85 research outputs found
η^2 bonded Nickel(0) thiophene π-complexes - identifying the missing link in catalyst transfer polymerization
<p>We have structurally characterized a nickel(0) thiophene complex that represents a close analog to the proposed intermediates in the mechanism of catalyst transfer polycondensation (CTP) of thiophenes. Additional studies on this intermediate allow us to determine the bonding in such complexes using a combination of nuclear magnetic resonance spectroscopy, X-ray absorption spectroscopy, and DFT-based computations. Our structure
of the catalyst agrees with previous postulates but provides important new
structural insights into the species. Furthermore, our studies explain why these complexes
actually exist and are stable enough to support living CTP.</p
A Terthiophene-Containing Alkynylplatinum Terpyridine Pacman Complex: Controllable Folding/Unfolding Modulated by Weak Intermolecular Interactions
Folded and unfolded solid-state structures
of a bimetallic alkynylplatinum terpyridine complex with a flexible
terthiophene linker have been obtained. Weak intermolecular interactions
including π–π stacking and C–H···O
and C–H···Cl interactions as well as Cl−π
interactions stabilize the folded structure. In solution, folding
is studied by electronic absorption spectroscopy and <sup>1</sup>H
and NOESY NMR experiments
Thiol, Disulfide, and Trisulfide Complexes of Ru Porphyrins: Potential Models for Iron–Sulfur Bonds in Heme Proteins
Thirty-two Ru(porp)L<sub>2</sub> complexes have been
synthesized, where porp = the dianion of <i>meso</i>-tetramesitylporphyrin
(TMP) or <i>meso</i>-tetrakis(4-methylphenyl)porphyrin (H<sub>2</sub>T-<i>p</i>Me-<i>P</i>P), and L = a thiol,
a sulfide, a disulfide, or a trisulfide. Species studied were with
RSH [R = Me, Et, <sup><i>n</i></sup>Pr, <sup><i>i</i></sup>Pr, <sup><i>t</i></sup>Bu, Bn (benzyl), and Ph],
RSR (R = Me, Bn), RSSR (R = Me, Et, <sup><i>n</i></sup>Pr,
Bn) and MeSS<sup><i>t</i></sup>Bu, and RSSSR (R = Me, Bn).
All the species except two, which were the isolated Ru(T-<i>p</i>Me-<i>P</i>P)(<sup><i>t</i></sup>BuSH)<sub>2</sub> and Ru(TMP)(MeSSMe)<sub>2</sub>, were characterized in situ. The
disulfide complex was characterized by X-ray analysis. <sup>1</sup>H NMR data for the coordinated thiols are the first reported within
metalloporphyrin systems, and are especially informative because of
the upfield shifts of the axial sulfur-containing ligands due to the
porphyrin π-ring current effect, which is also present in the
di- and trisulfide species. The disulfide in the solid state structure
of Ru(TMP)(MeSSMe)<sub>2</sub> is η<sup>1</sup>(end-on) coordinated,
the first example of such bonding in a nontethered, acyclic dialkyl
disulfide; <sup>1</sup>H–<sup>1</sup>H EXSY NMR data in solution
show that the species undergoes 1,2-S-metallotropic shifts. Stepwise
formation of the bis(disulfide) complex from Ru(TMP)(MeCN)<sub>2</sub> in solution occurs with a cooperativity effect, resembling behavior
of Fe<sup>II</sup>–porphyrin systems where crystal field effects
dominate, but ligand trans-effects are more likely in the Ru system.
