Energetics of the Inclusion of Organic Molecules by Rigidified Cone Calix[4]arenes in Carbon Tetrachloride

The complexation of acetonitrile and nitromethane by the calix[4]biscrowns 1, 2, 3 and the upper rim pyridine brigded calix[4]arene 4, all fixed in a C4v structure, was studied by 1H NMR and calorimetry in CCl4. The spectroscopic data show that the inclusion of the two organic guests occurs via their -CH3 moiety in all cases. For all the investigated hosts the binding constants with both guests determined by both 1H NMR and calorimetric titrations, show that the inclusion is favoured by the presence of functionalizing groups at the upper rim. Furthermore logK values indicate that no stability difference was found between the two guests. Calorimetric data show that the inclusion is enthalpically driven in all cases. The favourable enthalpic contribution is to be attributed mainly to the CH- π interactions between the π-donor aromatic moieties of the hosts and the activated methyl residue of the guests. The combination of 1H NMR and calorimetric information allows for a more detailed description of the inclusion process.