Lanthanide Complexes of the alpha-1 Isomer of the [P(2)W(17)O(61)](10-) Heteropolytungstate: Preparation, Stoichiometry, and Structural Characterization by (183)W and (31)P NMR Spectroscopy and Europium(III) Luminescence Spectroscopy.

The alpha-1 and alpha-2 P(2)W(17)O(61)(-) isomers, derivatives of the Wells-Dawson molecule, alpha-P(2)W(18)O(62)(-), may be useful ligands for stabilizing high-valent metal ions and lanthanides and actinides. However, the potential utility of the alpha1-P(2)W(17)O(61)(-) ligand has not been realized. Specifically, for the lanthanides, the stoichiometry, structure, and purity of the lanthanide complexes of the alpha1-P(2)W(17)O(61)(-) isomer are ambiguous. We have prepared lanthanide (Ln) complexes of the alpha1-P(2)W(17)O(61)(-) isomer in >/=98% isomeric purity, according to (31)P NMR data. (183)W NMR data clearly showed, for the first time, that the C(1) symmetry of the alpha1-P(2)W(17)O(61)(-) lanthanide complexes was maintained in solution. We determined the stoichiometry of the lanthanide complexes of the alpha1-P(2)W(17)O(61)(-) isomer in solution by two different methods: a complexometric titration method and excited state lifetime measurements and luminescence titrations for the europium(III) analogue. All experiments show a 1:1 Ln:alpha1-P(2)W(17)O(61)(-) ratio. The (31)P NMR data showed that the lanthanides with smaller ionic radii (higher charge-size ratio) form stable complexes, even surviving crystallization from hot water. On the other hand, the lanthanum analogues were not stable in solutions of high lithium content. The tetrabutylammonium salt of the Lu(alpha1-P(2)W(17)O(61))(-) complex showed >/=98% isomeric purity and the C(1) symmetry required for a derivative of alpha1-P(2)W(17)O(61)(-). Also the tetrabutylammonium cation stabilized the Lu(alpha1-P(2)W(17)O(61))(-) complex; a mixed tetrabutylammonium, lithium salt was stable in water for weeks according to (31)P NMR spectroscopy.