71 research outputs found
El tercer sector es posa al dia amb la creació d'aplicacions mòbils socials
A series
of mononuclear nickelÂ(II) thiolate complexes (Et<sub>4</sub>N)ÂNiÂ(X-pyS)<sub>3</sub> (Et<sub>4</sub>N = tetraethylammonium; X
= 5-H (<b>1a</b>), 5-Cl (<b>1b</b>), 5-CF<sub>3</sub> (<b>1c</b>), 6-CH<sub>3</sub> (<b>1d</b>); pyS = pyridine-2-thiolate),
NiÂ(pySH)<sub>4</sub>(NO<sub>3</sub>)<sub>2</sub> (<b>2</b>),
(Et<sub>4</sub>N)ÂNiÂ(4,6-Y<sub>2</sub>-pymS)<sub>3</sub> (Y = H (<b>3a</b>), CH<sub>3</sub> (<b>3b</b>); pymS = pyrimidine-2-thiolate),
and NiÂ(4,4′-Z-2,2′-bpy)Â(pyS)<sub>2</sub> (Z = H (<b>4a</b>), CH<sub>3</sub> (<b>4b</b>), OCH<sub>3</sub> (<b>4c</b>); bpy = bipyridine) have been synthesized in high yield
and characterized. X-ray diffraction studies show that <b>2</b> is square planar, while the other complexes possess tris-chelated
distorted-octahedral geometries. All of the complexes are active catalysts
for both the photocatalytic and electrocatalytic production of hydrogen
in 1/1 EtOH/H<sub>2</sub>O. When coupled with fluorescein (Fl) as
the photosensitizer (PS) and triethylamine (TEA) as the sacrificial
electron donor, these complexes exhibit activity for light-driven
hydrogen generation that correlates with ligand electron donor ability.
Complex <b>4c</b> achieves over 7300 turnovers of H<sub>2</sub> in 30 h, which is among the highest reported for a molecular noble
metal-free system. The initial photochemical step is reductive quenching
of Fl* by TEA because of the latter’s greater concentration.
When system concentrations are modified so that oxidative quenching
of Fl* by catalyst becomes more dominant, system durability increases,
with a system lifetime of over 60 h. System variations and cyclic
voltammetry experiments are consistent with a CECE mechanism that
is common to electrocatalytic and photocatalytic hydrogen production.
This mechanism involves initial protonation of the catalyst followed
by reduction and then additional protonation and reduction steps to
give a key Ni–H<sup>–</sup>/N–H<sup>+</sup> intermediate
that forms the H–H bond in the turnover-limiting step of the
catalytic cycle. A key to the activity of these catalysts is the reversible
dechelation and protonation of the pyridine N atoms, which enable
an internal heterocoupling of a metal hydride and an N-bound proton
to produce H<sub>2</sub>
Effect of Carboxylate Ligands on Alkane Dehydrogenation with (<sup><i>dm</i></sup>Phebox)Ir Complexes
A series
of carboxylate-ligated iridium complexes (<sup><i>dm</i></sup>Phebox)ÂIrÂ(O<sub>2</sub>CR)<sub>2</sub>(H<sub>2</sub>O) (R =
−CH<sub>3</sub>, −CH<sub>2</sub>CH<sub>3</sub>, −CMe<sub>3</sub>, −CH<sub>2</sub>C<sub>6</sub>H<sub>5</sub>, and −CHCMe<sub>2</sub>) were designed and
synthesized to understand the carboxylate ligand effects on the reactivity
of the complex for alkane dehydrogenation. Kinetic studies showed
that the different R groups of the carboxylate iridium complexes can
affect the reactivity with octane in the β-H elimination step.
The rate constants for octene formation with different carboxylate
ligands follow the order R = −CHCMe<sub>2</sub> >
−CMe<sub>3</sub> > −CH<sub>2</sub>CH<sub>3</sub> >
−CH<sub>3</sub> > −CH<sub>2</sub>C<sub>6</sub>H<sub>5</sub>. In contrast,
there is no significant effect of carboxylate ligand on the rate of
the C–H activation step at 160 °C. These experimental
results support the findings in the previously reported density functional
theory study of the (<sup><i>dm</i></sup>Phebox)Ir complex
in alkane C–H activation
Effect of Carboxylate Ligands on Alkane Dehydrogenation with (<sup><i>dm</i></sup>Phebox)Ir Complexes
A series
of carboxylate-ligated iridium complexes (<sup><i>dm</i></sup>Phebox)ÂIrÂ(O<sub>2</sub>CR)<sub>2</sub>(H<sub>2</sub>O) (R =
−CH<sub>3</sub>, −CH<sub>2</sub>CH<sub>3</sub>, −CMe<sub>3</sub>, −CH<sub>2</sub>C<sub>6</sub>H<sub>5</sub>, and −CHCMe<sub>2</sub>) were designed and
synthesized to understand the carboxylate ligand effects on the reactivity
of the complex for alkane dehydrogenation. Kinetic studies showed
that the different R groups of the carboxylate iridium complexes can
affect the reactivity with octane in the β-H elimination step.
