Manganese-Based Catalysts with Varying Ligand Substituents for the Electrochemical Reduction of CO₂ to CO

A series of manganese complexes were synthesized with a variety of ligands and ligand substituents. These complexes were then studied using ultraviolet–visible spectroscopy, cyclic voltammetry, density functional theory calculations, and bulk electrolysis. The UV–vis, cyclic voltammetry, and calculation data show that the bipyridine π* level is modulated by the incorporation of different substituents on the bipyridine and through this interaction moderates the observed catalytic activity of the complex toward CO₂ reduction. The calculations were correlated to the experimental UV–vis data and cyclic voltammetry data to demonstrate the relationship among these data, and a Hammett plot showed a good correlation between the substituent identity and the MLCT wavelength from UV–vis (R² = 0.96). When aliphatic substituents were placed on the 4,4′-positions of the bipyridine, the location of the bpy π* was not significantly altered. However, when more electron withdrawing substituents were placed on the 4,4′-positions the bpy π* level was altered more significantly. This alteration in the bpy π* level had a profound effect on the rate of CO production determined from bulk electrolysis. While complexes whose bpy π* level were similar or more blue shifted in comparison to the parent manganese complex did not display significantly altered efficiencies or rates for the conversion of CO₂ to CO, those species whose bpy π* energies were significantly red shifted in comparison to the parent manganese complex displayed far poorer catalysis. This is postulated to be a combination of two factors. First, the singly reduced complex’s ability to lose the axial bromide ligand is diminished when electron-withdrawing groups are placed on the bpy ligand due to an increasing gap between the bpy π* and the Mn–Br σ*. Second, the decreased electron density of the HOMO of the doubly reduced complex with electron-withdrawing groups makes the binding of a molecule of CO₂ less energetically favorable.