Dissociation kinetics of cerium(III) complexes of macrocyclic polyaza polycarboxylate ligands TETA and DOTA

The acid-catalyzed dissociation rate constants of the cerium(III) complexes of 1,4,8,11-tetraazacyclotetradecane-1,4, 8,11-tetraacetic acid (TETA) and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) have been determined at four different temperatures (i.e., 25.0 degreesC, 32.0 degree sC, 39.0 degreesC, 45.0 degreesC) in aqueous media (mu = 0.10 M, HCl/KCl) t o obtain additional kinetic data and to evaluate possible effects of ligand pre-organization for metal ion complexation. The rates are much faster for Ce(TETA)(-) than for Ce(DOTA)(-), indicating the lower thermodynamic stabi lity of the former. In the presence of excess strong acid, 0.1 - 1.0 M HCl, the dissociation reactions follow the rate law: -d[ML]T/dt = (k(d) + k(H)[ H+])[ML](T) and -d[ML]T/dt = (k(H)[H+] + k(H2)[H+](2))[ML](T), respectively , where k(d) is acid-independent dissociation reaction rate constant and k( H) and k(H2) are the respective dissociation rate constants for the pathway s involving monoprotonated and diprotonated species. The rate activation pa rameters, DeltaH(not equal), DeltaS(not equal) and DeltaG(not equal), for e ach dissociation pathway have been obtained and their values are consistent with the proposed mechanisms. In particular, the rate difference between C e(TETA)(-) and Ce(DOTA)(-) for the monoprotonated complex dissociation path way is mainly due to difference in DeltaH(H)(not equal). If has been conclu ded that ligand pre-organization results in more stable complexes and slowe r complex dissociation rates.