Do mono- or diphenol substitutions in phenanthroline-based ligands serve in effective separation of Am³⁺/Eu³⁺ ions?- Insights from DFT calculations

ABSTRACT

Partitioning of trivalent actinides (An(III)) from lanthanides (Ln(III)) during Spent Nuclear Fuel (SNF) reprocessing presents a major challenge due to the exhibition of very similar chemical properties. Although an array of phenanthroline-based ligands have surfaced to solve this problem, the recently reported soft – hard-donor-combined N, N’-diethyl-N.N’-ditolyl-2,9-diamide-1,10-phenanthroline (Et-Tol-DAPhen) ligand has gained much attention. In this work, Et-Tol-DAPhen has been electronically modified with di and mono substitutions of phenol at the 4^th and 5^th positions. The structural properties and extraction abilities of these modified ligands have been studied using DFT calculations. The results reveal that the ligand (ETDP3) with diphenol substitutions at the 4^th position has an excellent extraction capability with high selectivity toward Am(III) over Eu(III) ion. Electronic structure and bonding analyses provide insights into the nature of metal-ligand bonds in the ten coordinated ML(NO₃)₃ complexes [M = Am and Eu] and convey that in all the complexes the M-O bonds are stronger than the M-N bonds. The quantum theory of atoms in molecules (QTAIM) reveals the weak C-H – O interactions in these ML(NO₃)₃ complexes. The charge analysis explains the exceptional complexation behavior by determining the donor–acceptor interactions in the Am complexes. ΔΔG values obtained from thermodynamic analysis indicate the preferential selectivity of these ligands toward trivalent Am over Eu ions. Around 22% increased covalency is observed between M(NO₃)₃ and L in the Am complexes compared to that of the Eu complexes. Overall, this computational study offers an understanding of how beneficial it is to have mono- and diphenol substitutions at the fourth and fifth position of the phenanthroline-based ligands for effective actinide/lanthanide separation.

KEYWORDS:

• Phenanthroline
• Et-Tol-DAPhen
• preorganization
• Am/Eu separation
• extraction and stripping
• phenol substitution

Acknowledgements

The Department of Science and Technology – Science and Engineering Research Board (DST-SERB) is acknowledged for the Early Career Research award (Ref. No. ECR/2017/001147) to RVS while CE thanks DST-SERB for the financial support from the ECR project. The authors thank DST-FIST sponsored Professor Edward Barnes Centre for Instrumentation and Computation Facility (PEBCIF) of the Department of Chemistry, Madras Christian College for the computational support. A part of this work done by Shruti Vinod has been included in the thesis for the award of Master of Science in Chemistry (2020–2022) from Madras Christian College (Affiliated to the University of Madras, Chennai, India).

Disclosure statement

No potential conflict of interest was reported by the authors.

Novelty statement

To avoid the adverse effects of radioactive SNF, a number of chelators and extractants have been used in the selective extraction of radioactive actinides from lanthanides in nuclear waste management. However, designing competent ligands for the partitioning of trivalent Am/Eu ions is challenging and requires a fundamental understanding on the origin of the selectivity of these ligands from different perspectives such as M-L bonding, thermodynamic properties, and feasibility of reaction and extraction capabilities. This work would help in designing better ligands for the Am/Eu partitioning process and will also help in extending our knowledge on the coordination chemistry of lanthanides and actinides.

Additional information

Funding

The work was supported by the DST-SERB-ECR [2017/001147].