Positive and negative allosteric effects of thiacalix[4]arene-based receptors having urea and crown-ether moieties

Heteroditopic receptors (4ₐ₋ₑ) based on a thiacalix[4]arene in the 1,3-alternate conformation, which have two urea moieties linking various phenyl groups substituted with either electron-donating or -withdrawing groups at their m-, or p-positions with a crown-ether moiety at the opposite side of the thiacalix[4]arene cavity, have been synthesized. The two examples with p-CH₃– (4b) and p-NO₂-substituted (4ₑ) phenyl groups have been characterized by X-ray crystallography. The binding properties of receptor 4ₑ were investigated by means of ¹H NMR spectroscopic and absorption titration experiments in CHCl₃–DMSO (10:1, v/v) solution in the presence of K⁺ ions and various anions. Interestingly, it was found that receptor 4ₑ, which possesses two p-nitrophenyl ureido moieties, can complex most efficiently in the urea cavity or the crown-ether moiety; and the plausible allosteric effect of receptor 4ₑ was also studied