The “Pull Effect” of a Hanging ZnII on Improving the Four-Electron Oxygen Reduction Selectivity with Co Porphyrin

Abstract

Due to the challenge of cleaving O−O bonds at single Co sites, mononuclear Co complexes typically show poor selectivity for the four-electron (4e⁻) oxygen reduction reaction (ORR). Herein, we report on selective 4e⁻ ORR catalyzed by a Co porphyrin with a hanged Zn^II ion. Inspired by Cu/Zn-superoxide dismutase, we designed and synthesized 1-CoZn with a hanging Zn^II at the second sphere of a Co porphyrin. Complex 1-CoZn is much more effective than its Zn-lacking analogues to catalyze the 4e⁻ ORR in neutral aqueous solutions, giving an electron number of 3.91 per O₂ reduction. With spectroscopic studies, the hanging Zn^II was demonstrated to be able to facilitate the electron transfer from Co^II to O₂, through an electronic “pull effect”, to give Co^III-superoxo. Theoretical studies further suggested that this “pull effect” plays crucial roles in assisting O−O bond cleavage. This work is significant to present a new strategy of hanging a Zn^II to improve O₂ activation and O−O bond cleavage.