Adducts of aqua complexes of Ln3+ with a di-hydroxylated symmetrical octamethyl-substituted cucurbituril: potential applications for isolation of heavier lanthanides

Abstract

The interaction of a series of lanthanide cations (Ln³⁺) with a di-hydroxylated symmetrical octamethyl-substituted cucurbituril {(OH)₂OMeQ[6]} has been investigated. Single-crystal X-ray diffraction analysis has revealed that the interaction results in the formation of adducts of (OH)₂OMeQ[6] with aqua complexes of lanthanide cations ([Ln(H₂O)₈]³⁺), Ln = Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu in (OH)₂OMeQ[6]–Ln(NO₃)₃–H₂O systems. However, no solid crystals were obtained from systems containing La, Ce, Pr, Nd and Sm. X-ray diffraction analysis has revealed that although the solid adducts fall into two isomorphous groups, there are no significant differences in the interactions between (OH)₂OMeQ[6] and [Ln(H₂O)₈]³⁺ complexes and in the corresponding supramolecular assemblies. Thermodynamic parameters for the interaction between (OH)₂OMeQ[6] and [Ln(H₂O)₈]³⁺ complexes based on isothermal titration calorimetry experiments show two periods corresponding to the above two systems, with the lighter lanthanide cations preferring to remain in solution and the heavier lanthanide cations forming crystalline solids. Energy dispersive spectroscopy (EDS) has shown that the interaction of OMeQ[6] with lanthanide cations could provide a means of isolating heavier lanthanide cations from their lighter counterparts.