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

Abstract

The inclusion of symmetrical tetramethylammonium cation (TEMA) by the water soluble calixarene hosts 1–8 was studied at neutral pH by ¹H 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 ¹H 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.