Dissecting Halide-Receptor Interactions in “Four Wall” Aryl-Extended Calix[4]pyrrole complexes: The Role of the Aromatic Walls.

We report cryogenic ion vibrational spectroscopy of the chloride, bromide, and iodide complexes of octa-methyl calix[4]pyrrole (omC4P) and α,α,α,α-isomer of the 4-nitro-aryl-extended calix[4]pyrrole (AEC4P). The experimentally determined infrared spectra are compared with those obtained using density functional theory calculations, providing molecular-level insights into the binding properties of these receptor-halide complexes. Halide binding is mainly driven by the symmetric interactions between the halide ion and the receptor’s four pyrrole NH groups. These interactions lock the receptors in cone conformation and are encoded in the NH stretching region of the infrared spectra of the complexes. In the case of the “four wall” AEC4P, the cone conformation receptor defines a deep, polar, aromatic cavity, which is closed at one end by the four converging pyrrole NHs. Upon inclusion of the halide (Cl−, Br−, or I−), we observed a redistribution of the negative charge of the bound anion into the meso-p-nitro-aryl groups of the receptor. The resulting electrostatic field pushes the ion deeper into the cavity of the AEC4P receptor. This contrasts with the more superficial binding geometry of the corresponding omC4P anionic complex, which lacks meso-aryl substituents. A detailed decomposition of the electrostatic forces from various parts of the meso-4-substituted aryl groups suggests design principles that could guide the development of other anion receptors with improved binding affinity and selectivity.