Efficient Photocatalytic CO₂ Reduction by a Ni(II) Complex Having Pyridine Pendants through Capturing a Mg²⁺ Ion as a Lewis-Acid Cocatalyst

We have synthesized a new Ni(II) complex having an S₂N₂-tetradentate ligand with two noncoordinating pyridine pendants as binding sites of Lewis-acidic metal ions in the vicinity of the Ni center, aiming at efficient CO production in photocatalytic CO₂ reduction. In the presence of Mg²⁺ ions, enhancement of selective CO formation was observed in photocatalytic CO₂ reduction by the Ni complex with the pyridine pendants through the formation of a Mg²⁺-bound species, as compared to the previously reported Ni complex without the Lewis-acid capturing sites. A higher quantum yield of CO evolution for the Mg²⁺-bound Ni complex was determined to be 11.1%. Even at lower CO₂ concentration (5%), the Ni complex with the pendants exhibited comparable CO production to that at the CO₂-saturated concentration (100%). The Mg²⁺-bound Ni complex was evidenced by mass spectrometry and ¹H NMR measurements. The enhancement of CO₂ reduction by the Mg²⁺-bound species should be derived from cooperativity between the Ni and Mg centers for the stabilization of a Ni-CO₂ intermediate by a Lewis-acidic Mg²⁺ ion captured in the vicinity of the Ni center, as supported by DFT calculations. The detailed mechanism of photocatalytic CO₂ reduction by the Ni complex with the pyridine pendants in the presence of Mg²⁺ ions is discussed based on spectroscopic detection of the intermediate and kinetic analysis.