Lone electron pair stereoactivity, cation arrangements and distortion of heteropolyhedral sheets in the structures of Tl-2[(UO2)(AO(4))(2)] (A = Cr, MO)

Two new Tl uranyl compounds, Tl-2(UO2)(MoO4)(2) and Tl-2(UO2)(CrO4)(2), have been prepared by high-temperature solid-state reaction and hydrothermal synthesis, respectively. The structure of Tl-2(UO2)(MoO4)(2) (orthorhombic, Pca2(1), a = 10.977(3), b = 14.004(3), c = 14.041(3) angstrom, V = 2158.5(8) angstrom(3), Z = 8) has been solved by direct methods and refined to R-1 = 0.051 on the basis of 5229 unique observed reflections (vertical bar F-O vertical bar >= 4 sigma(F)). The structure of Tl-2(UO2)(CrO4)(2) (orthorhombic, Pca2(1), a = 10.7034(5), b = 13.4252(6), c = 13.9364(6) angstrom, V = 2002.6(2) angstrom(3), Z = 8) has been solved by direct methods and refined to R-1 = 0.035 on the basis of 6330 unique observed reflections (vertical bar F-O vertical bar >= 4 sigma(F)). The compounds are isostructural and are based upon [(UO2)(AO(4))(2)] sheets (A = Cr, Mo) of UrO(5) pentagonal bipyramids (Ur = U-2(2+), uranyl ion) that share equatorial corners with AO(4) tetrahedra. The sheets are parallel to (001) and are linked by TV cations located in the interlayer. Stereoactive lone electron pairs on the Tl+ cations form lone pair-containing cavities that form channels parallel to the a axis and are located at the level of y similar to 0.5. The repulsion of the Tl+ cations across the cavities leads to the distortion of the cation arrangements and, as a consequence, distortion of the flexible [(UO2)(AO(4))(2)] uranyl molybdate/chromate sheets.