A High-Valent Iron–Oxo Corrolazine Activates C–H Bonds via Hydrogen-Atom Transfer

Oxidation of the Fe^III complex (TBP₈Cz)­Fe^III [TBP₈Cz = octakis­(4-<i>tert</i>-butylphenyl)­corrolazinate] with O-atom transfer oxidants under a variety of conditions gives the reactive high-valent Fe­(O) complex (TBP₈Cz^+•)­Fe^IV(O) (<b>2</b>). The solution state structure of <b>2</b> was characterized by XAS [<i>d</i>(Fe–O) = 1.64 Å]. This complex is competent to oxidize a range of C–H substrates. Product analyses and kinetic data show that these reactions occur via rate-determining hydrogen-atom transfer (HAT), with a linear correlation for log <i>k</i> versus BDE­(C–H), and the following activation parameters for xanthene (Xn) substrate: Δ<i>H</i>^⧧ = 12.7 ± 0.8 kcal mol^–1, Δ<i>S</i>^⧧ = −9 ± 3 cal K^–1 mol^–1, and KIE = 5.7. Rebound hydroxylation versus radical dimerization for Xn is favored by lowering the reaction temperature. These findings provide insights into the factors that control the intrinsic reactivity of Compound I heme analogues

Noticiero de Vigo : diario independiente de la mañana: Ano XXVIII Número 11530 - 1913 setembro 21

The generation of a new high-valent iron terminal imido complex prepared with a corrolazine macrocycle is reported. The reaction of [Fe^III(TBP₈Cz)] (TBP₈Cz = octakis­(4<i>-tert</i>-butylphenyl)­corrolazinato) with the commercially available chloramine-T (Na⁺TsNCl^–) leads to oxidative N-tosyl transfer to afford [Fe^IV(TBP₈Cz^+•)­(NTs)] in dichloromethane/acetonitrile at room temperature. This complex was characterized by UV–vis, Mössbauer (δ = −0.05 mm s^–1, Δ<i>E</i>_Q = 2.94 mm s^–1), and EPR (X-band (15 K), <i>g</i> = 2.10, 2.00) spectroscopies, and together with reactivity patterns and DFT calculations has been established as an iron­(IV) species antiferromagnetically coupled with a Cz-π-cation-radical (<i>S</i>_total = ¹/₂ ground state). Reactivity studies with triphenylphosphine as substrate show that [Fe^IV(TBP₈Cz^+•)­(NTs)] is an efficient NTs transfer agent, affording the phospharane product Ph₃PNTs under both stoichiometric and catalytic conditions. Kinetic analysis of this reaction supports a bimolecular NTs transfer mechanism with rate constant of 70(15) M^–1 s^–1. These data indicate that [Fe^IV(TBP₈Cz^+•)­(NTs)] reacts about 100 times faster than analogous Mn terminal arylimido corrole analogues. It was found that two products crystallize from the same reaction mixture of Fe^III(TBP₈Cz) + chloramine-T + PPh₃, [Fe^IV(TBP₈Cz)­(NPPh₃)] and [Fe^III(TBP₈Cz)­(OPPh₃)], which were definitively characterized by X-ray crystallography. The sequential production of Ph₃PNTs, Ph₃PNH, and Ph₃PO was observed by ³¹P NMR spectroscopy and led to a proposed mechanism that accounts for all of the observed products. The latter Fe^III complex was then rationally synthesized and structurally characterized from Fe^III(TBP₈Cz) and OPPh₃, providing an important benchmark compound for spectroscopic studies. A combination of Mössbauer and EPR spectroscopies led to the characterization of both intermediate spin (<i>S</i> = ³/₂) and low spin (<i>S</i> = ¹/₂) Fe^III corrolazines, as well as a formally Fe^IV corrolazine which may also be described by its valence tautomer Fe^III(Cz^+•)

High-Valent Manganese–Oxo Valence Tautomers and the Influence of Lewis/Brönsted Acids on C–H Bond Cleavage

The addition of Lewis or Brönsted acids (LA = Zn­(OTf)₂, B­(C₆F₅)₃, HBAr^F, TFA) to the high-valent manganese–oxo complex Mn^V(O)­(TBP₈Cz) results in the stabilization of a valence tautomer Mn^IV(O-LA)­(TBP₈Cz^•+). The Zn^II and B­(C₆F₅)₃ complexes were characterized by manganese K-edge X-ray absorption spectroscopy (XAS). The position of the edge energies and the intensities of the pre-edge (1s to 3d) peaks confirm that the Mn ion is in the +4 oxidation state. Fitting of the extended X-ray absorption fine structure (EXAFS) region reveals 4 N/O ligands at Mn–N_ave = 1.89 Å and a fifth N/O ligand at 1.61 Å, corresponding to the terminal oxo ligand. This Mn–O bond length is elongated compared to the Mn^V(O) starting material (Mn–O = 1.55 Å). The reactivity of Mn^IV(O-LA)­(TBP₈Cz^•+) toward C–H substrates was examined, and it was found that H^• abstraction from C–H bonds occurs in a 1:1 stoichiometry, giving a Mn^IV complex and the dehydrogenated organic product. The rates of C–H cleavage are accelerated for the Mn^IV(O-LA)­(TBP₈Cz^•+) valence tautomer as compared to the Mn^V(O) valence tautomer when LA = Zn^II, B­(C₆F₅)₃, and HBAr^F, whereas for LA = TFA, the C–H cleavage rate is slightly slower than when compared to Mn^V(O). A large, nonclassical kinetic isotope effect of <i>k</i>_H/<i>k</i>_D = 25–27 was observed for LA = B­(C₆F₅)₃ and HBAr^F, indicating that H-atom transfer (HAT) is the rate-limiting step in the C–H cleavage reaction and implicating a potential tunneling mechanism for HAT. The reactivity of Mn^IV(O-LA)­(TBP₈Cz^•+) toward C–H bonds depends on the strength of the Lewis acid. The HAT reactivity is compared with the analogous corrole complex Mn^IV(O–H)­(tpfc^•+) recently reported (<i>J. Am. Chem. Soc.</i> <b>2015</b>, 137, 14481–14487)

