Computational investigation of ethene trimerisation catalysed by cyclopentadienyl chromium complexes

Intermediates and transition states of the reaction cycle for the trimerisation of ethene catalysed by the initial catalyst precursor [η⁵-CpCrCl₂]₂ have been characterised by modeling, starting from the species η⁵-CpCrMe₂. This is a simplified model system of the actual catalytically active system containing bulky cyclopentadienyls. The ground-state multiplicity configuration was determined to be that of a quartet, in the case of non-chlorinated Cr(III) species, and a triplet for corresponding chlorinated Cr(IV) analogues. Geometry optimizations were performed on all intermediates, using their ground-state multiplicity, and all relevant transition states were located and subsequently optimised. The effect of an additional chlorine ligand on the chromium centre (viz. species of the form η⁵-CpCrClL_n) on the activation energy barriers was also determined for two key high energy transformations. It was found that the activation energy barriers are lowered significantly upon the addition of a chlorine ligand to the chromium centre. The rate determining step for the non-chlorinated, Cr(III) system, was calculated as requiring a free energy value of 88 kJ mol⁻¹, with the chlorinated Cr(IV) analogue at 56 kJ mol⁻¹ in the same step. The process of ethene tetramerisation was found to be unfeasible with the system, with a free energy barrier of 162 kJ mol⁻¹ associated with this transformation.