General H2 Activation Modes for Lewis Acid–Transition Metal Bifunctional Catalysts

A general mechanism for H2 activation by Lewis acid–transition metal (LA-TM) bifunctional catalysts has been presented via density functional theory (DFT) studies on a representative nickel borane system, (PhDPBPh)Ni. There are four typical H2 activation modes for LA-TM bifunctional catalysts: (1) the cis homolytic mode, (2) the trans homolytic mode, (3) the synergetic heterolytic mode, and (4) the dissociative heterolytic mode. The feature of each activation mode has been characterized by key transition state structures and natural bond orbital analysis. Among these four typical modes, (PhDPBPh)Ni catalyst most prefers the synergetic heterolytic mode (ΔG‡ = 29.7 kcal/mol); however the cis homolytic mode cannot be totally disregarded (ΔG‡ = 33.7 kcal/mol). In contrast, the trans homolytic mode and dissociative heterolytic mode are less feasible (ΔG‡ = ∼42 kcal/mol). The general mechanistic picture presented here is fundamentally important for the development and rational design of LA-TM catalysts in the future.