Boryl–Metal Bonds Facilitate Cobalt/Nickel-Catalyzed Olefin Hydrogenation

New approaches toward the generation of late first-row metal catalysts that efficiently facilitate two-electron reductive transformations (e.g., hydrogenation) more typical of noble-metal catalysts is an important goal. Herein we describe the synthesis of a structurally unusual <i>S</i> = 1 bimetallic Co complex, <b>[(</b>^<b>Cy</b><b>PBP)­CoH]</b>_<b>2</b> (<b>1</b>), supported by bis­(phosphino)­boryl and bis­(phosphino)­hydridoborane ligands. This complex reacts reversibly with a second equivalent of H₂ (1 atm) and serves as an olefin hydrogenation catalyst under mild conditions (room temperature, 1 atm H₂). A bimetallic Co species is invoked in the rate-determining step of the catalysis according to kinetic studies. A structurally related Ni^INi^I dimer, <b>[(</b>^<b>Ph</b><b>PBP)­Ni]</b>_<b>2</b> (<b>3</b>), has also been prepared. Like Co catalyst <b>1</b>, Ni complex <b>3</b> displays reversible reactivity toward H₂, affording the bimetallic complex <b>[(</b>^<b>Ph</b><b>PBHP)­NiH]</b>_<b>2</b> (<b>4</b>). This reversible behavior is unprecedented for Ni^I species and is attributed to the presence of a boryl–Ni bond. Lastly, a series of monomeric <b>(</b>^<b>tBu</b><b>PBP)­NiX</b> complexes (X = Cl (<b>5</b>), OTf (<b>6</b>), H (<b>7</b>), OC­(H)O (<b>8</b>)) have been prepared. The complex <b>(</b>^<b>tBu</b><b>PBP)­NiH</b> (<b>7</b>) shows enhanced catalytic olefin hydrogenation activity when directly compared with its isoelectronic/isostructural analogues where the boryl unit is substituted by a phenyl or amine donor, a phenomenon that we posit is related to the strong trans influence exerted by the boryl ligand

Two-Electron Redox Chemistry at the Dinuclear Core of a TePt Platform: Chlorine Photoreductive Elimination and Isolation of a Te^VPt^I Complex

As part of our interest in novel redox-active main group/transition metal platforms for energy applications, we have synthesized the chloride salt of [Te^IIIPt^ICl­(<i>o</i>-dppp)₂]⁺ ([<b>1</b>]⁺, <i>o</i>-dppp = <i>o</i>-(Ph₂P)­C₆H₄) by reaction of the new bis­(phosphino) telluroether (<i>o</i>-(Ph₂P)­C₆H₄)₂Te with (Et₂S)₂PtCl₂. Complex [<b>1</b>]⁺ is chemically robust and undergoes a clean two-electron oxidation reaction in the presence of PhICl₂ to afford ClTe^IIIPt^IIICl₃(<i>o</i>-dppp)₂ (<b>2</b>), a complex combining a hypervalent four-coordinate tellurium atom and an octahedral platinum center. While the Te–Pt bond length is only slightly affected by the oxidation state of the TePt platform, DFT and NBO calculations show that this central linkage undergoes an umpolung from Te→Pt in [<b>1</b>]⁺ to Te←Pt in <b>2</b>. This umpolung signals an increase in the electron releasing ability of the tellurium center upon switching from an eight-electron configuration in [<b>1</b>]⁺ to a hypervalent configuration in <b>2</b>. Remarkably, the two-electron redox chemistry displayed by this new dinuclear platform is reversible as shown by the photoreductive elimination of a Cl₂ equivalent when <b>2</b> is irradiated at 350 nm in the presence of a radical trap such as 2,3-dimethyl-1,3-butadiene. This photoreductive elimination, which affords [<b>1</b>]­[Cl] with a maximum quantum yield of 4.4%, shows that main group/late transition metal complexes can mimic the behavior of their transition metal-only analogues and, in particular, undergo halogen photoelimination from the oxidized state. A last notable outcome of this study is the isolation and characterization of F­(MeO)₂Te^VPt^ICl­(<i>o</i>-dppp)₂ (<b>4</b>), the first metalated hexavalent tellurium compound, which is formed by reaction of <b>2</b> with KF in the presence of MeOH

Boryl-Mediated Reversible H₂ Activation at Cobalt: Catalytic Hydrogenation, Dehydrogenation, and Transfer Hydrogenation

