5 research outputs found
Recommended from our members
Initiator Control of Conjugated Polymer Topology in Ring-Opening Alkyne Metathesis Polymerization
No full textMolybdenum
carbyne complexes [RCMo(OC(CH<sub>3</sub>)(CF<sub>3</sub>)<sub>2</sub>)<sub>3</sub>] featuring a mesityl (R
= Mes) or an ethyl (R = Et) substituent initiate the living ring-opening
alkyne metathesis polymerization of the strained cyclic alkyne, 5,6,11,12-tetradehydrobenzo[<i>a</i>,<i>e</i>][8]annulene, to yield fully conjugated
poly(<i>o</i>-phenylene ethynylene). The difference in the
steric demand of the polymer end-group (Mes vs Et) transferred during
the initiation step determines the topology of the resulting polymer
chain. While [MesCMo(OC(CH<sub>3</sub>)(CF<sub>3</sub>)<sub>2</sub>)<sub>3</sub>] exclusively yields linear poly(<i>o</i>-phenylene ethynylene), polymerization initiated by [EtCMo(OC(CH<sub>3</sub>)(CF<sub>3</sub>)<sub>2</sub>)<sub>3</sub>] results in cyclic
polymers ranging in size from <i>n</i> = 5 to 20 monomer
units. Kinetic studies reveal that the propagating species emerging
from [EtCMo(OC(CH<sub>3</sub>)(CF<sub>3</sub>)<sub>2</sub>)<sub>3</sub>] undergoes a highly selective intramolecular backbiting
into the butynyl end-group
Recommended from our members
Initiator Control of Conjugated Polymer Topology in Ring-Opening Alkyne Metathesis Polymerization
No full textMolybdenum
carbyne complexes [RCMo(OC(CH<sub>3</sub>)(CF<sub>3</sub>)<sub>2</sub>)<sub>3</sub>] featuring a mesityl (R
= Mes) or an ethyl (R = Et) substituent initiate the living ring-opening
alkyne metathesis polymerization of the strained cyclic alkyne, 5,6,11,12-tetradehydrobenzo[<i>a</i>,<i>e</i>][8]annulene, to yield fully conjugated
poly(<i>o</i>-phenylene ethynylene). The difference in the
steric demand of the polymer end-group (Mes vs Et) transferred during
the initiation step determines the topology of the resulting polymer
chain. While [MesCMo(OC(CH<sub>3</sub>)(CF<sub>3</sub>)<sub>2</sub>)<sub>3</sub>] exclusively yields linear poly(<i>o</i>-phenylene ethynylene), polymerization initiated by [EtCMo(OC(CH<sub>3</sub>)(CF<sub>3</sub>)<sub>2</sub>)<sub>3</sub>] results in cyclic
polymers ranging in size from <i>n</i> = 5 to 20 monomer
units. Kinetic studies reveal that the propagating species emerging
from [EtCMo(OC(CH<sub>3</sub>)(CF<sub>3</sub>)<sub>2</sub>)<sub>3</sub>] undergoes a highly selective intramolecular backbiting
into the butynyl end-group
Mechanistic Investigations of Water Oxidation by a Molecular Cobalt Oxide Analogue: Evidence for a Highly Oxidized Intermediate and Exclusive Terminal Oxo Participation
No full textArtificial photosynthesis (AP) promises
to replace society’s
dependence on fossil energy resources via conversion of sunlight into
sustainable, carbon-neutral fuels. However, large-scale AP implementation
remains impeded by a dearth of cheap, efficient catalysts for the
oxygen evolution reaction (OER). Cobalt oxide materials can catalyze
the OER and are potentially scalable due to the abundance of cobalt
in the Earth’s crust; unfortunately, the activity of these
materials is insufficient for practical AP implementation. Attempts
to improve cobalt oxide’s activity have been stymied by limited
mechanistic understanding that stems from the inherent difficulty
of characterizing structure and reactivity at surfaces of heterogeneous
materials. While previous studies on cobalt oxide revealed the intermediacy
of the unusual Co(IV) oxidation state, much remains unknown, including
whether bridging or terminal oxo ligands form O<sub>2</sub> and what
the relevant oxidation states are. We have addressed these issues
by employing a homogeneous model for cobalt oxide, the [Co(III)<sub>4</sub>] cubane (Co<sub>4</sub>O<sub>4</sub>(OAc)<sub>4</sub>py<sub>4</sub>, py = pyridine, OAc = acetate), that can be
oxidized to the [Co(IV)Co(III)<sub>3</sub>] state. Upon addition
of 1 equiv of sodium hydroxide, the [Co(III)<sub>4</sub>] cubane is
regenerated with stoichiometric formation of O<sub>2</sub>.
