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

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 [TolCMo­(ONO)­(OR)]<b>·</b>KOR (R = CCH₃(CF₃)₂) <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>_p/<i>k</i>_i ∼ 10^–3). The reversible coordination of KOCCH₃(CF₃)₂ 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 [TolCMo­(ONO)­(OR)]<b>·</b>KOR (R = CCH₃(CF₃)₂) <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>_p/<i>k</i>_i ∼ 10^–3). The reversible coordination of KOCCH₃(CF₃)₂ 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

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[<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 [MesCMo­(OC­(CH₃)­(CF₃)₂)₃] exclusively yields linear poly­(<i>o</i>-phenylene ethynylene), polymerization initiated by [EtCMo­(OC­(CH₃)­(CF₃)₂)₃] 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 [EtCMo­(OC­(CH₃)­(CF₃)₂)₃] undergoes a highly selective intramolecular backbiting into the butynyl end-group

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[<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 [MesCMo­(OC­(CH₃)­(CF₃)₂)₃] exclusively yields linear poly­(<i>o</i>-phenylene ethynylene), polymerization initiated by [EtCMo­(OC­(CH₃)­(CF₃)₂)₃] 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 [EtCMo­(OC­(CH₃)­(CF₃)₂)₃] 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 Co­(IV) oxidation state, much remains unknown, including whether bridging or terminal oxo ligands form O₂ and what the relevant oxidation states are. We have addressed these issues by employing a homogeneous model for cobalt oxide, the [Co­(III)₄] cubane (Co₄O₄­(OAc)₄­py₄, py = pyridine, OAc = acetate), that can be oxidized to the [Co­(IV)­Co­(III)₃] state. Upon addition of 1 equiv of sodium hydroxide, the [Co­(III)₄] cubane is regenerated with stoichio­metric formation of O₂. Oxygen isotopic labeling experiments demonstrate that the cubane core remains intact during this stoichio­metric OER, implying that terminal oxo ligands are responsible for forming O₂. The OER is also examined with stopped-flow UV–visible spectroscopy, and its kinetic behavior is modeled, to surprisingly reveal that O₂ formation requires disproportionation of the [Co­(IV)­Co­(III)₃] state to generate an even higher oxidation state, formally [Co­(V)­Co­(III)₃] or [Co­(IV)₂­Co­(III)₂]. 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 Co­(IV) oxidation state, much remains unknown, including whether bridging or terminal oxo ligands form O₂ and what the relevant oxidation states are. We have addressed these issues by employing a homogeneous model for cobalt oxide, the [Co­(III)₄] cubane (Co₄O₄­(OAc)₄­py₄, py = pyridine, OAc = acetate), that can be oxidized to the [Co­(IV)­Co­(III)₃] state. Upon addition of 1 equiv of sodium hydroxide, the [Co­(III)₄] cubane is regenerated with stoichio­metric formation of O₂. Oxygen isotopic labeling experiments demonstrate that the cubane core remains intact during this stoichio­metric OER, implying that terminal oxo ligands are responsible for forming O₂. The OER is also examined with stopped-flow UV–visible spectroscopy, and its kinetic behavior is modeled, to surprisingly reveal that O₂ formation requires disproportionation of the [Co­(IV)­Co­(III)₃] state to generate an even higher oxidation state, formally [Co­(V)­Co­(III)₃] or [Co­(IV)₂­Co­(III)₂]. The mechanistic understanding provided by these results should accelerate the development of OER catalysts leading to increasingly efficient AP systems