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

Abstract

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 OAnO¹⁺ linear dioxo actinide moiety. Previously we reported that B-α-[BiW₉O₃₃]⁹⁻ and B-α-[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 Np^V atoms interact with a central W^VI atom through bridging oxo groups. These bridging oxygen atoms come from one of the two axial oxygens in ONpO¹⁺ 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]·62H₂O crystallising from solution in high yield. At lower pH and [BiW₉O₃₃]⁹⁻ : NpO₂¹⁺ ratios additional Np^V species could be observed in solution. Stabilization of [(Np₃W₄O₁₅)(H₂O)₃(SbW₉O₃₃)₃]¹⁸⁻ in solution proved more challenging, with several distinctive Np^V near infra-red transitions observed in solution. Slow complexation kinetics and reduction to Np^IV 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 Pu^Vvs.Np^V. Np L_II-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.