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The Chemistry of Carbene-Stabilized

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The Chemistry of Carbene-Stabilized THE CHEMISTRY OF CARBENE-STABILIZED MAIN GROUP DIATOMIC ALLOTROPES by MARIHAM ABRAHAM (Under the Direction of Gregory H. Robinson) ABSTRACT The syntheses and molecular structures of carbene-stabilized arsenic derivatives of 1 1 i 1 1 AsCl3 (L :AsCl3 (1); L : = :C{N(2,6- Pr2C6H3)CH}2), and As2 (L :As–As:L (2)), are presented herein. The potassium graphite reduction of 1 afforded the carbene-stabilized diarsenic complex, 2. Notably, compound 2 is the first Lewis base stabilized diatomic molecule of the Group 13–15 elements, in the formal oxidation state of zero, in the fourth period or lower of the Periodic Table. Compound 2 contains one As–As σ-bond and two lone pairs of electrons on each arsenic atom. In an effort to study the chemistry of the electron-rich compound 2, it was combined with an electron-deficient Lewis acid, GaCl3. The addition of two equivalents of GaCl3 to 2 resulted in one-electron oxidation of 2 to 1 1 •+ – •+ – give [L :As As:L ] [GaCl4] (6 [GaCl4] ). Conversely, the addition of four equivalents of GaCl3 to 2 resulted in two- electron oxidation of 2 to give 1 1 2+ – 2+ – •+ [L :As=As:L ] [GaCl4 ]2 (6 [GaCl4 ]2). Strikingly, 6 represents the first arsenic radical to be structurally characterized in the solid state. The research project also explored the reactivity of carbene-stabilized disilicon, (L1:Si=Si:L1 (7)), with borane. The reaction of 7 with BH3·THF afforded two unique compounds: one containing a parent silylene (:SiH2) unit (8), and another containing a three-membered silylene ring (9). Notably, 8 is the first example of a “push–pull” stabilized parent silylene, while 9 is the first example of a three-membered cyclic silylene, both of which are persistent under ambient conditions. INDEX WORDS: arsenic, N-heterocyclic carbene, diatomic, gallium, metal–metal bonds, main-group, oxidation, oxidation-state, “push-pull”, radical, reduction, silicon, silylene, sterically demanding ligands THE CHEMISTRY OF CARBENE-STABILIZED MAIN GROUP DIATOMIC ALLOTROPES by MARIHAM ABRAHAM B.S., Louisiana State University, 2008 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 2013 2013 Mariham Abraham All Rights Reserved THE CHEMISTRY OF CARBENE-STABILIZED MAIN GROUP DIATOMIC ALLOTROPES by MARIHAM ABRAHAM Major Professor: Gregory H. Robinson Committee: George F. Majetich Robert S. Phillips Electronic version approved: Maureen Grasso Dean of the Graduate School The University of Georgia August 2013 DEDICATION I dedicate my dissertation to my parents, Feleklek Mengistu and Yohannes Abraham, for their endless love, support and sacrifices they had to make to provide for my education. Their continued words of encouragement and faith in me have given me the strength to achieve this goal. My dedication extends to my brothers, Abraham Yohannes and Daniel Yohannes, and my sister, Hemen Yohannes, who have also been extremely supportive and my grandmother, Gorfenesh Abayere, whose love, encouragement, and prayers have always been my valuable assets. I also dedicate my dissertation to my uncle, Dr. Ashagre Yigletu, his wife, Menbere Netere, and their children, Agereneshe Ashagre and Yonas Ashagre, who provided me a loving home when I moved to the United States. They have been a constant source of support, advice, and motivation and have instilled in me the importance of hard work and higher education. I am forever thankful for their kindness and words of wisdom that have enabled me to achieve my goals. This dissertation is also lovingly dedicated in memory of my great grandparents, Azmara Akalu and Shambele Zeleke Gelaye, my grandparents, Amsale Yigletu and Abraham Hailu, and my uncle, Solomon Abraham. Despite being unable to see the completion of my degree, I have no doubt they are smiling down on me. I am forever grateful to Emaye Azmara and Emaye Amsale whose prayers, and support have strengthened me. I praise God for giving me such a wonderful family, and I thank them for all the love and care they have given me throughout my life. iv ACKNOWLEDGEMENTS First and foremost, I would like to thank God for directing my life. I thank him for all the people he has led me to meet that helped me in numerous ways. I have come this far in life due to his many blessings. I would like to sincerely thank my advisor, Dr. Gregory H. Robinson, for giving me the opportunity to be a part of his research team. His support, guidance, and encouragement during my moments of frustration have enabled me to accomplish my goal. I would also like to thank my committee members, Dr. George F. Majetich and Dr. Robert S. Philips, for their helpful advice and inputs. I am very grateful to Dr. Yuzhong Wang for his guidance in