The η<sup>1</sup>(end-on) coordination mode is also favored
for the trisulfide ligand. Discussed also are the remarkable linear
correlations that exist between the ring-current shielding shifts
for the axial ligand C<sup>1</sup> protons of Ru(porp)(RS<sub><i>x</i></sub>R)<sub>2</sub> and <i>x</i> (the number
of S atoms). The Introduction briefly reviews literature on Ru- and
Fe porphyrins (including heme proteins) with sulfur-containing ligands
or substrates, and relationships between our findings and this literature
are discussed throughout the paper
Thiol, Disulfide, and Trisulfide Complexes of Ru Porphyrins: Potential Models for Iron–Sulfur Bonds in Heme Proteins
Thirty-two Ru(porp)L<sub>2</sub> complexes have been
synthesized, where porp = the dianion of <i>meso</i>-tetramesitylporphyrin
(TMP) or <i>meso</i>-tetrakis(4-methylphenyl)porphyrin (H<sub>2</sub>T-<i>p</i>Me-<i>P</i>P), and L = a thiol,
a sulfide, a disulfide, or a trisulfide. Species studied were with
RSH [R = Me, Et, <sup><i>n</i></sup>Pr, <sup><i>i</i></sup>Pr, <sup><i>t</i></sup>Bu, Bn (benzyl), and Ph],
RSR (R = Me, Bn), RSSR (R = Me, Et, <sup><i>n</i></sup>Pr,
Bn) and MeSS<sup><i>t</i></sup>Bu, and RSSSR (R = Me, Bn).
All the species except two, which were the isolated Ru(T-<i>p</i>Me-<i>P</i>P)(<sup><i>t</i></sup>BuSH)<sub>2</sub> and Ru(TMP)(MeSSMe)<sub>2</sub>, were characterized in situ. The
disulfide complex was characterized by X-ray analysis. <sup>1</sup>H NMR data for the coordinated thiols are the first reported within
metalloporphyrin systems, and are especially informative because of
the upfield shifts of the axial sulfur-containing ligands due to the
porphyrin π-ring current effect, which is also present in the
di- and trisulfide species. The disulfide in the solid state structure
of Ru(TMP)(MeSSMe)<sub>2</sub> is η<sup>1</sup>(end-on) coordinated,
the first example of such bonding in a nontethered, acyclic dialkyl
disulfide; <sup>1</sup>H–<sup>1</sup>H EXSY NMR data in solution
show that the species undergoes 1,2-S-metallotropic shifts. Stepwise
formation of the bis(disulfide) complex from Ru(TMP)(MeCN)<sub>2</sub> in solution occurs with a cooperativity effect, resembling behavior
of Fe<sup>II</sup>–porphyrin systems where crystal field effects
dominate, but ligand trans-effects are more likely in the Ru system.
The η<sup>1</sup>(end-on) coordination mode is also favored
for the trisulfide ligand. Discussed also are the remarkable linear
correlations that exist between the ring-current shielding shifts
for the axial ligand C<sup>1</sup> protons of Ru(porp)(RS<sub><i>x</i></sub>R)<sub>2</sub> and <i>x</i> (the number
of S atoms). The Introduction briefly reviews literature on Ru- and
Fe porphyrins (including heme proteins) with sulfur-containing ligands
or substrates, and relationships between our findings and this literature
are discussed throughout the paper
Intermolecular C–H Activations of Hydrocarbons Initiated by a Tungsten Trimethylsilylallyl Complex
Thermolysis of Cp*W(NO)(Npt)(η<sup>3</sup>-CH<sub>2</sub>CHCHSiMe<sub>3</sub>) [Cp* = η<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>; Npt = CH<sub>2</sub>CMe<sub>3</sub>] at 55 °C
leads to the loss of neopentane and the formation of the 16-electron
η<sup>2</sup>-allene intermediate Cp*W(NO)(η<sup>2</sup>-CH<sub>2</sub>CCHSiMe<sub>3</sub>), which activates
hydrocarbons at their methyl groups. In the case of linear alkanes,
only terminal C–H activation occurs. This selectivity persists
in the presence of an ether functionality, but not with other oxygen-containing
substrates such as aldehydes and alcohols. With these latter substrates,
the organometallic complex is oxidized to Cp*W(O)<sub>2</sub>(Npt).