The rate constants for octene formation with different carboxylate
ligands follow the order R = −CHCMe<sub>2</sub> >
−CMe<sub>3</sub> > −CH<sub>2</sub>CH<sub>3</sub> >
−CH<sub>3</sub> > −CH<sub>2</sub>C<sub>6</sub>H<sub>5</sub>. In contrast,
there is no significant effect of carboxylate ligand on the rate of
the C–H activation step at 160 °C. These experimental
results support the findings in the previously reported density functional
theory study of the (<sup><i>dm</i></sup>Phebox)Ir complex
in alkane C–H activation
Room-Temperature Carbon–Sulfur Bond Activation by a Reactive (dippe)Pd Fragment
The reactivity of [PdÂ(dippe)Â(μ-H)]<sub>2</sub> (<b>1</b>) and [(μ-dippe)ÂPd]<sub>2</sub> (<b>2</b>) (dippe = 1,2-bisÂ(diisopropylphosphino)Âethane)
toward C–S bonds in thiophene derivatives and thioethers was
investigated, which led to C–S bond activation products. The
thiapalladacycles derived from thiophenic substrates were fully characterized
by <sup>1</sup>H, <sup>31</sup>P, and <sup>13</sup>C NMR spectroscopy,
elemental analysis, and X-ray diffraction. The stability of the C–S
insertion products was probed by performing competition experiments
which follow the thermodynamic stability order (dippe)ÂPdÂ(κ<sup>2</sup><i>C</i>,<i>S</i>-benzothiophene) (<b>6</b>) > (dippe)ÂPdÂ(κ<sup>2</sup><i>C</i>,<i>S</i>-dibenzothiophene) (<b>8</b>) > (dippe)ÂPdÂ(κ<sup>2</sup><i>C</i>,<i>S</i>-thiophene) (<b>3</b>). The reactivity of the thiapalladacycles with small molecules such
as H<sub>2</sub>, CO, and alkynes was investigated
Light-Driven Hydrogen Production from Aqueous Protons using Molybdenum Catalysts
Homogeneous light-driven systems
employing molecular molybdenum
catalysts for hydrogen production are described. The specific Mo complexes
studied are six-coordinate bisÂ(benzenedithiolate) derivatives having
two additional isocyanide or phosphine ligands to complete the coordination
sphere. Each of the complexes possesses a trigonal prismatic coordination
geometry. The complexes were investigated as proton reduction catalysts
in the presence of [RuÂ(bpy)<sub>3</sub>]<sup>2+</sup>, ascorbic acid,
and visible light. Over 500 TON are obtained over 24 h. Electrocatalysis
occurs between the Mo<sup>IV</sup>/Mo<sup>III</sup> and Mo<sup>III</sup>/Mo<sup>II</sup> redox couples, around 1.0 V vs SCE. Mechanistic
studies by <sup>1</sup>H NMR spectroscopy show that upon two-electron
reduction the MoÂ(CNR)<sub>2</sub>(bdt)<sub>2</sub> complex dissociates
the isocyanide ligands, followed by addition of acid to result in
the formation of molecular hydrogen and the MoÂ(bdt)<sub>2</sub> complex
Light-Driven Hydrogen Production from Aqueous Protons using Molybdenum Catalysts
Homogeneous light-driven systems
employing molecular molybdenum
catalysts for hydrogen production are described. The specific Mo complexes
studied are six-coordinate bisÂ(benzenedithiolate) derivatives having
two additional isocyanide or phosphine ligands to complete the coordination
sphere. Each of the complexes possesses a trigonal prismatic coordination
geometry. The complexes were investigated as proton reduction catalysts
in the presence of [RuÂ(bpy)<sub>3</sub>]<sup>2+</sup>, ascorbic acid,
and visible light. Over 500 TON are obtained over 24 h. Electrocatalysis
occurs between the Mo<sup>IV</sup>/Mo<sup>III</sup> and Mo<sup>III</sup>/Mo<sup>II</sup> redox couples, around 1.0 V vs SCE. Mechanistic
studies by <sup>1</sup>H NMR spectroscopy show that upon two-electron
reduction the MoÂ(CNR)<sub>2</sub>(bdt)<sub>2</sub> complex dissociates
the isocyanide ligands, followed by addition of acid to result in
the formation of molecular hydrogen and the MoÂ(bdt)<sub>2</sub> complex
Investigation of C–C Bond Activation of sp–sp<sup>2</sup> C–C Bonds of Acetylene Derivatives via Photolysis of Pt Complexes
Carbon–carbon bond activation
reactions of acetylene derivatives featuring sp–sp<sup>3</sup> C–C bonds or both sp–sp<sup>2</sup> and sp–sp
single C–C bonds were studied via photolysis of platinum compounds.
