Binding of ion pairs and neutral guests by aryl-extended meso‑p-hydroxyphenyl calix[4]pyrrole : The interplay between three binding sites

An aryl-extended calix[4]pyrrole with four meso‑p-hydroxyphenyl substituents was investigated as a host for chloride, acetate, and benzoate anions. Crystal structures of pyridinium and imidazolium chloride complexes were obtained in which chloride ions are hydrogen bonded exo-cavity to the upper rim hydroxyl groups, and the aromatic cations are bound to the shallow cavity of the host. Furthermore, the calix[4]pyrrole formed a hydrogen bonded dimeric capsule templated by inclusion of adiponitrile guest in the endo-cavity binding site. NMR titrations revealed the preference of the OH groups of the host to bind anionic guests in solution. Benzoate anion had the highest binding constant (4 700 M−1) in acetonitrile. Density functional theory (DFT) calculations indicated that the exo-cavity complex with chloride anions was favoured by 23.2 kJ/mol over the endo-cavity complex, whereas the energies of endo- and exo-cavity benzoate complexes were of similar magnitude due to dispersion interactions between the host and the guest.