Complexation of Hexavalent Neptunium(VI) with Oxydiacetic Acid and Its Amide Derivatives in Aqueous Solution: Spectrophotometry and DFT Calculations

Lei Xu; Xiao-Kun Zhao; Hong Cao; Han-Shi Hu; Jun Li; Jing Chen; Chao Xu

doi:10.1021/acs.inorgchem.3c04221

Complexation of Hexavalent Neptunium(VI) with Oxydiacetic Acid and Its Amide Derivatives in Aqueous Solution: Spectrophotometry and DFT Calculations

Inorg Chem. 2024 Apr 8;63(14):6173-6183. doi: 10.1021/acs.inorgchem.3c04221. Epub 2024 Mar 26.

Authors

Lei Xu^{1

2}, Xiao-Kun Zhao³, Hong Cao¹, Han-Shi Hu⁴, Jun Li⁴, Jing Chen¹, Chao Xu¹

Affiliations

¹ Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China.
² Institute of Nuclear Agricultural Science, Key Laboratory of Nuclear Agricultural Sciences of Ministry of Agriculture and Zhejiang Province, Zhejiang University, Hangzhou 310058, China.
³ Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055, China.
⁴ Key Laboratory of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, China.

PMID: 38530927
DOI: 10.1021/acs.inorgchem.3c04221

Abstract

Unfolding the solution coordination chemistry of high-valent transuranium elements with the "CHON"-type ligands is important to understand the fundamental chemistry of actinides and to design more efficient extractants for partitioning of transuranium elements in advanced nuclear fuel cycles. Here, the complexation of a hexavalent neptunyl ion (NpO₂²⁺ or Np(VI)) with oxydiacetic acid (ODA) has been systematically investigated in comparison with its amide analogues N,N-dimethyl-3-oxa-glutaramic acid (DMOGA) and N,N,N',N'-tetramethyl-3-oxa-glutaramide (TMOGA) both experimentally and computationally. The formation of both 1:1 and 1:2 complexes between Np(VI) and the three ligands was identified by spectrophotometry, and their stability constants were obtained and compared with those of hexavalent U(VI) and Pu(VI). The corresponding bonding nature is elucidated by using energy decomposition analysis (EDA), electrostatic potential (ESP), ELF contours, and natural orbitals for chemical valence (NOCV) methods, which shows that the Np-O bonds are essentially ionic in character and the unoccupied 6d orbitals of Np play a key role in enhancing the covalent interactions between Np(VI) and the three ligands.