CH688236A5 - New hydrophobic liq. imidazolium salt - Google Patents

Abstract

Hydrophobic liq. salts of formula (I) are new: R1,R3 = 1-8C (un)branched alkyl, fluoroalkyl, or alkoxyalkyl; R2,R4,R5 = H or 1-3C alkyl. Also claimed is: (a) the prepn. of such salts; (b) an electrolytic compsn. contg. (I) as aprotic polar solvent; and (c) an electrochemical photovoltaic cell contg. the electrolytic compsn..

Description

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CH 688 236 A5

Description

The present invention relates to new hydrophobic salts, liquids and of low viscosity at room temperature, having for cation an N, N'-dialkylimidazolium and for anion bis-trifluorometha-nesulfonylamide (CF3SC> 2) 2N- (otherwise called bis -triflylamidure), as well as their manufacturing process.

The invention also relates to the application of said salts as hydrophobic polar solvents for electrochemical or synthetic applications, for example as a solvent in an electrolytic composition for electrochemical photovoltaic cells.

Already known liquid salts at room temperature having for cation and for anion respectively a substituted imidazolium and trifluoromethane sulfonate (otherwise called tritiate). Such compounds are described in a publication by E. J. Cooper and E. J. M. O'Sullivan (The Electrochem. Soc., Proceeding Vol. 92-16 (1992)). These compounds have various viscosities, but all are miscible with water (hydrophilic) with the exception of dodecyl-1-ethyl-3-imidazolium tritiate, which is otherwise solid at room temperature (melting point 32 ° C) .

Document US Pat. No. 4,505,997 also describes, as new chemical compounds, bis-tri-flylamides of alkali metals, obtained by ion exchange from the corresponding tetrabutylammonium. These salts are solid compounds intended to be incorporated into the matrix of a polymer to form solid electrolytes making it possible to produce primary or secondary electrochemical generators. In addition, these salts are described as highly hydrophilic, a property resulting, according to the authors of the invention, essentially from a bis-triflamide flag.

Surprisingly, it has now been discovered that a salt formed from a substituted imidazolium cation and from the bis-triflylamide anion is liquid at room temperature, hydrophobic and has a low viscosity favorable to ionic mobility, a fairly high conductivity. , high thermal stability (at least up to 300 ° C), insignificant vapor pressure at room temperature and still less than 0.1 mbar at 150 ° C, a wide range of electrochemical stability (at least -2 V at 2 V relative to the iodine / triiodide pair), a set of properties making it possible to use such salts advantageously as a solvent in an electrochemical system, such as electrochemical photovoltaic cells, or for syntheses requiring polar aprotic solvents.

The compounds according to the invention can be represented by the formula below:

wherein Ri and R3 are the same or different and each represents a linear or branched alkyl radical, a fluoroalkyl radical or an alkoxyalkyl radical of 1 to 8 carbon atoms; and R2, R4 and R5 are the same or different and each represents a hydrogen atom or an alkyl radical of 1 to 3 carbon atoms.

Preferably R1 or R3 represents a methyl, ethyl, butyl, isobutyl, octyl, methoxyethyl or 2,2,2-trifluoroethyl radical and R2, R4 or R5 represents hydrogen or a methyl or ethyl radical.

The invention also relates to a process for obtaining said compounds by ion exchange in an aqueous medium between a bis-triflylamide of alkali metal, preferably lithium, and a substituted imidazolium halide, preferably bromide or iodide, according to the reaction scheme below:

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CH 688 236 A5

R

2

R] N0N - S02) ^ N ♦ L i B r

/ \

A:

R

AT

Lithium bis-triflylamidide is a commercially available compound, for example from the company 3M (St-Paul Minnesota-USA).

The bis-triflylamidides of an alkali metal can also be prepared according to methods known from the prior art, such as those described in document EP 0 096 629 from a perfluoroalkanesulfonic anhydride, or by JR Foropoulos and DD Desmarteau ( Inorg Chem 22, 3 720-3723 (1984)) from a fluoride of perfluoroalkanesulfonic acid.

The substituted imidazolium halide is obtained according to a known process by the action, in an organic solvent with stirring, of an alkyl halide R3X on an imidazole suitably substituted with R1, R2, R4 and R5.

The examples below give, by way of illustration and without limitation, the preparation of a certain number of compounds according to the invention, as well as their physicochemical and electrochemical properties making them suitable for being advantageously used as an electrolytic solvent.

