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Lanthanide Complexes with Cucurbit[n]urils (n = 5, 6, 7) and Perrhenate Ligands: New Examples of Encapsulation of Perrhenate Anions

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posted on 18.05.2009, 00:00 by Pierre Thuéry
The reaction of lanthanide nitrates with cucurbit[n]urils (CBn, n = 5, 6, 7) under hydrothermal conditions in the presence of perrhenic acid and, for some of them, other reactants such as KNO3 yielded 14 new complexes which were crystallographically characterized and present some original features. Four of the five complexes with CB5 are capsules closed by a lanthanide ion (Ce, Sm, Gd) on one side and potassium on the other and have an encapsulated bridging nitrate ion, but the fifth is a monodimensional polymer with both bidentate portals of CB5 complexed to ytterbium. All eight CB6 complexes (Ce, Pr, Sm, Gd, Yb, Lu) and the single ytterbium complex of CB7 (the first lanthanide complex of CB7 to be characterized) involve perrhenate encapsulation, in a form which is either bridging the lanthanide ions coordinated at each tridentate portal, or terminal when the CB is bidentate and the cation is further from the portal, or even noncoordinating in one case. The orientation of the cation in the cavity varies depending upon its coordination mode, as well as the nature of the shortest contacts in the cavity. Some original architectures are described, in particular a sinuous chain, ribbonlike assembly in a cerium complex of CB6 and a novel samarium−CB6 sandwich complex. The ubiquitous encapsulation of ReO4 in CB6 and CB7 may open new perspectives for the investigation of anion complexation by these macrocycles.