Chemical Hydrogen Storage: Ammonia Borane Dehydrogenation Catalyzed by NP3Ruthenium Hydrides (NP3=N(CH2CH2PPh2)3)

Abstract

Two novel RuII hydrides containing the tripodal polyphosphine ligand NP3 (N(CH2CH2PPh2)3) have been prepared and characterized: (κ4-NP3)Ru(H)2 (1) and [(κ4-NP3)Ru(H)(η2-H2)]BArCl4 (2; ArCl=3,5-dichlorophenyl, C6H3Cl2). The classical and nonclassical nature of the hydride ligand in 1 and 2, respectively, has been confirmed by solution NMR studies and single-crystal X-ray diffraction. The reaction of 1 and 2 with excess ammonia borane (NH3⋅BH3, AB) has been analyzed experimentally through variable-temperature multinuclear (11B, 31P, 1H) NMR spectroscopy, kinetic rate measurements, and kinetic isotope effect determination with deuterated AB isotopologues. Both complexes are active in AB dehydrogenation, with release of two H2 equivalents per AB equivalent, with concomitant polyborazylene and cyclic polyaminoborane formation as the final “spent fuel”. A DFT modeling of the AB first dehydrogenation step has been finally performed, at the M06//6-31G* level of theory. The combination of the kinetic and computational data reveals that the initial AB activation step is different for the two catalysts: a simultaneous BH/NH activation occurs in the presence of 1, whereas the NH activation only is the rate-determining step if 2 is employed as a catalyst.