Thiolate (RS–) ligands have been shown to lower the activation barrier to O2 binding, and facilitate peroxo O-O bond cleavage and HAT reactions. We will describe a new structurally characterized alkyl thiolate-ligated Fe(II) complex, which reacts with dioxygen (O2) to form an unprecedented example of a reactive iron superoxo (RS-Fe(III)-O2) intermediate that is capable of cleaving strong C-H bonds. A thiolate-ligated iron superoxo is proposed to play a key role in the biosynthesis of β-lactam antibiotics, as well as the prevention of cancerous tumor metastases. The former is catalyzed by the iron enzyme isopenicillin N-Synthase (IPNS), while the latter by the iron enzyme cysteine dioxygenase (CDO). Very few iron superoxo compounds have been reported, and none are capable of cleaving strong C-H bonds. Spectroscopic characterization of our RS-Fe(III)-O2, which also forms in the reaction between KO2 and the Fe(III) derivative, as well as calibrated DFT and TD-DFT calculations, show that the frontier orbitals consist of two strongly coupled unpaired electrons of opposite spin, one in a superoxo ?*(O-O) orbital, and the other in an Fe(dxy) orbital. Unpaired spin on the superoxo likely contributes to the ability of our RS-Fe(III)-O2 to abstract H-atoms. Both the calculated and experimental electronic absorption spectrum of our RS-Fe-O2 are similar to that of the putative IPNS superoxo intermediate, as well as CDO. The rate at which our superoxo converts to a second observable intermediate, proposed to be an iron hydroperoxo (Fe(III)-OOH), depends on the C-H bond strength of the solvent or sacrificial H-atom donor, and a deuterium isotope effect (kH/kD= 4.8), comparable to that of IPNS (kH/kD= 5.6), is observed. As demonstrated by the presence of a low energy thiolate à Fe-O2 charge transfer transition, the electron-rich alkyl thiolate likely plays a role in increasing reactivity by creating a more basic superoxo. The bond dissociation energy (BDE) of the C-H bonds cleaved by our RS-Fe-O2 superoxo compound (92 kcal/mol) are comparable to those cleaved by the enzyme IPNS (93 kcal/mol)

Formation of a reactive, alkyl thiolate-ligated FeIII-superoxo intermediate derived from dioxygen

2019 Spring National Meeting

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