Phase behavior of binary mixtures of tetraglyme (G4) and Mg­[TFSA]2 (TFSA: bis­(trifluoro­methane­sulfonyl)­amide) was investigated. In a 1:1 molar ratio, G4 and Mg­[TFSA]2 formed a stable complex with a melting point of 137 °C. X-ray crystallography of a single crystal of the complex grown from a G4-Mg­[TFSA]2 binary mixture revealed that the G4 molecule wraps around Mg2+ to form a complex [Mg­(G4)]2+ cation, and the two [TFSA]− anions also participate in the Mg2+ coordination in the crystal. The thermal stability of [Mg­(G4)]­[TFSA]2 was examined by thermogravimetry, and it was found that the complex is stable up to 250 °C. Above 250 °C, desolvation of the Mg2+ ion takes place and G4 evaporates. On the other hand, the weight loss starts at around 140 °C in solutions containing excess G4 (n > 1 in Mg­[TFSA]2:G4 = 1:n) due to the evaporation of free (uncoordinated) G4. The suppression of G4 volatility in the [Mg­(G4)]­[TFSA]2 complex is attributed to strong electrostatic and induction interactions between divalent Mg2+ and G4. In addition, complexation of G4 with Mg2+ is effective in enhancing the oxidative stability of G4. Linear sweep voltammetry revealed that the oxidative decomposition of [Mg­(G4)]­[TFSA]2 occurs at electrode potentials >5 V vs Li/Li+, while the oxidation of uncoordinated G4 occurs at around 4.0 V. This oxidative stability enhancement occurs because the HOMO energy level of G4 is reduced by complexation with Mg2+, which is supported by the ab initio calculations.

The interactions of glymes with alkali or alkaline earth metal cations depend strongly on the metal cations. For example, the stabilization energies (E) calculated for the formation of cation-triglyme (G3) complexes with Li, Na, K, Mg, and Ca at the MP2/6-311G** level were -95.6, -66.4, -52.5, -255.0, and -185.0 kcal mol, respectively, and those for the cation-tetraglyme (G4) complexes were -107.7, -76.3, -60.9, -288.3 and -215.0 kcal mol, respectively. The electrostatic and induction interactions are the major source of the attraction in the complexes; the contribution of the induction interactions to the attraction is especially significant in the divalent cation-glyme complexes. The binding energies of the cation-G3 complexes with Li, Na, K, Mg, and Ca and the bis(trifluoromethylsulfonyl)amide anion ([TFSA]) were -83.9, -86.6, -80.0, -196.1, and -189.5 kcal mol, respectively, and they are larger than the binding energies of the corresponding cation-G4 complexes (-73.6, -75.0, -77.4, -172.1, and -177.2 kcal mol, respectively). The binding energies and conformational flexibility of the cation-glyme complexes also affect the melting points of equimolar mixtures of glyme and TFSA salts. Furthermore, the interactions of the metal cations with the oxygen atoms of glymes significantly decrease the HOMO energy levels of glymes. The HOMO energy levels of glymes in the cation-glyme-TFSA complexes are lower than those of isolated glymes, although they are higher than those of the cation-glyme complexes.

The phase behavior of binary mixtures of triglyme (G3) and Mg[TFSA] (TFSA: bis(trifluoromethanesulfonyl)amide) was investigated, towards the development of a Mg-based room-temperature solvate ionic liquid (SIL) electrolyte. In a 1 : 1 molar ratio, G3 and Mg[TFSA] form a thermally stable complex (decomposition temperature, T: 240 °C) with a melting point (T) of 70 °C, which is considerably lower than that of the analogous tetraglyme (G4) system (137 °C). X-ray crystallography of a single crystal of [Mg(G3)][TFSA] revealed that a single Mg cation is coordinated by a single, distorted, tetradentate G3 molecule from one side, and two monodentate [TFSA] anions, with transoid conformation, from the reverse side to form an ion pair. Raman spectra of [Mg(G3)][TFSA] in the molten state revealed the presence of different coordination structures, as the liquid exhibits changes in the vibrational modes corresponding to G3 and the [TFSA] anion compared to those observed for the solid. Investigation of the ion pair stabilization energies by DFT calculations suggests that higher stability cation complexes and ion pairs co-exist in the molten state than those observed in the crystalline state. These results imply that the coordination structures of the ion pairs play a key role in providing SILs with low T. To decrease the T further, several asymmetric homologues of G3, which have higher conformational flexibility than G3, were investigated. Notably, a 1 : 1 mixture of Mg[TFSA] with G3Bu (where one of the terminal methyl groups of G3 is substituted for a butyl group) formed a thermally stable complex (T: 251 °C) without any distinct T and showed reasonable ionic conductivity at room-temperature, indicating partial dissociation of ions. In this electrolyte, which showed high oxidative stability, quasi-reversible Mg deposition/dissolution was achieved, indicating that Mg-based room-temperature SILs can be utilized as a new class of Mg electrolyte.

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