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>

A Reactive Manganese(IV)–Hydroxide Complex: A Missing Intermediate in Hydrogen Atom Transfer by High-Valent Metal-Oxo Porphyrinoid Compounds

High-valent metal-hydroxide species are invoked as critical intermediates in both catalytic, metal-mediated O₂ activation (e.g., by Fe porphyrin in Cytochrome P450) and O₂ production (e.g., by the Mn cluster in Photosystem II). However, well-characterized mononuclear M^IV(OH) complexes remain a rarity. Herein we describe the synthesis of Mn^IV(OH)­(ttppc) (<b>3</b>) (ttppc = tris­(2,4,6-triphenylphenyl) corrole), which has been characterized by X-ray diffraction (XRD). The large steric encumbrance of the ttppc ligand allowed for isolation of <b>3</b>. The complexes Mn^V(O)­(ttppc) (<b>4</b>) and Mn^III(H₂O)­(ttppc) (<b>1</b>·H₂O) were also synthesized and structurally characterized, providing a series of Mn complexes related only by the transfer of hydrogen atoms. Both <b>3</b> and <b>4</b> abstract an H atom from the O–H bond of 2,4-di-<i>tert</i>-butylphenol (2,4-DTBP) to give a radical coupling product in good yield (<b>3</b> = 90(2)%, <b>4</b> = 91(5)%). Complex <b>3</b> reacts with 2,4-DTBP with a rate constant of <i>k</i>₂ = 2.73(12) × 10⁴ M^–1 s^–1, which is ∼3 orders of magnitude larger than <b>4</b> (<i>k</i>₂ = 17.4(1) M^–1 s^–1). Reaction of <b>3</b> with a series of <i>para</i>-substituted 2,6-di-<i>tert</i>-butylphenol derivatives (4-X-2,6-DTBP; X = OMe, Me, <i>t</i>Bu, H) gives rate constants in the range <i>k</i>₂ = 510(10)–36(1.4) M^–1 s^–1 and led to Hammett and Marcus plot correlations. Together with kinetic isotope effect measurements, it is concluded that O–H cleavage occurs by a concerted H atom transfer (HAT) mechanism and that the Mn^IV(OH) complex is a much more powerful H atom abstractor than the higher-valent Mn^V(O) complex, or even some Fe^IV(O) complexes

A Reactive Manganese(IV)–Hydroxide Complex: A Missing Intermediate in Hydrogen Atom Transfer by High-Valent Metal-Oxo Porphyrinoid Compounds

High-valent metal-hydroxide species are invoked as critical intermediates in both catalytic, metal-mediated O₂ activation (e.g., by Fe porphyrin in Cytochrome P450) and O₂ production (e.g., by the Mn cluster in Photosystem II). However, well-characterized mononuclear M^IV(OH) complexes remain a rarity. Herein we describe the synthesis of Mn^IV(OH)­(ttppc) (<b>3</b>) (ttppc = tris­(2,4,6-triphenylphenyl) corrole), which has been characterized by X-ray diffraction (XRD). The large steric encumbrance of the ttppc ligand allowed for isolation of <b>3</b>. The complexes Mn^V(O)­(ttppc) (<b>4</b>) and Mn^III(H₂O)­(ttppc) (<b>1</b>·H₂O) were also synthesized and structurally characterized, providing a series of Mn complexes related only by the transfer of hydrogen atoms. Both <b>3</b> and <b>4</b> abstract an H atom from the O–H bond of 2,4-di-<i>tert</i>-butylphenol (2,4-DTBP) to give a radical coupling product in good yield (<b>3</b> = 90(2)%, <b>4</b> = 91(5)%). Complex <b>3</b> reacts with 2,4-DTBP with a rate constant of <i>k</i>₂ = 2.73(12) × 10⁴ M^–1 s^–1, which is ∼3 orders of magnitude larger than <b>4</b> (<i>k</i>₂ = 17.4(1) M^–1 s^–1). Reaction of <b>3</b> with a series of <i>para</i>-substituted 2,6-di-<i>tert</i>-butylphenol derivatives (4-X-2,6-DTBP; X = OMe, Me, <i>t</i>Bu, H) gives rate constants in the range <i>k</i>₂ = 510(10)–36(1.4) M^–1 s^–1 and led to Hammett and Marcus plot correlations. Together with kinetic isotope effect measurements, it is concluded that O–H cleavage occurs by a concerted H atom transfer (HAT) mechanism and that the Mn^IV(OH) complex is a much more powerful H atom abstractor than the higher-valent Mn^V(O) complex, or even some Fe^IV(O) complexes

