A computational study on the coordination modes and electron absorption spectra of the complexes U(iv) with N,N,N′,N′-tetramethyl-diglycolamide and anions

Abstract

Lipophilic N,N,N′,N′-tetraalkyl-diglycolamides (TRDGAs) are promising extractants for actinides separation in spent nuclear fuel reprocessing. Usually, in the extracted complexes of actinide and lanthanide ions of various oxidation states, the metal ions are completely surrounded by 2 or 3 TRDGA molecules, and the counter anions do not directly coordinate with them. In contrast, the extracted complexes of U(IV) from different media presenting different absorption spectra indicate that the anions (Cl⁻ and NO₃⁻) are directly involved in the coordination with U(IV) in the first inner sphere. Based on this exceptional observation in solvent extraction, taking the coordination of U(IV) with N,N,N′,N′-tetramethyl-diglycolamide (TMDGA, the smallest analogue of TRDGA) as the research object, we mimic the behaviours of counterions (Cl⁻ and NO₃⁻) and the water molecule during coordination of TMDGA with U(IV), especially combining with the simulation of the absorption spectra. We demonstrate that during the complexing of TMDGA to U(IV), the counterion Cl⁻ will occupy one coordination number in the inner coordination sphere, and NO₃⁻ will occupy two by bidentate type; however, the ubiquitous water cannot squeeze in the inner coordination sphere. In addition, the coordination of Cl⁻ and NO₃⁻ is proved to favour the extraction with the lower binding energy. Moreover, the simulation of absorption spectra is in good agreement with the observation from experiments, further verifying the aforementioned conclusion. This work in some way will provide guidance to improve the computation methods in research of actinides by mimicking the absorption spectra of actinide ions in different complexes.