Mechanistic DFT Study on Ethylene Trimerization of Chromium Catalysts Supported by a Versatile Pyrrole Ligand System

To elucidate fundamental mechanistic aspects of the landmark Chevron–Phillips ethylene trimerization system, a detailed theoretical study has been carried out by DFT methods on an aluminum pyrrolyl chromium catalyst. Reaction pathways for selective ethylene oligomerization have been successfully located on the basis of the metallacycle mechanism. Consistent with experimental results, for the model system ethylene trimerization was proven to be energetically preferred in comparison to ethylene dimerization or further ring expansion toward the formation of higher α-olefins. The Cr­(I/III) redox couple was found to be the most likely for the catalytic ethylene trimerization. A careful electronic configuration analysis has been conducted, and the ground state of all active species involved in the catalytic cycle is identified to be <i>S</i> = ³/₂ except for the bare active species, which favors a high spin state of <i>S</i> = ⁵/₂. The role of a pendant chlorine functionality has been investigated as well. Variable Cr–Cl bond distance and NBO charge analysis of every intermediate clearly exhibit the hemilabile behavior of the chlorine. This unique hemilability is considered to be a key factor for the selectivity toward 1-hexene formation