Characterization of Proton Coupled Electron Transfer in a Biomimetic Oxomanganese Complex: Evaluation of the DFT B3LYP Level of Theory

Abstract

The capabilities and limitations of the Becke-3-Lee−Yang−Parr (B3LYP) density functional theory (DFT) for modeling proton coupled electron transfer (PCET) in the mixed-valence oxomanganese complex [(bpy)₂Mn^III(μ-O)₂Mn^IV(bpy)₂]³⁺ (1; bpy = 2,2′-bipyridyl) are analyzed. Complex 1 serves as a prototypical synthetic model for studies of redox processes analogous to those responsible for water oxidation in the oxygen-evolving complex (OEC) of photosystem II (PSII). DFT B3LYP free energy calculations of redox potentials and pK_a’s are obtained according to the thermodynamic cycle formalism applied in conjunction with a continuum solvation model. We find that the pK_a’s of the oxo-ligands depend strongly on the oxidation states of the complex, changing by approximately 10 pH units (i.e., from pH ∼ 2 to pH ∼ 12) upon III,IV → III,III reduction of complex 1. These computational results are consistent with the experimental pK_a’s determined by solution magnetic susceptibility and near-IR spectroscopy as well as with the pH dependence of the redox potential reported by cyclic voltammogram measurements, suggesting that the III,IV → III,III reduction of complex 1 is coupled to protonation of the di-μ-oxo bridge as follows: [(bpy)₂Mn^III(μ-O)₂Mn^IV(bpy)₂]³⁺ + H⁺ + e⁻ → [(bpy)₂Mn^III(μ-O)(μ-OH)Mn^III(bpy)₂]³⁺. It is thus natural to expect that analogous redox processes might strongly modulate the pK_a’s of oxo and hydroxo/water ligands in the OEC of PSII, leading to deprotonation of the OEC upon oxidation state transitions.