Dipyrrolylmethane-based Macrobicyclic Azacryptand: Synthesis, X-ray Structures, Conformational and Anion Binding Properties

A new class of macrobicyclic azacryptand containing dipyrrolylmethane
subunits with nitrogen bridgeheads was synthesized by the Mannich
reaction of the dipyrrolylmethane in the presence of aqueous ammonia.
The azacryptand exhibits a staggered conformation in the solid state,
but is in a dynamic equilibrium with the eclipsed conformation in
solution studied by the variable-temperature 1H NMR methods.
The azacryptand has a specific size suitable only for fluoride ion;
large anions such as NO3– bind in the
clefts of the macrobicycle as shown by the X-ray structures of its
fluoride ion inclusion and the nitrate anion complexes. The anion
binding studies showed that it has high selectivity and affinity for
fluoride ion in acetone over other anions studied, which was supported
by 1H and 19F NMR methods. The azacryptand has
fast fluoride ion-mediated proton–deuterium exchanges with
acetone-d6 studied by the 19F NMR method.