The term “ionic liquid” (IL) is usually used to refer to a salt which is liquid at temperatures below 100° C., in particular at room temperature. Such liquid salts typically comprise organic cations and organic or inorganic anions, and are described inter alia in P. Wasserscheid et al., Angew. Chem., 2000, 112, 3926-3945.
Ionic liquids have a series of interesting properties: Usually, they are thermally stable, relatively non-flammable and have a low vapor pressure. They show good solvability for numerous organic and inorganic substances. In addition, ionic liquids have interesting electrochemical properties, for example electrical conductivity which is often accompanied by a high electrochemical stability.
These attributes give rise to many applications of ionic liquids: They can be used foremost as solvent in synthesis, as electrolyte, as lubricant and as hydraulic fluid. Moreover they serve as phase-transfer catalyst, as extraction medium, as heat-transfer medium, as surface-active substance, as plasticizer, as conductive salt, organic salt or additive in electrochemical cells, as electrolyte, as component in electrolyte formulations, wherein such electrolyte formulation comprising an ionic liquid is preferably used in electrochemical and/or optoelectronic device such as a photovoltaic cell, a light emitting device, an electrochromic or photo-electrochromic device, an electrochemical sensor and/or biosensor, particularly preferred in a dye sensitized solar cell.
Therefore, there is a fundamental need for ionic liquids having a variety of properties which open up additional opportunities for their use.
An interesting family of ionic liquids contains tetravalent boron anions. Tetrafluoroborate containing ionic liquids were among the first of this new generation of compounds and 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIm][BF4]) was prepared via metathesis of [EMIm]I with Ag[BF4] in methanol as disclosed by J. S. Wilkes et al., J. Chem. Soc. Chem. Commun. 1990, 965.
E. Bernhardt, Z. Anorg. Allg. Chem. 2003, 629, 677-685, discloses the reaction of M[BF4] (M=Li, K) with (CH3)3SiCN (TMSCN). The preparation of Li[BF(CN)3] is disclosed to take 7 days, that of K[BF(CN)3] takes one month. The yield of K[BF(CN)3] was 60%, the product contained 5% K[BF2(CN)2]. The molar ratio of [BF4]−:TMSCN was 1:7.8.
US 2011/150736 A1 discloses as a “Third Production Method” a reaction of three compounds: TMSCN, an amine or ammonium salt, and a boron compound.
EP 2 327 707 A claims in claim 7 a method for producing an ionic compound represented by the general formula (I), comprising a step of reacting starting materials containing a cyanide and a boron compound. General formula (I) is a salt of a cation Ktm+ with [B(CN)4]−.
The examples disclose various methods for preparing tetrabutylammonium tetracyanoborate, for example:    1) Example 1-1 of EP 2 327 707 A discloses a reaction of tetrabutylammonium bromide, zinc (II) cyanide and boron tribromide in toluene at 130° C. for 2 days, with a yield of 35%. The molar ratio of boron compound:TMSCN was 1:5.5.    2) Example 2-1 of EP 2 327 707 A discloses a reaction of tetrabutylammonium bromide, tetrabutylammonium cyanide and boron tribromide in toluene at 130° C. for 2 days, with a yield of 77%. The molar ratio of boron compound:tetrabutylammonium cyanide was 1:7.1.    3) Example 3-3 of EP 2 327 707 A discloses a reaction of tetrabutylammonium bromide, trimethylsilyl cyanide and boron trichloride in p-xylene at 150° C. for 30 hours, with a yield of 98%. The molar ratio of boron compound:TMSCN was 1:5.5.    4) Example 3-11 of EP 2 327 707 A discloses a reaction of boron trifluoride diethyl ether, tetrabutylammonium bromide and trimethylsilylcyanide at 170° C. for 30 hours, with a yield of 75%.But not all embodiments which fall under claim 7 actually work well: Example 3 of the instant invention shows one embodiment also starting with boron trifluoride diethyl ether, which falls under claim 7, but produces the desired [B(CN)4] salt only as a by-product in negligible amounts, the main product is a [BF(CN)3] salt.
There was a need for a simplified method with high yield and satisfactory purity for the preparation of fluoro cyanide compounds of the 13th group of the periodic table with the anion having the general formula [(Z1F4−m(CN)m)−] with Z1 is B, Al, Ga, In or Tl and m being 1, 2, 3 or 4. The boron source should be a readily available compound with low costs. The cyanide source should not be a metal cyanide to avoid its negative impact on the environment. The number of reactants should be small and the method should allow the conversion without the presence of a solvent. The content of Cl and Br in the final product should be low. Also the content of Si and cyanide in the final product should be low. The method should require as few steps as possible. The method should allow also the preparation of compounds with m being 1, 2, 3 or 4 and not only of either a compound with m being 3 or a compound with m being 4. The method should avoid the use of Cl2, AgCN or AgBF4. The method should provide stable compounds of said formula which can be used as ionic liquids or as precursors of ionic liquids and can be used e.g. in electrolyte formulations and in electrochemical or optoelectronic devices. These compounds should be able to be disposed of in an environmentally friendly manner after use.
