Development of a water-soluble ouroboros-like calix[6]arene-trisimidazole-based ligand for enhanced binding of zinc

Increasing the thermodynamic stability of metal complexes in water is a key challenge for various applications of coordination chemistry. In this study, a calix[6]arene macrocycle functionalized with three imidazole units on the narrow rim and three primary amino legs on the wide rim was synthesized and studied. This ligand coordinates a Zn²⁺ cation in water, within a pH window centered around 7, through the three imidazole units and one self-included amino leg occupying the coordination site within the macrocycle cavity. The coordination process benefits from a favourable entropic effect, conferring greater thermodynamic stability to the resulting ouroboros-like complex compared to the one stabilized by an exogenous amine. Additionally, the intramolecular coordination ensures a supramolecular protection, which can be exploited for the site-selective functionalization of two out of the three amino groups. These findings demonstrate that the use of macrocyclic ligands capable of forming ouroboros-like molecular complexes represents a valuable strategy for stabilizing metal ions in water.