N-Heterocylic Carbene-Based Mn Electrocatalyst for Two-Electron CO₂ Reduction over Proton Reduction

A bifunctional, chelating N-heterocyclic carbene-pyridine (NHC-pyridine) containing Mn(I) complex [MnBr(NHC-pyridine)(CO)₃] displays a strong selectivity for CO₂ reduction over proton reduction. Interestingly, the two-electron reduction of this complex occurs at a single potential, as opposed to MnBr(bpy)(CO)₃, which is reduced by two electrons in two separate one-electron reductions. Here, the Gibbs free energy barriers, reduction potentials, rate constants, and pK_a values are predicted with theory to understand the one- vs two-electron reduction mechanism. The effects of weak and strong Brønsted acids [HCl, TFE (2,2,2-trifluoroethanol), PhOH, CH₃OH, and H₂O] are studied to gauge the preference for CO₂ vs proton reduction; water is found to be an ideal proton donor that allows for strong CO₂ selectivity.