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Ionic liquid

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An ionic liquid is a liquid that contains essentially only ions. Some ionic liquids, such as ethylammonium nitrate, are in a dynamic equilibrium where at any time more than 99.99% of the liquid is made up of ionic rather than molecular species. In the broad sense, the term includes all molten salts, for instance, sodium chloride at temperatures higher than 800 °C. Today, however, the term "ionic liquid" is commonly used for salts whose melting point is relatively low (below 100 °C). In particular, the salts that are liquid at room temperature are called room-temperature ionic liquids, or RTILs.

1-butyl-3-methylimidazolium salts or bmim

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[edit] History

Whereas the date of discovery, as well as discoverer, of the "first" ionic liquid is disputed, one of the earlier known ionic liquids was [EtNH3]+ [NO3]- (m.p. 12 °C), the synthesis of which was published in 1914.<ref>P. Walden, Bull. Acad. Sci. St. Petersburg 1914, 405-422</ref> Much later, series of ionic liquids based on mixtures of 1,3-dialkylimidazolium or 1-alkylpyridinium halides and trihalogenoaluminates, initially developed for use as electrolytes, were to follow.<ref>Electrochemical scrutiny of organometallic iron complexes and hexamethylbenzene in a room temperature molten salt H. L. Chum, V. R. Koch, L. L. Miller, R. A. Osteryoung Journal of the American Chemical Society 1975, 97, 3264.DOI</ref>,<ref>Dialkylimidazolium chloroaluminate melts: a new class of room-temperature ionic liquids for electrochemistry, spectroscopy and synthesis J. S. Wilkes, J. A. Levisky, R. A. Wilson, C. L. Hussey Inorganic Chemistry 1982, 21, 1263-1264. DOI</ref> An important property of the imidazolium halogenoaluminate salts was that they were tuneable – viscosity, melting point and the acidity of the melt could be adjusted by changing the alkyl substituents and the ratio of imidazolium or pyridinium halide to halogenoaluminate.<ref>Potentiometric investigation of dialuminum heptachloride formation in aluminum chloride-1-butylpyridinium chloride mixtures R. J. Gale, R. A. Osteryoung Inorganic Chemistry 1979, 18, 1603.DOI</ref>

A major drawback was their moisture sensitivity and, though to a somewhat lesser extent, their acidity/basicity, the latter which can sometimes be used to an advantage. In 1992, Wilkes and Zawarotko reported the preparation of ionic liquids with alternative, 'neutral' anions such as hexafluorophosphate ([PF6]-) and tetrafluoroborate ([BF4])-, allowing a much wider range of applications for ionic liquids.<ref>J. S. Wilkes, M. J. Zaworotko Chemical Communications 1992, 965-967</ref> It was not until recently that a class of new, air- and moisture stable, neutral ionic liquids, was available that the field attracted significant interest from the wider scientific community.

More recently, people have been moving away from [PF6]- and [BF4]- since they are highly toxic, and towards new anions such as bistriflimide [(CF3SO2)2N]- or even away from halogenated compounds completely. Moves towards less toxic cations have also been growing, with compounds like ammonium salts (such as choline) being just as flexible a scaffold as imidazole.

[edit] Characteristics

The notable characteristics of ionic liquids are their inherrant conductivity (they are Coulombic fluids) as well as non-measurable vapor pressure (recent papers have actually shown that there is a small but non negligible vapour pressure <ref>The distillation and volatility of ionic liquids Martyn J. Earle, José M.S.S. Esperança, Manuela A. Gilea, José N. Canongia Lopes, Luís P.N. Rebelo, Joseph W. Magee, Kenneth R. Seddon and Jason A. Widegren Nature, 2006, 439, 831 [1]</ref>, which will not be a surprise to anybody who has worked with them, since many have a noticeable smell, although this is most likely due to impurities in the materials rather than the minimal vapour pressures of the ionic liquids themselves under ambient conditions), non-flammability, thermal stability, wide liquid range, and solvating properties for diverse kinds of materials. It is known that, as in conventional organic solvents, many kinds of chemical reactions such as Diels-Alder reactions and Friedel-Crafts reactions occur in ionic liquids. More recently, there has been a lot of work showing ionic liquids to be very suitable solvents for biocatalysis <ref>Cofactor-dependent enzyme catalysis in functionalized ionic solvents Adam J. Walker and Neil C. Bruce Chemical Communications, 2004, 22, 2570 DOI abstract</ref>. Another important feature of ionic liquids is their tunability: miscibility with water or organic solvents can be tuned through sidechain lengths on the cation and choice of anion. Furthermore, they can be functionalized to act as acids, bases or ligands, and have been used as precusor salts in the preparation of stable carbenes. Because of their properties, ionic liquids attract great attention in many fields, including organic chemistry, electrochemistry, catalysis, physical chemistry, and engineering; see for instance magnetic ionic liquid.

It should be noted that if the term 'ionic liquid' is used to denote the subset of liquids which are entirely ionic, then all other generic statements of characteristics are incorrect - there are examples which are instable, volatile, flammable, reactive etc.

There are indications that under certain conditions some ionic liquids can be distilled; some ionic liquids generate flammable gases on thermal decomposition (such as 1-butyl-3-methylimidazolium nitrate); and the thermal stability as well as melting point depend on the components of the ionic liquid and thus affect the liquid window to some extent. These exceptions illustrate the nature of ionic liquids as a diverse class of fluids, rather than a small group of individual examples.

