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.