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Synthesis, Reactivity, and Coordination Chemistry

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Synthesis, Reactivity, and Coordination Chemistry SYNTHESIS, REACTIVITY, AND COORDINATION CHEMISTRY RELEVANT TO THE COPOLYMERIZATION OF CO2 AND EPOXIDES BY FIRST ROW TRANSITION METAL SCHIFF BASE COMPLEXES A Dissertation by ERIC BENJAMIN FRANTZ Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY August 2008 Major Subject: Chemistry SYNTHESIS, REACTIVITY, AND COORDINATION CHEMISTRY RELEVANT TO THE COPOLYMERIZATION OF CO2 AND EPOXIDES BY FIRST ROW TRANSITION METAL SCHIFF BASE COMPLEXES A Dissertation by ERIC BENJAMIN FRANTZ Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Approved by: Chair of Committee, Donald J. Darensbourg Committee Members, Abraham Clearfield Edward D. Harris Francois P. Gabbai Head of Department, David H. Russell August 2008 Major Subject: Chemistry iii ABSTRACT Synthesis, Reactivity, and Coordination Chemistry Relevant to the Copolymerization of CO2 and Epoxides by First Row Transition Metal Schiff Base Complexes. (August 2008) Eric Benjamin Frantz, B.S., University of Houston Chair of Advisory Committee: Dr. Donald J. Darensbourg Excepting agricultural based products, which themselves require a great deal of energy to produce, our supply of natural resources such as minerals, metal ore, fresh water, coal, oil and natural gas are all limited in supply. The depletion of these substances is imminent and this knowledge weighs heavily on humankind. The utilization of CO2 for the production of polycarbonates is one attempt at exploiting a profoundly abundant and renewable resource. The importance of research in this and similar fields justifies the detailed study of the chemicals and procedures involved with this chemistry. This current work concentrates on the fundamental study of transition metal Schiff base complexes that have shown a great deal of promise in their ability to catalyze the copolymerization of CO2 and epoxide to form aliphatic polycarbonates. A new chromium(III) Schiff base complex has been synthesized and evaluated for its ability to catalyze the formation of polymer. The ligand employed bears an N2O2 coordination sphere identical to the widely utilized chromium(III) and cobalt(III) salen catalysts. This complex was shown to be active towards the copolymerization of CO2 iv and cyclohexene oxide. Although the activity was less than that seen with chromium(III) salen complex, the study demonstrates that new ligand systems are available beyond salen and deserve further attention. A class of manganese(III) Schiff base complexes was also synthesized and evaluated as catalysts. Although crystallographic data has shown that these complexes are structural analogs to chromium(III) salens, the difference in metal center leads to a nearly complete elimination of catalytic activity. Such a marked difference has been taken advantage of by using this very low activity to study the ring-opening of epoxide in the initial step of the copolymerization both mechanistically and kinetically. It has also been utilized in an evaluation of the coordination chemistry of the polymerization process. This has led to some valuable conclusions about the nature and role of the metal center that previously have not been studied. Manganese(III) salen complexes were also synthesized and evaluated in an effort to compare these important ligands to other Schiff bases and confirm the findings mentioned above. v DEDICATION I dedicate this dissertation to the two women who have and always will be the most important people in my life. My sister has always led the way for me, be it in art, music, education, profession or life. She has set the standard for me and at all times has never hesitated to share what she knows and to treat me as a person of value and worth, even when so few others have. My mother has always been the foundation of my life. She has taught me that the ability to think independently is the greatest and most important gift an individual has. She has also taught me that a Godly life is the only life worth living. Central to her teaching is the knowledge that our stay on earth is short and only fools spend their time like lottery winnings, easy come, easy go, and that an hour watching meaningless TV is like rebuffing God and his gift of thought and breath and love. vi ACKNOWLEDGEMENTS Since my diagnosis in the 4th grade my life has been dominated by one word; dyslexia. I have eaten, drank and slept with this malodorous word foremost in my mind for too long. It has embarrassed me a countless number of times, given me mistakes where there shouldn’t have been any and made me confused and frightened where I shouldn’t have been and it, most of all, has kept me away from associating with other talented and intelligent