Contributions to the stereochemistry of zirconium oxysalts—part I: syntheses and crystal structures of novel Zr(SeO4)2·H2O and Zr(SeO4)2·4H2O

Abstract

The new compounds Zr(SeO4)2·H2O and Zr(SeO4)2·4H2O were synthesized at low-hydrothermal conditions (teflon-lined steel vessels, 220 °C) from mixtures of Zr2O2(CO3)(OH)2, H2SeO4, and minor contents of water. Colorless single crystals up to 0.2 mm in length, obtained within 1 week, were studied by single-crystal X-ray techniques. Zr(SeO4)2·H2O crystallizes in the monoclinic space group P21/n (no. 14), with a = 5.332(1) A, b = 7.962(2) A, c = 16.256(3) A, β = 92.19(1)°, V = 689.6(3) A3, Z = 4, and R1 = 0.0195. Zr(SeO4)2·H2O represents a new structure type built from isolated ZrO7 pentagonal dipyramids, further cornerlinked via SeO4 groups to a three-dimensional framework. One H2O group acts as a ligand of the ZrO7 coordination. Zr(SeO4)2·4H2O [orthorhombic, space group Fddd (no. 70), with a = 5.651(1) A, b = 11.964(3) A, c = 26.611(6) A, V = 1799.1(4) A3, Z = 8, and R1 = 0.0177] is isotypic with the compounds M(SO4)2·4H2O; M = Zr, Hf, Ce, and Pu. The structure is built up by sheets within (001), linked by hydrogen bonds only. The zirconium atom is 8-coordinated to oxygen atoms of four H2O molecules and of SeO4 tetrahedra, forming a distorted tetragonal antiprism. For both structures, mean cation-oxygen bond lengths are in accordance with the literature: Zr[7]: 2.137 A, Zr[8]: 2.188 A, and Se[4]: 1.631–1.635 A. Observed hydrogen bonds show donor–acceptor distances within the range of 2.65–2.79 A.