Side Arm Twist on Zn-Catalyzed Hydrosilylative Reduction of CO₂ to Formate and Methanol Equivalents with High Selectivity and Activity

Abstract

Reductive hydrosilylation of CO2 by catalysts based on environmentally benign metals such as zinc still presents an unmet challenge in terms of catalyst selectivity and activity. Here we show that tetra-coordinated neutral zinc siloxide complexes supported by the β-diketiminato ligand bearing a pendant amine arm exhibit unprecedentedly high catalytic activity toward the selective reduction of CO2 by a hydrosilane to silyl formate. More significantly, this zinc catalyst system can also effectively catalyze the subsequent reduction of silyl formate into methoxysilane in quantitative yield. To understand the mechanism of the CO2 hydrosilylation and the influence of the side arm, a series of catalytic active species (or intermediates), zinc hydrides, and formates, has been synthesized and structurally characterized; the relevant stoichiometric reactions and kinetic study were performed. DFT calculations were also carried out to determine the reaction profiles for the CO2 hydrosilylation and to explain the relative activity of the zinc catalysts employed in this study. This combined experimental and theoretical study gives insights into the critical effects of the pendant amine donor arm and the size of the alkyl group of the amine on the relative energetics of the species involved in the proposed catalytic cycle.