Preparation and characterization of new low melting ammonium-based ionic liquids with ether functionality Minna Kärnä, Manu Lahtinen * , Jussi Valkonen Department of Chemistry, University of Jyväskylä, P.O. Box 35, FI-40014 Jyväskylä, Finland article info Article history: Received 3 November 2008 Received in revised form 19 January 2009 Accepted 20 January 2009 Available online 31 January 2009 Keywords: Quaternary ammonium bromide Tetrafluoroborate Hexafluorophosphate Ionic liquid Thermal analysis X-ray diffraction abstract Eighteen new and three previously known but insufficiently characterized RR ´ 3 N + A  and R 2 R ´ 2 N + A  type (R = 2-ethoxyethyl or 4-methoxybenzyl, R ´ = methyl, ethyl, n-propyl, n-butyl, n-pentyl or n-hexyl, A = Br, BF 4 or PF 6 ) quaternary ammonium (QA) salts were synthesized and characterized by using 1 H and 13 C NMR techniques, mass spectroscopy and elemental analysis. The bromide salts were synthesized either by treating dimethyl formamide with an ether functionalized alkyl bromide in the presence of potassium carbonate or by treating a tertiary amine with an ether functionalized alkyl bromide. The QA tetrafluoro- borates and hexafluorophosphates were synthesized by metathesis reaction between a prepared QA bro- mide and HBF 4 or KPF 6 . The crystal structures of four compounds were determined by X-ray single crystal diffraction and powder diffraction was used to study the crystallinity of the solid compounds and to com- pare the structural similarities between the single crystals and the microcrystalline bulk form. Thermal properties of all compounds were studied by using TG/DTA and DSC methods. The anion exchange had a clear lowering effect on the melting points and enhanced the thermal stability of the BF 4  and PF 6  salts compared to the analogous bromides. Most of the compounds melted clearly below 100 °C, of which four are liquid at room temperature. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction The seemingly ever growing interest on room temperature ionic liquids (RTILs) has turned the spotlight, along with many others, to quaternary ammonium-based compounds. Ionic liquids are of interest because of their unique characteristics, for example, low vapor pressure and extraordinary solvent properties, which have enabled their use in numerous applications and which have been widely discussed in several excellent publications or reviews and references therein [1–6]. Traditionally, the most widespread applications of ionic liquids lie in organic synthesis as solvents and catalysts [7,8]. Ionic liquids have in fact been described as designer solvents since their proper- ties can be tuned by careful selection of ionic species [1,9]. They have also various electrochemical applications: in electric double layer capacitors (energy storage device) ionic liquids have been used as electrolytes due to their high conductivity and some qua- ternary ammonium-based ionic liquids have been reported to have more potential than the traditional aromatic type ionic liquids such as imidazolium or pyridinium due to their higher cathodic stability [10,11]. The quest for new energy sources has also led a great attention to dye-sensitized solar cells (DSSCs) in which ionic liquids could be used to replace the organic liquid electrolytes due to their non-volatility, non-flammability and high ionic conductiv- ity, among other characteristics [12]. In addition to the most stud- ied imidazolium-based ionic liquids, quaternary ammonium polyiodides have been tested as electrolytes in DSSCs [13]. Another solar application is the use of ionic liquids as heat-transfer fluids in electric power plants using parabolic trough solar collector tech- nology [14–16]. Lately, some studies have suggested that ionic liq- uids could be used as a new class of lubricants or as additives in conventional lubricants. Certain quaternary ammonium-based io- nic liquids have proven to be even more efficient lubricants than conventional hydrocarbon oils in reciprocal sliding tests [17]. Some ionic liquids containing a large amount of nitrogen are highly ener- getic materials which may be used in industrial or military appli- cations [18]. A large number of tested applications consider only imidazolium-based ionic liquids which unfortunately are still rather expensive especially for large scale industrial applications. Quaternary ammonium and phosphonium-based ionic liquids, on the other hand, can also be made from readily available lower cost materials and thereby will be more applicable to industrial use if synthesized with reasonable yields [19–22]. Furthermore, the sat- urated quaternary ammonium cations are generally more resistant against oxidation and reduction than 1,3-substituted imidazolium cations that have certain degree of electrochemical instability and have some usability limitations in organic reactions [23,24]. Intro- ducing an oxygen-containing side chain has proven to reduce the toxicity of imidazolium-based ionic liquids [25,26] and it might 0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2009.01.041 * Corresponding author. Tel.: +358 14 2602624; fax: +358 14 2602501. E-mail address: manu.lahtinen@jyu.fi (M. Lahtinen). Journal of Molecular Structure 922 (2009) 64–76 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc