Probing and Evaluating the Anion Binding Studies in Novel Coumarin Based Strapped Calix[4]Pyrrole

A novel coumarin-based strapped calix[4]pyrrole, C4P9, was synthesized through an acid cyclization reaction followed by the reaction of coumarin with dipyrromethane. The structure of C4P9 was confirmed using 1H-NMR,13C-NMR, and mass spectrometry. Structural analysis revealed a pre-organized conformation for anion recognition, facilitated by intramolecular hydrogen bonding and the rigid strapping unit. An in-depth investigation of anion binding with various species such as F−, Br−, Cl−, I−, AcO−, NO3−, HSO4−, SCN−, and H2PO4− was conducted using UV-vis and NMR spectroscopy. The anion binding studies were validated by comparing 1H-NMR spectra of free receptor C4P9 with that of C4P9@F. The results from the online supramolecular Bindfit v0.5 program and Job’s plots indicated a 1:1 stoichiometry for the complexation between C4P9 and anions. The coumarin strap enhanced the binding ability and selectivity toward anions, as evidenced by calculations of binding constants. This was observed through interactions such as anion—π (involving aromatic moieties at the meso-position) and C–H—anion (involving the strapped linker and aromatic moiety) as well as N–H—anion interactions. This study demonstrates the potential of coumarin-based strapped calix[4]pyrrole as a versatile platform for developing functional anion receptors, particularly in anion/ion-pair coordination chemistry and supramolecular chemistry in general.