Density functional theory study of the mechanism for Ni(NHC)2 catalyzed dehydrogenation of ammonia–borane for chemical hydrogen storage

The mechanism for catalytic dehydrogenation of ammonia–borane (AB = H3N–BH3), a promising candidate for chemical hydrogen storage, by the Ni(NHC)2 complexes was studied by using density functional theory at the non-empirical meta-GGA level, Tao–Perdew–Staroverov–Scuseria (TPSS) functional, with all-electron correlation-consistent polarized valence double-zeta (cc-pVDZ) basis set. The mechanism for both the first and the second H2 release from AB was studied for the first time. Several unusual aspects of this catalytic mechanism were revealed through our calculations. First, the first H2 release begins with proton transfer from nitrogen to the Ni bound carbene carbon, forming a new C–H bond, instead of the previously hypothesized direct B–H or N–H bond activation. Second, this new C–H bond is activated by the metal, transferring the H to Ni, then forming the H2 molecule by transferring another H from B to Ni, rather than β-H transfer. Third, the second H2 release from H2N–BH2 begins with the breaking of a 3-center, 2-electron Ni–H–B bridging structure with the assistance of the unsaturated carbene carbon atom to form a B–C bond. Fourth, a nearly rhombic N2B2H6 structure is formed to help the regeneration of the catalyst Ni(NHC)2. These reaction pathways explain the importance of NHC ligands in this catalytic process and yield lower energy barriers than those mechanisms that begin with N–H or B–H activations catalyzed by the metal atoms. The predicted reaction mechanism which features unexpected ligand participation points the way to finding new catalysts with higher efficiency, as partial unsaturation of the M–L bond may be essential for low energy H transfers.