Hydrogen Atom Abstraction by a High-Valent Manganese(V)−Oxo Corrolazine

High-valent metal−oxo complexes are postulated as key intermediates for a wide range of enzymatic and synthetic processes. To gain an understanding of these processes, the reactivity of an isolated, well-characterized MnV−oxo complex, (TBP8Cz)MnV⋮O (1), (TBP8Cz = octakis(para-tert-butylphenyl)corrolazinato3-) has been examined. This complex has been shown to oxidize a series of substituted phenols (4-X-2,6-t-Bu2C6H2OH, X = C(CH3)3 (3), H, Me, OMe, CN), resulting in the production of phenoxyl radicals and the MnIII complex [(TBP8Cz)MnIII] (2). Kinetic studies have led to the determination of second-order rate constants for the phenol substrates, which give a Hammett correlation ((log k’’x/k’’H) vs σp+) with ρ = −1.26. A plot of log k versus BDE(O−H) also reveals a linear correlation. These data, combined with a KIE of 5.9 for 3−OD, provide strong evidence for a concerted hydrogen-atom-abstraction mechanism. Substrates with C−H bonds (1,4-cyclohexadiene and 9,10-dihydroanthracene) are also oxidized via H-atom abstraction by 1, although at a much slower rate. Given the stability of 1, and in particular its low redox potential, (−0.05 V vs SCE), the observed H atom abstraction ability is surprising. These findings support a hypothesis regarding how certain heme enzymes can perform difficult H-atom abstractions while avoiding the generation of high-valent metal−oxo intermediates with oxidation potentials that would lead to the destruction of the surrounding protein environment

Epoxidations Catalyzed by Manganese(V) Oxo and Imido Complexes: Role of the Oxidant−Mn−Oxo (Imido) Intermediate

The manganese(V) oxo complex (TBP8Cz)MnV(O) (1) is shown to catalyze the epoxidation of alkenes with a series of iodosylarenes (ArIO) as oxidants. Competition experiments reveal that the identity of ArIO influences the product ratios, implicating an unusual coordinated oxo−metal−ArIO intermediate (1-OIAr) as the active catalytic species. The isoelectronic manganese(V) imido complex (TBP8Cz)MnV(NMes) (2) does not participate in NR transfer but does catalyze epoxidations with ArIO as the O-atom source, suggesting a mechanism similar to that seen for 1. Direct evidence (ESIMS) is obtained for 1-OIMes

Generation of an Isolable, Monomeric Manganese(V)–Oxo Complex from O₂ and Visible Light

The direct conversion of a Mn^III complex [(TBP₈Cz)­Mn^III (<b>1</b>)] to a Mn^V–oxo complex [(TBP₈Cz)­Mn^V(O) (<b>2</b>)] with O₂ and visible light is reported. Complex <b>1</b> is also shown to function as an active photocatalyst for the oxidation of PPh₃ to OPPh₃. Mechanistic studies indicate that the photogeneration of <b>2</b> does not involve singlet oxygen but rather likely occurs via a free-radical mechanism upon photoactivation of <b>1</b>

Synthesis of the First Corrolazine: A New Member of the Porphyrinoid Family

Synthesis of the First Corrolazine:  A New Member of the Porphyrinoid Famil

Synthesis of the First Corrolazine: A New Member of the Porphyrinoid Family

Synthesis of the First Corrolazine:  A New Member of the Porphyrinoid Famil

O₂ Activation by Bis(imino)pyridine Iron(II)−Thiolate Complexes

The new iron(II)−thiolate complexes [(iPrBIP)FeII(SPh)(Cl)] (1) and [(iPrBIP)FeII(SPh)(OTf)] (2) [BIP = bis(imino)pyridine] were prepared as models for cysteine dioxygenase (CDO), which converts Cys to Cys-SO2H at a (His)3FeII center. Reaction of 1 and 2 with O2 leads to Fe-oxygenation and S-oxygenation, respectively. For 1 + O2, the spectroscopic and reactivity data, including 18O isotope studies, are consistent with an assignment of an iron(IV)−oxo complex, [(iPrBIP)FeIV(O)(Cl)]+ (3), as the product of oxygenation. In contrast, 2 + O2 results in direct S-oxygenation to give a sulfonato product, PhSO3−. The positioning of the thiolate ligand in 1 versus 2 appears to play a critical role in determining the outcome of O2 activation. The thiolate ligands in 1 and 2 are essential for O2 reactivity and exhibit an important influence over the FeIII/FeII redox potential

