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

Tylan Watkins, Daniel Buttry

Research output: Contribution to journalArticlepeer-review

46 Scopus citations

Abstract

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).

Original languageEnglish (US)
Pages (from-to)7003-7014
Number of pages12
JournalJournal of Physical Chemistry B
Volume119
Issue number23
DOIs
StatePublished - Jun 11 2015

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Surfaces, Coatings and Films
  • Materials Chemistry

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