Luminescence and Potentiometric Studies on the Complexation of Europium(III) by Picolinate in an Aqueous Solution

Aoyagi Aoyagi

doi:10.6000/1929-5030.2012.01.02.11

Abstract

The complexation of Eu3+ aquo ions by picolinic acid in an aqueous solution was investigated to describe the formation of polynuclear complexes that are uncommon for trivalent lanthanides. Potentiometric titration indicated that no polynuclear complexes formed via hydroxide bridges, even at high pH, and that the monomeric form [Eu(Pic)4_(H2O)] was present when a large excess of a picolinate ligand was used. In addition, lifetime analysis of Eu3+ luminescence via time-resolved laser-induced fluorescence spectroscopy was performed under conditions where Eu(pic)2+ appears as the secondary dominant species. Strong quenching of the luminescence was detected, suggesting that polymeric complexes ([Eu(Pic)]m2m+) form, even at low pH, owing to a vicinal Eu-Eu interaction.

Highlights

Understanding of the coordination chemistry for lanthanide(III)–organic ligand complexes in an aqueous solution plays a significant role in the current design and synthesis of fascinating hybrid materials, such as porous coordination polymers, metal organic frameworks, and supramolecular polymetallic assemblies of lanthanides [1,2,3]
The analytical methods used for investigating such materials, for example, time-resolved laser-induced fluorescence spectroscopy (TRLFS) at low concentration, are useful for examining chemical speciation under the wide-ranging experimental conditions assumed to be present in the terrestrial environment [4,5,6]
TRLFS measurements were performed under conditions where Eu(Pic)2+ and Eu3+ coexist, and the formation of polymeric complexes was confirmed from the spectral analysis of the quenching behavior

Summary

INTRODUCTION

Understanding of the coordination chemistry for lanthanide(III)–organic ligand complexes in an aqueous solution plays a significant role in the current design and synthesis of fascinating hybrid materials, such as porous coordination polymers, metal organic frameworks, and supramolecular polymetallic assemblies of lanthanides [1,2,3]. Recent studies have revealed the polynuclear complex formation of tetravalent actinide and trivalent lanthanide–glycolate (HO-CH2-COO) and 5sulfosalicylate (HO-C6H3SO3-COO) systems in aqueous solutions [10,11,12] as metal-OH-ligand-type ternary complexes. Both the glycolate and 5sulfosalicylate ligands form chelate complexes via -OH and -COOH functional groups. We investigated the complexation behavior of Eu3+ with picolinate as a representative system for trivalent lanthanide-N/O-type donor group systems and explored the possibility of polynuclear complex formation in aqueous solutions. To confirm the formation of the polymetallic form using TRLFS, we conducted two experimental series in which the predominant chemical species was different

Materials

Apparatus

Complexation in the Alkaline Region with a Large Excess of Ligand

CONCLUSION