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 Ir^III 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 Ir^III(H)(NCMe)₂ ⁺) and turnover frequency. We find that several thermodynamically favorable pathways exist for the hydrogen evolution reaction (HER) from both Ir^III(H)₂ and Ir^I-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 Ir^I and Ir^III intermediates illustrate different strategies for positioning the hydricity relative to the thermodynamic hydricities of H₂/H⁺ and HCOO^-/CO_2. 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.