Oxidation State Determines Solvent Structure Around a Manganese–Vanadium Polyoxometalate Water‐Oxidation Catalyst

Understanding how microsolvation influences the reactivity and stability of molecular water-oxidation catalysts remains a central challenge in artificial photosynthesis. Here, we combine molecular dynamics (MD) simulations with spectroscopic and electrochemical experiments to elucidate how acetonitrile/water mixtures organize around the mixed-valence polyoxometalate [(Mn₄O₄) (V₄O₁₃) (OAc)₃]n- across catalytically relevant redox states. Our results reveal a pronounced oxidation-state dependence: the reduced { MnV } 3 - \³⁻ species is surrounded by a dense, highly structured hydration shell even at low water contents, preferentially engaging terminal vanadate oxygen sites and partially displacing acetonitrile from the first solvation shell. By contrast, the oxidized { MnV } 2 - \²⁻ and { MnV } 1 - \¹⁻ species show substantially weaker water structuring and largely oxidation state-insensitive acetonitrile organization. These microscopic solvation motifs are directly reflected experimentally: spectroscopic titrations reveal the emergence of hydrogen-bond formation at V═O groups and Jahn-Teller-driven asymmetric solvent accumulation accompanied by ligand exchange, while electrochemical measurements indicate reduced diffusion coefficients and diminished redox features at higher water contents, consistent with ion pair-mediated aggregation observed in MD simulations. Together, these results establish oxidation state as a key control parameter for solvent organization around polyoxometallate water-oxidation catalysts and provide a molecular rationale for the enhanced activity yet limited stability window of { MnV } n - \ⁿ⁻ species in acetonitrile/water mixtures.