KINETICS AND MECHANISM OF MACROCYCLIC COMPLEX-FORMATION

Macrocyclic ligands form highly selective and kinetically labile complexes with a wide range of cations, including alkali metal and alkaline earth cations. Formation rate constants are high and remarkably insensitive to ligand structure, but decrease systematically with increasing charge density of the cation - this behaviour is qualitatively similar to that expected for the dissociative interchange mechanism (I(d)) common to many complexation reactions of simple monodentate ligands. The dissociation rate constants reflect directly the changes in complex stabilities within a related series of cations and over a wide range of ligand structures and solvents. A good illustration of these properties is provided by kinetic studies of the macrocyclic and macrobicyclic effects, which show that the major differences in complexation specificities and thermodynamic stabilies between complexes of open-chain and cyclic ligands are reflected almost entirely in the respective dissociation reactions. The results are considered in terms of a formal, fully-stepwise complexation model in which each of the solvent molecules in contact with the cation is replaced successively by a donor atom of the ligand.