Uranyl–organic bilayer assemblies with flexible aromatic di-, tri- and tetracarboxylic acids

Abstract

The reaction of uranyl nitrate hexahydrate with three flexible aromatic polycarboxylic acids under hydrothermal conditions has been found to give 2-D assemblies in all three cases. The complex formed with phenylsuccinic acid (H₂L¹), [UO₂(L¹)(H₂O)] (1), comprises a planar grid-like assembly with all the phenyl substituents directed outwards on the same side. These layers are organized in a face-to-face fashion by inter-layer π-stacking interactions. 1,3,5-Benzenetriacetic acid (H₃L²) gives the compound [UO₂(H₂O)₅][UO₂(L²)]₂·5H₂O (2), in which a novel bilayer arrangement results from both the uranyl ions and the ligand behaving as trigonal nodes. Interdigitation of the anionic bilayers leaves spaces occupied by the [UO₂(H₂O)₅]²⁺ counter ions. Complex 2 is a novel example of a molecular bilayer involving other building blocks than the usual T-shaped nodes and linear spacers. Finally, [(UO₂)₃(HL³)₂(H₂O)]·3H₂O (3) was obtained with benzophenone-3,3′,4,4′-tetracarboxylic acid (H₄L³) generated in situ from the corresponding dianhydride. The high overall denticity of the ligand (bound to as much as six cations) and the twisting of the two aromatic rings around the central bonds result in the formation of a pillared bilayer of the “double floor” type, with channels containing water molecules.