Example 1

Methvl-1-butvl-3-imidazolium bis-triflvlamide

In a reactor, 0.1 mol (8.2 g) of 1-methylimidazole was dissolved in 200 ml of trichloroethane, then a solution of 0.12 mol was added dropwise at room temperature with vigorous stirring. 16.44 g) butyl bromide in 100 ml of trichloroethane. It was then heated at reflux for 2 hours, then the mixture allowed to settle. Methyl-1-butyl-3-imidazolium bromide separates as a liquid. The bromide is then washed twice with 100 ml of 50 ° C. trichloroethane, then dried for one hour in a Rotavapor at 150 ° C. under reduced pressure of 0.1 mbar to give 15.3 g (70% yield) of methyl bromide. 1 -butyl-3-imidazolium.

50 mmol (11 g) of the bromide obtained in the previous step were then dissolved in 50 ml of water, then a solution of 50 mmol (14.35 g) of lithium bis-triflylamide was added at once. in 50 ml of water. The solution immediately becomes cloudy and after decantation separates into two phases. After removal of the supernatant aqueous phase, the liquid salt was washed twice with 50 ml of water, then dried for one hour at 150 ° C. under reduced pressure of 0.1 mbar. There was thus obtained 18.9 g of title product (yield 90%) in the form of a colorless liquid immiscible with water, having the refractive index no = 1.4271. The freezing point of this liquid is less than -25 ° C. The hydrophobic character of the product obtained is confirmed by a titration according to Karl Fisher of the product previously saturated with water showing a content of only 1.4% by mass.

A measurement of the density of the title compound carried out at 20 ° C. gives the value d = 1.429 g cm-3, and a measurement of kinematic viscosity, carried out at 20 ° C. with a Haake ball microviscosimeter makes it possible to determine the value of the coefficient viscosity 1 = 52.4 cP.

By way of comparison, the homologous compound known from the prior art, methyl-1-butyl-3-imidazolium tritiate is a compound having a melting point of 16 ° C, soluble in water, and of viscosity 1 = 90 cP.

The title compound has also been shown to have high thermal stability up to 300 ° C and very low vapor pressure (less than 0.1 mbar at 150 ° C).

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CH 688 236 A5

The co-conductivity of the liquid salt measured at 20 ° C with an Autolab frequency analyzer potentiostat has the value a = 3.88 mS cm-1, that is to say a value of the same order of magnitude as the triflate counterpart ( a = 3.68 mS cm-1).

The cyclovoltammogram shown in fig. 1, to be carried out at 20 = C, with a platinum working electrode with a surface of 0.78 mm 2 and a reference electrode of this same metal in contact with the iodine / triiodide couple in the same liquid salt, with a sweep 50 mVs-1, shows that the title compound has a high electrochemical stability in a window between -2.0 V and +2.2 V.

Example 2

Methvl-1-ethvl-3 imidazolium bis-triflvlamide

According to a two-step process, comparable to that of Example 1, but replacing in the first step 0.12 mole of butyl bromide with 0.25 mole (27.75 g) of ethyl bromide, we have 54 g (78% yield) obtained of the title product in the form of a colorless liquid having the following physicochemical characteristics:

Freezing point <-25 ° C;

Water content (Karl Fisher): 1.4%

d2 ° = 1.52 g cm-3 n = 34.3 cP

np = 1.4231 0 = 8.84 mS cm-1

A cyclovoltammogram produced under the same conditions as in Example 1 shows that the title compound has an electrochemical window of between -2.0 V and +2.2 V.

Example 3

Ethyl-1-bis-triflylamide-butyl-3-imidazolium

By proceeding as indicated in Example 1, but starting from 0.1 mole (9.6 g) of ethyl-1-imidazole, 13 g (yield 60%) of the title compound were obtained in the form of a colorless liquid having the following physicochemical characteristics:

Freezing point <-25 ° C

Water content (Karl Fisher): 1.3%

d20 = 1.404 g cm-3 n = 47.6 cP

iD = 1.4285 a = 4.12 mS cm-1

In the same way as in Examples 1 to 3, but starting from differently substituted imidazoles, the triflylamides of the following cations were also prepared:

methyl-1-methyl-3-imidazolium; methyl-1-methoxyethyl-3-imidazolium; 1-methyl -2,2,2-trifluoroethyl-3-imi-dazolium; ethyl-1-ethyl-3-imidazolium; methyl-1-methyl-2-ethyl-3-imidazolium; ethyl-1-methyl-2-ethyl-3-imi-dazolium; methyl-1-ethyl-3-methyl-5-imidazolium; ethyl-1-ethyl-3-methyl-5-imidazolium; 1-ethyl-isobutyl-3-imidazolium; 1-methyl-octyl-3-imidazolium.

