Aromatic Chloride Catcher

To grab hold of an anion, an organic host molecule usually uses strong hydrogen-bond donors, such as N–H bonds or C–H bonds adjacent to a cationic center. Now, chemists at Indiana University have shown that a neutral macrocycle equipped with only aromatic C–H bonds (which are weak H–bond donors) can snatch up a chloride anion with an affinity akin to its stronger H–bonding brethren (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200704717). Using "click chemistry" and other reactions, Amar H. Flood and Yongjun Li created a neutral macrocycle wherein four phenyl rings alternate with four triazoles. Each of these eight rings donates one C–H bond for catching the chloride ion (complex shown). Flood and Li attribute the macrocycle's surprisingly good affinity for chloride ions to the triazole moieties' strong dipoles and to a central cavity that's the perfect size for the anion. The click chemistry-based synthesis makes it easy to create libraries of such macrocycles, which could be used to sequester other anions, Flood says.