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

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, O 2 -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) O 2 to [Fe II (S 2 Me2 N 3 (Pr,Pr))] (1) or (b) superoxide to [Fe III (S 2 Me2 N 3 (Pr,Pr))] + (3) is shown to provide insight into the mechanism of O 2 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 O 2 binding kinetics and reversibility. Dioxygen binds irreversibly to [Fe II (S 2 Me2 N 3 (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 O 2 binding in methanol vs. tetrahydrofuran. Dioxygen release from [Fe III (S 2 Me2 N 3 (Pr,Pr))O 2 ] (2) is shown to be 24 kJ mol -1 higher relative to previously reported [Fe III (S Me2 N 4 (tren))(O 2 )] + (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.