Redox Isomerism in the S3 State of the Oxygen-Evolving Complex Resolved by Coupled Cluster Theory

The electronic and geometric structures of the water-oxidizing complex of photosystem II in the steps of the catalytic cycle that precede dioxygen evolution remain hotly debated. Recent structural and spectroscopic investigations support contradictory redox formulations for the active-site Mn₄CaO_x cofactor in the final metastable S₃ state. These range from the widely accepted Mn^IV₄ oxo-hydroxo model, which presumes that O−O bond formation occurs in the ultimate transient intermediate (S₄) of the catalytic cycle, to a Mn^III₂Mn^IV₂ peroxo model representative of the contrasting “early-onset” O−O bond formation hypothesis. Density functional theory energetics of suggested S₃ redox isomers are inconclusive because of extreme functional dependence. Here, we use the power of the domain-based local pair natural orbital approach to coupled cluster theory, DLPNO-CCSD(T), to present the first correlated wave function theory calculations of relative stabilities for distinct redox-isomeric forms of the S₃ state. Our results enabled us to evaluate conflicting models for the S₃ state of the oxygen-evolving complex (OEC) and to quantify the accuracy of lower-level theoretical approaches. Our assessment of the relevance of distinct redox-isomeric forms for the mechanism of biological water oxidation strongly disfavors the scenario of early-onset O−O formation advanced by literal interpretations of certain crystallographic models. This work serves as a case study in the application of modern coupled cluster implementations to redox isomerism problems in oligonuclear transition metal systems.