CuSeO4 and Cu(SeO3OH)2·6H2O, two novel copper–selenium(VI) oxysalts

The two new copper(II) salts Cu(SeO₄) and Cu(SeO₃OH)₂·6H₂O were synthesized at low-temperature hydrothermal conditions (220 °C), and room temperature, respectively. Their atomic arrangements were studied based on single-crystal X-ray investigations [P2₁/n, a = 4.823(1), b = 8.957(2), c = 6.953(1) Å, β = 94.82(1)°, Z = 4; \(P{\overline 1}\), a = 6.133(1), b = 6.303(1), c = 8.648(2) Å, α = 70.45(1), β = 84.60(1), γ = 73.44(1)°, Z = 1]. Cu(SeO₄) adopts the MnAsO₄ structure type. It exhibits structural as well as topological relations with two formerly known isochemical compounds. They crystallize in the structure type ZnSO₄ (mineral name zincosite, Pnma) respectively NiSO₄ (Cmcm). The two minerals dravertite, CuMg(SO₄)₂, and hermannjahnite, CuZn(SO₄)₂, are isotypic with CuSeO₄-P2₁/n; interestingly, also α-NaCu(PO₄) belongs to this structure type: some rotation of the XO₄ group allows a supplementary position for the Na atom. — Cu(SeO₃OH)₂·6H₂O represents a new structure type. The protonated selenate group shows an extended Se—O_h bond distance (1.695 Å) as compared to the other Se—O bonds (1.614 to 1.626 Å). One OH dipole of the three independent H₂O molecules represents a rather free hydrogen bond. For the other H atoms, the O—H···O lengths vary from 2.585 to 2.799 Å. Interestingly, the distance O_w7···O_w7 of only 2.791 Å does not represent an edge in a coordination polyhedron and it is not preliminary involved in the hydrogen bond scheme. All Cu²⁺ ions in the two title compounds are in a pronounced [4 + 2] coordination. The Cu^2+[4+2] atoms in Cu(SeO₄) are linked to chains along [100]; in Cu(SeO₃OH)₂·6H₂O they are not connected among each other.