Pyrrole- and Naphthobipyrrole-Strapped Calix[4]pyrroles as Azide Anion Receptors

The
binding interactions between the azide anion (N3–) and the strapped calix[4]­pyrroles 2 and 3 bearing auxiliary hydrogen bonding donors on
the bridging moieties, as well as of normal calix[4]­pyrrole 1, were investigated via 1H NMR spectroscopic and
isothermal titration calorimetry analyses. The resulting data revealed
that receptors 2 and 3 have significantly
higher affinities for the azide anion in organic media as compared
with the unfunctionalized calix[4]­pyrrole 1 and other
azide receptors reported to date. Single crystal X-ray diffraction
analyses and calculations using density functional theory revealed
that receptor 2 binds CsN3 in two distinct
structural forms. As judged from the metric parameters, in the resulting
complexes one limiting azide anion resonance contributor is favored
over the other, with the specifics depending on the binding mode.
In contrast to what is seen for 2, receptor 3 forms a CsN3 complex in 20% CD3OD in CDCl3, wherein the azide anion is bound only vertically to the
NH protons of the calix[4]­pyrrole and the cesium cation is complexed
within the cone shaped-calix[4]­pyrrole bowl. The bound cesium cation
is also in close proximity to a naphthobipyrrole subunit present in
a different molecule, forming an apparent cation-π complex.