Magnesium bis(trifluoromethanesulfonyl)amide complexes with triglyme and asymmetric homologues: phase behavior, coordination structures and melting point reduction

Abstract

The phase behavior of binary mixtures of triglyme (G3) and Mg[TFSA]₂ (TFSA: bis(trifluoromethanesulfonyl)amide) was investigated, towards the development of a Mg²⁺-based room-temperature solvate ionic liquid (SIL) electrolyte. In a 1 : 1 molar ratio, G3 and Mg[TFSA]₂ form a thermally stable complex (decomposition temperature, T_d: 240 °C) with a melting point (T_m) 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]₂ revealed that a single Mg²⁺ 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]₂ 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_m. To decrease the T_m further, several asymmetric homologues of G3, which have higher conformational flexibility than G3, were investigated. Notably, a 1 : 1 mixture of Mg[TFSA]₂ with G3Bu (where one of the terminal methyl groups of G3 is substituted for a butyl group) formed a thermally stable complex (T_d: 251 °C) without any distinct T_m 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 Mg²⁺-based room-temperature SILs can be utilized as a new class of Mg electrolyte.