Oxygen isotopic labeling experiments demonstrate that the cubane core
remains intact during this stoichiometric OER, implying that
terminal oxo ligands are responsible for forming O<sub>2</sub>. The
OER is also examined with stopped-flow UV–visible spectroscopy,
and its kinetic behavior is modeled, to surprisingly reveal that O<sub>2</sub> formation requires disproportionation of the [Co(IV)Co(III)<sub>3</sub>] state to generate an even higher oxidation state, formally
[Co(V)Co(III)<sub>3</sub>] or [Co(IV)<sub>2</sub>Co(III)<sub>2</sub>]. The mechanistic understanding provided by these results
should accelerate the development of OER catalysts leading to increasingly
efficient AP systems
Mechanistic Investigations of Water Oxidation by a Molecular Cobalt Oxide Analogue: Evidence for a Highly Oxidized Intermediate and Exclusive Terminal Oxo Participation
No full textArtificial photosynthesis (AP) promises
to replace society’s
dependence on fossil energy resources via conversion of sunlight into
sustainable, carbon-neutral fuels. However, large-scale AP implementation
remains impeded by a dearth of cheap, efficient catalysts for the
oxygen evolution reaction (OER). Cobalt oxide materials can catalyze
the OER and are potentially scalable due to the abundance of cobalt
in the Earth’s crust; unfortunately, the activity of these
materials is insufficient for practical AP implementation. Attempts
to improve cobalt oxide’s activity have been stymied by limited
mechanistic understanding that stems from the inherent difficulty
of characterizing structure and reactivity at surfaces of heterogeneous
materials. While previous studies on cobalt oxide revealed the intermediacy
of the unusual Co(IV) oxidation state, much remains unknown, including
whether bridging or terminal oxo ligands form O<sub>2</sub> and what
the relevant oxidation states are. We have addressed these issues
by employing a homogeneous model for cobalt oxide, the [Co(III)<sub>4</sub>] cubane (Co<sub>4</sub>O<sub>4</sub>(OAc)<sub>4</sub>py<sub>4</sub>, py = pyridine, OAc = acetate), that can be
oxidized to the [Co(IV)Co(III)<sub>3</sub>] state. Upon addition
of 1 equiv of sodium hydroxide, the [Co(III)<sub>4</sub>] cubane is
regenerated with stoichiometric formation of O<sub>2</sub>.
Oxygen isotopic labeling experiments demonstrate that the cubane core
remains intact during this stoichiometric OER, implying that
terminal oxo ligands are responsible for forming O<sub>2</sub>. The
OER is also examined with stopped-flow UV–visible spectroscopy,
and its kinetic behavior is modeled, to surprisingly reveal that O<sub>2</sub> formation requires disproportionation of the [Co(IV)Co(III)<sub>3</sub>] state to generate an even higher oxidation state, formally
[Co(V)Co(III)<sub>3</sub>] or [Co(IV)<sub>2</sub>Co(III)<sub>2</sub>]. The mechanistic understanding provided by these results
should accelerate the development of OER catalysts leading to increasingly
efficient AP systems
Mechanistic Investigations of Water Oxidation by a Molecular Cobalt Oxide Analogue: Evidence for a Highly Oxidized Intermediate and Exclusive Terminal Oxo Participation
No full textArtificial photosynthesis (AP) promises
to replace society’s
dependence on fossil energy resources via conversion of sunlight into
sustainable, carbon-neutral fuels. However, large-scale AP implementation
remains impeded by a dearth of cheap, efficient catalysts for the
oxygen evolution reaction (OER). Cobalt oxide materials can catalyze
the OER and are potentially scalable due to the abundance of cobalt
in the Earth’s crust; unfortunately, the activity of these
materials is insufficient for practical AP implementation. Attempts
to improve cobalt oxide’s activity have been stymied by limited
mechanistic understanding that stems from the inherent difficulty
of characterizing structure and reactivity at surfaces of heterogeneous
materials. While previous studies on cobalt oxide revealed the intermediacy
of the unusual Co(IV) oxidation state, much remains unknown, including
whether bridging or terminal oxo ligands form O<sub>2</sub> and what
the relevant oxidation states are. We have addressed these issues
by employing a homogeneous model for cobalt oxide, the [Co(III)<sub>4</sub>] cubane (Co<sub>4</sub>O<sub>4</sub>(OAc)<sub>4</sub>py<sub>4</sub>, py = pyridine, OAc = acetate), that can be
oxidized to the [Co(IV)Co(III)<sub>3</sub>] state. Upon addition
of 1 equiv of sodium hydroxide, the [Co(III)<sub>4</sub>] cubane is
regenerated with stoichiometric formation of O<sub>2</sub>.
Oxygen isotopic labeling experiments demonstrate that the cubane core
remains intact during this stoichiometric OER, implying that
terminal oxo ligands are responsible for forming O<sub>2</sub>. The
OER is also examined with stopped-flow UV–visible spectroscopy,
and its kinetic behavior is modeled, to surprisingly reveal that O<sub>2</sub> formation requires disproportionation of the [Co(IV)Co(III)<sub>3</sub>] state to generate an even higher oxidation state, formally
[Co(V)Co(III)<sub>3</sub>] or [Co(IV)<sub>2</sub>Co(III)<sub>2</sub>]. The mechanistic understanding provided by these results
should accelerate the development of OER catalysts leading to increasingly
efficient AP systems
