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 [FE_max] of 16 ± 6%) and HCOO⁻ (FE_max = 20%) and produces H₂ (FE_max = 56 ± 4%). Without added MeOH, 1 reacts with adventitious water to form H₂ (FE_max = 85 ± 1%), HCOO⁻ (FE_max = 7.8%), and CO (FE_max = 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⁻.