By: Roland St. Kalb and Michael J. Kotschan, Aldrich ChemFiles 2006, 6.9, 8.
Aldrich ChemFiles 2006, 6.9, 8.
A Revolutionary Tool for Ionic Liquid Synthesis
Roland St. Kalb and Michael J. Kotschan, proionic Production of Ionic Substances GmbH, Leoben, Austria; www.proionic.at; email@example.com
“When I first started to work with ionic liquids years ago, it always was an unfulfilled dream to investigate the most interesting ionic liquid structures in my field of research; either the substances were commercially not available in a suitable quality, were not even available at all, or there was no predictable synthetic route I trusted. It always took a long time to develop a new synthesis and to realize a new structure; the throughput of my screening was terrible,” said Roland Kalb, organic chemist and scientific director of the Austrian company proionic Production of Ionic Substances GmbH. Michael Kotschan, managing director of proionic GmbH: “With CBILS© – Carbonate Based Ionic Liquid Synthesis – we bring a revolutionary tool to the public, which stood the test of time perfectly in our own R&D for years. We are able to synthesize more than 20 brand new ionic liquids per person a day with ease, just using simple standard laboratory equipment.”
How Does CBILS© Work?
Conventional ionic liquid synthesis is complicated, and often suffers from halogen impurities. In general, there was no systematic way to predicitably produce high-quality ionic liquids. CBILS© now offers the simplest synthetic method to Sigma-Aldrich customers.
For example, the synthesis of 1-vinyl-3-methylimidazolium acetate is performed by addition of one equivalent of acetic acid to the corresponding CBILS© precursor 1-vinyl-3- methylimidazolium hydrogencarbonate.
The anion is ultimately removed as carbon dioxide gas and exchanged by acetate. This type of reaction always works quantitatively with every Brønsted acid available, even very insoluble ones. The ionic liquid is isolated by simple evaporative removal of the solvent and the reaction by-product (water).
One Hundred Syntheses in One Week?
By using the CBILS© route for the synthesis of ionic liquids and analogous structures, it is possible for a single person, in one week’s time, to synthesize 100 new substances via the combination of 10 precursors with 10 Brønsted acids. There is no need for the use of dry solvents, no halogen impurities or by-products are formed, no guesswork about the synthetic route, and no waste is generated. Just select the cation precursor and the anion (via the acid) of your choice, and make your dream ionic liquid a reality!
Details about CBILS© – Carbonate-Based Ionic Liquid Synthesis
Sigma-Aldrich now offers a selection of eleven CBILS© precursors for the synthesis of imidazolium, ammonium, phosphonium, pyrrolidinium, piperidinium, and morpholinium type ionic liquids in the form of their methylcarbonates and hydrogencarbonates. To synthesize your desired structure all you simply need to do is:
1. Choose the cation in the form of the corresponding CBILS© precursor.
2. Choose the anion in the form of the corresponding Brønsted acid.
3. Calculate the necessary stoichiometry to form the ionic liquid.
4. Mix the CBILS© precursor and the Brønsted acid and stir until CO2 generation subsides – depending on the batch and the substance, this typically requires 5 minutes to 1 hour.
5. Remove the solvent and reaction by-product (water or methanol) by rotary evaporation.
When performing your reaction, reserve extra head space in the reaction vessel (typically 2x the volume of the reaction mixture) to allow room for CO2 foaming, especially for long-chained structures with detergent-like properties. Adding solvents such as ethanol or 2-propanol helps to prevent foaming. If you wish to avoid discoloration during the evaporation of the solvent, we recommend working under inert gas, otherwise weak discoloration is possible. Even so, discoloration does not generally affect the ionic liquids quality unless it is to be used in optical applications in the visible spectrum.
Now you can choose your anion out of thousands of commercially available Brønsted acids – from simple mineral acids to the most complicated, chiral and functionalized acids, from strong to weak ones, and from soluble to nearly insoluble ones. Because the chemical equilibrium is shifted continuously by the formation of gaseous carbon dioxide, the reaction always achieves a 100% conversion.
You can also use Brønsted acid precursors such as organic or inorganic anhydrides. For example, we successfully synthesized ionic liquid molybdates and tungstates using molybdenum oxide and tungsten oxide respectively, both of which are very insoluble in aqueous media. Although sluggish, both ionic liquids were formed at 100% conversion as evidenced by the slow, but continuous evolution of CO2. Also the solubility, viscosity or aggregate state of the synthesized ionic liquid does not limit the reaction. Even water insoluble solids are formed perfectly and precipitate smoothly.
Calculating the exact reaction stoichiometry is critical, because the ionic liquid may be sensitive to unreacted free acid or carbonate precursor. The concentration of CBILS© precursors is indicated on the label and is lot specific. The concentration of the acid you are using must be known exactly via the certificate of analysis or alkalimetric titration. It should be known to within 1% accuracy.
By changing the stoichiometry of a diprotic Brønsted acid, you can synthesize hydrogenated anions with ease:
Some anions are not accessible via Brønsted acids because the free acid is not stable or not known at all. An illustrative example is the thiocyanate anion SCN–. Free thiocyanic acid (HSCN) is only stable at 0 °C in 5% aqueous solution and is not commercially available. In this case, you can use the corresponding ammonium salt, gently heating the reaction mixture to 60 °C for at least 30 minutes and removing the solvents at a temperature of at least 70 °C:
Again, all reaction by-products are readily removed and the reaction reaches a 100% conversion. (Caution! Ammonia is harmful and corrosive. Work in an efficient fume hood.)
A second alternative to Brønsted acids is the reaction of a CBILS© carbonate precursor with a calcium, zinc, manganese or other metal salt, which results in the formation of an insoluble metal carbonate. For example, if you react calcium thiocyanate (Ca(SCN)2) with a carbonate precursor, the desired thiocyanatebased ionic liquid is formed and calcium carbonate precipitates. The resultant solid can easily be removed by filtration or centrifugation.
Stability, Handling, and Storage
All CBILS© ionic liquid precursors are delivered in 40 to 60% aqueous or aqueous/methanolic solutions and are stable at room temperatures for years, unless stated otherwise (e.g. vinyl derivatives). The exact concentration is marked on the label and is lot specific. To maintain this concentration, the bottle should be closed immediately after use. Due to the methanol content, the solutions are weakly toxic. CBILS© ionic liquid precursors typically have pH values of 8 to 11 and are irritants, especially to the eyes.
Removal of the solvent to isolate the pure CBILS© ionic liquid precursors prior to the synthesis of an ionic liquid is not advised for any dialkylimidazolium hydrogencarbonate (1,3-dimethylimidazolium hydrogencarbonate, 1-ethyl- 3-methylimidazolium hydrogencarbonate, 1-butyl-3- methylimidazolium hydrogencarbonate, 1-vinyl-3-methylimidazolium hydrogencarbonate) due to the irreversible formation of dialkylimidazolium carboxylates