A Reactive Manganese(IV)–Hydroxide Complex: A Missing Intermediate in Hydrogen Atom Transfer by High-Valent Metal-Oxo Porphyrinoid Compounds

High-valent metal-hydroxide species are invoked as critical intermediates in both catalytic, metal-mediated O₂ activation (e.g., by Fe porphyrin in Cytochrome P450) and O₂ production (e.g., by the Mn cluster in Photosystem II). However, well-characterized mononuclear M^IV(OH) complexes remain a rarity. Herein we describe the synthesis of Mn^IV(OH)­(ttppc) (<b>3</b>) (ttppc = tris­(2,4,6-triphenylphenyl) corrole), which has been characterized by X-ray diffraction (XRD). The large steric encumbrance of the ttppc ligand allowed for isolation of <b>3</b>. The complexes Mn^V(O)­(ttppc) (<b>4</b>) and Mn^III(H₂O)­(ttppc) (<b>1</b>·H₂O) were also synthesized and structurally characterized, providing a series of Mn complexes related only by the transfer of hydrogen atoms. Both <b>3</b> and <b>4</b> abstract an H atom from the O–H bond of 2,4-di-<i>tert</i>-butylphenol (2,4-DTBP) to give a radical coupling product in good yield (<b>3</b> = 90(2)%, <b>4</b> = 91(5)%). Complex <b>3</b> reacts with 2,4-DTBP with a rate constant of <i>k</i>₂ = 2.73(12) × 10⁴ M^–1 s^–1, which is ∼3 orders of magnitude larger than <b>4</b> (<i>k</i>₂ = 17.4(1) M^–1 s^–1). Reaction of <b>3</b> with a series of <i>para</i>-substituted 2,6-di-<i>tert</i>-butylphenol derivatives (4-X-2,6-DTBP; X = OMe, Me, <i>t</i>Bu, H) gives rate constants in the range <i>k</i>₂ = 510(10)–36(1.4) M^–1 s^–1 and led to Hammett and Marcus plot correlations. Together with kinetic isotope effect measurements, it is concluded that O–H cleavage occurs by a concerted H atom transfer (HAT) mechanism and that the Mn^IV(OH) complex is a much more powerful H atom abstractor than the higher-valent Mn^V(O) complex, or even some Fe^IV(O) complexes

A Reactive Manganese(IV)–Hydroxide Complex: A Missing Intermediate in Hydrogen Atom Transfer by High-Valent Metal-Oxo Porphyrinoid Compounds

High-valent metal-hydroxide species are invoked as critical intermediates in both catalytic, metal-mediated O₂ activation (e.g., by Fe porphyrin in Cytochrome P450) and O₂ production (e.g., by the Mn cluster in Photosystem II). However, well-characterized mononuclear M^IV(OH) complexes remain a rarity. Herein we describe the synthesis of Mn^IV(OH)­(ttppc) (<b>3</b>) (ttppc = tris­(2,4,6-triphenylphenyl) corrole), which has been characterized by X-ray diffraction (XRD). The large steric encumbrance of the ttppc ligand allowed for isolation of <b>3</b>. The complexes Mn^V(O)­(ttppc) (<b>4</b>) and Mn^III(H₂O)­(ttppc) (<b>1</b>·H₂O) were also synthesized and structurally characterized, providing a series of Mn complexes related only by the transfer of hydrogen atoms. Both <b>3</b> and <b>4</b> abstract an H atom from the O–H bond of 2,4-di-<i>tert</i>-butylphenol (2,4-DTBP) to give a radical coupling product in good yield (<b>3</b> = 90(2)%, <b>4</b> = 91(5)%). Complex <b>3</b> reacts with 2,4-DTBP with a rate constant of <i>k</i>₂ = 2.73(12) × 10⁴ M^–1 s^–1, which is ∼3 orders of magnitude larger than <b>4</b> (<i>k</i>₂ = 17.4(1) M^–1 s^–1). Reaction of <b>3</b> with a series of <i>para</i>-substituted 2,6-di-<i>tert</i>-butylphenol derivatives (4-X-2,6-DTBP; X = OMe, Me, <i>t</i>Bu, H) gives rate constants in the range <i>k</i>₂ = 510(10)–36(1.4) M^–1 s^–1 and led to Hammett and Marcus plot correlations. Together with kinetic isotope effect measurements, it is concluded that O–H cleavage occurs by a concerted H atom transfer (HAT) mechanism and that the Mn^IV(OH) complex is a much more powerful H atom abstractor than the higher-valent Mn^V(O) complex, or even some Fe^IV(O) complexes

A Reactive Manganese(IV)–Hydroxide Complex: A Missing Intermediate in Hydrogen Atom Transfer by High-Valent Metal-Oxo Porphyrinoid Compounds

