Formation and dissociation kinetics of triaza-crown-alkanoic acid complexes of transition metal(II) and lanthanide (III)

Abstract

The formation and dissociation rates of some transition metal(II) and lanthanide(III) complexes of the 1,7,13-triaza-4,10,16-trioxacyclooctadecane N′, N″, N‴-triacetic acid ( 1) and 1,7,13-triaza-4,10,16-trioxacyclooctadecane- N′, N″, N‴- trimethylacetic acid ( 2) have been measured by the use of stopped-flow and conventional spectrophotometry. Experimental observations were made at 25.0 ± 0.1°C and at an ionic strength of 0.10 M KCl. The complexation of Zn 2+ and Cu 2+ ions with 1 and 2 proceeds through the formation of an intermediate complex (MH 3L +∗) in which the metal ion is incompletely coordinated. This may then lead to a final product in the rate-determining step. Between pH 4.68 and 5.55, the diprotonated (H 2L −) form is revealed to be a kinetically active species despite its low concentration. The stability constants ( log K MH 3 L +∗ ) and specific base-catalyzed rate constants ( k OH) of intermediate complexes have been determined from the kinetic data. The dissociation reactions of 1 and 2 complexes of Co 2+, Ni 2+, Zn 2+, Ce 3+, Eu 3+ and Yb 3+ were investigated with Cu 2+ ions as a scavenger in acetate buffer. All complexes exhibit acid-independent and acid-catalyzed contributions. The buffer and Cu 2+ concentration dependence on the dissociation rate has also been investigated. The metal and ligand effects on the dissociation rate of some transition metal(II) and lanthanide(III) complexes are discussed in terms of the ionic radius of the metal ions, the side-pendant arms and the rigidity of the ligands.