The homolytic activation of alkyl hydroperoxides with transition metal ions able to undergo one-electron switches such as CoII/III, FeII/III, plays an important role in many industrially relevant oxidation processes (for instance, the oxidation of p-xylene to therephatlic acid, and cyclohexane to cyclohexanol/cyclohexanone). Despite their great importance, and the ongoing trend to synthesize heterogeneous analogues to the homogeneous catalysts, stable under reaction conditions, the elementary reaction steps in the mechanism are still poorly understood. According to the textbook mechanisms, CoII, as an archetypical example, is able to generate radicals from alkyl hydroperoxides following the Haber-Weiss cycle (or better, Fenton reaction):
CoII + ROOH → CoIIIOH + RO●
CoIIIOH + ROOH → CoII + H2O + ROO●
This mechanism implies the CoIIIOH to be an important reaction intermediate. However, very recent quantum-chemical calculations performed at our laboratory demonstrated that CoIIIOH species react very fast with ROOH due to two-state-reactivity.[1] These new computational results put forward Co-OOR as the crucial intermediate. This species can either eliminate ROO● and regenerate the active CoII species, or trigger deactivation of the cycle. The computed barrier for Co-OOR dissociation is in quantitative agreement with the measured activation energy of the overall reaction, supporting our hypothesis that this is the rate-determining step. Various approaches to the quantification of the reaction kinetics will be discussed, ranging from titrations, to in situ UV-Vis spectroscopy.[1,2] Furthermore, experimental evidence, supported by quantum-chemical calculations, shows that dimeric cobalt species are responsible for the appearance of a maximum in the ROOH-decomposition activity versus cobalt concentration. These findings provide a crucial step towards fully understanding this relevant system.
1. E. Spier, U. Neuenschwander, I. Hermans, submitted.
2. N. Turrà, U. Neuenschwander, A. Baiker, J. Peeters, I. Hermans, Chemistry, a European Journal 2010, 16, 13226-13235.