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 <b>C4P9</b> 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 <b>C4P9</b> with that of <b>C4P9@F</b>. The results from the online supramolecular Bindfit v0.5 program and Job’s plots indicated a 1:1 stoichiometry for the complexation between <b>C4P9</b> 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 (<i>involving aromatic moieties at the meso-position</i>) and C–H—anion (<i>involving the strapped linker and aromatic moiety</i>) 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.