Electrochemical CO2 reduction by phosphine-modified [FeFe]-hydrogenase models.

[FeFe]-hydrogenase model complexes of the type [Fe2(µ-RS2)CO6], featuring rigid dithiolate ligands, have recently been identified as efficient and selective electrocatalysts for the reduction of CO2 to formate. While substitution of carbonyl ligands by phosphines has been extensively explored to tune the reactivity of related complexes for proton reduction, its impact on CO2 reduction remains largely unexplored. Herein, we investigate the effect of replacing a single CO ligand with triphenylphosphine (1), diphenyl-2-pyridylphosphine (2), or its cationic N-methylated analog (3). These modifications alter the electronic properties of the FeFe core while introducing secondary coordination-sphere functionalities. In the presence of CO2 and methanol, all three complexes selectively generate formate as the major product, with H2 and CO detected as minor products. Among the series, complex 2 exhibits the highest turnover frequency for formate production, likely reflecting beneficial secondary-sphere effects imparted by the pendant pyridine unit of the phosphine ligand. In contrast, despite the electrostatic influence exerted on the FeFe core by the cationic N-methylpyridinium group in complex 3, no enhancement in CO2RR activity is observed. Unlike the hexacarbonyl series, the pentacarbonyl complexes are proposed to bind CO2 upon one-electron reduction, revealing distinct initial mechanistic pathways for CO2RR in the two series.