Chemical equilibria in the UO22+–H2O2–F−/OH− systems and possible solution precursors for the formation of [Na6(OH2)8]@[UO2(O2)F]2418− and [Na6(OH2)8]@[UO2(O2)OH]2418− clusters

Abstract

The focus of this study is on the relationship between uranyl(VI) poly-peroxo clusters in the solid state and their possible precursors in solution. For this purpose, the complex formation in the ternary U(VI)–H₂O₂–F⁻ system has been studied by potentiometric titrations, measuring p[H⁺] and p[F⁻], revealing that significant amounts of ternary uranyl(VI)–peroxide–fluoride complexes are formed. Based on the analysis of these data we find that there are two models consistent with structure data and previous speciation in the uranyl(VI)–peroxide–carbonate system (Dalton. Trans., 2012, 41, 11635–11641). One model contains ternary complexes (UO₂)₄(O₂)₄F⁻ and (UO₂)₄(O₂)₄F₂²⁻ and the other (UO₂)₄(O₂)₄F⁻ and (UO₂)₅(O₂)₅F₃³⁻; we have chosen the second model as the one most consistent with available information. We suggest that (UO₂)₄(O₂)₄F⁻ is a building block in the U-24 cluster, [Na₆(OH₂)₈]@[UO₂(O₂)F]₂₄¹⁸⁻ identified in a single-crystal X-ray diffraction study of the solid phase that slowly precipitates from the slightly acidic test solutions. At p[H⁺] ≈ 9.5, a new solid phase is formed that contains the cluster [Na₆(OH₂)₈]@[UO₂(O₂)OH]₂₄¹⁸⁻, also identified from an X-ray structure. Both structures contain η²–η² bridging peroxide and η² bridging fluoride or hydroxide ions, respectively. As fluoride bridges are unknown in solution coordination chemistry, it is unlikely that the U-24 fluoride cluster is formed in solution. We suggest that both the solid state fluoride and hydroxide clusters are formed in the crystallization from smaller precursors identified in solution. The study illustrates the importance of accurate control of the solution chemistry when preparing poly-peroxo-metallate clusters and also that the mechanism of their formation is still an open field of research.