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 <b>2</b> and <b>3</b> bearing auxiliary hydrogen bonding donors on the bridging moieties, as well as of normal calix[4]­pyrrole <b>1</b>, were investigated via ¹H NMR spectroscopic and isothermal titration calorimetry analyses. The resulting data revealed that receptors <b>2</b> and <b>3</b> have significantly higher affinities for the azide anion in organic media as compared with the unfunctionalized calix[4]­pyrrole <b>1</b> and other azide receptors reported to date. Single crystal X-ray diffraction analyses and calculations using density functional theory revealed that receptor <b>2</b> 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 <b>2</b>, receptor <b>3</b> 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