NIO2ACTIVATION · O2-Activation at nickel complexes and their use as catalysts for environmentally friendly oxidation technologies

O2-Activation at nickel complexes and their use as catalysts for environmentally friendly oxidation technologies

The chemistry of dioxygen activation by metal complexes is of great importance. On one hand, such compounds serve as synthetic models to get insight into the peculiar reaction mechanisms operating in dioxygen-activating metalloproteins found in biological systems and on the other hand, they can be applied as catalysts in oxidative transformations. While the bioinspired investigation concerning iron- and copper-mediated dioxygen-activation has led to numerous stoichiometric and catalytic transformations of organic substrates, the chemistry of non-iron and non-copper systems remains largely unexplored. In particular, this type of chemistry for nickel centres is still in its infancy. Moreover, studies regarding the oxidizing ability of the resulting nickel-dioxygen species towards exogenous substrates are very scarce. The main objective of this research project was the study of the ability of nickel to activate molecular oxygen and the oxidizing potential of the resulting nickel- dioxygen species towards organic substrates. We have studied in detail the reactivity of an isolable and thermally stable nickel-superoxo compound [NiII(beta-diketiminato)(O2)] (LNiO2) towards different types of organic substrates. Interestingly, LNiO2 performs hydrogen-atom abstraction from O-H and N-H bonds and most importantly it shows an unprecedented dioxygenase-like activity in the oxidation of 2,4,6-tri-tert-butylphenol. The reaction occurs with full conversion into the dioxygenated product and the origin of the oxygen-atoms that end up into the oxidised product was unambiguously established by the use of 18O2 in combination with mass-spectrometry. From these experiments it was clear that the two oxygens incorporated into the final product originate from a single nickel-superoxo unit (dioxygenase-like activity). This type of reaction shows no precedent in the literature and it seems to be exclusive of nickel because the corresponding metal-superoxo compounds based on copper and cobalt do not exhibit this behaviour. It is postulated that this reaction occurs through the mediation of a putative [Ni(III)-oxo] intermediate, which is energetically plausible on the basis of theoretical calculations. On the other hand, LNiO2 can also interact with an iron(I) complex to generate an heterobimetallic compound with a NiO2Fe core. This compound forms a highly reactive heterobimetallic core that can even perform the intramolecular ligand monohydroxylation, thus exhibiting monooxygenase activity (see scheme). The reactive intermediate responsible for the observed chemistry is postulated to be a Ni-Fe bis(mu-oxo) species on the basis of theoretical calculations. Overall, the results obtained in this work evidence the potential oxidizing power of Ni-dioxygen species and further support the viability to use this metal in oxidation catalysts analogously to its heavy metal congeners, palladium and platinum. The findings unravelled in this project help in laying the basis for the use of nickel in oxidation catalysis, which nowadays constitutes a fundamental issue in industrial processes.

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