Premium
Dissociation Kinetics of Cerium(lll) Complexes of Macrocyclic Polyaza Polycarboxylate Ligands TETA and DOTA
Author(s) -
Chang C. Allen,
Liu YuhLiang
Publication year - 2000
Publication title -
journal of the chinese chemical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.329
H-Index - 45
eISSN - 2192-6549
pISSN - 0009-4536
DOI - 10.1002/jccs.200000139
Subject(s) - chemistry , dissociation (chemistry) , dota , reaction rate constant , macrocyclic ligand , ligand (biochemistry) , cerium , kinetics , lanthanide , medicinal chemistry , dissociation constant , metal , chelation , stereochemistry , inorganic chemistry , ion , organic chemistry , biochemistry , physics , receptor , quantum mechanics
Abstract The acid‐catalyzed dissociation rate constants of the cerium(III) complexes of 1,4,8,11‐tetraazacyclo‐tetradecane‐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°C,32.0°C,39.0°C,45.0°C) in aqueous media (μ = 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 MHCl, 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, ΔH ≠ , ΔS ≠ and ΔG ≠ , 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 ΔH H ≠ . It has been concluded that ligand pre‐organization results in more stable complexes and slower complex dissociation rates.