A tripyrrolylmethane-based macrobicyclic triazacryptand: X-ray structure, size-selective anion binding, and...

A new class of tripyrrolylmethane-based triazacryptand with bridgehead carbons and acyclic molecules were synthesized by the Mannich reaction of tripyrrolylmethane with primary or secondary amine hydrochloride and formaldehyde, respectively. The structure of the triazacryptand was determined by X-ray diffraction (XRD) method. The anion binding properties of both the bicyclic and acyclic receptors were studied by (1)H NMR titration method. The binding studies showed that both receptors exhibit very high affinity and bind strongly with the F(-) ion in DMSO-d(6). However, the binding constant of azacryptand with F(-) is much higher than that of the acyclic receptor. This is attributed to the preorganization of the azacryptand having a specific cavity size, and the strength and the number of hydrogen bonds formed by the F(-) ion. This is supported by the crystal structures of F(-), Cl(-), and Br(-) ion complexes of the bicyclic receptor and by DFT calculations. The X-ray structures showed that the azacryptand receptor forms an inclusion complex with only the F(-) ion; other anions bind in the clefts of the macrobicycle, thus supporting a size-selective anion binding behavior. The high affinity and the selectivity of the macrobicycle as a neutral receptor of the F(-) ion in the presence of other competitive anions in DMSO-d(6) were confirmed by (1)H NMR spectroscopy. Furthermore, the F(-)-ion-mediated hydrogen-deuterium exchanges were monitored by (19)F NMR spectroscopy, showing multiplets based on the formation of all possible deuterium-exchanged fluoride complexes in solution.