Article,
Perfluoroalkyltricyanoborate and Perfluoroalkylcyanofluoroborate Anions: Building Blocks for Low-Viscosity Ionic Liquids
Chemistry – A European Journal, 24 (3): 608--623 (January 2018)
DOI: 10.1002/chem.201703685
Abstract
The potassium perfluoroalkyltricyanoborates KCnF2 n+1B(CN)3 n=1 (1 d), 2 (2 d) and the potassium mono(perfluoroalkyl)cyanofluoroborates KCnF2 n+1BF(CN)2 n=1 (1 c), 2 (2 c) and CnF2 n+1BF2(CN)− n=1 (1 b), 2 (2 b), 3 (3 b), 4 (4 b) are accessible with perfect selectivities on multi-gram scales starting from KCnF2 n+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 5.8 V. The RTILs described herein, exhibit very low viscosities with a minimum of 14.0 mPa s at 20 °C for EMIm1 c, low melting points down to −57 °C for EMIm2 b and extraordinary high conductivities up to 17.6 mS cm−1 at 20 °C for EMIm1 c. 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, Grätzel 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 1 b–d, 2 d, 3 b and 4 c as well as of nBu4N2 c have been studied by X-ray diffraction.
