Pyrrole-based acyclic hexapodal aldehyde for anion recognition

Abstract

A novel acyclic hexapodal host (4a) featuring with multiple formyl groups, pyrrole cores, and ester linkages has been prepared and characterized through 1H NMR, 13C NMR, high-resolution mass spectrometry and single-crystal X-ray diffraction techniques. This versatile host molecule exhibits robust anion recognition for CN−, F−, SO42−, HSO4−, H2PO4−, and HP2O73− with the binding constants spanning 0.18 × 105 M−1 to 7.72 × 105 M−1. Interactions with these anions were elucidated through UV–visible and 1H NMR titrations, complemented by DFT calculations. At low guest concentrations (5- to 100-fold excess), a 1:1 complexation is predominant. However, at high concentrations (1000-fold excess), the host's flexibility promotes the assembly of other host-guest complexes. Comparative studies with related pyrrole-based hexapodal ester 4b and tetrapodal aldehyde 4c highlight the importance of peripheral formyl groups, pyrrolic NH groups, the number of pendant arms, and conformational adaptability in anion recognition. These insights underscore the potential of acyclic multipodal hosts in advancing supramolecular chemistry and anion recognition.