Debashis Kundu

Quantum chemical and experimental insights for the ionic liquid facilitated thermal dehydrogenation of ethylene diamine bisborane

The current work reports the judicial selection and subsequent dehydrogenation reaction with ionic liquid (IL) facilitated ethylene diamine bisborane (EDAB). Quantum chemical based COSMO-SAC (COnductor like Screening MOdel Segment Activity Coefficient) model was initially used to screen the ILs as available from Sigma Aldrich. LUMO–HOMO calculation was then performed to analyze the stability of EDAB/IL complexes. The molecular modeling studies converged on the two ILs, namely 1-ethyl-3-methyl imidazolium acetate ([EMIM][OAc]) and 1-butyl-3-methyl imidazolium acetate ([BMIM][OAc]), which were subsequently chosen for the dehydrogenation experiments. The thermal dehydrogenation of EDAB was carried out at 95 °C and 105 °C under vacuum so as to prevent generation of oxygen moieties. A total of 3.96 and 3.52 equivalents of hydrogen were released from the desorption of EDAB/[BMIM][OAc] and EDAB/[EMIM][OAc], respectively, at 105 °C. The purity of released gas was confirmed by gas chromatographic analysis, while the catalytic activity of ILs was confirmed by 1H NMR characterization of pure EDAB, ILs and EDAB/IL complexes both before and after the reaction. 11B NMR analysis confirms the presence of trigonal boron (sp2) BH2 group as the only hydrogen containing boron moiety in dehydrogenated EDAB. Further, the two-stage release mechanism of EDAB was also verified by thermogravimetric analysis. High resolution mass spectrometry was able to detect the mass of cyclic repeat units in the polymeric chain containing an sp2 BH2 group.

Ionic Liquid Facilitated Dehydrogenation of tert-Butylamine Borane

The current work reports ionic liquid (IL) facilitated dehydrogenation of tert-butylamine borane (TBAB) at 90 and 105 °C. For the screening of potential IL solvent, solubility predictions of TBAB in ILs were performed by the conductor-like screening model segment activity coefficient (COSMO-SAC) model. The COSMO-SAC model predicted a logarithmic infinite dilution activity coefficient of −6.66 and −7.31 for TBAB in 1-butyl-3-methylimidazolium acetate [BMIM][OAc] and trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate [TDTHP][Phosph], respectively. Hydrogen (1.95 equiv) was seen to release from TBAB/[BMIM][OAc] at 105 °C, whereas TBAB/[TDTHP][Phosph] produced 1.63 equiv of hydrogen after 360 min of dehydrogenation. The proton nuclear magnetic resonance (1H NMR) characterization of TBAB/IL systems revealed the structural integrity of ILs during dehydrogenation. Further characterization through the boron NMR (11B NMR) technique disclosed the time-resolved formation and stability of the starting compound, intermediate boron moieties, and product distribution. The 11B NMR characterization also revealed the fact that the TBAB/[TDTHP][Phosph] mixture dehydrogenates via bimolecular addition of TBAB by forming borohydride anion (−BH4–). It was seen to oligomerize with the subsequent addition of TBAB in the oligomer chain. For the TBAB/[BMIM][OAc] system, the 11B NMR characterization could not identify the borohydride anion but confirmed a faster formation of the B=N moiety when compared to the TBAB/[TDTHP][Phosph] system. On the basis of the NMR characterization, IL-facilitated dehydrogenation mechanism of TBAB is proposed.