Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 81:401–405 (2006) DOI: 10.1002/jctb.1411 Synthesis and application of dicationic ionic liquids Qingbin Liu, † Fred van Rantwijk and Roger A Sheldon ∗ Laboratory of Biocatalysis and Organic Chemistry, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands Abstract: Ionic liquids were synthesised that consisted of two methylimidazolium rings joined by a spacer 1–12 carbon atoms in length. The bis(triflic)imides with spacer ≤C 4 were solids; the others were liquid at room temperature. The polarity of the dication liquids and their performance in the extraction of phenol from water were compared with those of similar monocationic liquids.  2005 Society of Chemical Industry Keywords: ionic liquids; dicationic liquids; phenol extraction; water INTRODUCTION The interest in ionic liquids as solvents without vapour pressure has stimulated an explosion of research into their properties. One aspect of ionic liquid chemistry that has not yet been studied very much is whether polyvalent ionic liquids, in which the mobility of the charges is restricted, have any special properties. A number of examples of di- and polycationic liquids have recently been published. Engel and co-workers have reported the synthesis and properties of a wide range of poly(tetraalkylammonium) phosphate ionic liquids. 1–4 The synthesis of bis(3-methylimidazolium- 1-yl)alkanes via a traditional procedure, 5 and also under microwave heating 6 and with sonication, 7 has been described and their use in microwave flash heat- ing has been demonstrated. 8 Holbrey et al . described the synthesis, crystal structure and mercury(II) com- plexing properties of bis(methylimidazolium) cations connected by ethylene glycol spacers. 9 Recently, ionic liquids based on quaternised 4,4 ′ -dipyridyl derivatives bearing fluorous ponytails on the nitro- gen atoms have been described. 10 The synthesis and characterisation of a wide variety of dica- tionic liquids containing alkane spacers was pub- lished by Anderson et al . 11 while our work was in progress. We wished to investigate the properties of bis(3- methylimidazolium-1-yl)alkane-type ionic liquids. We have synthesised a range of such ionic liquids with the bis(triflic)imide counterion and have investigated their solvatochromic properties and their behaviour as extractants. MATERIALS AND METHODS All chemicals were purchased from Aldrich. NMR spectra were measured on a Varian VXR400 spec- trometer. UV measurements were performed on a Cary 3 spectrometer. Compounds 1,1-Bis(3-methylimidazolium-1-yl)methane iodide (1a) 12 1-Methylimidazole (16.4 g, 0.2 mol) was mixed with CH 2 I 2 (26.7 g, 0.1 mol) in 100 ml of toluene and refluxed for 2 h; the mixture was cooled to room temperature and filtered. The solids were washed with toluene and dried; yield 21.2 g (49.2% of theory) of 1a, m.p. >260 ◦ C. 1 H NMR (DMSO-d 6 ), δ (ppm) 9.37 (s, 2H), 7.97 (s, 2H), 7.81 (s, 2H), 6.66 (S, 2H), 3.91 (s, 6H). 1,1-Bis(3-methylimidazolium-1-yl)methane bis(triflic)imide (1b) Compound 1a (4.31 g, 10 mmol) was dissolved in 25 mL of water and Li N(SO 2 CF 3 ) 2 (5.74 g, 20 mmol) was added. After stirring at room temperature for 3 h, a white solid formed which was collected by filtration, washed with water and dried; yield 6.48 g (87.9% of theory) of 1b, m.p. 90–94 ◦ C. 1 H NMR (DMSO-d 6 ), δ (ppm) 9.31 (s, 2H), 7.92 (s, 2H), 7.78 (s, 2H), 6.60 (s, 2H), 3.90 (s, 6H). 13 C NMR (DMSO-d 6 ), δ (ppm) 137.9, 124.3, 121.8, 119.2 (q, J C–F = 322.5 Hz), 58.1, 36.1. MS (−ve) m/z 280.38 [100%, − N(SO 2 CF 3 ) 2 ]. 1,4-Bis(3-methylimidazolium-1-yl)butane chloride (2a) 1-Methylimidazole (32.8 g, 0.4 mol) and 1,4- dichlorobutane (25.4 g, 0.2 mol) were heated at 100 ◦ C ∗ Correspondence to: Roger A Sheldon, Laboratory of Biocatalysis and Organic Chemistry, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands E-mail: r.a.sheldon@tnw.tudelft.nl † Visiting scholar from China, financially supported by the CSC. Current address: Institute of Chemistry and Chemical Engineering, Hebei Normal University, Hebei, Shijiazhuang 050 091, China. (Received 23 December 2004; revised version received 19 July 2005; accepted 17 August 2005) Published online 25 November 2005  2005 Society of Chemical Industry. J Chem Technol Biotechnol 0268–2575/2005/$30.00 401