(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 1H 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.