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Reactivities of non‐heme iron(IV)‐oxo complexes are mostly controlled by the ligands. Complexes with tetradentate ligands such as [(TPA)FeO]²⁺(TPA=tris(2‐pyridylmethyl)amine) belong to the most reactive ones. Here, we show a fine‐tuning of the reactivity of [(TPA)FeO]²⁺by an additional ligand X (X=CH₃CN, CF₃SO₃⁻, ArI, and ArIO; ArI=2‐(^tBuSO₂)C₆H₄I) attached in solution and reveal a thus far unknown role of the ArIO oxidant. The HAT reactivity of [(TPA)FeO(X)]^+/2+decreases in the order of X: ArIO > MeCN > ArI ≈ TfO⁻. Hence, ArIO is not just a mere oxidant of the iron(II) complex, but it can also increase the reactivity of the iron(IV)‐oxo complex as a labile ligand. The detected HAT reactivities of the [(TPA)FeO(X)]^+/2+complexes correlate with the Fe=O and FeO−H stretching vibrations of the reactants and the respective products as determined by infrared photodissociation spectroscopy. Hence, the most reactive [(TPA)FeO(ArIO)]²⁺adduct in the series has the weakest Fe=O bond and forms the strongest FeO−H bond in the HAT reaction.

Reactivities of non‐heme iron(IV)‐oxo complexes are mostly controlled by the ligands. Complexes with tetradentate ligands such as [(TPA)FeO]²⁺(TPA=tris(2‐pyridylmethyl)amine) belong to the most reactive ones. Here, we show a fine‐tuning of the reactivity of [(TPA)FeO]²⁺by an additional ligand X (X=CH₃CN, CF₃SO₃⁻, ArI, and ArIO; ArI=2‐(^tBuSO₂)C₆H₄I) attached in solution and reveal a thus far unknown role of the ArIO oxidant. The HAT reactivity of [(TPA)FeO(X)]^+/2+decreases in the order of X: ArIO > MeCN > ArI ≈ TfO⁻. Hence, ArIO is not just a mere oxidant of the iron(II) complex, but it can also increase the reactivity of the iron(IV)‐oxo complex as a labile ligand. The detected HAT reactivities of the [(TPA)FeO(X)]^+/2+complexes correlate with the Fe=O and FeO−H stretching vibrations of the reactants and the respective products as determined by infrared photodissociation spectroscopy. Hence, the most reactive [(TPA)FeO(ArIO)]²⁺adduct in the series has the weakest Fe=O bond and forms the strongest FeO−H bond in the HAT reaction.

Nonheme iron enzymes generate powerful and versatile oxidants that perform a wide range of oxidation reactions, including the functionalization of inert C−H bonds, which is a major challenge for chemists. The oxidative abilities of these enzymes have inspired bioinorganic chemists to design synthetic models to mimic their ability to perform some of the most difficult oxidation reactions and study the mechanisms of such transformations. Iron‐oxygen intermediates like iron(III)‐hydroperoxo and high‐valent iron‐oxo species have been trapped and identified in investigations of these bio‐inspired catalytic systems, with the latter proposed to be the active oxidant for most of these systems. In this Review, we highlight the recent spectroscopic and mechanistic advances that have shed light on the various pathways that can be accessed by bio‐inspired nonheme iron systems to form the high‐valent iron‐oxo intermediates.

Nicht‐Häm‐Eisenenzyme generieren leistungsstarke und vielfältige Oxidantien, die eine Vielzahl an Oxidationsreaktionen vermitteln, einschließlich der Funktionalisierung von inerten C‐H‐Bindungen, die eine anspruchsvolle Aufgabe für Chemiker darstellt. Die oxidativen Fähigkeiten dieser Enzyme inspirieren Bioorganiker zur Entwicklung synthetischer Modelle, um die Fähigkeit solcher Enzyme zur Vermittlung schwierigster Oxidationsreaktionen nachzuahmen und die Mechanismen derartiger Transformationen zu erforschen. Eisen‐Sauerstoff‐Intermediate wie Eisen(III)‐Hydroperoxo‐Spezies und hochvalente Eisen‐Sauerstoff‐Spezies wurden bei Untersuchungen dieser bioinspirierten katalytischen Systemen abgefangen und identifiziert, wobei Letztgenannte als aktives Oxidationsmittel für die meisten dieser Systeme vorgeschlagen wurden. In diesem Aufsatz heben wir die jüngsten spektroskopischen und mechanistischen Fortschritte hervor, die Licht auf die verschiedenen Wege geworfen haben, die von bioinspirierten Nicht‐Häm‐Eisensystemen zur Bildung der hochvalenten Eisen‐Sauerstoff‐Zwischenprodukte genutzt werden können.

Thesynandantiisomers of [Fe^IV(O)(TMC)]²⁺(TMC=tetramethylcyclam) represent the first isolated pair of synthetic non‐heme oxoiron(IV) complexes with identical ligand topology, differing only in the position of the oxo unit bound to the iron center. Both isomers have previously been characterized. Reported here is that thesynisomer [Fe^IV(O_syn)(TMC)(NCMe)]²⁺(2) converts into itsantiform [Fe^IV(O_anti)(TMC)(NCMe)]²⁺(1) in MeCN, an isomerization facilitated by water and monitored most readily by¹H NMR and Raman spectroscopy. Indeed, when H₂¹⁸O is introduced to2, the nascent1becomes¹⁸O‐labeled. These results provide compelling evidence for a mechanism involving direct binding of a water moleculetransto the oxo atom in2with subsequent oxo–hydroxo tautomerism for its incorporation as the oxo atom of1. The nonplanar nature of the TMC supporting ligand makes this isomerization an irreversible transformation, unlike for their planar heme counterparts.

Thesynandantiisomers of [Fe^IV(O)(TMC)]²⁺(TMC=tetramethylcyclam) represent the first isolated pair of synthetic non‐heme oxoiron(IV) complexes with identical ligand topology, differing only in the position of the oxo unit bound to the iron center. Both isomers have previously been characterized. Reported here is that thesynisomer [Fe^IV(O_syn)(TMC)(NCMe)]²⁺(2) converts into itsantiform [Fe^IV(O_anti)(TMC)(NCMe)]²⁺(1) in MeCN, an isomerization facilitated by water and monitored most readily by¹H NMR and Raman spectroscopy. Indeed, when H₂¹⁸O is introduced to2, the nascent1becomes¹⁸O‐labeled. These results provide compelling evidence for a mechanism involving direct binding of a water moleculetransto the oxo atom in2with subsequent oxo–hydroxo tautomerism for its incorporation as the oxo atom of1. The nonplanar nature of the TMC supporting ligand makes this isomerization an irreversible transformation, unlike for their planar heme counterparts.