Uranyl ion complexes of cucurbit[7]uril with zero-, one- and two-dimensionality

Abstract

Three novel uranyl ion complexes with cucurbit[7]uril (CB7) as a ligand have been synthesized under hydrothermal conditions and crystallographically characterized. In [UO₂(CB7)(NO₃)(H₂O)₂](ReO₄)·10H₂O (1), obtained from uranyl nitrate in the presence of perrhenic acid, each cation is bound to two monodentate CB7 ligands, giving rise to a 1-D polymer in which the CB7 molecules are arranged in herringbone fashion and are connected by CH⋯O hydrogen bonds. The nitrate ion is chelating, with the NOO plane parallel to the uranyl axis, the uranium coordination environment being thus pentagonal bipyramidal with a split equatorial vertex. The complex [{(UO₂)₂(OH)₂(H₂O)₄}₂(CB7)](NO₃)₄·9H₂O (2) is a 2-D framework in which each CB7 connects four bis(µ₂-hydroxo)-bridged dinuclear uranyl motifs. Finally, CB7 reacts with uranium trioxide UO₃ in the presence of formic acid to give the discrete molecular complex [(UO₂)₂(CB7)(OH)₂(HCOO)₂(H₂O)₃]·8H₂O (3), in which each CB7 is bound to one bis(µ₂-hydroxo)-bridged dinuclear uranyl motif and each uranium atom is coordinated to one terminal monodentate formate ligand. The packing in 3 consists in chains in which neighbouring complexes are held in herringbone geometry by OH⋯O and CH⋯O hydrogen bonds, with an overall topology similar to that observed in compound 1. Apart from a uranyl sulfate/CB7 complex reported previously, these are the only examples of metal ion complexes of CB7.