Solvation of uranyl–CMPO complexes in dry vs. humid forms of the [BMI][PF6] ionic liquid. A molecular dynamics study†
Abstract
The solvation of the [UO2(NO3)(CMPO)]+ and [UO2(NO3)2(CMPO)2] complexes (CMPO = octyl(phenyl)-N,N-diisobutylmethylcarbamoyl phosphine oxide) is investigated by molecular dynamics in the “dry” and “humid” forms of a room temperature ionic liquid (IL) based on the 1-butyl-3-methylimidazolium (BMI+) cation and the hexafluorophosphate (PF6−) anion. The simulations reveal the importance of the solvent anions in “dry” conditions and of water molecules in the “humid” solvent. For the [UO2(NO3)(CMPO)]+ complex, the monodentate vs. bidentate coordination modes of CMPO are compared, and the first solvation shell of uranyl is completed by 1–3 PF6− anions in the dry IL and by 2–3 water molecules in the humid IL, leading to a total coordination number close to 5. The energy analysis shows that interactions with the IL stabilize the [UO2(NO3)bi(CMPO)mono]+ form (with bidentate nitrate and monodentate CMPO) in the dry IL and the [UO2(NO3)mono(CMPO)mono]+ form (with monodentate nitrate and CMPO) in the humid IL. The extracted compound characterized by EXAFS is thus proposed to be the [UO2(NO3)mono(CMPO)mono(H2O)3]+ species. Furthermore we compare the [UO2(NO3)2(CMPO)2] complex in its associated and dissociated forms ([UO2(NO3)mono(CMPO)mono]+ + CMPO + NO3−) and discuss the results in the context of uranyl extraction by CMPO to ionic liquids.