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 (AnCl4) and phthalic (1,2-H2bdc) or mellitic (H6mel)
acid in aqueous media at 130 °C. With the phthalate ligand, two
analogous assemblies ([AnO­(H2O)­(1,2-bdc)]2·H2O; An = U4+ (1) or Np4+ (2)) have been isolated, in which the square-antiprismatic
polyhedra of AnO8 are linked to each other via μ3-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 [U2(OH)2(H2O)2(mel)] (3) exhibits
a three-dimensional (3D) structure composed of the dinuclear units
of UO8 polyhedra (square antiprism), which are further
linked via the μ2-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
([(NpO2)10(H2O)14(Hmel)2]·12H2O (4)) reveals oxidation
of the initial NpIV to NpV under the applied
hydrothermal synthetic conditions, yielding the neptunyl­(V) (NpO2+) unit with a pentagonal-bipyramidal NpO7 environment. This further leads to the formation of a layered assembly
of the square-frame NpO7 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 NpO7 sheets by using four of their carboxylage groups,
eventually building up a 3D structure.