Mechanistic information on ligand-substitution reactions of europium(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 Eu^III with the polyaminecarboxylates dtpa (diethylenetriaminepentaacetate) and edta (ethylenediaminetetraacetate) were studied as a function of polyaminecarboxylate concentration, pH, temperature, pressure and ionic strength by stopped-flow spectrophotometry within the acidity range 3.61 < pH < 5.56 and between 17.5 and 40 °C. Under all experimental conditions two consecutive steps (k₁ and k₂) were observed over different time-scales. For the fast step (k₁) the substitution rate of Eu^IIIL₂(H₂O)(L = arsenazo III) by dtpa and edta (L′) increases with increasing acidity of the medium, k₁=k_a+k_b[H⁺], and depends on the L′ concentration, i.e. k₁=k_c+k_d[L′]. For the slow step (k₂) plots of k₂versus[H⁺] are linear and exhibit no intercept and k₂ is independent of L′ concentration. Ionic strength studies indicated no significant dependence of k₁ and k₂ on [NaClO₄]. Rate constant k₁ increases with increasing pressure, and there is a very good linear relationship between ln k₁ and pressure at any temperature, ligand concentration, pH and ionic strength. In contrast, k₂ exhibits almost no pressure dependence for any temperature, pH and ligand concentration. It is suggested that this latter step involves acid-catalysed dechelation of arsenazo III accompanied by chelation of L′. In terms of intrinsic volume changes, bond formation and breakage contributions seem to cancel in order to account for the zero ΔV₂^‡ values. The substitution of arsenazo III by dtpa on Eu^III is much faster than the same process on Gd^III.