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

Abstract

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 was 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–, B zO– 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 thestrongly competitive solvent mixture DMSO/0.5% H2O.