Keggin cluster formation by hydrothermal reaction of tungsten trioxide with methyl substituted ammonium: The crystal structure of two novel compounds, [NH₂(CH₃)₂]₆H₂W ₁₂O₄₀ · ∼4H₂O and [N(CH₃)₄]₆H₂W₁₂O ₄₀ · 2H₂O

Tungsten(VI) oxides can be formed by mild hydrothermal reactions, with the exact crystalline structure being determined by the cations in solution. The impact on the reaction between tungstic acid and ammonium ions as the hydrogen atoms are systematically substituted by methyl groups is investigated. Ammonium and methylammonium (MA) cations form the pyrochlore structure type (NH₄)_xWO_3+x/2 · nH₂O (A) and (MA)_xWO_3+x/2 · nH₂O (B). Ammonium may also form the hexagonal bronze tunnel structure. Using dimethylammonium (DMA), trimethylammonium (TrMA), and tetramethylammonium (TMA) leads to the formation of well crystallized Keggin structures: DMA₆[H₂W₁₂O₄₀] · nH₂O (C), DMA₆[H₂W₁₂O₄₀] · ∼4H₂O (D), TrMA₆[H₂W₁₂O₄₀] (E), TrMA₆[H₂W₁₂O₄₀] · ∼2H₂O (F), TMA₆[H₂W₁₂O₄₀] · 2H₂O (G), and in some cases hexagonal tungsten bronzes. Characterization was done using powder and single crystal X-ray analysis, as well as IR and TGA methods. The pyrochlore structures have been refined by the Rietveld method. The powder patterns were indexed to the orthorhombic system (space group Pnma, a = 21.886(4), b = 21.469(4), c = 13.232(3) A) for E and monoclinic system (P2₁/n, a = 19.273(4), b = 12.017(2), c = 12.253(2) Å, β = 90.70(1)°) in the case of F. The crystal structures of D and G were determined by single crystal X-ray analysis. Blue crystals of D are orthorhombic, space group Pnma, a = 22.446(3), b = 19.810(3), c = 12.854(2) A, V_cell = 5716 ų, Z = 4, d_clc = 3.72 g/cm³ (MoKα radiation, 2θ_max. = 55°, R = 0.077 for 2076 reflections). Dark blue crystals of G are orthorhombic, space group Pnma, a = 21.90(1), b = 21.507(7), c = 13.27(1) Å, V_cell = 6250 ų, Z = 4, d_clc = 3.54 g/cm³ (MoKα radiation, 2θ_max. = 55°, R = 0.052 for 4488 reflections). Both structures consist of a hexagonal close packed arrangement of the almost spherical Keggin anions [H₂W₁₂O₄₀]^-6. They are not isostructural because of a different orientation of the tungsten clusters.