Crystalline Complexes of Pyr_12O1TFSI-Based Ionic Liquid Electrolytes

Abstract

This study examines the formation of previously unreported crystalline phases of <i>N</i>-methoxyethyl-<i>N</i>-methylpyrrolidinium bis­(trifluoromethanesulfonyl)­imide (Pyr_12O1TFSI). The melting point of pristine Pyr_12O1TFSI, determined by conductivity measurements, is between −20 and −17.5 °C. Formation of this crystalline phase is difficult and only occurs under specific conditions. Pyr_12O1TFSI readily forms 1:1 phases with both NaTFSI and Mg­(TFSI)_2. The results of single crystal structure determinations are presented. The Na⁺ crystalline phase provides clear evidence that the Pyr_12O1⁺ cation can coordinate some metal ions, but this coordinative interaction does not occur with all metal cations, e.g., Mg²⁺, and in all states of matter, e.g., Na⁺-IL solutions. The TFSI^– ions are found in two different aggregate solvates in the Pyr_12O1TFSI:NaTFSI 1:1 phase and in contact ion pair and aggregate solvates in the Pyr_12O1TFSI:Mg­(TFSI)₂ 1:1 phase. The Pyr_12O1TFSI:Mg­(TFSI)₂ crystalline phase gives insight into the local structure of the liquid electrolyte, where it is likely that a maximum of approximately 30% of the total TFSI^– can likely be coordinated in a bridging geometry, and the rest are in a bidentate coordination geometry. This ratio is determined from both the crystal structure and the Raman spectroscopy results