Synthesis and characterization of open and sandwich-type polyoxometalates reveals visible-light-driven water oxidation via POM-photosensitizer complexes

The trivacant triruthenium-substituted polyoxometalate alpha-K6Na[{Ru3O3(H2O)Cl-2}(SiW9O34)]center dot 17H(2)O (1) and two sandwich-type Co- and Ni-containing silicotungstates K11Na1[M-4(H2O)(2)(SiW9O34)(2)]center dot nH(2)O (M = Co2+, Ni2+) (2, 3) based on the same Keggin-type building block were obtained through variation of a solution-based protocol. All compounds were investigated with respect to their performance as visible-light-driven water oxidation catalysts (WOCs). Analytical characterizations and photocatalytic reactivity of (1) are correlated with results of B97D/Def2-TZVP computation. The WOC activity of (1) and (2) was found to arise from their immobilization into a recyclable POM-photosensitizer complex (POM-PS). These POM-PS complexes are formed in the initial phase of the reaction, and they prevent POM decomposition into colloidal oxide catalysts.