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 alkalin e 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 outsi de, the cavity. The benzo substituents have no influence upon the enth alpy of the protonation. The entropy of the protonation decreases slig htly with the number of benzo substituents due to the lower flexibilit y of the cryptands. The increasing number of benzo groups leads to low er stability constants for the alkali and alkaline earth cations. Thes e cryptands shield the complexed cations more efficiently from the sol vent. Comparing the complexation entropies of the cryptands of barium ion with those of potassium ion it is shown that in the case of the ba rium ion one more water molecule is released compared with the potassi um ion during the complex formation with all three cryptands examined. (C) 1998 Elsevier Science Ltd. All rights reserved.