25 research outputs found
Swallow-Tailed Alkyl and Linear Alkoxy-Substituted Dibenzocoronene Tetracarboxdiimide Derivatives: Synthesis, Photophysical Properties, and Thermotropic Behaviors
A series
of dibenzocoronene tetracarboxdiimide derivatives decorated
with alkyl swallow-tail and alkoxy moieties were synthesized, and
their structures were characterized. 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ) as an effective oxidant was first used in the benzannulation
of perylene diimides with the almost quantitative yield. The thermotropic
behavior was investigated using differential scanning calorimetry
(DSC) and polarization optical microscopy (POM). The introduction
of alkyl swallow-tail and alkoxy substituents facilitates thermotropic
liquid crystalline behavior. The branching site of alkyl swallow-tail
units at the Ī± position and the longer alkoxy chains played
a similar role in lowering the mesophase transition as well as isotropization
transition temperatures. The UVāvis absorption spectra of all
compounds appeared as absorption in 425ā600 nm region, and
POM images of certain compounds exhibited characteristic columnar
hexagonal (Col<sub>h</sub>) packing and readily self-assembled into
a homeotropic alignment toward the substrate
New Reactions of Terminal Hydrides on a Diiron Dithiolate
Mechanisms
for biological and bioinspired dihydrogen activation
and production often invoke the intermediacy of diiron dithiolato
dihydrides. The first example of such a Fe<sub>2</sub>(SR)<sub>2</sub>H<sub>2</sub> species is provided by the complex [(<i>term</i>-H)Ā(Ī¼-H)ĀFe<sub>2</sub>(pdt)Ā(CO)Ā(dppv)<sub>2</sub>] ([H<b>1</b>H]<sup>0</sup>). Spectroscopic and computational studies
indicate that [H<b>1</b>H]<sup>0</sup> contains both a bridging
hydride and a terminal hydride, which, notably, occupies a basal site.
The synthesis begins with [(Ī¼-H)ĀFe<sub>2</sub>(pdt)Ā(CO)<sub>2</sub>(dppv)<sub>2</sub>]<sup>+</sup> ([H<b>1</b>(CO)]<sup>+</sup>), which undergoes substitution to afford [(Ī¼-H)ĀFe<sub>2</sub>(pdt)Ā(CO)Ā(NCMe)Ā(dppv)<sub>2</sub>]<sup>+</sup> ([H<b>1</b>(NCMe)]<sup>+</sup>). Upon treatment of [H<b>1</b>(NCMe)]<sup>+</sup> with borohydride salts, the MeCN ligand is displaced to afford
[H<b>1</b>H]<sup>0</sup>. DNMR (EXSY, SST) experiments on this
complex show that the terminal and bridging hydride ligands interchange
intramolecularly at a rate of 1 s<sup>ā1</sup> at ā40
Ā°C. The compound reacts with D<sub>2</sub> to afford [D<b>1</b>D]<sup>0</sup>, but not mixed isotopomers such as [H<b>1</b>D]<sup>0</sup>. The dihydride undergoes oxidation with Fc<sup>+</sup> under CO to give [<b>1</b>(CO)]<sup>+</sup> and H<sub>2</sub>. Protonation in MeCN solution gives [H<b>1</b>(NCMe)]<sup>+</sup> and H<sub>2</sub>. Carbonylation converts [H<b>1</b>H]<sup>0</sup> into [<b>1</b>(CO)]<sup>0</sup>
Unsensitized Photochemical Hydrogen Production Catalyzed by Diiron Hydrides
The diiron hydride [(Ī¼-H)ĀFe<sub>2</sub>(pdt)Ā(CO)<sub>4</sub>(dppv)]<sup>+</sup> ([H<b>2</b>]<sup>+</sup>, dppv = <i>cis</i>-1,2-C<sub>2</sub>H<sub>2</sub>(PPh<sub>2</sub>)<sub>2</sub>) is shown to be an effective photocatalyst for the H<sub>2</sub> evolution reaction (HER). These experiments establish the
role of hydrides in photocatalysis by biomimetic diiron complexes.
