Speciation in iron epoxidation catalysis: A perspective on the discovery and role of non-heme iron(III)-hydroperoxo species in iron-catalyzed oxidation reactions

Oxidation reactions catalyzed by enzymes with central iron atoms are essential to life. The nature of these systems has evoked great interest to understand and model their reactivity. Considerable efforts have been made to elucidate the mechanisms that underlie the catalytic transformations. Many simplified, mononuclear model systems are known that allow for the spectroscopic and crystallographic characterization of intermediates relevant to this chemistry, such as iron(IV)-oxo, iron(V)-hydroxo oxo, iron(III)-peroxo, or iron(III)-superoxo complexes. One of the less researched, but still very decisive species is the iron(III)-hydroperoxo moiety that is assumed to be a crucial part of catalytic cycles during oxidation reactions with agents such as hydrogen peroxide, superoxides, or molecular oxygen. Although its molecular structure has not been resolved yet, a multitude of spectroscopic data unequivocally supports the existence of the hydroperoxo ligated iron center and its essential role in oxidation catalysis. The comprehension of the catalytic processes led to the development of powerful homogeneous catalysts for different oxidative conversions, such as dihydroxylation and epoxidation reactions.