Uncovering the impact of 'capsule' shaped amine-type ligands on Am(III)/Eu(III) separation

Separation of trivalent actinides (An(III)) and lanthanides (Ln(III)) in spent nuclear fuel reprocessing is extremely challenging mainly owing to their similar chemical properties. Two amine-type reagents, tetrakis(2-pyridyl-methyl)-1,2-ethylenediamine (TPEN) and its hydrophobic derivative N,N,N',N-tetrakis((4-butoxypyridin-2-yl)methyl)-ethylenediamine (TBPEN), have been identified to possess a selectivity for Am(III) over Eu(III). In this work, the structures, bonding nature, and thermodynamic behaviors of the Am(III) and Eu(III) complexes with these two ligands have been systematically studied via scalar relativistic density functional theory (DFT) calculations. According to Mayer bond order and the quantum theory of atoms in molecules (QTAIM) analyses, interactions between the ligands and metal cations exhibit some degree of covalent character with relatively more covalency for Am(III) complexes. In comparison with TPEN, TBPEN has better extractability but worse separation ability for Am(III) and Eu(III). Four nitrogen atoms in pyridine moieties may be responsible for the different extraction abilities of TPEN and TBPEN, while two nitrogen atoms in amine chains of these ligands appear to play more important roles in the separation of Am(III)/Eu(III). These different extraction behaviors may be attributed to the longer and thinner 'capsule' shaped TBPEN ligand compared to TPEN. Our study might provide new insights into understanding the selectivity of the amine-type ligands toward minor actinides, and pave the way for designing new TPEN derivatives for extraction and separation of An(III)/Ln(III).