Selective aqueous anion recognition in an anionic host.

Water-soluble FeL cages can be synthesized in a multicomponent self-assembly process exploiting functionalized trigonal ligands, Fe salts, and water-soluble sulfonated formylpyridine components. The cages are soluble in purely aqueous solution and display an overall 4- charge, but are capable of binding suitably sized non-coordinating anions in the host cavity despite their anionic nature. Anions such as PF or AsF occupy the internal cavity, whereas anions that are too small (BF ) or too large (NTf ) are not encapsulated. The external anionic charge and sterically blocked ligand cores limit the exchange rate of bound anions, as no exchange is seen over a period of weeks with the anion-filled cages, and internalization of added PF by an empty cage takes multiple weeks, despite the strong affinity of the cavity for PF ions. In the future, this recognition mechanism could be used to control release of anions for environmental applications.