Novel Pt<sup>0</sup>–acetylene complexes with η<sup>2</sup> coordination of the alkynes were synthesized and characterized.
Irradiation of (dtbpe)ÂPtÂ(η<sup>2</sup>-H<sub>3</sub>CCî—¼CCH<sub>3</sub>) (<b>1</b>), (dtbpe)ÂPtÂ[η<sup>2</sup>-(H<sub>3</sub>C)<sub>3</sub>CCî—¼CCÂ(CH<sub>3</sub>)<sub>3</sub>] (<b>3</b>), and [(dtbpe)ÂPt]<sub>2</sub>(μ<sub>2</sub>-η<sup>2</sup>:η<sup>2</sup>-H<sub>3</sub>CCî—¼CCî—¼CCH<sub>3</sub>) (<b>6</b>) with UV light (λ >300 nm) produced the
activation product (dtbpe)ÂPtÂ(D)Â(C<sub>6</sub>D<sub>5</sub>) (<b>2</b>) as a result of C–D bond activation of the solvent
(C<sub>6</sub>D<sub>6</sub>), whereas (dtbpe)ÂPtÂ(η<sup>2</sup>-F<sub>3</sub>CCî—¼CCF<sub>3</sub>) (<b>4</b>) and (dippe)ÂPtÂ(η<sup>2</sup>-F<sub>3</sub>CCî—¼CCF<sub>3</sub>) (<b>5</b>)
remained unchanged upon irradiation for 22 h. Photolysis of [(dtbpe)ÂPt]<sub>2</sub>(μ<sub>2</sub>-η<sup>2</sup>:η<sup>2</sup>-PhCî—¼CCî—¼CPh) (<b>7</b>) and (dippe)ÂPtÂ(η<sup>2</sup>-PhCî—¼CCî—¼CPh) (<b>9</b>) resulted in [(dtbpe)Â(Ph)ÂPt]<sub>2</sub>(μ-Cî—¼CCî—¼C−) (<b>8</b>) and
(dippe)ÂPtÂ(Ph)Â(Cî—¼CCî—¼CPh) (<b>10</b>), respectively,
showing exclusive C–C bond activation through sp–sp<sup>2</sup> type C–C bonds. Both of the products stayed unchanged
upon heating to 150 °C overnight
Oxidative Addition of Chlorohydrocarbons to a Rhodium Tris(pyrazolyl)borate Complex
The
reactive fragment [Tp′RhÂ(PMe<sub>3</sub>)], generated from
the thermal precursor Tp′RhÂ(PMe<sub>3</sub>)Â(Me)ÂH, is found
to cleave the C–Cl bonds of chlorohydrocarbons under mild conditions.
Reaction with chloromethane gives clean formation of an initial C–H
activation product, which rearranges to form the C–Cl activation
product at 30 °C. Reaction with dichloromethane or benzyl chloride
gives a mixture of C–Cl activation products as well as products
from chlorination. Reaction with chlorocyclohexane gives a mixture
of intermediates from C–H activation, which react further upon
heating to give a C–Cl cleavage product as well as the β-chloride
elimination product Tp′RhÂ(PMe<sub>3</sub>)Â(Cl)H plus cyclohexene.
