Variable Dimensionality and New Uranium Oxide Topologies in the Alkaline-Earth Metal Uranyl Selenites AE[(UO2)(SeO3)2] (AE=Ca, Ba) and Sr[(UO2)(SeO3)2] · 2H2O

Three new alkaline-earth metal uranyl selenites, Ca[(UO2)(SeO3)2] (1), Sr[(UO2)(SeO3)2] · 2H2O (2), and Ba[(UO2)(SeO3)2] (3), have been prepared from the reactions of CaCO3 and Ca(OH)2, SrCl2 and Sr(OH)2, or BaCl2 and Ba(OH)2 with UO3 and SeO2 under mild hydrothermal conditions. Single-crystal X-ray diffraction experiments reveal that the structures of 1-3 differ in both connectivity and dimensionality even though all contain the same fundamental building unit, namely [UO2(SeO3)4]. This polyhedron consists of a linear uranyl A unit that is bound by one chelating and three bridging selenite anions creating a pentagonal bipyramidal environment around the U(VI) center. The crystal structure of 1 contains onedimensional ribbons where the edges are terminated by monodentate selenite anions. The interior of the ribbons are constructed from edge-sharing pentagonal bipyramidal UO7 units. The structure of 2 is also one-dimensional; however, here there are chains of edge-sharing pentagonal bipyramidal UO7 dimers that are connected by bridging selenite anions. Ba[(UO2) (SeO3)2] (3) is two-dimensional, and the highly ruffled anionic sheets present in this structure are formed from both bridging and chelating/bridging selenite anions bound to uranyl moieties. The anionic substructures in 1-3 are separated by Ca2+, Sr2+, or Ba2+ cations. Crystallographic data (193 K, MoKalpha, lambda=0.71073): 1, triclinic, space group P , a= 5.5502(6) ?, b=6.6415(7) ?, c=11.013(1) ?, alpha=104.055(2)°, beta=93.342(2)°, gamma= 110.589(2)°, Z=2, R(F)=4.56% for 100 parameters with 1530 reflections with I>2sigma(I); 2, triclinic, space group P , a=7.0545(5) ?, b=7.4656(5) ?, c=10.0484(6) ?, alpha= 106.995(1)°, beta=108.028(1)°, gamma=98.875(1)°, Z=2, R(F)= 2.43% for 128 parameters with 2187 reflections with I>2sigma(I); 3, monoclinic, space group P21/c, a=7.3067(6) ?, b= 8.1239(7) ?, c=13.651(1) ?, beta=100.375(2)°, Z=4, R(F)=4.31% for 105 parameters with 1824 reflections with I>2sigma(I).