posted on 2012-04-02, 00:00authored byKarah
E. Knope, Monica Vasiliu, David A. Dixon, L. Soderholm
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+.