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

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

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

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

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

Bimetallic Zirconium Amine Bis(phenolate) Polymerization Catalysts: Enhanced Activity and Tacticity Control for Polyolefin Synthesis

Binucleating multidentate amine bis(phenolate) ligands with rigid terphenyl backbones were designed to support two zirconium centers locked in close proximity. Polymerizations of propylene or 1-hexene with the synthesized bimetallic precatalysts resulted in polymers with significantly higher isotacticity (up to 79% mmmm) in comparison to the stereoirregular polymers produced with previously reported C_s-symmetric monometallic analogues. The bimetallic precatalysts also display higher activity (up to 124 kg of poly(1-hexene) (mmol of Zr)^(−1) h^(–1)), in comparison to the monometallic analogues, and among the highest activities reported for nonmetallocene catalysts. The stereocontrol is consistent with a bimetallic mechanism involving remote steric interactions with the ligand sphere of the second metal center

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

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

A Terminal Fe^(III)-Oxo in a Tetranuclear Cluster: Effects of Distal Metal Centers on Structure and Reactivity

Tetranuclear Fe clusters have been synthesized bearing a terminal Fe^(III)–oxo center stabilized by hydrogen-bonding interactions from pendant (tert-butylamino)pyrazolate ligands. This motif was supported in multiple Fe oxidation states, ranging from [Fe^(II)_2Fe^(III)_2] to [Fe^(III)_4]; two oxidation states were structurally characterized by single-crystal X-ray diffraction. The reactivity of the Fe^(III)–oxo center in proton-coupled electron transfer with X–H (X = C, O) bonds of various strengths was studied in conjunction with analysis of thermodynamic square schemes of the cluster oxidation states. These results demonstrate the important role of distal metal centers in modulating the reactivity of a terminal metal–oxo

Ligand architecture for triangular metal complexes: a high oxidation state Ni₃ cluster with proximal metal arrangement

A new multidentate tetraanionic ligand platform for supporting trinuclear transition metal clusters has been developed. Two trisphenoxide phosphinimide ligands bind three Ni centers in a triangular arrangement. The phosphinimide donors bridge in µ₃ fashion and the phenoxides complete a pseudo-square planar coordination sphere around each metal center. Electrochemical studies reveal two pseudo-reversible oxidation events at notably low potentials (−0.80 V and +0.05 V). The one electron oxidized species was characterized structurally, and it is assigned as a Ni^(III)-containing cluster