Issue 32, 2024

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 FeIII-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 [FeII(S2Me2N3(Pr,Pr))] (1) or (b) superoxide to [FeIII(S2Me2N3(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 [FeII(S2Me2N3(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 FeIII-superoxo, as shown by the 10 kJ mol−1 increase in the kinetic barrier to O2 binding in methanol vs. tetrahydrofuran. Dioxygen release from [FeIII(S2Me2N3(Pr,Pr))O2] (2) is shown to be 24 kJ mol−1 higher relative to previously reported [FeIII(SMe2N4(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.

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

Supplementary files

Article information

Article type
Edge Article
Submitted
26 apr 2024
Accepted
05 jul 2024
First published
09 jul 2024
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY-NC license

Chem. Sci., 2024,15, 12710-12720

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

M. N. Lundahl, M. B. Greiner, M. C. Piquette, P. M. Gannon, W. Kaminsky and J. A. Kovacs, Chem. Sci., 2024, 15, 12710 DOI: 10.1039/D4SC02787F

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