Trends in redox potentials suggests that other unsymmetrically substituted
diiron hydrides are promising catalysts. Unlike previous catalysts
for photo-HER, [H<b>2</b>]<sup>+</sup> functions without sensitizers:
irradiation of [H<b>2</b>]<sup>+</sup> in the presence of triflic
acid (HOTf) efficiently affords H<sub>2</sub>. Instead of sacrificial
electron donors, ferrocenes can be used as recyclable electron donors
for the photocatalyzed HER, resulting in 4 turnovers
Isolation of a Mixed Valence Diiron Hydride: Evidence for a Spectator Hydride in Hydrogen Evolution Catalysis
The mixed-valence
diiron hydrido complex (Ī¼-H)ĀFe<sub>2</sub>(pdt)Ā(CO)<sub>2</sub>(dppv)<sub>2</sub> ([H<b>1</b>]<sup>0</sup>, where pdt =1,3-propanedithiolate
and dppv = <i>cis</i>-1,2-C<sub>2</sub>H<sub>2</sub>(PPh<sub>2</sub>)<sub>2</sub>), was
generated by reduction of the differous hydride [H<b>1</b>]<sup>+</sup> using decamethylcobaltocene. Crystallographic analysis shows
that [H<b>1</b>]<sup>0</sup> retains the stereochemistry of
its precursor, where one dppv ligand spans two basal sites and the
other spans apical and basal positions. The Fe---Fe bond elongates
to 2.80 from 2.66 Ć
, but the FeāP bonds only change subtly.
Although the FeāH distances are indistinguishable in the precursor,
they differ by 0.2 Ć
in [H<b>1</b>]<sup>0</sup>. The X-band
electron paramagnetic resonance (EPR) spectrum reveals the presence
of two stereoisomers, the one characterized crystallographically and
a contribution of about 10% from a second symmetrical (<i>sym</i>) isomer wherein both dppv ligands occupy apicalābasal sites.
The unsymmetrical (<i>unsym</i>) arrangement of the dppv
ligands is reflected in the values of <i>A</i>(<sup>31</sup>P), which range from 31 MHz for the basal phosphines to 284 MHz for
the apical phosphine. Density functional theory calculations were
employed to rationalize the electronic structure of [H<b>1</b>]<sup>0</sup> and to facilitate spectral simulation and assignment
of EPR parameters including <sup>1</sup>H and <sup>31</sup>P hyperfine
couplings. The EPR spectra of [H<b>1</b>]<sup>0</sup> and [D<b>1</b>]<sup>0</sup> demonstrate that the singly occupied molecular
orbital is primarily localized on the Fe center with the longer bond
to H, that is, Fe<sup>II</sup>āHĀ·Ā·Ā·Fe<sup>I</sup>. The coupling to the hydride is <i>A</i>(<sup>1</sup>H)
= 55 and 74 MHz for <i>unsym</i>- amd <i>sym</i>-[H<b>1</b>]<sup>0</sup>, respectively. Treatment of [H<b>1</b>]<sup>0</sup> with H<sup>+</sup> gives 0.5 equiv of H<sub>2</sub> and [H<b>1</b>]<sup>+</sup>. Reduction of D<sup>+</sup> affords D<sub>2</sub>, leaving the hydride ligand intact. These
experiments demonstrate that the bridging hydride ligand in this complex
is a spectator in the hydrogen evolution reaction
Computational Investigation of [FeFe]-Hydrogenase Models: Characterization of Singly and Doubly Protonated Intermediates and Mechanistic Insights
The [FeFe]-hydrogenase enzymes catalyze
hydrogen oxidation and production efficiently with binuclear Fe metal
centers. Recently the bioinspired H<sub>2</sub>-producing model system
Fe<sub>2</sub>(adt)Ā(CO)<sub>2</sub>(dppv)<sub>2</sub> (adt=azadithiolate
and dppv=diphosphine) was synthesized and studied experimentally.
In this system, the azadithiolate bridge facilitates the formation
of a doubly protonated ammonium-hydride species through a proton relay.