We describe the synthesis of a cobalt­(I)–N₂ complex (<b>2</b>) supported by a meridional bis-phosphino-boryl (PBP) ligand. Complex <b>2</b> undergoes a clean reaction with 2 equiv of dihydrogen to afford a dihydrido­borato­cobalt dihydride (<b>3</b>). The ability of boron to switch between a boryl and a dihydrido­borate conformation makes possible the reversible conversion of <b>2</b> and <b>3</b>. Complex <b>3</b> reacts with HMe₂N–BH₃ to give a hydrido­borane cobalt tetra­hydrido­borate complex. We explore this boryl–cobalt system in the context of catalytic olefin hydrogenation as well as amine–borane dehydrogenation/transfer hydrogenation

Two-Electron Redox Chemistry at the Dinuclear Core of a TePt Platform: Chlorine Photoreductive Elimination and Isolation of a Te^VPt^I Complex

As part of our interest in novel redox-active main group/transition metal platforms for energy applications, we have synthesized the chloride salt of [Te^IIIPt^ICl­(<i>o</i>-dppp)₂]⁺ ([<b>1</b>]⁺, <i>o</i>-dppp = <i>o</i>-(Ph₂P)­C₆H₄) by reaction of the new bis­(phosphino) telluroether (<i>o</i>-(Ph₂P)­C₆H₄)₂Te with (Et₂S)₂PtCl₂. Complex [<b>1</b>]⁺ is chemically robust and undergoes a clean two-electron oxidation reaction in the presence of PhICl₂ to afford ClTe^IIIPt^IIICl₃(<i>o</i>-dppp)₂ (<b>2</b>), a complex combining a hypervalent four-coordinate tellurium atom and an octahedral platinum center. While the Te–Pt bond length is only slightly affected by the oxidation state of the TePt platform, DFT and NBO calculations show that this central linkage undergoes an umpolung from Te→Pt in [<b>1</b>]⁺ to Te←Pt in <b>2</b>. This umpolung signals an increase in the electron releasing ability of the tellurium center upon switching from an eight-electron configuration in [<b>1</b>]⁺ to a hypervalent configuration in <b>2</b>. Remarkably, the two-electron redox chemistry displayed by this new dinuclear platform is reversible as shown by the photoreductive elimination of a Cl₂ equivalent when <b>2</b> is irradiated at 350 nm in the presence of a radical trap such as 2,3-dimethyl-1,3-butadiene. This photoreductive elimination, which affords [<b>1</b>]­[Cl] with a maximum quantum yield of 4.4%, shows that main group/late transition metal complexes can mimic the behavior of their transition metal-only analogues and, in particular, undergo halogen photoelimination from the oxidized state. A last notable outcome of this study is the isolation and characterization of F­(MeO)₂Te^VPt^ICl­(<i>o</i>-dppp)₂ (<b>4</b>), the first metalated hexavalent tellurium compound, which is formed by reaction of <b>2</b> with KF in the presence of MeOH

Telluroether to Telluroxide Conversion in the Coordination Sphere of a Metal: Oxidation-Induced Umpolung of a Te–Au Bond

While Ph₂Te is oxidized into Ph₂TeCl₂ in the presence of (tht)­AuCl (tht = tetrahydrothiophene), reaction of (<i>o</i>-(Ph₂P)­C₆H₄)₂Te (<b>L</b>^TeP2) with the same gold reagent affords the complex <b>L</b>^TeP2AuCl (<b>1</b>). Upon reaction with Ph₃P, this complex is converted into [<b>L</b>^TeP2AuPPh₃]­[Cl] ([<b>2</b>]­[Cl]). Complex <b>1</b> also reacts with H₂O₂ to afford the telluroxide gold chloride complex (<i>o</i>-(Ph₂P)­C₆H₄)₂Te­(O)­AuCl (<b>3</b>). The three new complexes have been fully characterized. Their crystal structures indicate the presence of a Te–Au bond with lengths of 2.874(4), 2.937(2), and 2.9864(5) Å, for <b>1</b>, [<b>2</b>]­[Cl], and <b>3</b>, respectively. Natural bond orbital analysis shows that oxidation of <b>1</b> into <b>3</b> results in an umpolung of the Te–Au bond which switches from Te→Au in <b>1</b> to Te←Au in <b>3</b>. These results show that the telluroxide moiety of <b>3</b> acts as a Z-type ligand

