7 research outputs found
Reductive Cleavage of CO<sub>2</sub> by Metal–Ligand-Cooperation Mediated by an Iridium Pincer Complex
A unique mode of
stoichiometric CO<sub>2</sub> activation and reductive
splitting based on metal–ligand-cooperation is described. The
novel Ir hydride complexes [(<sup><i>t</i></sup>Bu-PNP*)ÂIrÂ(H)<sub>2</sub>] (<b>2</b>) (<sup><i>t</i></sup>Bu-PNP*,
deprotonated <sup><i>t</i></sup>Bu-PNP ligand) and [(<sup><i>t</i></sup>Bu-PNP)ÂIrÂ(H)] (<b>3</b>) react with
CO<sub>2</sub> to give the dearomatized complex [(<sup><i>t</i></sup>Bu-PNP*)ÂIrÂ(CO)] (<b>4</b>) and water. Mechanistic studies
have identified an adduct in which CO<sub>2</sub> is bound to the
ligand and metal, [(<sup><i>t</i></sup>Bu-PNP-COO)ÂIrÂ(H)<sub>2</sub>] (<b>5</b>), and a di-CO<sub>2</sub> iridacycle [(<sup><i>t</i></sup>Bu-PNP)ÂIrÂ(H)Â(C<sub>2</sub>O<sub>4</sub>-Îş<sub>C,O</sub>)] (<b>6</b>). DFT calculations confirm the formation
of <b>5</b> and <b>6</b> as reversibly formed side products,
and suggest an η<sup>1</sup>-CO<sub>2</sub> intermediate leading
to the thermodynamic product <b>4</b>. The calculations support
a metal–ligand-cooperation pathway in which an internal deprotonation
of the benzylic position by the η<sup>1</sup>-CO<sub>2</sub> ligand leads to a carboxylate intermediate, which further reacts
with the hydride ligand to give complex <b>4</b> and water
Study of Precatalyst Degradation Leading to the Discovery of a New Ru<sup>0</sup> Precatalyst for Hydrogenation and Dehydrogenation
The complex Ru-MACHO
(<b>1</b>) is a widely used precatalyst
for hydrogenation and dehydrogenation reactions under basic conditions.
In an attempt to identify the active catalyst form, <b>1</b> was reacted with a strong base. The formation of previously unreported
species was observed by NMR and mass spectrometry. This observation
indicated that complex <b>1</b> quickly degraded under basic
conditions when no substrate was present. X-ray crystallography enabled
the identification of three complexes as products of this degradation
of complex <b>1</b>. These complexes suggested degradation pathways
which included ligand cleavage and reassembly, along with reduction
of the ruthenium atom. One of the decomposition products, the Ru<sup>0</sup> complex [RuÂ(NÂ(CH<sub>2</sub>CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>)ÂCO] (<b>5</b>), was prepared independently and
studied. <b>5</b> was found to be active, entirely additive-free,
in the acceptorless dehydrogenation of aliphatic alcohols to esters.
The hydrogenation of esters catalyzed by <b>5</b> was also demonstrated
under base-free conditions with methanol as an additive. Protic substrates
were shown to add reversibly to complex <b>5</b>, generating
Ru<sup>II</sup>–hydrido species, thus presenting a rare example
of reversible oxidative addition from Ru<sup>0</sup> to Ru<sup>II</sup> and reductive elimination from Ru<sup>II</sup> to Ru<sup>0</sup>
B–H Bond Cleavage via Metal–Ligand Cooperation by Dearomatized Ruthenium Pincer Complexes
Organic derivatives of boronic acid
are widely used reagents useful in various synthetic applications.
A fundamental understanding and the exploration of new reaction pathways
of boronic reagents with organometallic systems hold promise for useful
advancement in chemical catalysis. Herein we present the reactions
of simple boranes with dearomatized ruthenium pincer complexes based
on PNP (2,6-bisÂ(di-<i>tert</i>-butylphosphinomethyl)Âpyridine)
or PNN (2-(di-<i>tert</i>-butylphosphinomethyl)-6-(diethylaminomethyl)Âpyridine)
ligands. NMR studies revealed dehydrogenative addition of the borane
B–H bond across the metal center and the ligand. Remarkably,
new complexes were observed, which contain the boryl moiety at the
benzylic carbon of the pincer ligand arm. X-ray crystal structures
of new dearomatized boryl pincer complexes were obtained, and DFT
calculations revealed mechanistic details of the adduct formation
process through a dehydrogenative pathway. In addition, catalytic
aryl–boron coupling reactions were explored. The new boryl
pincer systems may possibly be useful in future postmodification techniques
for ruthenium pincer complexes, as well as in catalytic B–B
and B–C coupling reactions
B–H Bond Cleavage via Metal–Ligand Cooperation by Dearomatized Ruthenium Pincer Complexes
Organic derivatives of boronic acid
are widely used reagents useful in various synthetic applications.
