A Reactive, Photogenerated High-Spin (<i>S</i>= 2) Fe^IV(O) Complex via O₂Activation

Abstract

Addition of dioxygen at low temperature to the non-heme ferrous complex Fe^II(Me₃TACN)((OSi^Ph2)₂O) (<b>1</b>) in 2-MeTHF produces a peroxo-bridged diferric complex Fe₂^III(μ-O₂)(Me₃TACN)₂((OSi^Ph2)₂O)₂ (<b>2</b>), which was characterized by UV-vis, resonance Raman, and variable field Mössbauer spectroscopies. Illumination of a frozen solution of <b>2</b> in THF with white light leads to homolytic O-O bond cleavage and generation of a Fe^IV(O) complex <b>4</b> (ν(Fe=O) = 818 cm^-1; δ = 0.22 mm s^-1, Δ<i>E</i>_Q = 0.23 mm s^-1). Variable field Mössbauer spectroscopy measurements show that <b>4</b> is a rare example of a high-spin <i>S</i> = 2 Fe^IV(O) complex and the first synthetic example to be generated directly from O₂. Complex <b>4</b> is highly reactive, as expected for a high-spin ferryl, and decays rapidly in fluid solution at cryogenic temperatures. This decay process in 2-MeTHF involves C-H cleavage of the solvent. However, the controlled photolysis of <b>2</b> <i>in situ</i> with visible light and excess phenol substrate leads to competitive phenol oxidation, via the proposed transient generation of <b>4</b> as the active oxidant.