Extraction and complexation of trivalent americium and lanthanides using an asymmetric picolinic acid-derived tridentate N,O-hybrid ligand†

Abstract

The extraction and complexation of trivalent americium (Am) and lanthanides (Ln) using an asymmetric picolinic acid-derived tridentate N,O-hybrid ligand, 6-(dioctylcarbamoyl)picolinic acid (DOAPA), have been studied through both experimental and theoretical methods. DOAPA exhibits effective and fast extraction of Am(III) and Ln(III). The extraction is driven by favorable enthalpy change. Slope analysis, absorption spectroscopy and NMR titration indicate that both Am(III) and Ln(III) coordinate with DOAPA to form complexes of a 1 : 3 stoichiometry (metal to ligand). Meanwhile, luminescence and mass spectral studies suggest that three deprotonated tridentate DOAPA ligands (L⁻) substitute all the H₂O molecules in the primary coordination sphere of Eu(III), resulting in the extraction of a neutral complex into the organic phase. Further theoretical calculations reveal that a more robust interaction between DOAPA and Am(III) relative to Eu(III) as well as a higher degree of covalence in Am–N/O bonds than in Eu–N/O bonds leads to slight selectivity of Am(III) over Eu(III).