On the coordination of Zn ion in TfN based ionic liquids: structural and dynamic properties depending on the nature of the organic cation.

A synergic approach combining molecular dynamics (MD) simulations and X-ray absorption spectroscopy has been used to investigate diluted solutions of zinc bis(trifluoromethanesulfonyl)imide (Zn(TfN)) in TfN based ionic liquids (ILs) having different organic cations, namely the 1-butyl-3-methylimidazolium ([C(mim)]), 1,8-bis(3-methylimidazolium-1-yl)octane ([C(mim)]), N,N,N-trimethyl-N-(2-hydroxyethyl)ammonium ([Choline]) and butyltrimethylammonium ([BTMA]) ions. All of the ILs tend to dissolve the Zn(TfN) species giving rise to a different structural arrangement around the Zn as compared to that of the salt crystallographic structure. A quantitative analysis of the Zn K-edge extended X-ray absorption fine structure (EXAFS) spectra of the solutions has been carried out based on the microscopic description of the systems derived from the MD simulations. A very good agreement between theoretical and experimental EXAFS signals has been obtained, allowing us to assess the reliability of the MD structural results for all the investigated solutions. The Zn ion has been shown to be coordinated by six oxygen atoms of the TfN anions arranged in an octahedral geometry in all the TfN based ILs, regardless of the organic cation of the IL solvent. However, the nature of the organic cation has a small influence on the overall spatial arrangement of the TfN anions in the Zn first solvation shell: two different Zn-TfN complexes are found in solution, a 5-fold one, with one bidentate and four monodentate TfN anions, and a 6-fold one with only monodentate ligands, with the ratio between the two species being slightly dependent on the IL cation. The IL ion three-dimensional arrangements in the different IL solutions were also investigated by carrying out a thorough analysis of the MD simulations, highlighting similarities and differences between imidazolium and ammonium based IL systems.