8 research outputs found
Asymmetric Epoxidation with H<sub>2</sub>O<sub>2</sub> by Manipulating the Electronic Properties of Non-heme Iron Catalysts
A non-heme
iron complex that catalyzes highly enantioselective
epoxidation of olefins with H<sub>2</sub>O<sub>2</sub> is described.
Improvement of enantiomeric excesses is attained by the use of catalytic
amounts of carboxylic acid additives. Electronic effects imposed by
the ligand on the iron center are shown to synergistically cooperate
with catalytic amounts of carboxylic acids in promoting efficient
OāO cleavage and creating highly chemo- and enantioselective
epoxidizing species which provide a broad range of epoxides in synthetically
valuable yields and short reaction times
Zirconium Hydrazides as Metallanitrene Synthons: Release of Molecular N<sub>2</sub> from a Hydrazinediido Complex Induced by Oxidative NāN Bond Cleavage
The NāN bond in the zirconium
hydrazinediido(2ā)
complex [ZrĀ(N<sub>2</sub><sup>TBS</sup>N<sub>py</sub>)Ā(NNPh<sub>2</sub>)Ā(py)] (<b>1</b>) is readily cleaved by one-electron oxidation.
Reacting [ZrĀ(N<sub>2</sub><sup>TBS</sup>N<sub>py</sub>)Ā(NNPh<sub>2</sub>)Ā(py)] (<b>1</b>) with 0.5 molar equiv of iodine led to the
release of molecular N<sub>2</sub> and yielded the mixed diphenylamido/iodo
complex [ZrĀ(N<sub>2</sub><sup>TBS</sup>N<sub>py</sub>NPh<sub>2</sub>)Ā(I)] (<b>2</b>). Exposure of hydrazinediide <b>1</b> to an excess of iodine resulted in further oxidation of the diphenylamido
ligand, yielding the diiodo complex <b>3</b> and tetraphenylhydrazine.
Similar reactivity was observed in the reaction of <b>1</b> with
diphenyl diselenide and diaryl disulfides, which reacted to give the
corresponding diphenylamido/arylchalcogenido complexes [ZrĀ(N<sub>2</sub><sup>TBS</sup>N<sub>py</sub>NPh<sub>2</sub>)Ā(SePh)] (<b>4a</b>) and [ZrĀ(N<sub>2</sub><sup>TBS</sup>N<sub>py</sub>)Ā(NPh<sub>2</sub>)Ā(SAr)] (Ar = Ph (<b>4b</b>), C<sub>6</sub>F<sub>5</sub> (<b>4c</b>)) along with N<sub>2</sub>. The reactions were also carried
out on an NMR scale with a <sup>15</sup>N<sub>Ī±</sub>-labeled
hydrazido complex (<b>1-</b><sup><b>15</b></sup><b>N</b>). In all cases a single <sup>15</sup>N NMR resonance at
310.16 ppm, assigned to <sup>15</sup>N<sub>2</sub>, indicated the
formation of dinitrogen from the N<sub>Ī±</sub> atom in the hydrazide.
A crossover labeling experiment employing a 1:1 mixture of <b>1</b> and <sup>15</sup>N<sub>Ī±</sub>-labeled <b>1-</b><sup><b>15</b></sup><b>N</b> revealed that the isotope distribution
is, as expected, statistical 1:2:1 (<sup>14</sup>N<sub>2</sub>: <sup>14/15</sup>N<sub>2</sub>: <sup>15</sup>N<sub>2</sub>), which is consistent
with a reaction pathway involving a dinuclear intermediate in the
dinitrogen-forming step. Complex <b>1</b> reacted with N<sub>2</sub>O to give a mixture of two compounds, the bisĀ(diphenylamido)
complex <b>6</b> and the doubly bridged Ī¼-oxo complex <b>7</b>. In contrast, reaction of <b>1</b> with 1 molar equiv
of pyridinium <i>N</i>-oxide only gave the doubly bridged
Ī¼-oxo complex <b>7</b> along with 2,2ā²-bipyridine
and diphenylamine
Highly Stereoselective Epoxidation with H<sub>2</sub>O<sub>2</sub> Catalyzed by Electron-Rich Aminopyridine Manganese Catalysts
Fast, efficient, and highly stereoselective epoxidation with H<sub>2</sub>O<sub>2</sub> is reached by manganese coordination complexes with e-rich aminopyridine tetradentate ligands. It is shown that the electronic properties of these catalysts vary systematically with the stereoselectivity of the O-atom transfer event and exert fine control over the activation of hydrogen peroxide, reducing the amount of carboxylic acid co-catalyst necessary for efficient operation
