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 ...