Rapid Metal Ions Shuttling through 1,3-Alternate Thiacalix[4]crown Tubes

Several new thiacalix[4]monocrown and thiacalix[4]biscrown compounds were synthesized. Their metal ion extractabilities are found to be lower than those of conventional calix[4]crowns. By use of X-ray crystal structures and 1H NMR spectroscopy, this result is explained by weaker electrostatic interactions of the polyether ring oxygen atoms with the metal ions and diminished π-metal ion interactions between the metal ions and the aromatic rings of the thiacalix[4]crowns. Temperature-dependent 1H NMR spectroscopic measurements reveal coalescence temperatures T c intra and T c inter for metal ion exchange. In one case, this exchange was rationalized as metal ion shuttling through a thiacalix[4]biscrown tube. Since the metal ions are less tightly complexed by the polyether units in thiacalix[4]biscrowns than in conventional calix[4]biscrowns, more facile metal ion exchange between the two polyether units takes place in the former.