A Ni(II) complex bearing an SN-type tetradentate ligand inspired by the active site of carbon monoxide dehydrogenase was found to selectively catalyze CO reduction to produce CO in a photocatalytic system using [Ru(bpy)] (bpy = 2,2'-bipyridine) as a photosensitizer and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as an electron donor. The Ni(II) complex shows a high turnover number over 700 with high CO selectivity of >99% and quantum yield of 1.42% in the photocatalytic system.

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.

We report homogeneous electrocatalytic and photocatalytic H evolution using two Ni(II) complexes with SN-type tetradentate ligands bearing two different sizes of chelate rings as catalysts. A Ni(II) complex with a five-membered SCS-Ni chelate ring (1) exhibited higher activity than that with a six-membered SCS-Ni chelate ring (2) in both electrocatalytic and photocatalytic H evolution despite both complexes showing the same reduction potentials. A stepwise reduction of the Ni center from Ni(II) to Ni(0) was observed in the electrochemical measurements; the first reduction is a pure electron transfer reaction to form a Ni(I) complex as confirmed by electron spin resonance measurements, and the second is a 1e/1H proton-coupled electron transfer reaction to afford a putative Ni(II)-hydrido (Ni-H) species. We also clarified that Ni(II) complexes can act as homogeneous catalysts in the electrocatalytic H evolution, in which complex 1 exhibited higher reactivity than that of 2. In the photocatalytic system using [Ru(bpy)] as a photosensitizer and sodium ascorbate as a reductant, complex 1 with the five-membered chelate ring also showed higher catalytic activity than that of 2 with the six-membered chelate ring, although the rates of photoinduced electron-transfer processes were comparable. The Ni-H bond cleavage in the putative Ni-H intermediate should be involved in the rate-limiting step as evidenced by kinetic isotope effects observed in both photocatalytic and electrocatalytic H evolution. Kinetic analysis and density functional theory calculations indicated that the difference in H evolution activity between the two complexes was derived from that of activation barriers of the reactions between the Ni-H intermediates and proton, which is consistent with the fact that increase of proton concentration accelerates the H evolution.