Electrochemical CO2 reduction by Mn-mesbpy complex: Tuning redox properties and mechanistic switching via ligand substitution

Density functional theory (DFT) calculations were used to investigate the effects of electron-donating and electron-withdrawing substituents on the mesbpy ligand of Mn-based complexes employed in the electrochemical reduction of CO₂. The influence of these substituents on reduction potentials, CO₂ binding affinities, and the overall catalytic mechanism was systematically examined. DFT results indicated that Mn complexes bearing electron-donating groups (–CH₃, –OH), neutral groups (–H), and moderately electron-withdrawing groups (–F) can bind CO₂ in the two-electron-reduced state. In contrast, Mn complexes with strongly electron-withdrawing substituents (–CF₃, –CN) facilitate the initial reduction step but fail to bind CO₂ at the same redox state, thus requiring a third electron for activation. However, these complexes become energetically inefficient as catalysts due to the high energy demand associated with the three-electron reduction. The mechanistic insights presented here, which distinguish Mn complexes bearing electron-donating or moderately electron-withdrawing substituents from those with strongly electron-withdrawing groups, provide valuable design principles for the development of energy-efficient Mn-based CO₂ reduction catalysts.