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 MnIV (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 pKas are obtained according to the thermodynamic cycle formalism applied in conjunction with a continuum solvation model. We find that the pKas 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 pKas 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 MnIII (μ-O)2 MnIV (bpy) 2]3+ + H+ + e- ⇁ [(bpy) 2 MnIII (μ-O)(μ-OH)MnIII (bpy) 2]3+. It is thus natural to expect that analogous redox processes might strongly modulate the pKas of oxo and hydroxo/water ligands in the OEC of PSII, leading to deprotonation of the OEC upon oxidation state transitions.
ASJC Scopus subject areas
- Computer Science Applications
- Physical and Theoretical Chemistry