Anion Binding Modes in meso-Substituted Hexapyrrolic Calix[4]pyrrole Isomers

We report on the synthesis of a new receptor for anions, meso-substituted hexapyrrolic calix[4]­pyrrole 1. 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 1 is formed as cis- and trans-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 cis-1 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 trans-1 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 1 using UV spectrophotometry and NMR titrations by anions reveal that cis-isomer 1 displays higher affinity (10⁵–10⁶ M^–1) and cross-reactivity for anions, while the trans-isomer 1 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.