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

Abstract

The potassium perfluoroalkyltricyanoborates K[C F B(CN) ] [n=1 (1 d), 2 (2 d)] and the potassium mono(perfluoroalkyl)cyanofluoroborates K[C F BF(CN) ] [n=1 (1 c), 2 (2 c)] and [C F BF (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[C F BF ] and Me SiCN. 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 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 [nBu N]2 c have been studied by X-ray diffraction.

Cyanohydridoborate Anions: Synthesis, Salts, and Low‐Viscosity Ionic Liquids

“…The conductivities of the EMIm‐ILs with the mixed cyanohydridoborate anions are exceptionally high (Table ). The specific conductivities ( σ ) of the three ILs [EMIm] 2 , [EMIm] 3 , and [EMIm] 4 exceed the highest value reported for [EMIm][CF 3 BF(CN) 2 ] (17.6 mS cm −1 ), earlier . [EMIm] 3 possesses the by far highest specific conductivity of 32.6 mS cm −1 at 20 °C known for non‐protic ILs, up to now .…”

“…[a] Dynamic viscosity η (values measured with an angle of 60°); density ρ ; concentration c ; diffusion coefficients of cations ( D + ) and anions ( D − ); specific conductivity σ measured via impedance spectroscopy; molar conductivities calcd by Λ NMR =( D + + D − )⋅ N A ⋅e 2 ⋅ k −1 ⋅ T −1 and Λ imp = σ ⋅ M ⋅ ρ −1 ; ionicity I = Λ imp ⋅ Λ NMR −1 ; cathodic and anodic limits E c and E a ; electrochemical window Δ E = E a — E c . [b] Literature values except for η (Slightly different values were reported, for example, 21.8 and 22.2 mPa s determined with a Stabinger viscometer) . [c] Literature values for [EMIm] 4 at 25 °C: η =19 mPa s and ρ =0.98 g cm ‐3 .…”

“…The specific conductivities ( σ ) of the three ILs [EMIm] 2 , [EMIm] 3 , and [EMIm] 4 exceed the highest value reported for [EMIm][CF 3 BF(CN) 2 ] (17.6 mS cm −1 ), earlier . [EMIm] 3 possesses the by far highest specific conductivity of 32.6 mS cm −1 at 20 °C known for non‐protic ILs, up to now . A combination of a high number of charge carrier ions, which is given by their molar concentration c in the IL, with a low dynamic viscosity ( η ) results in a high specific conductivity .…”

“…All ILs studied, qualify as good ILs according to Walden plots, they possess remarkably low dynamic viscosities and high specific conductivities. [EMIm][BH 2 (CN) 2 ] ([EMIm] 3 ) provides the lowest dynamic viscosity (10.2 mPa s) in combination with the highest specific conductivity (32.6 mS cm −1 ) of a non‐protic IL at 20 °C, known to date . Similar appealing properties were found for [EMIm][BH(CN) 3 ] ([EMIm] 2 ), which is thermally and electrochemically more stable than [EMIm] 3 , making it a remarkable candidate for potential applications, for example in electrochemical devices such as dye‐sensitized solar cells (Grätzel cells).…”

“…The conductivities of the EMIm-ILs with the mixed cyanohydridoborate anionsa re exceptionally high ( [38] [EMIm]3 possesses the by far highest specific conductivityo f3 2.6 mS cm À1 at 20 8C knownf or non-protic ILs, up to now. [38] Ac ombination of a high number of chargec arrieri ons, which is given by their molar concentration c in the IL, with al ow dynamic viscosity (h)r esults in ah igh specificc onductivity.…”