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The radioactive element technetium-99 (99Tc, half-life = 2.1 × 105 years, ß- of 253 keV), is a major byproduct of 235U fission in the nuclear fuel cycle. 99Tc is also found in radioactive waste tanks and in the environment at National Lab sites and fuel reprocessing centers. Separation and storage of the long-lived 99Tc in an appropriate and stable waste-form is an important issue that needs to be addressed. Considering metal oxide solid-state materials as potential storage matrixes for Tc, we are examining the redox speciation of Tc on the molecular level using polyoxometalates (POMs) as models. In this study we investigate the electrochemistry of Tc complexes of the monovacant Wells-Dawson isomers, α1-P2W17O61l0- (α1) and α2-P2W17O6110- (α2) to identify features of metal oxide materials that can sta ilize the immobile Tc(IV) oxidation state accessed from the synthesized Tc(V)O species and to interrogate other possible oxidation states available to Tc within these materials. The experimental results are consistent with density functional theory (DFT) calculations. Electrochemistry of K7-nHn[TcVO(α1P2W17O61)] (TcVO-αl), K7_nHn[TcVO(α2-P2W17O61)] (TcVO-α2) and their rhenium analogues as a function of pH show that the Tc-containing derivatives are always more readily reduced than their Re analogues. Both Tc and Re are reduced more readily in the lacunary αl site as compared to the a2 site. The DFT calculations elucidate that the highest oxidation state attainable for Re is VII while, under the same electrochemistry conditions, the highest oxidation state for Tc is VL The MV?MIV reduction processes for TcVOαl are not pH dependent or only slightly pH dependent suggesting that protonation does not accompany reduction of this species unlike the MVO-α2 (M = 99Tc, Re) and ReVO-αl where MV/IV reduction process must occur hand in hand with protonation of the terminal M=O to make the π*(M=O) orbitals accessible to the addition of electrons. This result is consistent with previous extended X-ray absorption fine structure (EXAES) and X-ray absorption near edge structure (XANES) data that reveal that the TcV is "pulled" into the -αl framework and that may facilitate the reduction of TcVO-αl and stabilize lower Tc oxidation states. This study highlights the inequivalency of the two sites, and their impact on the chemical properties of the Tc substituted in these positions. [ABSTRACT FROM AUTHOR]

The tetra-n-butylammonium (TBA) salts for a series of lanthanoid(III) (Ln = Nd, Sm, Eu, Tb, Dy, Yb, and Y) complexes with the α-1-isomer of the Wells-Dawson heteropolyoxoanion, α-1-[P2W17O61]10−, were prepared and characterized by voltammetry, controlled-potential bulk electrolysis, Eu L3-edge XANES spectroelectrochemistry, and Ln L3-edge X-ray absorption spectroscopy. Aspects of the series behavior across the 4f period for the complex anions with nominal 1 : 1 Ln(iii):α-1-[P2W17O61]10−stoichiometries are detailed. The voltammetric response of the α-1 ligand alone in dry acetonitrile (with 0.1 M (TBA)PF6electrolyte) is concentration independent and remarkably well-defined with five waves attributable to W-based redox processes. The formation of heteropoly blue solutions upon electrochemical reduction results in chemical instabilities and isomerization. The deliberate addition of water turns an otherwise ideal response into a broad and poorly resolved one, wherein the first reduction process is shifted 150 mV to more positive electrode potentials. Upon its coordination with Ln(iii) ions, the voltammetric response develops notable complexities with as many as ten concentration-dependent couples attributable to W-based redox processes of the Ln:α-1 complexes. The results from in situEu L3-edge XANES of the Eu(iii):α-1 complex provide no evidence for the one-electron reduction of Eu(iii) at controlled electrode potentials comparable to those that were previously found to form Eu(ii) in the potassium salt of the corresponding Eu(iii) complex upon reduction in an aqueous electrolyte. To explain the contrasting system behaviors, the Ln(iii) coordination environments in the TBA5H2[(H2O)nLn(α-1-P2W17O61)] solid salts, including the extent of Ln hydration (n) upon their dissolution in acetonitrile, were determined through use of EXAFS, which demonstrates a structural uniformity among the salts and their acetonitrile solutions, wherein the average Ln-O interatomic distances and O coordination numbers reveal variations that are consistent with the effects of the lanthanoid contraction. The side-by-side comparison of the solid and solution data provides evidence that is consistent with a partial solvent (H2O–CH3CN) exchange upon dissolution in MeCN. Details of the Ln(iii) coordination chemistry, wherein the decrease in the ionic radius from the large, light Nd(iii) to the small, heavy Yb(iii), are presented in the context of known structural and physical phenomena of acetonitrile solvates, aqua ions, and the corresponding water-soluble K7[(H2O)4Eu(α-1-P2W17O61)] complex. [ABSTRACT FROM AUTHOR]

The redox speciation of Eu(III) in the 1:1 stoichiometric complex with the α-1 isomer of the Wells-Dawson anion, [α-1-P2W17O61]10-, was studied by electrochemical techniques (cyclic voltammetry and bulk electrolysis), in situ XAFS (X-ray absorption fine structure) spectroelectrochemistry, NMR spectroscopy (31P), and optical luminescence. Solutions of K7[(H2O)4Eu(α-1-P2W17O61)] in a 0.2 M Li2SO4 aqueous electrolyte (pH 3.0) show a pronounced concentration dependence to the voltammetric response. The fully oxidized anion and its reduced forms were probed by Eu L3-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, α-[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) Å, which is 0.17 Å longer than that for the oxidized anion, and consistent with the 0.184 Å difference between the Eu(II) and Eu(III) ionic radii. The reduction of Eu(III) is unusual among POM complexes with Lindqvist and Å-2 isomers of Wells-Dawson anions, that is, [Eu(W5O18)2]9- and [Eu(Å-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. [ABSTRACT FROM AUTHOR]