Highly Efficient Catalytic Two-Electron Two-Proton Reduction of Dioxygen to Hydrogen Peroxide with a Cobalt Corrole Complex

Abstract

Selective and efficient catalytic two-electron two-proton (2e(-)/2H(+)) reduction of dioxygen (O-2) to hydrogen peroxide (H2O2) has been achieved less successfully than four-electron four-proton (4e(-)/4H(+)) reduction of O-2 because of the instability of the H2O2 product in the presence of metal catalysts. We report herein a highly efficient 2e(-)/2H(+) reduction of O-2 by ferrocene (Fc; one-electron reductant) and 9,10-dihydro-10-methylacridine (AcrH(2); two-electron reductant and an NADH analog), with a cobalt corrole complex, [Co-III(tpfc)(Py)(2)] (1), in the presence of HClO4 in CH3CN at 298 K, affording a large TON (50,000) and a high turnover frequency (275 s(-1)) with 100% selectivity in producing H2O2. The H2O2 product yielded in the 2e(-)/2H(+) reduction of O-2 using AcrH(2) as a reductant was stable for more than 5 h even in the presence of 1. Detailed kinetic analysis revealed that the rate-determining step (rds) for the 2e(-)/2H(+) reduction of O-2 by Fc with 1 in the presence of HClO4 in CH3CN was the proton-coupled electron transfer from Co-III(tpfc) (2) to O-2 to produce a cobalt(III) corrole radical cation, [CoIII(tpfc(+))](+) (3), which was detected by EPR. When ferrocene was replaced by AcrH(2), the rds became the electron transfer from AcrH(2) to 3, coupled with the deprotonation of AcrH(2)(center dot+) (ET/PT), followed by fast ET from AcrH(center dot) to 3. To the best of our knowledge, this is a report of a highly efficient Co-corrole catalyst that retains its cobalt(III) oxidation state for the 2e(-)/2H(+) reduction of O-2 in producing H2O2 with a high selectivity.