Efficient Photocatalytic CO 2 Reduction by a Ni(II) Complex Having Pyridine Pendants through Capturing a Mg 2+ Ion as a Lewis-Acid Cocatalyst

• Published in: Journal of the American Chemical Society Vol. 141; no. 51; pp. 20309 - 20317
• Main Authors: Hong, Dachao, Kawanishi, Takuya, Tsukakoshi, Yuto, Kotani, Hiroaki, Ishizuka, Tomoya, Kojima, Takahiko
• Format: Journal Article
• Language: English
• Published: United States 26.12.2019
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.9b10597

We have synthesized a new Ni(II) complex having an S N -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 CO reduction. In the presence of Mg ions, enhancement of selective CO formation was observed in photocatalytic CO reduction by the Ni complex with the pyridine pendants through the formation of a Mg -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 Mg -bound Ni complex was determined to be 11.1%. Even at lower CO concentration (5%), the Ni complex with the pendants exhibited comparable CO production to that at the CO -saturated concentration (100%). The Mg -bound Ni complex was evidenced by mass spectrometry and H NMR measurements. The enhancement of CO reduction by the Mg -bound species should be derived from cooperativity between the Ni and Mg centers for the stabilization of a Ni-CO intermediate by a Lewis-acidic Mg ion captured in the vicinity of the Ni center, as supported by DFT calculations. The detailed mechanism of photocatalytic CO reduction by the Ni complex with the pyridine pendants in the presence of Mg ions is discussed based on spectroscopic detection of the intermediate and kinetic analysis.