Mechanism of the Catalytic Deperoxidation of tert-Butylhydroperoxide with Cobalt(II) Acetylacetonate

The Co-II/Co-III-induced decomposition of hydroperoxides is an important reaction in many industrial processes and is referred to as deperoxidation In the first step of the so-called Haber-Weiss cycle alkoxyl radicals and Co-III-OH species are generated upon the reaction of the Coll ion with ROOH The catalytic cycle is closed upon the regeneration of the Coll ion through the reaction of the Cow-OH species with a second ROOH molecule, thus producing one equivalent of the peroxyl radicals Herein, the deperoxidation of tert butylhydroperoxide by dissolved cobalt(II) acetylacetonate is studied by using UV/Vis spectroscopy in situ with a noninteracting solvent, namely, cyclohexane Kinetic information extracted from experiments together with quantum chemical calculations, led to new mechanistic hypotheses Even under anaerobic conditions, the Haber-Weiss cycle initiates a radical chain destruction of ROOH propagated by both alkoxyl and peroxyl radicals This chain mechanism rationalizes the high deperoxidation rates, which are directly proportional to the cobalt concentration up to approximately 75 mu m at 333 K However, at higher cobalt concentrations, a remarkable de crease of the rate is observed The hypothesis put forward herein is that this remarkable autoinhibition effect could be explained by the hitherto over looked chain termination of two Co-III-OH species The direct competition be tween the first-order Haber-Weiss initiation and the second-order termination can indeed explain this peculiar kinetic behavior of this homogeneous de peroxidation system