people that should have been my companions all my life. And now, as I leave graduate school some eight years older than my cohorts, roughened, aged and grizzled from my long war with this condition I can finally say, “Veni, Vidi, Vici”. In truth, I struggle on, just as I always will, and without the help and patience of some very important people I would not have been able to complete my graduate education. First and foremost, I must thank my advisor and committee chair Professor Donald J. Darensbourg. He has supported me both as a teacher and as an employer for five long years and I hope has been rewarded for the unreasonable amount of time and patience he has given me. I’d like to thank Professors Marcetta Y. Darensbourg, Abraham Clearfield, Edward D. Harris, Francois P. Gabbai, and Steven A. Miller for also being involved in my education, attending my presentations and serving on my committee. My thanks also goes to Dr. Joseph H. Reibenspies and Dr. Nattamai Bhuvanesh (Bhuv) for their enduring patience with the 13 crystal structures I present in this work and the countless ones that never quite made it out of the refinement, data collection or crystal mounting stages of fruition. And of course, my thanks must go out vii to my colleague and friend Susan Winters. It would take a plate full of cookies who knows how many stories high to express my gratitude for the help she has given me on the countless number of annoying and should-be-obvious issues I have had. Now I must embark on thanking those comrades who have stood with me side- by-side though my time here at Texas A&M. Firstly, I’d like thank those Darensbourgers who came before and mentored me, namely, Drs. Rosario Mejia- Rodriquez, Melissa L. Golden, Joey Chiang, Jesse Tye, Marilyn Rampersad, Stephen Jeffery, Jody Rodgers, Ryan Mackiewicz, Damon Billodeaux, and Andrea Phelps. I want to give special thanks to the two DJD students who later preceded me, my good friends Poulomi Ganguly and Wonsook Choi. They not only paved the way immediately in front of me, but encouraged me and instilled in me an appreciation of scientists from other nations. Next, I thank the current DJD and MYD group members Mike Singleton, Roxanne Jenkins, Elky Almaraz, William Foley, Jen Hess, Dr. Kayla Green, Dr. Christine Thomas, Tianbiao "Leo" Liu, Scott Brothers, Adriana Moncada, Osit “Pop” Karroonnirun, Ross Poland, Shawn Fitch, and Jeremy Andreatta. Particular thanks goes to Adriana, Jeremy and Shawn for listening to my ramblings about chemistry and helping me with my many presentations, papers and other materials. I honesly do proff red my writting beforhand, beleave it or knot. Also, I want to express my gratitude to Scott and Jen for joining the group, being my friends and making life as a Darensbourger a heck of a lot more fun. And lastly, I want to thank two exceptionally talented and hard working women who have aided me immensely over the time I’ve known them. Chris, you have helped guide me over the last two years and acted both as viii a friend and deputy advisor at times. Kayla, you are a great listener, colleague and friend (and cook). Both of you will kick butt in your field and I look forward to hearing of your many successes. ix TABLE OF CONTENTS Page ABSTRACT.............................................................................................................. iii DEDICATION .......................................................................................................... v ACKNOWLEDGEMENTS ...................................................................................... vi TABLE OF CONTENTS.......................................................................................... ix LIST OF FIGURES................................................................................................... xi LIST OF TABLES .................................................................................................... xiv CHAPTER I INTRODUCTION................................................................................ 1 Statement About Green Chemistry ................................................ 1 Commercial Utilization of Polycarbonates .................................... 2 Synthesis of Polycarbonate from CO2 and Epoxide....................... 6 Salen-Based Catalytic Systems ...................................................... 8 Application of Salen Complexes Towards Polycarbonate Synthesis......................................................................................... 11 Mechanism of the Copolymerization Process................................ 16 II THE COPOLYMERIZATION OF CYCLOHEXENE OXIDE AND CARBON DIOXIDE CATALYZED BY A CHROMIUM ACACEN COMPLEX..........................................................................................
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