Entropic origin of the sulfonate groups' electrostatic assistance in the complexation of quaternary ammonium cations by water soluble calix[4]arenes

The inclusion of symmetrical tetramethylammonium cation (TEMA) by the water soluble calixarene hosts 1-8 was studied at neutral pH by H-1 NMR spectroscopy and compared with that of the ditopic trimethylanilinium cation (TMA). The hosts blocked in the cone conformation and bearing sulfonate groups at the upper rim (2, 3, 5, 7 and 8) bind selectively the aromatic portion of TMA, whereas compound 4 which lacks sulfonate groups interacts only with the charged head group of TMA. The conformationally mobile compound 1 and the partial cone calixarene 6 include TMA cation in an unselective fashion. TEMA is complexed by hosts 1-7, but not by the tetraether-tetrasulfonate receptor 8. The binding constants for all the systems, as determined by H-1 NMR spectroscopy, show that inclusion is favoured by the presence of the sulfonate groups and that the complexes of the conformationally mobile receptor 1 with both guests are more stable. The thermodynamic parameters of inclusion determined by direct calorimetry for 2-TEMA and 4-TEMA systems show that in both cases the inclusion process is enthalpically driven and that the greater stability constant observed for 2-TEMA with respect to that of 4-TEMA mainly results from a less unfavourable entropic contribution, suggesting that in the 2-TEMA complex the charged sulfonate groups cause a better desolvation of the host-guest system upon inclusion.