Herein computational methods are utilized to examine this system in
the various oxidation states and protonation states along proposed
mechanistic pathways for H<sub>2</sub> production. The calculated
results agree well with the experimental data for the geometries,
CO vibrational stretching frequencies, and reduction potentials. The
calculations illustrate that the NHĀ·Ā·Ā·HFe dihydrogen
bonding distance in the doubly protonated species is highly sensitive
to the effects of ion-pairing between the ammonium and BF<sub>4</sub><sup>ā</sup> counterions, which are present in the crystal
structure, in that the inclusion of BF<sub>4</sub><sup>ā</sup> counterions leads to a significantly longer dihydrogen bond. The
non-hydride Fe center was found to be the site of reduction for terminal
hydride species and unsymmetric bridging hydride species, whereas
the reduced symmetric bridging hydride species exhibited spin delocalization
between the Fe centers. According to both experimental measurements
and theoretical calculations of the relative p<i>K</i><sub>a</sub> values, the Fe<sub>d</sub> center of the neutral species
is more basic than the amine, and the bridging hydride species is
more thermodynamically stable than the terminal hydride species. The
calculations implicate a possible pathway for H<sub>2</sub> evolution
that involves an intermediate with H<sub>2</sub> weakly bonded to
one Fe, a short H<sub>2</sub> distance similar to the molecular bond
length, the spin density delocalized over the two Fe centers, and
a nearly symmetrically bridged CO ligand. Overall, this study illustrates
the mechanistic roles of the ammonium-hydride interaction, flexibility
of the bridging CO ligand, and intramolecular electron transfer between
the Fe centers in the catalytic cycle. Such insights will assist in
the design of more effective bioinspired catalysts for H<sub>2</sub> production
Iron-Catalyzed 1,2-Selective Hydroboration of <i>N</i>āHeteroarenes
A N<sub>2</sub>-bridged diiron complex
[Cp*Ā(Ph<sub>2</sub>PC<sub>6</sub>ĀH<sub>4</sub>S)ĀFe]<sub>2</sub>Ā(Ī¼-N<sub>2</sub>) (<b>1</b>) has been found
to catalyze the hydroboration
of <i>N</i>-heteroarenes with pinacolborane, giving <i>N</i>-borylated 1,2-reduced products with high regioselectivity.
The catalysis is initiated by coordination of <i>N</i>-heteroarenes
to the iron center, while the BāH bond cleavage is the rate-determining
step
Hydride Transfer from Iron(II) Hydride Compounds to NAD(P)<sup>+</sup> Analogues
IronĀ(II) hydride complexes of the
āpiano-stoolā type, Cp*Ā(P-P)ĀFeH (P-P = dppe (<b>1H</b>), dppbz (<b>2H</b>), dppm (<b>3H</b>), dcpe (<b>4H</b>)) were examined as hydride donors in the reduction of <i>N</i>-benzylpyridinium and acridinium salts. Two pathways of hydride transfer,
āsingle-step H<sup>ā</sup>ā transfer to pyridinium
and a ātwo-step (e<sup>ā</sup>/H<sup>ā¢</sup>)ā
transfer for acridinium reduction, were observed. When 1-benzylnicotinamide
cation (BNA<sup>+</sup>) was used as an H<sup>ā</sup> acceptor,
kinetic studies suggested that <b>BNA</b><sup><b>+</b></sup> was reduced at the C6 position, affording 1,6-BNAH, which
can be converted to the more thermally stable 1,4-product. The rate
constant <i>k</i> of H<sup>ā</sup> transfer was very
sensitive to the bite angle of PāFeāP in Cp*Ā(P-P)ĀFeH
and ranged from 3.23 Ć 10<sup>ā3</sup> M<sup>ā1</sup> s<sup>ā1</sup> for dppe to 1.74 Ć 10<sup>ā1</sup> M<sup>ā1</sup> s<sup>ā1</sup> for dppm. The results
obtained from reduction of a range of <i>N</i>-benzylpyridinium
derivatives suggest that H<sup>ā</sup> transfer is more likely
to be charge controlled. In the reduction of 10-methylacridinium ion
(<b>Acr</b><sup><b>+</b></sup>), the reaction was initiated
by an e<sup>ā</sup> transfer (ET) process and then followed
by rapid disproportionation reactions to produce <b>Acr</b><sub><b>2</b></sub> dimer and release of H<sub>2</sub>. To achieve
H<sup>ā¢</sup> transfer after ET from [Cp*Ā(P-P)ĀFeH]<sup>+</sup> to acridine radicals, the bulkier acridinium salt 9-phenyl-10-methylacridinium
(<b>PhAcr</b><sup><b>+</b></sup>) was selected as an acceptor.
