Determination of Mg²⁺ Speciation in a TFSI^--Based Ionic Liquid with and Without Chelating Ethers Using Raman Spectroscopy

Raman spectroscopy was employed to assess the complex environment of magnesium salts in the n-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPyrTFSI) room-temperature ionic liquid (RTIL). At room temperature, Mg(TFSI)₂ was miscible with BMPyrTFSI and formulated by [Mg(TFSI)₂]_x[BMPyrTFSI]_1-x (x ≤ 0.55). Results suggest that at low concentrations of Mg(TFSI)₂, anionic complexes in which Mg²⁺ is surrounded by at least four TFSI^- were formed. Above x = 0.2 an average of three TFSI^- surround each Mg²⁺. Below x = 0.12, there is a greater number of monodentate interactions between TFSI^- oxygens and Mg²⁺ cations, whereas above x = 0.12 bidentate ligands dominate. The fraction of TFSI^- existing in the cis conformation increased with increasing Mg²⁺ concentration. Mg(ClO₄)₂ was also studied as a Mg²⁺ source. At equivalent mole fractions to those of the Mg(TFSI)₂ salt, Mg²⁺ from Mg(ClO₄)₂ was surrounded by only two TFSI^- anions as ClO₄^- appeared to compete with TFSI^- for coordination with Mg²⁺. Similar behavior was also observed for the less soluble halide salts MgX₂ (X = Cl, Br, I). Additions of chelating ligands were shown to effectively reduce the average number of TFSI^- around Mg²⁺ in a manner consistent with maintaining a sixfold oxygen coordination number around Mg²⁺. Furthermore, an alternative class of ionic liquids, known as "solvate" ionic liquids, were produced. In this case glymes (Gm, m + 1 ether oxygens) were mixed with Mg(TFSI)₂ so that glymes chelated Mg²⁺, creating Mg(Gm)_y²⁺ complexes. The general formula was given by Mg(Gm)_y(TFSI)₂. These solvate ILs melt between 40 and 80 °C. Raman spectra clearly showed the glyme chelating ability and stronger coordination with Mg²⁺ with respect to TFSI^-. Finally, linear sweep voltammograms showed the anodic stability of the glymes to improve due to coordination with Mg²⁺. (Graph Presented).