Determination of Mg<sup>2+</sup> Speciation in a TFSI<sup>-</sup>-Based Ionic Liquid with and Without Chelating Ethers Using Raman Spectroscopy

Tylan Watkins, Daniel Buttry

Research output: Contribution to journalArticle

37 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)<inf>2</inf> was miscible with BMPyrTFSI and formulated by [Mg(TFSI)<inf>2</inf>]<inf>x</inf>[BMPyrTFSI]<inf>1-x</inf> (x ≤ 0.55). Results suggest that at low concentrations of Mg(TFSI)<inf>2</inf>, anionic complexes in which Mg<sup>2+</sup> is surrounded by at least four TFSI<sup>-</sup> were formed. Above x = 0.2 an average of three TFSI<sup>-</sup> surround each Mg<sup>2+</sup>. Below x = 0.12, there is a greater number of monodentate interactions between TFSI<sup>-</sup> oxygens and Mg<sup>2+</sup> cations, whereas above x = 0.12 bidentate ligands dominate. The fraction of TFSI<sup>-</sup> existing in the cis conformation increased with increasing Mg<sup>2+</sup> concentration. Mg(ClO<inf>4</inf>)<inf>2</inf> was also studied as a Mg<sup>2+</sup> source. At equivalent mole fractions to those of the Mg(TFSI)<inf>2</inf> salt, Mg<sup>2+</sup> from Mg(ClO<inf>4</inf>)<inf>2</inf> was surrounded by only two TFSI<sup>-</sup> anions as ClO<inf>4</inf><sup>-</sup> appeared to compete with TFSI<sup>-</sup> for coordination with Mg<sup>2+</sup>. Similar behavior was also observed for the less soluble halide salts MgX<inf>2</inf> (X = Cl, Br, I). Additions of chelating ligands were shown to effectively reduce the average number of TFSI<sup>-</sup> around Mg<sup>2+</sup> in a manner consistent with maintaining a sixfold oxygen coordination number around Mg<sup>2+</sup>. 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)<inf>2</inf> so that glymes chelated Mg<sup>2+</sup>, creating Mg(Gm)<inf>y</inf><sup>2+</sup> complexes. The general formula was given by Mg(Gm)<inf>y</inf>(TFSI)<inf>2</inf>. These solvate ILs melt between 40 and 80 °C. Raman spectra clearly showed the glyme chelating ability and stronger coordination with Mg<sup>2+</sup> with respect to TFSI<sup>-</sup>. Finally, linear sweep voltammograms showed the anodic stability of the glymes to improve due to coordination with Mg<sup>2+</sup>. (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
  • Materials Chemistry
  • Surfaces, Coatings and Films

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