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（AnCl₄）与邻苯二甲酸（1,2-H₂bdc）或苯六甲酸（H₆mel）为反应物，在130 ℃的水相体系中经水热法合成了四种含铀或镎的金属有机配位聚合物（metal–organic coordination polymers）。
当采用邻苯二甲酸根配体时，分离得到两种同构组装体([AnO(H₂O)(1,2-bdc)]₂·H₂O；An = U⁴⁺(1)或Np⁴⁺(2))，其中AnO₈的四方反棱柱（square-antiprismatic）多面体通过μ₃-氧原子（μ₃-oxo groups）以边共享模式相互连接，形成无限锯齿形链。邻苯二甲酸根分子用于连接相邻的锯齿形链，构建出二维（2D）网络。水分子既可与锕系中心配位，也可插层于层间。
当采用苯六甲酸根配体时，得到两种不同的结构。含铀的化合物[U₂(OH)₂(H₂O)₂(mel)](3)呈现三维（3D）结构，其由UO₈四方反棱柱（square antiprism）多面体构成的双核单元组成，该双核单元进一步通过μ₂-羟基（μ₂-hydroxo groups）连接。苯六甲酸根配体利用其羧酸根基团连接双核单元，最终构筑出三维骨架。
针对镎的苯六甲酸根配合物[(NpO₂)₁₀(H₂O)₁₄(Hmel)₂]·12H₂O(4)显示，在水热合成条件下，初始的Np⁴⁺被氧化为Np⁵⁺，形成具有五角双锥（pentagonal-bipyramidal）NpO₇配位环境的镎酰(V)（NpO₂⁺）单元。该单元进一步通过镎酰氧原子的桥连氧原子形成由方形框架NpO₇片层构成的层状组装体，这些桥连氧原子同时与相邻镎单元的五边形赤道配位平面配位（即阳离子-阳离子相互作用（cation–cation interactions））。在化合物4中，苯六甲酸分子通过四个羧酸根基团作为NpO₇片层间的桥连配体，最终构筑出三维结构。