Nitrile (Cyano) Functionalized Ionic Liquids

By: Dongbin Zhao, Zhaofu Fei, and Paul J. Dyson, chemfiles volume 6 article 9

Dongbin Zhao, Zhaofu Fei, and Paul J. Dyson, Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne EPFL-BCH, CH-1015 Lausanne, Switzerland.

Without a doubt, ionic liquids have captured the imagination of chemists as well as researchers in other scientific disciplines.1 The principle application of ionic liquids in chemistry has been as alternative solvents for synthesis and catalysis. With this in mind, the search for ionic liquids with superior physical and chemical properties to those most frequently used has been met with considerable enthusiasm, especially with respect to task specific ionic liquids.2 Of the numerous ionic liquids developed in this regard, a comparatively simple class of ionic liquid with alkyl-nitrile chains attached to the ionic liquid cation (see product list on page 15) have proven to be particularly useful, based on a series of impressive applications.

While the nitrile functionality is only weakly basic due to the presence of the ionic liquid cation (typically an imidazolium or pyridinium cation), it can nevertheless weakly coordinate to metal centers.3 The net effect of such weak coordination is four-fold:

1. To facilitate the solubility of metal salts in the ionic liquids.
2. To be sufficiently labile so as not to suppress catalytic activity by blocking coordination sites during reaction.
3. To stabilize the catalyst, facilitate the formation of transition states, and therefore increase its lifetime.
4. To enhance the immobilization of the catalyst in the ionic liquid during product extraction, thereby improving recycling.

These advantages have been demonstrated to the greatest extent in palladium catalyzed C–C coupling reactions such as Suzuki, Heck, and Stille reactions.4 Figure 1 compares ionic liquid solutions following catalysis that illustrate the benefit of using the nitrile-functionalized system. Moreover, palladium nanoparticles separated after catalysis were analyzed by TEM and the images indicate that the nitrile-functionalized ionic liquid stabilizes palladium nanoparticles which are present as Pd(0) reservoirs. ICP analysis of the organic phase for palladium residues is often below the detection limit (1 ppm) suggesting that such ionic liquids could be useful in the synthesis of pharmaceuticals and liquid crystals where trace metal impurities must be extremely low.


Figure 1: Comparison of ionic liquid-organic biphases after catalysis and TEM images of nanoparticles extracted from the ionic liquid phase; (left) [C4py][Tf2N] and (right) [C3CNpy][Tf2N]. Note the absence of precipitates and clarity of the organic phase with the nitrile-functionalized ionic liquid.

Nitrile-functionalized ionic liquids are considerably more effective for the immobilization of palladium catalysts for the transfer of a vinyl group in Stille reactions with respect to alkyl-substituted ionic liquids (Scheme 1).5 Again, TEM analysis of nanoparticles extracted from the ionic liquids provide evidence for the stabilizing effect exerted by the nitrile pendant group on the metal center.


Scheme 1

While model substrates were used in the above examples, desulfitative Mizoroki-Heck-type arylation of alkenes using complex precursors can be performed efficiently in nitrilefunctionalized ionic liquids (Scheme 2), again proving superior to the commonly used organic solvents and simple ionic liquids.6


Scheme 2

In a quite different application, ionic liquids are being used as non-volatile electrolytes in dye-sensitized solar cells, and those containing the nitrile functional group attached to imidazolium cations have been evaluated as electolytes in neat or binary systems (Figure 2).7 Excellent energy conversion efficiencies were observed; ca. 8% in low light irradiations in the binary systems. Furthermore, excellent light soaking stability was observed during 1,000 hours of aging, indicating quite an extraordinary robustness.


Figure 2: Current density-voltage characteristic of a DSC device containing a [C3CNC1im]-based ionic liquid electrolyte under AM 1.5 simulated full sunlight (100 mW cm-2) illumination.

In general, these nitrile-derivatized ionic liquids can act as acetonitrile replacements wherever the advantage of having a non-volatile equivalent can be envisaged. If one searches the literature for reactions where optimum results are obtained in acetonitrile or other nitrile based solvents, then it is clear that these ionic versions offer considerable potential in synthesis and catalysis.

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References

  1. For example see: (a) Seddon, K. R. J. Chem. Technol. Biotechnol. 1997, 68, 351. (b) Welton, T. Chem. Rev. 1999, 99, 2071. (c) Wasserscheid, P.; Keim, W. Angew. Chem. Int., Ed. Engl. 2000, 39, 3772. (d) Dupont, J.; Souza, R. F. D.; Suarez, P. A. Z. Chem. Rev. 2002, 102, 3667. (e) Chiappe C.; Pieraccini, D.; J. Phys. Org. Chem. 2005, 18, 275.
  2. (a) Davis Jr., J. H. Chem. Lett. 2004, 33, 1072. (b) Fei, Z.; Geldbach, T. J.; Zhao, D.; Dyson, P. J. Chem. Eur. J. 2006, 12, 2122.
  3. Zhao, D.; Fei, Z.; Scopelliti, R.; Dyson, P. J. Inorg. Chem. 2004, 43, 2197.
  4. Zhao, D.; Fei, Z.; Geldbach, T. J.; Scopelliti, R.; Dyson, P. J. J. Am. Chem. Soc. 2004, 126, 15876.
  5. Chiappe, C.; Pieraccini, D.; Zhao, D.; Fei, Z.; Dyson, P. J. Adv. Synth. Catal. 2006, 348, 68.
  6. Reddy, D. S.; Zhao, D.; Fei, Z.; Rao Volla, C. M.; Dyson, P. J.; Vogel, P. Synlett 2006, 3155.
  7. Mazille, F.; Fei, Z.; Kuang, D.; Zhao, D.; Zakeeruddin, S. M.; Grätzel, M.; Dyson, P. J. Inorg. Chem. 2006, 45, 1585.

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