Characterization of proton coupled electron transfer in a biomimetic oxomanganese complex: Evaluation of the DFT B3LYP level of theory

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 mixedvalence oxomanganese complex [(bpy)₂ Mn ^III (μ-O)₂ Mn^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_as are obtained according to the thermodynamic cycle formalism applied in conjunction with a continuum solvation model. We find that the pK_as of the oxo-ligands depend strongly on the oxidation states of the complex, changing by approximately 10 pH units (i.e., from pH ~ ∼ 2to pH ~∼ 12) upon III,IV ⇁ III,III reduction of complex 1. These computational results are consistent with the experimental pK_as 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) ₂ Mn^III (μ-O)₂ Mn^IV (bpy) ₂]³⁺ + H⁺ + e^- ⇁ [(bpy) ₂ Mn^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_as of oxo and hydroxo/water ligands in the OEC of PSII, leading to deprotonation of the OEC upon oxidation state transitions.