Complexation of a Nitrilotriacetate-Derived Triamide Ligand with Trivalent Lanthanides: A Thermodynamic and Crystallographic Study

Abstract

Non-heterocyclic N-donor nitrilotriacetate-derived triamide ligands are one of the most promising extractants for the selective extraction separation of trivalent actinides over lanthanides, but the thermodynamics and mechanism of the complexation of this kind of ligand with actinides and lanthanides are still not clear. In this work, the complexation behaviors of N,N,N′,N′,N″,N″-hexaethylnitrilotriacetamide (NTAamide(Et)) with four representative trivalent lanthanides (La 3+ , Nd 3+ , Eu 3+ , and Lu 3+ ) were systematically investigated by using 1 H nuclear magnetic resonance ( 1 H NMR), ultraviolet−visible (UV−vis) and fluorescence spectrophotometry, microcalorimetry, and single-crystal X-ray diffractometry. 1 H NMR spectroscopic titration of La 3+ and Lu 3+ indicates that two species of 1:2 and 1:1 metal−ligand complexes were formed in NO 3 − and ClO 4 − media. The stability constants of NTAamide(Et) with Nd 3+ and Eu 3+ obtained by UV−vis and fluorescence titration show that the complexing strength of NTAamide(Et) with Nd 3+ is lower than that with Eu 3+ in the same anionic medium, while that of the same lanthanide complex is higher in ClO 4 − medium than in NO 3 − medium. Meanwhile, the formation reactions for all metal−ligand complexes are driven by both enthalpy and entropy. The structures of lanthanide complexes in the single ClO 4 − and NO 3 − medium and the mixed one were determined to be [LnL 2 (MeOH)](ClO 4 ) 3 (Ln = La, Nd, Eu, and Lu), [LaL 2 (EtOH) 2 ][La(NO 3 ) 6 ], and [LaL 2 (NO 3 )](ClO 4 ) 2 , separately. The average bond lengths of lanthanide complexes decrease gradually with the decrease in ionic radii of Ln 3+ , indicating that heavier lanthanides form stronger complexes due to the lanthanide contraction effect, which coincides with the trend of the complexing strength obtained by spectroscopic titration. This work not only reveals the thermodynamics and mechanism of the complexation between NTAamide ligands and lanthanides but also obtains the periodic tendency of complexation between them, which may facilitate the separation of trivalent lanthanides from actinides.

Enhancing the Selectivity of Trivalent Actinide over Lanthanide Using Asymmetrical Phenanthroline Diamide Ligands

“…The calculated enthalpy of the complexation reaction involves the released coordination enthalpy of the metal with the ligand and the consumed desolvation enthalpy of the metal ions and ligands . The negative values of Δ H suggest that the energy released from the complexation reaction of the metal with the ligand is more than the energy needed to realize the desolvation, which agrees with the energetics of the complexation of lanthanides with other amide ligands. , The complexation reaction of Eu­(III) with DE-ET-DAPhen (Δ H = −29.25 kJ/mol) is less exothermic than that with DO-ET-DAPhen (Δ H = −25.90 kJ/mol). Eu­(III) in the aqueous phase is surrounded by water molecules.…”

Theoretical investigation on the ligands constructed from phenanthroline and five-membered N-heterocyclic rings for bonding and separation properties of Am(iii) and Eu(iii)

“…According to the hard-soft acid-base principle, the relatively softer nature of actinide(III) ions compared to lanthanide(III) ions makes them more inclined to bind to soft N donors. 7,8 Recently, N-donor ligands, notably N-heterocyclic ligands like those based on six-membered triazinyl N-heterocyclic rings (such as 2,6-bis(5,6-dialkyl-1,2,4-triazin-3-yl) pyridines (BTPs), 9,10 6,6 0bis(5,6-dialkyl-1,2,4-triazin-3-yl)-2,2 0 -bipyridines (BTBPs), 11,12 2,9-bis(1,2,4-triazin-3-yl)-1,10-phenanthrolines (BTPhens), 13 and 6 0 -bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-benzo [1,2,4]triazin-3-yl)-[2,2 0 ]bipyridine (CyMe 4 -BTBP)) (Scheme 1), have been studied for their advantages like easy synthesis, acid resistance, and high selectivity in nitric acid solutions, [9][10][11][12][13][14][15] and their derivatives, show good performance and are promising for extraction separation of lanthanides and actinides. These ligands have a wide range of acidity tolerance and exhibit high selectivity in nitric acid solutions, with a separation factor SF Am/Eu 4 100 for the extraction separation of Am(III) from Eu(III).…”

Heterocyclic Ligands with Different N/O Donor Modes for Am(III)/Eu(III) Separation: A Theoretical Perspective

“…Theoretically, both An­(III) and Ln­(III) belong to the group of hard Lewis acids; the former is relatively softer than the latter because the shielding effect of the 5f orbital is weaker than that of the 4f orbital . On the basis of the hard–soft acid–base (HSAB) principle, a large number of soft N − or S donor , ligands have been designed for An­(III)/Ln­(III) separation. For instance, 2,9-bis­(5,5,8,8-tetramethyl-5,6,7,8-tetrahydrobenzo­[1,2,4]­triazin-3-yl)­1,10-phenanthroline with four coordinated N atoms [CyMe 4 -BTPhen, L 1 (Scheme )] shows excellent Am­(III)/Eu­(III) selectivity with a separation factor (SF Am/Eu ) of >200 in 0.5–4 M HNO 3 .…”