Solvent Dependence of the Kinetics of Formation and Dissociation of Cryptate Complexes

Article abstract of DOI:10.1021/ja00395a010

The rates of dissociation of a variety of alkali metal cations and Ca²⁺ cryptates have been measured in several solvents.These have been combined with measured stability constants to give the corresponding formation rates.The dissociation rates are very sensitive to solvent variation, covering a range of more than 9 orders of magnitude.Except for (2,1,1) cryptates, formation rates are all within the range 10⁶ - 10⁹ M^-1 s^-1.Changes in stability constants, whether from a change in the cation, ligand, or solvent, are largely reflected in changes in dissociation rates.The properties of the transition state, particularly with the respect to solvent variation, most closely resemble those of the reactants, suggesting that the transition state lies close to the reactants.The dissociation rates increase sharply with increasing donor number of the solvent, whereas the formation rates decrease but are much less sensitive to solvent variation.On the basis of these correlations, formation rates in water are much lower than expected and dissociation rates much higher than expected.It is suggested that this is due to the H-bonded interactions between water and the electronegative atoms (O and N) of the ligands.