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 (Ln(3+)) 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 Ln(3+)-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 Ln(3+) cations coordinated to Q[6] to form zigzag coordination polymers that fill the [CdCl4](2-) honeycombs. For the Ln(3+)-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 Ln(3+)-Q[6]-[CdCl4](2-) or -[ZnCl4](2-) systems when the Ln(3+) cations were La, Ce, Pr or Nd. This may offer a means of separating light lanthanides cations from their heavier counterparts.