Unfolding the complexation and extraction of Am3+ and Eu3+ using N-heterocyclic aromatic diphosphonic acids: a combined experimental and DFT study

Abstract

Phosphonate based ligands are well known for the extraction of ‘f’ block elements. Three N,O-donor N-heterocyclic aromatic diphosphonate ligands were evaluated in the present work for the extraction/separation studies of Am³⁺ and Eu³⁺. Complexation studies in aqueous medium using luminescence titration indicated the formation of anionic complexes in the case of Eu³⁺. Two phase liquid–liquid extraction studies were, therefore, carried out by employing Aliquat-336 as the liquid anion exchanger. The results indicated the formation of a species with a metal–ligand stoichiometry of 1 : 3 in the case of pyridine-2,6-diphosphonic acid (PyPOH). In the case of 2,2′-bipyridine-6,6′-diphosphonic acid (BipyPOH), however, a 1 : 2 complex was extracted and 1,10-phenanthroline-2,9-diphosphonic acid (PhenPOH) extracts the Am³⁺ and Eu³⁺ ions by forming both 1 : 2 and 1 : 3 complexes. Formation of these kinds of anionic complexes was further confirmed using electrospray ionization mass spectrometry (ESI-MS). DFT calculations predicted the structure of the anionic complexes. The non-selectivity of these kinds of ligands between Am³⁺ and Eu³⁺ was attributed to the presence of unfavorable covalent interactions in the metal–ligand bonds.