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

Abstract

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)3SiCN, and KCN were developed. Direct conversion of Na5 into K2 was accomplished with (CH3)3SiCN and (CH3)3SiCl 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)3SiCN 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 BH2Cl such as the commercial 1,4‐dioxane adducts that react with KCN and (CH3)3SiCN. 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).

Cyanoborates

“…Reactions of Na[BH 2 (CN) 2 ] with amine hydrochlorides to yield the corresponding quaternary ammonium salts with the DHB anion as major products were studied earlier . DHB ‐ILs are even less viscous than the corresponding ILs with the [BF 2 (CN) 2 ] – anion, but less thermally and electrochemically stable (chapter 7, vide infra ) . DHB ‐ILs were tested as component of electrolytes for DSSCs and in electrolyte compositions for semi‐transparent photovoltaic cells in blue optoelectronic devices.…”

“…Low viscosity ionic liquids and organic salts with the [BH 2 (CN) 2 ] – anion were found to have limited thermal and electrochemical stabilities . These disadvantages were addressed by the introduction of an additional cyano group at boron.…”

“…Similar to the syntheses of dicyanodihydridoborates starting from BH 2 Cl with an O ‐donor molecule, the complex of dichloroborane with 1,4‐dioxane BHCl 2 · C 4 H 8 O 2 was treated with trimethylsilyl cyanide (CH 3 ) 3 SiCN and KCN to yield MHB ‐salts (Scheme , B ). This three‐component reaction proceeded at 80 °C within 3 h in THF yielding K[BH(CN) 3 ] in 89 % , . The preparation of BHCl 2 · C 4 H 8 O 2 was described in the literature and was most conveniently performed in situ…”

“…Even more feasible syntheses for salts of the [BH(CN) 3 ] – anion are based on inexpensive and commercially available Na[BH 4 ] as starting material. Na[BH 4 ] was treated with an excess of trimethylsilyl cyanide in an autoclave in the presence of (CH 3 ) 3 SiCl at 200–230 °C to give Na[BH(CN) 3 ] in 56 % yield and after aqueous work‐up K[BH(CN) 3 ] in 44 % yield (Scheme , C ) , . Similarly, Na[BH 3 CN] and Na[BH 2 (CN) 2 ] were used as starting materials but the MHB yields were lower than starting from Na[BH 4 ] (Scheme , C ) , .…”

“…Na[BH 4 ] was treated with an excess of trimethylsilyl cyanide in an autoclave in the presence of (CH 3 ) 3 SiCl at 200–230 °C to give Na[BH(CN) 3 ] in 56 % yield and after aqueous work‐up K[BH(CN) 3 ] in 44 % yield (Scheme , C ) , . Similarly, Na[BH 3 CN] and Na[BH 2 (CN) 2 ] were used as starting materials but the MHB yields were lower than starting from Na[BH 4 ] (Scheme , C ) , . Drawbacks of this procedure were the relatively high pressure required and the formation of highly volatile and flammable (CH 3 ) 3 SiH as side product.…”