Adducts of aqua complexes of Ln3+ with hexahydroxyhexamethylcucurbit[6]uril: potential application in the isolation of heavy lanthanides

Abstract

The interactions of a series of lanthanide cations (Ln³⁺) with hexahydroxyhexamethylcucurbit[6]uril (HHQ[6]) have been investigated. The interaction results in the formation of solid crystals of adducts of HHQ[6] with aqua complexes of lanthanide cations of type ([Ln(H₂O)₈]³⁺) for Ln = Gd–Lu to yield HHQ[6]–Ln(NO₃)₃–CdCl₂–H₂O species in neutral solution. Conversely, no solid crystals were obtained for systems containing La, Ce, Pr, Nd, Sm, or Eu. Unit cell determination by X-ray diffraction shows all of these adducts to be isomorphous. Thermodynamic parameters, K_a, ΔH, and ΔS determined by ITC, show clear differences in the systems involving the six lightest lanthanides compared to those involving the eight heavier lanthanides. Energy-dispersive spectrometry indicated that the lighter or heavier lanthanide cations could be isolated from their heavier or lighter counterparts through interaction with HHQ[6].