Exploring the influence of H-bonding and ligand constraints on thiolate ligated non-heme iron mediated dioxygen activation

Abstract

Converting triplet dioxygen into a powerful oxidant is fundamentally important to life. The study reported herein quantitatively examines the formation of a well-characterized, reactive, O₂-derived thiolate ligated Fe^III-superoxo using low-temperature stopped-flow kinetics. Comparison of the kinetic barriers to the formation of this species via two routes, involving either the addition of (a) O₂ to [Fe^II(S₂^Me2N₃(Pr,Pr))] (1) or (b) superoxide to [Fe^III(S₂^Me2N₃(Pr,Pr))]⁺ (3) is shown to provide insight into the mechanism of O₂ activation. Route (b) was shown to be significantly slower, and the kinetic barrier 14.9 kJ mol⁻¹ higher than route (a), implying that dioxygen activation involves inner-sphere, as opposed to outer sphere, electron transfer from Fe(II). H-bond donors and ligand constraints are shown to dramatically influence O₂ binding kinetics and reversibility. Dioxygen binds irreversibly to [Fe^II(S₂^Me2N₃(Pr,Pr))] (1) in tetrahydrofuran, but reversibly in methanol. Hydrogen bonding decreases the ability of the thiolate sulfur to stabilize the transition state and the Fe^III-superoxo, as shown by the 10 kJ mol⁻¹ increase in the kinetic barrier to O₂ binding in methanol vs. tetrahydrofuran. Dioxygen release from [Fe^III(S₂^Me2N₃(Pr,Pr))O₂] (2) is shown to be 24 kJ mol⁻¹ higher relative to previously reported [Fe^III(S^Me2N₄(tren))(O₂)]⁺ (5), the latter of which contains a more flexible ligand. These kinetic results afford an experimentally determined reaction coordinate that illustrates the influence of H-bonding and ligand constraints on the kinetic barrier to dioxygen activation an essential step in biosynthetic pathways critical to life.