Solvation of uranyl-CMPO complexes in dry vs. humid forms of the [BMI][PF6] ionic liquid.: A molecular dynamics study

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.