Selective aqueous anion recognition in an anionic host

Water-soluble Fe₄L₄ ^4- cages can be synthesized in a multicomponent self-assembly process exploiting functionalized trigonal ligands, Fe^II 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 PF₆ ^- or AsF₆ ^- occupy the internal cavity, whereas anions that are too small (BF₄ ^-) or too large (NTf₂ ^-) 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 PF₆ ^- by an empty cage takes multiple weeks, despite the strong affinity of the cavity for PF₆ ^- ions. In the future, this recognition mechanism could be used to control release of anions for environmental applications.