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

Non-heme diiron enzymes (NHFe 2 ) activate O 2 by reaction with a reduced Fe II Fe II form, providing peroxo Fe III Fe III intermediates that are capable of electrophilic substrate oxidations. In contrast, the peroxo Fe III Fe 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 NHFe 2 in situ by the reaction of the {Fe II (μ-OH) 2 Fe II } precursor with O 2 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 NEt 3 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 PPh 3 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.