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-CpCrClLn) 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.

Graphical abstractA simplified model system for chromium cyclopentadienyl ethene trimerisation was studied fully using density functional theory methods. Reactants and products involved in this process were optimized. All relevant transition states were located and optimised. Activation barriers were determined for all steps in the catalytic process, in addition to the rate determining step. It was also shown that the process of tetramerisation is not feasible with this system.Figure optionsDownload full-size imageDownload as PowerPoint slide