There is still an evident need for selective and stable ligands able to separate actinide(III) from lanthanide(III) metal ions in view of the treatment of the accumulated radioactive waste and of the recycling of minor actinides. We have herein demonstrated that hydrophilic 2,6-bis-triazolyl-pyridines are able to strip all actinides in all the different oxidation states from a diglycolamide-containing kerosene solution into an acidic aqueous phase. The ascertained high actinide selectivity, efficiency, extraction kinetics, and chemical/radiolytic stability spotlight this hydrophilic class of ligands as exceptional candidates for advanced separation processes fundamental for closing the nuclear fuel cycle and solving the environmental issues related to the management of existing nuclear waste.

To quantify the impact of the N-donor softness on the coordination of f elements in aqueous solution, and in particular on the selectivity for Am(III) over Eu(III), we have designed the two tetrapodal hexadentate ligands N,N-bis(2-pyridylmethyl)ethylenediamine-N′,N′-diacetic acid (L py ) and N,N-bis(2-pyrazylmethyl)ethylenediamine-N′,N′diacetic acid (L pz ). These ligands bear two hard acetate groups to provide stability to the An(III) and Ln(III) complexes and two N-heterocyclic soft groups to provide Am(III) versus Eu(III) selectivity. They only differ in their N-donor moieties, pyridine or pyrazine. The proton NMR and potentiometric analyses performed on the lanthanide complexes of the two ligands indicate that a unique metallic complex, LnL, is formed and that LnL py+ and LnL pz+ have the same structure in water. Furthermore, the hydration numbers of the europium and terbium ions in these complexes, measured by luminescence decay, have the same value (q ) 3), indicating that the two ligands act as hexadentate donors in both systems. As expected, the softer pyrazine-based ligand gives less stable complexes than the pyridinebased ligand with the hard Ln(III) cations. The fragment N(CH 2 pz) 2 containing two pyrazine functions has a very low contribution to the stability of the lanthanide complexes, even though the pyrazine groups are coordinated to the cation in water. The stabilities of the americium(III) complexes were determined by potentiometry and are greater than those found for the isoelectronic europium complexes. The selectivity for Am(III) over Eu(III) increases from 60 to 500 when the pyridine-containing fragment N(CH 2 py) 2 is substituted by the pyrazine-containing fragment N(CH 2 pz) 2 , which demonstrates that the selectivity for Am(III) over Eu(III) is significantly enhanced when the softness of the N-heterocycle increases from pyridine to pyrazine. These new hydrophilic ligands present attractive selectivities for Am(III) over Eu(III) that could make them good candidates for the selective back extraction of Am(III) from organic solutions containing 4f and 5f elements.