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 ${\left\{\mathrm{MnV}\right\}}^{3-}$ 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 ${\left\{\mathrm{MnV}\right\}}^{2-}$ and ${\left\{\mathrm{MnV}\right\}}^{1-}$ 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 ${\left\{\mathrm{MnV}\right\}}^{n-}$ species in acetonitrile/water mixtures.