Complexation of divalent and trivalent nickel and copper ions by rigid and flexible dioxo tetraaza macrocycles

Potentiometric, spectrophotometric, kinetic, and electrochem. studies on the Ni2+ and Cu2+ complexes with I and II were performed to study the importance of the flexibility of the macrocyclic ring to the properties of the corresponding complexes. The more flexible ligand I gives a series of Cu2+ complexes with variable protonation stoichiometry, whereas ligand II (H2L) only forms a Cu2+ complex of the type [CuL]. The 2 ligands also differ in their complexation kinetics, the reactivity of I being higher than that of II. Thus from its equil. and kinetics, ligand I closely resembles open-chain ligands, whereas the reactivity of II is typical for macrocycles. On the other hand, the redox potential of the M(III)/M(II) couple is practically independent of the rigidity of the macrocyclic ring.