Mechanistic insights of electrocatalytic CO2 reduction by Mn complexes: synergistic effects of the ligands

Abstract

The electrocatalytic mechanisms of CO₂ reduction catalyzed by pyridine-oxazoline (pyrox)-based Mn catalysts were investigated by DFT calculations. In-depth comparative analyses of pyrox-based and bipyridine-based Mn complexes were carried out. C–OH cleavage is the rate-determining step for both the protonation-first path and the reduction-first path. The free energy of CO₂ activation (ΔG₁) and the electrons donated by CO ligands in this step are effective descriptors in regulating the C–OH cleavage barrier. The reduction of carboxylate complex 6 (E₆) is the potential-determining step for the reduction-first path. Meanwhile, for the protonation-first path, the initial generation (E₂) or the regeneration (E₈) of active catalyst might be potential-determining. Hirshfeld charge and orbital contribution analysis indicate that E₆ is definitely based on the heterocyclic ligand and E₂ is related to both the heterocyclic ligand and three CO ligands. Therefore, replacement of the CO ligand by a stronger electron donating ligand can effectively boost the catalytic activity of CO₂ reduction without increasing the overpotential in the reduction-first path. This hypothesis is supported by the mechanism calculations of the Mn complex in which the axial CO ligand is replaced by a pyridine or PMe₃.