A Layer Structure Encapsuling Interlayer K⁺ Cations: Hydrothermal Synthesis and Crystal Structure of Vanadyl(IV) Arsenate K₂[(VO)₂(AsO₄)(HAsO₄)(H₂AsO₄)(H₂O)].0.5H₂O

A new vanadyl(IV) arsenate, K₂[(VO)₂(AsO₄)(HAsO₄)(H₂AsO₄)(H₂O)].0.5H₂O, 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) ų Z = 2 and R = 0.0428 for 2114 unique reflections. The structure consists of V^IVO₆ octahedra and tetrahedra of AsO₄³⁻ HAsO₄²⁻, and H₂AsO₄⁻ 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 K₂(VO)₂(P₃O₉)(OH)₃‐1.125H₂O (Lii et al. Inorg. Chem. 1991, 30, 446), that was synthesized at 450 °C. In contrast to the phosphate, tetranedra of three groups, AsO₄³⁻, HAsO₄²⁻, and H₂AsO₄⁻, 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 K₂[(VO)₂(AsO₄)(HAsO₄)(H₂AsO₄)(H₂O)].0.5H₂O. The structures of this arsenate and K₂(VO)₂(P₃O₉)(OH)₃·1.125H₂O are discussed.