Redox Self-Equilibration in Molecular Vanadium Oxide Mixtures Enables Multi-Electron Storage

Polyoxometalates (POMs) are ideal components for reversible multi-electron storage in energy technologies. To-date, most redox-applications employ only single, individual POM species, which limits the number of electrons that can be stored within a given potential window. Here, we report that spontaneous redox self-equilibration during cluster synthesis leads to the formation of two structurally related polyoxovanadates which subsequently aggregate into co-crystals. This results in systems with significantly increased redox reactivity. The mixed POM system was formed by non-aqueous self-assembly of a vanadate precursor in the presence of Mg²⁺, resulting in two mixed-valent (V^IV/V) species, [(MgOH)V₁₃O₃₃Cl]^4- (={MgV₁₃}) and the di-vanadium-functionalized species [V₁₄O₃₄Cl]^4- (={V₁₄}), which co-crystallize in a 1 : 1 molar stoichiometry. Experimental data indicate that in the native state, {MgV₁₃} is reduced by three electrons, and {V₁₄} is reduced by five electrons. Electrochemical studies in solution show, that the system can reversibly undergo up to fourteen redox transitions (tentatively assigned to twelve 1-electron processes and two 2-electron processes) in the potential range between -2.15 V to +1.35 V (vs Fc⁺/Fc). The study demonstrates how highly redox-active, well-defined molecular mixtures of mixed-valent molecular metal oxides can be accessed by redox-equilibration during synthesis, opening new avenues for molecular energy storage.