Perfluoroalkyltricyanoborate and Perfluoroalkylcyanofluoroborate Anions: Building Blocks for Low-Viscosity Ionic Liquids

The potassium perfluoroalkyltricyanoborates K[CnF2n+1B(CN)(3)] [n=1 (1d), 2 (2d)] and the potassium mono(perfluoroalkyl)cyanofluoroborates K[CnF2n+1BF(CN)(2)] [n=1 (1c), 2 (2c)] and [CnF2n+1BF2(CN)](-) [n=1 (1b), 2 (2b), 3 (3b), 4 (4b)] are accessible with perfect selectivities on multi-gram scales starting from K[CnF2n+1BF3] and Me3SiCN. The K+ salts are starting materials for the preparation of salts with organic cations, for example, [EMIm](+) (EMIm=1-ethyl-3-methylimidazolium). These [EMIm](+) salts are hydrophobic room-temperature ionic liquids (RTILs) that are thermally, chemically and electrochemically very robust, offering electrochemical windows up to 5.8V. The RTILs described herein, exhibit very low viscosities with a minimum of 14.0mPas at 20 degrees C for [EMIm]1c, low melting points down to -57 degrees C for [EMIm]2b and extraordinary high conductivities up to 17.6mScm(-1) at 20 degrees C for [EMIm]1c. The combination of these properties makes these ILs promising materials for electrochemical devices as exemplified by the application of selected RTILs as component of electrolytes in dye-sensitised solar cells (DSSCs, Gratzel cells). The efficiency of the DSSCs was found to increase with a decreasing viscosity of the neat ionic liquid. In addition to the spectroscopic characterisation, single crystals of the potassium salts of the anions 1b-d, 2d, 3b and 4c as well as of [nBu(4)N]2c have been studied by X-ray diffraction.