On the coordination of Zn2+ ion in Tf2N− 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(Tf2N)2) in Tf2N− based ionic liquids (ILs) having different organic cations, namely the 1-butyl-3-methylimidazolium ([C4(mim)]+), 1,8-bis(3-methylimidazolium-1-yl)octane ([C8(mim)2]2+), N,N,N-trimethyl-N-(2-hydroxyethyl)ammonium ([Choline]+) and butyltrimethylammonium ([BTMA]+) ions. All of the ILs tend to dissolve the Zn(Tf2N)2 species giving rise to a different structural arrangement around the Zn2+ 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 Zn2+ ion has been shown to be coordinated by six oxygen atoms of the Tf2N− anions arranged in an octahedral geometry in all the Tf2N− 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 Tf2N− anions in the Zn2+ first solvation shell: two different Zn–Tf2N complexes are found in solution, a 5-fold one, with one bidentate and four monodentate Tf2N− 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.