Unfolding the complexation and extraction of Am and Eu using N-heterocyclic aromatic diphosphonic acids: a combined experimental and DFT study

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 Am 3+ and Eu 3+ . Complexation studies in aqueous medium using luminescence titration indicated the formation of anionic complexes in the case of Eu 3+ . Two phase liquidliquid 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 metalligand 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 Am 3+ and Eu 3+ 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 Am 3+ and Eu 3+ was attributed to the presence of unfavorable covalent interactions in the metalligand bonds. Extraction behaviour of Am 3+ with three N,O-donor N-heterocyclic aromatic diphosphonates is explained using spectroscopic and DFT studies.