Inner to outer-sphere coordination of plutonium(iv) with N,N-dialkyl amide: influence of nitric acid

Abstract

N,N-Dialkylamides are extensively studied as alternative organic ligands to achieve the extraction and separation of uranium(vi) and plutonium(iv). We report here the coordination structures of the plutonium(iv) ion with N,N-di(2-ethylhexyl)-n-butanamide as a function of nitric acid concentration in the aqueous phase. The coordination structure of Pu(iv) evolves gradually with increasing nitric acid concentration from an inner-sphere with two coordinated amide ligands toward an outer-sphere hexanitrate complex with only nitrate ions in the first coordination sphere and protonated amide ligands in the outer shell.

Hexavalent Actinide Extraction Using N,N-Dialkyl Amides

“…Recent studies have suggested high nitric acid concentration increases the amount of p rotonated monoamide ligands (HL) in the organic phase, bringing about an outer-sphere coordination environment. 15,16 Similarly, elevated nitric acid conditions give rise to anionic trinitrate uranyl species, [UO 2 (NO 3 ) 3 ] -. 17,18 The mixed 1:1 and 1:2 stoichiometry could be due a protonated monoamide complexing with anionic uranyl (UO 2 (NO 3 ) 3 HL) and neptunyl species while the 1:2 ratio is synonymous with two ligands extracting the neutral hexavalent actinide dinitrate species (UO 2 (NO 3 ) 2 L 2 ).…”

X-ray absorption spectroscopy and actinide electrochemistry: a setup dedicated to radioactive samples applied to neptunium chemistry

“…Another way to constrain the accurate coordination number is to associate EXAFS analysis with quantum chemical calculations (Vila et al, 2012). For some actinide molecular compounds, it is possible to generate EXAFS metrical parameters from a DFT calculation (Acher et al, 2016(Acher et al, , 2017 2. The accuracy of the optimized geometry was checked by direct comparison of Np-O eq bond lengths with the measurements.…”

Insights from quantum chemical calculations into inner and outer-sphere complexation of plutonium(iv) by monoamide and carbamide extractants

“…It was shown that the Pu(IV) coordination structure can switch from inner to outer complexation either by varying the experimental conditions (such as nitric acid concentration) or by slightly altering the amide alkyl group. 19,[23][24][25] Furthermore, the recent study of Pu(IV) with linear alkyl chains carbamides demonstrated that carbamides are strong extractants at high nitric acidity while promoting outersphere coordination. As a result, the strong variation of extraction properties cannot be solely rationalized from steric hindrance in the inner coordination sphere.…”

“…The aim of this work is to explore and rationalize Pu(IV) inner and outer-sphere interactions with amide and carbamide derivatives from scalar relativistic Density Functional Theory (DFT) calculations, that have proven to be reliable at predicting the geometries of Pu(IV) complexes. 24,[26][27][28] While the structures of the inner-sphere complexes with amide and carbamide ligands have been well characterized in the solid state from XRD and in solution from EXAFS, the structures of outer-sphere complexes are largely unknown, though carbamides are believed to promote the formation of outer-sphere complexes. 11 DFT geometry optimizations and complexation energetics shall help shedding light on their 3D structures and relative stabilities.…”