Computational Study of Photocatalytic CO₂ Reduction by a Ni(II) Complex Bearing an S₂N₂‑Type Ligand

Photocatalytic CO₂ reduction to CO by a Ni­(II) complex with an S₂N₂-type tetradentate ligand exhibits high efficiency and selectivity. Here, a density functional theory (DFT) study of the reaction mechanism is presented. Our calculations support that the four-coordinated Ni⁰ species formed through a photoinduced process participates in the actual catalytic reaction, which reduces CO₂ to CO and then regenerates the Ni^II species. A CO₂ adduct with the η²_CO binding mode is the precursor for the formation of CO, and once its first protonation is completed, further C–O bond cleavage is most likely to occur directly, yielding the CO molecule. Product selectivity calculations suggest that the Ni hydride complex is a crucial intermediate for the generation of H₂ and formic acid, while the former is more favorable than the latter. The high CO selectivity is mainly attributed to the fact that a much lower concentration of protons in comparison to that of CO₂ restricts the formation of the Ni hydride intermediate. In addition, we found that sulfur coordination could promote the formation of a CO₂ adduct.