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 using spectroscopic, analytic, and crystallographic methods. The electrochemical properties of all complexes were studied by cyclic voltammetry under N2 and CO₂ atmospheres. Photocatalytic reduction of CO₂ to CO was analyzed using all complexes in the presence and absence of an external photosensitizer (PS). All of these complexes are active as photocatalysts for CO₂ reduction in the presence of PS with low to moderate turnover numbers (TON). Notably, three Ni(II) CNC-pincer complexes (3^OMe, 3^H, and 4^Me) 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 with a TON nearly seven times higher than its TON in the presence of a photosensitizer. 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 halide loss to occur later to allow for a 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 Ni⁰ catalytic cycles involving CO₂ binding, protonation of bound CO₂, and proton-coupled electron transfer leading to CO formation.

Sonya Manafe

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