Manganese-Based Catalysts with Varying Ligand Substituents for the Electrochemical Reduction of CO ₂ 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 CO ₂ 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 (R ² = 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 CO ₂ 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 CO ₂ less energetically favorable.