Fast and reversible insertion of carbon dioxide into zirconocene–alkoxide bonds. A mechanistic study

Abstract

In two consecutive equilibria the compound (Cp*)₂Zr(OMe)₂ undergoes insertion of CO₂ to form the mono- and bis-hemicarbonates. Both equilibria are exothermic but entropically disfavoured. Magnetisation transfer experiments gave kinetic data for the first equilibrium showing that the rate of insertion is overall second order with a rate constant of 3.20 ± 0.12 M⁻¹ s⁻¹, which is substantially higher than those reported for other early transition metal alkoxides, which are currently the best homogeneous catalysts for dimethyl carbonate formation from methanol and CO₂. Activation parameters for the insertion reaction point to a highly ordered transition state and we interpret that as there being a substantial interaction between the CO₂ and the metal during the C–O bond formation. This is supported by DFT calculations showing the lateral attack by CO₂ to have the lowest energy transition state.