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

ABSTRACT: 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 1 [(bpy)(2)Mn(III)(mu-O)(2)Mn(IV)(bpy)(2)](3+) (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-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 pKa's of oxo and hydroxo/water ligands in the OEC of PSII, leading to deprotonation of the OEC upon oxidation state transitions.

SUBMITTER: Wang T

PROVIDER: S-EPMC2896228 | biostudies-literature | 2010 Jan

REPOSITORIES: biostudies-literature

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

Wang Ting T Brudvig Gary G Batista Victor S VS

Journal of chemical theory and computation 20100101 3

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 1 [(bpy)(2)Mn(III)(mu-O)(2)Mn(IV)(bpy)(2)](3+) (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 c ...[more]

PMID: 20607115

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Project description:Designing molecular platforms for controlling proton and electron movement in artificial photosynthetic systems is crucial to efficient catalysis and solar energy conversion. The transfer of both protons and electrons during a reaction is known as proton-coupled electron transfer (PCET) and is used by nature in myriad ways to provide low overpotential pathways for redox reactions and redox leveling, as well as to generate bioenergetic proton currents. Herein, we describe theoretical and electrochemical studies of a series of bioinspired benzimidazole-phenol (BIP) derivatives and a series of dibenzimidazole-phenol (BI2P) analogs with each series bearing the same set of terminal proton-accepting (TPA) groups. The set of TPAs spans more than 6 pK a units. These compounds have been designed to explore the role of the bridging benzimidazole(s) in a one-electron oxidation process coupled to intramolecular proton translocation across either two (the BIP series) or three (the BI2P series) acid/base sites. These molecular constructs feature an electrochemically active phenol connected to the TPA group through a benzimidazole-based bridge, which together with the phenol and TPA group form a covalent framework supporting a Grotthuss-type hydrogen-bonded network. Infrared spectroelectrochemistry demonstrates that upon oxidation of the phenol, protons translocate across this well-defined hydrogen-bonded network to a TPA group. The experimental data show the benzimidazole bridges are non-innocent participants in the PCET process in that the addition of each benzimidazole unit lowers the redox potential of the phenoxyl radical/phenol couple by 60 mV, regardless of the nature of the TPA group. Using a series of hypothetical thermodynamic steps, density functional theory calculations correctly predicted the dependence of the redox potential of the phenoxyl radical/phenol couple on the nature of the final protonated species and provided insight into the thermodynamic role of dibenzimidazole units in the PCET process. This information is crucial for developing molecular "dry proton wires" with these moieties, which can transfer protons via a Grotthuss-type mechanism over long distances without the intervention of water molecules.

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Dataset Information

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

Publications

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

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