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 ion 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. Combined experimental and computational studies show iron(III)‐oxo complexes can hydrogen atoms from toluene efficiently, but thermochemically fail to do OH rebound and as such tolyl radicals are released and react separately. As a consequence iron(IV)‐oxo and iron(III)‐oxo generate different products in a reaction with toluene.