How Does the Nickel Pincer Complex Catalyze the Conversion of CO₂ to a Methanol Derivative? A Computational Mechanistic Study

The mechanistic details of nickel-catalyzed reduction of CO2 with catecholborane (HBcat) have been studied by DFT calculations. The nickel pincer hydride complex ({2,6-C6H3(OPtBu2)2}NiH = [Ni]H) has been shown to catalyze the sequential reduction from CO2 to HCOOBcat, then to CH2O, and finally to CH3OBcat. Each process is accomplished by a two-step sequence at the nickel center: the insertion of a C═O bond into [Ni]H, followed by the reaction of the insertion product with HBcat. Calculations have predicted the difficulties of observing the possible intermediates such as [Ni]OCH2OBcat, [Ni]OBcat, and [Ni]OCH3, based on the low kinetic barriers and favorable thermodynamics for the decomposition of [Ni]OCH2OBcat, as well as the reactions of [Ni]OBcat and [Ni]OCH3 with HBcat. Compared to the uncatalyzed reactions of HBcat with CO2, HCOOBcat, and CH2O, the nickel hydride catalyst accelerates the Hδ− transfer by lowering the barriers by 30.1, 12.4, and 19.6 kcal/mol, respectively. In general, the catalytic role of the nickel hydride is similar to that of N-heterocyclic carbene (NHC) catalyst in the hydrosilylation of CO2. However, the Hδ− transfer mechanisms used by the two catalysts are completely different. The Hδ− transfer catalyzed by [Ni]H can be described as hydrogen being shuttled from HBcat to nickel center and then to the C═O bond, and the catalyst changes its integrity during catalysis. In contrast, the NHC catalyst simply exerts an electronic influence to activate either the silane or CO2, and the integrity of the catalyst remains intact throughout the catalytic cycle. The comparison between [Ni]H and Cp2Zr(H)Cl in the stoichiometric reduction of CO2 has suggested that ligand sterics and metal electronic properties play critical roles in controlling the outcome of the reaction. A bridging methylene diolate complex has been previously observed in the zirconium system, whereas the analogous [Ni]OCH2O[Ni] is not a viable intermediate, both kinetically and thermodynamically. Replacing HBcat with PhSiH3 in the nickel-catalyzed reduction of CO2 results in a high kinetic barrier for the reaction of [Ni]OOCH with PhSiH3. Switching silanes to HBcat in NHC-catalyzed reduction of CO2 generates a very stable NHC adduct of HCOOBcat, which makes the release of NHC less favorable.