Electronic effects promoted the catalytic activities of binuclear Co(II) complexes for visible-light-driven CO2 reduction in a water-containing system.

Under the action of a catalyst, the photoinduced reduction of CO2 to chemicals and fuels is one of the greenest and environment-friendly approaches for decreasing atmospheric CO2 emissions. Since the environment was affected by the greenhouse effect, scientists have never stopped exploring efficient photoinduced CO2 reduction systems, particularly the highly desired non-noble metal complexes. Most of the currently reported complexes based on non-noble metals exhibit low catalytic activity, selectivity, and stability in aqueous systems under the irradiation of visible light. Herein, we report a new binuclear cobalt complex [Co2(L1)(OAc)2](OAc) (Co2L1, HL1 = 2,6-bis((bis(pyridin-2-ylmethyl)amino)methyl)-4-methoxyphenol), which accelerates the visible-light-driven conversion of CO2 to CO in acetonitrile/water (4/1, v/v) nearly 40% more than that for the previously reported [Co2(L2)(OAc)2](OAc) (Co2L2, HL2 = 2, 6-bis((bis(pyridin-2-ylmethyl)amino)methyl)-4-(tert-butyl)phenol) by our research group. It has an excellent CO selectivity of 98%, and the TONCO is as high as 5920. Experimental results and DFT calculations showed that the enhanced catalytic performance of Co2L1 is due to the electron-donating effect of a methoxy group (-OCH3) in Co2L1 compared to a tertiary butyl group (-C(CH3)3) in Co2L2, which reduces the energy barrier of the rate-limiting CO2 coordination step in the visible-light-driven CO2 reduction process.

Highly Efficient Visible-Light-Driven CO2-to-CO Conversion by Coordinatively Unsaturated Co-Salen Complexes in a Water-Containing System.

The development of cost-effective catalysts for CO2 reduction is highly desired but remains a significant challenge. The unsaturated coordination metal center in a catalyst is favorable for the process of catalytic CO2 reduction. In this paper, two asymmetric salen ligands were used to synthesize two coordinatively unsaturated Co-salen complexes. The two Co-salen complexes exhibit an unsaturated coordination pattern and display high activity and CO selectivity for visible-light-driven CO2 reduction in a water-containing system. The photocatalytic performance of 2 is higher than that of 1 because the reduction potential of the catalytic CoII center and the energy barrier of the catalytic transition states of 2 are lower than those of 1, with turnover numbers (TONCO), turnover frequencies (TOF), and CO selectivity values of 8640, 0.24 s-1, and 97% for 2, respectively. The photocatalytic reduction of CO2 to CO for 2 is well supported by control experiments and density functional theory (DFT) calculations.