Complete conversion from a C–H activation product to a C–Cl
activation product was observed in the reaction with 1,2-dichloroethylene,
where β-elimination is circumvented. Activation of 1-chlorobutane,
1,2-dichloroethane, or 1,4-dichlorobutane gives a mixture of C–Cl
activation products as well as Tp′RhÂ(PMe<sub>3</sub>)Â(Cl)ÂH
plus olefin. Similar to the case for activation of methylene chloride,
C–Cl activation and hydride/chloride exchange was observed
in the reaction with benzyl chloride, where C–H activation
was not seen. The reaction with chlorobenzene gives isomeric species
resulting from C–H activation, which react further to give
the corresponding chloride derivatives upon heating. Reaction with
pentachlorobenzene gives a cyclometalated product from C–H
bond cleavage in the phosphine ligand. These reactions are compared
and contrasted with related photoreactions with the [Tp′RhÂ(CNneopentyl)]
analogue, where C–H activation is solely observed in most cases.
Mechanistic studies suggest the spectator ligand dependent reactivity
relies greatly on the dissociation energy of the Tp′Rh–L
bond
Mechanistic Insights in the Exchange of Arylthiolate Groups in Aryl(arylthiolato)palladium Complexes Supported by a Dippe Ligand
Carbon–sulfur
activation of phenyl <i>p</i>-tolyl
sulfide by a mixture of [PdÂ(dippe)Â(μ-H)]<sub>2</sub> (<b>1a</b>) and dinuclear Pd(0), [(μ-dippe)ÂPd]<sub>2</sub> (<b>1b</b>) (dippe = 1,2-bisÂ(diisopropylphosphino)Âethane), to yield
four carbon–sulfur activation products, (dippe)ÂPdÂ(<i>p-</i>tolyl)Â(SPh) (<b>3a</b>), (dippe)ÂPdÂ(Ph)Â(S-<i>p</i>-tolyl) (<b>3b</b>), (dippe)ÂPdÂ(SPh)Â(Ph)Â(<b>3c</b>), and
(dippe)ÂPdÂ(<i>p-</i>tolyl)Â(S-<i>p</i>-tolyl) (<b>3d</b>), was investigated. The carbon–sulfur complexes <b>3a</b>–<b>3d</b> were completely characterized by <sup>1</sup>H, <sup>31</sup>P, and <sup>13</sup>C NMR spectroscopy, elemental
analysis, and X-ray diffraction. Exchange interactions between arylthiolate
groups in (dippe)ÂPdÂ(Ar)Â(SAr′) (<b>3a</b>–<b>3d</b>) were investigated, leading to understanding the mechanism
of interconversions among the complexes
Oxidative Addition of Chlorohydrocarbons to a Rhodium Tris(pyrazolyl)borate Complex
The
reactive fragment [Tp′RhÂ(PMe<sub>3</sub>)], generated from
the thermal precursor Tp′RhÂ(PMe<sub>3</sub>)Â(Me)ÂH, is found
to cleave the C–Cl bonds of chlorohydrocarbons under mild conditions.
Reaction with chloromethane gives clean formation of an initial C–H
activation product, which rearranges to form the C–Cl activation
product at 30 °C. Reaction with dichloromethane or benzyl chloride
gives a mixture of C–Cl activation products as well as products
from chlorination. Reaction with chlorocyclohexane gives a mixture
of intermediates from C–H activation, which react further upon
heating to give a C–Cl cleavage product as well as the β-chloride
elimination product Tp′RhÂ(PMe<sub>3</sub>)Â(Cl)H plus cyclohexene.
Complete conversion from a C–H activation product to a C–Cl
activation product was observed in the reaction with 1,2-dichloroethylene,
where β-elimination is circumvented. Activation of 1-chlorobutane,
1,2-dichloroethane, or 1,4-dichlorobutane gives a mixture of C–Cl
activation products as well as Tp′RhÂ(PMe<sub>3</sub>)Â(Cl)ÂH
plus olefin. Similar to the case for activation of methylene chloride,
C–Cl activation and hydride/chloride exchange was observed
in the reaction with benzyl chloride, where C–H activation
was not seen. The reaction with chlorobenzene gives isomeric species
resulting from C–H activation, which react further to give
the corresponding chloride derivatives upon heating. Reaction with
pentachlorobenzene gives a cyclometalated product from C–H
bond cleavage in the phosphine ligand. These reactions are compared
and contrasted with related photoreactions with the [Tp′RhÂ(CNneopentyl)]
analogue, where C–H activation is solely observed in most cases.
Mechanistic studies suggest the spectator ligand dependent reactivity
relies greatly on the dissociation energy of the Tp′Rh–L
bond
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