Selectivity for HCO2– over H2 in the Electrochemical Catalytic Reduction of CO2 by (POCOP)IrH2

It has been demonstrated experimentally that electrochemical CO2 reduction catalyzed by (POCOP)­IrH2 ([C6H3-2,6-[OP­(tBu)2]2]­IrH2) produces formate without significant H2. We use first-principles density functional theory (M06) including Poisson–Boltzmann solvation to determine the detailed atomistic mechanism and illuminate strategies for designing formate-selective catalysts. A mechanism involving hydride transfer from IrIII dihydride explains the selectivity for formate over H2 and is corroborated by reduction potential (irreversible reduction of (POCOP)­Ir­(H)­(NCMe)2+ at ca. −1.3 V vs NHE, in comparison to −1.31 V vs NHE calculated for one-electron reduction of IrIII(H)­(NCMe)2+) and turnover frequency. We find that several thermodynamically favorable pathways exist for the hydrogen evolution reaction (HER) from both IrIII(H)2 and IrI–H– but are kinetically hindered, posing computed activation barriers above 25 kcal/mol at pH 7. However, with formate or bicarbonate acting as cocatalyst, the barriers are lowered to 18.8 kcal/mol. The preference of (POCOP)Ir to form a dihydride instead of a dihydrogen adduct also disfavors the HER and facilitates catalyst regeneration. In contrast, substituting cobalt for iridium raises the barrier for hydride transfer to CO2 by 12.0 kcal/mol and lowers the required reduction potential to −1.65 V vs NHE. Calculated driving forces for hydride transfer from IrI and IrIII intermediates illustrate different strategies for positioning the hydricity relative to the thermodynamic hydricities of H2/H+ and HCOO–/CO2. The data support an approach of selecting a hydricity that is just thermodynamically able to reduce CO2. The effect of solvation on calculated driving forces for hydride transfer is also discussed.