Synthesis and characterization of boranate ionic liquids (BILs)

Straightforward access to hydridoborate-based ionic liquids (BILs) is provided. They fall into a barely developed area of research and are of interest as, for example, reagents for organic synthesis. A series of pure [BH(4)](-) ILs with 1-butyl-2,3-dimethylimidazolium (BMMIM), 1-ethyl-3-methylimidazolium (EMMIM), 1-propyl-1-methylpiperidinium (PropMPip), and1-butyl-1-methylpyrrolidinium (BMP) cations were prepared. All synthesized ILs are well soluble in CH(2)Cl(2). We developed a procedure that gives clean products with correct elemental analyses. In contrast to earlier reports, which when conducted by us yielded only mixtures of the boranate anion with major halide contamination (maximum [BH(4)](-) content: 77.5 %). These materials can be viewed as the starting material for the (hypothetical) hydrogen-storage redox shuttling sequence between [BH(4)](-) and [B(12)H(12)](2-), in which the triboranate anion [B(3)H(8)](-) is a formal intermediate. Here we also developed a facile route to [B(3)H(8)](-) ILs with [BMMIM](+), [EMMIM](+), [PropMPip](+), and [NBu(4)](+), in which Na[BH(4)] reacts in situ (enhanced by ultrasound) with the solvent CH(2)Cl(2) as the oxidizing agent to give the triboranate IL in high yield and purity according to the equation: 3 [BH(4)](-)+2 CH(2)Cl(2)+[Cat](+)→[B(3)H(8)](-)[Cat](+)+H(2)+2 CH(3)Cl+2 Cl(-). We further investigated this reaction path by additional NMR spectroscopic experiments, powder-XRD analysis, and quantum chemical DFT calculations.