12 research outputs found
Spectroscopic and magnetic properties of an iodo Co^I tripodal phosphine complex
Reaction of the tripodal phosphine ligand 1,1,1-tris((diphenylphosphino)phenyl)ethane (PhP3) with CoI_2 spontaneously generates a one-electron reduced complex, [(PhP3)Co^(I)(I)] (1). The crystal structure of 1 reveals a distorted tetrahedral environment, with an apical Co–I bond distance of ~2.52 Å. Co^(II/I) redox occurs at an unusually high potential (+0.38 V vs. SCE). The electronic absorption spectrum of 1 exhibits an MLCT peak at 320 nm (ε = 8790 M^(−1) cm^(−1)) and a d–d feature at 850 nm (ε = 840 M^(−1) cm^(−1)). Two more d–d bands are observed in the NIR region, 8650 (ε = 450) and 7950 cm−1 (ε = 430 M−1 cm^(−1)). Temperature dependent magnetic measurements (SQUID) on 1 (solid state, 20–300 K) give μ_eff = 2.99(6) μB, consistent with an S = 1 ground state. Magnetic susceptibilities below 20 K are consistent with a zero field splitting (zfs) |D| = 8 cm^(−1). DFT calculations also support a spin-triplet ground state for 1, as optimized (6-31G*/PW91) geometries (S = 1) closely match the X-ray structure. EPR measurements performed in parallel mode (X-band; 0–15 000 G, 15 K) on polycrystalline 1 or frozen solutions of 1 (THF/toluene) exhibit a feature at g ≈ 4 that arises from a (Δm = 2) transition within the MS = manifold. Below 10 K, the EPR signal decreases significantly, consistent with a solution zfs parameter (|D| ≈ 8 cm^(−1)) similar to that obtained from SQUID measurements. Our work provides an EPR signature for high-spin Co^I in trigonal ligation
Highly Selective Molybdenum ONO Pincer Complex Initiates the Living Ring-Opening Metathesis Polymerization of Strained Alkynes with Exceptionally Low Polydispersity Indices
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Initiator Control of Conjugated Polymer Topology in Ring-Opening Alkyne Metathesis Polymerization.
Molybdenum carbyne complexes [RC≡Mo(OC(CH3)(CF3)2)3] 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[a,e][8]annulene, to yield fully conjugated poly(o-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 [MesC≡Mo(OC(CH3)(CF3)2)3] exclusively yields linear poly(o-phenylene ethynylene), polymerization initiated by [EtC≡Mo(OC(CH3)(CF3)2)3] results in cyclic polymers ranging in size from n = 5 to 20 monomer units. Kinetic studies reveal that the propagating species emerging from [EtC≡Mo(OC(CH3)(CF3)2)3] undergoes a highly selective intramolecular backbiting into the butynyl end-group
Highly Selective Molybdenum ONO Pincer Complex Initiates the Living Ring-Opening Metathesis Polymerization of Strained Alkynes with Exceptionally Low Polydispersity Indices
The pseudo-octahedral
molybdenum benzylidyne complex [TolCî—¼MoÂ(ONO)Â(OR)]<b>·</b>KOR (R = CCH<sub>3</sub>(CF<sub>3</sub>)<sub>2</sub>) <b>1</b>, featuring a stabilizing ONO pincer ligand, initiates
the controlled living polymerization of strained dibenzocyclooctynes
at <i>T</i> > 60 °C to give high molecular weight
polymers
with exceptionally low polydispersities (PDI ∼ 1.02). Kinetic
analyses reveal that the growing polymer chain attached to the propagating
catalyst efficiently limits the rate of propagation with respect to
the rate of initiation (<i>k</i><sub>p</sub>/<i>k</i><sub>i</sub> ∼ 10<sup>–3</sup>). The reversible coordination
of KOCCH<sub>3</sub>(CF<sub>3</sub>)<sub>2</sub> to the propagating
catalyst prevents undesired chain-termination and -transfer processes.
