Hydrothermal Synthesis and Structure of [H₄As^III₂As^VMo^V₈Mo^VI₄O₄₀]⁻, A Bicapped, Reduced Keggin Species

The hydrothermal reaction of As₂O₃, MoO₃, NaCl, and water in a mole ratio of 5:10:10:380 for 50 h at 150 °C yielded a dark blue solution, which upon standing produced blue-black crystals of Na_0.5H_4.5[As₃Mo₁₂O₄₀]·4H₂O (1). The structure of the anion of 1 [H₄As₃M0₁₂I₄₀]⁻ consists of an α-Keggin core {AsMo₁₂O₄₀} capped on opposite {MO₄O₄} faces by As(III) groups. The anion is a mixed-valence species with eight Mo(V) and four MO(VI) centers. Crystal data for As₃Mo₁₂Na_0.5O₄₄H_12.5: Triclinic space group P1̄, a = 13.609(3) Å, b = 14.177(3) Å, c = 10.584(2) Å, α = 111.02(1)°, β = 98.95(2)°, γ = 90.15(1)°, V= 1879.2(9) ų, Z = 2, D_calc = 3.711 g cm⁻³; structure solution and refinement were based on 7301 reflections converging at R = 0.0518.