Inorganic anion-aided coordination of lanthanide metal ions to cucurbituril and supramolecular self-assembly: potential applications in the separation of light lanthanides

Linear coordination polymers have been assembled from lanthanide cations (Ln3+) and cucurbituril (Q[6]) in the presence of [CdCl4]2− or [ZnCl4]2− anions as inorganic structure inducers in HCl solution. X-ray diffraction analysis has revealed honeycomb-patterned supramolecular assemblies, in which [CdCl4]2− or [ZnCl4]2− anions form the honeycomb and the hexagonal cells are occupied by Ln3+–Q[6] linear coordination polymers. For the Ln3+–Q[6]–[CdCl4]2− systems, the solid-state structures of the obtained compounds were all isomorphous, with the Ln3+ cations coordinated to Q[6] to form zigzag coordination polymers that fill the [CdCl4]2− honeycombs. For the Ln3+–Q[6]–[ZnCl4]2− systems, the obtained compounds were divided into two groups of four and five isomorphous complexes, respectively. Besides the zigzag coordination polymers, tubular coordination polymers occupying the [ZnCl4]2− honeycombs were also observed. The most remarkable phenomenon in the present work was the rapid precipitation of the Ln3+–Q[6]–[CdCl4]2− or –[ZnCl4]2− systems when the Ln3+ cations were La, Ce, Pr or Nd. This may offer a means of separating light lanthanides cations from their heavier counterparts.