Earth-Abundant Mixed-Metal Catalysts for Hydrocarbon Oxygenation

The oxygenation of aliphatic and aromatic hydrocarbons using earth-abundant Fe and Cu catalysts and "green" oxidants such as hydrogen peroxide is becoming increasingly important to atom-economical chemical processing. In light of this, we describe that dinuclear Cu^II complexes of pyrrolic Schiff-base macrocycles, in combination with ferric chloride (FeCl₃), catalyze the oxygenation of π-activated benzylic substrates with hydroperoxide oxidants at room temperature and low loadings, representing a novel design in oxidation catalysis. Mass spectrometry and extended X-ray absorption fine structure analysis indicate that a cooperative action between Cu^II and Fe^III occurs, most likely because of the interaction of FeCl₃ or FeCl₄^- with the dinuclear Cu^II macrocycle. Voltammetric measurements highlight a modulation of both Cu^II and Fe^III redox potentials in this adduct, but electron paramagnetic resonance spectroscopy indicates that any Cu-Fe intermetallic interaction is weak. High ketone/alcohol product ratios, a small reaction constant (Hammett analysis), and small kinetic isotope effect for H-atom abstraction point toward a free-radical reaction. However, the lack of reactivity with cyclohexane, oxidation of 9,10-dihydroanthracene, oxygenation by the hydroperoxide MPPH (radical mechanistic probe), and oxygenation in dinitrogen-purge experiments indicate a metal-based reaction. Through detailed reaction monitoring and associated kinetic modeling, a network of oxidation pathways is proposed that includes "well-disguised" radical chemistry via the formation of metal-associated radical intermediates.