Synthesis and Reactivity of Organometallic Complexes of Scandium and Titanium
Total Page：16
File Type：pdf, Size：1020Kb
University of Calgary PRISM: University of Calgary's Digital Repository Graduate Studies The Vault: Electronic Theses and Dissertations 2019-09-11 Synthesis and Reactivity of Organometallic Complexes of Scandium and Titanium Beh, Daniel Wei Ming Beh, D. W. M. (2019). Synthesis and Reactivity of Organometallic Complexes of Scandium and Titanium (Unpublished doctoral thesis). University of Calgary, Calgary, AB. http://hdl.handle.net/1880/110922 doctoral thesis University of Calgary graduate students retain copyright ownership and moral rights for their thesis. You may use this material in any way that is permitted by the Copyright Act or through licensing that has been assigned to the document. For uses that are not allowable under copyright legislation or licensing, you are required to seek permission. Downloaded from PRISM: https://prism.ucalgary.ca UNIVERSITY OF CALGARY Synthesis and Reactivity of Organometallic Complexes of Scandium and Titanium by Daniel Wei Ming Beh A THESIS SUBMITTED TO THE FACULTY OF GRADUATE STUDIES IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY GRADUATE PROGRAM IN CHEMISTRY CALGARY, ALBERTA SEPTEMBER, 2019 © Daniel Wei Ming Beh 2019 Abstract Rare-earth metal alkyl and hydrides are highly reactive species that can act as a metal precatalyst or reactive intermediate in a variety of homogeneous catalytic reactions such as hydroelementation and polymerization of olefins. Rare-earth metal hydrides play an important role for researchers to study sigma bond metathesis reactions and develop better homogeneous catalysts. However, these complexes tend to oligomerize in order to stabilize the highly polarizable, soft hydride at the Lewis acidic metal center. This thesis presents the synthesis various scandium complexes supported by the tetrapodal pentadentate B2Pz4Py ligand with the aim to stabilize these reactive intermediates and to isolate their monomeric species. The single site of reactivity at the apical position allows for controlled reactivity and a better understanding of these transformations. The scandium alkyl complexes were shown to be highly robust and do not undergo sigma bond metathesis to afford the hydride complex. Alternatively, the scandium hydride was synthesized from the reaction between a ligated scandium chloro complex and NaHBEt3 and was found to exist as a dimeric species. These complexes reacted with small molecules like H2O, N2O and CO2, and the mechanism for CO2 insertion was studied by DFT calculations. Cationic scandium complexes were also generated by alkyl or hydride abstraction with B(C6F5)3 and employed as a catalyst for hydrosilylation of CO2. Efforts to synthesize an anionic scandium terminal oxo complex showed that the complex is highly basic and immediately scavenges any acidic protons to form the corresponding scandium hydroxo complex which condenses to form the more thermodynamically stable scandium µ-O dimer. ii The analogous B2Pz4Py titanium complexes were also synthesized which showed some differences in the coordination chemistry. The dimeric titanium hydride complex is further stabilized by π stacking interactions which made it unreactive to N2O and CO2. Although the titanium µ-O dimer is once again a common by-product in the presence of adventitious moisture, it can be further oxidized by O2 to form other peroxo and oxo products. iii Acknowledgements First and foremost, I would like to thank my supervisor, Professor Warren Piers, for this invaluable experience and opportunity to work in such an amazing research group. His continuous guidance and support helped me to develop my personal and professional character, and I learned so much in the process. Many thanks to Prof. Roland Roesler and Prof. Gregory Welch for being part of my supervisory committee and providing helpful feedback over the years. I would also like to acknowledge Prof. Thomas Back and Prof. George Shimizu for participating in my Qualifying examination, and Prof. Stephen Larter for reading this thesis and taking part in my defense. Thank you to Prof. Ian Tonks for taking the time to review this thesis and serving as my external examiner. I would also like to extend my gratitude to the instrumentation staff at the University of Calgary who assisted in collecting various spectroscopic data: Michelle Forgeron, Michelle Thibault, Wade White and Johnson Li. Also thank you to our collaborators Drs. Laurent Maron and Iker del Rosal (Université de Toulouse) who helped with DFT calculations. A big thank you to the X-ray crystallographers Drs. Chris Gendy Denis Spasyuk, Benjamin Gelfand, Jian-Bin Li and David Bi who helped with obtaining crystal structures and putting up with the countless times I gave them crystals of µ-O dimers or free ligand. It was a great pleasure to work with many past and present members of the Piers group. They participated in helpful discussion and were always there to support one another along. They are a fun group of people to hang out with both in and out of the lab. iv My friends and family have continuously supported me throughout this journey ever since I started undergraduate studies abroad, and I will be forever grateful for all the opportunities that came along the way. Finally, I would like to thank the University of Calgary and the Department of Chemistry for financial support. v Table of Contents Abstract .............................................................................................................................. ii Acknowledgements .......................................................................................................... iv Table of Contents ............................................................................................................. vi List of Tables .................................................................................................................. viii List of Figures .....................................................................................................................x List of Schemes ............................................................................................................... xiv List of Symbols, Abbreviations and Nomenclature .................................................... xvi List of Numbered Compounds .................................................................................... xviii Chapter 1: Introduction ....................................................................................................1 1.1 Transition Metal Alkyl Complexes .......................................................................1 1.2 Transition Metal Hydride ......................................................................................6 1.3 Rare-earth M=E Complexes ..................................................................................9 1.4 Pentadentate Ligand Systems ..............................................................................16 1.4.1 Dianionic B2Pz4Py Ligand ................................................................................20 1.5 Thesis Goals .........................................................................................................22 Chapter 2: B2Pz4Py Supported Scandium Alkyl and Hydride Complexes ................24 2.1 Introduction ..........................................................................................................24 2.2 Synthesis of B2Pz4Py Scandium Chloro Complex ..............................................24 2.3 Synthesis of B2Pz4Py Scandium Alkyl Complexes .............................................28 2.4 Synthesis of B2Pz4Py Scandium Hydride Complex ............................................36 2.5 Reactivity Studies with Small Molecules ............................................................41 2.5.1 DFT Studies on CO2 Insertion...........................................................................45 2.6 Generation of Cationic Scandium Complexes .....................................................48 2.6.1 Contact Ion Pair Complex .................................................................................48 2.6.2 Scandium Formatoborate Complex ...................................................................51 2.6.3 Preliminary Studies on Hydrosilylation of CO2 ................................................59 2.7 Summary and Conclusions ..................................................................................66 Chapter 3: Scandium Oxo and Hydroxo Complexes ...................................................67 3.1 Introduction ..........................................................................................................67 3.2 Towards Scandium Anionic Terminal Oxo Complex .........................................70 3.3 Scandium Hydroxo Complexes ...........................................................................74 3.4 Summary and Conclusions ..................................................................................87 Chapter 4: Comparative Studies with Analogous B2Pz4Py Titanium Complexes .....88 4.1 Introduction ..........................................................................................................88 4.2 Synthesis of B2Pz4Py Titanium Chloro Complex ................................................90 4.3 Synthesis of B2Pz4Py Titanium Alkyl and Hydride Complexes .........................94 4.4 Generation of Cationic Titanium Complexes ....................................................105 4.4.1 Titanium Formatoborate Complex ..................................................................105