Example 4

Electrochemical photovoltaic cell before a hydrophobic liquid salt as an electrolytic solvent

An electrochemical photovoltaic cell of the type described in international patent application WO94 / 04497 was produced, consisting of two sets of electrodes 20 μm apart, one of the electrodes being coated with a nanoparticulate layer of TÌO2 of 0, 3 µm thick on which the cis-dithiocyanato-bis (2,2'-bipyridyl-4,4'-dicarboxylate ruthenium II) is absorbed as a sensitizer in an amount such that the absorption of visible light is only 5% around 520 nm. The free space between the electrodes was filled with an electrolyte composed of the liquid salt of Example 1 as solvent, 10% by weight of methylhexyl imidazolium iodide and 10 mmol of iodine.

Under an illumination corresponding to 1/100 of the standard solar illumination (AM1), an open circuit voltage of 530 mV and a short-circuit current of 27 nA ■ cm-2 were obtained.

By repeating the same experiment with the liquid salt of Example 2 as solvent, 10% by weight of methylhexyl imidazolium iodide and 5 mmol of iodine, an open circuit voltage of 550 mV and a short current were obtained. -25 ^ A circuit. cm-2.

It was also observed that the hydrophobic nature of the liquid salt used as electrolyte solvent made it possible to reduce the desorption of the sensitizer of the nanoparticulate TÌO2 layer. Con4

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CH 688 236 A5

Unlike many aprotic polar solvents, a liquid salt according to the invention is inert towards triiodide.

This example is given by way of illustration and the person skilled in the art is able to use the hydrophobic liquid salts according to the invention in other applications in the electrochemical field without departing from the scope of the invention.

Claims (9)

Claims 1. Hydrophobic liquid salts of general formula R. wherein Ri and R3 are the same or different and each represents a linear or branched alkyl radical, a fluoroalkyl radical or an alkoxyalkyl radical of 1 to 8 carbon atoms; and R2, R4 and R5 are the same or different and each represents a hydrogen atom or an alkyl radical of 1 to 3 carbon atoms. 2. hydrophobic liquid salts according to claim 1, characterized in that Ri or R3 represents a methyl, ethyl, butyl, isobutyl, octyl, methoxyethyl or 2,2,2-trifluoroethyl radical and R2, R4 or Rs represents hydrogen or a methyl or ethyl radical. 3. hydrophobic liquid salt according to claim 1, characterized in that it corresponds to the bis-triflylamidide of methyl-1-butyl-3-imidazolium. 4. hydrophobic liquid salt according to claim 1, characterized in that it corresponds to the bis-triflylamide of methyl-1-ethyl-3-imidazolium. 5. Hydrophobic liquid salt according to claim 1, characterized in that it corresponds to the bis-triflylami-dur of ethyl-1-butyl-3-imidazolium. 6. A process for the preparation of a hydrophobic liquid salt according to claim 1, characterized in that an imidazolium halide substituted with Ri, R2, R3, R4 and R5 is reacted in an aqueous medium on an alkaline bis-triflylamide, and separating the product obtained by decantation. 7. Preparation process according to claim 6 characterized in that the halide is a bromide or an iodide and the alkali metal is lithium. 8. Electrolytic composition containing, as aprotic polar solvent, a hydrophobic liquid salt of general formula. R. V ^ -R3 / \ R5 R4 (CFg SO2 ^ N in which R 1 represents an alkyl radical of 1 to 5 carbon atoms, R 3 represents a linear or branched alkyl radical, a fluoroalkyl radical or an alkoxyalkyl radical of 1 to 8 carbon atoms; and R2, R4 and R5 represent independently a hydrogen atom or an alkyl radical of 1 to 3 carbon atoms. 9. An electrochemical photovoltaic cell containing an electrolytic composition according to claim 8 associated with an iodine / triiodide couple. 5