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

Abstract

Non-heme thiolate-ligated metalloenzymes such as isopenicillin N synthase (IPNS) and cysteine dioxygenase (CDO) active dioxygen to produce catalytic oxidative intermediates including metal -superoxos, -hydroperoxos, and high-valent oxos. Thiolate-ligated metalloenzymes have been shown to form highly covalent metal ligand bonds and in return lower the activation energy barrier to dioxygen binding. Kinetic investigations of dioxygen binding to metalloenzymes provide insights into the order of the mechanism of the enzyme and whether geometric rearrangements, spin-state changes, or solvent interactions affect dioxygen binding. This dissertation investigates the barrier to dioxygen activation of the non-heme alkyl thiolate-ligated iron complex ([FeII(SMe2N4(tren))]+) with variable low temperature stopped-flow kinetics. The reaction of [FeII(SMe2N4(tren))]+ with dioxygen under pseudo first order conditions with excess dioxygen is shown to be first order with respect to dioxygen, and second order with respect to iron. These reaction orders are consistent with the formation of the peroxo-bridged diferric species [(SMe2N4(tren))FeIII]2(μ-O2)]. However, when the conditions are switched, under excess Fe, the first dioxygen derived intermediate FeIII-superoxo is observed, [FeIII(SMe2N4(tren))(O2)]+. The global fitting of experimental kinetic data determined that the rate determining step was the FeIII-superoxo reacting with another FeII molecule to form the peroxo-bridge species, [(SMe2N4(tren))FeIII]2(μ-O2)].. Low-temperature stopped-flow kinetic studies investigated the reversibility and mechanism of a well-characterized dioxygen derived FeIII-superoxo, [FeIII(S2Me2N2NH(Pr,Pr))(O2)], in aprotic and protic solvents (THF and MeOH). The different environments provided insights into ligand constraints and H-bond donors which have been shown to influence dioxygen binding kinetics and reversibility. The reaction is faster in aprotic THF versus protic MeOH, as well as irreversible in THF and reversible in MeOH. Hydrogen bonding in MeOH to the thiolate sulfur indicates destabilization of the transition state, and an increase in the activation energy of dioxygen binding to FeII. The reaction between potassium superoxide and oxidized [FeIII(S2Me2N2NH(Pr,Pr))]+ was also investigated and the resulting kinetics support the dioxygen binding mechanism to [FeII(S2Me2N2NH(Pr,Pr))]+ to involve inner-sphere, as opposed to outer-sphere, electron transfer. The well characterized FeIII-superoxo [FeIII(S2Me2N2NH(Pr,Pr))(O2)] was shown to abstract external hydrogens with a BDFE of 93 kcal/mol with a KIE of 4.8 comparable to the strength of theβ-hydrogen abstraction the FeIII-superoxo in IPNS performs. In order to investigate a closer biomimetic model to IPNS, the gem-dimethyl groups adjacent to the thiolate sulfurs were removed to incorporate β-hydrogens to the sulfur to form the complex, [FeII(S2β-H2N2NH(Pr,Pr))]. The structure of [FeII(S2β-H2N2NH(Pr,Pr))]can more closely mimics the internal hydrogen abstraction mechanism proposed in the native enzyme. The change in the electronics affected the longevity of the FeIII-superoxo intermediate formation and the addition of external hydrogen donors resulted in other Fe intermediates. To further explore and isolate the FeIII-superoxo the β-hydrogens were exchanged for deuteriums in order to investigate whether an internal or external hydrogen atom abstraction occurs and results in the shorter lived [FeII(S2β-H2N2NH(Pr,Pr))(O2). The synthesis of the β-deuterium ligand will be discussed as well as preliminary characterizations of the complex and it’s reactivity.. The [FeII(Cyclam-PrS)]+ complex was shown to react with potassium superoxide in protic conditions to form a high-spin FeIII-hydroperoxo. The hydroperoxo is proposed to be trans to the thiolate and is rare for small molecules as the electron density donated to the metal center from the sulfur tend to push ligands away. Furthermore, when [FeII(Cyclam-PrS)]+ is reacted with nitric oxide, a mimic to dioxygen binding, a crystal structure of [FeIII(Cyclam-PrS(NO))]+ with the NO ligand bound cis to the thiolate. DFT is used to confirm if the FeIII-hydroperoxo is trans or cis to the thiolate. Further investigation of the FeIII-hydroperoxo utilizes dioxygen in protic solvent and alkylperoxos to observe if [FeIII(Cyclam-PrS(OOH))]+ can be synthesized with the shunt pathway.