CCDC 964044: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures

CCDC 964043: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures

Generation of a High-Valent Iron Imido Corrolazine Complex and NR Group Transfer Reactivity

The generation of a new high-valent iron terminal imido complex prepared with a corrolazine macrocycle is reported. The reaction of [Fe^III(TBP₈Cz)] (TBP₈Cz = octakis­(4<i>-tert</i>-butylphenyl)­corrolazinato) with the commercially available chloramine-T (Na⁺TsNCl^–) leads to oxidative N-tosyl transfer to afford [Fe^IV(TBP₈Cz^+•)­(NTs)] in dichloromethane/acetonitrile at room temperature. This complex was characterized by UV–vis, Mössbauer (δ = −0.05 mm s^–1, Δ<i>E</i>_Q = 2.94 mm s^–1), and EPR (X-band (15 K), <i>g</i> = 2.10, 2.00) spectroscopies, and together with reactivity patterns and DFT calculations has been established as an iron­(IV) species antiferromagnetically coupled with a Cz-π-cation-radical (<i>S</i>_total = ¹/₂ ground state). Reactivity studies with triphenylphosphine as substrate show that [Fe^IV(TBP₈Cz^+•)­(NTs)] is an efficient NTs transfer agent, affording the phospharane product Ph₃PNTs under both stoichiometric and catalytic conditions. Kinetic analysis of this reaction supports a bimolecular NTs transfer mechanism with rate constant of 70(15) M^–1 s^–1. These data indicate that [Fe^IV(TBP₈Cz^+•)­(NTs)] reacts about 100 times faster than analogous Mn terminal arylimido corrole analogues. It was found that two products crystallize from the same reaction mixture of Fe^III(TBP₈Cz) + chloramine-T + PPh₃, [Fe^IV(TBP₈Cz)­(NPPh₃)] and [Fe^III(TBP₈Cz)­(OPPh₃)], which were definitively characterized by X-ray crystallography. The sequential production of Ph₃PNTs, Ph₃PNH, and Ph₃PO was observed by ³¹P NMR spectroscopy and led to a proposed mechanism that accounts for all of the observed products. The latter Fe^III complex was then rationally synthesized and structurally characterized from Fe^III(TBP₈Cz) and OPPh₃, providing an important benchmark compound for spectroscopic studies. A combination of Mössbauer and EPR spectroscopies led to the characterization of both intermediate spin (<i>S</i> = ³/₂) and low spin (<i>S</i> = ¹/₂) Fe^III corrolazines, as well as a formally Fe^IV corrolazine which may also be described by its valence tautomer Fe^III(Cz^+•)

Generation of a High-Valent Iron Imido Corrolazine Complex and NR Group Transfer Reactivity

The generation of a new high-valent iron terminal imido complex prepared with a corrolazine macrocycle is reported. The reaction of [Fe^III(TBP₈Cz)] (TBP₈Cz = octakis­(4<i>-tert</i>-butylphenyl)­corrolazinato) with the commercially available chloramine-T (Na⁺TsNCl^–) leads to oxidative N-tosyl transfer to afford [Fe^IV(TBP₈Cz^+•)­(NTs)] in dichloromethane/acetonitrile at room temperature. This complex was characterized by UV–vis, Mössbauer (δ = −0.05 mm s^–1, Δ<i>E</i>_Q = 2.94 mm s^–1), and EPR (X-band (15 K), <i>g</i> = 2.10, 2.00) spectroscopies, and together with reactivity patterns and DFT calculations has been established as an iron­(IV) species antiferromagnetically coupled with a Cz-π-cation-radical (<i>S</i>_total = ¹/₂ ground state). Reactivity studies with triphenylphosphine as substrate show that [Fe^IV(TBP₈Cz^+•)­(NTs)] is an efficient NTs transfer agent, affording the phospharane product Ph₃PNTs under both stoichiometric and catalytic conditions. Kinetic analysis of this reaction supports a bimolecular NTs transfer mechanism with rate constant of 70(15) M^–1 s^–1. These data indicate that [Fe^IV(TBP₈Cz^+•)­(NTs)] reacts about 100 times faster than analogous Mn terminal arylimido corrole analogues. It was found that two products crystallize from the same reaction mixture of Fe^III(TBP₈Cz) + chloramine-T + PPh₃, [Fe^IV(TBP₈Cz)­(NPPh₃)] and [Fe^III(TBP₈Cz)­(OPPh₃)], which were definitively characterized by X-ray crystallography. The sequential production of Ph₃PNTs, Ph₃PNH, and Ph₃PO was observed by ³¹P NMR spectroscopy and led to a proposed mechanism that accounts for all of the observed products. The latter Fe^III complex was then rationally synthesized and structurally characterized from Fe^III(TBP₈Cz) and OPPh₃, providing an important benchmark compound for spectroscopic studies. A combination of Mössbauer and EPR spectroscopies led to the characterization of both intermediate spin (<i>S</i> = ³/₂) and low spin (<i>S</i> = ¹/₂) Fe^III corrolazines, as well as a formally Fe^IV corrolazine which may also be described by its valence tautomer Fe^III(Cz^+•)