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