Uranyl-Pyromellitate Coordination Polymers: Toward Three-Dimensional Open Frameworks with Large Channel Systems

Five coordination polymers based on uranyl-pyromellitates have been hydrothermally synthesized and their single-crystal XRD structures have been analyzed. These different compounds, obtained with different ammonia concentrations, exhibit either three-dimensional (3D) or two-dimensional (2D) networks. Complex 1, (UO2)(2)(H2O)(2)(btec)center dot H2O, is a quite compact 3D structure containing isolated 7-fold coordinated uranyl cations linked through the pyromellitate (noted btec) and encapsulating free water species. Phase (NH4)[(UO2)(2)(OH)(H2O)(btec)]center dot 1.75H(2)O (2) offers a second 3D architecture built up from dinuclear 7-fold coordinated uranyl units and mononuclear 8-fold coordinated uranyl units linked through the organic ligands. This framework is slightly more open because narrow one-dimensional (1D) channels trapping water species are visible Phase 3, (NH4)(2)[(UO2)(6)O-2(OH)(4)(btec)(1.5)]center dot 11H(2)O, consists of large 1D Iozenge-shaped channels (8.2 angstrom x 8.6 angstrom) delimited by infinite ribbons (composed of 7-fold coordinated uranyl polyhedra sharing edges) and pyromellitate ligands. Ammonium cations as well as water molecules are trapped within the channels. The fourth compound, (NH4)(6)[(UO2)(3)(btec)(3)]center dot 12H(2)O (4) is lamellar with sheets containing 8-fold coordinated uranyl centers linked through the btec molecules, which have two nonbonded carboxylate functions interacting with the intercalated ammonium cations. Compound 5 also consists of a layered structure, (UO2)(3)(OH)(2)(H2O)(btec), with uncommon trinuclear building blocks containing 7-fold (x2) and 8-fold (x1) coordinated uranyl centers, linked through the btec molecules. Fluorescence spectra of compounds 1, 3, and 4 are also measured.