Lanthanide complexes of the alpha-1 isomer of the [P2W17O61](10-) heteropolytungstate: Preparation, stoichiometry, and structural characterization byW-183 and P-31 NMR spectroscopy and europium(III) luminescence spectroscopy

The alpha-1 and alpha-2 [P2W17O61](10-) isomers, derivatives of the Wells-D awson molecule, [alpha-P2W18O62](6-), may be useful ligands for stabilizing high-valent metal ions and lanthanides and actinides. However, the potenti al utility of the [alpha 1-P2W17O61](10-) ligand has not been realized. Spe cifically, for the lanthanides, the stoichiometry, structure, and purity of the lanthanide complexes of the [alpha 1-P2W17O61](10-) isomer are ambiguo us. We have prepared lanthanide (Ln) complexes of the [alpha 1-P2W17O61](10 -) isomer in greater than or equal to 98% isomeric purity, according to P-3 1 NMR data. W-183 NMR data clearly showed, for the first time, that the C-1 symmetry of the [alpha 1-P2W17O61](10-) lanthanide complexes was maintaine d in solution. We determined the stoichiometry of the lanthanide complexes of the [alpha 1-P2W17O61](10-) 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:[alpha 1-P2W17O61](10-) ratio. The P-31 NMR data showed that the lanthanides with smaller ionic radii (higher charge-size ratio) form s table complexes, even surviving crystallization from hot water. On the othe r hand, the lanthanum analogues were not stable in solutions of high lithiu m content. The tetrabutylammonium salt of the [Lu(alpha 1-P2W17O61)](7-) co mplex showed greater than or equal to 98% isomeric purity and the C-1 symme try required for a derivative of [alpha 1-P2W17O61](10-). Also the tetrabut ylammonium cation stabilized the [Lu(alpha 1-P2W17O61)](7-) complex; a mixe d tetrabutylammonium, lithium salt was stable in water for weeks according to P-31 NMR spectroscopy.