Ethylene trimerisation with Cr-PNP catalysts: a theoretical benchmarking study and assessment of catalyst oxidation state

A model for the homogeneous Cr-PNP (PNP = diphosphinoamine) ethylene trimerisation and tetramerisation catalyst system has been studied theoretically, with the aim of identifying suitable density functional theory methods for treatment of this catalyst, and evaluating the likely oxidation and spin states of the active species. Benchmarking studies involving high-level treatment reveal the difficulty of accurately calculating the thermochemistry of this system, and suggest that local density functionals, such as M06L, probably provide the best option. Density functional theory modelling of catalyst activation and the first steps of oligomerisation up until 1-hexene formation appears to favour a Cr^I–Cr^III mechanism, involving spin surface crossing from sextet to quartet states.