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

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journal contribution
posted on 09.03.2010, 00:00 by Ting Wang, Gary Brudvig, Victor S. Batista
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)2MnIII(μ-O)2MnIV(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 pKa’s are obtained according to the thermodynamic cycle formalism applied in conjunction with a continuum solvation model. We find that the pKa’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 pKa’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)2MnIII(μ-O)2MnIV(bpy)2]3+ + H+ + e → [(bpy)2MnIII(μ-O)(μ-OH)MnIII(bpy)2]3+. It is thus natural to expect that analogous redox processes might strongly modulate the pKa’s of oxo and hydroxo/water ligands in the OEC of PSII, leading to deprotonation of the OEC upon oxidation state transitions.