Ether-Functionalized Imidazolium Hexafluorophosphate Ionic Liquids for Improved Water Miscibilities Henri S. Schrekker,* ,†,‡ Marcelo P. Stracke, † Clarissa M. L. Schrekker, † and Jairton Dupont* ,† Laboratory of Molecular Catalysis, Institute of Chemistry, UFRGS, AV. Bento Gonc ¸ alVes 9500, Porto Alegre-RS, CEP:91501-970, P.O. Box 15003, Brazil, and Laboratory of Technological Processes and Catalysis, Institute of Chemistry, UFRGS, AV. Bento Gonc ¸ alVes 9500, Porto Alegre-RS, CEP:91501-970, P.O. Box 15003, Brazil The hexafluorophosphate anion of 1-alkyl-3-methylimidazolium ionic liquids is responsible for their poor water miscibility. Transformation of the hexafluorophosphate ionic liquids into water miscible liquids by structural modifications in the imidazolium cation could result in new applications. Improved water miscibilities were achieved with 1-alkyl ether-3-methylimidazolium hexafluorophosphate ionic liquids. Especially, ionic liquid 1-triethylene glycol monomethyl ether-3-methylimidazolium hexafluorophosphate 5 showed a strongly increased water miscibility range at 30 °C, which was further enhanced in the presence of ethanol as cosolvent. Besides, a complete water miscibility of 5 was observed at an elevated temperature of 50 °C. This knowledge may facilitate the predictive development of new task-specific ionic liquids. 1. Introduction Ionic liquids (ILs) that are liquid at or below 25 °C are a special class of ILs and are referred to as room-temperature ionic liquids (RTILs). 1 In general, ILs consist of a large organic cation together with an organic or inorganic anion. Especially, the class of imidazolium cation-based ILs has proven to be highly attractive and versatile. Frequently encountered favorable characteristics of imidazolium ILs are, for instance, high thermal stability, being liquid over a wide temperature range, air and moisture stability, very low vapor pressure, wide electrochemical window, high conductivity and ionic mobility, easy recycling, and being a good solvent for a wide variety of organic and inorganic chemical compounds. 1-5 Besides, imidazolium ILs are “designable” because structural modifications in both the cation (especially the 1 and 3 positions of the imidazolium ring) and anion permit the tuning of properties like, e.g., miscibility with water and organic solvents, melting point, and viscosity. 1 As a result, applications of imidazolium ILs are numerous and found in the fields of extraction and separation processes, 3,6,7 synthetic chemistry, 2,3 catalysis (organometallic 2,4,8,9 /transition- metal nanoparticle 8-13 /bio), 14 materials science, 3,15 and electro- chemistry. 16,17 The use of imidazolium ILs in combination with water can provide beneficial circumstances. Imidazolium ILs are being adopted as replacements for volatile organic solvents. 1 Water is, without a doubt, the most green solvent. As a consequence, an important property for the design of new processes with IL- water mixtures is their miscibility. 18-22 Dependent on the application, either a monophasic or a biphasic system could be desired. Mainly the 1-alkyl-3-methylimidazolium ILs have been studied to understand the relation between structural modifica- tions and water miscibility. 23-27 This relation is characterized by a highly pronounced anion effect. Halide-, nitrate-, etha- noate-, and trifluoroacetate-based 1-alkyl-3-methylimidazolium ILs are fully miscible with water at ambient temperature. However, water miscibility is drastically reduced to immiscible for their “hydrophobic” hexafluorophosphate- and bis(trifluo- romethanesulfonyl)imide-equivalents, including IL 1 (Figure 1). 28,29 An intermediate behavior has been observed for the tetrafluoroborate ILs. 18 Their miscibility can be fine-tuned by the alkyl chain length of the imidazolium cation. Tetrafluo- roborate ILs with a short 1-alkyl chain (up to butyl) are completely miscible, and those with a longer 1-alkyl chain form biphasic systems. Biphasic IL-water systems have been the basis for innovative liquid-liquid separation and extraction techniques. 3,6,7 Interest- ingly, alcohols like ethanol have a strong cosolvent effect, which furnishes fully miscible ternary IL-water-ethanol mixtures. 30,31 However, the use of a cosolvent could be undesired and is not necessarily a general solution for all “hydrophobic” anion- specific (e.g., hexafluorophosphate and bis(trifluoromethane- sulfonyl)imide) applications that require monophasic IL-water mixtures. Avoidance of a cosolvent would require the trans- formation of a hydrophobic IL into a hydrophilic one by structural modifications in the imidazolium cation. ILs that contain a specific functionality covalently incorporated in either the cation or the anion are denominated task-specific ILs. 32-35 This task-specific IL approach is the reason for the rapidly expanding application scope of ILs. For example, improved HgCl 2 solubilities and extractions of HgCl 2 from aqueous solutions have been reported for ethylene glycol functionalized imidazolium ILs (Figure 1: cations [C 2 O 1 MIm], [C 3 O 1 MIm], and [C 5 O 2 MIm]) and ethylene glycol bridged bis-imidazolium ILs (Figure 1: cation [MImC 6 O 2 MIm]) when compared to their alkyl analogs. 36,37 The ethylene glycol spacer of bis-imidazolium bis(trifluoromethanesulfonyl)imide IL 6 (Figure 1) was respon- sible for an increased saturating water content. 37 An increased polarity and a slightly higher saturating water content were determined for the monoethylene glycol monomethyl ether- functionalized bis(trifluoromethanesulfonyl)imide IL 3b (Figure 1). 25,38 Branco et al. reported about the full miscibility of the monoethylene glycol- and monoethylene glycol monomethyl ether-functionalized hexafluorophosphate ILs 2 and 3a in water (Figure 1). 36 However, attachment of the longer diethylene * Corresponding authors. E-mail: schrekker@iq.ufrgs.br (H.S.S.); dupont@iq.ufrgs.br (J.D.). Tel.: +55-51-3308-6284 (H.S.S.); +55-51- 3308-6321 (J.D.). Fax: +55-51-3308-7304 (H.S.S.); +55-51-3308-7304 (J.D.). † Laboratory of Molecular Catalysis, Institute of Chemistry. ‡ Laboratory of Technological Processes and Catalysis, Institute of Chemistry. 7389 Ind. Eng. Chem. Res. 2007, 46, 7389-7392 10.1021/ie0709685 CCC: $37.00 © 2007 American Chemical Society Published on Web 09/26/2007