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

Abstract

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 degreesC, 39.0 degreesC, 45.0 degreesC) in aqueous media (mu = 0.10 M, HCl/KCl) to 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 stability 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 pathways involving monoprotonated and diprotonated species. The rate activation parameters, DeltaH(not equal), DeltaS(not equal) and DeltaG(not equal), for each dissociation pathway have been obtained and their values are consistent with the proposed mechanisms. In particular, the rate difference between Ce(TETA)(-) and Ce(DOTA)(-) for the monoprotonated complex dissociation pathway is mainly due to difference in DeltaH(H)(not equal). If has been concluded that ligand pre-organization results in more stable complexes and slower complex dissociation rates.