Towards the Mechanism(s) of Early Transition Metal Promoted Selective Olefin Oligomerisation Catalysis - Synthesis, Spectroscopic and XAFS Studies

The quest for more highly selective, cleaner and more efficient catalysts e.g for olefin trimerisation or tetramerisation remains a high priority for the chemical industry. Achieving these targets demands a detailed understanding of the catalytic cycle(s) and the nature of the active species. Characterisation of the individual stages in a homogeneous catalytic cycle is not easily achieved since the active species are likely to be highly reactive and often very transient, making their crystallographic characterisation highly unlikely. Furthermore, for the paramagnetic e.g. Cr-based catalysts NMR spectroscopy is not informative. Under this project we will develop and use a unique freeze-quench cell to allow the transient and active species to be trapped at various selected stages through the cycle, allowing in situ spectroscopic analysis by extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopy techniques. We will prepare and characterise a series of related metal complexes based on Cr, Mo and Sc in the presence of a selected set of N-, S-, N/S- and N/P-donor ligands, including complexes of the industrially important NH(CH2CH2Sdecyl)2 and iPrN(PPh2)2. Using a range of techniques (UV-visible, EPR, 45Sc NMR spectroscopy), in conjunction with XAFS and XANES data using the set-up described above, we will probe in detail the oxidation state and structures at various stages through the activation and catalysis to provide a much more detailed understanding of the mechanisms at work. We also expect to demonstrate the potential of the new rapid (millisecond) freeze-quench XAFS/XANES approach much more widely to provide key information regarding other homogeneous catalysis systems.