Dissecting halide-receptor interactions in “four wall” aryl-extended calix[4]pyrrole complexes: the role of the aromatic walls

Abstract

We report cryogenic ion vibrational spectroscopy of chloride, bromide, and iodide complexes with octa-methyl calix[4]pyrrole (omC4P) and with the tetra-α “four wall” 4-nitroaryl-extended calix[4]pyrrole (AEC4P). Experimental infrared spectra are compared with those obtained using density functional theory calculations to elucidate the binding motifs of these receptor-halide complexes. For both receptors, halide binding is mainly driven by symmetric interactions between the halide ion and the receptor's four pyrrole NH groups. These interactions lock the receptor in cone conformation and are encoded in the NH stretching region of their infrared spectra. For the “four wall” AEC4P, the cone conformation defines a deep, polar, aromatic cavity closed at one end by the converging pyrrole NH groups. Halide binding (Cl⁻, Br⁻, or I⁻) redistributes negative charge from the bound anion onto the meso-p-nitroaryl substituents, generating an electrostatic field that drives the ion deeper into the cavity. In contrast, the anionic complexes of the omC4P receptor, which lacks meso-aryl substituents, feature a shallower binding geometry for the bound halide.