The solubility of different species in imidazolium ionic liquids tends to be dependent mainly on polarity and hydrogen bonding ability. Simple aliphatic compounds are generally only sparingly soluble in ionic liquids, whereas olefins show somewhat greater solubility, and aldehydes can be completely miscible. This can be exploited in biphasic catalysis, such as hydrogenation and hydrocarbonylation processes, allowing for relatively easy separation of products and/or unreacted substrate(s). Gas solubility follows the same trend, with carbon dioxide gas showing exceptional solubility in many ionic liquids, carbon monoxide being less soluble in ionic liquids than in many popular organic solvents, and hydrogen being only slightly soluble, similar to the solubility in water, and probably varying relatively little between the more popular ionic liquids. Different groups have, however, employed different analytical techniques, which in turn have yielded somewhat different absolute solubility values.

[edit] Room temperature ionic liquids

Room temperature ionic liquids consist of bulky and asymmetric organic cations such as 1-alkyl-3-methylimidazolium, 1-alkylpyridinium, N-methyl-N-alkylpyrrolidinium and ammonium ions. A wide range of anions is employed, from simple halides, which generally inflect high melting points, to inorganic anions such as tetrafluoroborate and hexafluorophosphate and to large organic anions like bis-trifluorsulfonimide, triflate or tosylate. There are also many interesting examples of uses of ionic liquids with simple non-halogenated organic anions such as formate, alkylsulfate, alkylphosphate or glycolate. As an example, the melting point of 1-butyl-3-methylimidazolium tetrafluoroborate or [bmim][BF4] with an imidazole skeleton is about -80 °C, and it is a colorless liquid with high viscosity at room temperature.

It has been pointed out that in many synthetic processes using transition metal catalyst, metal nanoparticles play an important role as the actual catalyst or as a catalyst res-ervoir. It also been shown that ionic liquids (ILs) are an appealing medium for the formation and stabilization of catalytically active transition metal nanoparticles. More importantly, ILs can be made that incorporate co-ordinating groups,<ref>Applications of Functionalized Ionic Liquids, X. Li, D. Zhao, Z. Fei, L. Wang, Science in China: B, 2006, 35, 181</ref>, for example, with nitrile groups on either the cation or anion (CN-IL). In various C-C coupling reactions catalyzed by palladium catalyst, it has been found the palladium nanoparticles are better stabilized in CN-IL compared to non-functionalized ionic liquids; thus enhanced catalytic activity and recy-clability are realized. <ref>Zhao, D.; Fei, Z.; Geldbach, T. J.; Scopelliti, R.; Dyson, P. J. J. Am. Chem. Soc. 2004, 126, 15876 http://www.esi-topics.com/nhp/2006/july-06-ZhaofuFei.html</ref>

[edit] Food science

The application range of ionic liquid also extends to food science. [bmim]Cl for instance is able to completely dissolve freeze dried banana pulp and the solution with an additional 15% DMSO lends itself to Carbon-13 NMR analysis. In this way the entire banana compositional makeup of starch, sucrose, glucose, and fructose can be monitored as a function of banana riping <ref>Use of ionic liquids in the study of fruit ripening by high-resolution 13C NMR spectroscopy: green solvents meet green bananas Diego A. Fort, Richard P. Swatloski, Patrick Moyna, Robin D. Rogers and Guillermo Moyna Chemical Communications, 2006, 714 - 716 DOI abstract</ref>.

[edit] Safety

Due to their non-volatility, effectively eliminating a major pathway for environmental release and contamination, ionic liquids have been considered as having a low impact on the environment and human health, and thus recognized as solvents for green chemistry. However, this is distinct from toxicity, and it remains to be seen how 'environmentally-friendly' ILs will be regarded once widely used by industry. Research into IL aquatic toxicity has shown them to be as toxic or more so than many current solvents already in use <ref>Acute toxicity of ionic liquids to the zebrafish (Danio rerio) C Pretti, C Chiappe, D Pieraccini, M Gregori, F Abramo, G Monni and L Intorre, Green Chem., 2005 DOI abstract</ref>. Available research also shows that mortality isn't necessarily the most important metric for measuring their impacts in aquatic environments, as sub-lethal concentrations have been shown to change organisms' life histories in meaningful ways. According to these researchers balancing between zero VOC emissions, and avoiding spills into waterways (via waste ponds/streams, etc.) should become a top priority. However, with the enormous diversity of substituents available to make useful ILs, it should be possible to design them with useful physical properties and less toxic chemical properties.

Despite their low vapor pressure many ionic liquids have also found to be combustable and therefore require careful handling <ref>Combustible ionic liquids by design: is laboratory safety another ionic liquid myth? Marcin Smiglak et al. Chemical Communications, 2006, (Advance Article) DOI:10.1039/b602086k</ref>. Brief exposure (5 to 7 seconds) to a flame torch will ignite these IL's and some of them are even completely consumed by combustion.

[edit] See also

[edit] External links

Major academic groups
Other

[edit] References

<references />
  • Air and water stable ionic liquids in physical chemistry F. Endres, S. Zein El Abedin, Phys. Chem. Chem. Phys., 8 (2006) 2101.[2]de:Ionische Flüssigkeit

es:líquido iónico ja:イオン液体 pl:Ciecz jonowa

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