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

The activation of dioxygen by Fe II (Me 3 TACN)(S 2 SiMe 2 ) ( 1 ) is reported. Reaction of 1 with O 2 at −135 °C in 2-MeTHF generates a thiolate-ligated (peroxo)diiron complex Fe III 2 (O 2 )(Me 3 TACN) 2 (S 2 SiMe 2 ) 2 ( 2 ) that was characterized by UV–vis (λ max = 300, 390, 530, 723 nm), Mössbauer (δ = 0.53, |Δ E 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, |Δ E 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)(Me 3 TACN)(S 2 SiMe 2 ) ( 3 ), which was identified by UV–vis (λ max = 385, 460, 890 nm), Mössbauer (δ = 0.21, |Δ E 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)(Me 3 TACN)(S 2 SiMe 2 ) ( 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 O 2 . It also demonstrates the feasibility of forming Fe/O 2 intermediates with strongly donating sulfur ligands while avoiding immediate sulfur oxidation.