(Thio)ureido Anion Receptors Based on a 1,3-Alternate Oxacalix[2]arene[2]pyrimidine Scaffold

In pursuit of highly preorganized macrocyclic host molecules for the complexation of anions, a series of oxacalix[2]arene[2]pyrimidine-based bis(thio)ureido receptors were synthesized and fully characterized. The pincer-like 1,3-alternate conformation of the oxacalix[4]arene scaffold, essential for an efficient host-guest interaction, was visualized by single-crystal X-ray analysis and supported by variable-temperature NMR studies. The anion binding properties of the receptors were evaluated via H-1 NMR titration experiments, showing intermolecular interactions with H2PO4-, AcO-, BzO(-), and Cl- ions. The host molecule bearing 4-nitrophenyl substituents on the bisurea binding pocket showed association constants in the range of 200-400 M-1 in the strongly competitive solvent mixture of DMSO/0.5% H2O.