Low- to Room-Temperature Dehydrogenation of Dimethylamine Borane Facilitated by Ionic Liquids: Molecular Modeling and Experimental Studies

Abstract

A novel low- and room-temperature dehydrogenation of dimethylamine borane (DMAB) is being reported using hydrogen sulfate-based ionic liquids (ILs), namely, 1-ethyl-3-methylimidazolium hydrogen sulfate ([EMIM][HSO4]) and 1-butyl-3-methylimidazolium hydrogen sulfate ([BMIM][HSO4]). The ILs are selected from a pool of potential ILs using a conductor-like screening model-segment activity coefficient (COSMO-SAC) model, and the dehydrogenation is performed at 0, 15, and 25 °C and a high vacuum of 4 × 10–2 mbar gauge pressure. The dehydrogenated products are characterized with 1H and 11B NMR. 1H NMR characterization reveals the structural integrity of ILs after the dehydrogenation. The 11B NMR characterization reveals the evolution of −BH2 and B═N moieties during dehydrogenation. Based on the 11B NMR study, the IL-assisted intramolecular and intermolecular dehydrogenation mechanism has been proposed. The intramolecular dehydrogenation pathway of dehydrogenation is proposed at 0 °C, and the intermolecular dehydrogenation pathway is at 15 and 25 °C. Formation of DMAB dimers has been confirmed for the intermolecular dehydrogenation pathway; on the contrary, DMAB does not oligomerize as other amine boranes. The interaction energy calculations reveal a greater stability of DMAB in [EMIM][HSO4] than in [BMIM][HSO4] IL. The anionic moiety of ILs is found to be the prominent pathway for the hydrogen bond interaction with protic and hydridic moieties of DMAB. It confirms the dominant role with respect to the selection of anion in hydrogen production.