A DFT study toward the mechanism of chromium-catalyzed ethylene trimerization

A theoretical study of a mechanism for ethylene trimerization, catalyzed by Cr-pyrrolyl complexes, is proposed and investigated with density functional theory methods. The selective formation of 1-hexene is generally accepted to follow a metallacycle mechanism. A detailed spin state analysis for active species in the mechanism shows that the triplet spin state represents the ground spin state for all stationary points. Complete Gibbs free energy (298.15 K) surfaces are mapped for both eta(5)- and sigma-bonding modes of pyrrole, as well as a stripped-down Cl anion model and a full ClAlMe3 anion model. From the calculated results it is shown that the proposed metallacycle mechanism is energetically favorable, with metallacycle growth identified as the rate-determining step. In addition, it is demonstrated that different bonding modes of pyrrole are preferred at different stages in the proposed mechanism, effectively suggesting that ring slippage of the pyrrole occurs on the minimum energy path on the potential energy surface. From the calculated results important insight is gained into the hemilabile nature of the pyrrole ring in the mechanism, which in turn sheds light on the general requirements for an effective ligand in Cr-catalyzed ethylene trimerization.