A fundamental understanding and the exploration of new reaction pathways
of boronic reagents with organometallic systems hold promise for useful
advancement in chemical catalysis. Herein we present the reactions
of simple boranes with dearomatized ruthenium pincer complexes based
on PNP (2,6-bisÂ(di-<i>tert</i>-butylphosphinomethyl)Âpyridine)
or PNN (2-(di-<i>tert</i>-butylphosphinomethyl)-6-(diethylaminomethyl)Âpyridine)
ligands. NMR studies revealed dehydrogenative addition of the borane
B–H bond across the metal center and the ligand. Remarkably,
new complexes were observed, which contain the boryl moiety at the
benzylic carbon of the pincer ligand arm. X-ray crystal structures
of new dearomatized boryl pincer complexes were obtained, and DFT
calculations revealed mechanistic details of the adduct formation
process through a dehydrogenative pathway. In addition, catalytic
aryl–boron coupling reactions were explored. The new boryl
pincer systems may possibly be useful in future postmodification techniques
for ruthenium pincer complexes, as well as in catalytic B–B
and B–C coupling reactions
B–H Bond Cleavage via Metal–Ligand Cooperation by Dearomatized Ruthenium Pincer Complexes
Organic derivatives of boronic acid
are widely used reagents useful in various synthetic applications.
A fundamental understanding and the exploration of new reaction pathways
of boronic reagents with organometallic systems hold promise for useful
advancement in chemical catalysis. Herein we present the reactions
of simple boranes with dearomatized ruthenium pincer complexes based
on PNP (2,6-bisÂ(di-<i>tert</i>-butylphosphinomethyl)Âpyridine)
or PNN (2-(di-<i>tert</i>-butylphosphinomethyl)-6-(diethylaminomethyl)Âpyridine)
ligands. NMR studies revealed dehydrogenative addition of the borane
B–H bond across the metal center and the ligand. Remarkably,
new complexes were observed, which contain the boryl moiety at the
benzylic carbon of the pincer ligand arm. X-ray crystal structures
of new dearomatized boryl pincer complexes were obtained, and DFT
calculations revealed mechanistic details of the adduct formation
process through a dehydrogenative pathway. In addition, catalytic
aryl–boron coupling reactions were explored. The new boryl
pincer systems may possibly be useful in future postmodification techniques
for ruthenium pincer complexes, as well as in catalytic B–B
and B–C coupling reactions
Bottom-Up Construction of a CO<sub>2</sub>‑Based Cycle for the Photocarbonylation of Benzene, Promoted by a Rhodium(I) Pincer Complex
The use of carbon
dioxide for synthetic applications presents a
major goal in modern homogeneous catalysis. Rhodium–hydride
PNP pincer complex <b>1</b> is shown to add CO<sub>2</sub> in
two disparate pathways: one is the expected insertion of CO<sub>2</sub> into the metal–hydride bond, and the other leads to reductive
cleavage of CO<sub>2</sub>, involving metal–ligand cooperation.
The resultant rhodium–carbonyl complex was found to be photoactive,
enabling the activation of benzene and formation of a new benzoyl
complex. Organometallic intermediate species were observed and characterized
by NMR spectroscopy and X-ray crystallography. Based on the series
of individual transformations, a sequence for the photocarbonylation
of benzene using CO<sub>2</sub> as the feedstock was constructed and
demonstrated for the production of benzaldehyde from benzene
Bottom-Up Construction of a CO<sub>2</sub>‑Based Cycle for the Photocarbonylation of Benzene, Promoted by a Rhodium(I) Pincer Complex
The use of carbon
dioxide for synthetic applications presents a
major goal in modern homogeneous catalysis. Rhodium–hydride
PNP pincer complex <b>1</b> is shown to add CO<sub>2</sub> in
two disparate pathways: one is the expected insertion of CO<sub>2</sub> into the metal–hydride bond, and the other leads to reductive
cleavage of CO<sub>2</sub>, involving metal–ligand cooperation.
The resultant rhodium–carbonyl complex was found to be photoactive,
enabling the activation of benzene and formation of a new benzoyl
complex. Organometallic intermediate species were observed and characterized
by NMR spectroscopy and X-ray crystallography. Based on the series
of individual transformations, a sequence for the photocarbonylation
of benzene using CO<sub>2</sub> as the feedstock was constructed and
demonstrated for the production of benzaldehyde from benzene