Kinetic deuterium solvent isotope effects on the general acid-catalysed dissociation of metal cryptates

Abstract

The dissociation of metal ions from cryptate complexes is subject to general acid catalysis in a number of cases. The dissociation of Ca(2,2,2)²⁺ has been studied in the presence of HCl and several carboxylic acids as catalysts, and kinetic solvent isotope effects measured for the HCl- and CH₃CO₂H-catalysed dissociation. The observation of general acid catalysis, combined with relatively large isotope effects (2.4 and 2.7, respectively), are consistent with a rate-determining proton transfer in the dissociation reaction. The H⁺-catalysed dissociation of Ag(2,2,2)⁺ shows a lower, but still significant, isotope effect (1.9) and most likely also involves a rate-determining proton transfer step. Li(2,1,1)⁺ shows somewhat different behaviour in that the apparent catalytic constant for acid catalysed dissociation decreases with increasing buffer concentration. The results are shown to be in quantitative accord with a reaction scheme involving protonation of Li(2,1,1)⁺ followed by competitive dissociation of Li⁺ from Li(2,1,1)H²⁺ and deprotonation by A^– of Li(2,1,1)H²⁺ to give (2,1,1)H⁺ or regenerate Li(2,1,1)⁺, respectively. Kinetic solvent deuterium isotope effects for the protonation and deprotonation–dissociation steps are close to unity.