A Reactive, Photogenerated High-Spin (S = 2) FeIV(O) Complex via O2 Activation

Addition of dioxygen at low temperature to the non-heme ferrous complex Fe^II(Me₃TACN)((OSi^Ph2)₂O) (1) in 2-MeTHF produces a peroxo-bridged diferric complex Fe₂^III(μ-O₂)(Me₃TACN)₂((OSi^Ph2)₂O)₂ (2), which was characterized by UV-vis, resonance Raman, and variable field Mössbauer spectroscopies. Illumination of a frozen solution of 2 in THF with white light leads to homolytic O-O bond cleavage and generation of a Fe^IV(O) complex 4 (ν(Fe=O) = 818 cm^-1; δ = 0.22 mm s^-1, ΔE_Q = 0.23 mm s^-1). Variable field Mössbauer spectroscopy measurements show that 4 is a rare example of a high-spin S = 2 Fe^IV(O) complex and the first synthetic example to be generated directly from O₂. Complex 4 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 2 in situ with visible light and excess phenol substrate leads to competitive phenol oxidation, via the proposed transient generation of 4 as the active oxidant.

Activation of Dioxygen by a Mononuclear Nonheme Iron Complex: Sequential Peroxo, Oxo, and Hydroxo Intermediates

The activation of dioxygen by Fe^II(Me₃TACN)(S₂SiMe₂) (1) is reported. Reaction of 1 with O₂ at -135 °C in 2-MeTHF generates a thiolate-ligated (peroxo)diiron complex Fe^III₂(O₂)(Me₃TACN)₂(S₂SiMe₂)₂ (2) that was characterized by UV-vis (λ_max = 300, 390, 530, 723 nm), Mössbauer (δ = 0.53, |ΔE_Q| = 0.76 mm s^-1), resonance Raman (RR) (ν(O-O) = 849 cm^-1), and X-ray absorption (XAS) spectroscopies. Complex 2 is distinct from the outer-sphere oxidation product 1^ox (UV-vis (λ_max = 435, 520, 600 nm), Mössbauer (δ = 0.45, |ΔE_Q| = 3.6 mm s^-1), and EPR (S = 5/2, g = [6.38, 5.53, 1.99])), obtained by one-electron oxidation of 1. Cleavage of the peroxo O-O bond can be initiated either photochemically or thermally to produce a new species assigned as an Fe^IV(O) complex, Fe^IV(O)(Me₃TACN)(S₂SiMe₂) (3), which was identified by UV-vis (λ_max = 385, 460, 890 nm), Mössbauer (δ = 0.21, |ΔE_Q| = 1.57 mm s^-1), RR (ν(Fe^IV═O) = 735 cm^-1), and X-ray absorption spectroscopies, as well as reactivity patterns. Reaction of 3 at low temperature with H atom donors gives a new species, Fe^III(OH)(Me₃TACN)(S₂SiMe₂) (4). Complex 4 was independently synthesized from 1 by the stoichiometric addition of a one-electron oxidant and a hydroxide source. This work provides a rare example of dioxygen activation at a mononuclear nonheme iron(II) complex that produces both Fe^III-O-O-Fe^III and Fe^IV(O) species in the same reaction with O₂. It also demonstrates the feasibility of forming Fe/O₂ intermediates with strongly donating sulfur ligands while avoiding immediate sulfur oxidation.