The Catalytic Dehydrogenation of Ammonia-Borane Involving an Unexpected Hydrogen Transfer to Ligated Carbene and Subsequent Carbon−Hydrogen Activation

Density functional Tao−Perdew−Staroverov−Scuseria calculations with all-electron correlation-consistent polarized valence double-ζ basis set demonstrate that N-heterocyclic carbene (NHC) nickel complexes catalyze the dehydrogenation of ammonia-borane, a candidate for chemical hydrogen storage, through proton transfer from nitrogen to the metal-bound carbene carbon, instead of the B−H or N−H bond activation. This new C−H bond is then activated by the metal, transferring the H to the metal, then forming the H2 by transferring a H from B to the metal, instead the β-H transfer. This reaction pathway explains the importance of the NHC ligands in the dehydrogenation and points the way to finding new catalyst with higher efficiency, as partial unsaturation of the M-L bond may be essential for rapid H transfers.