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Boron-Bridged Constrained Geometry Complexes

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Boron-Bridged Constrained Geometry Complexes Boron-bridged constrained geometry complexes and related compounds A Thesis presented by Frank Michael Breitling In partial fulfilment of the requirements for the award of Doctor of Philosophy of University of London and Diploma of Imperial College London August 2005 Department of Chemistry Imperial College London - 1 - - 2 - For Spyridoula - 3 - Acknowledgements I would like to thank my supervisor, Prof. Dr Holger Braunschweig, for giving me the opportunity to work in his group, for providing me with a very interesting project, for his encouragement and for giving me freedom in the way I carried out my research. My thanks also go to my second supervisor, Prof. Dr Tom Welton, for giving me all the support I needed after the move of my initial group, and to his group for ‘adopting’ me. Dr Christian Burschka, Dr Krzysztof Radacki, Dr David Scheschkewitz, Fabian Seeler, Dr Andrew J. P. White and Prof. Dr David J. Williams are thanked for performing X-ray diffraction experiments of my compounds. Dr Rüdiger Bertermann, Dr Carsten Kollann, Marie-Luise Schäfer, Dick Sheppard and Peter Haycock are acknowledged for performing countless NMR experiments on my behalf. I also have to thank John Barton, Dr Stephan Wagner and Dr Justin Wolf for conduct various MS and GC-MS analyses. In addition, I want to thank Richhilde Schedl for carrying out thermogravimetric measurements and Dr Steve Holding of RAPRA Technology for performing numerous GPC polymer analyses. The Fonds der Chemischen Industrie (FCI) is thanked for financial support of my PhD studies by means of a Kekulé scholarship. I am particularly grateful to Dr Mario Kraft who generously paved my way both before my arrival in London and Würzburg, for sharing the good and bad times of a PhD studentship and his friendship. I also want to specially acknowledge Sascha Stellwag for his relentless help in lab 204 and Carina Grimmer and Viktor Weber for performing polymerisation experiments. Many thanks go to Dr Melanie Homberger, for lots of helpful discussions particularly during the early stages of my project. Dr Guy Clentsmith and Dr George Whittell are also gratefully acknowledged for their very thorough and patient proof-reading of this manuscript. Furthermore, I wish to thank all my past and present colleagues in the group, André, Carina, Carsten, Daniela, David, Emanuel, Fabian, George, Giovanni, Guy, Justin B., Justin W., Katharina, Krzysztof, Mario, Martina, Matthias, Melanie H., - 4 - Melanie L., Natalia, Nele, Stefan, Thomas and Viktor, for their help and assistance when needed and just a great time. Finally, I want to thank my parents and family for their continued support during my education. And a special thanks goes to Spyridoula Ntella for her loving support and care throughout my PhD studies, especially during the writing up of my thesis that was testing her patience much more than I ever envisaged. - 5 - The work described in this Thesis was carried out in part at Imperial College, Department of Chemistry, University of London, from September 2001 to September 2003, under the supervision of Prof. Holger Braunschweig and Prof. Tom Welton, and in part at the Institut für Anorganische Chemie, Bayerische Julius-Maximilians- Universität Würzburg, Germany, from October 2003 to February 2005, under the supervision of Prof. Holger Braunschweig. The research described in this Thesis is original, unless otherwise stated and to my knowledge it has not been submitted previously for a degree at this or any other University. Frank Michael Breitling August 2005 - 6 - Abstract Boron-bridged constrained geometry complexes and related compounds Group 3 and 4 complexes bearing linked cyclopentadienyl amido ligands, often referred to as constrained geometry complexes (CGCs), have experienced considerable interest due to their superior ability to copolymerise ethylene and higher α-olefins when activated with suitable co-catalyst. The work presented in this thesis aimed to replace the most commonly applied bridge in CGCs, which is silicon based, by one containing boron. The potential of the bridging element to have Lewis acidic character was expected to positively alter the catalytic activity of the activated species and possibly allowing for self-activation. Synthetic approaches to ligand precursors based on aminoboranes, diaminodiboranes(4) and ferrocenylboranes are described. Starting from the dihalo derivatives of these boranes, sequential substitution of the halides by one equivalent each of a cyclopentadienide derivative and an amide allowed the synthesis and isolation of a broad range of new CGC ligand precursors. Complexation of these ligand precursors to Group 4 metals was studied by utilising various