Investigations on Low-Valent Group 8 and 9
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INVESTIGATIONS ON LOW-VALENT GROUP 8 AND 9 METALLORADICALS Thesis by Ayumi Takaoka In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy California Institute of Technology Pasadena, CA 2012 (Defended January 17, 2012) ii © 2012 Ayumi Takaoka All Rights Reserved iii Acknowledgments I would like to first and foremost thank my research advisor, Professor Jonas C. Peters, for taking me into his group in 2007 and advising me throughout my graduate career. In addition to being a truly brilliant principle investigator, Jonas has always been incredibly patient in teaching me the fundamental process of research, and I am extremely grateful for this. As previous graduates of this research group have also stated, much of my scientific thought process operates according to how I believe Jonas would proceed in solving difficult problems. Indeed, I believe that I have been able to conduct research most efficiently by thinking about the types of questions Jonas would ask during our group meetings. Simply put, I could not have asked for a better advisor. Jonas also gave me the opportunity to pursue my graduate studies at two of the best research institutions in the world, for which I feel very fortunate. This unique experience gave me a chance to work at two very distinct environments and provided me with the confidence in taking an industrial position in Japan, which is a place where I have not lived for over 13 years and expect to be quite different from both MIT and Caltech. Professors Dick Schrock and Theo Agapie were the chairs of my thesis committee at MIT and Caltech, respectively. Dick was always supportive of the chemistry I was pursuing at MIT, and was especially excited about the dinitrogen complexes that I was synthesizing at the time. He was always available for scientific discussion, and he even spent time with me for our annual thesis chair meeting before our group left for Caltech, even through he no longer had any obligations to do so. Theo was also very supportive of my research at Caltech and was a great thesis chair to have. His criticism was extremely constructive, and his assessment of my research was always positive, regardless of iv content. Many of the questions that he raised during our meetings helped me think more deeply about my research. Theo also happens to be married to one of my very good friends, Smaranda, and was kind enough to invite me to their wedding. I wish the best to both Theo and Smaranda in the future. Other members of my two committees, Professors Steve Lippard at MIT and Harry Gray and Mitchio Okumura at Caltech, were also instrumental in providing me with great advice through the years. I owe a tremendous amount to my classmates, Samantha MacMillan, Seth Mickenberg, Daniel Suess, and Charlene Tsay, who joined the Peters group with me in 2007. Each and every member of this group has been a great friend and has helped me keep my sanity through tough times. I will greatly miss them. I have also been fortunate to overlap with a large number of brilliant co-workers. I would like to name a few in detail that have particularly been helpful. Aside from Jonas, Dr. Neal Mankad probably influenced my approach to research the most, and I am honored to coauthor a publication with him. He also introduced me to fantasy football, which has been one of my favorite pastimes outside of research. Dr. Arjun Mendiratta and Laura Gerber also coauthored publications with me. Professor Yunho Lee was an incredible boxmate who taught me many experimental techniques. Dr. Marc-Etienne Moret was one of my best sources of chemical knowledge, and was also a great lunch buddy. Kenny Lotito and Jon Rittle were my other boxmates who had to endure my presence while conducting research inside the glovebox. Dr. Hill Harman, Alicia Chang, and Henry Fong are friends from my undergraduate research group, and I will miss not seeing them everyday. John Anderson was my fantasy football rival and was kind enough to invite me to his residence for multiple football games. I would also like to thank my other co-workers over the years, v Professor Nate Sczymczak, Professor Louise Berben, Dr. Ning Jin, Dr. Caroline Sauoma, Dr. Chris Uyeda, Dr. Charles McCrory, and Sid Creutz. I could not have performed my research without the competence of the technical staff at both MIT and Caltech. Dr. Peter Müller and Larry Henling were the X-ray crystallographers that helped me at the two institutions, respectively. Dr. Jeff Simpson, Anne Gorham, and Dr. David Vandervelde were staff at the NMR facilities and were always helpful in setting up NMR experiments that were beyond my understanding. Dr. Angelo DiBilio taught me how to measure an EPR spectrum. I have been very fortunate to be surrounded by a great group of friends outside the Peters group at both MIT and Caltech. Julia Kozhukh and Shuang Liu were my friends at MIT, and I wish the best for both of them in their future. Rachel Klet was a person I first met at the Caltech visiting weekend, and she introduced me to numerous delicious cuisines throughout this area. Hidehiko Inagaki, Akihisa Goban, and Taisuke Imai are members of the Japanese community at Caltech whom I have had a great time with. In addition to these people, countless others have been kind and helpful to me. I would also like to thank the entire MIT and Caltech community. Both institutions were incredible research environments. In fact, I had such a nice time at MIT that I was somewhat hesitant to move from Cambridge to Pasadena because I thought it could only get worse. I am fortunate that I was wrong and that the Caltech community as a whole has been extremely welcoming and friendly. Finally, I could not have accomplished anything that I have these past few years without the support of my parents, Jun and Tomoko Takaoka, and my sister, Haruka vi Takaoka. I look forward to living closer to my family in the coming years. Last but not least, I am indebted to Hazuki Nakatani for providing moral support over the years. The past few years have been absolutely incredible. This experience will probably be one of the biggest highlights of my life. vii Abstract Tetradentate, monoanionic, tris(phosphino)silyl ligands were chelated to group 8 and 9 transition metals to stabilize complexes with unusual oxidation states and/or Ph − geometries. Initial studies with the [SiP 3] ligand on ruthenium established the flexibility of this ancillary ligand in stabilizing complexes with strongly trans influencing iPr − ligands in trans dispositions. A related ligand scaffold, [SiP 3] , was subsequently used to stabilize mononuclear complexes of Ru(I) and Os(I), the first examples to be isolated and thoroughly chracterized. EPR spectroscopy and DFT calculations supported their metalloradical character, and further studies highlighted their reactivity in both one- and iPr − 7 two-electron redox processes. The ability of the [SiP 3] scaffold to stabilize d metalloradicals of group 8 metals was extended to group 9 metals, and a series of d7 complexes of cobalt, rhodium, and iridium were synthesized in which their ancillary ligands, oxidation states, spin states, and geometry are conserved. Similar to the iPr iPr previously reported [SiP 3]Fe(N2) complex, the related [SiP 3]Ru(N2) complex was shown to exhibit N−N coupling of organic azides to yield azoarenes catalytically. Detailed mechanistic studies conclusively showed that the Ru(III) imide species, whose iPr iron analog is the key intermediate in the [SiP 3]Fe system, is not involved in the iPr mechanism for the [SiP 3]Ru system. Instead, a mechanism in which free nitrene is released during the catalytic cyle is favored. Finally, hybrid ligands with multiple R − thioether donors in place of phosphine donors on the [SiP 3] scaffold were synthesized to stabilize a number of dinitrogen complex of iron. These complexes featured rare examples of S−Fe−N2 linkages. viii Table of Contents Acknowledgments ......................................................................................................................... iii Abstract .................................................................................................................................. vii Table of Contents ........................................................................................................................ viii List of Figures ............................................................................................................................... xii List of Tables .............................................................................................................................. xvii List of Abbreviations .................................................................................................................. xix Chapter 1: Introduction ................................................................................................................ 1 1.1 Opening Remarks....................................................................................................................... 2 1.2 The Tris(phosphino)silyl Ligand ............................................................................................... 5 1.3 d7 Complexes of Group 8 and 9 ................................................................................................. 6 1.4 Reactivity of a Ru(I) Metalloradical .......................................................................................... 7 1.5 Hybrid Phosphine/Thioether Ligands .......................................................................................