DFT-based investigation of solvation of Nd(III)/Yb(III) cations in room-temperature ionic-liquid

We report a DFT study on the solvation of bare rare-earth (RE) cations [Nd(III) and Yb(III)] in the room-temperature ionic-liquid (RTIL) based on 1,3-dimethyl-imidazolium+, PF6-, [MMI][PF6]. In the present article, we aim to investigate whether the chosen RTIL can preferentially solvate any of the RE cations under study. First, the RTIL binding to the RE cation is studied in the gas phase, and then explored with continuum solvation models employing RTIL parameters. First, we considered the formation of the first solvation shell, where the PF6- anions directly bond with the Ln(III) cation. We then further surrounded the first solvation shell with [MMI]+ cations to form the second solvation shell. The stability of PF6- coordinated complexes is studied by calculating the binding energy values. The number of PF6- anions for both Ln(III) cations starts with three ligands and goes until the maximum binding energy in the RTIL medium is reached. The coordination number and binding modes of PF6- depend on the size of the RE cation. Comparing the binding energy values between the RE cations for the first solvation shell complexes, it is noticed that Yb-complexes have relatively greater binding energy than Nd-complexes. We have further performed a Boltzmann analysis to find out the most dominant conformer among each category of PF6- coordinated complexes. A study using Natural Population Analysis shows that after complexation with PF6- the charge on the Yb-centers lower to a greater extent than Nd-centers, implying stronger bonding between Yb and PF6-, which is in harmony with the binding energy analysis. Overall, the computational investigation provides an in-depth understanding of the complexation phenomena of RE cations in RTIL, which can be useful for the practical separation of RE cations using RTIL.