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

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.

本研究测定了水热条件下，铀酰离子与4-氨基苯甲酸（HL1）、4-氨基-3-甲基苯甲酸（HL2）、4-(氨甲基)苯甲酸（HL3）及3-氨基-5-羟基苯甲酸（HL4）在葫芦[6]脲（cucurbit[6]uril，CB6）存在时所形成配合物的晶体结构。选取上述配体的依据在于，其两性离子形式同时具备可与金属配位的羧酸根基团，以及可通过离子-偶极作用与氢键相互作用与CB6结合的铵基。在N,N-二甲基甲酰胺（dimethylformamide，DMF）存在下可原位生成二甲基铵离子，由此得到配合物[H2NMe2]2[(UO2)2(L1)2O(OH)(H2O)]2·CB6·15H2O（1）与[H2NMe2]2[(UO2)2(L2)2O(OH)(H2O)]2·CB6·17H2O（2）。二甲基铵离子被固定于CB6的端口处，而带有氨基苯甲酸根阴离子的四核铀酰配合物并未与CB6结合。配体的中性含铵形式存在于配合物[UO2(HL3)(OH)(HCOO)(H2O)]2·2CB6·2DMF·14H2O（3）中，该双核铀酰配合物通过双μ2-羟基桥联，带有两个呈发散排布的单齿HL3配体。HL3配体与两个CB6分子结合，形成哑铃形超分子组装体。在配合物[(UO2)4(HL3)2(L3)O2(OH)2(H2O)4]·2CB6·0.5CB8·HL3·NO3·20H2O（4）中，三个CB6分子组装于四核铀酰配合物周围，其中两个CB6作为桥联单元，构筑得到一维线性超分子结构。最后，3-氨基取代配体HL4得到高度对称的配合物[UO2(HL4)(L4)2]·3CB6·16H2O（5），其中铀酰离子被三个羧酸根基团螯合。三个CB6分子环绕该平面配合物组装，形成三角形离散物种。在化合物3至5中，CB6分子通常的柱状或层状堆积方式并未得到保留——这一现象在铀酰配合物参与的体系中并不常见。其根源在于异双功能配体的CB6组装作用：此类配体将CB6分子固定于具有典型平面几何结构的单核、双核或四核铀酰配合物的外围。