Uranyl Ion Complexation by Aliphatic Dicarboxylic Acids in the Presence of Cucurbiturils as Additional Ligands or Structure-Directing Agents

A series of nine complexes has been obtained from the hydrothermal reaction of uranyl nitrate with long-chain alkyldicarboxylic acids (HOOC-(CH2)n–2-COOH, denoted H2Cn) in the presence of cucurbit[6]uril (CB6), dimethylformamide (DMF), which generates dimethylammonium cations in situ, and additional alkaline-earth metal ions in some cases. The crystal structures of the three complexes [H2NMe2]2[(UO2)4O2(OH)2(C8)2(CB6)]·8H2O (1), [H2NMe2]2[(UO2)4O2(OH)2(C9)2(CB6)]·5H2O (2), and [H2NMe2]2[(UO2)4O2(OH)2(C9)2(CB6)]·8H2O (3) are closely related, since all contain the same tetranuclear motif held by two μ3-oxo and two μ2-hydroxo ions, and attached to CB6 by uranyl-O-carbonyl bonds. Two-dimensional assemblies are formed in 1 and 2 and a three-dimensional framework in 3. The cucurbituril molecule is not coordinated in compounds 4–9 but is present as a structure-directing agent. The complex [H2NMe2]2[(UO2)4O2(C6)3]·CB8·8H2O (4), in which cucurbit[8]uril (CB8) comes from the initial CB6, displays a stacking of planar uranyl–organic two-dimensional layers with sheets of CB8 molecules. An analogous overall arrangement involving one-dimensional ribbons based on a uranyl trinuclear motif is found in the complex [H2NMe2][(UO2)3O(OH)2(C7)(HC7)(H2O)0.5]·CB6·4H2O (5). A helicoidal one-dimensional polymer is formed in [H2NMe2]4[(UO2)4(C10)6]·CB6·H2O (6), in which the packing is quite intricate. Finally, the last three complexes, [Ca2(CB6)(H2O)7(DMF)][(UO2)8O4(C8)6(H2O)]·6H2O (7), [Sr2(CB6)(H2O)7(DMF)3][(UO2)6O2(C8)6]·2H2O (8), and [Sr2(CB6)(H2O)8(DMF)2][(UO2)8O4(C10)6] (9), include columns built from alkaline-earth ion complexes with CB6, which are either molecular (7) or one-dimensional polymeric (8 and 9). In complexes 7 and 9, these columns are separated from one another and surrounded by the corrugated two-dimensional uranyl–organic assemblies, while, in 8, the latter are planar grids whose stacking defines channels in which the columns are nestled. These results provide new examples of the potential of cucurbiturils as uranyl complexants or as structure-directing agents. The sheets or columns readily formed by CBs, either free or complexed, appear as particularly well suited to associate with the two-dimensional or ribbon-like topologies most common among uranyl–organic species, which opens perspectives for the design of hybrid layered materials.