Polarizing a Hydrophobic Cavity for the Efficient Binding of Organic Guests:The Case of Calix[6]tren,a Highly Efficient and Versatile Receptor for Neutral or Cationic Species

摘要

The host-guest properties of calix[6]tren 1 have been evaluated.The receptor is based on a calix[6]arene that is covalently capped at the narrow rim by a tren unit.As a result,the system presents a concave hydrophobic cavity with,at its bottom,a grid-like nitrogenous core.Despite its well-defined cavity and opening to the outside at the large rim,1 did not behave as a good receptor for neutral molecules in chloroform.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 complexation is driven by a strong charge-dipole interaction and hydrogen bonds between the polar guest and the tetracationic cap of the calixarene.Finally,coordination of Zn~(2+)to the tren core led to the asymmetrization of calixarene cavity and to the strong but selective endo-binding of neutral ligands.This study emphasizes the efficiency of a receptor presenting a concave hydrophobic cavity that is polarized at its bottom.The resulting combination of charge-dipole,hydrogen bonding,CH-pi,and van der Waals interactions highly stabilizes the supramolecular architectures.Also,importantly,the tren cap allows the tuning of the polarization,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 simple addition of protons,bases,or metal ions.