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

Abstract

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 β₁₁₀ = 6.99–9.3 (La–Lu). On the other hand, the softest molecule L^3pz forms much less stable complexes with log β₁₁₀ = 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 ≈ 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.