100 research outputs found
Dinuclear Nickel Complexes of Thiolate-Functionalized Carbene Ligands and Their Electrochemical Properties
Four
dimeric nickelĀ(II) complexes [Ni<sub>2</sub>Cl<sub>2</sub>(BnC<sub>2</sub>S)<sub>2</sub>] [<b>1</b>], [Ni<sub>2</sub>Cl<sub>2</sub>(BnC<sub>3</sub>S)<sub>2</sub>] [<b>2</b>], [Ni<sub>2</sub>(PyC<sub>2</sub>S)<sub>2</sub>]ĀBr<sub>2</sub> [<b>3</b>]ĀBr<sub>2</sub>, and [Ni<sub>2</sub>(PyC<sub>3</sub>S)<sub>2</sub>]ĀBr<sub>2</sub> [<b>4</b>]ĀBr<sub>2</sub> 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
āC<sub>2</sub>ā or three āC<sub>3</sub>ā
carbon atoms) and the second functionality at the NHC being a benzyl
(Bn) or a pyridylmethyl (Py) group. The nickelĀ(II) ions are coordinated
to the NHC carbon atom and the pendent thiolate group, which bridges
to the second nickelĀ(II) ion creating the dinuclear structure. Additionally,
in compounds [<b>1</b>] and [<b>2</b>], the fourth coordination
position of the square-planar NiĀ(II) centers is occupied by the halide
ions, whereas in [<b>3</b>]<sup>2+</sup> and [<b>4</b>]<sup>2+</sup>, 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>]ĀBr<sub>2</sub> and [<b>4</b>]ĀBr<sub>2</sub>, 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<sub><i>a</i>3</sub>/Cu<sub>B</sub> 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 NO<sub>2</sub><sup>ā</sup> ion, producing nitric oxide. The heme serves as the reductant, but
the Cu<sup>II</sup> ion is also required. In turn, a Ī¼-oxo heme-Fe<sup>III</sup>āOāCu<sup>II</sup> complex facilitates NO
oxidation to nitrite; the final products are the reduced heme and
Cu<sup>II</sup>ānitrito complexes
Heme/Copper Assembly Mediated Nitrite and Nitric Oxide Interconversion
The heme<sub><i>a</i>3</sub>/Cu<sub>B</sub> 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 NO<sub>2</sub><sup>ā</sup> ion, producing nitric oxide. The heme serves as the reductant, but
the Cu<sup>II</sup> ion is also required. In turn, a Ī¼-oxo heme-Fe<sup>III</sup>āOāCu<sup>II</sup> complex facilitates NO
oxidation to nitrite; the final products are the reduced heme and
Cu<sup>II</sup>ānitrito complexes
Heme/Copper Assembly Mediated Nitrite and Nitric Oxide Interconversion
The heme<sub><i>a</i>3</sub>/Cu<sub>B</sub> 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 NO<sub>2</sub><sup>ā</sup> ion, producing nitric oxide. The heme serves as the reductant, but
the Cu<sup>II</sup> ion is also required. In turn, a Ī¼-oxo heme-Fe<sup>III</sup>āOāCu<sup>II</sup> complex facilitates NO
oxidation to nitrite; the final products are the reduced heme and
Cu<sup>II</sup>ānitrito complexes
Intermolecular Aliphatic CāFĀ·Ā·Ā·HāC Interaction in the Presence of āStrongerā Hydrogen Bond Acceptors: Crystallographic, Computational, and IR Studies
An
unprecedented intermolecular aliphatic CāFĀ·Ā·Ā·HāC
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 CāH stretch vibration frequency
blue-shifted by 14 cm<sup>ā1</sup> and <i>d</i>(FāH)
= 2.13 Ć
. In addition, the aliphatic CāF 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>]Āborepins 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 ĻāElectron 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. Boronās 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 Ļ-substituents,
leading to compounds featuring multistep electrochemical reductions
and polarizable electronic structures. As rare examples of directly
functionalizable, Ļ-conjugated, boron-containing polycyclic
aromatics, DTBs are promising building blocks for the next generation
of organoboron Ļ-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>]Āborepins 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)āHydroxide 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 O<sub>2</sub> activation (e.g.,
by Fe porphyrin in Cytochrome P450) and O<sub>2</sub> production (e.g.,
by the Mn cluster in Photosystem II). However, well-characterized
mononuclear M<sup>IV</sup>(OH) complexes remain a rarity. Herein we
describe the synthesis of Mn<sup>IV</sup>(OH)Ā(ttppc) (<b>3</b>) (ttppc = trisĀ(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<sup>V</sup>(O)Ā(ttppc) (<b>4</b>) and Mn<sup>III</sup>(H<sub>2</sub>O)Ā(ttppc) (<b>1</b>Ā·H<sub>2</sub>O) 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 OāH 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><sub>2</sub> = 2.73(12) Ć 10<sup>4</sup> M<sup>ā1</sup> s<sup>ā1</sup>, which is ā¼3
orders of magnitude larger than <b>4</b> (<i>k</i><sub>2</sub> = 17.4(1) M<sup>ā1</sup> s<sup>ā1</sup>). 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><sub>2</sub> = 510(10)ā36(1.4)
M<sup>ā1</sup> s<sup>ā1</sup> and led to Hammett and
Marcus plot correlations. Together with kinetic isotope effect measurements,
it is concluded that OāH cleavage occurs by a concerted H atom
transfer (HAT) mechanism and that the Mn<sup>IV</sup>(OH) complex
is a much more powerful H atom abstractor than the higher-valent Mn<sup>V</sup>(O) complex, or even some Fe<sup>IV</sup>(O) complexes
A Reactive Manganese(IV)āHydroxide 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 O<sub>2</sub> activation (e.g.,
by Fe porphyrin in Cytochrome P450) and O<sub>2</sub> production (e.g.,
by the Mn cluster in Photosystem II). However, well-characterized
mononuclear M<sup>IV</sup>(OH) complexes remain a rarity. Herein we
describe the synthesis of Mn<sup>IV</sup>(OH)Ā(ttppc) (<b>3</b>) (ttppc = trisĀ(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<sup>V</sup>(O)Ā(ttppc) (<b>4</b>) and Mn<sup>III</sup>(H<sub>2</sub>O)Ā(ttppc) (<b>1</b>Ā·H<sub>2</sub>O) 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 OāH 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><sub>2</sub> = 2.73(12) Ć 10<sup>4</sup> M<sup>ā1</sup> s<sup>ā1</sup>, which is ā¼3
orders of magnitude larger than <b>4</b> (<i>k</i><sub>2</sub> = 17.4(1) M<sup>ā1</sup> s<sup>ā1</sup>). 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><sub>2</sub> = 510(10)ā36(1.4)
M<sup>ā1</sup> s<sup>ā1</sup> and led to Hammett and
Marcus plot correlations. Together with kinetic isotope effect measurements,
it is concluded that OāH cleavage occurs by a concerted H atom
transfer (HAT) mechanism and that the Mn<sup>IV</sup>(OH) complex
is a much more powerful H atom abstractor than the higher-valent Mn<sup>V</sup>(O) complex, or even some Fe<sup>IV</sup>(O) complexes
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