Kinetics and mechanism of the acid dissociation of the Cobalt(II) complex of novel C-functionalized 13-membered macrocyclic dioxotetraamines

Abstract

The kinetics of acid dissociation of cobalt(II) complexes of novel C-functionalized 13-membered macrocyclic dioxotetraamines was studied using stopped-flow spectrophotometry at different temperatures. The results indicated the dissociation rate follows the law vd = CcomkK1K2[H]2/(1 + K1[H] + K1K2[H]2). On the basis of the experiment facts obtained, the dissociation kinetics is interpreted by a mechanism involving the negatively charged carbonyl oxygen of the complex being rapidly protonated in a pre-rate-determining step, the rate-determining step being intramolecular hydrogen (enolic tautomer) migration (to imine nitrogen). The dissociation rate reached a plateau in strongly acidic solution. K1 and K2 of the pre-rate-determining steps and k of the rate-determining step were obtained by means of non-linear least-squares fitting method, and corresponding activation parameters were also obtained by means of temperature coefficient method. The influence of the substituents to the acid dissociation rates has been discussed. The Bronsted type linear-free energy relationship does exist in these C-functionalized dioxotetraamine cobalt(II) complexes. It is first found that the linear relationship between the ΔH≠ and ΔS≠ of the rate-determining step does exist in this and another analogous system. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 804–809, 1999