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

Abstract

Pacman dinuclear Co^II triphenylporphyrin-tri(pentafluorophenyl)porphyrin <b>1</b> and dinuclear Co^II bis-tri(pentafluorophenyl)porphyrin <b>2</b>, anchored at the two <i>meso</i>-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 <b>3</b> and Co^II tetraphenylporphyrin <b>4</b> 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, <b>1</b> outperforms <b>2</b>, in terms of larger catalytic ORR currents and lower overpotentials. Electrochemical studies showed different redox behaviors of the two Co sites of <b>1</b>: 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 <i>c</i> oxidase (C<i>c</i>O), 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.