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 [UO(2)(NO(3))(CMPO)](+) and [UO(2)(NO(3))(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 (PF(6)(-)) anion. The simulations reveal the importance of the solvent anions in "dry" conditions and of water molecules in the "humid" solvent. For the [UO(2)(NO(3))(CMPO)](+) complex, the monodentate vs. bidentate coordination modes of CMPO are compared, and the first solvation shell of uranyl is completed by 1-3 PF(6)(-) 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 [UO(2)(NO(3))(bi)(CMPO)(mono)](+) form (with bidentate nitrate and monodentate CMPO) in the dry IL and the [UO(2)(NO(3))(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 [UO(2)(NO(3))(mono)(CMPO)(mono)(H(2)O)(3)](+) species. Furthermore we compare the [UO(2)(NO(3))(2)(CMPO)(2)] complex in its associated and dissociated forms ([UO(2)(NO(3))(mono)(CMPO)(mono)](+) + CMPO + NO(3)(-)) and discuss the results in the context of uranyl extraction by CMPO to ionic liquids.

Insights into the Mechanism of Extraction of Uranium (VI) from Nitric Acid Solution into an Ionic Liquid by using Tri‐n‐butyl phosphate

“…Therefore, am aximum of two TBP molecules may complex to uranyl in the RTIL, with the equatorial sphereb eing additionally filled with two or three Tf 2 Ni onsi no rder to obtain at otal coordination number of five atoms around UO 2 2 + .T he fit of the Sample As hows that the extracteds peciesa tl ow acidic concentrations do not contain any nitrateg roups, anda re of the type [UO 2 .H owever,T f 2 N À anions are weaker ligands than water.T his has been evidencede xperimentally, [44] but also by molecular dynamics calculation. Chaumont and Wipff [45] 2 + .T his confirms that at low acid concentrationt he uranium (VI) extraction occurs by ac ation exchange.…”

Insights into Uranyl Chemistry from Molecular Dynamics Simulations

“…Comparing the [UO 2 + complex further binds 1-3 H 2 O molecules, depending on the denticity of the nitrate and CMPO ligands. [68] 2 2 + , NO 3 À and Cl À ions in the same ratio (1:2:6), but prepared differently, display different UV/Vis spectra, likely due to very slow equilibration kinetics in the viscous ionic liquid. [70] It would thus be quite questionable to attempt to reproduce experimental data by a single simulation alone.…”

Exploring Actinide Materials Through Synchrotron Radiation Techniques

“…The EXAFS fitting showed an average equatorial coordination number of about 4.5 oxygen atoms with a dominant species of [UO 2 (NO 3 )(CMPO)] + , but uncertainty for the denticity of CMPO and NO 3 − . In further studies, by directly dissolving the [UO 2 (NO 3 )(CMPO)] + and [UO 2 (NO 3 ) 2 (CMPO) 2 ] complexes in [BMIm][PF 6 ] ionic liquid, the Chaumont group proved that the species obtained was [UO 2 (NO 3 ) mono (CMPO) mono (H 2 O) 3 ] + .…”

“…Guilbaud and co‐workers proposed a new force field of the uranyl ion in water, and tested it on some uranyl complexes. They also investigated the solvation of CMPO and [UO 2 (NO 3 )(CMPO)] + as well as UO 2 (NO 3 ) 2 (CMPO) 2 in the ionic liquid based on BMIm + and PF 6 − by MD simulations. It has been suggested that humidity was important for the solvation of these species in ionic liquids.…”