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

ABSTRACT 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 (Mn 4 O 4 ) (V 4 O 13 ) (OAc) 3 n− across catalytically relevant redox states. Our results reveal a pronounced oxidation‐state dependence: the reduced 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 and 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 species in acetonitrile/water mixtures.