Kinetics and thermodynamics of complex formation between ions and macrobicyclic ligands in several solvents

Abstract

This work deals with the variation of the stability constants and formation and dissociation rate constants with solvent, of complexes (cryptates) formed between macrobicyclic polyether diamines (cryptands) and metal cations, chiefly alkali and alkaline-earth metal cations. Stability and rate constants were determined for complexes of the cryptands (2,1,1), (2,2,1) and (2,2,2) in the following solvents: ethanol, dimethyl sulphoxide, dimethylformamide, N-methylpropionamide, propylene carbonate and acetonitrile (stability constants only). Complexes of monobenzo and dibenzo derivatives of the (2,2,2) cryptand were also studied in propylene carbonate. The results show a wide range of stability constants(from less than 10 M-1 to more than 1018 M-1 ) and dissociation rate constants (from less than 10-5 s-1 to more than 104 s-1 ). However, formation rates vary only over few orders of magnitude and, when the cryptand is sufficiently flexible and the metal ion is in a poorly solvating medium, are close to the rates of solvent exchange in the inner coordination shell of the cations. The variations in cryptate stability with metal ion and solvent are almost entirely reflected in the dissociation rather than in the formation rates. This suggests that the transition state lies closer to the reactants (metal ion and cryptand) than to the complex. A quantitative analysis of the results shows a correlation between the free energies of transfer of the reactants and transition state. A combination of the stability constants and free energies of transfer of the free cations, shows that there is a more favourable transport (in terms of free energy) from water to non-aqueous media of cryptates involving optimum fit of the cation into the ligand cavity. Finally, results obtained for the acid-catalysed dissociation of cryptates suggest that catalysis is more important for small cations, but also depends upon ligand flexibility and the solvent.