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Significantly improved electrocatalytic oxygen reduction by an asymmetrical Pacman dinuclear cobalt(ii) porphyrin–porphyrin dyad

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Abstract

Pacman dinuclear CoII triphenylporphyrin-tri(pentafluorophenyl)porphyrin 1 and dinuclear CoII bis-tri(pentafluorophenyl)porphyrin 2, anchored at the two meso-positions of a benzene linker, are synthesized and examined as electrocatalysts for the oxygen reduction reaction (ORR). Both dinuclear Co bisporphyrins are more efficient and selective than corresponding mononuclear CoII tetra(pentafluorophenyl)porphyrin 3 and CoII tetraphenylporphyrin 4 for the four-electron electrocatalytic reduction of O2 to water. Significantly, although the ORR selectivities of the two dinuclear Co bisporphyrins are similar to each other, 1 outperforms 2, in terms of larger catalytic ORR currents and lower overpotentials. Electrochemical studies showed different redox behaviors of the two Co sites of 1: the CoIII/CoII reduction of the Co-TPP (TPP = triphenylporphyrin) site is well-behind that of the Co-TPFP (TPFP = tri(pentafluorophenyl)porphyrin) site by 440 mV. This difference indicated their different roles in the ORR: CoII-TPFP is likely the O2 binding and reduction site, while CoIII-TPP, which is generated by the oxidation of CoII-TPP on electrodes, may function as a Lewis acid to assist the O2 binding and activation. The positively charged CoIII-TPP will have through-space charge interactions with the negatively charged O2-adduct unit, which will reduce the activation energy barrier for the ORR. This effect of Co-TPP closely resembles that of the CuB site of metalloenzyme cytochrome c oxidase (CcO), which catalyzes the biological reduction of O2. This work represents a rare example of asymmetrical dinuclear metal catalysts, which can catalyze the 4e reduction of O2 with high selectivity and significantly improved activity.

Graphical abstract: Significantly improved electrocatalytic oxygen reduction by an asymmetrical Pacman dinuclear cobalt(ii) porphyrin–porphyrin dyad

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Publication details

The article was received on 07 Oct 2019, accepted on 03 Nov 2019 and first published on 04 Nov 2019


Article type: Edge Article
DOI: 10.1039/C9SC05041H
Chem. Sci., 2020, Advance Article
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    Significantly improved electrocatalytic oxygen reduction by an asymmetrical Pacman dinuclear cobalt(II) porphyrin–porphyrin dyad

    Y. Liu, G. Zhou, Z. Zhang, H. Lei, Z. Yao, J. Li, J. Lin and R. Cao, Chem. Sci., 2020, Advance Article , DOI: 10.1039/C9SC05041H

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