Determination of Stability Constants between Lanthanoid(III) Chelates and Oxygen-Containing Organic Compounds by Synergistic Extraction

Abstract

The stability of adduct complexes between lanthanoid(III) chelates and oxygen-containing organic compounds was examined by synergistic extraction using hexafluoroacetylacetone (Hhfa) as the chelating agent. Pr(III) and Eu(III) were extracted into toluene in the presence of polyethylene glycol 400 dimethyl ether (PEG-dm), di(2-ethylhexyl) phthalate (DEHP), and polyethylene glycol mono-p-isooctylphenyl ether (Tx100). Significant synergistic enhancement was observed in all systems, indicating adduct formation in the organic phase. PEG-dm formed a 1:1 adduct, Ln(hfa)₃L, with log β_s,1 values of 5.56 ± 0.04 for Pr(III) and 5.27 ± 0.02 for Eu(III). In contrast, DEHP and Tx100 produced both 1:1 and 1:2 adducts, with smaller differences between β_s,1 and β_s,2 for Eu(III), suggesting steric hindrance due to its smaller ionic radius. The obtained constants were comparable to those reported for crown ether systems, confirming the reliability of the extraction approach. These results demonstrate that synergistic extraction with Hhfa provides a simple and effective approach for evaluating the coordination ability of neutral oxygen donors toward lanthanoid(III) chelates.