From Generation of Peroxo and High-Valent Diiron Complexes to Enantioselective C-H Oxidation Catalysts

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Enzymes with a non-heme diiron active site (NHFe2) activate dioxygen in biological systems to catalyze a variety of oxidation and/or oxygenation reactions of organic substrates, that are also interesting and promising for synthetic applications. One such reactivity is the selective C-H hydroxylation of not activated organic molecules including the least reactive substrate CH4 used by methane monooxygenase (sMMO). The catalytic cycles of NHFe2 generally employ a diferrous form that reacts with dioxygen to a peroxo-diferric intermediate. The active species is supposed to be either this peroxo-diferric species or a species derived from it. In sMMO, the peroxo intermediate, P, converts to a high-valent Fe(IV)Fe(IV) active species, Q. Despite strong worldwide efforts to obtain functional bioinspired diiron models for this reactivity, the actual catalytic performances are not good enough for application. Most ligand systems used so far are mainly comprised of nitrogen-containing donors. However, in sMMO there is a carboxylate-rich coordination environment, that is also characteristic for many other diiron enzymes. This implies, that the reactivity of these otherwise structurally very similar diiron active sites depends strongly on the terminal donors. In this respect, we want to establish a family of diiron complexes with dinucleating ligands that vary in their terminal donors including pyridine, imidazole, phenolate, and carboxylate and all kind of mixed forms to study their reactivity for building peroxo and/or high-valent diiron species. Besides the elucidation of the molecular and electronic structures of these peroxo and/or high-valent species, we will investigate their reactivities in the hydroxylation of selected substrates. As it is thought that the molecular and electronic structure dictate reactivity, we intend to obtain correlations between the molecular and electronic structures with their reactivities so that we can identify the best catalytic systems for selective C-H oxidations. For this, we will synthesize new derivatives of our family of dinucleating ligands with different terminal donors, we will synthesize their diiron complexes, we will try to obtain high-valent and peroxo species, and we will study their reactivites and catalytic abilities. However, we don’t want to stay with traditional substrates for C-H activation as cyclohexane, but we also want to introduce an enantioselectivity to this C-H bond activation. Therefore, we will try to introduce chirality into the dinucleating ligand system so that we can obtain chiral catalysts for the enantioselective C-H activation of carefully chosen substrates.