More evidence for this ātwo-step (e<sup>ā</sup>/H<sup>ā¢</sup>)ā process was derived from the characterization
of <b>PhAcr<sup>ā¢</sup></b> and [<b>4H</b>]<sup><b>+</b></sup> radicals by EPR spectra and by the crystallographic
structure confirmation of [<b>4H</b>]<sup><b>+</b></sup>. Our mechanistic understanding of fundamental H<sup>ā</sup> transfer from ironĀ(II) hydrides to NAD<sup>+</sup> analogues provides
insight into establishing attractive bio-organometallic transformation
cycles driven by iron catalysis
Hydride Transfer from Iron(II) Hydride Compounds to NAD(P)<sup>+</sup> Analogues
IronĀ(II) hydride complexes of the
āpiano-stoolā type, Cp*Ā(P-P)ĀFeH (P-P = dppe (<b>1H</b>), dppbz (<b>2H</b>), dppm (<b>3H</b>), dcpe (<b>4H</b>)) were examined as hydride donors in the reduction of <i>N</i>-benzylpyridinium and acridinium salts. Two pathways of hydride transfer,
āsingle-step H<sup>ā</sup>ā transfer to pyridinium
and a ātwo-step (e<sup>ā</sup>/H<sup>ā¢</sup>)ā
transfer for acridinium reduction, were observed. When 1-benzylnicotinamide
cation (BNA<sup>+</sup>) was used as an H<sup>ā</sup> acceptor,
kinetic studies suggested that <b>BNA</b><sup><b>+</b></sup> was reduced at the C6 position, affording 1,6-BNAH, which
can be converted to the more thermally stable 1,4-product. The rate
constant <i>k</i> of H<sup>ā</sup> transfer was very
sensitive to the bite angle of PāFeāP in Cp*Ā(P-P)ĀFeH
and ranged from 3.23 Ć 10<sup>ā3</sup> M<sup>ā1</sup> s<sup>ā1</sup> for dppe to 1.74 Ć 10<sup>ā1</sup> M<sup>ā1</sup> s<sup>ā1</sup> for dppm. The results
obtained from reduction of a range of <i>N</i>-benzylpyridinium
derivatives suggest that H<sup>ā</sup> transfer is more likely
to be charge controlled. In the reduction of 10-methylacridinium ion
(<b>Acr</b><sup><b>+</b></sup>), the reaction was initiated
by an e<sup>ā</sup> transfer (ET) process and then followed
by rapid disproportionation reactions to produce <b>Acr</b><sub><b>2</b></sub> dimer and release of H<sub>2</sub>. To achieve
H<sup>ā¢</sup> transfer after ET from [Cp*Ā(P-P)ĀFeH]<sup>+</sup> to acridine radicals, the bulkier acridinium salt 9-phenyl-10-methylacridinium
(<b>PhAcr</b><sup><b>+</b></sup>) was selected as an acceptor.
More evidence for this ātwo-step (e<sup>ā</sup>/H<sup>ā¢</sup>)ā process was derived from the characterization
of <b>PhAcr<sup>ā¢</sup></b> and [<b>4H</b>]<sup><b>+</b></sup> radicals by EPR spectra and by the crystallographic
structure confirmation of [<b>4H</b>]<sup><b>+</b></sup>. Our mechanistic understanding of fundamental H<sup>ā</sup> transfer from ironĀ(II) hydrides to NAD<sup>+</sup> analogues provides
insight into establishing attractive bio-organometallic transformation
cycles driven by iron catalysis
Classification of imaging results based on the WHO/IWG-E classification of cystic echinococcosis (nā=ā27).
<p>Classification of imaging results based on the WHO/IWG-E classification of cystic echinococcosis (nā=ā27).</p