A Layer Structure Encapsuling Interlayer K + Cations: Hydrothermal Synthesis and Crystal Structure of Vanadyl(IV) Arsenate K 2 [(VO) 2 (AsO 4 )(HAsO 4 )(H 2 AsO 4 )(H 2 O)].0.5H 2 O

A new vanadyl(IV) arsenate, K 2 [(VO) 2 (AsO 4 )(HAsO 4 )(H 2 AsO 4 )(H 2 O)].0.5H 2 O, has been synthesized hydrothermally at 230 °C and structurally characterized by single‐crystal X‐ray diffraction. The compound crystallizes in triclinic space group P1 with a= 6.533(2) Å, b = 9.062(3) Å, c = 12.582(5) Å, α = 106.29(3), β = 98.64(3), γ = 103.09(2)° V = 678.0(4) Å 3 Z = 2 and R = 0.0428 for 2114 unique reflections. The structure consists of V IV O 6 octahedra and tetrahedra of AsO 4 3− HAsO 4 2− , and H 2 AsO 4 − groups to form layers along the [001] direction with K + cations between the layers. Building units of the layers are 4‐connected ribbons connected to each other by dihydrogen arsenate groups to form large tunnels perpendicular to the layers. Cations and water molecules reside in the tunnels. The title compound is isotypic with phosphate K 2 (VO) 2 (P 3 O 9 )(OH) 3 ‐1.125H 2 O (Lii et al. Inorg. Chem . 1991 , 30 , 446), that was synthesized at 450 °C. In contrast to the phosphate, tetranedra of three groups, AsO 4 3− , HAsO 4 2− , and H 2 AsO 4 − , were distinctly identified in the arsenate. The sites for lattice water are essentially fully occupied due to the larger space provided by the framework of K 2 [(VO) 2 (AsO 4 )(HAsO 4 )(H 2 AsO 4 )(H 2 O)].0.5H 2 O. The structures of this arsenate and K 2 (VO) 2 (P 3 O 9 )(OH) 3 ·1.125H 2 O are discussed.