Formation Pathways of an Electrophilic μ-1,2-Peroxo Fe^IIIFe^III Complex: Spectroscopic Characterization and Reactivity

Non-heme diiron enzymes (NHFe₂) activate O₂ by reaction with a reduced Fe^IIFe^II form, providing peroxo Fe^IIIFe^III intermediates that are capable of electrophilic substrate oxidations. In contrast, the peroxo Fe^IIIFe^III model complexes reported so far do not show this reactivity. Using a bis(tetradentate) dinucleating ligand, we report here an electrophilic {Fe^III(μ-1,2-peroxo)(μ-O)Fe^III} complex that was generated in a manner analogous to NHFe₂ in situ by the reaction of the {Fe^II(μ-OH)₂Fe^II} precursor with O₂ in the presence of a base and characterized by cryospray-ionization mass spectrometry and UV-vis-NIR, Mössbauer, resonance Raman, and X-ray absorption spectroscopies in conjunction with DFT calculations. In the absence of a base, an end-on hydroperoxo intermediate {Fe^III(μ-O)Fe^III(OOH)} is formed, which reacts slowly with the weak base NEt₃ to give {Fe^III(μ-1,2-peroxo)(μ-O)Fe^III} without observable intermediates, while the strong base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) deprotonates the end-on hydroperoxo in a first fast step to form a side-on peroxo that converts in a second slower step to the μ-1,2-peroxo bridge. The {Fe^III(μ-1,2-peroxo)(μ-O)Fe^III} complex exhibits unprecedented electrophilic reactivity in oxygen atom transfer with PPh₃ and hydrogen atom transfer with 9,10-dihydroanthracene (DHA) and benzyl alcohol. This reactivity is discussed in terms of the ligand's electron-donating properties.