Intramolecular Arene C–H to C–P Functionalization Mediated by Nickel(II) and Palladium(II)

A tris­(phosphine) ligand with a triarylbenzene backbone was employed to support mono-nickel­(II) and -palladium­(II) complexes. Two phosphine arms coordinated to the metal center, while the third phosphine was found to form a C–P bond with dearomatization of the central arene. Deprotonation effected the rearomatization of the central ring and metal reduction from M­(II) to M(0). The overall conversion corresponds to a functionalization of an unactivated arene C–H bond to a C–P bond. This transformation represents a rare type of mechanism of C–H functionalization, facilitated by the interactions of the group 10 metal with the arene π system. This conversion is reminiscent of and expands the scope of recently reported intramolecular rearrangements of biaryl phosphine ligands common in group 10 catalysis

Tetranuclear Fe Clusters with a Varied Interstitial Ligand: Effects on the Structure, Redox Properties, and Nitric Oxide Activation

A new series of tetranuclear Fe clusters displaying an interstitial μ₄-F ligand was prepared for a comparison to previously reported μ₄-O analogues. With a single nitric oxide (NO) coordinated as a reporter of small-molecule activation, the μ₄-F clusters were characterized in <i>five</i> redox states, from Fe^II₃{FeNO}⁸ to Fe^III₃{FeNO}⁷, with NO stretching frequencies ranging from 1680 to 1855 cm^–1, respectively. Despite accessing more reduced states with an F^– bridge, a two-electron reduction of the distal Fe centers is necessary for the μ₄-F clusters to activate NO to the same degree as the μ₄-O system; consequently, NO reactivity is observed at more positive potentials with μ₄-O than μ₄-F. Moreover, the μ₄-O ligand better translates redox changes of remote metal centers to diatomic ligand activation. The implication for biological active sites is that the higher-charge bridging ligand is more effective in tuning cluster properties, including the involvement of remote metal centers, for small-molecule activation

Stoichiometrically Activated Catalysts for Ethylene Tetramerization using Diphosphinoamine-Ligated Cr Tris(hydrocarbyl) Complexes

A new, stoichiometric activation mode is presented for Cr-PNP (PNP = diphosphinoamine) complexes for ethylene tetramerization catalysis. To access suitable precatalysts, two robust Cr­(III) multiaryl compounds were synthesized as THF adducts. These complexes are supported by a facially coordinated bis­(aryl) ligand with an additional ether donor. From these precursors, Cr-PNP tris­(hydrocarbyl) complexes were synthesized. Using 1 equiv of Brønsted acid as an activator, an active species for the catalytic tetramerization of ethylene was produced, without the need for excess alkylaluminum reagents

A <i>trans</i>-Hyponitrite Intermediate in the Reductive Coupling and Deoxygenation of Nitric Oxide by a Tricopper–Lewis Acid Complex

The reduction of nitric oxide (NO) to nitrous oxide (N₂O) is a process relevant to biological chemistry as well as to the abatement of certain environmental pollutants. One of the proposed key intermediates in NO reduction is hyponitrite (N₂O₂^2–), the product of reductive coupling of two NO molecules. We report the reductive coupling of NO by an yttrium–tricopper complex generating a <i>trans</i>-hyponitrite moiety supported by two μ-O-bimetallic (Y,Cu) cores, a previously unreported coordination mode. Reaction of the hyponitrite species with Brønsted acids leads to the generation of N₂O, demonstrating the viability of the hyponitrite complex as an intermediate in NO reduction to N₂O. The additional reducing equivalents stored in each tricopper unit are employed in a subsequent step for N₂O reduction to N₂, for an overall (partial) conversion of NO to N₂. The combination of Lewis acid and multiple redox active metals facilitates this four electron conversion via an isolable hyponitrite intermediate

Combination of Redox-Active Ligand and Lewis Acid for Dioxygen Reduction with π‑Bound Molybdenum−Quinonoid Complexes

