Reactivity Differences of Trigonal Pyramidal Nonheme Iron(IV)-Oxo and Iron(III)-Oxo Complexes: Experiment and Theory

High-valent metal-oxo species play critical roles in enzymatic catalysis yet their properties are still poorly understood. In this work we report a combined experimental and computational study into biomimetic iron(IV)-oxo and iron(III)-oxo complexes with tight second-coordination sphere environments that restrict substrate access. The work shows that the second-coordination sphere slows the hydrogen atom abstraction step from toluene dramatically and the kinetics is zeroth order in substrate. However, the iron(II)-hydroxo that is formed has a low reduction potential and hence cannot do OH rebound favorably. The tolyl radical in solution then reacts further with alternative reaction partners. By contrast, the iron(IV)-oxo species reacts predominantly through OH rebound to form alcohol products. Our studies show that the oxidation state of the metal influences reactivities and selectivities with substrate dramatically and that enzymes will likely need an iron(IV) center to catalyze C-H hydroxylation reactions.

Automated summary

In this study, a combination of experimental and computational methods was used to investigate biomimetic iron(IV)-oxo and iron(III)-oxo complexes. It was found that the second-coordination sphere significantly slows down the hydrogen atom abstraction reaction from toluene and the kinetics is zeroth order in substrate. However, the generated iron(II)-hydroxo has a low reduction potential, leading to unfavorability of OH rebound. On the other hand, the tolyl radical in solution reacts further with alternative reaction partners. In contrast, the iron(IV)-oxo species predominantly reacts through OH rebound to form alcohol products. The research highlights the importance of the metal oxidation state in determining reactivities and selectivities, suggesting that enzymes may require an iron(IV) center for efficient C-H hydroxylation reactions.