Water-soluble tetrapodal N,O ligands incorporating soft N-heterocycles for the selective complexation of Am(III) over Ln(III)

A series of four water-soluble N,O-tetrapodal ligands derived from ethylenediamine, bearing hard acetate groups and soft N-heterocycles, either pyridine or pyrazine, was developed to study the impact of the softness of N-donors on the complexation properties with trivalent f ions. Two novel ligands of enhanced soft character, bearing three pyridines (L-3py) or three pyrazines (L-3pz), were synthesized and the related lanthanide complexes were studied in solution. The ligand containing three pyridylmethyl moieties L-3py gives complexes with a coordination similar to EDTA, i.e. a hexadentate coordination mode as indicated by NMR and luminescence decays (q = 3) and stability constants in the range log beta(110) = 6.99-9.3 (La-Lu). On the other hand, the softest molecule L-3pz forms much less stable complexes with log beta(110) = 4.0-4.4 (La-Eu). The selective back-extraction of Am(III) from organic solutions containing 4f and 5f elements was tested with the four water-soluble complexing agents. The ligand L-3pz demonstrates poor stripping ability and selectivity. In contrast, the three ligands L-py, L-pz and L-3py give interesting back-extraction results with Eu/Am separation factors ranging from 36 to 46, which are significantly higher than with HEDTA. This exemplifies the role of the N-heterocycle softness in enhancing the separation between Am(III) and Eu(III). Interestingly, the pyrazine-based ligand, L-pz, demonstrates the best stripping properties, with a distribution factor that approaches that of HEDTA in the same conditions (D-Am approximate to 0.3). This molecule is a good compromise between softness and hardness and forms complexes still stable at pH 3 due to its low basicity.