Mixed ligand complexes of lanthanides with macrocyclic and open-chained polyaminopolycarboxylic acids and acetylacetone

Abstract

Studies of mixed ligand complex formation stabilities and dissociation kinetics have been performed on lanthanide ions with macrocyclic and open- chained polyaminopolycarboxylic acids ( i.e. DAPDA, DACDA, EDDA, and EDTA) and acetylacetone (acac). From UV spectroscopic evidence, it was found that Ln(DACDA) + and Ln(EDTA) − complexes do not form mixed ligand complexes with acac under the set conditions, i.e. pH = 7.2 and complex concentration of 1 x 10 −4 M. On the other hand, formation of Ln(DAPDA)(acac) and Ln(EDDA)(acac) 2 − complexes were readily detectable. The mixed complex forma- tion constants,β 1, for the equilibrium Ln(L) + + acac − ⇌ Ln(L)(acac), and β 2, for the equilibrium Ln(L) + + 2 acac − ⇌ Ln(L)(acac) 2 − were determined by potentiometric titration technique where possible. It was found that β 1 values were in general greater for Ln(EDDA) + complexes than for Ln(DAPDA) + complexes indicating the resulting reduced charge density at the lanthanide ion of Ln(DAPDA) + and that less space is available for the acetylacetone moiety to coordinate to the Ln(DAPDA) + complexes due to the large size and the greater number of coordination atoms of DAPDA. The hydrolysis constants of Ln(EDDA)(H 2O) n + species were also determined and were found to be increasing with increasing atomic number of Ln. Attempts to measure the acid assisted mixed ligand complex dissociation rates by a stopped-flow spectrophotometer were not fruitful due to the much faster rates.