Anion Binding Modes in <i>meso</i>-Substituted Hexapyrrolic Calix[4]pyrrole Isomers

We report on the synthesis of a new receptor for anions, <i>meso</i>-substituted hexapyrrolic calix[4]­pyrrole <b>1</b>. The calix[4]­pyrrole’s core features two additional pyrrole side-arms suspended above or below the calix[4]­pyrrole core. This hexapyrrolic calix[4]­pyrrole <b>1</b> is formed as <i>cis</i>- and <i>trans</i>-configurational isomers, the structures of which have been determined by single crystal X-ray diffraction. The anion binding experiments revealed interesting difference in the binding mode: The <i>cis</i>-<b>1</b> isomer binds anions in a mixed binding mode featuring a combination of hydrogen bonding and anion−π interactions resulting in an unexpected strong binding. On the other hand, the <i>trans</i>-<b>1</b> isomer displays only hydrogen bonding and lower affinity for anions. This is unexpected as one would assume both isomers to display the same binding modes. Overall, the titrations of <b>1</b> using UV spectrophotometry and NMR titrations by anions reveal that <i>cis</i>-isomer <b>1</b> displays higher affinity (10⁵–10⁶ M^–1) and cross-reactivity for anions, while the <i>trans</i>-isomer <b>1</b> shows a more selective response to anions. Such differences in binding mode in configurational isomers are so far unexplored and a feature deserving further study.