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

摘要

In two consecutive equilibria the compound (Cp*)2Zr(OMe)2 undergoes insertion of CO_2 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 CO_2. 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 CO_2 and the metal during the C-O bond formation. This is supported by DFT calculations showing the lateral attack by CO_2 to have the lowest energy transition state.