A theoretical probe of high-valence uranium and transuranium silylamides: Structural and redox properties
Graphical abstract
A mixed character involving both metal and pyridines has been proposed for the reduction process of AnpyNH via relativistic DFT calculations, but a metal-based reduction mechanism suggested for thf-coordinated complexes. The solvent polarity and equatorial coordination environment play a significant role in calculating the reduction potentials (E0). The marked E0 shift for the thf-coordinated actinyl silylamides is elucidated by the different electron-donating ability of equatorial NSiH3 and NSiMe3 groups.
Introduction
Redox chemistry of uranium and transuranium complexes plays a crucial role in pollution remediation and nuclear fuel processing [1], [2], [3], [4]. With oxidation states of VI and V, their species feature approximately linear dioxo cations like AnO22+ and AnO2+ (An = U, Np and Pu) [1], [2], [3], [4], [5], [6]. The hexavalent uranyl (UO22+) appears as one of the most stable and the most prevalent forms of uranium [7], [8], [9], [10], [11], [12], [13], [14], [15]. Its high solubility aggravates the migration of uranium contaminant into aquifers and other groundwater resources [2], [3], [4], [5], [6]. Comparatively, the UO2+ can be stabilized in the lab but is not found in the natural environment. Transuranium actinyl(V) ions like NpO2+ and PuO2+, however, are stable and relatively mobile in the environment [1], [2], [3], [4], [5], [6].
The redox property of actinyl species has been found to be sensitive to the coordination environment around metal center [1], [2], [3], [4]. The uranyl complexes commonly show 4–6-fold metal coordination in the equatorial plane [6], [7], [8], [9], [10], [11], [12]. More and more uranyl analogues with a variety of ancillary ions and molecules such as halides, tetrahydrofuran (thf), pyridine (py) and silylamide have been structurally characterized in the non-aqueous solution [6], [16]. The silylamide [N(SiMe3)2]− (Me = methyl), for instance, can stabilize a wide range of uranium oxidation states. It has been used for the preparation of uranyl(VI) derivatives aside from the U(III) and U(IV) species. So the redox chemistry of uranyl silylamides is becoming a burgeoning area of research [16], [17]. Comparatively, studies of transuranium analogues are fewer because of chemical toxicity, radioactivity, scarcity and synthetic difficulty [1], [2]. Therefore, a comprehensive and systematic study including the U, Np and Pu series is highly demanded.
To address above points, we embarked theoretical study of a series of prototypical actinyl silylamides. It includes a systematic variation of metals (U, Np and Pu), metal formal oxidation states (VI and V) and equatorial ligands (thf and py as well as N(SiMe3)2 and N(SiH3)2). Their reduction potential was calculated, and corresponding reduction mechanism proposed.
Section snippets
Computational details
Actinyl complexes, [AnVIO2(thf)2(N(SiMe3)2)2] and [AnVO2(thf)2(N(SiMe3)2)2]− (An = U, Np and Pu), were examined in theory. The hexavalent uranyl one has been structurally characterized using X-ray crystal diffraction [11]. By comparison, we designed complexes, [AnVIO2(Sol)2(N(SiH3)2)2] and [AnVO2(Sol)2(N(SiH3)2)2]− (Sol = thf and py) in silico, aiming to unravel effects of the N(SiMe3)2 → N(SiH3)2 and thf → py variations on molecular properties. See their abbreviations in Table 1.
Structural
Structural property
Eighteen actinyl silylamides, AnmthfNMe, AnmthfNH and AnmpyNH (m = VI and V; An = U, Np and Pu) were optimized (Figure 1). It yields UO bond lengths of 1.82 Å for UVIthfNMe (Table 2), which is comparable to the experimental value of 1.78 Å [11]. The present calculated UO distances fall within the range of the experimentally known values [7], [8], [9], [10], [11], [12], [13], [14], [15] and theoretically studied ones [33], [34]. The 0.04 Å UO lengthening relative to experimental value is found. This is
Conclusions
A series of actinyl silylamides have been examined using relativistic DFT. Calculations of AnVI/VthfNH and AnVI/VW couples and comparison with available experimental results reveal a reliable theoretical approach, i.e. GGA-PBE functional and L1 basis sets together with implicit solvation and spin–orbit/multiplet corrections.
The inspection of electron-spin density proposed a mixed character involving both metal and pyridines for the reduction process of AnpyNH, but suggested a metal-based
Acknowledgements
This work is supported by the National Natural Science Foundation of China (21273063). The Natural Science Foundation of Heilongjiang Province (B201318) and the Program for Innovative Research Team in University (IRT-1237) are greatly acknowledged. The authors are grateful to Dr. Dimitri Laikov for providing us with the Priroda code. We acknowledge Dr. Zheng-Dong Li for his helpful discussion and critical review of the manuscript.
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