Selective Electrochemical CO2 Reduction to Formate: Mechanistic Insights into [FeFe]-Hydrogenase Models with Tunable Bridging Ligands

The selective electrochemical reduction of CO2 to formate is a key challenge in developing carbon-recycling chemical processes, with potential applications in energy storage and renewable feedstock production. Inspired by the active site of [FeFe]-hydrogenases, we synthesized and characterized three derivatives of the [(μ-bdt)-Fe2(CO)6] complex (1, bdt = benzene-1,2-dithiolate), incorporating nitrogen-containing functional groups into the aromatic ring of the bdt ligand. These complexes exhibit 100% Faradaic efficiency for CO2 reduction in the presence of MeOH, with formate selectivity reaching up to 62%, alongside H2 and CO as side products. While bdt ligand functionalization has a minimal impact on the electronic properties of the FeFe core, it moderately enhances turnover frequencies (TOFmax) for CO2 reduction. The catalysis depends on the MeOH concentration, with the functionalized complexes reaching their maximum catalytic current at lower MeOH concentrations with respect to complex 1. By combining infrared spectroelectrochemistry (IR-SEC) and density functional theory (DFT) calculations, we revisited the reaction mechanism of this class of catalysts, demonstrating the involvement of a key metal-hydride intermediate. Our findings explain the observed selectivity and open the possibility of further design of secondary-sphere interactions to enhance CO2 reduction kinetics.