Metal-Free, Ionic Liquid-Mediated Hydrogen Release From Amine Borane Complexes: An Experimental And Density Functional Theory Investigation

This work reports the thermal dehydrogenation of chemical hydrides, namely, ammonia borane (AB) and ethylene diamine bisborane (EDAB), in the presence of neoteric ionic liquids (ILs) based on methyl carbonate anions. Initially, the COSMO-SAC model was performed to predict the infinite dilution activity coefficient values for the solubility of AB and EDAB on the pyrrolidinium- and ammonium-based cations. Based on the screening study, 1-butyl-1-methylpyrrolidinium methyl carbonate[Bmpyr][CH3CO3] and tributylmethylammonium methyl carbonate [TBMA][CH3CO3] were selected for our dehydrogenation studies. It was observed that the latter performed remarkably well in terms of equivalents of hydrogen released, which is primarily due to the higher stability of the intermediate in the polar medium of ILs. Here, [TBMA][CH3CO3] gave a cumulative release of 3.50 equiv of hydrogen with EDAB at 105 degrees C. The H-1 NMR spectroscopy technique confirmed the catalytic sum solvent role of ILs. The electronic structure elucidation of individual ILs and IL-chemical hydride complexes was then performed at the M06-2X/6-311++G(d,p) level of theory. The DFT calculations, along with the highest occupied molecular orbital-lowest unoccupied molecular orbital analysis, pointed out the fact that the active sites mainly existed within the methyl carbonate anions. Overall, the dehydrogenation pathway was initiated by the formation of hydrogen-bonded interactions between the protic moieties of the hydrides and the anionic part of the ILs, respectively.