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.