Kinetic studies of the on/off reaction of the amino group in the side chain of Cu(ii), Ni(ii), and Co(ii) complexes with 14-membered tetraazamacrocycles

Abstract

The kinetics of the on/off reaction of the amino group in the side chain of tetraazamacrocyclic Cu²⁺, Ni²⁺ and Co²⁺ complexes has been measured. The rate law k_obs = k₀ + k_H[H⁺] + k_OH/[H⁺], the sum of the forward and reverse reaction, gives rise to u-shaped pH dependences from which the three rate constants can be determined. k_H describes the proton assisted dissociation of the amino group bound to the metal ion and is roughly correlated to the equilibrium constant of the reaction. k_OH is determined by the protonation constant of the free amino group and the rate constant describing the binding of the amino group to the metal ion. k₀ is composed of the rate constant for the opening of the chelate ring without proton assistance and the rate for the reactivity of the ammonium group in the formation of the chelate ring. Our results show that the rates of the opening and closing of the chelate ring are very little dependent on the nature of the metal ion.