Room temperature ionic liquids anions

Kurig, H., M. Vestli, K. Tonurist, A. Janes, and E. Lust. 2012. Influence of room temperature ionic liquid anion chemical composition and electrical charge delocalization on the supercapacitor properties. Journal of the Electrochemical Society 159 A944-A951. 

However, ionic liquids containing other classes of organic cations are known. Room-temperature ionic liquids containing organic cations including quaternary ammonium, phosphonium, pyridinium, and - in particular - imidazolium salts are currently available in combination with a variety of anions (Figure 3.1-1 provides some common examples) and have been studied for applications in electrochemistry [7, 8] and in synthesis [9-11]. 

In recent years ionic liquids have also been employed as media for reactions catalyzed both by isolated enzymes and by whole cells, and excellent reviews on this topic are already available [47]. Biocatalysis has been mainly conducted in those room-temperature ionic liquids that are composed of a 1,3-dialkylimidazolium or N-alkylpyridinium cation and a noncoordinating anion [47aj. 

Ionic liquids, having per definition a melting point below 100 °C, and especially room temperature ionic liquids (RTIL) have attracted much interest in recent years as novel solvents for reactions and electrochemical processes [164], Some of these liquids are considered to be green solvents [165]. The scope of ionic liquids based on various combinations of cations and anions has dramatically increased, and continuously new salts [166-168] and solvent mixtures [169] are discovered. The most commonly used liquids are based on imidazolium cations like l-butyl-3-methylimidazolium [bmim] with an appropriate counter anion like hexafluorophos-phate [PFg]. Salts with the latter anion are moisture stable and are sometimes called third generation ionic liquids. 

The temperature dependence of viscosity of 23 room-temperature ionic liquids was investigated. The size and symmetry of the cations and anions were shown to have a marked effect on viscosity (79). 

Sprunger, L. et al., Characterization of room temperature ionic liquids by the Abraham model cation-specific and anion-specific equation coefficients, /. Chem. Info. Model, 47,1123,2007. 

Fujii, K., Fujimori, T., Takamuku, T., Kanzaki, R., Umebayashi, Y., and Ishiguro, S., Conformational equilibrium of bis(trifluoromethanesulfonyl) imide anion of a room-temperature ionic liquid Raman spectroscopic study and DFT calculations, /. Phys. Ghem. B, 110,8179-8183,2006. 

Tokuda, H., Hayamizu, K., Ishii, K. et al.. Physicochemical properties and structures of room temperature ionic liquids. 1. variation of anionic species, /. Phys. Chem. B, 108,16593,2004. 

Room-temperature ionic liquids are attractive due to their chemical and thermal stability, negligible vapor pressure, high ionic conductivity, and ample electrochemical window. Their properties can be varied by a rational choice of the cations and of the anions and can represent an important iodide source for an I /I3 -based electrolyte (Fig. 17.12). 

Another type of room temperature ionic liquids are typically the salts between cations like 1-butyl-3-methylimidazolium (BMI+), l-ethyl-3-methylimidazolium, and 1-butyl-pyridinium (see Scheme) and anions like BF7, PFq, CF3COO , CF3S03 and (CF3S02)2] T [29, 30]. By suitably selecting the cation and the anion, we can design ionic liquids that are nonvolatile, nonflammable, chemically stable, highly... 

Introduction of room-temperature ionic liquids (RTIL) as electrochemical media promises to enhance the utility of fuel-cell-type sensors (Buzzeo et al., 2004). These highly versatile solvents have nearly ideal properties for the realization of fuelcell-type amperometric sensors. Their electrochemical window extends up to 5 V and they have near-zero vapor pressure. There are typically two cations used in RTIL V-dialkyl immidazolium and A-alkyl pyridinium cations. Their properties are controlled mostly by the anion (Table 7.4). The lower diffusion coefficient and lower solubility for some species is offset by the possibility of operation at higher temperatures. 

Separation of C02 from N2 or CH4 comprises an area of critical industrial, social, and environmental importance where room-temperature ionic liquids (RTILs) are showing great potential [3, 63, 64], The commonly used ILs for this purpose are l-alkyl-3-methylimidazolium salts, represented by the formula [C MIm][X, where C is an n-alkyl chain of varying length and X is typically a molecular anion with a delocalized negative charge, such as bis(trifluorome-thane)sulfonimide, [Tf2N ] (Scheme 4.4) [65-71]. 

Pyridinium hydrobromide perbromide salt was introduced by Djerassi and Scholz as an alternative brominating agent to bromine in 1948. Salazar and Dorta rationalized that since alkylpyridinium salts are well documented and commercially available room temperature ionic liquids, a combination of an alkylpyridinium cation with tribromide anion 1 should therefore lead to a room temperature ionic liquid bromine analogue (Equation 1). 

Ionic liquids are quite simply liquids that are composed entirely of ions [96, 97]. They are generally salts of organic cations, e.g. tetraalkylammonium, alkylpyridi-nium, 1,3-dialkylimidazolium, tetraalkylphosphonium (Fig. 7.28). Room temperature ionic liquids exhibit certain properties which make them attractive media for performing green catalytic reactions. They have essentially no vapor pressure and are thermally robust with liquid ranges of e.g. 300 °C, compared to 100 °C for water. Polarity and hydrophilicity/hydrophobicity can be tuned by a suitable combination of cation and anion, which has earned them the accolade, designer solvents . 

Separation science focuses on room temperature ionic liquids (RTlLs), salts that are liquid at ambient temperature. They have been studied as extracting solvents, stationary and mobile phases, mobile phase additives, and other uses. Common RTILs consist of a bulky nitrogen- or phosphorus-containing organic cation (pyridinium or pyrrolidinium, alkyl-imidazolium, ammonium or phosphonium) and a variety of organic and inorganic anions (triflate, dicyanamide, trifluoroacetate, acetate trifluo-romethylsulfate, nitrate, perchlorate, bromide, chloride, chloroaluminate, tetrafluo-roborate, hexafluorophosphate). 

Ethylammonium nitrate (entry 18 in Table 3-1) was shown in 1914 to have m.p. 12 °C and was hence the first room temperature ionic Hquid [156] this was followed in 1967 by tetra- -hexylammonium benzoate with m.p. —50 °C (entry 26) [169], Ambient-temperature ionic liquids based on l-alkyl-3-methylimidazolium salts (entries 19-24) were first reported by Wilkes et al. in 1982 as tetrachloroaluminates [162a], Replacement of this moisture-sensitive anion by the tetrafluoroborate ion and other anions led, in 1992, to air- and water-stable, room temperature ionic liquids [162b], which have since found increasing application as reaction media for various kinds of organic reactions, mainly owing to the work of Seddon [167, 190] and Hussey [187], Suitably selected... 

Solvent free methods have also impacted on the preparation of other alternative reaction media. Namely, a range of ionic liquids (ILs) was prepared (including imidazolium, pyridinium and phosphonium salts) through halidetrapping anion metathesis reactions (Figure 2.17). The alkyl halide by-product was easily removed by vacuum or distillation and the products were obtained quantitatively in high purity. In addition to being solvent free, this route is more atom economic than the usual route to room temperature ionic liquids (RTILs) as it does not use silver(i), alkali metal or ammonium salts which are normally used in an anion metathesis reaction. 

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