Orientational Jahn–Teller Isomerism in the Dark‐Stable State of Nature's Water Oxidase

Water Oxidation Catalyzed by a Bioinspired Tetranuclear Manganese Complex: Mechanistic Study and Prediction

Density functional theory calculations were utilized to elucidate the water oxidation mechanism catalyzed by polyanionic tetramanganese complex a [Mn^III ₃ Mn^IV O₃ (CH₃ COO)₃ (A-α-SiW₉ O₃₄ )]^6- . Theoretical results indicated that catalytic active species 1 (Mn₄ ^{III,III,IV,IV} ) was formed after O₂ formation in the first turnover. From 1, three sequential proton-coupled electron transfer (PCET) oxidations led to the Mn^IV -oxyl radical 4 (Mn₄ ^IV,IV,IV,IV -O⋅). Importantly, 4 had an unusual butterfly-shaped Mn₂ O₂ core for the two substrate-coordinated Mn sites, which facilitated O-O bond formation via direct coupling of the oxyl radical and the adjacent Mn^IV -coordinated hydroxide to produce the hydroperoxide intermediate Int1 (Mn₄ ^III,IV,IV,IV -OOH). This step had an overall energy barrier of 24.9 kcal mol^-1 . Subsequent PCET oxidation of Int1 to Int2 (Mn₄ ^III,IV,IV,IV -O₂ ⋅) enabled the O₂ release in a facile process. Furthermore, apart from the Si-centered complex, computational study suggested that tetramanganese polyoxometalates with Ge, P, and S could also catalyze the water oxidation process, where those bearing P and S likely present higher activities.

Jahn-Teller Effects in a Vanadate-Stabilized Manganese-Oxo Cubane Water Oxidation Catalyst

Heuristic rules that allow identifying the preferred mixed‐valence isomers and Jahn‐Teller axis arrangements in the water oxidation catalyst [(Mn₄O₄)(V₄O₁₃)(OAc)₃]ⁿ⁻ and its activated form [(Mn₄O₄)(V₄O₁₃)(OAc)₂(H₂O)(OH)]ⁿ⁻ are derived. These rules are based on computing all combinatorially possible mixed‐valence isomers and Jahn‐Teller axis arrangements of the Mn^III atoms, and associate energetic costs with some structural features, like crossings of multiple Jahn‐Teller axes, the location of these axes, or the involved ligands. It is found that the different oxidation states localize on different Mn centers, giving rise to clear Jahn‐Teller distortions, unlike in previous crystallographic findings where an apparent valence delocalization was found. The low barriers that connect different Jahn‐Teller axis arrangements suggest that the system quickly interconverts between them, leading to the observation of averaged bond lengths in the crystal structure. We conclude that the combination of cubane‐vanadate bonds that are chemically inert, cubane‐acetate/water bonds that can be activated through a Jahn‐Teller axis, and low activation barriers for intramolecular rearrangement of the Jahn‐Teller axes plays an essential role in the reactivity of this and probably related compounds.

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 Mn4 CaOx cofactor in the final metastable S3 state. These range from the widely accepted MnIV 4 oxo-hydroxo model, which presumes that O-O bond formation occurs in the ultimate transient intermediate (S4 ) of the catalytic cycle, to a MnIII 2 MnIV 2 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.