Insights into Dioxygen Activation by Biomimetic Alkyl Thiolate-Ligated Iron Complexes

Abstract

Dioxygen activation by non-heme thiolate-ligated Fe-enzymes, such as isopenicillin N synthase (IPNS) and cysteine dioxygenase (CDO) are believed to proceed through several intermediates, including an Fe-superoxo, -hydroperoxo, and high-valent oxo. Thiolate (RS-) ligands are predicted lower the activation barrier to O2 binding, facilitate peroxo O-O bond cleavage via the formation of highly covalent FeIII−SR bonds, and H-atom abstraction reactions. There are few reported examples of well-characterized RS-Fe-superoxo, or -hydroperoxo intermediates. This dissertation describes a new structurally characterized alkyl thiolate-ligated FeII complex, [FeII(S2Me2N3(Pr,Pr)], which reacts with dioxygen (O2) to form an unprecedented example of a reactive RS-FeIII-superoxo intermediate The kinetics of formation of the thiolate ligated FeIII-superoxo intermediate [FeIII(S2Me2N3(Pr,Pr)(O2)] formed via the addition of O2 to [FeII(S2Me2N3(Pr,Pr)], and the addition of potassium superoxide (KO2) to [FeIII(S2Me2N3(Pr,Pr)]+, are presented. This RS-FeIII-superoxo intermediate goes on to cleave strong C-H bonds converting to a second observable intermediate, proposed to be an FeIII-hydroperoxo, en route to a rare example of a crystallographically characterized η2-coordinated sulfenate (RSO-) complex. A look into the mechanism of the formation of the rare stable sulfenate was carried out using oxo atom donors added to the oxidized [FeIII(S2Me2N3(Pr,Pr)]+. The formation of oxo atom donor adducts is observed in these reactions at low temperatures, which then convert to the singly oxygenated sulfenate. This implicates a metal-mediated pathway for sulfur oxygenation, as opposed to direct attack at the sulfur, and possibly an unobservable FeV-oxo as the active oxidant. Finally, synthetic routes to new alkyl-thiolate ligated Fe complexes are discussed due to their potential to expand our understanding of how thiolates influence reactivity.