The "Pull Effect" of a Hanging Zn II on Improving the Four-Electron Oxygen Reduction Selectivity with Co Porphyrin.

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 O 2 reduction. With spectroscopic studies, the hanging Zn II was demonstrated to be able to facilitate the electron transfer from Co II to O 2 , 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 O 2 activation and O-O bond cleavage.