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.