High-valent metal-hydroxide species are invoked as critical intermediates in both catalytic, metal-mediated O₂ activation (e.g., by Fe porphyrin in Cytochrome P450) and O₂ production (e.g., by the Mn cluster in Photosystem II). However, well-characterized mononuclear M^IV(OH) complexes remain a rarity. Herein we describe the synthesis of Mn^IV(OH)­(ttppc) (<b>3</b>) (ttppc = tris­(2,4,6-triphenylphenyl) corrole), which has been characterized by X-ray diffraction (XRD). The large steric encumbrance of the ttppc ligand allowed for isolation of <b>3</b>. The complexes Mn^V(O)­(ttppc) (<b>4</b>) and Mn^III(H₂O)­(ttppc) (<b>1</b>·H₂O) were also synthesized and structurally characterized, providing a series of Mn complexes related only by the transfer of hydrogen atoms. Both <b>3</b> and <b>4</b> abstract an H atom from the O–H bond of 2,4-di-<i>tert</i>-butylphenol (2,4-DTBP) to give a radical coupling product in good yield (<b>3</b> = 90(2)%, <b>4</b> = 91(5)%). Complex <b>3</b> reacts with 2,4-DTBP with a rate constant of <i>k</i>₂ = 2.73(12) × 10⁴ M^–1 s^–1, which is ∼3 orders of magnitude larger than <b>4</b> (<i>k</i>₂ = 17.4(1) M^–1 s^–1). Reaction of <b>3</b> with a series of <i>para</i>-substituted 2,6-di-<i>tert</i>-butylphenol derivatives (4-X-2,6-DTBP; X = OMe, Me, <i>t</i>Bu, H) gives rate constants in the range <i>k</i>₂ = 510(10)–36(1.4) M^–1 s^–1 and led to Hammett and Marcus plot correlations. Together with kinetic isotope effect measurements, it is concluded that O–H cleavage occurs by a concerted H atom transfer (HAT) mechanism and that the Mn^IV(OH) complex is a much more powerful H atom abstractor than the higher-valent Mn^V(O) complex, or even some Fe^IV(O) complexes

A Reactive Manganese(IV)–Hydroxide Complex: A Missing Intermediate in Hydrogen Atom Transfer by High-Valent Metal-Oxo Porphyrinoid Compounds

High-valent metal-hydroxide species are invoked as critical intermediates in both catalytic, metal-mediated O₂ activation (e.g., by Fe porphyrin in Cytochrome P450) and O₂ production (e.g., by the Mn cluster in Photosystem II). However, well-characterized mononuclear M^IV(OH) complexes remain a rarity. Herein we describe the synthesis of Mn^IV(OH)­(ttppc) (<b>3</b>) (ttppc = tris­(2,4,6-triphenylphenyl) corrole), which has been characterized by X-ray diffraction (XRD). The large steric encumbrance of the ttppc ligand allowed for isolation of <b>3</b>. The complexes Mn^V(O)­(ttppc) (<b>4</b>) and Mn^III(H₂O)­(ttppc) (<b>1</b>·H₂O) were also synthesized and structurally characterized, providing a series of Mn complexes related only by the transfer of hydrogen atoms. Both <b>3</b> and <b>4</b> abstract an H atom from the O–H bond of 2,4-di-<i>tert</i>-butylphenol (2,4-DTBP) to give a radical coupling product in good yield (<b>3</b> = 90(2)%, <b>4</b> = 91(5)%). Complex <b>3</b> reacts with 2,4-DTBP with a rate constant of <i>k</i>₂ = 2.73(12) × 10⁴ M^–1 s^–1, which is ∼3 orders of magnitude larger than <b>4</b> (<i>k</i>₂ = 17.4(1) M^–1 s^–1). Reaction of <b>3</b> with a series of <i>para</i>-substituted 2,6-di-<i>tert</i>-butylphenol derivatives (4-X-2,6-DTBP; X = OMe, Me, <i>t</i>Bu, H) gives rate constants in the range <i>k</i>₂ = 510(10)–36(1.4) M^–1 s^–1 and led to Hammett and Marcus plot correlations. Together with kinetic isotope effect measurements, it is concluded that O–H cleavage occurs by a concerted H atom transfer (HAT) mechanism and that the Mn^IV(OH) complex is a much more powerful H atom abstractor than the higher-valent Mn^V(O) complex, or even some Fe^IV(O) complexes

A Reactive Manganese(IV)–Hydroxide Complex: A Missing Intermediate in Hydrogen Atom Transfer by High-Valent Metal-Oxo Porphyrinoid Compounds

