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DIAZABOROLE-DERIVED CARBENE COMPLEXES of BORON by HUNTER PATRICK HICK

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DIAZABOROLE-DERIVED CARBENE COMPLEXES of BORON by HUNTER PATRICK HICK SYNTHESIS OF NOVEL CARBENE-STABILIZED SILYLENES AND 1,3,2- DIAZABOROLE-DERIVED CARBENE COMPLEXES OF BORON by HUNTER PATRICK HICKOX (Under the Direction of Gregory H. Robinson) ABSTRACT The investigation of novel silylene (:SiR2) species is presented herein. The reaction of carbene-stabilized disilicon(0) with one equivalent of Fe(CO)5 at room temperature yields the carbene-stabilized Si2Fe(CO)4 complex (72). Further reaction of 72 with an additional equivalent of Fe(CO)5 at raised temperatures affords the carbene-stabilized silylene-iron carbonyl cluster, Si[μ-Fe2(CO)6](μ-CO)Si (73), through the insertion of a CO and a Fe2(CO)6 unit into the Si=Si double bond. Each silylene center in 73 is covalently bonded to one Fe(CO)3 center, and datively bonded to the other Fe(CO)3 using the silicon-based electron lone pair. Notably, compound 73 represents the first example of direct cleavage of a Si=Si double bond by a transition metal species. Subsequently, the reactivity of complex 72 was studied. The reaction of 72 with HCl·NC5H5 yields a “push-pull” stabilized parent monochlorosilylene [:Si(H)Cl] (78), with addition of an HCl unit to each silicon atom, and both silicon-based electron lone pairs coordinating a central Fe(CO)3. Importantly, compound 78 is the first stabilized parent monochlorosilylene isolated at ambient conditions. In addition, the reaction of carbene-stabilized diiodo-bis-silylene with an imidazole- based thiolate ligand in toluene or THF gives a five-membered (82) and four-membered (83) cyclic silylene via unexpected C–H and C–N bond activation, respectively. Compounds 82 and 83 represent the first cyclic silylenes containing a silicon-silicon bond, and compound 83 is the first example of silicon(I)-mediated C–N bond cleavage of N-heterocyclic carbenes (NHCs). Recently, the synthesis of 1,2-azaborole-derived cyclic (alkyl)(amino)carbene (CAAC)-borane adducts were reported, via a 1,2-hydrogen migration. In an effort to extend this chemistry to the diazocyclo- borole system, we reported the synthesis of 1,3,2-diazaborole-derived NHC-boron halide complexes, via a 1,2-hydrogen migration. A reaction of 2-bromo-1,3,2-diazaborole with excess 1 BX3 (X = Br, I) affords the BBr3 (87) and BI3 (88) complexes in quantitative yield (by H NMR). Interestingly, both 87 and 88 are in equilibrium in solution, likely due to the weak electron- donating properties of the 1,3,2-diazaborole-derived carbene. However, the equilibrium of 87 favors the formation of the reactants, while the equilibrium of 88 favors the formation of the products, which could be ascribed to the relative Lewis acidity of BBr3 when compared to BI3. INDEX WORDS: silylene, N-heterocyclic carbene, disilicon, silicon, “push-pull”, main group, iron carbonyl, monochlorosilylene, bond cleavage, cyclic, boron, borole SYNTHESIS OF NOVEL CARBENE-STABILIZED SILYLENES AND 1,3,2- DIAZABOROLE-DERIVED CARBENE COMPLEXES OF BORON by HUNTER PATRICK HICKOX B.S., Auburn University, 2013 A Dissertation Submitted to the Graduate Faculty of The University of Georgia in Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY ATHENS, GEORGIA 2018 © 2018 Hunter Patrick Hickox All Rights Reserved SYNTHESIS OF NOVEL CARBENE-STABILIZED SILYLENES AND 1,3,2- DIAZABOROLE-DERIVED CARBENE COMPLEXES OF BORON by HUNTER PATRICK HICKOX Major Professor: Gregory H. Robinson Committee: Tina T. Salguero Henry F. Schaefer, III Electronic Version Approved: Suzanne Barbour Dean of the Graduate School The University of Georgia May 2018 DEDICATION I dedicate this dissertation to my parents, Patrick and Heather Hickox, for their constant love and support and I thank them for helping me to become the person I am today. I would not have made it half this far without them. iv ACKNOWLEDGEMENTS First, I would like to thank my advisor, Dr. Gregory H. Robinson. His mentorship has allowed me the opportunity to become a successful scientist, and his consistent standards have taught me the importance of high-quality work and attention to detail. I would also like to thank my committee members, Dr. Tina T. Salguero, and Dr. Henry F. Schaefer, III, for guiding me throughout my time in graduate school. Thanks especially to Dr. Yuzhong Wang; his instruction in the laboratory has been invaluable, and I would not have enjoyed the success I have had without his support and guidance. Dr. Pingrong Wei has provided his vital X-ray crystallography experience, and Dr. Dongtao Cui has assisted with numerous NMR samples, and these projects could not have been done without their help. Thank you also to the undergrads that I have worked with during my time at UGA, Katie Luedecke, Deidrah Carrillo, and Nathan Dominique; I