Electronic Structure and Reactivity of Dioxygen-Derived Aliphatic Thiolate-Ligated Fe-Peroxo and Fe(IV) Oxo Compounds

Herein, we examine the electronic and geometric structural properties of O 2 -derived aliphatic thiolate-ligated Fe-peroxo, Fe-hydroxo, and Fe(IV) oxo compounds. The latter cleaves strong C-H bonds (96 kcal mol -1 ) on par with the valine C-H bond cleaved by isopencillin N synthase (IPNS). Stopped-flow kinetics studies indicate that the barrier to O 2 binding to [Fe II (S Me2 N 4 (tren))] + ( 3 ) is extremely low ( E a = 36(2) kJ mol -1 ), as theoretically predicted for IPNS. Dioxygen binding to 3 is shown to be reversible, and a superoxo intermediate, [Fe III (S Me2 N 4 (tren))(O 2 )] + ( 6 ), forms in the first 25 ms of the reaction at -40 °C prior to the rate-determining ( E a = 46(2) kJ mol -1 ) formation of peroxo-bridged [(S Me2 N 4 (tren))Fe(III)] 2 (μ-O 2 ) 2+ ( 7 ). A log( k obs ) vs log([Fe]) plot for the formation of 7 is consistent with the second-order dependence on iron, and H 2 O 2 assays are consistent with a 2:1 ratio of Fe/H 2 O 2 . Peroxo 7 is shown to convert to ferric-hydroxo [Fe III (S Me2 N(tren))(OH)] + ( 9 , g ⊥ = 2.24, g ∥ = 1.96), the identity of which was determined via its independent synthesis. Rates of the conversion 7 → 9 are shown to be dependent on the X-H bond strength of the H-atom donor, with a k H / k D = 4 when CD 3 OD is used in place of CH 3 OH as a solvent. A crystallographically characterized cis thiolate-ligated high-valent iron oxo, [Fe IV (O)(S Me2 N 4 (tren))] + ( 11 ), is shown to form en route to hydroxo 9 . Electronic structure calculations were shown to be consistent with 11 being an S = 1 Fe(IV)═O with an unusually high ν Fe-O stretching frequency at 918 cm -1 in line with the extremely short Fe-O bond (1.603(7) Å).