Inverse Isotope Effects of a Late Transition Metal Olefin Polymerization Catalyst: A DFT Study

Abstract

An unusual inverse KIE of 0.48 was observed for the catalyst [C5Me5(P(OCH3)3)CoCH2CH2-μ-H]+ in the polymerization of ethylene, which prompted the investigation of one full catalytic cycle by density functional theory calculations at the B3LYP/6-311+G(d,p) level of theory. In agreement with results from previous NMR investigations, the alkyl complex exhibiting a strong β-agostic interaction in the ground state could be identified as the catalyst resting state. This interaction leads to a smaller zero-point energy difference in the ground state and to the quantum mechanically calculated free activation energy difference of 0.6 kcal/mol between the hydrogen and the deuterium isotopomer. The migratory insertion step corresponds to the rate-determining transition state with a total activation barrier of 28.6 kcal/mol. The overall reaction profile is exergonic by −7.0 kcal/mol. The calculated inverse kinetic isotope effect (kH/kD = 0.37) is consistent with the experimental value of 0.48. Additionally the equili...