Reactivity, Formation, and Solubility of Polyoxometalates Probed by Calorimetry.

Room temperature calorimetry methods were developed to describe the energy landscapes of six polyoxometalates (POMs), Li-U24, Li-U28, K-U28, Li/K-U60, Mo132, and Mo154, in terms of three components: enthalpy of dissolution (ΔHdiss), enthalpy of formation of aqueous POMs (ΔHf,(aq)), and enthalpy of formation of POM crystals (ΔHf,(c)). ΔHdiss is controlled by a combination of cation solvation enthalpy and the favorability of cation interactions with binding sites on the POM. In the case of the four uranyl peroxide POMs studied, clusters with hydroxide bridges have lower ΔHf,(aq) and are more stable than those containing only peroxide bridges. In general for POMs, the combination of calorimetric results and synthetic observations suggest that spherical topologies may be more stable than wheel-like clusters, and ΔHf,(aq) can be accurately estimated using only ΔHf,(c) values owing to the dominance of the clusters in determining the energetics of POM crystals.

Crystal Chemistry and Structural Complexity of Uranium(IV) Sulfates: Synthesis of U3H2(SO4)7(H2O)5·3H2O and U3(UO2)0.2(SO4)6(OH)0.4·2.3H2O with Framework Structures by the Photochemical Reduction of Uranyl.

Two uranium(IV) sulfate framework compounds were crystallized at room temperature from aqueous solutions containing uranyl ions by photochemical reduction in the presence of 2-propanol. U3H2(SO4)7(H2O)5·3H2O (1) crystallizes in space group P65 with a = 9.3052(17) Å, c = 53.515(10) Å, V = 4012.9(13) Å3, and Z = 6, and U3(UO2)0.2(SO4)6(OH)0.4·2.3H2O (2) is tetragonal, with space group P42/nmc, a = 25.624(3) Å, c = 8.9435(10) Å, V = 5872.2(11) Å3, and Z = 8. The structures of 1 and 2 are the most complex among uranium(IV) sulfates.

Sulfate-Centered Sodium-Icosahedron-Templated Uranyl Peroxide Phosphate Cages with Uranyl Bridged by µ-η1:η2 Peroxide.

Two novel hybrid uranyl peroxide phosphate cage clusters, designated U20P6 and U20P12, contain peroxide bridges between uranyl in an unusual µ-η1:η2 configuration, as well as the common µ-η2:η2 configuration. These appear to be the only high-nuclearity metal peroxide complexes containing µ-η1:η2 peroxide bridges, and they are unique among uranyl peroxide cages. Both clusters contain 20 uranyl polyhedra, and U20P6 and U20P12 contain 6 and 12 phosphate tetrahedra, respectively. The 20 uranyl polyhedra in both cages are arranged on the vertices of distorted topological dodecahedrons (20 vertex fullerenes). Each cage is completed by phosphate tetrahedra and is templated by a sulfate-centered Na12 cluster with the Na cations defining a regular convex isocahedron. Whereas µ-η2:η2 peroxides are essential features of uranyl peroxide cages, where they form equatorial edges of uranyl hexagonal bipyramids, the µ-η1:η2 peroxide groups in U20P6 and U20P12 are associated with strong distortions of the uranyl polyhedra. Formation of U20P6 and U20P12 is a further demonstration of the pliable nature of uranyl polyhedra, which contributes to the tremendous topological variability of uranyl compounds. Despite the unusual structure and highly distorted polyhedral geometries of U20P6, small-angle X-ray scattering and Raman spectra suggest its stability in the aqueous solution and solid state.