Chapter 22
Synthesis and Application of New Ionic Liquids with Tris(perfluoroalkyl)trifluorophosphate Anions
Downloaded by STANFORD UNIV GREEN LIBR on August 6, 2012 | http://pubs.acs.org Publication Date: January 18, 2007 | doi: 10.1021/bk-2007-0950.ch022
Ν. V. Ignat'ev*, W.-R. Pitner, and U. Welz-Biermann Merck KGaA, New Venture-Materials (NV-M), Darmstadt, D-64271, Germany (email:
[email protected];
[email protected])
The synthesis of new ionic liquids with tris(perfluoroalkyl)trifluorophosphate (FAP) anions is described. The physicochemical properties of this new generation of ionic liquids are discussed. FAP ionic liquids show an excellent hydrolytic stability, low viscosity and high thermal stability, that makes them attractive for the application as new reaction media in organic synthesis.
1. Introduction. Ionic Liquids are the modern approach to replace volatile and flammable organic compounds, which are used as solvents in organic synthesis. The application of ionic liquids as novel reaction media may have a grate impact on the future development of chemical technologies and chemical engineering (1). Ionic liquids as novel reaction media have a clear advantages in comparison to traditional organic solvents. They are not volatile and not flammable. The properties of ionic liquids (thermal and electrochemical stability, viscosity and solution-ability) can be tuned simply by the variation of the cation-anion pair. But, ionic liquids are still relatively expensive. From this point of view, the application of ionic liquids as reaction media makes sense if the yield, quality of the product or efficiency of an expensive catalyst is greatly improved. Ionic liquids can also make an improvement in the process if the product isolation is simplified and ionic liquids themselves can be reused several times without substantial loosing of the process efficiency. © 2007 American Chemical Society
In Ionic Liquids in Organic Synthesis; Malhotra, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.
281
282 Both of these approaches are reasonable, but in the case of continued use of ionic liquids their lifetime is a crucial point. Long-term applicability of ionic liquids is directly related to their thermal and hydrolytical stability. First (tetrachloroaluminates) and second (tetrafluoro-borates and hexafluorophosphates) generations of ionic liquids are moisture sensitive. Commonly used ionic liquids with hexafluorophosphate, [PF ]~ anion are hydrolytically unstable, especially at elevated temperature (2). To address this disadvantage of the hexafluorophosphate anion, the replacement of some fluorine atoms by hydrophobic perfluoroalkyl-groups looks promising to increase the hydrolytic stability of fluorophosphates. Downloaded by STANFORD UNIV GREEN LIBR on August 6, 2012 | http://pubs.acs.org Publication Date: January 18, 2007 | doi: 10.1021/bk-2007-0950.ch022
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2. Results and Discussion. 2.1. Synthesis of ionic liquids with FAP - anion. Recently Merck KGaA (Darmstadt, Germany) has developed convenient synthesis of ionic liquids with tris(perfluoroalkyl)trifluorophospate (FAP) -anion (water insoluble) (3,4)], as replacement for hydrolytically unstable PF ". Preparation of the ionic liquids with FAP - anion is based on the industrial process - electrochemical fluorination of trialkylphosphines, resulting in the formation of tris(perfluoroalkyl)difluorophosphoranes of type 1 in high yield (5). 6
1
2
Phosphoranes of type 1 can be easily converted into tris(perfluoroalkyl)trifluorophosphoric acid (HFAP) of type 2 by the action of HF in water or in organic solvents [4]. Acid 2 is the new strong acid which shows better hydrolytic stability in comparison to HPF and serves as convenient starting material for the synthesis of various salts with tris(perfluoroalkyl)-trifluorophosphate anion (FAP-anion), for example (Scheme 1) [4]: 6
H+[(C F ) PF J 2
5
3
(water solution)
3
+
c ^ f f Ö - ' CH ( ter solution) wa
3
cr
C H -N@N-CH |[C F ) PF ] 6
13
3
3
2
5
3
3
Liquid at room temperature
Scheme 1
In Ionic Liquids in Organic Synthesis; Malhotra, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.
+ HCl
283 2.2. Typical procedure for the preparation of ionic liquids with FAP-anion. 5.02 mol of tris(pentafluoroethyl)trifluorophosphoric acid pentahydrate, H[(C F ) PF ] · 5H 0, were added at room temperature to the stirred solution of 5.0 mol of Ν,Ν'-dialkylimidazolium chloride in 1.5 liter of water. The mixture was left stirring for 0.5 hour. The bottom phase was separated and washed with water until pH 6-7 and the test on chloride with silver nitrate was negative. The residue was dried at reduced pressure of 10-20 Pa at 80° - 90° C within 10-20 hours. The yield of Ν,Ν'-dialkylimidazolium tris(pentafluoroethyl)trifluorophosphate (usually liquid) is close to quantitative. Downloaded by STANFORD UNIV GREEN LIBR on August 6, 2012 | http://pubs.acs.org Publication Date: January 18, 2007 | doi: 10.1021/bk-2007-0950.ch022
2
5
3
3
2
3. Physico-Chemical Properties of Ionic Liquids with FAPanion. 3. l.Hydrolytic stability of FAP-ionic liquids. The FAP-ionic liquids are not mixable with water, but mixable with polar organic solvents. The water uptake (saturation with water) for these ionic liquids is much less then for the popular ionic liquids with the bis(trifluoromethylsulfonyl)imide-anion and more than ten time less in comparison to the commonly used ionic liquids with the [PF ]" anion. That allows access to FAP-ionic liquids with a very low content of residual water, ΙΟ Ι 5 ppm, or even less. FAP ionic liquids show an excellent hydrolytic stability. For example, no HF formation was detected for l-hexyl-3-methylimidazolium FAP after five hours in boiled water (data from QUILL, Queen's University, Belfast, UK). 6
3.2. Thermal stability. Typically ionic liquids with FAP-anion are liquids at room temperature and show a low melting point (glass transition point), typically below -50° C (see Table 1). FAP Ionic liquids also possess a high thermal stability. The data in Table 1 show that imidazolium based ionic liquids with FAP-anion start to decompose at temperatures of above 280° C.
3.3. Viscosity. Viscosity is a very important for the practical application. Ionic liquids are more viscous then conventional organic solvents. By this reason the kinetic of
In Ionic Liquids in Organic Synthesis; Malhotra, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.
284 Table 1 Ionic Liquid
M.P., °C
-37
300
1-Pentyl-3-methylimidazolium FAP