The existence of the allene intermediate has been confirmed by its
reaction with PMe<sub>3</sub> to form the 18-electron adduct and by
its diagnostic reaction with cyclohexene. Carbonylation of Cp*W(NO)(Npt)(η<sup>3</sup>-CH<sub>2</sub>CHCHSiMe<sub>3</sub>) with CO (550 psig) at
room temperature results in the clean formation of the corresponding
Cp*W(NO)(η<sup>1</sup>-C(O)Npt)(η<sup>3</sup>-CH<sub>2</sub>CHCHSiMe<sub>3</sub>) complex, which exists as a mixture of
two interconverting isomers differing in their modes of attachment
of the (η<sup>3</sup>-CH<sub>2</sub>CHCHSiMe<sub>3</sub>) ligands
to the tungsten centers. The congeneric molybdenum complex, Cp*Mo(NO)(Npt)(η<sup>3</sup>-CH<sub>2</sub>CHCHSiMe<sub>3</sub>), has also been synthesized,
and although it generates the requisite η<sup>2</sup>-allene
intermediate upon thermolysis, its preferred mode of reactivity is
coupling of the allyl and alkyl ligands. Consequently, the molybdenum
complex is inferior to the tungsten system for effecting C–H
activations. All new complexes have been characterized by conventional
spectroscopic and analytical methods, and the solid-state molecular
structures of most of them have been established by single-crystal
X-ray crystallographic analyses
Thermal Chemistry of a Tungsten Trimethylsilylallyl Complex in Benzene and Fluorobenzenes
The thermolysis of Cp*W(NO)(Npt)(η<sup>3</sup>-CH<sub>2</sub>CHCHSiMe<sub>3</sub>) (<b>1</b>; Cp* = η<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>; Npt = CH<sub>2</sub>CMe<sub>3</sub>) in benzene
at 55 °C generates three isomeric products having the composition
Cp*W(NO)(H)(η<sup>3</sup>-Me<sub>3</sub>SiCHCHCHPh) (<b>2</b>). These are isomers of the expected Cp*W(NO)(Ph)(η<sup>3</sup>-Me<sub>3</sub>SiCHCHCH<sub>2</sub>) compound and result from an
intramolecular Ph/allyl H exchange. Thermolysis of <b>2</b> in
the presence of pyridine produces the η<sup>2</sup>-olefin pyridine
adduct Cp*W(NO)(η<sup>2</sup>-Me<sub>3</sub>SiCH<sub>2</sub>CHCHPh)(C<sub>5</sub>H<sub>5</sub>N) (<b>3</b>). However,
when the same reaction is carried out in deuterobenzene with 10 equiv
of pyridine, NMR spectroscopic data suggest that the meso hydrogen
of the allyl ligand is exchanged for a deuterium atom before pyridine
trapping occurs. The activation of fluorobenzenes (i.e., pentafluorobenzene, <i>p</i>-difluorobenzene, and <i>o</i>-difluorobenzene)
by Cp*W(NO)(Npt)(η<sup>3</sup>-CH<sub>2</sub>CHCHSiMe<sub>3</sub>) has also been studied, and for these substrates, C–H bond
activation occurs exclusively. Selectivity for the activation of these
C–H bonds appears to be determined by sterics. Intramolecular
migration of the newly formed fluoroaryl ligands onto the allyl ligands
does not occur when there is a fluorine atom in the position ortho
to the newly formed W–C bond. This behavior is probably a manifestation
of the fact that metal–<i>o</i>-fluoroaryl bonds
tend to be stronger than metal–aryl linkages. All new complexes
have been characterized by conventional spectroscopic and analytical
methods, and the solid-state molecular structures of most of them
have been established by single-crystal X-ray crystallographic analyses
Thermal Chemistry of a Tungsten Trimethylsilylallyl Complex in Benzene and Fluorobenzenes
The thermolysis of Cp*W(NO)(Npt)(η<sup>3</sup>-CH<sub>2</sub>CHCHSiMe<sub>3</sub>) (<b>1</b>; Cp* = η<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>; Npt = CH<sub>2</sub>CMe<sub>3</sub>) in benzene
at 55 °C generates three isomeric products having the composition
Cp*W(NO)(H)(η<sup>3</sup>-Me<sub>3</sub>SiCHCHCHPh) (<b>2</b>). These are isomers of the expected Cp*W(NO)(Ph)(η<sup>3</sup>-Me<sub>3</sub>SiCHCHCH<sub>2</sub>) compound and result from an
intramolecular Ph/allyl H exchange. Thermolysis of <b>2</b> in
the presence of pyridine produces the η<sup>2</sup>-olefin pyridine
adduct Cp*W(NO)(η<sup>2</sup>-Me<sub>3</sub>SiCH<sub>2</sub>CHCHPh)(C<sub>5</sub>H<sub>5</sub>N) (<b>3</b>). However,
