Spin Surface Crossing between Chromium(I)/Sextet and Chromium(III)/Quartet without Deprotonation in SNS-Cr Mediated Ethylene Trimerization

Abstract

Experimentally unsolved problems, including the oxidation states of active species and the occurrence of ligand deprotonation in the SNS-Cr ethylene trimerization system, were studied using the density functional theory (DFT) method. The full catalytic cycle was calculated on the basis of the metallacycle mechanism, and Gibbs free energy surfaces of the trimerization reaction were completely located. A detailed spin state analysis revealed that the ground states of intermediates change along the redox cycle and the spin surface crossing occurring at the minimum energy crossing point (MECP) before metallacyclopentane formation was found to open up a much lower energy pathway by spin acceleration. Formation of metallacycloheptane was identified as the rate-determining step in this system. By comparison of the activation energies of the rate-determining step, Cr(I)/Cr(III) active species bearing nondeprotonated ligands were proposed to be most plausibly responsible for ethylene trimerization. Frontier orbitals and natural population analysis were also determined to further elucidate the reason for high 1-hexene selectivity in this system.