The method should allow the preparation of the desired compounds in high yields and under mild conditions with respect to methods disclosed in the prior art.
This object is achieved by a method using trimethylsilylcyanide as CN source and by doing the reaction in the presence of a Lewis acid. No Cl2, AgCN or AgBF4 is required. The content of Cl, Br, Si and cyanide in the final product is low. Another advantage is that the reaction does not require an extra solvent. The method has a reduced number of steps compared to the methods known from the prior art. The method allows for the preparation not only of compounds with m being only 3 or only 4, but for compounds with n being 1, 2, 3 or 4. These compounds can be prepared specifically and individually, and not only as mixtures. The reaction can be done under milder conditions than those used in the methods of the prior art, the reaction can be done at lower temperature or in shorter time.
In this text, the following meanings are used, if not otherwise stated:                alkyl linear or branched alkyl;        C1-q alkyl refers to any alkyl residue which contains from 1 to q carbon atoms; for example C1-6 alkyl encompasses inter alia methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, isopentyl (3-methylbutyl), neopentyl (2,2-dimethylpropyl), n-hexyl and isohexyl (4-methylpentyl);        C2-q alkenyl refers to an alkenyl residue which contains from 2 to q carbon atoms and contains at least one double bond, the carbon chain can be linear or branched; for example C2-4 alkenyl encompasses inter alia ethenyl, 1-methylethenyl, prop-1-enyl, prop-2-enyl, 2-methylprop-2-enyl and buta-1,3-dienyl;        C2-q alkynyl refers to an alkynyl residue which contains from 2 to q carbon atoms and contains at least one triple bond, the carbon chain can be linear or branched; for example C2-4 alkynyl encompasses inter alia ethynyl, prop-1-ynyl and prop-2-ynyl;        C6-10 aryl refers to an aryl residue which has from 6 to 10 carbon atoms and is unsubstituted or substituted by 1, 2, 3 or 4 identical or different substituents independently from each other selected from the group consisting of C1-4 alkyl and C1-4 alkoxy; for example C6-10 aryl encompasses inter alia phenyl, methylphenyl, methoxyphenyl, dimethylphenyl, ethylmethylphenyl, diethylphenyl and naphthyl;        cyclic alkyl or cycloalkyl include cyclo and polycyclo, such as bicyclo or tricyclo, aliphatic residues;        C3-q cycloalkyl refers to a cycloalkyl group having from 3 to q carbon atoms; for example C3-10 cycloalkyl encompasses inter alia cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononyl and cyclodecyl;        C1-q alkoxy refers to an linear or branched alkoxy group having from 1 to q carbon atoms; for example C1-20 alkoxy encompasses inter alia methoxy, ethoxy, propoxy, isopropoxy, n-butoxy, isobutoxy, sec-butoxy, tert-butoxy, pentyloxy, 1,4-dimethylpentyloxy, hexyloxy, heptyloxy, octyloxy, 1,5-dimethylhexyloxy, nonyloxy, decyloxy, 4-ethyl-1,5-dimethylhexyloxy, undecyloxy, dodecyloxy, tridecyloxy, tetradecyloxy and eicosyloxy;        alkylene means a linear or branched alkylene group; e.g. propylene, and e.g.        
propylene can be connected via its C1 and C2 carbon atoms (a branched alkylene group), or via its C1 and C3 carbon atoms (linear alkylene group);                BMMIm n-Butyl-2-methyl-3-methylimidazolium        
                BMPy n-Butylmethylpyridinium        
                BMPyrr n-Butylmethylpyrrolidinium        
                BMPip n-Butylmethylpiperidinium        
                DCM dichloromethane;        EMIm 1-ethyl-3-methylimidazolium        
                eq. molar equivalent;        halide F−, Cl−, Br− or I−, preferably F−, Cl− or Br−, more preferably Cl−;        halogen F, Cl, Br or I; preferably F, Cl or Br;        HEIm 1-ethylimidazolium        
                IL ionic liquid;        “linear” and “n-” are used synonymously with respect to the respective isomers of alkanes;        RT room temperature, it is used synonymously with the expression ambient temperature;        Tdec decomposition temperature;        THF tetrahydrofuran;        TMSCN (CH3)3SiCN, i.e. trimethylsilylcyanide;        Trityl means the trityl cation, i.e. [Ph3C+]        “wt %”, “% by weight” and “weight-%” are used synonymously and mean percent by weight.        The expressions dye sensitized solar cell and photosensitized solar cell are used synonymously.