Bis-(1,2,4-triazin-3-yl) ligand structure driven selectivity reversal between Am3+ and Cm3+: solvent extraction and DFT studies

Abstract

Selectivity between Am³⁺ and Cm³⁺ was investigated after their aqueous complexation with three structurally tailored hydrophilic bis-(1,2,4-triazin-3-yl) ligands followed by their extraction with N,N,N′N′-tetraoctyl diglycolamide (TODGA) dissolved in an ionic liquid (C₄mim·Tf₂N). The three hydrophilic ligands used were SO₃PhBTP, SO₃PhBTBP, and SO₃PhBTPhen. It was evident from the solvent extraction studies that SO₃PhBTP formed a stronger complex with Cm³⁺ than with Am³⁺, but SO₃PhBTPhen showed better complexation ability for Am³⁺ than for Cm³⁺, and SO₃PhBTBP showed no selectivity for the two actinide ions. DFT calculations indicated that the coordinating ‘N’ atoms in BTP were more co-planar in the complex and this co-planarity was higher in the Cm³⁺ complex as compared to that in Am³⁺. In the case of BTBP and BTPhen ligands, on the other hand, the co-planarity was more pronounced in the Am³⁺ complexes. Mayer's bond order calculations of M–N bonds in the complexes also indicated a reversal of the complexation ability of the BTP and BTPhen ligands for Am³⁺ and Cm³⁺. Calculations of the complexation energies further supported the higher selectivity of the BTP ligand for Am³⁺ by −52.0 kJ mol⁻¹, and better selectivity of the BTPhen ligand for Cm³⁺ by −24.7 kJ mol⁻¹.