Interfacial catalysis of formation and dissociation of tervalent lanthanide complexes in two-phase systems

The metallochromic indicator method which was previously developed for studying in micelles metal complex formation and dissociation reactions without an adequate spectral change has been adapted here for the organic-aqueous phase pair to discern the factors affecting the efficiencies of separations of metal ions by centrifugal partition chromatography (CPC). A novel hydrophilic phase separator capable of operating under high aqueous phase low rates has been introduced to accomplish these studies. The kinetics of formation and dissociation of representative tervalent lanthanide (M3+=Pr3+, Eu3+, Tb3+, Ho3+, Yb3+)-acylpyrazone (HL equals 1phenyl-3-methyl-4-benzoyl-5-pyrazolone (HPMBP) or 1-phenyl-3-methyl-4-capryloyl-5-pyrazolone (HPMCP)) complexes (ML3) in the toluene-water phase pair have been studied by monitoring the fate of the M3+-arsenazo III (AZ) complex (MAZ) in the aqueous phase with the hydrophilic phase separator. Mechanistic studies indicate that the formation 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 reactions in the two-phase systems. The formation and dissociation of the ML2+ complex are rate-limiting in the ML3 formation and dissociation reactions, respectively, and the rate constants for the formation reactions increase while those for the dissociation reactions decrease from light to heavy lanthanides. The variations in the formation and dissociation rate constants from the light to the heavy lanthanides are less than an order of magnitude for both ligands, with the rate constants for a given M3+ being larger for HPMCP than for HPMBP. The dissociations of ML3 complexes were also studied in micelles formed from the neutral 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 those at the toluene-water interface. Interestingly, the metallochromic indicator does not catalyze the dissociation reactions to a significant extent in the micellar pseudophase.