when the same reaction is carried out in deuterobenzene with 10 equiv
of pyridine, NMR spectroscopic data suggest that the meso hydrogen
of the allyl ligand is exchanged for a deuterium atom before pyridine
trapping occurs. The activation of fluorobenzenes (i.e., pentafluorobenzene, <i>p</i>-difluorobenzene, and <i>o</i>-difluorobenzene)
by Cp*W(NO)(Npt)(η<sup>3</sup>-CH<sub>2</sub>CHCHSiMe<sub>3</sub>) has also been studied, and for these substrates, C–H bond
activation occurs exclusively. Selectivity for the activation of these
C–H bonds appears to be determined by sterics. Intramolecular
migration of the newly formed fluoroaryl ligands onto the allyl ligands
does not occur when there is a fluorine atom in the position ortho
to the newly formed W–C bond. This behavior is probably a manifestation
of the fact that metal–<i>o</i>-fluoroaryl bonds
tend to be stronger than metal–aryl linkages. All new complexes
have been characterized by conventional spectroscopic and analytical
methods, and the solid-state molecular structures of most of them
have been established by single-crystal X-ray crystallographic analyses
Polyannulated Bis(N-heterocyclic carbene)palladium Pincer Complexes for Electrocatalytic CO<sub>2</sub> Reduction
Phenanthro-
and pyreno-annulated N-heterocyclic carbenes (NHCs) have been incorporated
into lutidine-linked bis-NHC Pd pincer complexes to investigate the
effect of these polyannulated NHCs on the ability of the complexes
to electrochemically reduce CO<sub>2</sub> to CO in the presence of
2,2,2-trifluoroacetic acid and 2,2,2-trifluoroethanol as proton sources.
These complexes are screened for their ability to reduce CO<sub>2</sub> and modeled using density functional theory calculations, where
the annulated phenanthrene and pyrene moieties are shown to be additional
sites for redox activity in the pincer ligand, enabling increased
electron donation. Electrochemical and computational studies are used
to gain an understanding of the chemical significance of redox events
for complexes of this type, highlighting the importance of anion binding
and dissociation
Electrocatalytic Reduction of CO<sub>2</sub> with Palladium Bis-N-heterocyclic Carbene Pincer Complexes
A series
of pyridine- and lutidine-linked bis-N-heterocyclic carbene (NHC)
palladium pincer complexes were electrochemically characterized and
screened for CO<sub>2</sub> reduction capability with 2,2,2-trifluoroethanol,
acetic acid, or 2,2,2-trifluoroacetic acid (TFA) as proton sources.
The lutidine-linked pincer complexes electrocatalytically reduce CO<sub>2</sub> to CO at potentials as low as −1.6 V versus Ag/AgNO<sub>3</sub> in the presence of TFA. The one-electron reduction of these
complexes is shown to be chemically reversible, yielding a monometallic
species, with density functional theory studies indicating charge
storage on the redox-active ligand, thus addressing a major source
of deactivation in earlier triphosphine electrocatalysts
Polyannulated Bis(N-heterocyclic carbene)palladium Pincer Complexes for Electrocatalytic CO<sub>2</sub> Reduction
Phenanthro-
and pyreno-annulated N-heterocyclic carbenes (NHCs) have been incorporated
into lutidine-linked bis-NHC Pd pincer complexes to investigate the
effect of these polyannulated NHCs on the ability of the complexes
to electrochemically reduce CO<sub>2</sub> to CO in the presence of
2,2,2-trifluoroacetic acid and 2,2,2-trifluoroethanol as proton sources.
These complexes are screened for their ability to reduce CO<sub>2</sub> and modeled using density functional theory calculations, where
the annulated phenanthrene and pyrene moieties are shown to be additional
sites for redox activity in the pincer ligand, enabling increased
electron donation. Electrochemical and computational studies are used
to gain an understanding of the chemical significance of redox events
for complexes of this type, highlighting the importance of anion binding
and dissociation
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