An uncommon Cu[2+4]O6coordination polyhedron in the crystal structure of KCu3(OH)2[(AsO4)H(AsO4)] (with a comparison to related structure types)

Crystals of KCu 3 (OH) 2 [(AsO 4 )H(AsO 4 )] were synthesized under hydrothermal conditions at ∼ 500 K. The crystal structure was solved from 1386 single crystal X-ray data: a = 10.292(5) Å, b = 5.983(3) Å, c = 7.877(4) Å, β = 117.86(2)°, space group C 2/ m , Z = 2, R = 0.033. One of the two crystallographically different copper coordination polyhedra is a “compressed octahedron” with two “short” (axial) and four “medium” (equatorial) Cu – O bonds [1.899(2) Å, 2× and 2.186(1) Å, 4×]. The other Cu atom as usual is [4 + 2] coordinated [Cu – O = 1.934(1) Å, 2×, 2.000(1) Å, 2×, and 2.428(2) Å, 2×]. Each two arsenate tetrahedra are connected to dimers by a symmetry restricted hydrogen bond of 2.490(6) Å. The structure type of KCu 3 (OH) 2 [(AsO 4 )H(AsO 4 )] is strongly related with that of NaCu 2 (H 3 O 2 )(SO 4 ) 2 (natrochalcite) and Cu 2 O(SO 4 ) (dolerophanite).