protocols. The reaction with Group 4 tetraamides via amine elimination was the most successful yielding numerous new boron-bridged CGCs and related complexes in which the boron-bridged ligand binds in a non-chelating fashion. The newly synthesised compounds were fully characterised by multinuclear NMR spectroscopy, supplemented by X-ray diffraction studies where applicable. Studies on the reactivity of boron-bridged CGCs in the presence of alkylating agents indicated susceptibility of the boron atom to nucleophilic attack resulting in a decomposition of the linking moiety between the cyclopentadienyl and amido - 7 - fragments. This is as well reflected in the data gathered from polymerisation experiments, in which methylaluminoxane activated boron-bridged CGCs displayed a low activity towards ethylene polymerisation, but a high activity towards styrene polymerisation. Such characteristics are comparable to unbridged compounds, e.g. [(η5- C5H5)TiCl3], rather than silicon-bridged CGCs, thus suggesting degradation of the boron-bridged CGCs to unbridged complexes under polymerisation conditions. - 8 - Table of contents List of abbreviations used ............................................................................................... 18 List of figures .................................................................................................................. 20 List of schemes................................................................................................................ 23 List of tables.................................................................................................................... 25 Chapter 1. Introduction................................................................................................ 29 1.1. Definition of Constrained Geometry Complex............................................ 31 1.2. Synthesis of CGCs ....................................................................................... 32 1.2.1. Synthesis of CGCs by complexation of the pre-assembled ansa-ligand precursor....................................................................................................... 32 1.2.1.1. Dimetalation/salt elimination sequence ....................................................... 32 1.2.1.2. Amine elimination........................................................................................ 34 1.2.1.3. Toluene elimination ..................................................................................... 35 1.2.1.4. Amine assisted HCl elimination................................................................... 36 1.2.1.5. Me3SiCl elimination..................................................................................... 36 1.2.1.6. Combined LiCl and Me3SiCl elimination.................................................... 37 1.2.2. Synthesis of CGCs by reaction in the ligand sphere of a transition metal... 37 1.3. Derivatisation of Group 4 CGCs.................................................................. 38 1.4. Modification of the ligand system with constrained geometry.................... 39 1.4.1. Variation of the cyclopentadienyl fragment................................................. 40 1.4.2. Variation of the amido fragment .................................................................. 42 1.4.3. Variation of the ansa-bridge ........................................................................ 44 1.4.4. Variation of the metal centre........................................................................ 47 1.5. Polymerisation with constrained geometry complexes................................ 49 1.5.1. Mechanism of the polymerisation reaction.................................................. 50 - 9 - 1.5.2. Activation of CGC for the polymerisation of α-olefins............................... 51 1.5.3. Zwitterionic CGCs as single component olefin polymerisation catalysts ... 54 1.5.4. Structure-activity relationship for constrained geometry complexes........... 55 1.5.5. Further aspects of (co)polymerisation of ethylene and α-olefins ................ 57 1.5.6. Polymerisation of monomers other than α-olefins....................................... 59 1.5.6.1. Styrene and derivatives ................................................................................ 59 1.5.6.2. Cyclic monomers ......................................................................................... 61 1.5.6.3. Conjugated dienes ........................................................................................ 63 1.5.6.4. Polar monomers ........................................................................................... 63 1.6. Other transformations catalysed by CGCs................................................... 65 1.7. Boron-bridged metallocenophanes and related compounds........................
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