Thermodynamic Origin of the Cryptate Effect in Complexes Involving Cryptand-2,2,2

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Abstract
This paper describes a titration calorimetric study of the heats of complex formation of the ligand bis(2-hydroxyethyl)-1,10-diaza-4,7,13,16-tetraoxocyclooctadecane (BHE-18-aneN2O4) with the metal ions Ag+, Pb2+, Sr2+, and Ba2+, and the ligand 1,10-diaza-4,7,13,16-tetraoxocyclooctadecane (18-aneN2O4) with the metal ions Ag+ and Pb2+. Thermodynamic quantities for complex formation of the ligand BHE-18-aneN2O4, which is a macrocycle with two pendant 2-hydroxyethyl groups, are compared with, corresponding quantities for the ligand 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane (cryptand-2,2,2), which is a cryptand consisting of the same basic macrocycle with a diether bridge. When the ‘cryptate effect’ is defined as the increase in complex stability that occurs when a cryptand ligand is produced by simple ring closure from a parent macrocycle with appropriate pendant chelating substituents, then it is demonstrated that, in the system studied, the cryptate effect is predominantly an enthalpy effect, attributable to preference of the metal ions for ethereal over alcoholic O-donors, and with a small but significant destabilizing entropy contribution. It is also demonstrated that the increase in complex stability that accompanies addition of pendant hydroxyethyl groups to a parent macrocycle is largely an entropy effect, related to desolvation of the terminal hydroxy groups.