Mechanistic understanding of chromium-based oligomerisation catalysts : an EPR and ENDOR investigation

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.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

A cw EPR and ENDOR investigation on a series of Cr(i) carbonyl complexes with relevance to alkene oligomerization catalysis: [Cr(CO)4L]+ (L = Ph2PN(R)PPh2, Ph2P(R)PPh2)

The preparation and characterisation of the Cr(I) complexes [Cr(CO)4L]+ (L = Ph2PN(R)PPh2, Ph2P(R)PPh2), which are used as pre-catalysts for the selective oligomerization of ethylene, are reported. The electronic properties and structural features of these complexes in frozen solution have been established via continuous wave X-band Electron Paramagnetic Resonance (cw-EPR) and continuous wave 1H, 14N and 31P Electron Nuclear Double Resonance (cw-ENDOR) spectroscopy. The EPR spectra are dominated by the g anisotropy, with notably large PA couplings from the two equivalent 31P nuclei. The spin Hamiltonian parameters (g⊥ (gxx = gyy) > ge > g∥ (gzz)) are consistent with a low-spin d5 system possessing C2v symmetry, with a SOMO where the metal contribution is primarily dxy for all complexes. The isotropic Fermi contact term (Paiso, determined by EPR and ENDOR) was found to be largest for complexes containing ligands e, d, f and g, indicating that the 31P 3 s character in the SOMO is higher for the PNP type ligands than the PCP type. Subtle structural differences in the complexes were also identified through variations in the Δg shifts (identified by EPR), and through differences in the phenyl ring conformations (identified by 1H ENDOR). Attempts to correlate trends in EPR-derived parameters with data measured for catalysis using these pre-catalysts are also made, but no clear connections were found