Highly Stereoselective Epoxidation with H<sub>2</sub>O<sub>2</sub> Catalyzed by Electron-Rich Aminopyridine Manganese Catalysts
Fast, efficient, and highly stereoselective epoxidation with H<sub>2</sub>O<sub>2</sub> is reached by manganese coordination complexes with e-rich aminopyridine tetradentate ligands. It is shown that the electronic properties of these catalysts vary systematically with the stereoselectivity of the O-atom transfer event and exert fine control over the activation of hydrogen peroxide, reducing the amount of carboxylic acid co-catalyst necessary for efficient operation
Toward the Understanding of the StructureāActivity Correlation in Single-Site Mn Covalent Organic Frameworks for Electrocatalytic CO<sub>2</sub> Reduction
The encapsulation of organometallic complexes into reticular
covalent
organic frameworks (COFs) represents an effective strategy for the
immobilization of molecular electrocatalysts. In particular, well-defined
polypyridyl Mn sites embedded into a crystalline COF backbone (COFbpyMn) were found to exhibit higher
selectivity and activity toward electrochemical CO2 reduction
compared to the parent molecular derivative noncovalently immobilized
on carbon electrodes. In situ mechanistic studies revealed that the
electronic and steric features of the reticular framework strongly
affect the redox mechanism of the Mn sites, stabilizing the formation
of a mononuclear Mn(I) radical anion intermediate over the most common
off-cycle Mn0āMn0 dimer. Herein, we report
the study of a Mn-based COF (COFPTMn), introducing a larger phenanthroline building block, to explore
how tuning the structural and electronic properties of the lattice
may affect the catalytic CO2 reduction performance and
the mechanism at the molecular level of the reticular system. The
Mn sites encapsulated into the reticular COFPTMn exhibited a remarkable enhancement in the intrinsic
catalytic CO2 reduction activity at near-neutral pH compared
to that of the corresponding noncovalently immobilized molecular derivative.
On the other hand, the poor crystallinity and porosity of COFPTMn, likely introduced by the lattice expansion
and spatial dynamics of the phenanthroline linker, were found to limit
its catalytic performances compared to those of the bipyridyl COFbpyMn analogue. ATR-IR spectroelectrochemistry
revealed that the higher spatial mobility of the Mn sites does not
completely suppress the Mn0āMn0 dimerization
upon the electrochemical reduction of the Mn sites at the COFbpyMn. This work highlights the positive
role of the reticular structure of the material in enhancing its catalytic
activity versus that of its molecular counterpart and provides useful
hints for the future design and development of efficient reticular
frameworks for electrocatalytic applications
Oxidant-Free Au(I)-Catalyzed Halide Exchange and C<sub>sp2</sub>āO Bond Forming Reactions
Au
has been demonstrated to mediate a number of organic transformations
through the utilization of its Ļ Lewis acid character, AuĀ(I)/AuĀ(III)
redox properties or a combination of both. As a result of the high
oxidation potential of the AuĀ(I)/AuĀ(III) couple, redox catalysis involving
Au typically requires the use of a strong external oxidant. This study
demonstrates unusual external oxidant-free AuĀ(I)-catalyzed halide
exchange (including fluorination) and C<sub>sp2</sub>āO bond
formation reactions utilizing a model aryl halide macrocyclic substrate.
Additionally, the halide exchange and C<sub>sp2</sub>āO coupling
reactivity could also be extrapolated to substrates bearing a single
chelating group, providing further insight into the reaction mechanism.
