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

Abstract

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.