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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. [ABSTRACT FROM AUTHOR]

High‐valent iron(IV)‐oxo intermediates are versatile oxidants in the biotransformation of various substrates by metalloenzymes and catalyze essential reactions for human health as well as in the biodegradation of toxic organic pollutants in the environment. Herein, we report a biomimetic system that efficiently reacts with halophenols through defluorination reactions and characterize various short‐lived intermediates along the reaction mechanism. We study the reactivity pattern of a nonheme iron(IV)‐oxo species with a series of trihalophenols (X=F, Cl, Br). A combined experimental and computational study reveals that the oxidative dehalogenation of 2,4,6‐trifluorophenol is initiated with an H‐atom abstraction from the phenolic group by the iron(IV)‐oxo species resulting in the formation of a phenolate radical and an iron(III)‐hydroxo species. This iron(III)‐hydroxo species forms an adduct with the oxidized substrate with λmax at 558 nm which subsequently decays to give quinones as products. [ABSTRACT FROM AUTHOR]

The biodegradation of compounds with C−F bonds is challenging due to the fact that these bonds are stronger than the C−H bond in methane. In this work, results on the unprecedented reactivity of a biomimetic model complex that contains an N‐bridged diiron‐phthalocyanine are presented; this model complex is shown to react with perfluorinated arenes under addition of H2O2 effectively. To get mechanistic insight into this unusual reactivity, detailed density functional theory calculations on the mechanism of C6F6 activation by an iron(IV)‐oxo active species of the N‐bridged diiron phthalocyanine system were performed. Our studies show that the reaction proceeds through a rate‐determining electrophilic C−O addition reaction followed by a 1,2‐fluoride shift to give the ketone product, which can further rearrange to the phenol. A thermochemical analysis shows that the weakest C−F bond is the aliphatic C−F bond in the ketone intermediate. The oxidative defluorination of perfluoroaromatics is demonstrated to proceed through a completely different mechanism compared to that of aromatic C−H hydroxylation by iron(IV)‐oxo intermediates such as cytochrome P450 Compound I. [ABSTRACT FROM AUTHOR]

Iron is an essential element in nonheme enzymes that plays a crucial role in many vital oxidative transformations and metabolic reactions in the human body. Many of those reactions are regio‐ and stereospecific and it is believed that the selectivity is guided by second‐coordination sphere effects in the protein. Here, results are shown of a few engineered biomimetic ligand frameworks based on the N4Py (N,N‐bis(2‐pyridylmethyl)‐N‐bis(2‐pyridyl)methylamine) scaffold and the second‐coordination sphere effects are studied. For the first time, selective substitutions in the ligand framework have been shown to tune the catalytic properties of the iron(IV)‐oxo complexes by regulating the steric and electronic factors. In particular, a better positioning of the oxidant and substrate in the rate‐determining transition state lowers the reaction barriers. Therefore, an optimum balance between steric and electronic factors mediates the ideal positioning of oxidant and substrate in the rate‐determining transition state that affects the reactivity of high‐valent reaction intermediates. It takes two: A combined spectroscopy and computational study on engineered iron(IV)‐oxo complexes identified a second‐coordination sphere effect that enables better positioning of the iron(IV)‐oxo and higher reactivity. [ABSTRACT FROM AUTHOR]

Catechol intradiol dioxygenation is a unique reaction catalyzed by iron-dependent enzymes and non-heme iron(III) complexes. The mechanism by which these systems activate dioxygen in this important metabolic process remains controversial. Using a combination of kinetic measurements and computational modelling of multiple iron(III) catecholato complexes, we have elucidated the catechol cleavage mechanism and show that oxygen binds the iron center by partial dissociation of the substrate from the iron complex. The iron(III) superoxide complex that is formed subsequently attacks the carbon atom of the substrate by a rate-determining CO bond formation step. [ABSTRACT FROM AUTHOR]

Cover Feature: Mechanism of Oxidative Activation of Fluorinated Aromatic Compounds by N-Bridged Diiron-Phthalocyanine: What Determines the Reactivity? Keywords: biomimetic models; enzyme models; epoxidation; hydroxylation; reaction mechanisms Biomimetic models, enzyme models, epoxidation, hydroxylation, reaction mechanisms. [Extracted from the article]

Enzyme Models Intradiol catechol dioxygenases are an important class of enzymes that catalyze the key step in the microbial degradation of aromatic compounds. In their Communication on page 15686, P. C. A. Bruijnincx and co ‐ workers describe their experimental and computational work on the intradiol cleavage mechanism of a biomimetic, non ‐ heme iron(III) complex. Surprisingly, the substrate was found to partially dissociate from the complex to enable the formation of an iron(III) superoxide intermediate. The ability of the iron(III) center to change from a high ‐ spin to an intermediate ‐ spin state was found to be crucial for activity in this formally spin ‐ forbidden reaction. [ABSTRACT FROM AUTHOR]