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A Dissertation Entitled Group V Imido Complexes Bearing Mono-Anionic A Dissertation entitled Group V Imido Complexes Bearing Mono-Anionic Acetophenone Imine Ligands by Abdollah Neshat Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Chemistry Dr. Joseph A. R. Schmidt, Committee Chair Dr. Mark R. Mason, Committee Memebr Dr. Viranga L. M. Tillekeratne, Committee Memebr Dr. Dean M. Giolando, Committee Memebr Dr. Patricia R. Komuniecki, Dean College of Graduate Studies The University of Toledo May 2011 An Abstract of Group V Imido Complexes Bearing Mono-Anionic Acetophenone Imine Ligands by Abdollah Neshat Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Chemistry The University of Toledo May 2011 Ortho-metalation through C-H bond activation has been widely used as a means to generate bidentate ligands that support metal complexes with catalytic activities in hydroamination and alkyne hydroalkoxylation reactions. For this reason, many ligand systems based on α,β-unsaturated ketones, esters and acetophenones have been the topic of intense research in recent years. With late transition metals, chelation of the acetophenone type ligands can be easily achieved through oxidative addition of the ortho C-H bond. However, due to the high oxidation states of early transition metals, this route cannot be utilized to successfully coordinate and synthesize analogous early transition metal complexes. As discussed in Chapter Two, to achieve site-specific activation, acetophenone imines were treated with nBuLi. The major goal of the research presented in this dissertation involved the synthesis and characterization of a series of niobium and tantalum imido complexes with these mono-anionic ortho-metalated acetophenone imine ligands. The products were characterized using NMR spectroscopy, mass spectrometry, and elemental analysis. iii These low symmetry complexes are produced with only one or two isomers in all cases and display interesting correlations between the steric bulk of the ligands employed and the isomers isolated. Crystal structures of several new niobium and tantalum complexes are presented as confirmation of the structural isomers produced. The reactions of the activated acetophenone imine ligands with several group V imido precursors demonstrate a few trends. For example, depending on the steric bulk of the imido group, more than one structural isomer was frequently generated and the ratio of these structural isomers varied depending upon the reaction temperature employed. In order to fully understand the steric and electronic effects of the acetophenone imine ligands on the structure and properties of the resulting metal complexes, more detailed studies were undertaken through the synthesis of a series of niobium and tantalum complexes by varying the size of the substituents on both imine and imido groups. Full details on this study are presented in Chapter Three. To delineate the reactivity of these new metal complexes and to obtain potential catalyst precursors, we also undertook an investigation of the substitution chemistry accessible by derivatization of the metal halogen bond. Each derivatized complex was produced as a single isomer and fully characterized using NMR spectroscopy, elemental analysis, and X-ray crystallography. The successful isolation and characterization of derivatization reaction products of the niobium imido complex L2NbCl(NAr) (L = ortho- i metalated acetophenone imine, Ar = 2,6- Pr2C6H3) with carbon and oxygen donor ligands such as Me, Me3SiCH2, PhCC, Me3SiCC, CF3SO3, and MeO are described. These more reactive -donor ligands were readily installed in place of the chloride ligand through salt metathesis reactions. The trimethylsilylmethyl complex showed significant iv -agostic interactions between the methylene group and the niobium center. Similar strategies to derivatize the chloride complex utilizing lithiated amides (LiNMe2, LiNEt2, i LiN Pr2, and LiNC5H10) resulted in the production of a niobium-hydride species due to - hydrogen elimination processes. All of the resulting C1 symmetric complexes were formed as predominantly a single isomer and fully characterized using a combination of 1H and 13C NMR spectroscopy, elemental analyses, and X-ray crystallography, when possible. v Acknowledgments Finally, it is time to close another important chapter in academic life before opening a new one and starting a new career. The past five years has been very challenging and rewarding times of my life. As it is hard to imagine the formation of any molecule without atoms (some sort of Chemistry rule, I guess), I can‟t imagine earning a PhD degree from UT without the support and guidance of the following people. I am greatly indebted to my research advisor Prof. Joseph Schmidt for being a passionate mentor and also a good