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

• Published in: Chemistry : a European journal 2018-01, Vol.24 (3), p.608-623
• Main Authors: Landmann, Johannes, Sprenger, Jan A. P., Hennig, Philipp T., Bertermann, Rüdiger, Grüne, Matthias, Würthner, Frank, Ignat'ev, Nikolai V., Finze, Maik
• Format: Article
• Language: English
• Subjects: Anions; borates; Cations; Chemistry; conducting materials; Crystals; Dye-sensitized solar cells; Electrochemistry; Electrolytic cells; Hydrophobicity; Ionic liquids; Ions; Melting points; perfluoroalkylborates; Photovoltaic cells; Potassium; Potassium salts; Salts; Single crystals; Solar cells; Solvents; Viscosity; X-ray diffraction
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201703685

The potassium perfluoroalkyltricyanoborates K[CnF2 n+1B(CN)3] [n=1 (1 d), 2 (2 d)] and the potassium mono(perfluoroalkyl)cyanofluoroborates K[CnF2 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 K[CnF2 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 [EMIm]1 c, low melting points down to −57 °C for [EMIm]2 b and extraordinary high conductivities up to 17.6 mS cm−1 at 20 °C for [EMIm]1 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 [nBu4N]2 c have been studied by X‐ray diffraction. Ionic liquids: Unique perfluoroalkylcyanoborate ionic liquids exhibiting high thermal, chemical and electrochemical stabilities and providing low viscosities and high conductivities were developed (see figure).