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1. Tuning the H‐Atom Transfer Reactivity of Iron(IV)‐Oxo Complexes as Probed by Infrared Photodissociation Spectroscopy

   https://doi.org/10.1002/anie.202016695  

   Tripodi, Guilherme L. ; Dekker, Magda M. J. ; Roithová, Jana ; Que, Jr., Lawrence ( February 2021 , Angewandte Chemie International Edition)

   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.

    

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2. Synthesis, Structure, and Bonding of d 3 Molybdenum–Oxo Complexes

   https://doi.org/10.1002/ange.202001379  

   Vibbert, Hunter B. ; Filatov, Alexander S. ; Hopkins, Michael D. ( April 2020 , Angewandte Chemie)

   Reduction of d²metal–oxo ions of the form [MO(PP)₂Cl]⁺(M=Mo, W; PP=chelating diphosphine) produces d³MO(PP)₂Cl complexes, which include the first isolated examples in group 6. The stability and reactivity of the MO(PP)₂Cl compounds are found to depend upon the steric bulk of the phosphine ligands: derivatives with bulky phosphines that shield the oxo ligand are stable enough to be isolated, whereas those with phosphines that leave the oxo ligand exposed are more reactive and observed transiently. Magnetic measurements and DFT calculations on MoO(dppe)₂Cl indicate the d³compounds are low spin with a²[(d_xy)²(π*(MoO))¹] configuration. X‐ray crystallographic and vibrational‐spectroscopic studies on d²and d³[MoO(dppe)₂Cl]^0/+establish that the d³compound possesses a reduced M−O bond order and significantly longer Mo−O bond, accounting for its greater reactivity. These results indicate that the oxo‐centered reactivity of d³complexes may be controlled through ligand variation.

    

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3. Synthesis, Structure, and Bonding of d 3 Molybdenum–Oxo Complexes

   https://doi.org/10.1002/anie.202001379  

   Vibbert, Hunter B. ; Filatov, Alexander S. ; Hopkins, Michael D. ( April 2020 , Angewandte Chemie International Edition)

   Reduction of d²metal–oxo ions of the form [MO(PP)₂Cl]⁺(M=Mo, W; PP=chelating diphosphine) produces d³MO(PP)₂Cl complexes, which include the first isolated examples in group 6. The stability and reactivity of the MO(PP)₂Cl compounds are found to depend upon the steric bulk of the phosphine ligands: derivatives with bulky phosphines that shield the oxo ligand are stable enough to be isolated, whereas those with phosphines that leave the oxo ligand exposed are more reactive and observed transiently. Magnetic measurements and DFT calculations on MoO(dppe)₂Cl indicate the d³compounds are low spin with a²[(d_xy)²(π*(MoO))¹] configuration. X‐ray crystallographic and vibrational‐spectroscopic studies on d²and d³[MoO(dppe)₂Cl]^0/+establish that the d³compound possesses a reduced M−O bond order and significantly longer Mo−O bond, accounting for its greater reactivity. These results indicate that the oxo‐centered reactivity of d³complexes may be controlled through ligand variation.

    

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4. Octahedral iron( iv )–tosylimido complexes exhibiting single electron-oxidation reactivity

   https://doi.org/10.1039/C9SC02526J  

   Sabenya, Gerard ; Gamba, Ilaria ; Gómez, Laura ; Clémancey, Martin ; Frisch, Jonathan R. ; Klinker, Eric J. ; Blondin, Geneviève ; Torelli, Stéphane ; Que, Lawrence ; Martin-Diaconescu, Vlad ; et al ( October 2019 , Chemical Science)

   High valent iron species are very reactive molecules involved in oxidation reactions of relevance to biology and chemical synthesis. Herein we describe iron( iv )–tosylimido complexes [Fe IV (NTs)(MePy 2 tacn)](OTf) 2 ( 1(IV)NTs ) and [Fe IV (NTs)(Me 2 (CHPy 2 )tacn)](OTf) 2 ( 2(IV)NTs ), (MePy 2 tacn = N -methyl- N , N -bis(2-picolyl)-1,4,7-triazacyclononane, and Me 2 (CHPy 2 )tacn = 1-(di(2-pyridyl)methyl)-4,7-dimethyl-1,4,7-triazacyclononane, Ts = Tosyl). 1(IV)NTs and 2(IV)NTs are rare examples of octahedral iron( iv )–imido complexes and are isoelectronic analogues of the recently described iron( iv )–oxo complexes [Fe IV (O)(L)] 2+ (L = MePy 2 tacn and Me 2 (CHPy 2 )tacn, respectively). 1(IV)NTs and 2(IV)NTs are metastable and have been spectroscopically characterized by HR-MS, UV-vis, 1 H-NMR, resonance Raman, Mössbauer, and X-ray absorption (XAS) spectroscopy as well as by DFT computational methods. Ferric complexes [Fe III (HNTs)(L)] 2+ , 1(III)–NHTs (L = MePy 2 tacn) and 2(III)–NHTs (L = Me 2 (CHPy 2 )tacn) have been isolated after the decay of 1(IV)NTs and 2(IV)NTs in solution, spectroscopically characterized, and the molecular structure of [Fe III (HNTs)(MePy 2 tacn)](SbF 6 ) 2 determined by single crystal X-ray diffraction. Reaction of 1(IV)NTs and 2(IV)NTs with different p -substituted thioanisoles results in the transfer of the tosylimido moiety to the sulphur atom producing sulfilimine products. In these reactions, 1(IV)NTs and 2(IV)NTs behave as single electron oxidants and Hammett analyses of reaction rates evidence that tosylimido transfer is more sensitive than oxo transfer to charge effects. In addition, reaction of 1(IV)NTs and 2(IV)NTs with hydrocarbons containing weak C–H bonds results in the formation of 1(III)–NHTs and 2(III)–NHTs respectively, along with the oxidized substrate. Kinetic analyses indicate that reactions proceed via a mechanistically unusual HAT reaction, where an association complex precedes hydrogen abstraction. 

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5. Ligand field effects on the ground and excited states of reactive FeO 2+ species

   https://doi.org/10.1039/c8cp05372c  

   Kirkland, Justin K. ; Khan, Shahriar N. ; Casale, Bryan ; Miliordos, Evangelos ; Vogiatzis, Konstantinos D. ( November 2018 , Physical Chemistry Chemical Physics)

   High-valent Fe( iv )-oxo species have been found to be key oxidizing intermediates in the mechanisms of mononuclear iron heme and non-heme enzymes that can functionalize strong C–H bonds. Biomimetic Fe( iv )-oxo molecular complexes have been successfully synthesized and characterized, but their catalytic reactivity is typically lower than that of the enzymatic analogues. The C–H activation step proceeds through two competitive mechanisms, named σ- and π-channels. We have performed high-level wave function theory calculations on bare FeO 2+ and a series of non-heme Fe( iv )-oxo model complexes in order to elucidate the electronic properties and the ligand field effects on those channels. Our results suggest that a coordination environment formed by a weak field gives access to both competitive channels, yielding more reactive Fe( iv )-oxo sites. In contrast, a strong ligand environment stabilizes only the σ-channel. Our concluding remarks will aid the derivation of new structure–reactivity descriptors that can contribute to the development of the next generation of functional catalysts. 

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