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

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

Dinuclear nickel phenoxyiminato olefin polymerization catalysts based on rigid p-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 (25-s and 25-a) 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 (7-a) allowed the calculation of the activation parameters for the isomerization process (ΔH^‡ = 28.0 ± 0.4 kcal × mol^(–1) and ΔS^‡ = −12.3 ± 0.4 cal mol^(–1) K^(–1)). The reported nickel complexes are active for ethylene polymerization (TOF up to 3700 (mol C_2H_4) (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 (26, 27-s, 27-a, 28, 30) 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

Dioxygen Reactivity with a Ferrocene–Lewis Acid Pairing: Reduction to a Boron Peroxide in the Presence of Tris(pentafluorophenyl)borane

Ferrocenes, which are typically air-stable outer-sphere single-electron transfer reagents, were found to react with dioxygen in the presence of B(C_6F_5)_3, a Lewis acid unreactive to O_2, to generate bis(borane) peroxide. Although several Group 13 peroxides have been reported, boron-supported peroxides are rare, with no structurally characterized examples of the BO_2B moiety. The synthesis of a bis(borane)-supported peroxide anion and its structural and electrochemical characterization are described

Carbon dioxide cleavage by a Ni_2 complex supported by a binucleating bis(N-heterocyclic carbene) framework

A binucleating bis(N-heterocyclic carbene) ligand was designed as a means to coordinate and proximally constrain two transition metal centers. Using an imidazopyridine-based NHC afforded a framework structurally related to previously reported para-terphenyl diphosphines. Bimetallic copper, cobalt, and nickel complexes supported by this framework were synthesized and structurally characterized. Strong interactions between the metal centers and the central arene were observed in all nickel complexes. Dinickel(0) complexes of this ligand framework were found to react with CO_2 to form a dicarbonyl-bridged dinickel(0) product, demonstrating facile CO_2 reduction

Dioxygen Reduction by a Pd(0)–Hydroquinone Diphosphine Complex

A novel p-terphenyl diphosphine ligand was synthesized with a noninnocent hydroquinone moiety as the central arene (1-H). Pseudo-tetrahedral 4-coordinate Ni^0 and Pd^0–quinone (2 and 3, respectively) complexes proved accessible by metalating 1-H with the corresponding M(OAc)_2 precursors. O_2 does not react with the Pd^0–quinone species (3) and protonation occurs at the quinone moiety indicating that the coordinated oxidized quinonoid moiety prevents reactivity at the metal. A 2-coordinate Pd^0–hydroquinone complex (4-H) was prepared using a one-pot metalation with Pd^(II) followed by reduction. The reduced quinonoid moiety in 4-H shows metal-coupled reactivity with small molecules. 4-H was capable of reducing a variety of substrates including dioxygen, nitric oxide, nitrous oxide, 1-azido adamantane, trimethylamine n-oxide, and 1,4-benzoquinone quantitatively producing 3 as the Pd-containing reaction product. Mechanistic investigations of dioxygen reduction revealed that the reaction proceeds through a η^2-peroxo intermediate (Int1) at low temperatures followed by subsequent ligand oxidation at higher temperatures in a reaction that consumed half an equivalent of O_2 and produced water as a final oxygenic byproduct. Control compounds with methyl protected phenolic moieties (4-Me), displaying a Ag^I center incapable of O_2 binding (7-H) or a cationic Pd–H motif (6-H) allowed for the independent examination of potential reaction pathways. The reaction of 4-Me with dioxygen at low temperature produces a species (8-Me) analogous to Int1 demonstrating that initial dioxygen activation is an inner sphere Pd-based process where the hydroquinone moiety only subsequently participates in the reduction of O_2, at higher temperatures, by H^+/e^– transfers

Redox Tuning via Ligand-Induced Geometric Distortions at a YMn₃O₄ Cubane Model of the Biological Oxygen Evolving Complex

The function of proteins involved in electron transfer is dependent on cofactors attaining the necessary reduction potentials. We establish a mode of cluster redox tuning through steric pressure on a synthetic model related to Photosystem II. Resembling the cuboidal [CaMn₃O₄] subsite of the biological oxygen evolving complex (OEC), [Mn4O4] and [YMn₃O₄] complexes featuring ligands of different basicity and chelating properties were characterized by cyclic voltammetry. In the absence of ligand-induced distortions, increasing the basicity of the ligands results in a decrease of cluster reduction potential. Contraction of Y-oxo/Y–Mn distances by 0.1/0.15 Å enforced by a chelating ligand results in an increase of cluster reduction potential even in the presence of strongly basic donors. Related protein-induced changes in Ca-oxo/Ca–Mn distances may have similar effects in tuning the redox potential of the OEC through entatic states and may explain the cation size dependence on the progression of the S-state cycle

Modulation of Proton-Coupled Electron Transfer through Molybdenum–Quinonoid Interactions

An expanded series of π-bound molybdenum–quinonoid complexes supported by pendant phosphines has been synthesized. These compounds formally span three protonation–oxidation states of the quinonoid fragment (catechol, semiquinone, quinone) and two different oxidation states of the metal (Mo^0, Mo^(II)), notably demonstrating a total of two protons and four electrons accessible in the system. Previously, the reduced Mo^0–catechol complex 1 and its reaction with dioxygen to yield the two-proton/two-electron oxidized Mo^0–quinone compound 4 was explored, while, herein, the expansion of the series to include the two-electron oxidized Mo^(II)–catechol complex 2, the one-proton/two-electron oxidized Mo–semiquinone complex 3, and the two-proton/four-electron oxidized MoII–quinone complexes 5 and 6 is reported. Transfer of multiple equivalents of protons and electrons from the Mo^0 and Mo^(II) catechol complexes, 1 and 2, to H atom acceptor TEMPO suggests the presence of weak O–H bonds. Although thermochemical analyses are hindered by the irreversibility of the electrochemistry of the present compounds, the reactivity observed suggests weaker O–H bonds compared to the free catechol, indicating that proton-coupled electron transfer can be facilitated significantly by the π-bound metal center

How calcium affects oxygen formation

Calcium is an essential component of the catalyst that forms oxygen from water during photosynthesis. It seems that part of calcium's job is to enable the release of oxygen from this catalyst

Cyclometalated Tantalum Diphenolate Pincer Complexes: Intramolecular C−H/M−CH_3σ-Bond Metathesis May Be Faster than O−H/M−CH_3 Protonolysis

A diphenol linked at the ortho positions to a benzene ring was metalated with TaCl_2(CH_3)_3. Deuterium labeling of the phenol hydrogens and of the linking 1,3-benzenediyl ring reveals an unexpected mechanism involving protonolysis of a methyl group, followed by C−H/Ta−CH_3 σ-bond metathesis, leading to cyclometalation of the linking ring and finally protonation of the cyclometalated group by the pendant phenol