Hydride Transfer-Based CO₂ Reduction Catalysis: Navigating Metal Hydride to Organic Hydride in the Catalytic Loop

ConspectusThe reductive conversion of carbon dioxide (CO₂) into value-added products is a process of immense importance. In the context of rising CO₂ concentration in the atmosphere and the detrimental effects it is having on the biosphere, use of alternative fuels which can offer a low-carbon or carbon-neutral pathway for storage and utilization of low-carbon energy by maintaining the net atmospheric CO₂ concentration might be a prospective solution. Among the wide variety of reduced products that can be obtained from CO₂, formic acid and formate salts are particularly important due to their ability to be used as an alternative fuel or a reversible hydrogen storage material. Utilization of molecular catalysts for CO₂ conversion offers several advantages such as high selectivity, mechanistic clarity, versatility, and stability, making them attractive for thermochemical and electro/photochemical CO₂ reduction processes. The presence of N-heterocyclic carbene (NHC) ligands in transition-metal-based molecular catalysts enhances the stability of the catalysts under harsh reaction conditions, such as high pressure, high temperature, and reductive environments, providing crucial benefits for sustained catalytic activity and longevity. Though the development of metal complex-based catalysts is essential to addressing the challenge of CO₂ reduction, the possibility of using purely organic compounds as catalysts for this transformation is lucrative from the aspect of developing a truly sustainable protocol with photosynthesis being its biggest inspiration. We begin this Account by examining our systematic development of molecular metal complexes based on NHC ligands for the chemical upgradation of CO₂ to formic acid/formate salt. In such cases, the ability of NHCs to act as strong σ-donor ligands for a greater hydride transfer propensity is discussed and analyzed. The reports range from catalytic ambient- and high-pressure CO₂ hydrogenation to CO₂ transfer-hydrogenation. Coupling of CO₂ capture methodologies with CO₂ conversion is also discussed. A case is made for the heterogenization of one of the highly efficient metal-NHC catalysts to develop a self-supported single-site catalyst for practical applications. Finally, our recent success of developing a novel organic catalyst system inspired from the natural NADP⁺/NADPH-based hydride-transfer redox couple that is active in photosynthetic CO₂ reduction has been discussed. This catalyst is designed based on a bis-imidazolium-embedded heterohelicene with a central pyridine ring and is capable of electrocatalytically converting CO₂ to HCO₂H with TON values 100-1000 times greater than the existing reported values achieved so far by organic catalysts. Overall, we believe that the results of hydride transfer-based CO₂ reduction catalysis presented in this Account hold significant implications beyond our work and have the potential for motivating future research toward further development in this important field.