Uranyl Ion Complexes with Ammoniobenzoates as Assemblers for Cucurbit[6]uril Molecules

Abstract

The crystal structures of the complexes formed under hydrothermal conditions by uranyl ions with 4-aminobenzoic (HL1), 4-amino-3-methylbenzoic (HL2), 4-(aminomethyl)benzoic (HL3), and 3-amino-5-hydroxybenzoic (HL4) acids, in the presence of cucurbit[6]uril (CB6), have been determined. These ligands have been chosen because, in their zwitterionic form, they display both a metal-complexing carboxylate group and an ammonium group able to associate with CB6 through ion-dipole and hydrogen bonding interactions. The complexes [H2NMe2]2[(UO2)2(L1)2O(OH)(H2O)]2·CB6·15H2O (1) and [H2NMe2]2[(UO2)2(L2)2O(OH)(H2O)]2·CB6·17H2O (2) were obtained in the presence of dimethylformamide, which gives dimethylammonium ions in situ. The latter are held at the CB6 portals, while the tetranuclear uranyl complex with the aminobenzoate anions is not bound to CB6. The neutral, ammonium-containing form of the ligand is present in [UO2(HL3)(OH)(HCOO)(H2O)]2·2CB6·2DMF·14H2O (3), in which the di(μ2-hydroxo)-bridged, dinuclear uranyl complex displays two diverging, monodentate HL3 ligands. The latter are associated with two CB6 molecules to give a dumbbell-shaped supramolecular assembly. Three CB6 molecules are assembled around a tetranuclear uranyl complex in [(UO2)4(HL3)2(L3)O2(OH)2(H2O)4]·2CB6·0.5CB8·HL3·NO3·20H2O (4), with two of them being bridging and giving rise to a one-dimensional, linear supramolecular architecture. Finally, the 3-amino substituted ligand HL4 gives the highly symmetric complex [UO2(HL4)(L4)2]·3CB6·16H2O (5), in which the uranyl ion is chelated by three carboxylate groups. Three CB6 molecules are assembled around the planar complex to give a triangular, discrete species. In compounds 3–5, the usual packing of CB6 molecules into columns or layers is not retained as it is frequently in the presence of uranyl complexes. This is due to the CB6-assembling role of the heterodifunctional ligands, which hold the CB6 molecules at the periphery of mono-, di-, or tetranuclear uranyl complexes of quite usual, planar geometry.