Effects of the benzosubstitution of cryptands for the complex formation between protons, alkali and alkaline earth cations in water

The protonation reactions of the cryptand (222), benzocryptand (222B) and dibenzocryptand (222BB) and the complexation of alkali and alkaline earth cations in aqueous solution have been studied using pH-metric and calorimetric titrations. The results show that the first proton of all protonated ligands is located inside, and the second proton outside, the cavity. The benzo substituents have no influence upon the enthalpy of the protonation. The entropy of the protonation decreases slightly with the number of benzo substituents due to the lower flexibility of the cryptands. The increasing number of benzo groups leads to lower stability constants for the alkali and alkaline earth cations. These cryptands shield the complexed cations more efficiently from the solvent. Comparing the complexation entropies of the cryptands of barium ion with those of potassium ion it is shown that in the case of the barium ion one more water molecule is released compared with the potassium ion during the complex formation with all three cryptands examined. (C) 1998 Elsevier Science Ltd. All rights reserved.