Preparation of novel, moisture-stable, Lewis-acidic ionic liquids containing quaternary ammonium salts with functional side chains

Abstract

Ionic liquids, or room temperature molten salts, have attracted increasing interest over recent years, particularly in the area of green chemistry, due to their advantageous properties including negligible vapour pressure and wide liquid range. 1-3 Work has focussed on liquids formed from dialkylimidazolium halides which are readily prepared from 1-methylimidazole and a slight excess of the desired haloalkane. 4 Addition of a suitable molar ratio of aluminium chloride to these salts gives rise to ionic liquids containing complex metal anions which have been used in a number of applications such as electroplating, electrochemical devices and catalysts for organic synthesis. 1,[4][5][6] The main disadvantage of imidazolium-based liquids is their relatively high cost for bulk applications, whilst chloroaluminate ionic liquids have the additional problem of their low tolerance to moisture, necessitating the use of glove box and Schlenk techniques to prepare and investigate their properties. The former problem may be overcome by using cheaper ammonium salts whereas the latter may be solved by replacing aluminium with less reactive metals.Recently, Freeman and coworkers have characterised ionic liquids formed from FeCl 2 or FeCl 3 and 1-butyl-3-methylimidazolium chloride. 7 However, other examples of ionic liquids made from metal chlorides other than aluminium are less well characterised and are mainly cited in conference proceedings 8 or in the patent literature. 9,10 Here we report the synthesis and characterisation of new moisture-stable, Lewis-acidic ionic liquids made from metal chlorides and quaternary ammonium salts that are commercially available or simple to synthesise. These offer the potential to tailor the physical properties e.g. melting point, viscosity and conductivity, and to tune the Lewis acidity by choosing a different metal or indeed combinations of metals.To investigate the parameters necessary for a salt to be liquid at or near room temperature we have heated a range of ammonium salts with zinc chloride in a 1+2 molar ratio and the results are shown in Since choline chloride, [Me 3 NC 2 H 4 OH]Cl, gave the lowest freezing point we have characterised this system in more detail. Heating mixtures of choline chloride and zinc chloride in molar ratios between 1+1 and 1+3 gave rise to clear colourless liquids, with the freezing points varying between ca. 65 °C (1+1), 25 °C (1+2) and 45 °C (1+3). Unlike analogous aluminium systems, 4,11,12 ratios with a molar excess of choline chloride, i.e. basic melts, do not form ambient temperature liquids in these systems. This would imply that complex zinc anions, in which the charge can be delocalised, are necessary in the formation of these ionic liquids. Further characterisation of the 1:2 liquid is described below.A 1 H NMR spectrum of the neat ionic liquid was recorded and shows resonances at d 2.97, 3.34 and 3.94 assigned to the methyl groups, N-CH 2 protons, and the CH 2 OH protons, respectively, of the choline cation. All signals were broad and poorly reso...

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