Molecular recognition of organic anions by a water-soluble calix[4]arene: Evidence for enthalpy–entropy compensation

The inclusion of organic anions by the water-soluble tetracationic calix[4]arene TAC4 has been studied at neutral pH by 1H NMR and isothermal titration calorimetry. The aromatic anions are better included than their aliphatic analogues. The ΔH° and ΔS° values show that the inclusion processes are mostly enthalpically driven and entropically unfavoured; they also show that an enthalpy–entropy compensation balances the gain in one contribution against a corresponding loss in the other.

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► The inclusion of organic anions by calix[4]arene was studied in water at neutral pH.
► Guests covering a range of sizes, shapes and functionalities have been examined.
► The aromatic anions are better included than their aliphatic analogues.
► The inclusion processes are mostly enthalpically driven and entropically unfavoured.
► ΔH° and ΔS° values indicate an enthalpy–entropy compensation effect.