Catalytic Dehydrogenation of Ammonia Borane at Ni Monocarbene and Dicarbene Catalysts

The development of ammonia borane (AB) as a promising hydrogen storage medium depends upon the ability to reversibly release H-2 from the system. We use density functional theory to investigate the mechanism of the catalytic dehydrogenation of AB by Ni N-heterocyclic carbene (NHC) complexes, which we show proceeds through Ni monocarbene and dicarbene species. Although Ni(NHC)(2) dehydrogenates AB, it competitively decomposes into a monocarbene species because AB readily displaces NHC from Ni(NHC)(2) and reaction of displaced NHC with abundant AB makes Ni monocarbene formation thermodynamically favored over the dicarbene catalyst. Prediction of NHC displacement by AB is consistent with the experimental observation of NHC-BH3. The Ni monocarbene species Ni(NHC)(NH2BH2) competitively dehydrogenates AB with barriers consistent with the experimental temperature required to obtain reasonable reaction rates. The Ni monocarbene pathway also involves rate-limiting steps that exhibit both N-H and B-H kinetic isotope effects (KIEs), as observed experimentally. The predicted N-H and B-H KlEs are also in quantitative agreement with experiment. In contrast, AB dehydrogenation by Ni(NHC)(2) does not exhibit a B-H KIE. Activation of AB at both mono- and dicarbene catalysts proceeds through cis-carbene, proton acceptance and involves transition states with significant electron delocalization over the pi-system of the carbene and its phenyl rings. NHC Ni catalysts involving carbenes with substituent groups containing steric factors that preclude planarity of the phenyl rings to the carbene aromatic system, such as the Imes and Idipp ligands, are predicted to have lower reactivity, in agreement with experiment. The addition of electron donating and withdrawing groups to the phenyl rings demonstrate the importance of pi-system electron delocalization by their influence on the barrier to cis-carbene proton acceptance.