Biomimetic Oxidation Catalysis

MSc Delia Hagemann, MSc Ronja Korbus, MSc Marwin Käthner, MSc Erik Niekamp, Dr. Jan Oldengott

The C-H bond is one of the most difficult to activate bonds in organic chemistry. The selective activation of inert C-H bonds are of strong interest for the preparation of basic and fine chemicals from natural oil, gas and coal resources and for the synthesis of complex agents, e. g. pharmaceutical products. Nature frequently uses dinuclear metallo-enzymes for selective C-H activations. The metal ions (mainly Fe and Cu) in the active sites of these enzymes store the oxidation and / or oxygenation equivalents of dioxygen during catalytic cycles. Examples for copper containing enzymes are tyrosinase and particulate methane monooxygenase, while soluble methane monooxygenase (sMMO) is the most prominent example for an iron containing enzyme for C-H activation. However, there is a growing class of non-heme diiron enzymes (NHFe2), which enable a breadth of divergent reactivities in natural product biosynthesis that emanate from a similar diiron core.
In general, the reduced FeIIFeII form reacts with O2 to yield a m-1,2-peroxo FeIIIFeIII intermediate (P), which is in most NHFe₂ enzymes the catalytically active species. In some cases, P intermediates undergo a proton-promoted activation step to yield P¢-type intermediates, for which FeIIIFeIII with m-1,1-peroxo, m-1,1-, or m-1,2-hydroperoxo ligands have been proposed. In sMMO, intermediate P converts in an also proton-promoted step to a high-valent FeIVFeIV active species (Q), which is generally believed to be the active intermediate for methane oxidation.

In order to develop better oxidation catalysts inspired by biology, we have developed a bis(tetradentate) dinucleating ligand system to generate dinuclear peroxo and high-valent species with a special focus on the correlation of their stabilities and reactivities as a function of the terminal donors.

The terminal donors ranges from carboxylates, phenolates, imidazoles, to differently substituted pyridines:

The ligand susan6-Me stabilizes the µ-1,2-peroxo complex [(susan6-Me){FeIII(µ-1,2-peroxo)(µ-O)FeIII}](ClO4)2, which was obtained from the reaction of [(susan6-Me){FeII(µ-OH)2FeII}]2+ with O2 and characterized in the solid state including sc-XRD and magnetism as well as in solution spectroscopically and electrochemically. The reversible oxidation provides the unprecedented high-valent peroxo complex [(susan6-Me){FeIV(µ-1,2-peroxo)(µ-O)FeIII}]3+ with a FeIV ion in local high spin state (Si = 2) and the reversible protonation to the also unprecedented µ-1,2-hydroperoxo complex [(susan6-Me){FeIII(µ-1,2-OOH)(µ-O)FeIII}]3+ as a model for P'-type intermediates.

In contrast, the µ-1,2-peroxo complex of susan is not stable but therefore reactive in HAT and OAT and is the first peroxo diferric model complexes showing oxidative reac-tivity as in the enzymes

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