Deprotonation of Water Ligands in V, Cr, Mn, Fe, and Co Complexes Reduces Oxidation-Driven Carboxylate Ligand Frequency Shifts

In Mn complexes, it has been shown that oxidation-driven changes in carboxylate ligand vibrations are suppressed, if a water or hydroxo ligand is simultaneously deprotonated. Deprotonation with oxidation has also been shown to greatly reduce the dependence of Mn complex redox energies on the oxidation state of the metal. We have here investigated the effect of oxidation with deprotonation on the carboxylate ligand frequencies of V, Cr, Mn, Fe, and Co complexes. The effects of anionic ligand substitution (instead of deprotonation) and solvent dielectric were also investigated to determine the mechanism that drives carboxylate frequency shifts. It is shown that the effect of deprotonation was similar for all of the metals tested in this study. C-O bond lengths and O-C-O angle changes in the carboxylate ligand were also reduced by deprotonation. Furthermore, the effect of anionic ligand substitution was similar to deprotonation in the suppression of carboxylate frequency shifts. These shifts were also reduced by increases in the solvent dielectric, in the absence of charge conservation through deprotonation. Therefore, we conclude that carboxylate frequency shifts are largely driven by electrostatic effects.