Applications of extended X-ray absorption fine structure spectroscopy to the study of polyoxometalates

Abstract

Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to characterise polyoxometalates of the type MmOyn– and XxMmOyn– in the solid state and in solution. The best fit to the low-temperature (80 K) niobium K-edge EXAFS of K7[HNb6O19]·13H2O was obtained with one terminal oxygen at 1.79 A(M–Ot), four doubly bridging oxygens at 1.97 A(M–Ob), four niobiums at 3.32 A and one niobium at 4.76 A. The Nb–Ob–Nb bond angle was determined as 113° following a multiple-scattering calculation for this unit. Analysis of the niobium K- and tungsten LIII-edge EXAFS of salts of NbxW6 –xO19(x+ 2)–(x= 2–4) yielded Nb–O bond lengths of 1.74–1.76 and 1.98–1.99 A, W–O bond lengths of 1.72–1.74 and 1.89–1.91 A, and non-bonded M ⋯ M distances of 3.31–3.35 and 4.74–4.78 A. The first co-ordination spheres also comprise one six-fold bridging oxygen at about 2.3 A but this shell is poorly defined due to static and thermal disorder in the bond. A combination of vanadium K- and tungsten LIII-edge EXAFS of [V2W4O19]4– yielded W ⋯ W separations of 3.27 A but shorter V ⋯ W separations of 3.20 A, showing a slight distortion in the hexametalate structure that is not observed for the niobotungstates. Molybdenum K-edge EXAFS analysis of [NBun4]3[PMo12O40] in the solid state at 80 K revealed systematic asymmetry in the Mo–Ob bonds with two oxygens at 1.81 and two at 1.97 A. The structure is not perturbed when the salt is dissolved in acetonitrile. Likewise the molybdenum K-edge EXAFS of [TeMo6O24]6– in water and [Mo6O19]2– in acetonitrile analysed for the same co-ordination spheres as those of the parent salts Na3[TeMo6O24]·nH2O and [NBun4]2[Mo6O19].