Contributions to the stereochemistry of zirconium oxysalts—part II: syntheses and crystal structures of Zr(SeO₃)(SeO₄), Zr₄(SeO₃)(SeO₄)₇, and Zr₃(SeO₃)(SeO₄)₅·2H₂O

Abstract

Three new compounds Zr(SeO₃)(SeO₄), Zr₄(SeO₃)(SeO₄)₇, and Zr₃(SeO₃)(SeO₄)₅·2H₂O 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 several tenth of a mm in size, obtained within 1 week, were studied by single crystal X-ray techniques. Zr(SeO₃)(SeO₄) crystallizes in the orthorhombic space group Pbca (No. 61), with a = 8.291 (2) Å, b = 9.458 (2) Å, c = 15.357 (3) Å, V = 1204.2 (5) ų, Z = 8, R1 = 0.0322. Zr₄(SeO₃)(SeO₄)₇ is monoclinic, space group P2₁/n (No. 14), with a = 5.313 (1) Å, b = 10.704 (2) Å, c = 10.484 (2) Å, β = 104.13 (1)°, V = 578.2 (1) ų, Z = 1, R1 = 0.0172. Two independent selenium atoms are present in this structure: one forming a SeO₄ tetrahedron, and the other one exhibiting mixed occupation by ¾ Se⁶⁺ and ¼ Se⁴⁺; its coordination is, therefore, partially disordered. Zr₃(SeO₃)(SeO₄)₅·2H₂O crystallizes in the triclinic space group P1 (No. 1), with a = 5.273 (1) Å, b = 8.079 (2) Å, c = 11.959 (2) Å, α = 82.60 (1)°, β = 88.27 (1)°, γ = 89.87 (1)°, V = 505.1 (1) ų, Z = 1, R1 = 0.0235, but exhibits strong centrosymmetric pseudosymmetry; the inversion center is violated only by replacement of one selenate(VI) tetrahedron by a trigonal pyramidal selenite(IV) group as pseudo-centric counterpart. Hydrogen bonds in this compound show donor–acceptor distances within the range of 2.67–2.81 Å. All three framework structures are unique and built up from corner-sharing polyhedra. In all three compounds, mean cation-oxygen bond lengths (Zr^[6]: 2.062 and 2.067 Å; Zr^[7]: 2.132, 2.137 and 2.139 Å; Se^4+[3]: 1.675 and 1.680 Å, excluding the disordered group; Se^6+[4]: 1.621–1.641 Å) are comparatively short, resulting in rather high bond valence sums.

Graphical abstract