A Tripyrrolylmethane-Based Macrobicyclic Triazacryptand: X‑ray Structure, Size-Selective Anion Binding, and Fluoride-Ion-Mediated Proton–Deuterium Exchange Studies

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 1H NMR titration method. The binding studies showed
that both receptors exhibit very high affinity and bind strongly with
the F– ion in DMSO-d6. 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-d6 were confirmed by 1H NMR spectroscopy. Furthermore,
the F–-ion-mediated hydrogen–deuterium exchanges
were monitored by 19F NMR spectroscopy, showing multiplets
based on the formation of all possible deuterium-exchanged fluoride
complexes in solution.