Mechanism of phenol oxidation by heterodinuclear Ni Cu bis(μ-oxo) complexes involving nucleophilic oxo groups

Abstract

Oxidation of phenols by heterodinuclear Cu(III)(μ-O)2Ni(III) complexes containing nucleophilic oxo groups occurs by both proton coupled electron transfer (PCET) and hydrogen atom transfer (HAT) mechanisms; the exact mechanism depends on the nature of the phenol as well as the substitution pattern of the ligand bound to Cu.

Figure 1

A) Changes in the absorption spectra associated with the reaction of 1 (0.2 mM in CH₂Cl₂) with 2,4-di-tert-butylphenol (100 equiv) at -90 °C. The inset shows the time trace of the decay of the 895 nm band upon addition of 2,4-di-tert-butylphenol, which can be fitted to a pseudo first-order kinetic model to obtain a first-order rate constant (k_obs). Figures 1B and 1C depict the linear dependence of k_obs on the concentrations of 2,4-di-tert-butylphenol and the mono-deuterated analogue for the reactions with 1 and 2, respectively.

Figure 2

Plots of (RT/F)ln(k₂) against the oxidation potential (E^ox) of ArOH for the reactions of ArOH with 1 (Figure 2A) and 2 (Figure 2B) in CH₂Cl₂.

Scheme 1

Possible reaction pathways for the oxidation of phenols.

Scheme 2

[(^RAN)Cu^III(μ-O)Ni^III] (^R = Me for 1 and ^R = H for 2) Complexes used in this study for the oxidation of phenols.