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.
