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

Abstract

Pacman dinuclear Co^II triphenylporphyrin-tri(pentafluorophenyl)porphyrin 1 and dinuclear Co^II 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 Co^II tetra(pentafluorophenyl)porphyrin 3 and Co^II 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 Co^III/Co^II 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: Co^II-TPFP is likely the O₂ binding and reduction site, while Co^III-TPP, which is generated by the oxidation of Co^II-TPP on electrodes, may function as a Lewis acid to assist the O₂ binding and activation. The positively charged Co^III-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 Cu_B 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.