Room-temperature ionic liquids definition

The first example of homogeneous transition metal catalysis in an ionic liquid was the platinum-catalyzed hydroformylation of ethene in tetraethylammonium trichlorostannate (mp. 78 °C), described by Parshall in 1972 (Scheme 5.2-1, a)) [1]. In 1987, Knifton reported the ruthenium- and cobalt-catalyzed hydroformylation of internal and terminal alkenes in molten [Bu4P]Br, a salt that falls under the now accepted definition for an ionic liquid (see Scheme 5.2-1, b)) [2]. The first applications of room-temperature ionic liquids in homogeneous transition metal catalysis were described in 1990 by Chauvin et al. and by Wilkes et ak. Wilkes et al. used weekly acidic chloroaluminate melts and studied ethylene polymerization in them with Ziegler-Natta catalysts (Scheme 5.2-1, c)) [3]. Chauvin s group dissolved nickel catalysts in weakly acidic chloroaluminate melts and investigated the resulting ionic catalyst solutions for the dimerization of propene (Scheme 5.2-1, d)) [4]. 

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. 

Ionic liquids (more specifically, room temperature ionic liquids (RTIL)) have attracted considerable interest over the last few years. Although the specific definition of what an RTIL is varies from person to person, the prevailing definition would be that it is a salt with a melting point below 100 °C. Such a broad definition leaves considerable room for flexibility, which contributed to labeling RTILs as "designer solvents." ... 

Ionic liquids (IL) are a new class of salt-like materials that are entirely composed of ions and that are liquid at unusually low temperatures. For the most commonly used definition of the term ionic liquid the boiling point of water was chosen as a reference point, most likely for emotional reasons The term ionic liquids refers to compounds consisting entirely of ions and existing in the liquid state below 100 °C. In many cases the melting point is even below room temperature. 

In order to generalize the definition of the solvent system for the case of ionic media, we shall analyse Franklin s definition. First, it should be noted that the term auto-ionization in this definition should be substituted by auto-dissociation or intrinsic acid-base equilibrium of the solvent , as a more common case of heterolytic break down of the constituent particles of a liquid. Indeed, for molecular solvents or those which are slightly ionized at room temperature, the terms autoionization and intrinsic acid-base equilibrium of the solvent , relate to the same process, whereas for ionic liquids they differ considerably. For example, although sodium nitrate (NaNOs) is subject to practically... 

After all, we have no problem in calling water, ethanol and ammonia molecular liquids, in spite of the fact that they all have autoionization reactions that lead to measurable concentrations of ions at room temperature. However, a comparison of the ion concentration between a saturated KQ solution in water and the typical ionic liquid [BMIM][PFe] immediately shows the unsuitability of such an approach. While a saturated KCl solution in water at 22 °C has an ion concentration of 4.6 mol 1 , the ion concentration of [BMIMjjPFs] is only 0.0054 mol 1 at the same temperature [32]. This means that a general ion concentration at which a system can be considered to be an ionic liquid certainly does not exist. Again, we find ourselves in a position where exploring the limits of a definition leads to it breaking down. 

Ionic liquids (ILs) in principle are a diverse group of salts which are liquid at ambient temperatures. General definition of ionic liquid is a liquid at room temperature that consists only ions. However, the term ionic liquid includes an additional special definition to distinguish it from the classical definition of a molten salt (e.g., NaCl) (Table 20.1). 

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