On the coordination of Zn²⁺ion in Tf₂N⁻based ionic liquids: structural and dynamic properties depending on the nature of the organic cation

Abstract

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.

Coordination polymer glass from a protic ionic liquid: proton conductivity and mechanical properties as an electrolyte

“…X-ray absorption ne structure (XAFS) analysis is oen used for consideration of the coordination geometry and average coordination number of Zn 2+ . [29][30][31] The XAFS spectra of the Kedge region of Zn 2+ for 1 and the reference samples were measured and analysed ( Fig. S6 †).…”

Coordination of the Co2+ and Ni2+ Ions in Tf2N– Based Ionic Liquids: A Combined X-ray Absorption and Molecular Dynamics Study

“…This function is defined in a wide interval of positive and negative asymmetry values and falls in the Gaussian limit for β → 0. 46 …”

“… 44 , 45 In particular, the combination between XAS and molecular dynamics (MD) simulations provides an invaluable tool for the structural characterization of metal ion solutions in ILs. 36 , 46 − 48 Through the synergic use of the XAS and MD techniques, one can also check the validity of the level of theory employed in the simulations directly on the experimental evidence, while having at the same time a reliable structural model to be used in the analysis of the experimental data.…”

Protic Ionic Liquid-Derived Coordination Polymer Glass for Anhydrous Proton Conductor with Moldability

“…The X-ray absorption fine structure (XAFS) analysis was used for coordination geometry and average coordination number of Zn 2+ in 1. [42][43][44] The XAFS spectra of the K-edge region of Zn 2+ for 1 and the reference samples were measured and analyzed ( Fig. S5).…”