Quantum chemical studies of redox properties and conformational changes of a four-center iron CO 2 reduction electrocatalyst

The CO2 reduction electrocatalyst [Fe4N(CO)12]− (abbrev. 1−) reduces CO2 to HCO2− in a two-electron, one-proton catalytic cycle. Here, we employ ab initio calculations to estimate the first two redox potentials of 1− and explore the pathway of a side reaction involving CO dissociation from 13−. Using the BP86 density functional approximation, the redox potentials were computed with a root mean squared error of 0.15 V with respect to experimental data. High temperature Born–Oppenheimer molecular dynamics was employed to discover a reaction pathway of CO dissociation from 13− with a reaction energy of +10.6 kcal mol−1 and an activation energy of 18.8 kcal mol−1; including harmonic free energy terms, this yields ΔGsep = 1.4 kcal mol−1 for fully separated species and ΔG‡ = +17.4 kcal mol−1, indicating CO dissociation is energetically accessible at ambient conditions. The analogous dissociation pathway from 12− has a reaction energy of 22.1 kcal mol−1 and an activation energy of 22.4 kcal mol−1 (ΔGsep = 12.8 kcal mol−1, ΔG‡ = +18.1 kcal mol−1). Our computed harmonic vibrational analysis of [Fe4N(CO)11]3− or 23− reveals a distinct CO-stretching peak red-shifted from the main CO-stretching band, pointing to a possible vibrational signature of dissociation. Multi-reference CASSCF calculations are used to check the assumptions of the density functional approximations that were used to obtain the majority of the results.