Effects of Grafting Density on Block Polymer Self-Assembly: From Linear to Bottlebrush

Grafting density is an important structural parameter that exerts significant influences over the physical properties of architecturally complex polymers. In this report, the physical consequences of varying the grafting density (<i>z</i>) were studied in the context of block polymer self-assembly. Well-defined block polymers spanning the linear, comb, and bottlebrush regimes (0 ≤ <i>z</i> ≤ 1) were prepared <i>via</i> grafting-through ring-opening-metathesis polymerization. ω-Norbornenyl poly­(d,l-lactide) and polystyrene macromonomers were copolymerized with discrete comonomers in different feed ratios, enabling precise control over both the grafting density and molecular weight. Small-angle X-ray scattering experiments demonstrate that these graft block polymers self-assemble into long-range-ordered lamellar structures. For 17 series of block polymers with variable <i>z</i>, the scaling of the lamellar period with the total backbone degree of polymerization (<i>d</i>* ∼ <i>N</i>_bb^α) was studied. The scaling exponent α monotonically decreases with decreasing <i>z</i> and exhibits an apparent transition at <i>z</i> ≈ 0.2, suggesting significant changes in the chain conformations. Comparison of two block polymer systems, one that is strongly segregated for all <i>z</i> (System I) and one that experiences weak segregation at low <i>z</i> (System II), indicates that the observed trends are primarily caused by the polymer architectures, not segregation effects. A model is proposed in which the characteristic ratio (<i>C</i>_∞), a proxy for the backbone stiffness, scales with <i>N</i>_bb as a function of the grafting density: <i>C</i>_∞ ∼ <i>N</i>_bb^{<i>f</i>(<i>z</i>)}. The scaling behavior disclosed herein provides valuable insights into conformational changes with grafting density, thus introducing opportunities for block polymer and material design

Disentangling Ligand Effects on Metathesis Catalyst Activity: Experimental and Computational Studies of Ruthenium–Aminophosphine Complexes

Second-generation ruthenium olefin metathesis catalysts bearing aminophosphine ligands were investigated with systematic variation of the ligand structure. The rates of phosphine dissociation (<i>k</i>₁; initiation rate) and relative phosphine reassociation (<i>k</i>_–1) were determined for two series of catalysts bearing cyclohexyl­(morpholino)­phosphine and cyclohexyl­(piperidino)­phosphine ligands. In both cases, incorporating P–N bonds into the architecture of the dissociating phosphine accelerates catalyst initiation relative to the parent [Ru]–PCy₃ complex; however, this effect is muted for the tris­(amino)­phosphine-ligated complexes, which exhibit higher ligand binding constants in comparison to those with phosphines containing one or two cyclohexyl substituents. These results, along with X-ray crystallographic data and DFT calculations, were used to understand the influence of ligand structure on catalyst activity. Especially noteworthy is the application of phosphines containing incongruent substituents (PR₁R′₂); detailed analyses of factors affecting ligand dissociation, including steric effects, inductive effects, and ligand conformation, are presented. Computational studies of the reaction coordinate for ligand dissociation reveal that ligand conformational changes contribute to the rapid dissociation for the fastest-initiating catalyst of these series, which bears a cyclohexyl-bis­(morpholino)­phosphine ligand. Furthermore, the effect of amine incorporation was examined in the context of ring-opening metathesis polymerization, and reaction rates were found to correlate well with catalyst initiation rates. The combined experimental and computational studies presented in this report reveal important considerations for designing efficient ruthenium olefin metathesis catalysts

Disentangling Ligand Effects on Metathesis Catalyst Activity: Experimental and Computational Studies of Ruthenium–Aminophosphine Complexes

