Europium(III) reduction and speciation within a Wells-Dawson heteropolytungstate

The redox speciation of Eu(III) in the 1:1 stoichiometric complex with the alpha-1 isomer of the Wells-Dawson anion, [alpha-1-P2W17O61](10-) was studied by electrochemical techniques (cyclic voltammetry and bulk electrolysis), in situ XAFS (X-ray absorption fine structure) spectroelectrochemistry, NMR spectroscopy (P-31), and optical luminescence. Solutions of K-7[(H2O)(4)Eu(alpha-1-P2W17O61)] in a 0.2 M Li2SO4 aqueous electrolyte (pH 3.0) show a pronounced concentration dependence to the voltarnmetric response. The fully oxidized anion and its reduced forms were probed by Eu L-3-edge XANES (X-ray absorption near edge structure) measurements in simultaneous combination with controlled potential electrolysis, demonstrating that Eu(III) in the original complex is reduced to Eu(II) in conjunction with the reduction of polyoxometalate (POM) ligand. After exhaustive reduction, the heteropoly blue species with Eu(II) is unstable with respect to cluster isomerization, fragmentation, and recombination to form three other Eu-POMs as well as the parent Wells-Dawson anion, alpha-[P2W18O62](6-). EXAFS data obtained for the reduced, metastable Eu(II)-POM before the onset of Eu(II) autoxidation provides an average Eu-O bond length of 2.55(4) A, which is 0.17 angstrom longer than that for the oxidized anion, and consistent with the 0.184 A difference between the Eu(II) and Eu(Ill) ionic radii. The reduction of Eu(III) is unusual among POM complexes with Lindqvist and alpha-2 isomers of Wells-Dawson anions, that is, [Eu(W5O18)](9-)- and [Eu(alpha-2-As2W17O61)(2)](17-), but not to the Preyssler complex anion, [EUP5W30O110](12-), and fundamental studies of materials based on coupling Eu and POM redox properties are still needed to address new avenues of research in europium hydrometallurgy, separations, and catalysis sciences.