Hydrothermal syntheses and characterization of uranyl tungstates with electro-neutral structural units

Abstract

Abstract Two uranyl tungstates, (UO2)(W2O7)(H2O)3 (1) and (UO2)3(W2O8)F2(H2O)3 (2), were synthesized under hydrothermal conditions at 220°C and were structurally, chemically, and spectroscopically characterized. 1 Crystallizes in space group Pbcm, a = 6.673(5) Å, b = 12.601(11) Å, c = 11.552 Å; 2 is in C2/m, a = 13.648(1) Å, b = 16.852(1) Å, c = 9.832(1) Å, β = 125.980(1)°. In 1 the U(VI) cations are present as (UO2)2+ uranyl ions that are coordinated by five oxygen atoms to give pentagonal bipyramids. These share two edges with two tungstate octahedra and single vertices with four additional octahedra, resulting in a sheet with the iriginite-type anion topology. Only water molecules are located in the interlayer. The structural units of 2 consist of (UO2)2+ uranyl oxy-fluoride pentagonal bipyramids present as either [UO2F2O3]–6 or [UO2FO4]–5 , and strongly distorted tungstate octahedra. The linkage of uranyl pentagonal bipyramids and tungstate octahedra gives a unique sheet anion topology consisting of pentagons, squares and triangles. In 2, the uranyl tungstates sheets are connected into a novel electro-neutral three-dimensional framework through dimers of uranyl pentagonal bipyramids. These dimers connecting the sheets share an edge defined by F anions. 2 is the first example of a uranyl tungstate oxy-fluoride, and 1 and 2 are rare examples of uranyl compounds containing electro-neutral structural units.