Super-reduction of Polyoxometalates: Unlocking the Oxidation State of Metalate Centers down to (III)

Predicting the number of electrons that polyoxometalates (POMs) can store, and whether these electrons are localized or delocalized, is a fundamental question with significant implications for energy storage and conversion. In this context, we report herein evidences that POM metalate centers can be reduced up to their trivalent state. This is exemplified by the super-reduction of the mixed metal Wells-Dawson POM [P₂W₁₅Mo₃O₆₂]⁶⁻ in acidic media, achieving up to 12 electrons per POM. Structural and electronic changes induced by the reduction process have been unveiled using an integrative approach combining in situ measurements (quick XANES/EXAFS analyzed with chemometric tools), ex situ techniques (X-ray diffraction, UV-vis spectroscopy, voltammetry, and multinuclear NMR), and in silico studies (DFT, TD-DFT, and CASSCF calculations). Our findings show a significant molecular transformation after the insertion of six electrons, leading to the MoIV-MoIV bonds formation within the {Mo₃O₁₃} unit. This adaptive behavior of the metal-oxo frameworks, assisted by multiple proton transfers, unlocks access to the trivalent state of Mo centers without decomposition of the initial Dawson-type POM structure. We anticipate that these insights will contribute to advancing the application of POMs in electrochemical energy storage systems.