A series of π-bound Mo−quinonoid complexes supported by pendant phosphines have been synthesized. Structural characterization revealed strong metal–arene interactions between Mo and the π system of the quinonoid fragment. The Mo–catechol complex (<b>2a</b>) was found to react within minutes with 0.5 equiv of O₂ to yield a Mo–quinone complex (<b>3</b>), H₂O, and CO. Si- and B-protected Mo–catecholate complexes also react with O₂ to yield <b>3</b> along with (R₂SiO)_<i>n</i> and (ArBO)₃ byproducts, respectively. Formally, the Mo–catecholate fragment provides two electrons, while the elements bound to the catecholate moiety act as acceptors for the O₂ oxygens. Unreactive by itself, the Mo–dimethyl catecholate analogue reduces O₂ in the presence of added Lewis acid, B­(C₆F₅)₃, to generate a Mo^I species and a bis­(borane)-supported peroxide dianion, [[(F₅C₆)₃B]₂O₂^2–], demonstrating single-electron-transfer chemistry from Mo to the O₂ moiety. The intramolecular combination of a molybdenum center, redox-active ligand, and Lewis acid reduces O₂ with pendant acids weaker than B­(C₆F₅)₃. Overall, the π-bound catecholate moiety acts as a two-electron donor. A mechanism is proposed in which O₂ is reduced through an initial one-electron transfer, coupled with transfer of the Lewis acidic moiety bound to the quinonoid oxygen atoms to the reduced O₂ species

A <i>trans</i>-Hyponitrite Intermediate in the Reductive Coupling and Deoxygenation of Nitric Oxide by a Tricopper–Lewis Acid Complex

The reduction of nitric oxide (NO) to nitrous oxide (N₂O) is a process relevant to biological chemistry as well as to the abatement of certain environmental pollutants. One of the proposed key intermediates in NO reduction is hyponitrite (N₂O₂^2–), the product of reductive coupling of two NO molecules. We report the reductive coupling of NO by an yttrium–tricopper complex generating a <i>trans</i>-hyponitrite moiety supported by two μ-O-bimetallic (Y,Cu) cores, a previously unreported coordination mode. Reaction of the hyponitrite species with Brønsted acids leads to the generation of N₂O, demonstrating the viability of the hyponitrite complex as an intermediate in NO reduction to N₂O. The additional reducing equivalents stored in each tricopper unit are employed in a subsequent step for N₂O reduction to N₂, for an overall (partial) conversion of NO to N₂. The combination of Lewis acid and multiple redox active metals facilitates this four electron conversion via an isolable hyponitrite intermediate

Dinickel Bisphenoxyiminato Complexes for the Polymerization of Ethylene and α-Olefins

Dinuclear nickel phenoxyiminato olefin polymerization catalysts based on rigid <i>p</i>-terphenyl frameworks are reported. Permethylation of the central arene of the terphenyl unit and oxygen substitution of the peripheral rings ortho to the aryl–aryl linkages blocks rotation around these linkages, allowing atropisomers of the ligand to be isolated. The corresponding syn and anti dinickel complexes (<b>25-s</b> and <b>25-a</b>) were synthesized and characterized by single-crystal X-ray diffraction. These frameworks limit the relative movement of the metal centers, restricting the metal–metal distance. Kinetics studies of isomerization of a ligand precursor (<b>7-a</b>) allowed the calculation of the activation parameters for the isomerization process (Δ<i>H</i>^⧧ = 28.0 ± 0.4 kcal × mol^–1 and Δ<i>S</i>^⧧ = −12.3 ± 0.4 cal mol^–1 K^–1). The reported nickel complexes are active for ethylene polymerization (TOF up to 3700 (mol C₂H₄) (mol Ni)⁻¹ h^–1) and ethylene/α-olefin copolymerization. Only methyl branches are observed in the polymerization of ethylene, while α-olefins are incorporated without apparent chain walking. These catalysts are active in the presence of polar additives and in neat tetrahydrofuran. The syn and anti isomers differ in polymerization activity, polymer branching, and polymer molecular weight. For comparison, a series of mononuclear nickel complexes (<b>26</b>, <b>27-s</b>, <b>27-a</b>, <b>28</b>, <b>30</b>) was prepared and studied. The effects of structure and catalyst nuclearity on reactivity are discussed

