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)(S2SiMe2) (1) is reported. Reaction of 1 with O-2 at -135 degrees C in 2-MeTHF generates a thiolate-ligated (peroxo)diiron complex Fe-2(III)(O-2)(Me(3)TACN)(2)(S2SiMe2)(2) (2) that was characterized by UV-vis (lambda(max) = 300, 390, 530, 723 nm), Mossbauer (delta = 0.53, vertical bar Delta E-Q vertical bar = 0.76 mm s(-1)), resonance Raman (RR) (nu(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 (lambda(max) = 435, 520, 600 nm), Mossbauer (delta = 0.45, vertical bar Delta E-Q vertical bar = 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)(S2SiMe2) (3), which was identified by UV-vis (lambda(max) = 385, 460, 890 nm), Mossbauer (delta = 0.21, vertical bar Delta E-Q vertical bar = 1.57 mm s(-1)), RR (nu(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)(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 O-2. It also demonstrates the feasibility of forming Fe/O-2 intermediates with strongly donating sulfur ligands while avoiding immediate sulfur oxidation.