Homoleptic octahedral coordination of CH₃CN to Mg²⁺ in the Mg[N(SO₂CF₃)₂]₂–CH₃CN system

Abstract

X-ray diffraction and Raman spectroscopic measurements revealed a homoleptic octahedral coordination of CH3CN to Mg(2+) in the [Mg(CH3CN)6][TFSA]2 (TFSA = bis(trifluoromethylsulfonyl)imide) crystal structure and in Mg[TFSA]2/CH3CN electrolytes.

Structural and Thermal Properties of Air‐Stable [Mg(1‐methylimidazole)6][N(SO2CF3)2]2

“…The TFSA – anion has few interactions with the Mg 2+ core and adopts the energetically favorable trans conformation. On the basis of Raman spectroscopy, the octahedral coordination of MIm that was observed for the Mg 2+ cation in the [Mg(MIm) 6 ][TFSA] 2 crystal structure is retained in MIm solutions, as was observed for the acetonitrile system , . This work complements previous studies of [Mg(L) n ][TFSA] 2 compounds, and the present MIm adduct is marked as the only air‐stable [Mg(L) n ][TFSA] 2 compound.…”

“…This trend was observed for all of the bands related to the TFSA – anion. The δ s (CF 3 ) + ν s (SNS) band discussed above appears at ν̃ = 741 cm –1 for all three solutions (Figure b), and the observation of this band indicates the absence of contacts between the Mg 2+ and TFSA – ions in the solutions and the homoleptic coordination of MIm ligands to Mg 2+ , as for the acetonitrile solutions . Although the Mg(MIm) 6 + moiety is considered to be preserved in solution, the weak ω(SO 2 ) bands for the 0.20 (ν̃ = 405 cm –1 ), 0.10 (ν̃ = 407 cm –1 ), and 0.05 m (ν̃ = 408 cm –1 ) solutions appear at slightly lower frequencies than that for the [Mg(MIm) 6 ][TFSA] 2 crystal.…”

“…For instance, molecular dynamics simulations by Rajput et al on solvation in different Mg‐based electrolytes including Mg[TFSA] 2 /acetonitrile suggested that the Mg 2+ ··· TFSA – interaction is dominant over the Mg 2+ ··· acetonitrile interaction. Our recent investigation showed that Mg is preferably coordinated by acetonitrile molecules, and there are few Mg 2+ ···TFSA – interactions in the crystal and solutions . Another stepwise mechanism of ligand attachment to the Mg 2+ ion in Mg[TFSA] 2 was recently reported for ethanol, ethyl acetate, and water …”

“…It should be noted that [Mg(CH 3 CN) 6 ][TFSA] 2 with the similarly hexacoordinate [Mg(CH 3 CN) 6 ] 2+ moiety has a much lower melting point (ca. 303 K) . Although the [Mg(CH 3 CN) 6 ] 2+ moiety is less bulky than the [Mg(MIm) 6 ] 2+ one, the balance of lattice energy and solvation energy results in a lower melting point for the acetonitrile adduct.…”

“…The Raman spectra of the [Mg(MIm) 6 ][TFSA] 2 crystal and Mg[TFSA] 2 powder in the wavenumber range 100–1500 cm –1 are shown in Figure {the full Raman spectrum of the [Mg(MIm) 6 ][TFSA] 2 crystal is shown in Figure S3}. The assignments were made on the basis of literature data and are summarized in Table , , , , . The full Raman spectrum of MIm liquid was recorded (see Figure S4) to enable the identification of the bands assigned to the MIm ligand.…”