Ind. Eng. Chem. Res. 2009, 48, 8757–8759
8757
Diol-Functionalized Imidazolium-Based Room-Temperature Ionic Liquids with Bis(trifluoromethanesulfonimide) Anions that Exhibit Variable Water Miscibility Andrew L. LaFrate,† Jason E. Bara,†,‡ Douglas L. Gin,*,†,‡ and Richard D. Noble*,†,‡ Department of Chemistry & Biochemistry and Department of Chemical & Biological Engineering, UniVersity of Colorado, Boulder, Colorado 80309
The synthesis and characterization of a new class of 1-(2,3-dihydroxypropyl)-3-alkylimidazolium bis(trifluoromethanesulfonimide) room-temperature ionic liquids (RTILs) with tunable water miscibility are reported. The presence of a vicinal diol substituent on these RTILs allows for variable water miscibility, depending on the nature of the alkyl substituent on the cation. Water-miscible, imidazolium-based RTILs with the bis(trifluoromethanesulfonimide) anion are unprecedented. Functionalized room-temperature ionic liquids (RTILs) have become increasingly popular in recent years because of their usefulness as new types of liquid-phase reaction and transport media in a number of important application areas including catalysis,1 gas separations,2 and removal of heavy metals from water.3 Generally, the structure and chemical or solubility characteristics of the RTIL cation are modified by adding functional groups such as oligo(ethylene glycol),4 nitriles,5 amines,6 isothiocyanates,7 and sulfonic acids,8 among others. The ability to mix or blend RTILs with aqueous solutions may be useful to reduce overall solvent volatility. Promising prior work in this area has focused on using water-miscible RTILs to separate aqueous azeotropic mixtures.9 There are several reports of imidazolium-based RTILs containing primary,10,11 secondary,12 and tertiary13 alcohol-functionalized cations. The primary alcohol functionality has been shown to influence the miscibility of imidazolium-based RTILs with 1° and 2° alkanolamines.11 However, RTILs containing a vicinal diol on the cation are much less common, although they have been used as aldehyde protecting groups14 and ligands for Pd catalysis.15 The vicinal diol-functionalized RTILs used in these studies employed the PF6 anion, which has the liability of hydrolyzing and generating HF under certain conditions.16 Herein, we report the design, synthesis, and adjustable water-miscibility of a new class of diol-functionalized alkylimidazolium RTILs containing the bis(trifluoromethanesulfonimide) (Tf2N) anion. We chose to prepare vicinal diol-functionalized imidazolium-based RTILs containing the Tf2N anion, because this anion does not form HF and often leads to RTILs with more favorable properties (e.g., lower viscosity, higher thermal stability).17 We found that by varying the length of the n-alkyl group on these diolfunctionalized alkylimidazolium RTILs, their miscibility in water can be adjusted to be completely water miscible or immiscible. The bulk hydrophobic or hydrophilic character of an RTIL is strongly influenced by the identity of the anion. Consider a family of salts based on the ubiquitous 1-butyl-3-methylimidazolium ([C4mim]) cation found in many studies on RTILs: The salts [C4mim][Cl], [C4mim][Br], and [C4mim][BF4] are completely miscible with water, while [C4mim][PF6] and [C4mim][Tf2N] are immiscible with water.18 The Tf2N anion has been described as a hydrophobic RTIL anion, although many
Tf2N-containing RTILs are somewhat hygroscopic.19 For this reason, we were surprised when certain imidazolium-based RTILs containing vicinal diol-functionalized cations and Tf2N anions (RTILs 7 and 9) were completely miscible with water. Furthermore, it was determined that the water-miscibility of these RTILs could be controlled by changing the length of the N-alkyl group on the cation or varying substitution at the 2-position of the imidazolium cation core. Curiosity about these diol-RTILs was driven by the attempted anion exchange of 1-(2,3-dihydroxypropyl)-3-methylimidazolium chloride (1) to the Tf2N anion using typical aqueous procedures which did not produce a water immiscible precipitate, as has come to be expected from imidazolium-based RTILs with Tf2N anions. This unusual result prompted the synthesis of derivatives 7-12 with varying substitution of R and R′. Compounds 1-6 were prepared in excellent yield by stirring 1-chloro-2,3-propanediol with the corresponding imidazole reagent at 130 °C for 48 h (Scheme 1). The reaction was attempted using various solvents (EtOH, CH3CN, toluene) at reflux and was incomplete after several days. However, Nalkylation proceeded most effectively as a neat (solvent-free) reaction. The reaction mixture was then placed under dynamic vacuum (
