The effects of chelating N4 ligand coordination on Co(ii)-catalysed photochemical conversion of CO2 to CO: reaction mechanism and DFT calculations. Issue 20 (15th August 2016)

• Record Type: Journal Article
• Authors: Wang, Feng; Cao, Bei; To, Wai-Pong; Tse, Chun-Wai; Li, Kai; Chang, Xiao-Yong; Zang, Chao; Chan, Sharon Lai-Fung; Che, Chi-Ming
• Is Part Of: Catalysis science & technology. Volume 6:Issue 20(2016)
• Page Start: 7408
• Page End: 7420
• Publication Date: 2016-08-15
• Subjects: Catalysis -- Periodicals; 541.395
• DOI: 10.1039/c6cy01265e
• Languages: English
• ISSNs: 2044-4753

Abstract

Abstract : cis -[Co(PDP)Cl2 ] complex mediated reduction conversion of CO2 to CO under photocatalytic or electrocatalytic conditions with high turnovers or Faraday efficiency. Abstract : Here we describe the synthesis and X-ray crystal structures of a panel of cis -[Co II (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 -[Co II (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 usingC1 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 -[Co II (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 toC1 is a key step in the generation of active Co(i ) species. The electronic structure and redox properties of the Co(i ) species, [Co I (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 [Co II (TPA)Cl] + (TPA = tris(2-pyridylmethyl)amine) catalyst was also computed. Subtle modifications of the chelating N4 ligand from cis -[Co II (N4 )Cl2 ] were found to have a profound effect on the efficiency of CO2 reduction by DFT calculations.