| Abstract
| - The host−guest properties of calix[6]tren 1 have been evaluated. The receptor is based on acalix[6]arene that is covalently capped at the narrow rim by a tren unit. As a result, the system presents aconcave hydrophobic cavity with, at its bottom, a grid-like nitrogenous core. Despite its well-defined cavityand opening to the outside at the large rim, 1 did not behave as a good receptor for neutral molecules inchloroform. However, it exhibited efficient endo-complexation of ammonium guests. By contrast, the per-protonated host, 1.4H+, behaved as a remarkable receptor for small organic molecules. The complexationis driven by a strong charge−dipole interaction and hydrogen bonds between the polar guest and thetetracationic cap of the calixarene. Finally, coordination of Zn2+ to the tren core led to the asymmetrizationof calixarene cavity and to the strong but selective endo-binding of neutral ligands. This study emphasizesthe efficiency of a receptor presenting a concave hydrophobic cavity that is polarized at its bottom. Theresulting combination of charge−dipole, hydrogen bonding, CH−π, and van der Waals interactions highlystabilizes the supramolecular architectures. Also, importantly, the tren cap allows the tuning of thepolarization, offering either a basic (1), a highly charged and acidic (1.4H+), or a coordination (1.Zn2+) site.As a result, the system proved to be highly versatile, tunable, and interconvertible in solution by simpleaddition of protons, bases, or metal ions.
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