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 EuIII 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 EuIIIL2(H2O)(L = arsenazo III) by dtpa and edta (L′) increases with increasing acidity of the medium, k1=ka+kb[H+], and depends on the L′ concentration, i.e. k1=kc+kd[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 EuIII is much faster than the same process on GdIII.