(101b) Rationalizing the Reactivity of Bimetallic Molecular Catalysts for CO2 Hydrogenation
We have recently reported the heterobimetallic nickelâgallium complex, NiGaL (where L represents the tris(phosphinoamido)amine ligand, [N(o-(NCH2Pi-Pr2) C6H4)3]3â), which is the most active Ni-based molecular catalyst for CO2 hydrogenation to date. Understanding the reaction mechanism of this catalytic system and identifying the factors that govern its catalytic activity are important in order to design even more efficient base metal catalysts. We will present a computational study of possible reaction pathways for CO2 hydrogenation catalyzed by NiGaL and to identify the most favorable pathway. We found the overall catalytic process has two main time periods which agrees well with experimental observations: an induction period, during which the deprotonation of the H2 adduct by exogenous base is predicted to be rate-limiting, followed by a subsequent period where the produced formate assists in deprotonation by acting as a proton shuttle between the H2 adduct and exogenous base. For H2 adduct deprotonation, the steric hindrance and basicity associated with the exogenous base, and formate assistance are important factors; a steady state, during which hydride transfer to CO2 is predicted to become rate-limiting. For hydride transfer to CO2, the free energy of activation was found to depend linearly on the thermodynamic hydricity for a series of bimetallic HM1M2Lâ complexes (M1 and M2 represent different transition metal), providing a simple and efficient strategy for screening other bimetallic catalysts. The predicted trends and structure-activity relationships arising from these computational calculations can be further utilized for the rational design of more efficient catalysts for CO2 hydrogenation and other hydride transfer processes for which reactive MâH species are generated in the presence of a Lewis base.
 Ryan C. Cammarota; Vollmer, M. V.; Xie, J.; Ye, J.; Linehan, J. C.; Burgess, S. A.; Appel, A. M.; Gagliardi, L.; Lu, C. C. J. Am. Chem. Soc. 2017, 139 (40), 14244â 14250.