Plutonium and Cerium Perrhenate/Pertechnetate Coordination Polymers and Frameworks

Spent nuclear fuel (SNF) contains transuranic and lanthanide species, which are sometimes recovered and repurposed. One particularly problematic fission product, ⁹⁹TcO₄^-, hampers this recovery via coextraction with high valence metals, perhaps by complexation during aqueous reprocessing of SNF. There is limited molecular-level knowledge concerning the coordination chemistry between TcO₄^- or its well-known surrogate ReO₄^- and transuranic/lanthanide species. In the current study, we investigated the coordination of ReO₄^-/TcO₄^- with plutonium and cerium cations by structural and chemical characterization of a series of isolated extended solids. In this study, Ce represents both trivalent lanthanides and is considered a surrogate for Pu, respectively, in its common trivalent and tetravalent oxidation states. The structural elucidation of the seven isolated crystalline solids revealed that ReO₄^-/TcO₄^- directly connects to Pu^IV, Pu^VIO₂²⁺, Ce^III, and Ce^IV in the terminal and bridging coordination modes, leading to 1-, 2-, and 3-dimensional frameworks. For example, ReO₄^- coordination to Pu(IV) formed a 1D chain or 2D framework, isostructural with previously isolated Th(IV) compounds. However, Pu^VIO₂²⁺ alternating with ReO₄^- led to a unique 1D chain, different from the prior-reported U(VI)/Np(VI)-ReO₄^-/TcO₄^- structures. Coordination of ReO₄^-/TcO₄^- with Ce(III) promotes the assembly of 3D frameworks. Finally, attempted synthesis of a Ce(IV)-ReO₄^- compound resulted in a 2D framework with a mixed-valence Ce^III/IV. The highly acidic reaction conditions supported the reduction of both Ce^IV and Tc^VII, challenging isolation of compounds featuring these species. Only one TcO₄-containing structure was obtained in this study (Ce^III-TcO₄ 3D framework), vs the six total Ce/Pu-ReO₄ compounds. Our three Pu-ReO₄ crystal structures are the first reported and translated to atomic-level information about Pu-TcO₄ coordination in nuclear fuel reprocessing scenarios, in addition to broadening our knowledge of bonding trends in the early, high-valence actinides.