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)(4)O-2(OH)(2)(-), (C8)(2)(CB6)]center dot 8H(2)O (1), [H2NMe2](2)[(UO2)(4)O-2(OH)(2)(C9)(2)(CB6)]center dot 5H(2)O (2), and [H2NMe2](2)[(UO2)(4)O-2(OH)(2)(C9)(2)(CB6)]center dot 8H(2)O (3) are closely related, since all contain the same tetranuclear motif held by two mu(3)-oxo and two mu(2)-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)(4)O-2(C6)(3)]center dot CB8 center dot 8H(2)O (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)(3)O(OH)(2)(C7)(HC7)(H2O)(0.5)]center dot CB6 center dot 4H(2)O (5). A helicoidal one-dimensional polymer is formed in [H2NMe2](4)[(UO2)(4)(C10)(6)]center dot CB6 center dot H2O (6), in which the packing is quite intricate. Finally, the last three complexes, [Ca-2(CB6)(H2O)(7)(DMF)][(UO2)(8)O-4(C8)(6)(H2O)]center dot 6H(2)O (7), [Sr-2(CB6)(H2O)(7)(DMF)(3)][(UO2)(6)O-2(C8)(6)]center dot 2H(2)O (8), and [Sr-2(CB6)(H2O)(8)(DMF)(2)][(UO2)(8)O-4(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.