Carboxymethyl-substituted benzo[3]uril and its application in ion-pair recognition

Carboxymethyl-substituted and ester-substituted benzo[3]urils have been successfully synthesized and investigated with regard to the supramolecular recognition of inorganic ions by means of fluorescence spectroscopy, 1H NMR spectrometry, and quantum chemical calculations. The results indicated that benzo[3]uril 4 bearing six carboxyl groups binds to Cu2+, Fe3+, F−, and CH3COO− with strong affinities. Moreover, this macrocyclic host could be used to recognize ion pairs comprising Cu2+ and CH3COO−, Cu2+ and F−, Fe3+ and CH3COO−, and Fe3+ and F− through its fluorescence response. In contrast, benzo[3]uril 3, bearing ester groups, showed only host–guest interactions with Cu2+ and Fe3+. The absence of binding of the macrocycle 3 to anions highlights the important role of the carboxyl groups on benzo[3]uril 4, which provide active protons to form hydrogen bonds between the host and anion guests.