Synthesis, Crystal Structure, and Solution Stability of Keggin-Type\nHeteropolytungstates (NH₄)₆Ni^II_0.5[α-Fe^IIIO₄W₁₁O₃₀Ni^IIO₅(OH₂)]·<i>n</i>H₂O,\n(NH₄)₇Zn_0.5[α-ZnO₄W₁₁O₃₀ZnO₅(OH₂)]·<i>n</i>H₂O, and\n(NH₄)₇Ni^II_0.5[α-ZnO₄W₁₁O₃₀Ni^IIO₅(OH₂)]·<i>n</i>H₂O (<i>n</i> ≈ 18)^†

Abstract

Reaction of acidified (pH ≈ 7) sodium tungstate solutions with transition metal cations (Fe³⁺, Ni²⁺, Zn²⁺, Co²⁺)\nleads to the formation of transition-metal-disubstituted Keggin-type heteropolytungstates with 3d-metal ions distributed\nover three different positions. A detailed investigation of the synthesis conditions confirmed that the complexes\ncould equally be obtained using aqueous solutions of either Na₂WO₄·2H₂O (sodium monotungstate) at pH ≈ 7,\nNa₆[W₇O₂₄]·∼14H₂O (sodium paratungstate A), or Na₁₀[H₂W₁₂O₄₂]·27H₂O (sodium paratungstate B) as starting\nmaterials. Three complexes, (NH₄)₆Ni^II_0.5[α-Fe^IIIO₄W₁₁O₃₀Ni^IIO₅(OH₂)]·18H₂O, (NH₄)₇Zn_0.5[α-ZnO₄W₁₁O₃₀ ZnO₅(OH₂)]·18H₂O, and (NH₄)₇Ni^II_0.5[α-ZnO₄W₁₁O₃₀Ni^IIO₅(OH₂)]·18H₂O were isolated in crystalline form. X-ray single-crystal\nstructure analysis revealed that the solid-state structures of the three compounds consist of four main structural\nfragments, namely [MO₄W₁₁O₃₀M‘O₅(OH₂)]<i>ⁿ</i>^- (Keggin-type, α-isomer) heteropolytungstates, hexaquo metal cations,\n[M‘ ‘(OH₂)₆]²⁺, ammonium−water cluster ions, [(NH₄⁺)₈(OH₂)₁₂], and additional ammonium cations and water molecules.\nThe 3d metals occupy the central (tetrahedral, M) and the peripheral (octahedral, M‘) positions of the Keggin\nanion, as well as cationic sites (M‘ ‘) outside of the polyoxotungstate framework. UV−vis spectroscopy, solution (¹H,\n¹⁸³W) and solid-state (¹H) NMR, and also chemical analysis data provided evidence that the 3d-metal-disubstituted\nKeggin anions do not exist in solution but are being formed only during the crystallization process. Investigations\nin the solid state and in solution were completed by ESR, IR, and Raman measurements.