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

Abstract

ABSTRACT 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.