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Structures and Reactivity of Transition-Metal Compounds STRUCTURES AND REACTIVITY OF TRANSITION-METAL COMPOUNDS FEATURING METAL-LIGAND MULTIPLE BONDS A Dissertation by ZHENGGANG XU Submitted to the Office of Graduate and Professional Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Chair of Committee, Michael B. Hall Committee Members, Roland E. Allen Robert R. Lucchese Simon W. North Head of Department, David H. Russell August 2014 Major Subject: Chemistry Copyright 2014 Zhenggang Xu ABSTRACT This dissertation presents the results from density functional theory (DFT) calculations on three major projects I have been working on over the past several years. The first system is focused on the structure and reactivity of a novel osmium silylyne compounds featuring an Os≡Si triple bond. NMR simulation confirmed the existence of this compounds and bonding analysis like NBO and ETS-NOCV proved its triple bond character. The structures of the [2+2] cyclo-addition product from silylyne and other small molecules (PhC≡CPh and P≡CtBu) were also determined from possible isomers by energetic results and NMR simulations. The cycloaddition reactions were found to be under kinetic control and steric effects should be a major reason for that. Furthermore, the geometric and electronic structures of the osmium silylyne analogues (M≡E, M = Ru and Os; E = Si, Ge and Sn) are studied computationally and their similarities and distinctions are discussed. Both the second and third systems are related to the formation of transition metal imido compound (M=NR). In the second system a cationic oxorhenium(V) complex reacts with a series of arylazides (N3Ar) to give cationic cis-rhenium(VII) oxo imido complexes. Inductive effect is found in the reaction rates but it leads to different trends for electron-donating and electron-withdrawing substituted phenyl azides. Computations found all the substituted phenyl imido products are formed through the same pathway. The reason for the different trend is due to the competitions between two major energy barriers along the reaction pathway. For the electron-withdrawing substituents, N2 ii extrusion is rate determining, while for the electron-donating substituents, the rate- determining step becomes the initial attack of the azide. The barriers for these two steps are inverted in their order with respect to the Hammett σ values; thus, the Hammett plot appears with a break in slope. In the third system a series of vanadium (III) terminal organoazides were converted to vanadium (V) imido compounds and small quantities of diazenes were found formed in the reactions. Computational studies were conducted to examine serveral possible pathways and a mechanism in which nitrene radicals released from metal-azide decompositions is supported based on two reasons: 1) The two-electron reduction on vanadium center during decomposition of azide generates two fragments both in their ground states (triplet states), which might drag the activation barrier down; 2) This conversion is spin-allowed since vanadium center and nitrene nitrogen are antiferromagnetically coupled in transition state. The ligand simplification study indicates that the bulky mesityl groups on the supporting ligand prevents the reactions from going through the direct N2 extrusion pathway. iii DEDICATION I dedicate this dissertation to my beloved and ever-supportive parents. iv ACKNOWLEDGEMENTS I am most grateful to my advisor Dr. Michael B. Hall, who guided me along from the first day I started at Texas A&M University to today. Over the past several years he was always willing to discuss any ideas I have in mind; he answered every single question with great patience; he showed me how to be a real scientist with genuine curiosity; he respected every decision I made about my life… I enjoy working with him and learning from him. I will benefit from this experience throughout my whole life. I also would like to thank my committee members. Dr. Robert R. Lucchese taught me quantum mechanics and Dr. Simon W. North taught me chemical kinetics. Both courses were the foundations on which I could better understand and appreciate the literatures and the computational tools I used in my research. Although I was not in Dr. Roland H. Allen’s statistical mechanics class, I was impressed by his passion in running. I am looking forward to being in the same race with him again, and I hope I can be as energetic as him throughout my entire life. I owe a special thanks to Dr. Lisa Perez, who was always around to facilitate my research with professional and instant support. I am sure the academic life will be a lot tougher without the help from her. I also feel lucky to have Monica Gonzales as our administrative secretary. She made all the sophisticated paperwork so simple that I have never had any trouble on those issues. v I am greatly indebted to all the group members with whom I shared a great time: Amanda, Jason, Xinzheng, Caiping, Justin, Ivica, Eszter, Li, Ana, Scott, Stella, Jia, Haixia, Shengda, Sonia, Lukasz and Qian. They are so kind and always willing to help, and I wish every one of them has a wonderful life and career in the future. I also owe a special thanks to my roommates and the good friends I met in College Station. You made the past five years so enjoyable that I cannot believe I will be leaving so soon. I will always treasure the days we shared together, and I send my best wishes along with you. At last, I would like to thank one more person. You are the reason I strive. vi TABLE OF CONTENTS Page ABSTRACT .......................................................................................................................ii DEDICATION .................................................................................................................. iv ACKNOWLEDGEMENTS ............................................................................................... v TABLE OF CONTENTS .................................................................................................vii LIST OF FIGURES ........................................................................................................... ix LIST OF TABLES .......................................................................................................... xiv CHAPTER I INTRODUCTION ....................................................................................... 1 1.1 Why transition metal complexes? ............................................................................ 1 1.2 Early understanding of transition metal complex and Werner’s contributions ........ 2 1.3 How do we understand the nature of transition metal compounds? ........................ 3 1.4 Back donation from metal center to the ligand ........................................................ 8 1.5 Multiple bonds between transition metal and ligand: carbene ............................... 10 1.6 More on metal ligand multiple bonds .................................................................... 17 CHAPTER II THEORETICAL BACKGROUND ......................................................... 21 2.1 Quantum mechanics and the Schrödinger equation ............................................... 21 2.2 Born-Oppenheimer approximation ........................................................................ 23 2.3 The Hartree-Fock approximation ........................................................................... 24 2.4 Basis set approximation ......................................................................................... 29 2.5 Electron correlation ................................................................................................ 31 2.6 Density functional theory ....................................................................................... 33 2.7 Wavefunction analysis ........................................................................................... 40 CHAPTER III STRUCTURE AND REACTIVITY OF AN OSMIUM SILYLYNE COMPOUND ................................................................................................................... 43 3.1 Introduction ............................................................................................................ 43 3.2 Experimental observations ..................................................................................... 45 3.3 Computational details ............................................................................................. 47 3.4 Computational results and discussion .................................................................... 48 3.5 Conclusion .............................................................................................................. 73 vii CHAPTER IV COMPUTATIONAL PREDICTION OF INDUCTIVE EFFECTS THAT CAUSE A CHANGE IN THE RATE-DETERMINING STEP FOR THE CONVERSION OF RHENIUM AZIDES TO IMIDO COMPLEXES ........................... 75 4.1 Introduction ............................................................................................................ 75 4.2 Experimental observations ..................................................................................... 80 4.3 Computational details ............................................................................................. 82 4.4 Computational results and discussion .................................................................... 84 4.5 Conclusion .............................................................................................................. 98 CHAPTER V A NITRENE-RELEASE
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