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.以四价锕系元素氯化物（tetravalent actinide chloride, AnCl₄）与邻苯二甲酸（phthalic acid, 1,2-H₂bdc）或苯六甲酸（mellitic acid, H₆mel）为原料，在130 ℃的水相介质中通过水热法合成了4种含铀（uranium）或镎（neptunium）的金属有机配位聚合物（metal–organic coordination polymers）。 采用邻苯二甲酸配体（phthalate ligand）时，分离得到2种同构组装体：[AnO(H₂O)(1,2-bdc)]₂·H₂O（An=U⁴⁺（1）或Np⁴⁺（2））。该类结构中，AnO₈四方反棱柱配位多面体（square-antiprismatic polyhedra）通过μ₃-氧桥基团（μ₃-oxo groups）以边共享模式（edge-sharing mode）相互连接，形成无限之字形链带（infinite zigzag ribbons）；邻苯二甲酸分子发挥连接作用，将相邻的之字形链构筑为二维（2D）网络（two-dimensional (2D) network）。水分子或与锕系中心配位，或插层于层间。 采用苯六甲酸配体（mellitate ligand）时，共得到2种不同的结构。含铀的化合物[U₂(OH)₂(H₂O)₂(mel)]（3）呈现三维（3D）结构（three-dimensional (3D) structure），其骨架由UO₈配位多面体（四方反棱柱构型）构成的双核单元（dinuclear units）组成，该双核单元进一步通过μ₂-羟桥基团（μ₂-hydroxo groups）相互连接；苯六甲酸配体利用其羧酸根基团（carboxylate groups）连接这些双核单元，最终构筑得到三维框架结构。 所得到的镎基苯六甲酸盐化合物[(NpO₂)₁₀(H₂O)₁₄(Hmel)₂]·12H₂O（4）中，初始的四价镎（Np⁴⁺）在水热合成条件下被氧化为五价镎（Np⁵⁺），形成双氧镎(V)（neptunyl(V), NpO₂⁺）结构单元，该单元中心镎离子处于五角双锥构型的NpO₇配位环境（pentagonal-bipyramidal NpO₇ environment）中。该单元进一步通过双氧镎的桥氧原子与相邻镎单元的赤道面五角配位平面配位，即阳离子-阳离子相互作用（cation–cation interactions），形成由方形框架NpO₇片层构成的层状组装体。在化合物4中，苯六甲酸分子通过其4个羧酸根基团作为桥连配体连接相邻的NpO₇片层，最终构筑得到三维结构。