Second-generation ruthenium olefin metathesis catalysts bearing aminophosphine ligands were investigated with systematic variation of the ligand structure. The rates of phosphine dissociation (<i>k</i>₁; initiation rate) and relative phosphine reassociation (<i>k</i>_–1) were determined for two series of catalysts bearing cyclohexyl­(morpholino)­phosphine and cyclohexyl­(piperidino)­phosphine ligands. In both cases, incorporating P–N bonds into the architecture of the dissociating phosphine accelerates catalyst initiation relative to the parent [Ru]–PCy₃ complex; however, this effect is muted for the tris­(amino)­phosphine-ligated complexes, which exhibit higher ligand binding constants in comparison to those with phosphines containing one or two cyclohexyl substituents. These results, along with X-ray crystallographic data and DFT calculations, were used to understand the influence of ligand structure on catalyst activity. Especially noteworthy is the application of phosphines containing incongruent substituents (PR₁R′₂); detailed analyses of factors affecting ligand dissociation, including steric effects, inductive effects, and ligand conformation, are presented. Computational studies of the reaction coordinate for ligand dissociation reveal that ligand conformational changes contribute to the rapid dissociation for the fastest-initiating catalyst of these series, which bears a cyclohexyl-bis­(morpholino)­phosphine ligand. Furthermore, the effect of amine incorporation was examined in the context of ring-opening metathesis polymerization, and reaction rates were found to correlate well with catalyst initiation rates. The combined experimental and computational studies presented in this report reveal important considerations for designing efficient ruthenium olefin metathesis catalysts

Tertiary and Quaternary Phosphonium Borane Bifunctional Catalysts for CO₂/Epoxide Copolymerization: A Mechanistic Investigation Using In Situ Raman Spectroscopy

Tertiary and quaternary phosphonium borane catalysts are employed as catalysts for CO2/epoxide copolymerization. Catalyst structures are strategically modified to gain insights into the intricate structure–activity relationship. To quantitatively and rigorously compare these catalysts, the copolymerization reactions were monitored by in situ Raman spectroscopy, allowing the determination of polymerization rate constants. The polymerization rates are very sensitive to perturbations in phosphonium/borane substituents as well as the tether length. To further evaluate catalysts, a nonisothermal kinetic technique has been developed, enabling direct mapping of polymerization rate constant (kp) as a function of polymerization temperatures. By applying this method, key intrinsic attributes governing catalyst performance, such as activation enthalpy (ΔH‡), entropy (ΔS‡), and optimal polymerization temperature (Topt), can be extracted in a single continuous temperature sweep experiment. In-depth analyses reveal intricate trends between ΔH‡, ΔS‡, and Lewis acidity (as determined using the Gutmann–Beckett method) with respect to structural variations. Collectively, these results are more consistent with the mechanistic proposal in which the resting state is a carbonate species, and the rate-determining step is the ring-opening of epoxide. In agreement with the experimental results, DFT calculations indicate the important contributions of noncovalent stabilizations exerted by the phosphonium moieties. Excitingly, these efforts identify tertiary phosphonium borane analogues, featuring an acidic phosphonium proton, as leading catalysts on the basis of kp and Topt. Mediated by phosphonium borane catalysts, epoxides such as butylene oxide (BO), n-butyl glycidyl ether (BGE), 4-vinyl cyclohexene oxide (VCHO), and cyclohexene oxide (CHO) were copolymerized with CO2 to form polyalkylene carbonate with >95% chemo-selectivity. The tertiary phosphonium catalysts maintain their high activity in the presence of large excess of di-alcohols as chain-transferring agents, affording well-defined telechelic polyols. The results presented herein shed light on the cooperative catalysis between phosphonium and borane

Consequences of Grafting Density on the Linear Viscoelastic Behavior of Graft Polymers

The linear viscoelastic behavior of poly­(norbornene)-<i>graft</i>-poly­(±-lactide) was investigated as a function of grafting density and overall molar mass. Eight sets of polymers with grafting densities ranging from 0 to 100% were synthesized by living ring-opening metathesis copolymerization. Within each set, the graft chain molar mass and spacing between grafts were fixed, while the total backbone length was varied. Dynamic master curves reveal that these polymers display Rouse and reptation dynamics with a sharp transition in the zero-shear viscosity data, demonstrating that grafting density strongly impacts the entanglement molar mass. The entanglement modulus (<i>G</i>_e) scales with inverse grafting density (<i>n</i>_g) as <i>G</i>_e ∼ <i>n</i>_g^1.2 and <i>G</i>_e ∼ <i>n</i>_g⁰ in accordance with scaling theory in the high and low grafting density limits, respectively. However, a sharp transition between these limiting behaviors occurs, which does not conform to existing theoretical models for graft polymers. A molecular interpretation based on thin flexible chains at low grafting density and thick semiflexible chains at high grafting density anticipates the sharp transition between the limiting dynamical regimes