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

摘要

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]~(2+) + 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.