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

High-yield syntheses up to molar scales for salts of [BH(CN)₃ ]^- (2) and [BH₂ (CN)₂ ]^- (3) starting from commercially available Na[BH₄ ] (Na5), Na[BH₃ (CN)] (Na4), BCl₃ , (CH₃ )₃ SiCN, and KCN were developed. Direct conversion of Na5 into K2 was accomplished with (CH₃ )₃ SiCN and (CH₃ )₃ SiCl as a catalyst in an autoclave. Alternatively, Na5 is converted into Na[BH{OC(O)R}₃ ] (R=alkyl) that is more reactive towards (CH₃ )₃ SiCN and thus provides an easy access to salts of 2. Some reaction intermediates were identified, for example, Na[BH(CN){OC(O)Et}₂ ] (Na7 b) and Na[BH(CN)₂ {OC(O)Et}] (Na8 b). A third entry to 2 and 3 uses ether adducts of BHCl₂ or BH₂ Cl such as the commercial 1,4-dioxane adducts that react with KCN and (CH₃ )₃ SiCN. Alkali metal salts of 2 and 3 are convenient starting materials for organic salts, especially for low viscosity ionic liquids (ILs). [EMIm]3 has the lowest viscosity and highest conductivity with 10.2 mPa s and 32.6 mS cm^-1 at 20 °C known for non-protic ILs. The ILs are thermally, chemically, and electrochemically robust. These properties are crucial for applications in electrochemical devices, for example, dye-sensitized solar cells (Grätzel cells).