A Reexamination of CO2 Reduction with Fe2S2 Hydrogenase Mimics: Lessons in Using a Hydrogen Evolution Reaction Catalyst for CO2 to Formate Catalysis

Recent reports show [FeFe] hydrogenase mimics are active for the electrochemical reduction of CO2 to formate (HCOO−). Herein, the electrochemical reduction of CO2 with the [FeFe] hydrogenase mimic [Fe2(μ‐pdt)(CO)6, 1, where pdt = propane‐1,3‐dithiolate] in acetonitrile is reported. In the presence of the weak acid, methanol (MeOH), 1 reduces CO2 to both CO (Faradaic Efficiency maximum [FEmax] of 16 ± 6%) and HCOO− (FEmax = 20%) and produces H2 (FEmax = 56 ± 4%). Without added MeOH, 1 reacts with adventitious water to form H2 (FEmax = 85 ± 1%), HCOO− (FEmax = 7.8%), and CO (FEmax = 7 ± 3%) with CO32− being detected by infrared spectroscopy.  Product formation is potential dependent: more negative potentials increases selectivity for HCOO− over CO. The first reduction of 1 forms a pdt‐bridged dimer, 2. However, the reduction of 2 at the potentials required for electrochemical CO2 reduction leads to two new species. Using density functional theory, and infrared spectroelectrochemistry (IR‐SEC), these structures are identified to be [Fe(CO)4]2− (3) and a trinuclear Fe3 species (4). While these species can reduce CO2 to CO and HCOO−, the predominant formation of H2 reveals kinetic issues in  CO2 reduction. The work offers to consider alternate competing mechanistic pathways and explains the lack of product selectivity when using hydrogen evolution reaction catalyst for CO2 reduction to HCOO−.