Equilibria and formation kinetics of some cyclen derivative complexes of lanthanides

The kinetics of the formation reactions between the lanthanide(III) ions Ce³⁺, Eu³⁺ and Yb³⁺ and four cyclen derivative ligands, DO3A-B, DO3A-ME, DO2A and DO2A-2B, were studied by spectrophotometry and a stopped-flow method at 25°C in 1.0 M KCl solutions. The reactions were found to be of first order, which was interpreted in terms of the formation of a diprotonated intermediate, Ln(H₂L)⁺. The formation of products occurs via deprotonation and rearrangement of the intermediate, characterised by the rate constant, k(r). The rate law obtained, k(r) = k(OH)[OH^-], is similar to those obtained for the formation reactions of DOTA and DOTA derivative complexes. The rate constants, k(OH), decrease with decrease in the number of charged carboxylate functional groups in the ligands; the lowest rates were found for the formation of the DO2A complexes. The formation rates increase significantly from Ce³⁺ to Yb³⁺. The direct proportionality between the formation rates and [H⁺]^- was interpreted by assuming the equilibrium formation of a monoprotonated intermediate, Ln(HL), which undergoes deprotonation in a slow, rate-determining step. The validity of general base catalysis was detected in the formation reactions, which supports the assumption of the rate-controlling role of deprotonation of the monoprotonated intermediate. The protonation constants of the ligands DO3A-ME and DO2A-2B and the stability constants of their complexes were also determined. The ligand DO2A forms the usual complexes Ln(DO2A)⁺ and the unusual species Ln(DO2A)₂^-. In the complexes Ln(DO2A)₂^-, the two ligands exhibit different coordination modes which were demonstrated by ¹H NMR spectroscopy. (C) 2000 Elsevier Science S.A.