INTERFACIAL CATALYSIS OF FORMATION AND DISSOCIATION OF TERVALENT LANTHANIDE COMPLEXES IN 2-PHASE SYSTEMS

The metallochromic indicator method which was previously developed for studying in micelles metal complex formation and dissociation reactio ns without an adequate spectral change has been adapted here for the o rganic-aqueous phase pair to discern the factors affecting the efficie ncies of separations of metal ions by centrifugal partition chromatogr aphy (CPC). A novel hydrophilic phase separator capable of operating u nder high aqueous phase low rates has been introduced to accomplish th ese studies. The kinetics of formation and dissociation of representat ive tervalent lanthanide (M3+=Pr3+, Eu3+, Tb3+, Ho3+, Yb3+)-acylpyrazo ne (HL equals 1phenyl-3-methyl-4-benzoyl-5-pyrazolone (HPMBP) or 1-phe nyl-3-methyl-4-capryloyl-5-pyrazolone (HPMCP)) complexes (ML3) in the toluene-water phase pair have been studied by monitoring the fate of t he M3+-arsenazo III (AZ) complex (MAZ) in the aqueous phase with the h ydrophilic phase separator. Mechanistic studies indicate that the form ation and dissociation reactions of ML3 which occur exclusively at the toluene-water interface are catalyzed by AZ through the adsorption of the MAZ complex at the interface. This is the first demonstration of interfacial catalysis of metal complex formation and dissociation reac tions in the two-phase systems. The formation and dissociation of the ML2+ complex are rate-limiting in the ML3 formation and dissociation r eactions, respectively, and the rate constants for the formation react ions increase while those for the dissociation reactions decrease from light to heavy lanthanides. The variations in the formation and disso ciation rate constants from the light to the heavy lanthanides are les s than an order of magnitude for both ligands, with the rate constants for a given M3+ being larger for HPMCP than for HPMBP. The dissociati ons of ML3 complexes were also studied in micelles formed from the neu tral surfactant poly(ethylene glycol) tert-octylphenyl ether (Triton X -100) using AZ, where also they occur at the aqueous-micelle interface , with these rate constants being 3 orders of magnitude larger than th ose at the toluene-water interface. Interestingly, the metallochromic indicator does not catalyze the dissociation reactions to a significan t extent in the micellar pseudophase.