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 H-2 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 H-2 activation and styrene hydrogenation. We found that H-2 activation by the nickel-borane complex is dominated by charge transfer from the sigma-bonding orbital of H-2 to the p-based vacant orbital of boron, whereas H-2 activation by the nickel-alane complex is governed by charge transfer from the d-based orbital of Ni to the sigma*-antibonding orbital of H-2. 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 H-2 activation and the subsequent hydrogenation of styrene. While the C6F5 electron-withdrawing substituent on boron of the nickel-borane complex facilitates H-2 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 H-2 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 sigma-acceptor ligand scaffold will assist with the development of metal-ligand bifunctional catalysts.