Interplay between Molecular Recognition and Acid-Base Reactivity in Tailored Metal-Based Calix[6]arene Receptors

Abstract

This study explores the interplay between molecular recognition and acid–base reactivity in two novel Zn–calix[6]arene funnel complexes featuring phenol units in place of the anisole units present in the parent systems. The two complexes differ in the accessibility of their third coordination sphere. NMR and X-ray crystallography revealed that, like their anisole-based predecessors, these new complexes can encapsulate neutral guests such as acetonitrile. However, in contrast to the parent systems, which strongly bind primary amines, the new phenol-based systems undergo a base-induced structural reorganization upon addition of propylamine, triggered by a selective metal-assisted phenol deprotonation. Moreover, the presence of phenol groups enables intra–cavity binding of anions, provided their structure allows for simultaneous stabilization through hydrogen bonding. Finally, differences in the accessibility of the third coordination sphere between the two complexes influenced their affinity for long linear anions. These findings underscore a delicate balance between coordination–driven recognition, hydrogen bonding, and acid-base chemistry in such systems. The work expands the understanding of first, second and third coordination sphere effects in metallo–receptors and demonstrates how subtle structural modifications can modulate binding modes. It also suggests new directions for designing responsive host systems capable of switching behavior under mild stimuli.