A computational study of electrocatalytic CO2 reduction by Mn(I) complexes: Role of bipyridine substituents

Abstract

Using density functional theory (DFT), a series of Mn(4,4′-R-bpy)(CO)3Br (bpy = 2,2′-bipyridine, R = CN, CF3, COOH, H, CH3, tBu, and OCH3) complexes has been studied for electrocatalytic CO2 to CO reduction. Here, we show that the formation of active catalytic species ([Mn(4,4′-R-bpy)(CO)3]-) proceeds via two different reduction pathways for electron-donating (OCH3, H, CH3, and tBu) and electron-withdrawing (CN, CF3, and COOH) bpy-substituents, respectively. Interestingly, electron-withdrawing bpy-substituents require lower reduction potentials compared to the electron-donating bpy-groups and the calculated reduction potentials agree well with the experimentally reported values. Our detailed study shows that electron-withdrawing bpy-substituents based Mn-complexes can be energetically favourable for two-electron reduction, but they are thermodynamically unfavourable for overall electrocatalytic CO2 to CO reduction. The reason being that such electron withdrawing substituents increase the electron density on the bpy ligand while decrease it on the Mn centre, which in turn make the CO2 binding to the Mn-complexes unfavourable compared to that in the presence of electron-donating substituents. Therefore, it can be concluded that electron-withdrawing substituents restrict the bpy ligand to act as an ideal redox ligand. On the other hand, though the electron-donating bpy-substituents based Mn-complexes require higher potential for reduction, but they can be promising for CO2 reduction and provide aplenty of scope for further improvements. © 2018 Elsevier Ltd