Kinetics and mechanism of metal complex formation with N 4-donor macrocycles of the cyclam type

Abstract

The N 4-donor macrocyclic ligand cyclam (1.4,8,11-tetraazacyclotetradecane) forms very stable complexes with nickel(II) and copper(II). With regard to kinetics and mechanism of complex formation, tetraaza cyclic ligands such as cyclam represent an interesting type of chelate ligands in that they are less flexible than aliphatic open-chain N-donor ligands and less rigid than cyclic N-donor ligands of the porphyrin type. The Eigen-Wilkins mechanism provides an adequate description of the kinetics of complex formation of nickel(II) and copper(II) with monodentate ligands and allows to correlate the rate of complex formation with the rate of solvent exchange on the solvated cations NiS 6 2+ and CuS 6 2+ (S = solvent). This review focuses on the factors controlling the rate and mechanism as well as the stereochemistry of complex formation of nickel(II) and copper(II) with N 4-donor macrocyclic ligands of the cyclam type. The kinetic results obtained in aqueous solution are briefly reviewed, although their mechanistic interpretation is hampered by ligand protonation. The kinetic studies carried out in aprotic polar solvents such as N,N-dimethylformamide are reviewed and discussed in detail. In comparison to the Eigen-Wilkins mechanism and Eigen-Winkler mechanism, the aspect of metal-based rate control versus ligand-based rate control is emphasized.