Dinuclear Nickel Complexes of Thiolate-Functionalized Carbene Ligands and Their Electrochemical Properties

Four dimeric nickel­(II) complexes [Ni₂Cl₂(BnC₂S)₂] [<b>1</b>], [Ni₂Cl₂(BnC₃S)₂] [<b>2</b>], [Ni₂(PyC₂S)₂]­Br₂ [<b>3</b>]­Br₂, and [Ni₂(PyC₃S)₂]­Br₂ [<b>4</b>]­Br₂ of four different thiolate-functionalized N-heterocyclic carbene (NHC) ligands were synthesized, and their structures have been determined by single-crystal X-ray crystallography. The four ligands differ by the alkyl chain length between the thiolate group and the benzimidazole nitrogen (two −C₂– or three −C₃– carbon atoms) and the second functionality at the NHC being a benzyl (Bn) or a pyridylmethyl (Py) group. The nickel­(II) ions are coordinated to the NHC carbon atom and the pendent thiolate group, which bridges to the second nickel­(II) ion creating the dinuclear structure. Additionally, in compounds [<b>1</b>] and [<b>2</b>], the fourth coordination position of the square-planar Ni­(II) centers is occupied by the halide ions, whereas in [<b>3</b>]²⁺ and [<b>4</b>]²⁺, the additional pendant pyridylmethyl groups complete the coordination spheres of the nickel ions. The electrochemical properties of the four complexes were studied using cyclic voltammetry and controlled-potential coulometry methods. The thiolate-functionalized carbene complexes [<b>1</b>] and [<b>2</b>] appear to be poor electrocatalysts for the hydrogen evolution reaction; the complexes [<b>3</b>]­Br₂ and [<b>4</b>]­Br₂, bearing an extra pyridylmethyl group, show higher catalytic activity in proton reduction, indicating that the pyridine group plays an important role in the catalytic cycle

Heme/Copper Assembly Mediated Nitrite and Nitric Oxide Interconversion

The heme_<i>a</i>3/Cu_B active site of cytochrome <i>c</i> oxidase is responsible for cellular nitrite reduction to nitric oxide; the same center can return NO to the nitrite pool via oxidative chemistry. Here, we show that a partially reduced heme/Cu assembly reduces NO₂^– ion, producing nitric oxide. The heme serves as the reductant, but the Cu^II ion is also required. In turn, a μ-oxo heme-Fe^III–O–Cu^II complex facilitates NO oxidation to nitrite; the final products are the reduced heme and Cu^II–nitrito complexes

Heme/Copper Assembly Mediated Nitrite and Nitric Oxide Interconversion

The heme_<i>a</i>3/Cu_B active site of cytochrome <i>c</i> oxidase is responsible for cellular nitrite reduction to nitric oxide; the same center can return NO to the nitrite pool via oxidative chemistry. Here, we show that a partially reduced heme/Cu assembly reduces NO₂^– ion, producing nitric oxide. The heme serves as the reductant, but the Cu^II ion is also required. In turn, a μ-oxo heme-Fe^III–O–Cu^II complex facilitates NO oxidation to nitrite; the final products are the reduced heme and Cu^II–nitrito complexes

Heme/Copper Assembly Mediated Nitrite and Nitric Oxide Interconversion

The heme_<i>a</i>3/Cu_B active site of cytochrome <i>c</i> oxidase is responsible for cellular nitrite reduction to nitric oxide; the same center can return NO to the nitrite pool via oxidative chemistry. Here, we show that a partially reduced heme/Cu assembly reduces NO₂^– ion, producing nitric oxide. The heme serves as the reductant, but the Cu^II ion is also required. In turn, a μ-oxo heme-Fe^III–O–Cu^II complex facilitates NO oxidation to nitrite; the final products are the reduced heme and Cu^II–nitrito complexes

Intermolecular Aliphatic C–F···H–C Interaction in the Presence of “Stronger” Hydrogen Bond Acceptors: Crystallographic, Computational, and IR Studies

An unprecedented intermolecular aliphatic C–F···H–C interaction was observed in the X-ray crystal structure of a fluorinated triterpenoid. Despite the notion of fluorine being a poor acceptor, computational and IR studies revealed this interaction to be a weak to moderate hydrogen bond with a C–H stretch vibration frequency blue-shifted by 14 cm^–1 and <i>d</i>(F–H) = 2.13 Å. In addition, the aliphatic C–F bond is the preferred acceptor in the presence of multiple, traditionally <i>stronger</i> oxygen-based hydrogen bond acceptors

Aromaticity Competition in Differentially Fused Borepin-Containing Polycyclic Aromatics

This report describes the synthesis and characterization of a series of borepin-based polycyclic aromatics bearing two different arene fusions. The borepin synthesis features streamlined Ti-mediated alkyne reduction, leading to <i>Z</i>-olefins, followed by direct lithiation and borepin formation. These molecules allow for an assessment of aromatic competition between the fused rings and the central borepin core. Crystallographic, magnetic, and computational studies yielded insights about the aromaticity of novel, differentially fused [<i>b</i>,<i>f</i>]­borepins and allowed for comparison to literature compounds. Multiple borepin motifs were also incorporated into polycyclic aromatics with five or six rings in the main backbone, and their properties were also evaluated

Thiophene-Fused Borepins As Directly Functionalizable Boron-Containing π‑Electron Systems

Synthetic protocols were developed for the gram-scale preparation of two isomeric dithienoborepins (DTBs), boron-containing polycyclic aromatics featuring the fusion of borepin and thiophene rings. DTBs exhibit reversible cathodic electrochemistry and boron-centered Lewis acidity in addition to enhanced electronic delocalization relative to benzo-fused analogues. Boron’s precise position within the conjugation pathway of DTBs significantly affected electronic structure, most clearly demonstrated by the variation in spectroscopic responses of each isomer to fluoride ion binding. In addition to excellent stability in the presence of air and moisture, DTBs could also be subjected to electrophilic aromatic substitution and metalation chemistry, the latter enabling the direct, regiospecific functionalization of the unsubstituted thiophene rings. Subsequent tuning of molecular properties was achieved through installation of donor and acceptor π-substituents, leading to compounds featuring multistep electrochemical reductions and polarizable electronic structures. As rare examples of directly functionalizable, π-conjugated, boron-containing polycyclic aromatics, DTBs are promising building blocks for the next generation of organoboron π-electron materials whose development will demand broad scope for molecular diversification in addition to chemical robustness

Aromaticity Competition in Differentially Fused Borepin-Containing Polycyclic Aromatics

This report describes the synthesis and characterization of a series of borepin-based polycyclic aromatics bearing two different arene fusions. The borepin synthesis features streamlined Ti-mediated alkyne reduction, leading to <i>Z</i>-olefins, followed by direct lithiation and borepin formation. These molecules allow for an assessment of aromatic competition between the fused rings and the central borepin core. Crystallographic, magnetic, and computational studies yielded insights about the aromaticity of novel, differentially fused [<i>b</i>,<i>f</i>]­borepins and allowed for comparison to literature compounds. Multiple borepin motifs were also incorporated into polycyclic aromatics with five or six rings in the main backbone, and their properties were also evaluated

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