10⁶-fold faster C–H bond hydroxylation by a Co^III,IV₂(µ-O)₂ complex [via a Co^III₂(µ-O)(µ-OH) intermediate] versus its Fe^IIIFe^IV analog

The hydroxylation of C–H bonds can be carried out by the high-valent Co^III,IV₂(µ-O)₂ complex 2a supported by the tetradentate tris(2-pyridylmethyl)amine ligand via a Co^III₂(µ-O)(µ-OH) intermediate (3a). Complex 3a can be independently generated either by H-atom transfer (HAT) in the reaction of 2a with phenols as the H-atom donor or protonation of its conjugate base, the Co^III₂(µ-O)₂ complex 1a. Resonance Raman spectra of these three complexes reveal oxygen-isotope-sensitive vibrations at 560 to 590 cm⁻¹ associated with the symmetric Co–O–Co stretching mode of the Co₂O₂ diamond core. Together with a Co•••Co distance of 2.78(2) Å previously identified for 1a and 2a by Extended X-ray Absorption Fine Structure (EXAFS) analysis, these results provide solid evidence for their “diamond core” structural assignments. The independent generation of 3a allows us to investigate HAT reactions of 2a with phenols in detail, measure the redox potential and pK_a of the system, and calculate the O–H bond strength (D_O–H) of 3a to shed light on the C–H bond activation reactivity of 2a. Complex 3a is found to be able to transfer its hydroxyl ligand onto the trityl radical to form the hydroxylated product, representing a direct experimental observation of such a reaction by a dinuclear cobalt complex. Surprisingly, reactivity comparisons reveal 2a to be 10⁶-fold more reactive in oxidizing hydrocarbon C–H bonds than corresponding Fe^III,IV₂(µ-O)₂ and Mn^III,IV₂(µ-O)₂ analogs, an unexpected outcome that raises the prospects for using Co^III,IV₂(µ-O)₂ species to oxidize alkane C–H bonds.