getting my graduate career started on the right foot, and for his much valued insight and motivation throughout my studies. I would also like to thank Dr. Pingrong Wei for his expertise in helping me obtain my X-ray crystallographic data. I would also like to thank my colleague, Robert Gilliard, for the enlightening discussions and helping me tirelessly with my writings. Our friendship has made my life in graduate school easier and memorable. I am very thankful for my friend, Dr. Magdelena Angelova, who constantly motivated me throughout my years at Louisiana State University and beyond. I would also like to thank Koustubh Dube, Vivian Ezeh, and Matthew Mendecino who eased my transition into graduate school with their friendship. A special thanks to Daniel Sexton, Jared Smith, Robert Heider, and Avery Wines for working with me on various projects. v I would like to thank my cousin, Hanna Yigletu, whose guidance ever since I moved to the United States was invaluable. I feel very fortunate to have her nearby not only in Louisiana but also in Georgia as I transitioned through various stages in my life. I thank her and her family for their love and support at all times. I would also like to thank my cousin, Saba McMillian, and her family who were also very supportive and Dr. Yaw Twum-Antwi for his care and encouragement throughout the years. I am very grateful to my aunt, Dr. Yeshihareg Abraham, and her family whose support, motivation, and prayers have always been uplifting. I would also like to thank my aunt, Sara Abraham, and her family who took time to visit me when I started my journey at The University of Georgia, and their valuable advice. I am also thankful to my aunts, Elizabeth Abraham and Nigist Fikru, and their families who always encouraged me throughout my studies. A special thanks to my cousins, Tizita Elias and Hanna Hailu, who shaped my attitude and view of life, and were great role models. I would also like to thank all the wonderful family in the Ethiopian Community Association of Baton Rouge for the memorable times I spent there. I am very thankful for Almaz Getachew, Getachew Ersido, Lemlem Getachew, Bealemlay Arega, Yibeltal Arega, and my God daughter, Sena Telila, for being a loving and supportive family. The outpouring love and kindness I received from Almaz and Yibeltal have always made me feel at home. Finally, I would like to thank a very special person in my life, Ashenafi Arega, for motivating me and being there for me and in all possible ways. His unwavering patience and advice have always put things back in perspective for me when I felt overwhelmed. I could not imagine going through this journey without him and I feel truly blessed to have him in my life. vi TABLE OF CONTENTS Page ACKNOWLEDGEMENTS……………………………………………………………….v LIST OF TABLES…………………………………………………………………….….ix LIST OF FIGURES…………………………………………………………………...….xi LIST OF SCHEMES……………………………………………………………………xiii CHAPTER 1 INTRODUCTION………………………………………………………………1 1.1 Purpose of Study…………………………………………...……….….1 1.2 Organometallic Chemistry……………………………………………..2 1.3 Synthetic Techniques of Organometallic Compounds……………........5 1.4 Low-Valent Main Group Element-Element Bonds…………...…….....6 1.5 Stable Carbenes……………………………………………………….12 1.6 Carbene-Stabilized Highly Reactive Molecules……………….….…..20 1.7 Conclusion…………………………………………………………….38 2 RESULTS AND DISCUSSIONS……………………………………………..39 2.1 Carbene-Stabilized Diarsenic………………………………..………..40 2.2 Oxidation of Carbene-Stabilized Diarsenic…………………………...49 2.3 Cleavage of Carbene-Stabilized Disilicon……………………............59 vii 3 CONCLUSION……………………………………………………….……….68 3.1 Concluding Remarks………………………………………………….68 4 EXPERIMENTAL………………………………………………………….…71 4.1 General Background………………………………………………..…71 4.2 Preparation of Starting Materials……………………………………..73 4.3 Syntheses of Carbene-Stabilized Diarsenic……………………….….74 4.4 Syntheses of Oxidized Diarsenic Derivatives………………………...75 4.5 Syntheses of “Push–Pull” Stabilized Silylenes…………………….....77 REFERENCES……………………………………………………………………...79 APPENDICES A CRYSTALLOGRAPHIC DATA…………………………………….…..94 B RESEARCH PUBLICATIONS…………………………………………138 viii LIST OF TABLES Table 1: Selected bond lengths [Å] and angles [°] of 1…………..…………….…...…..44 Table 2: Selected bond lengths [Å] and angles [°] of 2…….………………………..….46 Table 3: Selected bond lengths [Å] and angles [°] of 6•+………………………………..52 Table 4: Selected bond lengths [Å] and angles [°] of 62+……………………….……….56 Table 5: Selected bond lengths [Å] and angles [°] of 8 …………….……………..……63 Table 6: Selected bond lengths [Å] and angles [°] of 9.…………………….….…….…66 Table 7: Crystal data and structure refinement of 1….………………………………… 94 Table 8: Atomic coordinates (x 104) and equivalent isotropic displacement parameters (Å x 103) of 1……….………………………………………………………..…95
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