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 mixed-valence oxomanganese complex [(bpy)(2)Mn-III(mu-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(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 similar to 2 to pH similar to 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-mu-oxo bridge as follows: [(bpy)(2)Mn-III(mu-O)(2)Mn-IV(bpy)(2)](3+) + H+ + e(-) -> [(bpy)(2)Mn-III(mu-O)(mu-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.