Uranyl Ion Complexation by Cucurbiturils in the Presence of Perrhenic, Phosphoric, or Polycarboxylic Acids. Novel Mixed-Ligand Uranyl−Organic Frameworks

Six novel complexes formed under hydrothermal conditions by reaction of uranium trioxide or uranyl nitrate with cucurbit[6]uril (CB6), or cucurbit[7]uril (CB7) in one case, in the presence of additional ligands, either tetrahedral oxoanions or polycarboxylic acids, have been structurally characterized. In [(UO2)2(CB6)(ReO4)4(H2O)3]·2HReO4·5H2O (1), three perrhenate ions are bound to uranyl ions and one is encapsulated in the CB6 cavity, while perrhenate acts as a counterion only in the CB7 complex [UO2(CB7)(NO3)(H2O)2](ReO4)·5H2O (2). 1 is a molecular complex and 2 a one-dimensional polymer. Complex 3, [UO2(H2PO4)2(H2O)]2·CB6·H3PO4·6H2O, obtained in the presence of orthophosphoric acid, comprises one-dimensional uranyl dihydrogen phosphate chains, and the CB6 molecules are uncoordinated, as in [(UO2)4(C2O4)3(NO3)2(H2O)6]·CB6·6H2O (4), which displays an unprecedented tetranuclear uranyl oxalate moiety hydrogen bonded to the CB6 molecules. With pyrazinetetracarboxylic acid (H4PZTC), a one-dimensional polymer with alternate PZTC4− and CB6 ligands is formed, [(UO2)2(PZTC)(CB6)(H2O)2]·12H2O (5). Finally, the complex [(UO2)5(cit)2(CB6)(NO3)2(H2O)6]·7H2O (6), synthesized in the presence of citric acid (H4cit), is an intricate three-dimensional framework in which the uranyl citrate part builds a three-dimensional subunit defining cagelike spaces where the CB6 ligands are located. These results evidence the interest of cucurbiturils as structure-directing agents (complexes 3 and 4) and as auxiliary ligands in mixed-ligand uranyl−organic frameworks (5 and 6).