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Title: Mechanistic understanding of chromium-based oligomerisation catalysts : an EPR and ENDOR investigation
Author: McDyre, Lucia E.
ISNI:       0000 0004 2752 0284
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2011
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Electron Paramagnetic Resonance (EPR) and Electron Nuclear Double Resonance (ENDOR) spectroscopies have been used to study the fundamental nature of chromium-based selective oligomerisation catalysts. A series of 'pre-catalyst' complexes were fully characterised CW-EPR revealed each complex to possess an axial g matrix (g > ge > g ) and superhyperfine coupling to two equivalent 31P nuclei, consistent with a low-spin cf species of approximate Cjv symmetry, where the metal contribution to the SOMO is primarily dxy. The isotropic component to the 31P coupling was of a larger magnitude in those Cr(I) complexes bearing PNP ligands than those bearing PCP, indicating that the phosphorus 3s character in the SOMO was higher for the former. CW-ENDOR demonstrated that subtle structural differences in the complexes, namely in the phenyl ring conformations, occurred as a function of ligand type. Pulsed experiments proved that the technique is valid and viable for further work on the activated system. Upon activation of the pre-catalyst with an alkylaluminium, four distinct paramagnetic centres were identified. A Cr(I) bis-arene complex was firstly detected it was found to form either via intramolecular co-ordination of the ligand phenyl groups, or preferentially via solvent-based arene co-ordination, if such groups were available. Two further species (I and III) were subsequently observed at low temperatures the spin Hamiltonian parameters extracted for both showed that a significant modification to the structure of the pre-catalyst had occurred. Half-field transitions indicated the possibility of a dimeric nature to Species III. ENDOR measurements detected an exceptionally large proton coupling in the activated system, possibly due to the co ordination of alkyl fragments to the metal centre. A final, fourth paramagnetic centre (Species IV), was detected and classed as an intermediate species, due to the greater similarity between its g and A matrices with those of the parent complex, than the other activated species. Finally, a preliminary investigation into analogous pre-catalyst complexes bearing N-heterocyclic carbene ligands was performed, due to their similar employment in oligomerisation catalysis CW-EPR spectra revealed information on both their electronic and structural natures.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available