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Issue 20, 2016
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The effects of chelating N4 ligand coordination on Co(II)-catalysed photochemical conversion of CO2 to CO: reaction mechanism and DFT calculations

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Abstract

Here we describe the synthesis and X-ray crystal structures of a panel of cis-[CoII(N4)Cl2] complexes (N4 = tetradentate N atom donor ligand). We also examine the catalytic activities of these complexes in the photochemical and electrochemical reduction of CO2 to CO using [Ir(ppy)3] as the photosensitizer. Among the complexes studied, cis-[CoII(PDP)Cl2] (C1) (PDP = 1,1′-bis(2-pyridinylmethyl)-2,2′-bipyrrolidine) displayed the highest catalytic activity. This Co(II) complex was able to effectively mediate the reduction of CO2 to CO under either electrochemical or visible light photocatalytic conditions. For the electrocatalysis, C1 catalysed CO2 to CO with up to 96% Faraday efficiency at −1.70 V (vs. SCE, SCE = saturated calomel electrode). A selectivity of up to 95% for CO production was achieved in a photocatalytic CO2 reduction system by using C1 as the catalyst, Ir(ppy)3 as the photosensitizer and triethylamine as the electron donor. The Co(I) species in situ generated by the one electron reduction of cis-[CoII(PDP)Cl]+ is suggested to be directly responsible for CO2 activation. Ultrafast time (ns) resolved absorption spectroscopy revealed that the photoinduced electron transfer from the triplet excited state of Ir(ppy)3 to C1 is a key step in the generation of active Co(I) species. The electronic structure and redox properties of the Co(I) species, [CoI(N4)Cl], as well as its role in the catalytic reaction were investigated by DFT calculations. The presence of one chloride ligand cis to the CO2 coordination site neutralizes the positive charge on the Co(I) centre, therefore assisting the bound CO2 molecule in attracting protons. The reaction mechanism for CO2 reduction to CO catalysed by the recently reported [CoII(TPA)Cl]+ (TPA = tris(2-pyridylmethyl)amine) catalyst was also computed. Subtle modifications of the chelating N4 ligand from cis-[CoII(N4)Cl2] were found to have a profound effect on the efficiency of CO2 reduction by DFT calculations.

Graphical abstract: The effects of chelating N4 ligand coordination on Co(ii)-catalysed photochemical conversion of CO2 to CO: reaction mechanism and DFT calculations

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Publication details

The article was received on 10 Jun 2016, accepted on 23 Jul 2016 and first published on 15 Aug 2016


Article type: Paper
DOI: 10.1039/C6CY01265E
Citation: Catal. Sci. Technol., 2016,6, 7408-7420

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    The effects of chelating N4 ligand coordination on Co(II)-catalysed photochemical conversion of CO2 to CO: reaction mechanism and DFT calculations

    F. Wang, B. Cao, W. To, C. Tse, K. Li, X. Chang, C. Zang, S. L. Chan and C. Che, Catal. Sci. Technol., 2016, 6, 7408
    DOI: 10.1039/C6CY01265E

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