Dinickel Bisphenoxyiminato Complexes for the Polymerization of Ethylene and α-Olefins

Dinuclear nickel phenoxyiminato olefin polymerization catalysts based on rigid <i>p</i>-terphenyl frameworks are reported. Permethylation of the central arene of the terphenyl unit and oxygen substitution of the peripheral rings ortho to the aryl–aryl linkages blocks rotation around these linkages, allowing atropisomers of the ligand to be isolated. The corresponding syn and anti dinickel complexes (<b>25-s</b> and <b>25-a</b>) were synthesized and characterized by single-crystal X-ray diffraction. These frameworks limit the relative movement of the metal centers, restricting the metal–metal distance. Kinetics studies of isomerization of a ligand precursor (<b>7-a</b>) allowed the calculation of the activation parameters for the isomerization process (Δ<i>H</i>^⧧ = 28.0 ± 0.4 kcal × mol^–1 and Δ<i>S</i>^⧧ = −12.3 ± 0.4 cal mol^–1 K^–1). The reported nickel complexes are active for ethylene polymerization (TOF up to 3700 (mol C₂H₄) (mol Ni)⁻¹ h^–1) and ethylene/α-olefin copolymerization. Only methyl branches are observed in the polymerization of ethylene, while α-olefins are incorporated without apparent chain walking. These catalysts are active in the presence of polar additives and in neat tetrahydrofuran. The syn and anti isomers differ in polymerization activity, polymer branching, and polymer molecular weight. For comparison, a series of mononuclear nickel complexes (<b>26</b>, <b>27-s</b>, <b>27-a</b>, <b>28</b>, <b>30</b>) was prepared and studied. The effects of structure and catalyst nuclearity on reactivity are discussed

Bimetallic Effects on Ethylene Polymerization in the Presence of Amines: Inhibition of the Deactivation by Lewis Bases

Dinickel complexes supported by terphenyl ligands appended with phenoxy and imine donors were synthesized. Full substitution of the central arene blocks rotation around the aryl–aryl bond and allows for the isolation of atropisomers. The reported complexes perform ethylene polymerization in the presence of amines. The inhibiting effect of polar additives is up to 250 times lower for the syn isomer than the anti isomer. Comparisons with mononuclear systems indicate that the proximity of the metal centers leads to the observed inhibitory effect on the deactivation of the catalysts

A <i>trans</i>-Hyponitrite Intermediate in the Reductive Coupling and Deoxygenation of Nitric Oxide by a Tricopper–Lewis Acid Complex

The reduction of nitric oxide (NO) to nitrous oxide (N₂O) is a process relevant to biological chemistry as well as to the abatement of certain environmental pollutants. One of the proposed key intermediates in NO reduction is hyponitrite (N₂O₂^2–), the product of reductive coupling of two NO molecules. We report the reductive coupling of NO by an yttrium–tricopper complex generating a <i>trans</i>-hyponitrite moiety supported by two μ-O-bimetallic (Y,Cu) cores, a previously unreported coordination mode. Reaction of the hyponitrite species with Brønsted acids leads to the generation of N₂O, demonstrating the viability of the hyponitrite complex as an intermediate in NO reduction to N₂O. The additional reducing equivalents stored in each tricopper unit are employed in a subsequent step for N₂O reduction to N₂, for an overall (partial) conversion of NO to N₂. The combination of Lewis acid and multiple redox active metals facilitates this four electron conversion via an isolable hyponitrite intermediate