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)·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 (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)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 (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)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.