Thorium(IV)–Selenate Clusters Containing an Octanuclear Th(IV) Hydroxide/Oxide Core

Four Th­(IV) hydroxide/oxide clusters have been synthesized from aqueous solution. The structures of [Th8(μ3-O)4(μ2-OH)8(H2O)15(SeO4)8·7.5H2O] (1), [Th8(μ3-O)4(μ2-OH)8(H2O)17(SeO4)8·nH2O] (2), [Th9(μ3-O)4(μ2-OH)8(H2O)21(SeO4)10] (3), and Th9(μ3-O)4(μ2-OH)8(H2O)21(SeO4)10·nH2O (4) were determined using single crystal X-ray diffraction. Each structure consists of an octanuclear core, [Th8O4(OH)8]16+, that is built from eight Th­(IV) atoms (four Th in a plane and two up and two down) linked by four “inner” μ3-O and eight “outer” μ2-OH groups. Compounds 3 and 4 additionally contain mononuclear [Th­(H2O)5(SeO4)4]4– units that link the octamers into an extended structure. The octanuclear units are invariably complexed by two selenate anions that sit in two cavities formed by four planar Th­(IV) and four extra-planar Th­(IV) atoms, thus making [Th8O4(OH)8(SeO4)2]12+ a common building block in 1–4. However, changes in hydration as well selenate coordination give rise to structural differences that are observed in the extended structures of 1–4. The compounds were also characterized by Raman spectroscopy. Density functional theory calculations were performed to predict the geometries, vibrational frequencies, and relative energies of different structures. Details of the calculated structures are in good agreement with experimental results, and the calculated frequencies were used to assign the experimental Raman spectra. On the basis of an analysis of the DFT results, the compound Th8O8(OH)4(SeO4)6 was predicted to be a strong gas phase acid but is reduced to a weak acid in aqueous solution. Of the species studied computationally, the dication Th8O6(OH)6(SeO6)62+ is predicted to be the most stable in aqueous solution at 298 K followed by the monocation Th8O7(OH)5(SeO6)6+.