Kinetics of ligand-exchange reactions of macrobicyclic cryptands

Abstract

The rate constants for exchange reactions between metal cryptate complexes, MCryⁿ⁺₁, and a free cryptand, Cry₂, have been determined in several solvents. Two mechanisms could be distinguished: a reaction proceeding via the free metal cation following dissociation of MCryⁿ⁺₁, and a direct, bimolecular cation exchange between the two ligands. In solvents such as water, dimethylsulphoxide and dimethylformamide, which interact strongly with cations, the former pathway predominates for alkali-metal and alkaline-earth-metal cryptates. In poorly solvating media such as methanol and propylene carbonate, where dissociation rate constants are very low, bimolecular pathways make an important contribution to the overall exchange pathway. Exchange reactions of Ag⁺ cryptates show ligand-dependent rates and strong saturation effects in both methanol and dimethylsulphoxide.