High-valent metal-hydroxide species are invoked as critical intermediates in both catalytic, metal-mediated O₂ activation (e.g., by Fe porphyrin in Cytochrome P450) and O₂ production (e.g., by the Mn cluster in Photosystem II). However, well-characterized mononuclear M^IV(OH) complexes remain a rarity. Herein we describe the synthesis of Mn^IV(OH)­(ttppc) (<b>3</b>) (ttppc = tris­(2,4,6-triphenylphenyl) corrole), which has been characterized by X-ray diffraction (XRD). The large steric encumbrance of the ttppc ligand allowed for isolation of <b>3</b>. The complexes Mn^V(O)­(ttppc) (<b>4</b>) and Mn^III(H₂O)­(ttppc) (<b>1</b>·H₂O) were also synthesized and structurally characterized, providing a series of Mn complexes related only by the transfer of hydrogen atoms. Both <b>3</b> and <b>4</b> abstract an H atom from the O–H bond of 2,4-di-<i>tert</i>-butylphenol (2,4-DTBP) to give a radical coupling product in good yield (<b>3</b> = 90(2)%, <b>4</b> = 91(5)%). Complex <b>3</b> reacts with 2,4-DTBP with a rate constant of <i>k</i>₂ = 2.73(12) × 10⁴ M^–1 s^–1, which is ∼3 orders of magnitude larger than <b>4</b> (<i>k</i>₂ = 17.4(1) M^–1 s^–1). Reaction of <b>3</b> with a series of <i>para</i>-substituted 2,6-di-<i>tert</i>-butylphenol derivatives (4-X-2,6-DTBP; X = OMe, Me, <i>t</i>Bu, H) gives rate constants in the range <i>k</i>₂ = 510(10)–36(1.4) M^–1 s^–1 and led to Hammett and Marcus plot correlations. Together with kinetic isotope effect measurements, it is concluded that O–H cleavage occurs by a concerted H atom transfer (HAT) mechanism and that the Mn^IV(OH) complex is a much more powerful H atom abstractor than the higher-valent Mn^V(O) complex, or even some Fe^IV(O) complexes

Addition of Dioxygen to an N₄S(thiolate) Iron(II) Cysteine Dioxygenase Model Gives a Structurally Characterized Sulfinato–Iron(II) Complex

The non-heme iron enzyme cysteine dioxygenase (CDO) catalyzes the S-oxygenation of cysteine by O₂ to give cysteine sulfinic acid. The synthesis of a new structural and functional model of the cysteine-bound CDO active site, [Fe^II(N3PyS)­(CH₃CN)]­BF₄ (<b>1</b>) is reported. This complex was prepared with a new facially chelating 4N/1S­(thiolate) pentadentate ligand. The reaction of <b>1</b> with O₂ resulted in oxygenation of the thiolate donor to afford the doubly oxygenated sulfinate product [Fe^II(N3PySO₂)­(NCS)] (<b>2</b>), which was crystallographically characterized. The thiolate donor provided by the new N3PyS ligand has a dramatic influence on the redox potential and O₂ reactivity of this Fe^II model complex

Mechanism of S-Oxygenation by a Cysteine Dioxygenase Model Complex

In this work, we present the first computational study on a biomimetic cysteine dioxygenase model complex, [Fe^II(LN₃S)]⁺, in which LN₃S is a tetradentate ligand with a bis(imino)pyridyl scaffold and a pendant arylthiolate group. The reaction mechanism of sulfur dioxygenation with O₂ was examined by density functional theory (DFT) methods and compared with results obtained for cysteine dioxygenase. The reaction proceeds via multistate reactivity patterns on competing singlet, triplet, and quintet spin state surfaces. The reaction mechanism is analogous to that found for cysteine dioxygenase enzymes (Kumar, D.; Thiel, W.; de Visser, S. P. <i>J. Am. Chem. Soc.</i> <b>2011</b>, <i>133</i>, 3869–3882); hence, the computations indicate that this complex can closely mimic the enzymatic process. The catalytic mechanism starts from an iron(III)–superoxo complex and the attack of the terminal oxygen atom of the superoxo group on the sulfur atom of the ligand. Subsequently, the dioxygen bond breaks to form an iron(IV)–oxo complex with a bound sulfenato group. After reorganization, the second oxygen atom is transferred to the substrate to give a sulfinic acid product. An alternative mechanism involving the direct attack of dioxygen on the sulfur, without involving any iron–oxygen intermediates, was also examined. Importantly, a significant energetic preference for dioxygen coordinating to the iron center prior to attack at sulfur was discovered and serves to elucidate the function of the metal ion in the reaction process. The computational results are in good agreement with experimental observations, and the differences and similarities of the biomimetic complex and the enzymatic cysteine dioxygenase center are highlighted