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

• Published in: Journal of molecular structure 2023-02, Vol.1273, p.134268, Article 134268
• Main Authors: Pamuła, Małgorzata, Bulatov, Evgeny, Helttunen, Kaisa
• Format: Article
• Language: English
• Subjects: Anions; Calixpyrroles; Host-guest systems; Supramolecular chemistry; X-ray crystallography
• ISSN: 0022-2860; 1872-8014
• DOI: 10.1016/j.molstruc.2022.134268

•Meso-p-hydroxyphenyl calix[4]pyrrole is a host for neutral molecules and ion pairs.•The binding properties were studied by a combination of DFT, NMR, and XRD methods.•The receptor possesses endo- and exo-cavity binding modes.•Neutral molecules preferentially bind endo-cavity, while anions bind exo-cavity.•The interplay between the binding modes leads to extended assemblies in crystals. 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. [Display omitted]