A Nonheme μ-1,2-Peroxo Dicobalt(III) Complex Acting as a Key Intermediate for Selective Two-Electron/Two-Proton Catalytic Reduction of Dioxygen to Hydrogen Peroxide

The two-electron/two-proton reduction of O2 to H2O2 has attracted considerable attention recently, as H2O2 is both a valuable oxidant and a potential fuel for H2O2 fuel cells. While metalloporphyrins have been extensively studied as efficient catalysts for this reaction, nonheme metal complexes remain largely underexplored and key catalytic intermediates and rate-determining steps have rarely been identified. Herein, we report the isolation, along with the structural and spectroscopic characterization, of a nonheme μ-1,2-peroxo dicobalt(III) complex, [Co2(μ-1,2-O2)(L)2]2+ (HL = 2-[benzyl[bis(pyridin-2-yl)methyl]amino]-N-(quinolin-8-yl)acetamide), which serves as a key intermediate in the selective two-electron/two-proton catalytic reduction of O2. In the presence of trifluoroacetic acid in acetonitrile at 298 K, ferrocene derivatives act as electron donors to produce H2O2 with 100(2)% selectivity; the generated H2O2 is stable for up to 3 h under the catalytic conditions. The catalytic cycle has been elucidated through an analysis of both overall kinetics and the kinetics of individual steps. The rate-determining step in the catalytic cycle is the electron transfer from 1,1′-dimethylferrocene (Me2Fc) to the mononuclear Co(III) complex, [CoIII(L)]2+, generating 1,1′-dimethylferrocenium ion (Me2Fc+) and [CoII(L)]+. The resulting Co(II) species reacts with O2 to form [CoIII2(μ-1,2-O2)(L)2]2+, which upon protonation yields H2O2 and regenerates two equivalents of [CoIII(L)]2+. This study establishes a new catalytic pathway via the μ-1,2-peroxo dicobalt(III) complex for the selective reduction of O2 to H2O2 using a nonheme metal complex with perfect selectivity. Furthermore, the one-electron oxidation of the μ-1,2-peroxo dicobalt(III) complex affords the corresponding μ-1,2-superoxo dicobalt(III) complex, which was characterized by EPR, CSI-MS, and resonance Raman spectroscopies.