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

Area of Science:

• Bioinorganic Chemistry
• Oxidation Catalysis
• Coordination Chemistry

Background:

• Non-heme diiron enzymes (NHFe2) activate O2 using a Fe(II)Fe(II) form, generating peroxo Fe(III)Fe(III) intermediates for electrophilic substrate oxidation.
• Existing peroxo Fe(III)Fe(III) model complexes lack the characteristic reactivity of NHFe2 enzymes.
• Understanding the mechanism of O2 activation and intermediate formation is crucial for designing synthetic catalysts.

Purpose of the Study:

• To synthesize and characterize an electrophilic {FeIII(μ-1,2-peroxo)(μ-O)FeIII} complex analogous to NHFe2 intermediates.
• To investigate the role of bases in the formation pathway of the peroxo intermediate.
• To explore the reactivity of the synthesized complex in oxygen and hydrogen atom transfer reactions.

Main Methods:

• Synthesis of a {FeII(μ-OH)2FeII} precursor using a bis(tetradentate) dinucleating ligand.
• In situ generation of the target complex via reaction with O2 in the presence and absence of bases (NEt3, DBU).
• Characterization using cryospray-ionization mass spectrometry, UV-vis-NIR, Mössbauer, resonance Raman, and X-ray absorption spectroscopies, supported by DFT calculations.
• Reactivity studies involving oxygen atom transfer with PPh3 and hydrogen atom transfer with 9,10-dihydroanthracene (DHA) and benzyl alcohol.

Main Results:

• An electrophilic {FeIII(μ-1,2-peroxo)(μ-O)FeIII} complex was successfully generated and characterized.
• The presence and strength of a base influenced the formation pathway, with strong bases like DBU promoting the desired μ-1,2-peroxo bridge formation.
• The synthesized complex exhibited unprecedented electrophilic reactivity in O-atom and H-atom transfer reactions, attributed to the ligand's electron-donating properties.

Conclusions:

• A synthetic model complex successfully mimics the reactive peroxo intermediate of non-heme diiron enzymes.
• Base-mediated deprotonation plays a critical role in generating the electrophilic μ-1,2-peroxo species.
• The study provides valuable insights into the mechanism of O2 activation by NHFe2 enzymes and demonstrates the potential of synthetic complexes in oxidation catalysis.