Calix[3]amide-based anion receptors: high affinity for fluoride ions and a twisted binding model

Tris-phenylurea-substituted calix[3]amides (3a, 3b) and tris-phenylthiourea-substituted calix[3]amides (4a, 4b) were synthesised by the reaction of amine with isocyanates and thioisocyanate, respectively. Single crystal X-ray analysis revealed that the cyclic trimers adopt a syn conformation with all of the urea groups on the same side. The binding affinities of these compounds towards anions were measured using 1H NMR titrations in DMF-d 7 , DMSO-d 6 or CDCl3. Each receptor was observed to bind halogen anions in a 1:1 stoichiometry, exclusively through hydrogen bonding, and the anion selectivity was in the order F− > Cl− ≫Br− > I− . DFT calculations revealed that the fluoride anion is anchored in the centre of the six N–H hydrogen atoms of 3a through H···F interactions.