This therefore gives us 81 ionic liquids for each cation type (N.B.: we only consider the even chain lengths, as Figure 5.6 shows us that there is no advantage to be gained in using the more expensive odd chain lengths). Taking only the previously mentioned imidazolium skeleton (and hence ignoring the dozens of combinations that could be generated with 2-, 4-, and 5-substitution patterns), pyridinium, 4-methylpyridinium, and 3-methylpyridinium, there are 324 ionic liquids already defined here. Well, actually 612, since we should consider all the carboxylates, not just ethanoate! Adding only four more anions (e.g., nitrate, hydrogentartrate, lactate, and hydrogensulfate) and four more series of cations (e.g., [PPh3R]+, [PPh2MeR]+, [NPh3R]+, and [NPh2MeR]+) generates a possible maximum of 1512 ionic liquids. If we now consider all tetra-alkylammonium and tetra-alkylphosphonium salts (and there are 13,122 cations, without considering either branched chains or enantiomers), the total possible number of ionic liquids is now 277,074. It only takes a little imagination to select a few more anions and cations (and we have not yet considered optical isomers, or any aromatic skeleton other than pyridine or imidazole), and it can be seen that it is not impossible that there will be at least one million (106) simple ionic liquids that can be easily prepared in the laboratory. But that total is just for simple systems. If there are one million possible simple systems, then there are one billion (1012) binary combinations of these, and one trillion (1018)11 ternary systems possible!59 And if this seems too far-fetched, it is entirely plausible that to generate the required combination of reactivity, solubility, and viscosity, industrial applications will need to work with ternary systems.57 A simple, but viable as a zeroth-order approximation, view is that the anion defines the chemistry that can be performed, and the cation controls the physical properties. Thus the simple ionic liquids are good, but unoptimized, first choices. The binary systems can be used to fine-tune the physical properties (such as viscosity, density, and miscibility), by adding (for example) a second ionic liquid with a common anion but different cation. The ternary systems can be used to minimize cost, by replacing an expensive component with a less expensive one, to the point where the desired chemistry ceases to be as effective.
Thus, it possible to consider an artist's palette of anions and cations (see Figure 5.7), being used to create a vast range of ionic liquids, the anions being chosen to control the chemistry, and the cations to engineer the physical properties. In reality, then, the descriptor of designer solvents is justified and apt. And the recent work coming from Gratzel's and MacFarlane's groups is rapidly increasing the ranges of both anions60 and cations,61 while Ohno's elegant work has expanded ionic liquids into amino acid derivatives.62 Moreover, the use of microwave synthesis has speeded up ionic liquid synthesis considerably, as well as making their synthesis a model of green chemistry, being stoicheiometric, atom
"American readers should note, en passant, that the original (European) definition of a trillion is a million million millions, that the definition of a billion is a million millions, and that what they refer to as a billion is correctly referred to as a milliard (how the desire to be a billionaire can distort language!).
economic, and quantitative.63 In addition, new routes to purer ionic liquids, not starting with haloalkanes, has increased the scope of the possible applications.64
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