Quantum chemical studies of redox properties and conformational changes of a four-center iron CO2 reduction electrocatalyst

Abstract

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 ΔG_sep = 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 (ΔG_sep = 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.