Insights into H₂ Activation and Styrene Hydrogenation by Nickel–Borane and Nickel–Alane Bifunctional Catalysts

Lewis acid–transition metal bifunctional complexes have recently emerged as a new class of catalysts. The nickel–borane complex, Ni­[(Mes)­B­(o-Ph₂PC₆H₄)₂], has been reported as an efficient catalyst for H₂ activation and styrene hydrogenation. Here, we performed density functional theory calculations to investigate the cooperation between nickel and a group 13 element (Z = B and Al), including the effect of its substituent (R = Mes, Ph, and C₆F₅ in Ni­[(R)­Z­(o-Ph₂PC₆H₄)₂]), on H₂ activation and styrene hydrogenation. We found that H₂ activation by the nickel–borane complex is dominated by charge transfer from the σ-bonding orbital of H₂ to the p-based vacant orbital of boron, whereas H₂ activation by the nickel–alane complex is governed by charge transfer from the d-based orbital of Ni to the σ*-antibonding orbital of H₂. The resulting trans-dihydride nickel–alane complex has higher negative charges on both terminal and bridging hydrogen atoms than the corresponding nickel–borane complex. This accounts for the lower energy barriers observed for the nickel–alane complex toward both H₂ activation and the subsequent hydrogenation of styrene. While the C₆F₅ electron-withdrawing substituent on boron of the nickel–borane complex facilitates H₂ activation and styrene hydrogenation better than the phenyl or mesityl substituent, the substituent on aluminum does not affect the reactivity of the nickel–alane complex. As H₂ activation and styrene hydrogenation by nickel–alane complexes proceed with lower energy barriers than those by nickel–borane complexes, the nickel–alane complex with [(R)­Z­(o-Ph₂PC₆H₄)₂] ligand scaffold should be more reactive than the nickel–borane counterpart. Insights into the role of Lewis acid in this Z-type σ-acceptor ligand scaffold will assist with the development of metal–ligand bifunctional catalysts.