Kinetics of formation and dissociation of triaza-crown-alkanoic acid complexes of cerium(III) and europium(III)

Abstract

The formation and dissociation rates of some lanthanide complexes of the 1,4,10-triaza-7,13-dioxacyclopentadecane- N, N′, N″-triacetic acid ( 1) and 1,4,10-triaza-7,13-dioxacyclopentadecane- N, N′, N″-tripropionic ( 2) have been measured by using stopped-flow and conventional spectrophotometry. Observations were made at 25.0±0.1°C and at an ionic strength of 0.1 M NaClO 4. The complexation of Ce 3+ and Eu 3+ ions by ligand 1 proceeds through the formation of an intermediate complex in which the metal ion is incompletely coordinated. This may then proceed to a final product. Between pH 4.57 and 5.61, the monoprotonated (HL 2−) form is the kinetically active species despite its low concentration. The dissociation reactions of Ln- 1 and Ln- 2 complexes were investigated with Cu 2+ ions as scavenger in an acetate buffer. All complexes exhibit acid-independent and acid-catalysed contributions. The buffer and Cu 2+ concentration dependent of the dissociation rate has also been investigated. The metal and ligand effects on the dissociation rate of some lanthanide complexes are discussed by comparing the rate constants with those of analogous linear and macrocyclic polyaminepolycarboxylate systems.