A dinuclear iron complex based on parallel malonate binding sites: cooperative activation of dioxygen and biomimetic ligand oxidation.

A ligand that offers two parallel malonate binding sites linked by a xanthene backbone, namely, Xanthmal2-, has been utilised to synthesise dinuclear FeII complex [Fe2(Xanthmal)2] (1). The reactivity of 1 in contact with O2 was investigated at -40 degrees C and room temperature. After activation of O2 through interaction with both iron centres the ligand is oxidised: at the Calpha position monooxygenation and peroxide formation occur, partially accompanied by C-C bond cleavage to yield alpha-keto ester groups. To reveal mechanistic details investigations concerning 1) peroxide decomposition, 2) the reactivity of a corresponding mononuclear complex, 3) the influence of monooxygenation of the ligand on the reactivity and 4) product formation in dependence on time were carried out. The results can be explained by postulating formation of high-valent Fe intermediates and ligand-to-metal electron transfer, and the mechanistic scheme derived includes several steps that mimic the (suggested) functioning of non-heme iron enzymes. In agreement with this proposal, ligand oxidation can also be performed catalytically. Furthermore, we show that via a competitive route [(Xanthmal)2Fe2O] (2) is formed, which is unreactive towards O2 and thus is a dead end with respect to ligand oxidation. Both compounds 1 and 2 were fully characterised, and their properties are discussed.