O₂ Activation by Bis(imino)pyridine Iron(II)−Thiolate Complexes

The new iron(II)−thiolate complexes [(iPrBIP)FeII(SPh)(Cl)] (1) and [(iPrBIP)FeII(SPh)(OTf)] (2) [BIP = bis(imino)pyridine] were prepared as models for cysteine dioxygenase (CDO), which converts Cys to Cys-SO2H at a (His)3FeII center. Reaction of 1 and 2 with O2 leads to Fe-oxygenation and S-oxygenation, respectively. For 1 + O2, the spectroscopic and reactivity data, including 18O isotope studies, are consistent with an assignment of an iron(IV)−oxo complex, [(iPrBIP)FeIV(O)(Cl)]+ (3), as the product of oxygenation. In contrast, 2 + O2 results in direct S-oxygenation to give a sulfonato product, PhSO3−. The positioning of the thiolate ligand in 1 versus 2 appears to play a critical role in determining the outcome of O2 activation. The thiolate ligands in 1 and 2 are essential for O2 reactivity and exhibit an important influence over the FeIII/FeII redox potential

Halogen Transfer to Carbon Radicals by High-Valent Iron Chloride and Iron Fluoride Corroles

High-valent iron halide corroles were examined to determine their reactivity with carbon radicals and their ability to undergo radical rebound-like processes. Beginning with Fe­(Cl)­(ttppc) (1) (ttppc = 5,10,15-tris­(2,4,6-triphenylphenyl)­corrolato3–), the new iron corroles Fe­(OTf)­(ttppc) (2), Fe­(OTf)­(ttppc)­(AgOTf) (3), and Fe­(F)­(ttppc) (4) were synthesized. Complexes 3 and 4 are the first iron triflate and iron fluoride corroles to be structurally characterized by single crystal X-ray diffraction. The structure of 3 reveals an AgI–pyrrole (η2–π) interaction. The Fe­(Cl)­(ttppc) and Fe­(F)­(ttppc) complexes undergo halogen transfer to triarylmethyl radicals, and kinetic analysis of the reaction between (p-OMe-C6H4)3C• and 1 gave k = 1.34(3) × 103 M–1 s–1 at 23 °C and 2.2(2) M–1 s–1 at −60 °C, ΔH⧧ = +9.8(3) kcal mol–1, and ΔS⧧ = −14(1) cal mol–1 K–1 through an Eyring analysis. Complex 4 is significantly more reactive, giving k = 1.16(6) × 105 M–1 s–1 at 23 °C. The data point to a concerted mechanism and show the trend X = F– > Cl– > OH– for Fe­(X)­(ttppc). This study provides mechanistic insights into halogen rebound for an iron porphyrinoid complex

O₂ Activation by Bis(imino)pyridine Iron(II)−Thiolate Complexes

The new iron(II)−thiolate complexes [(iPrBIP)FeII(SPh)(Cl)] (1) and [(iPrBIP)FeII(SPh)(OTf)] (2) [BIP = bis(imino)pyridine] were prepared as models for cysteine dioxygenase (CDO), which converts Cys to Cys-SO2H at a (His)3FeII center. Reaction of 1 and 2 with O2 leads to Fe-oxygenation and S-oxygenation, respectively. For 1 + O2, the spectroscopic and reactivity data, including 18O isotope studies, are consistent with an assignment of an iron(IV)−oxo complex, [(iPrBIP)FeIV(O)(Cl)]+ (3), as the product of oxygenation. In contrast, 2 + O2 results in direct S-oxygenation to give a sulfonato product, PhSO3−. The positioning of the thiolate ligand in 1 versus 2 appears to play a critical role in determining the outcome of O2 activation. The thiolate ligands in 1 and 2 are essential for O2 reactivity and exhibit an important influence over the FeIII/FeII redox potential

O₂ Activation by Bis(imino)pyridine Iron(II)−Thiolate Complexes

The new iron(II)−thiolate complexes [(iPrBIP)FeII(SPh)(Cl)] (1) and [(iPrBIP)FeII(SPh)(OTf)] (2) [BIP = bis(imino)pyridine] were prepared as models for cysteine dioxygenase (CDO), which converts Cys to Cys-SO2H at a (His)3FeII center. Reaction of 1 and 2 with O2 leads to Fe-oxygenation and S-oxygenation, respectively. For 1 + O2, the spectroscopic and reactivity data, including 18O isotope studies, are consistent with an assignment of an iron(IV)−oxo complex, [(iPrBIP)FeIV(O)(Cl)]+ (3), as the product of oxygenation. In contrast, 2 + O2 results in direct S-oxygenation to give a sulfonato product, PhSO3−. The positioning of the thiolate ligand in 1 versus 2 appears to play a critical role in determining the outcome of O2 activation. The thiolate ligands in 1 and 2 are essential for O2 reactivity and exhibit an important influence over the FeIII/FeII redox potential