Acid Catalyzed Dissociation Kinetics of The Copper(II) and Mercury(II) Complexes of a Pendant-Arm Macrocycle

Spectrophotometry and stopped-flow spectroscopy was used to study the kinetics of the acid catalyzed dissociation reactions of the complex cations of copper(II) and mercury(II) of the macrocycle cyclam with four N-pendant amide groups, (N,N’,N’’,N’’’- tetrakis (2-carbamoylethyl)- 1,4,8,11-tetraazacyclotetradecane) (TCEC), at [H+] = 4.0 M, I = 4M (NaCl) and T= 35 °C(Cu) and at [H+] = 1.0 M, I = 1.0 M (LiClO4) and T= 25 °C (Hg). The reaction rate for the Hg(II) complex cation of TCEC is composite consisting of two overlapping processes, corresponding to the existence of two isomers. Each process displays acid dependence at low acidities and acid independence at high acidities over the acid range of study complying with the saturation kinetics mechanism which involves a rabid protonation preequilibrium, with rate-determining dissociation of this protonated species. The dissociation of the Cu(II) complex cation of TCEC in acidic medium occur in two separable consecutive reactions, first the complex isomerizes from blue to red and then the intermediate red complex dissociates to the final products Cu(II) and the protonated TCEC. Each of these consecutive reactions are composite consisting of two overlapping processes corresponding to the existence of two isomers. In the acid isomerization reaction of the blue Cu(II)-TCEC cation, both isomers show linear dependence on [H+]. Where as in the acid dissociation reaction of the intermediate red Cu(II)-TCEC cation, both isomers show non linear dependence on [H+] (a behavior similar to the dissociation of the Hg(II)-TCEC cation). Comparison between the results obtained in this study and previous studies is discu