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

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 (Delta G(1)) 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 (E-6) is the potential-determining step for the reduction-first path. Meanwhile, for the protonation-first path, the initial generation (E-2) or the regeneration (E-8) of active catalyst might be potential-determining. Hirshfeld charge and orbital contribution analysis indicate that E6 is definitely based on the heterocyclic ligand and E-2 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.