Beyond the Oxo-Wall: Synthesis and Reactivity of a Six-Coordinate Formally Co^IV-Oxo Complex

Despite their implication in a number of oxidation processes, there is a paucity of well-characterized high-valent cobalt-oxo complexes. In this study, we present the first example of a six-coordinate formally Co(IV)-oxo complex, [Co(TQA)(O)(OH)]⁺ (1, TQA = tris(2-quinolylmethyl)amine). CSI-MS shows the incorporation of an oxygen atom from a peracid oxidant and an additional one from water. EPR spectroscopy shows that 1 contains an S = ³/₂ cobalt-center, consistent with the Co^IV oxidation state, while both resonance Raman (ν = 775 cm^-1) and EXAFS (Co─O = 1.69 Å) indicate the formation of a Co-oxo moiety. CASSCF(11,10)/NEVPT2 calculations suggest that 1 is best described as a Co(IV)-oxo compound with significant Co(III)-oxyl character contributing to the ground state wavefunction. Complex 1 is relatively kinetically stable at -85°C but decays at elevated temperatures (> -50°C). We find that 1 is a competent hydrogen atom transfer (HAT) and oxygen atom transfer (OAT) reagent capable of oxidizing a wide variety of substrates including toluene and N,N-dimethylaniline. Although H/D-kinetic isotope studies demonstrate that proton movement takes place during the HAT rate limiting step, the substrate oxidation rate depends on substrate oxidation potential and not the C─H BDFE, suggesting these reactions are controlled by the underlying electron-transfer process.