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Structural Studies of Keggin-Type Polyoxotungstates by Extended X-ray Absorption Fine Structure Spectroscopy



Extended X-ray absorption fine structure spectroscopy(EXAFS) was used to characterize several Keggin-type polyoxotungstates (tetrabutylammonium salts) in the solid state and in acetonitrile solution. The polyoxometalates studied were [PW12O40]3– (the parent Keggin anion), [PW11O39]7– (a lacunary anion), [PW11M(H2O)O39]4–, M = Fe, Ru, and [XW11MnIII(H2O)O39]n, X = P, Si, B (transition-metal-substituted anions). The role of multiple-scattering contributions was evaluated for [PW12O40]3– for which an accurate structure determination was available. The results from the multiple-scattering analyses were compared with the single-scattering method. The average W-shell inter-atomic distances were determined in relation to a selected tungsten atom (Wc) by W LIII edge transmission analysis. Several shells of up to 3.8–4.0 Å were identified. For the anion [PW12O40]3–, the obtained inter-atomic distances in the solid sample and in acetonitrile were identical but for the lacunary anion the EXAFS spectrum of the solid was different from that in solution (the number of atoms coordinated to Wc in [PW11O39]7– was the same in both cases, but the calculated distances for the shells were slightly different). Metal-substituted anions were studied by W LIII edge transmission and by Fe, Mn, or Ru K edge fluorescence analysis. The average coordination sphere of tungsten was affected by the replacement of a W=O by a M–OH2 group. The EXAFS results indicate that on average the MnO6 octahedron is larger than FeO6 or RuO6 and also larger than the average of all WO6 octahedra in [PW11M(H2O)O39]4–. This was attributed to the occurrence of the Jahn–Teller effect in the coordination sphere of MnIII. A comparison of the W LIII edge analysis EXAFS data for three MnIII-substituted anions (X = P, Si, B) suggested that the average size of the W3O13 groups did not depend on X. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)