This work provides the first examples of external oxidant-free AuĀ(I)-catalyzed
carbonāheteroatom cross-coupling reactions
Triggering the Generation of an Iron(IV)-Oxo Compound and Its Reactivity toward Sulfides by Ru<sup>II</sup> Photocatalysis
The
preparation of [Fe<sup>IV</sup>(O)Ā(MePy<sub>2</sub>tacn)]<sup>2+</sup> (<b>2</b>, MePy<sub>2</sub>tacn = <i>N</i>-methyl-<i>N</i>,<i>N</i>-bisĀ(2-picolyl)-1,4,7-triazacyclononane)
by reaction of [Fe<sup>II</sup>(MePy<sub>2</sub>tacn)Ā(solvent)]<sup>2+</sup> (<b>1</b>) and PhIO in CH<sub>3</sub>CN and its full
characterization are described. This compound can also be prepared
photochemically from its ironĀ(II) precursor by irradiation at 447
nm in the presence of catalytic amounts of [Ru<sup>II</sup>(bpy)<sub>3</sub>]<sup>2+</sup> as photosensitizer and a sacrificial electron
acceptor (Na<sub>2</sub>S<sub>2</sub>O<sub>8</sub>). Remarkably, the
rate of the reaction of the photochemically prepared compound <b>2</b> toward sulfides increases 150-fold under irradiation, and <b>2</b> is partially regenerated after the sulfide has been consumed;
hence, the process can be repeated several times. The origin of this
rate enhancement has been established by studying the reaction of
chemically generated compound <b>2</b> with sulfides under different
conditions, which demonstrated that both light and [Ru<sup>II</sup>(bpy)<sub>3</sub>]<sup>2+</sup> are necessary for the observed increase
in the reaction rate. A combination of nanosecond time-resolved absorption
spectroscopy with laser pulse excitation and other mechanistic studies
has led to the conclusion that an electron transfer mechanism is the
most plausible explanation for the observed rate enhancement. According
to this mechanism, the in-situ-generated [Ru<sup>III</sup>(bpy)<sub>3</sub>]<sup>3+</sup> oxidizes the sulfide to form the corresponding
radical cation, which is eventually oxidized by <b>2</b> to
the corresponding sulfoxide
Spectroscopic and DFT Characterization of a Highly Reactive Nonheme Fe<sup>V</sup>āOxo Intermediate
The
reaction of [(PyNMe<sub>3</sub>)ĀFe<sup>II</sup>(CF<sub>3</sub>SO<sub>3</sub>)<sub>2</sub>], <b>1</b>, with excess peracetic
acid at ā40 Ā°C generates a highly reactive intermediate, <b>2b</b>(PAA), that has the fastest rate to date for oxidizing cyclohexane
by a nonheme iron species. It exhibits an intense 490 nm chromophore
associated with an <i>S</i> = 1/2 EPR signal having <i>g</i>-values at 2.07, 2.01, and 1.94. This species was shown
to be in a fast equilibrium with a second <i>S</i> = 1/2
species, <b>2a</b>(PAA), assigned to a low-spin acylperoxoironĀ(III)
center. Unfortunately, contaminants accompanying the <b>2</b>(PAA) samples prevented determination of the iron oxidation state
by MoĢssbauer spectroscopy. Use of MeO-PyNMe<sub>3</sub> (an
electron-enriched version of PyNMe<sub>3</sub>) and cyclohexyl peroxycarboxylic
acid as oxidant affords intermediate <b>3b</b>(CPCA) with a
MoĢssbauer isomer shift Ī“ = ā0.08 mm/s that indicates
an ironĀ(V) oxidation state. Analysis of the MoĢssbauer and EPR
spectra, combined with DFT studies, demonstrates that the electronic
ground state of <b>3b</b>(CPCA) is best described as a quantum
mechanical mixture of [(MeO-PyNMe<sub>3</sub>)ĀFe<sup>V</sup>(O)Ā(OCĀ(O)ĀR)]<sup>2+</sup> (ā¼75%) with some Fe<sup>IV</sup>(O)Ā(<sup>ā¢</sup>OCĀ(O)ĀR) and Fe<sup>III</sup>(OOCĀ(O)ĀR) character. DFT studies of <b>3b</b>(CPCA) reveal that the unbound oxygen of the carboxylate
ligand, O2, is only 2.04 Ć
away from the oxo group, O1, corresponding
to a Wiberg bond order for the O1āO2 bond of 0.35. This unusual
geometry facilitates reversible O1āO2 bond formation and cleavage
and accounts for the high reactivity of the intermediate when compared
to the rates of hydrogen atom transfer and oxygen atom transfer reactions
of Fe<sup>III</sup>(OCĀ(O)ĀR) ferric acyl peroxides and Fe<sup>IV</sup>(O) complexes. The interaction of O2 with O1 leads to a significant
downshift of the FeāO1 Raman frequency (815 cm<sup>ā1</sup>) relative to the 903 cm<sup>ā1</sup> value predicted for
the hypothetical [(MeO-PyNMe<sub>3</sub>)ĀFe<sup>V</sup>(O)Ā(NCMe)]<sup>3+</sup> complex