Mechanism of Ni-NHC CO2 Reduction Catalysis Predominantly Affording Formate via Attack of Metal Hydride to CO2

The catalytic role of hydride intermediate in the CO2 reduction to formate (HCOO−) by NiII-NHC complexes is investigated in detail by density functional theory (DFT) calculations. It is found that a NiII-hydride is sufficiently hydridic to facilitate the efficient transfer of hydride to the carbon center of CO2, leading to the HCOO− production. Importantly, the direct hydride transfer path proposed here bypasses the conventional CO2 insertion into a metal-hydride bond. This mechanism is elucidated through a detailed analysis of the free energy changes of the reaction and the activation barriers, where key parameters such as reduction potentials, pKa values, and the thermodynamics of hydride transfer are thoroughly evaluated. The thermodynamic hydricity of the NiII-hydride, calculated to be ΔGH− = 19.2 kcal/mol, is in sharp contrast with the less effective NiIII-hydride with ΔGH− = 52.4 kcal/mol, highlighting the enhanced reactivity of NiII-hydride in HCOO− formation. Additionally, an examination of the competitive formation of CO and H2 reveals the preferential tendency of NiII-hydride to produce HCOO− over these byproducts. Insights into the influence of the pKa for the proton source on the feasibility of H2 production and HCOO− selectivity are also provided, suggesting a way to optimize reaction conditions for improved selectivity and efficiency. Our findings provide a comprehensive understanding of the CO2 reduction to HCOO− by NiII-NHC catalysts, emphasizing the direct hydride transfer mechanism rather than the classical CO2 insertion mechanism.