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

The CO2 reduction electrocatalyst [Fe4 N(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 1 3- . 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 1 3- 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 Δ G sep = 1.4 kcal mol-1 for fully separated species and Δ G ‡ = +17.4 kcal mol-1 , indicating CO dissociation is energetically accessible at ambient conditions. The analogous dissociation pathway from 1 2- has a reaction energy of 22.1 kcal mol-1 and an activation energy of 22.4 kcal mol-1 (Δ G sep = 12.8 kcal mol-1 , Δ G ‡ = +18.1 kcal mol-1 ). Our computed harmonic vibrational analysis of [Fe4 N(CO)11 ]3- or 2 3- 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.