Uranyl Ion Complexes with Cucurbit[<i>n</i>]urils (<i>n</i> = 6, 7, and 8): A New Family of Uranyl-Organic Frameworks

Uranyl ion complexation by cucurbit[n]urils (CBn, n = 6, 7, 8) was investigated, mostly under hydrothermal conditions, and the crystal structure of six novel complexes was determined. In compounds [UO2(H2O)5](NO3)2·CB6·4H2O (1) and [UO2(NO3)2(H2O)2]·CB6·7H2O (2), CB6 acts as a second-sphere ligand through hydrogen bonding to uranyl aquo ligands, while in all other complexes, [(UO2)4(CB6)O2(OH)2(H2O)6](NO3)2·CB6·8H2O (3), [(UO2)2(CB6)3(H2O)6][(UO2)2(NO3)4(C10H16O4)]2·CB6·8H2O (4), [(UO2)2(CB7)(SO4)2(H2O)2]·4H2O (5), and [(UO2)6(CB8)(SO4)6(H2O)10]·18H2O (6), uranyl coordination to one or two CB carbonyl groups is present. A one-dimensional polymer with tetranuclear, μ3-oxo-bridged subunits and a three-decker dinuclear complex are formed in 3 and 4, respectively, with in both cases numerous CH···O interactions between CB6 molecules, either within the complex or involving other CB6 molecules present as guests. The CB7 and CB8 complexes 5 and 6, both obtained in the presence of sulfuric acid, involve either dimeric (5) or polymeric (6) uranyl sulfate subunits, which are further connected through tetradentate CB molecules to give two-dimensional frameworks. Cylindrical channels result from the stacking of CB7 molecules in complex 5.