Anion binding, recognition and separation by urea-based receptors

Anion coordination chemistry has attracted much attention because anions play important roles in biology, medicine, catalysis, and environmental science. A large number of anion receptors that employ ureas, amides, indoles, pyrroles and other groups as the binding sites have been reported. Urea can be used as an effective unit to build anion receptors because of its ability of binding anions by donating two parallel hydrogen bonds to oxoanions. Thus the urea-based receptors have been greatly explored in recent years. By proper functionalization of urea-based receptors, they are supposed to be applied in environmental monitoring, membrane transporters, organocatalysis, supramolecular gelation and soft materials. This review highlights the developments of anion binding, recognition and separation by mono-, di-, and poly-ureas, and metal-assisted urea-containing receptors in recent years.