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, (UO 2 ) 2 (H 2 O) 2 (btec)·H 2 O, 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 (NH 4 )[(UO 2 ) 2 (OH)(H 2 O)(btec)]·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, (NH 4 ) 2 [(UO 2 ) 6 O 2 (OH) 4 (btec) 1.5 ]·11H 2 O, consists of large 1D lozenge-shaped channels (8.2 Å × 8.6 Å) 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, (NH 4 ) 6 [(UO 2 ) 3 (btec) 3 ]·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, (UO 2 ) 3 (OH) 2 (H 2 O) 2 (btec), with uncommon trinuclear building blocks containing 7-fold (×2) and 8-fold (×1) coordinated uranyl centers, linked through the btec molecules. Fluorescence spectra of compounds 1, 3, and 4 are also measured.