Experimentally Validated Cobalt Phosphinothiolate Hydride Intermediate Governs the Selectivity Switch between Formate and Syngas in CO2 Reduction

Abstract Controlling product selectivity in homogeneous electrocatalytic CO2 reduction remains a significant challenge, often requiring complicated catalyst redesigning. Herein, we identify and validate a cobalt-hydride intermediate that governs the selectivity switch for CO2 reduction to either formate or synthesis gas (syngas), a mixture of H2 and CO, for a cobalt 2-diisopropylphosphinothiolate complex (CoPS) upon modifying the concentration of the proton donor, water, without altering the catalyst structure. At low water concentrations (0.01 M), CoPS achieves up to 90% faradaic efficiency for formate production, whereas at higher water concentrations (0.5 M), syngas is favored. Spectroscopic characterization techniques reveal the formation of a rate-limiting cobalt-hydride, with density functional theory calculations predicting a hydricity value of 44.3 kcal/mol, which coincides with the thermodynamic threshold of CO2 to formate interconversion in acetonitrile, positioning this intermediate at the thermodynamic boundary of the two reaction pathways. Electrochemical and mechanistic studies reveal that the concentration of the proton donor drives the competition between hydride transfer to CO2 (formate pathway) and protonation (hydrogen evolution pathway). These findings establish the local proton environment as a practical switch between catalytic pathways and underscore the mechanistic insights into the associated CO2 reduction reaction, highlighting broad applicability for tunable selectivity in homogeneous catalysis.