Formation of a Reactive, Alkyl Thiolate-Ligated Fe(III)-Superoxo Intermediate Derived from Dioxygen

Herein, we describe an alkyl thiolate-ligated iron complex that reacts with dioxygen to form an unprecedented example of an iron superoxo (O2•–) intermediate, [FeIII(S2Me2N3(Pr,Pr))(O2)] (4), which is capable of cleaving strong C–H bonds. A cysteinate-ligated iron superoxo intermediate is proposed to play a key role in the biosynthesis of β-lactam antibiotics by isopenicillin N-synthase (IPNS). Superoxo 4 converts to a metastable putative Fe(III)–OOH intermediate, at rates that are dependent on the C–H bond strength of the H atom donor, with a kinetic isotope effect (kH/kD = 4.8) comparable to that of IPNS (kH/kD = 5.6). The bond dissociation energy of the C–H bonds cleaved by 4 (92 kcal/mol) is comparable to C–H bonds cleaved by IPNS (93 kcal/mol). Both the calculated and experimental electronic absorption spectra of 4 are comparable to those of the putative IPNS superoxo intermediate, and are shown to involve RS– → Fe–O2•– and O2•– → Fe charge transfer transitions. The π-back-donation by the electron-ri...