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Group 9 and 10 transition metal n-heterocyclic carbene complexes in catalysis.

Hawkes, Kirsty June. 2006. Group 9 and 10 transition metal n-heterocyclic carbene complexes in catalysis. PhD Thesis, Cardiff University.

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Abstract

This thesis describes the theoretical and experimental study of group 9 and 10 transition metal N-heterocyclic carbene complexes in catalytic reactions. In order to overcome decomposition reactions discovered in the use of carbene complexes for carbon monoxide/ethylene copolymerisation, chelating thiazolium salts were prepared for the synthesis of corresponding palladium complexes. Complex formation proved difficult and experimental attempts to overcome possible side reactions caused by reactant-metal interactions were unsuccessful. Theoretical studies indicated a sulfur-palladium interaction may be contributing to alternative products, with the use of the bulky lBu coordination at the thiazolium 5 position likely to block this interaction enough to allow C2 carbene formation. Theoretical calculations for the oxidative addition of azolium salts to a model Wilkinson's catalyst (RhCl(PH3)3) is described. According to free energy calculations, a six ligand associative route with a concerted three-centred transition structure may be competitive to a route in which phosphine predissociation occurs. Exchange of the phosphine molecule on the metal centre with trimethylphosphine had a significant effect in lowering the barrier to oxidative addition and decreasing the endothermicity of the reaction, while explicit and bulk solvation was found to have a moderate effect on the overall reaction. Extension of the oxidative addition of azolium salts to rhodium carbene complexes have been examined, in which a range of ligands is described from the pi-acidic carbon monoxide ligand to multiple carbene ligands. Increasing basicity decreases activation barriers while increasing the exothermicity of the overall reaction for C-H activation, however the complex most successful at C-H activation was not considered hospitable enough for related C-C activation of 2-methylazolium salts. Switching to iridium indicated a large benefit in C-H activation. Unfortunately, C-C activation remained unfavourable for iridium due to a high barrier to reaction. A mechanism for the experimentally successful C-C coupling of azolium salts to alkenes by nickel complexes is studied, indicating an oxidative addition, alkene insertion and reductive elimination cycle seems likely. Experimentally, the switching of catalytically active phosphine ligands to the related carbenes causes the reaction to be halted. Theoretical calculations imply minor changes to reaction conditions may significantly affect the outcome of catalytic reactions by stabilisation of important reaction intermediates. Further studies of the alternative C4 activation of the azolium salts and use of related azoles show C4 activation and coupling may be possible, while the unactivated azoles are unlikely to be coupled using the same mechanism. (Abstract shortened by UMI.)

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Chemistry
Subjects: Q Science > QD Chemistry
ISBN: 9781303204722
Funders: School of Chemistry, University of Wales, Cardiff
Date of First Compliant Deposit: 30 March 2016
Last Modified: 10 Jan 2018 02:27
URI: http://orca.cardiff.ac.uk/id/eprint/56061

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