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

Selective and efficient catalytic two-electron two-proton (2e-/2H+) reduction of dioxygen (O2) to hydrogen peroxide (H2O2) has been achieved less successfully than four-electron four-proton (4e-/4H+) reduction of O2 because of the instability of the H2O2 product in the presence of metal catalysts. We report herein a highly efficient 2e-/2H+ reduction of O2 by ferrocene (Fc; one-electron reductant) and 9,10-dihydro-10-methylacridine (AcrH2; two-electron reductant and an NADH analog), with a cobalt corrole complex, [CoIII(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 O2 using AcrH2 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 O2 by Fc with 1 in the presence of HClO4 in CH3CN was the proton-coupled electron transfer from CoIII(tpfc) (2) to O2 to produce a cobalt(III) corrole radical cation, [CoIII(tpfc•+)]+ (3), which was detected by EPR. When ferrocene was replaced by AcrH2, the rds became the electron transfer from AcrH2 to 3, coupled with the deprotonation of AcrH2•+ (ET/PT), followed by fast ET from AcrH• 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 O2 in producing H2O2 with a high selectivity.