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

Two novel Ru-II hydrides containing the tripodal polyphosphine ligand NP3 (N(CH2CH2PPh2)(3)) have been prepared and characterized: (kappa(4)-NP3)Ru(H)(2) (1) and [(kappa(4)-NP3)Ru(H)(eta(2)-H-2)] BAr4Cl (2; Ar-Cl = 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 center dot BH3, AB) has been analyzed experimentally through variable-temperature multinuclear (B-11, P-31, H-1) 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 H-2 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 B-H/N-H activation occurs in the presence of 1, whereas the N-H activation only is the rate-determining step if 2 is employed as a catalyst.