The ring-opening alkyne metathesis polymerization with <b>1</b> has all the characteristics of a living polymerization and enables,
for the first time, the controlled synthesis of amphiphilic block
copolymers via ROAMP
Highly Selective Molybdenum ONO Pincer Complex Initiates the Living Ring-Opening Metathesis Polymerization of Strained Alkynes with Exceptionally Low Polydispersity Indices
The pseudo-octahedral
molybdenum benzylidyne complex [TolCî—¼MoÂ(ONO)Â(OR)]<b>·</b>KOR (R = CCH<sub>3</sub>(CF<sub>3</sub>)<sub>2</sub>) <b>1</b>, featuring a stabilizing ONO pincer ligand, initiates
the controlled living polymerization of strained dibenzocyclooctynes
at <i>T</i> > 60 °C to give high molecular weight
polymers
with exceptionally low polydispersities (PDI ∼ 1.02). Kinetic
analyses reveal that the growing polymer chain attached to the propagating
catalyst efficiently limits the rate of propagation with respect to
the rate of initiation (<i>k</i><sub>p</sub>/<i>k</i><sub>i</sub> ∼ 10<sup>–3</sup>). The reversible coordination
of KOCCH<sub>3</sub>(CF<sub>3</sub>)<sub>2</sub> to the propagating
catalyst prevents undesired chain-termination and -transfer processes.
The ring-opening alkyne metathesis polymerization with <b>1</b> has all the characteristics of a living polymerization and enables,
for the first time, the controlled synthesis of amphiphilic block
copolymers via ROAMP
Recommended from our members
Initiator Control of Conjugated Polymer Topology in Ring-Opening Alkyne Metathesis Polymerization
Molybdenum
carbyne complexes [RCî—¼MoÂ(OCÂ(CH<sub>3</sub>)Â(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-tetradehydrobenzoÂ[<i>a</i>,<i>e</i>]Â[8]Âannulene, to yield fully conjugated
polyÂ(<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 [MesCî—¼MoÂ(OCÂ(CH<sub>3</sub>)Â(CF<sub>3</sub>)<sub>2</sub>)<sub>3</sub>] exclusively yields linear polyÂ(<i>o</i>-phenylene ethynylene), polymerization initiated by [EtCî—¼MoÂ(OCÂ(CH<sub>3</sub>)Â(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 [EtCî—¼MoÂ(OCÂ(CH<sub>3</sub>)Â(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
Molybdenum
carbyne complexes [RCî—¼MoÂ(OCÂ(CH<sub>3</sub>)Â(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-tetradehydrobenzoÂ[<i>a</i>,<i>e</i>]Â[8]Âannulene, to yield fully conjugated
polyÂ(<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 [MesCî—¼MoÂ(OCÂ(CH<sub>3</sub>)Â(CF<sub>3</sub>)<sub>2</sub>)<sub>3</sub>] exclusively yields linear polyÂ(<i>o</i>-phenylene ethynylene), polymerization initiated by [EtCî—¼MoÂ(OCÂ(CH<sub>3</sub>)Â(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 [EtCî—¼MoÂ(OCÂ(CH<sub>3</sub>)Â(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
Artificial 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 CoÂ(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 [CoÂ(III)<sub>4</sub>] cubane (Co<sub>4</sub>O<sub>4</sub>Â(OAc)<sub>4</sub>Âpy<sub>4</sub>, py = pyridine, OAc = acetate), that can be
oxidized to the [CoÂ(IV)ÂCoÂ(III)<sub>3</sub>] state. Upon addition
of 1 equiv of sodium hydroxide, the [CoÂ(III)<sub>4</sub>] cubane is
regenerated with stoichioÂmetric formation of O<sub>2</sub>.
Oxygen isotopic labeling experiments demonstrate that the cubane core
remains intact during this stoichioÂmetric 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 [CoÂ(IV)ÂCoÂ(III)<sub>3</sub>] state to generate an even higher oxidation state, formally
[CoÂ(V)ÂCoÂ(III)<sub>3</sub>] or [CoÂ(IV)<sub>2</sub>ÂCoÂ(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
Artificial 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 CoÂ(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 [CoÂ(III)<sub>4</sub>] cubane (Co<sub>4</sub>O<sub>4</sub>Â(OAc)<sub>4</sub>Âpy<sub>4</sub>, py = pyridine, OAc = acetate), that can be
oxidized to the [CoÂ(IV)ÂCoÂ(III)<sub>3</sub>] state. Upon addition
of 1 equiv of sodium hydroxide, the [CoÂ(III)<sub>4</sub>] cubane is
regenerated with stoichioÂmetric formation of O<sub>2</sub>.
Oxygen isotopic labeling experiments demonstrate that the cubane core
remains intact during this stoichioÂmetric 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 [CoÂ(IV)ÂCoÂ(III)<sub>3</sub>] state to generate an even higher oxidation state, formally
[CoÂ(V)ÂCoÂ(III)<sub>3</sub>] or [CoÂ(IV)<sub>2</sub>ÂCoÂ(III)<sub>2</sub>]. The mechanistic understanding provided by these results
should accelerate the development of OER catalysts leading to increasingly
efficient AP systems