Ionic Liquids in Catalysis


Many transition metal complexes dissolve readily in ionic liquids which enables their use as solvents for transition metal catalysis. Sufficient solubility for a wide range of catalyst complexes is an obvious, but not trivial, prerequisite for a versatile solvent for homogenous catalysis. There are many other good reasons to apply ionic liquids as alternative solvents in transition metal catalyzed reactions. Besides their very low vapor pressure and their good thermal stability an important advantage is the possibility to tune their solubility and acidity/coordination properties by varying the nature of the anions and cations systematically. The possibility of adjusting solubility properties is of particular importance for liquid-liquid biphasic catalysis.

Principle of biphasic catalysis using ionic liquids for catalyst immobilization

Liquid-liquid catalysis can be realized when the ionic liquid is able to dissolve the catalyst, displays a partial solubility with the substrates and a poor solubility with the reaction products. Under these conditions, the product phase, containing also the unconverted reactants, is removed by simple phase decantation, and the ionic liquid containing the catalyst can be recycled.
The use of a liquid, ionic catalyst support in the immobilized ionic liquid systems allows preservation of the advantages of traditional homogeneous catalysis including for example the usually high reaction selectivity governed by dissolved of well-defined transition metal complexes. Additionally, reaction conditions in ionic liquid catalysis are generally mild and comparable to those in homogeneous catalysis. The almost unlimited combinations of cation-anion pairs further allow the synthesis of tailor made ionic liquids that can stabilize catalytic species.
The use of ionic liquids as solvents for homogeneous transition metal catalysts was described by Chauvin et al. and by Wilkes et al. in 1990. Since then, ionic liquids have been used as alternative solvents for a large number of homogeneously catalyzed reactions.

References

  • Metal Catalysed Reactions in Ionic Liquids. Catalysis by Metal Complexes (Eds.: Paul J. Dyson, Tilmann J. Geldbach), Springer Netherlands, 2006.
  • Ionic Liquids - "New Solutions" for Transition Metal Catalysis, Peter Wasserscheid, Willi Keim, Angew. Chem. Int. Ed. 2000, 39, 3772.