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 NiII species. A CO2 adduct with the η2CO 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.