Molecular Recognition of Alkylammonium Contact Ion-Pairs Using a Ditopic Receptor

A ditopic, macrobicyclic receptor with adjacent anion and cation binding sites is able to distinguish between various monoalkylammonium salts by binding them as contact ion-pairs. The affinity for linear n-propylammonium chloride is at least 2 orders of magnitude greater than that for n-propylammonium acetate, n-propylammonium p-toluenesulfonate, and branched isopropylammonium chloride. An X-ray structure of the receptor complexed with methylammonium chloride illuminates the basis of the molecular recognition.