Synthesis of Coordination Polymers of Tetravalent Actinides (Uranium and Neptunium) with a Phthalate or Mellitate Ligand in an Aqueous Medium.

Four metal-organic coordination polymers bearing uranium or neptunium have been hydrothermally synthesized from a tetravalent actinide chloride (AnCl) and phthalic (1,2-Hbdc) or mellitic (Hmel) acid in aqueous media at 130 °C. With the phthalate ligand, two analogous assemblies ([AnO(HO)(1,2-bdc)]·HO; An = U (1) or Np (2)) have been isolated, in which the square-antiprismatic polyhedra of AnO are linked to each other via μ-oxo groups with an edge-sharing mode to materialize infinite zigzag ribbons. The phthalate molecules play a role in connecting the adjacent zigzag chains to build a two-dimensional (2D) network. Water molecules are bonded to the actinide center or found intercalated between the layers. With the mellitate ligand, two distinct structures have been identified. The uranium-based compound [U(OH)(HO)(mel)] (3) exhibits a three-dimensional (3D) structure composed of the dinuclear units of UO polyhedra (square antiprism), which are further linked via the μ-hydroxo groups. The mellitate linkers use their carboxylate groups to connect the dinuclear units, eventually building a 3D framework. The compound obtained for the neptunium mellitate ([(NpO)(HO)(Hmel)]·12HO (4)) reveals oxidation of the initial Np to Np under the applied hydrothermal synthetic conditions, yielding the neptunyl(V) (NpO) unit with a pentagonal-bipyramidal NpO environment. This further leads to the formation of a layered assembly of the square-frame NpO sheets via the bridging oxygen atoms from the neptunyl oxo groups, which further coordinate to the pentagonal equatorial coordination plane of the adjacent neptunium unit (i.e., cation-cation interactions). In compound 4, the mellitate molecules act as bridging linkers between the NpO sheets by using four of their carboxylage groups, eventually building up a 3D structure.