 4.8 ((CH3)3Si), 4.3 ((CH3)3Si). 29
Si{1H} NMR (99 MHz, C6D6): δ 8.6 (s, (CH3)3Si), 5.7
(s, (CH3)3Si). Elemental Analysis for C29H55BN2Si2: C, 69.84; H, 11.12; N, 5.62. Found: C,
69.39; H, 11.50; N, 5.80.
201
Figure 6.4.1 1H NMR spectrum of 6-2 in C6D6.
Figure 6.4.2 11
B{1H} NMR spectrum of 6-2 in C6D6.
202
Figure 6.4.3 13
C{1H} NMR spectrum of 6-2 in C6D6.
Figure 6.4.4 29
Si{1H} NMR spectrum of 6-2 in C6D6.
203
6.4.3 X-ray Crystallography
6.4.3.1 X-ray Data Collection and Reduction
Crystals were coated in Paratone-N oil in the glove-box, mounted on a MiTegen Micromount
and placed under an N2 stream, thus maintaining a dry, O2-free environment for each crystal. The
data for crystals were collected on a Bruker Apex II diffractometer employing Mo Kα radiation
(λ = 0.71073 Å). The data were collected at 150(±2) K for all crystals. The frames were
integrated with the Bruker SAINT software package using a narrow-frame algorithm. Data were
corrected for absorption effects using the empirical multi-scan method (SADABS).46
6.4.3.2 X-ray Data Solution and Refinement
Non-hydrogen atomic scattering factors were taken from the literature tabulations.47
The heavy
atom positions were determined using direct methods employing the SHELXTL direct methods
routine. The remaining non-hydrogen atoms were located from successive difference Fourier
map calculations. The refinements were carried out by using full-matrix least squares techniques
on F, minimizing the function (Fo–Fc)2 where the weight is defined as 4Fo2/2 (Fo
2) and Fo
and Fc are the observed and calculated structure factor amplitudes, respectively. In the final
cycles of each refinement, all non-hydrogen atoms were assigned anisotropic temperature factors
in the absence of disorder or insufficient data. In the latter cases atoms were treated isotropically.
C-H atom positions were calculated and allowed to ride on the carbon to which they are bonded
assuming a C-H bond length of 0.95 Å. H-atom temperature factors were fixed at 1.10 times the
isotropic temperature factor of the C-atom to which they are bonded. The H-atom contributions
were calculated, but not refined. The locations of the largest peaks in the final difference Fourier
map calculation as well as the magnitude of the residual electron densities in each case were of
no chemical significance.
204
Table 6.4.1 Select crystallographic parameters for 6-1 to 6-5.
6-1 6-2 6-3 6-5
Formula C29H53BClN2Si2 C29H53BN2Si2 C30H53BN2OSi2 C29H55BN2Si2
Wt 532.17 496.72 524.73 498.74
Cryst. syst. Monoclinic Monoclinic Triclinic Monoclinic
Space group Cc P21/n P-1 P21/n
a(Å) 9.5806(7) 11.3825(12) 9.4580(8) 11.2220(8)
b(Å) 21.5850(19) 17.672(2) 11.4105(9) 17.4420(12)
c(Å) 15.5951(11) 15.5106(15) 16.9815(13) 15.9634(11)
(deg) 90.00 90.00 102.223(4) 90.00
(deg) 97.295(4) 92.331(6) 96.357(4) 95.365(2)
(deg) 90.00 90.00 114.021(4) 90.00
V(Å3) 3198.9(4) 3117.4(6) 1590.6(2) 3110.9(4)
Z 4 4 2 4
d(calc) gcm–3
1.105 1.058 1.096 1.065
R(int) 0.0255 0.1143 0.0635 0.0392
, mm–1
0.214 0.132 0.135 0.133
Total data 6923 5492 5595 7139
>2(FO2) 6421 3413 3616 5659
Variables 317 307 325 315
R (>2) 0.0446 0.0628 0.0593 0.0391
Rw 0.1111 0.1778 0.1492 0.1034
GOF 1.035 1.018 1.019 1.023
205
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208
Chapter 7 Summary
7.1 Summary
A new method of preparing ruthenium alkylidene complexes starting with bis-carbene RuHCl
species and alkenyl sulfides was developed. This new method is safe, high yielding, and uses
inexpensive starting materials. It also provides a route to bis-mixed carbene ruthenium
alkylidene complexes with a hemilabile tridentate carbene and conveniently installs both an
alkylidene fragment and a thiolate in one simple step. The use of ethyl vinyl sulfide, on the other
hand, resulted in the formation of Ru-alkyl and Ru-vinyl species.
The Ru-alkylidene complexes bearing the hemilabile tridentate NHC were either inactive or
minimally active for the standard metathesis tests. The species generated by the addition of one
equivalent of BCl3, however, showed improved activity for RCM, ROMP and CM either at room
temperature or at slightly elevated temperatures. In general, the catalysts which contain more
electron donating carbenes were more active and the catalysts with S(C6F5), as one of the anionic
ligands, were most active compared to the catalysts with the PhS- ligand. These species showed
activity in the cross metathesis of NBR and 1-hexene at different conditions. While they were
active, higher catalyst loadings and elevated temperatures were required to achieve similar
conversions as Grubbs II catalyst at room temperature.
Exchanging a chloride for an iodide resulted in enhanced metathesis activity for the standard
tests as well as for the CM of NBR with 1-hexene where catalytic olefin metathesis was observed
at room temperature. The bis-halide containing complexes showed the highest activity for all
standard metathesis tests without the need for a Lewis acid to initiate. This is presumably due to
the presence of the bulky iodides on the metal center which results in faster initiation. These
systems, however, too closely resemble Grubbs’ catalysts and were therefore not patented and
not tested further for the CM of NBR with 1-hexene.
Cyclic (alkylamino)carbenes were shown to stabilize iminoboryl moieties which have only been
previously stabilized in the coordination sphere of transition metals. These species were
characterized crystallographically and, depending on the halide size, directly formed through
elimination of Me3SiI, or through halide exchange for a non-coordinating anion after Me3SiX
209
elimination (X = Br, Cl). Some of the species were also shown to undergo [2+2] cycloaddition
with CO2.
CAACs were also used for the synthesis of a group-13 derivative, which is isoelectronic with
singlet carbenes, namely a borylene. This compound, which is stabilized by a push-pull effect,
was formed by the double reduction of a CAAC adduct of bis(trimethylsilylamino)
dichloroborane, going through a stable boron based radical. Similarly to singlet carbenes, it
reacted with carbon monoxide and hydrogen, but in contrast with the former, the latter acts as an
electrophile and therefore mimics the behavior of metals.