Synthesis and Characterization of Boranate Ionic Liquids (BILs)

Abstract

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 [BH₄]⁻ 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 CH₂Cl₂. 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 [BH₄]⁻ content: 77.5 %). These materials can be viewed as the starting material for the (hypothetical) hydrogen-storage redox shuttling sequence between [BH₄]⁻ and [B₁₂H₁₂]²⁻, in which the triboranate anion [B₃H₈]⁻ is a formal intermediate. Here we also developed a facile route to [B₃H₈]⁻ ILs with [BMMIM]⁺, [EMMIM]⁺, [PropMPip]⁺, and [NBu₄]⁺, in which Na[BH₄] reacts in situ (enhanced by ultrasound) with the solvent CH₂Cl₂ as the oxidizing agent to give the triboranate IL in high yield and purity according to the equation: 3 [BH₄]⁻+2 CH₂Cl₂+[Cat]⁺→[B₃H₈]⁻[Cat]⁺+H₂+2 CH₃Cl+2 Cl⁻. We further investigated this reaction path by additional NMR spectroscopic experiments, powder-XRD analysis, and quantum chemical DFT calculations.