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. 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. Here, we present a computational study of possible reaction pathways for CO2 hydrogenation catalyzed by NiGaL. The most favorable predicted pathway for formate production agrees well with key experimental observations and is defined by four elementary steps: (1) H2 binding to the Ni center, (2) deprotonation of the H2 adduct, (3) hydride transfer to CO2 to form a formate adduct, and (4) formate release to regenerate NiGaL. The overall catalytic process has two main time periods: 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. The barrier for H2 adduct deprotonation is governed predominantly by the steric hindrance associated with the exogenous base and is found to be dramatically lowered by formate assistance. Once sufficient formate has been generated, the catalysis enters the steady-state period, during which hydride transfer to CO2 is predicted to become rate-limiting once sufficient formate has been generated and the reaction rate remains constant until the base is nearly consumed. 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, providing a simple and efficient strategy for screening other bimetallic catalysts. Furthermore, the relative binding energies of H2 and formate were analyzed to predict the ability of the bimetallics to facilitate the catalytic turnover. 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.