Allosteric binding properties of a 1,3-alternate thiacalix[4]arene-based receptor having phenylthiolurea and 2-pyridylmethyl moieties on opposite faces

The synthesis of three new heteroditopic receptors (5a–c) which are based on thiacalix[4]arenes in the 1,3-alternate conformation are reported herein. These new receptors each have two thiourea moieties linking phenyl groups two of which are substituted with electron-withdrawing groups at their para-positions, and at the opposite side of the thiacalix[4]arene cavity, with two 2-pyridylmethyl groups. One example (5a) was also characterized by X-ray crystallography. A limited 1H-NMR and Uv-vis anion complexation study was conducted. DFT computational determinations indicated that 5c which has the strong electron–withdrawing NO2 groups had the most effective recognition ability towards the selected anions. The binding of Ag+ at the 2-pyridyl moieties and the binding of the anions at the two thiourea NH groups of the p-substituted phenylthioureido moieties, respectively, was also investigated. The appearance of a positive allosteric effect with receptor 5b was also found using 1H-NMR titration experiments.