Sensitized and Self-Sensitized Photocatalytic CO₂ Reduction to HCO₂ ^- and CO under Visible Light with Ni(II) CNC-Pincer Catalysts

Robust earth-abundant transition metal-based photocatalysts are needed for photocatalytic CO2 reduction. A series of six Ni(II) complexes have been synthesized with a tridentate CNC pincer ligand composed of two imidazole or benzimidazole-derived N-heterocyclic carbene (NHC) rings and a pyridyl ring with different R substituents (R = OMe, Me, H) para to N of the pyridine ring. These complexes have been characterized by using spectroscopic, analytic, and crystallographic methods. The electrochemical properties of all complexes were studied by cyclic voltammetry under N-2 and CO2 atmospheres. Photocatalytic reduction of CO2 to CO and HCO2- was analyzed using all of the complexes in the presence and absence of an external photosensitizer (PS). All of these complexes are active as photocatalysts for CO2 reduction with and without the presence of an external PS with appreciable turnover numbers (TONs) for formate (HCO2-) production and typically lower amounts of CO. Notably, all Ni(II) CNC-pincer complexes in this series are also active as self-sensitized photocatalysts. Complex 4(Me) with a benzimidazole-derived CNC pincer ligand was found to be the most active self-sensitized photocatalyst. Ultrafast transient absorption spectroscopy (TAS) experiments and computational studies were performed to understand the mechanism of these catalysts. Whereas sensitized catalysis involves halide loss to produce more active complexes, self-sensitized catalysis requires some halide to remain coordinated to allow for favorable electron transfer between the excited nickel complex and the sacrificial electron donor. This then allows the nickel complex to undergo CO2 reduction catalysis via Ni-I or Ni-0 catalytic cycles. The two active species (Ni-I and Ni-0) demonstrate distinct reactivity and selectivity which influences the formation of CO vs formate as the product.