| Abstract
| - Transition-metal-disubstituted Keggin-type heteropolytungstates were obtained by reaction of Fe3+, Ni2+, Zn2+, and Co2+ with aqueous acidified Na2WO4 (pH ≈ 7), Na6[W7O24]·∼14H2O (sodium paratungstate A), or Na10[H2W12O42]·27H2O (sodium paratungstate B). The 3d metals are distributed over the central (tetrahedral) and peripheral (octahedral) positions of the Keggin anions and cationic positions outside of the polyoxotungstate framework. Spectroscopic and analytical data provided evidence that the 3d-metal-disubstituted Keggin anions studied do not exist in solution but are being formed during the crystallization process.
- Reaction of acidified (pH ≈ 7) sodium tungstate solutions with transition metal cations (Fe3+, Ni2+, Zn2+, Co2+)leads to the formation of transition-metal-disubstituted Keggin-type heteropolytungstates with 3d-metal ions distributedover three different positions. A detailed investigation of the synthesis conditions confirmed that the complexescould equally be obtained using aqueous solutions of either Na2WO4·2H2O (sodium monotungstate) at pH ≈ 7,Na6[W7O24]·∼14H2O (sodium paratungstate A), or Na10[H2W12O42]·27H2O (sodium paratungstate B) as startingmaterials. Three complexes, (NH4)6NiII0.5[α-FeIIIO4W11O30NiIIO5(OH2)]·18H2O, (NH4)7Zn0.5[α-ZnO4W11O30 ZnO5(OH2)]·18H2O, and (NH4)7NiII0.5[α-ZnO4W11O30NiIIO5(OH2)]·18H2O were isolated in crystalline form. X-ray single-crystalstructure analysis revealed that the solid-state structures of the three compounds consist of four main structuralfragments, namely [MO4W11O30M‘O5(OH2)]n- (Keggin-type, α-isomer) heteropolytungstates, hexaquo metal cations,[M‘ ‘(OH2)6]2+, ammonium−water cluster ions, [(NH4+)8(OH2)12], and additional ammonium cations and water molecules.The 3d metals occupy the central (tetrahedral, M) and the peripheral (octahedral, M‘) positions of the Kegginanion, as well as cationic sites (M‘ ‘) outside of the polyoxotungstate framework. UV−vis spectroscopy, solution (1H,183W) and solid-state (1H) NMR, and also chemical analysis data provided evidence that the 3d-metal-disubstitutedKeggin anions do not exist in solution but are being formed only during the crystallization process. Investigationsin the solid state and in solution were completed by ESR, IR, and Raman measurements.
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