role model to properly conduct research and teaching. I would like to thank my committee members Dr. Mark Mason, Dr. Dean Giolando, and Dr. Viranga Tillekeratne. Also, I would like to thank all the hard-working friends and lab-mates I met and had the pleasure to work with during the time I spent in the Schmidt group. I am especially indebted to Mr. John Beck, Mr. Andy Behrle, Dr. Andy Schaffer, and Dr. Glenn Kuchenbeizer for their assistance. Thanks to Mr. Matthias Hanauer for sharing good times during his short stay at UT. Thanks to Dr. Kim for his assistance with NMR spectroscopy and Dr. Kirschbaum for her guidance with both practical and theoretical aspects of X-ray crystallography. I also like to acknowledge Mr. Steve Moder for supplying and repairing our glasswares as well as supplying hot cups of coffee. And last, but not least, many thanks are due to my family members in Iran, Sheikhi family members in US, and my close friends Prof. Naz. M. Agh-Atabay and Prof. Cahide Agh-Atabay in Istanbul for their continual support. vi Table of Contents Abstract...............................................................................................................................iii Acknowledgements.............................................................................................................vi Table of Contents...............................................................................................................vii List of Tables....................................................................................................................x List of Figures.................................................................................................................xi List of Schemes................................................................................................................xiii 1. A Brief Introduction to Bidentate Ligands, Ortho-Metalation, Hydroamination Reactions, and Group V Metal Imido Complexes...............................................….1 1.1. Bidentate Ligands in Organometallic Chemistry………………………...…...1 1.2. Ortho-Metalation...............................................................................................4 1.3. Hydroamination.................................................................................................7 1.4. Hydroamination Catalysts Based on Early Transition Metals…………..…..12 1.5. The Importance of Group (V) Metal Imido Complexes…..……………...…19 1.6. The Aim and Outline of the Work Presented Herei.………………………...23 2. Mono-Anionic Acetophenone Imine Ligands: Synthesis, Ortho-Lithiation and Spectroscopic Studies…………….……………….....……........………………….25 2.1. Introduction.....................................................................................................25 2.2. Results and Discussion………………...…………………………………….28 2.3. Experimental….……………………………….…………………………….36 vii 2.3.1. Ligand Synthesis….............…………………………………………..36 2.4. Crystallography…...…………………………………………………………40 2.5. Conclusions…...…………………...…………………...........................……43 3. Synthesis and Characterization of Niobium and Tantalum Imido Complexes Bearing Mono-Anionic Acetophenone Imine Ligands…………..........…………...44 3.1. Introduction…...………………………………………………..……………44 3.2. Results and Discussion………….………...…………………………………47 3.2.1. Formation of Niobium and Tantalum Complexes………...………...47 3.3. The Chemistry of a Vanadium Phenyl-imido Complex with Ortho-lithiated Acetophenone Imine Ligands…................................................................………...61 3.4. Experimental….………………………………..…………….……................64 3.4.1. General Considerations……………..…………………..……..……..64 3.4.2. Metal Precursor Synthesis……………………………………...…….65 3.4.3. Metal Complex Synthesis………………………………..………..…..66 3.5. Crystallography….……………..………………………………………....….83 3.6. Conclusions………………….……………………………………..……..….90 4. Derivatization of Niobium Complexes Bearing Imido and Acetophenone Imine Ligands............................................................................................………………..91 4.1. Introduction…………………………................……...…..……...…………..92 4.2. Results and Discussion……………………..……......................................…94 viii 4.3. A Cationic Niobium Imido Complex.............................................................107 4.4. Experimental..................................................................................................110 4.5. Crystallography..............................................................................................124 4.6. Conclusions....................................................................................................129 References........................................................................................................................130
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