(101b) Rationalizing the Reactivity of Bimetallic Molecular Catalysts for CO2 Hydrogenation

We have recently reported the heterobimetallic nickel–gallium complex, NiGaL (where L represents the tris(phosphinoamido)amine ligand, [N(o-(NCH₂Pi-Pr₂) C₆H₄)₃]^3−), which is the most active Ni-based molecular catalyst for CO₂ hydrogenation to date.[1] 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 HM₁M₂L^− 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 M−H species are generated in the presence of a Lewis base.

[1] 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), 14244– 14250.