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 <b>1</b>, (UO₂)₂(H₂O)₂(btec)·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)]·1.75H₂O (<b>2</b>) 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 <b>3</b>, (NH₄)₂[(UO₂)₆O₂(OH)₄(btec)_1.5]·11H₂O, 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, (NH₄)₆[(UO₂)₃(btec)₃]·12H₂O (<b>4</b>), 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 <b>5</b> also consists of a layered structure, (UO₂)₃(OH)₂(H₂O)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 <b>1</b>, <b>3</b>, and <b>4</b> are also measured