Stabilising pentavalent actinides-visible-near infrared and X-ray absorption spectroscopic studies of the utility of the [(Np3W4O15)(H2O)3(MW9O33)3]18- (M = Sb, Bi) structural type

We report the interaction between B-type tri-lacunary heteropolyoxotungstate anions and actinyl(v) cations in aqueous solution, yielding a greater understanding of the stability of the O[triple bond, length as m-dash]An[triple bond, length as m-dash]O1+ linear dioxo actinide moiety. Previously we reported that B-[small alpha]-[BiW9O33]9- and B-[small alpha]-[SbW9O33]9- will react with NpO21+ to yield [(Np3W4O15)(H2O)3(MW9O33)3]18- (M = Bi, or Sb). Single crystal structural characterisation of salts of these complexes revealed a core in which three NpV atoms interact with a central WVI atom through bridging oxo groups. These bridging oxygen atoms come from one of the two axial oxygens in O[triple bond, length as m-dash]Np[triple bond, length as m-dash]O1+ and represent a highly unusual interaction for a discrete molecular species. In this study visible/near infra-red spectroscopy indicates that [(Np3W4O15)(H2O)3(BiW9O33)3]18- could be readily stabilized in solution at near neutral pH for several months, with (NH4)14Na4[(Np3W4O15)(H2O)39BiW9O33)3][middle dot]62H2O crystallising from solution in high yield. At lower pH and [BiW9O33]9- : NpO21+ ratios additional NpV species could be observed in solution. Stabilization of [(Np3W4O15)(H2O)3(SbW9O33)3]18- in solution proved more challenging, with several distinctive NpV near infra-red transitions observed in solution. Slow complexation kinetics and reduction to NpIV was also observed. High [SbW9O33]9- : NpO21+ molar ratios and careful control of solution pH was required to prepare solutions in which [(Np3W4O15)(H2O)3(SbW9O33)3]18- was the only neptunium containing species. In stark contrast to the NpO21+ chemistry, [BiW9O33]9- readily oxidizes PuO21+ to PuO22+ yielding further evidence of the decreased stability of PuVvs. NpV. Np LII-edge XAFS measurement revealed very good agreement with single crystal diffraction data for the Np structural environment for [(Np3W4O15)(H2O)3(MW9O33)3]18- (M = Bi, or Sb) in the solid state. There was also good agreement between coordination shells for [(Np3W4O15)(H2O)3(BiW9O33)3]18- in the solid state and in solution, yielding further confirmation of the high stability of this particular cluster.