Solvothermal synthesis of uranium(VI) phases with aromatic carboxylate ligands : a dinuclear complex with 4-hydroxybenzoic acid and a 3D framework with terephthalic acid

With the coordination of dimethylformamide (DMF), two new uranium(VI) complexes with either 4-hydroxybenzoic acid (H 2phb) or terephthalic acid (H 2tph) have been synthesized under solvothermal conditions and structurally characterized. [(UO 2) 2(Hphb) 2(phb)(DMF)(H 2O) 3]·4H 2O (1) has a dinuclear structure constructed with both pentagonal and hexagonal bipyramidal uranium polyhedra linked through a μ 2-bridging ligand via both chelating carboxylate arm and alcohol oxygen bonding, first observation of such a coordination mode of 4-hydroxybenzoate for 5 f ions. [(UO 2)(tph)(DMF)] (2) has a three-dimensional (3D) framework built with pentagonal bipyramidal uranium polyhedra linked with μ 4-terephthalate ligands. The 3D channeled structure is facilitated by the unique carboxylate bonding with nearly linear C-O-U angles and the coordination of DMF molecules. The presence of phb ligands in different coordination modes, uranyl ions in diverse environments and DMF in complex 1, and tph ligand, DMF and uranyl ion in complex 2 has been confirmed by Raman spectroscopy. In addition, their thermal stability and photoluminescence properties have been investigated.