Stereochemistry and thermodynamics of the inclusion of aliphatic and aromatic anionic guests in a tetracationic calix[4]arene in acidic and neutral aqueous solutions

A cationic calix[4]arene derivative binds both aliphatic and aromatic anions in aqueous solution thanks to concerted electrostatic and hydrophobic interactions. 1H NMR and potentiometric datashow that the host flexibility changes as a function of pH. 1H NMR and isothermal titration calorimetry (ITC) studies, also supported by ESI-MS data, show that inclusion yields supramolecular complexes of the guestChost type only. Binding becomes weaker as the pH increases due to rigidification of the host in neutral aqueous solutions, and is enthalpically favoured for all the supramolecular complexes, regardless of pH.