Mechanistic information on ligand-substitution reactions of gadolinium(III) in aqueous solution from high-pressure stopped-flow experiments

Abstract

Substitution reactions of arsenazo III [3,6-bis(o-arsonophenylazo)-4,5-dihydroxynaphthalene-2,7-disulfonic acid] complexes of Gd^III with the polyaminocarboxylates ethylenediaminetetraacetate and diethylenetriaminepentaacetate were studied as a function of ligand concentration, pH, temperature and pressure. Under all experimental conditions two consecutive reaction steps were observed, for which the observed first-order rate constants are independent of aminocarboxylate concentration and show significant acid catalysis. The observed kinetic behaviour and the activation parameters determined under limiting conditions, i.e. low and high [H⁺], can be accounted for in terms of dechelation of the arsenazo III ligand being the rate-determining step. The existence of 1 : 1 and 1 : 2 Gd^III:arsenazo III complexes is suggested to account for the fast and slow substitution reactions, respectively. The observed volumes of activation can mainly be ascribed to large solvational changes associated with the acid-catalysed dechelation process.