Concerted proton-electron transfer oxidation of phenols and hydrocarbons by a high-valent nickel complex

The high-valent nickel(III) complex Ni(pyalk)₂⁺ (2) was prepared by oxidation of a nickel(II) complex, Ni(pyalk)₂ (1) (pyalk = 2-pyridyl-2-propanoate). 2 and derivatives were fully characterized by mass spectrometry and x-ray crystallography. Electron paramagnetic resonance spectroscopy and x-ray photoelectron spectroscopy confirm that the oxidation is metal-centered. 2 was found to react with a variety of phenolic and hydrocarbon substrates. A linear correlation between measured rate constant and substrate bond dissociation enthalpy (BDE) was found for both phenolic and hydrocarbon substrates. Large H/D kinetic isotope effects were also observed for both sets of substrates. These results suggest that 2 reacts through concerted proton-electron transfer (CPET). Analysis of measured thermodynamic parameters allows us to calculate a bond dissociation free energy (BDFE) of ~91 kcal/mol for the O-H bond of the bound pyalk ligand. These findings may shed light onto CPET steps in oxidative catalysis and have implications for ligand design in catalytic systems.