Kinetics and mechanism of the dissociation of the Cu2+ and Ni2+ complexes with monooxo‐ and dioxomacrocyclic tetraamines

The kinetics of the acid‐induced dissociation of the Ni2+ and Cu2+ complexes of a monooxo and two dioxo tetraaza macrocycles were studied by stopped‐flow spectrophotometry at single wavelengths and using a photodiode array, For the monooxo derivative the dissociation rate follows the law vd = k1Ccom [H+]/(K2 + [H+]), whereas for the dioxo derivatives the law is vd = k2Ccom [H+]2/(K1 + K2 [H+] + [H+]2). The dissociation kinetics are interpreted by a mechanism in which protonated intermediates are formed in a rapid pre‐equibrium, the rate‐determining step being the dissociation of this species. Comparison of the spectral results from multi‐wavelength stopped‐flow measurements with those acquired in the equilibium study using spectrophotometric titrations allows the nature and structure of these intermediates to be discussed. The order of the dissociation rates in strongly acidic solution, where a plateau in the pH dependence is reached, was determined and it demonstrates the importance of the ring size on one hand and of the number of amide groups on the other.