am grateful for all your help, and I know you all will go on to do great things. I would also like to thank Dr. Melinda Oliver, who showed me that chemistry is exciting and worth pursuing. I would not be where I am without Dr. Oliver’s friendship in my early academic years. I would, of course, like to thank all my family and friends for helping me throughout my academic career. Thanks to Hayley Broussard, whose love and support was a rock for me to hang on to when things were hard. Thanks to my parents, Heather and Patrick, to whom this dissertation is dedicated. You both taught me so many different lessons that helped me through graduate school, and you’re the reason that I am successful. Dad, you showed me the value of hard work and dedication and are an example of what kind of person I want to be. Mom, your love and compassion has helped to keep me grounded, and I’m so thankful that you have always been there v for me. I would be remiss if I did not mention my grandparents. I would like to thank the late Skeet Hickox, who was the hardest working man I have ever met and showed me what can be accomplished when you don’t quit. Thanks to the late Ruby Hickox, whose perseverance and grit was only matched by her exceeding kindness. I also want to say thank you to the late Bill Meeks, who’s quiet intelligence taught me that listening is oftentimes more valuable than speaking. Of course, I would like to thank Sadie Meeks for her continuing love, and who has shown me that even though you get older, you never have to grow up. vi TABLE OF CONTENTS Page ACKNOWLEDGEMENTS .............................................................................................................v LIST OF TABLES ......................................................................................................................... ix LIST OF FIGURES ...................................................................................................................... xii LIST OF SCHEMES.................................................................................................................... xiv CHAPTER 1 INTRODUCTION .........................................................................................................1 1.1 Purpose of Study ................................................................................................1 1.2 Organosilicon Chemistry ...................................................................................2 1.3 Stable Silylenes ................................................................................................10 1.4 Stable Carbenes ................................................................................................29 1.5 Conclusion .......................................................................................................36 2 RESULTS AND DISCUSSIONS ................................................................................39 2.1 Transition-Metal-Mediated Cleavage of a Si=Si Double Bond .......................39 2.2 Push-Pull Stabilization of a Parent Monochlorosilylene .................................48 2.3 Facile Conversion of Bis-Silylene to Cyclic Silylene Isomers: Unexpected C–N and C–H Bond Cleavage .........................................................................54 2.4 1,3,2-Diazaborole-Derived Carbene Complexes of Boron ..............................64 3 CONCLUSION ............................................................................................................73 3.1 Concluding Remarks ........................................................................................73 vii 4 EXPERIMENTAL .......................................................................................................76 4.1 General Background ........................................................................................76 4.2 Preparation of Starting Materials .....................................................................77 4.3 Synthesis of Fe(CO)4-Modified Carbene-Stabilized Disilicon(0) Derivatives ................................................................................................................................80 4.4 Synthesis of the Parent Monochlorosilylene [:Si(H)Cl] ..................................82 4.5 Synthesis of Cyclic Silylenes Containing a Silicon-Silicon Bond...................83 4.6 Synthesis of 1,3,2-Diazaborole-Derived Carbene Complexes of Boron .........85 REFERENCES ..............................................................................................................................87 APPENDICES A CRYSTALLOGRAPHIC AND COMPUTATIONAL DATA .................................103 B RESEARCH PUBLICATIONS .................................................................................171 viii LIST OF TABLES Page Table 1: Coordinates of the B3LYP/6-311+G** geometry of 72-Me
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