PNNL

Abstract

Alkali-countercations have emerging roles in polyoxometalate chemistry, directing reactivity in addition to structure. Here, we show the reactivity of uranyl triperoxide monomer building blocks of uranyl peroxide polyoxometalates is profoundly affected by the alkali-countercation. We isolate and structurally characterize the Cs4[UO2(O2)3] and Rb4[UO2(O2)3] monomers and document their spontaneous solution and solid-state polymerization (leading to polyoxometalates in solution) via small-angle X-ray scattering and Raman spectroscopy. We also document alkali hydroxide-promoted peroxide deprotonation and decomposition. Peroxide reactivity increases with increasing alkali size, with and without the uranyl. The Cs4[UO2(O2)3] crystals are extremely reactive, only surviving 30 minutes outside its mother liquor, rapidly converting to CsUVO3 and then to Cs4[UO2(CO3)3]. The former reaction involves reduction of UVI to UV, most likely by peroxide oxidation. Therefore, one possible emergent role of alkali-cations in peroxo-polyoxometalate systems is influencing the redox reactions of peroxide, which can act as either a reducing or an oxidizing reagent.