Dehydrogenation of ammonia-borane by cationic Pd(ii) and Ni(ii) complexes in a nitromethane medium: hydrogen release and spent fuel characterization

Abstract

A highly electrophilic cationic Pd^II complex, [Pd(MeCN)₄][BF₄]₂ (1), brings about the preferential activation of the B–H bond in ammonia-borane (NH₃·BH₃, AB). At room temperature, the reaction between 1 in CH₃NO₂ and AB in tetraglyme leads to Pd nanoparticles and formation of spent fuels of the general formula MeNH_xBO_y as reaction byproducts, while 2 equiv. of H₂ is efficiently released per AB equiv. at room temperature within 60 seconds. For a mechanistic understanding of dehydrogenation by 1, the chemical structures of spent fuels were intensely characterized by a series of analyses such as elemental analysis (EA), X-ray photoelectron spectroscopy (XPS), solid state magic-angle-spinning (MAS) NMR spectra (²H, ¹³C, ¹⁵N, and ¹¹B), and cross polarization (CP) MAS methods. During AB dehydrogenation, the involvement of MeNO₂ in the spent fuels showed that the mechanism of dehydrogenation catalyzed by 1 is different from that found in the previously reported results. This AB dehydrogenation derived from MeNO₂ is supported by a subsequent digestion experiment of the AB spent fuel: B(OMe)₃ and N-methylhydroxylamine ([Me(OH)N]₂CH₂), which are formed by the methanolysis of the AB spent fuel (MeNH_xBO_y), were identified by means of ¹¹B NMR and single crystal structural analysis, respectively. A similar catalytic behavior was also observed in the AB dehydrogenation catalyzed by a nickel catalyst, [Ni(MeCN)₆][BF₄]₂ (2).