Bis(iminopyridyl)phthalazine as a sterically hindered compartmental ligand for an M-2 (M = Co, Ni, Fe, Zn) centre; Applications in ethylene oligomerisation

The new bis(iminopyridyl)phthalazine ligand, 1,4-{(2,6-i-Pr2C6H3)Ndouble bond; length as m-dashCMe)C5H3N}2C8H4N2 (L), has been prepared in good yield using a combination of palladium-mediated cross coupling and condensation strategies. Reaction of L with three equivalents of CoX2 (X = Cl, Br) in n-BuOH at elevated temperature generates, on crystallisation from bench acetonitrile, the paramagnetic tetrahalocobaltate salts [(L)Co2X(μ-X)(NCMe)m(OH2)n](CoX4) (X = Cl, m = 2, n = 1 1a; X = Br, m = 2, n = 0 1b) as acetonitrile or mixed acetonitrile/aqua adducts; a similar product is obtained from the reaction of FeCl2 with L and has been tentatively assigned as [(L)Fe2Cl(μ-Cl)(OH2)3](FeCl4) (2). By contrast, reaction of L with NiX2(DME) (X = Cl, Br; DME = 1,2-dimethoxyethane), under similar reaction conditions, affords the halide salts [(L)Ni2X2(μ-X)(OH2)2](X) (X = Cl 3a, X = Br 3b) as aqua adducts. Structural determinations on 1 and 3 reveal L to adopt a bis(tridentate) bonding mode allowing the halide-bridged metal centres to assemble in close proximity (M⋯M range: 3.437–3.596 Å). Unexpectedly, on reaction of L with ZnCl2, the neutral bimetallic [(L)Zn2Cl4] (4b) complex is formed in which the ZnCl2 units fill inequivalent binding sites within L (viz. the Nphth,Npy,Nim and Npy,Nim pockets). Complex 4b could also be obtained by the sequential addition of ZnCl2 to L to form firstly monometallic [(L)ZnCl2] (4a) and then on further ZnCl2 addition 4b; the fluxional behaviour of diamagnetic 4a and 4b is also reported. On activation with excess methylaluminoxane (MAO), 1–3 display modest activities for ethylene oligomerisation forming low molecular weight waxes with methyl-branched products predominating for the nickel systems (3). On the other hand, the iron catalyst (2) gives exclusively α-olefins while the cobalt systems (1) are much less selective affording equal mixtures of α-olefins and internal olefins along with lower levels of vinylidenes and tri-substituted alkenes. Single crystal X-ray structures are reported for L, 1a, 1b, 3a, 3b and 4

Synthesis, Structure, and Luminescent Behavior of Anionic Oligomeric and Polymeric Ag₂Au₂ Clusters

Mixtures of silver salts AgX (X = NO3, CF3CO2, CF3SO3) with M­[Au­(C6F5)2] (M = NBu4, PPh4) gave respectively the ionic mixed-metal clusters [M2{(C6F5)4Au2Ag2X2}]n (1, X = NO3; a, M = NBu4; b, M = PPh4) and [M­{(C6F5)4Au2Ag2X}]n, (2a,b, X = CF3CO2; 3a,b, X = CF3SO3). The degree of aggregation n of these cluster compounds depends strongly on the method of isolation (solvent evaporation or precipitation); for example, recrystallization of 1a gave a crystalline salt of the tetraanion [(C6F5)4Au2Ag2X2]24– as well as the polymer [(NBu4)2{(C6F5)4Au2Ag2(NO3)2}]n. The aurophilic Au···Au interactions strongly influence the photoemission wavelength. The anion X has remarkably little effect on the luminescence color but strongly influences the conformation of the polyanionic chains, leading to a variety of solid-state structures, from well-defined dimers (1a1) to linear (1b) and curved (1a2, 2a) polymeric chain aggregates

Synthesis, Structure, and Luminescent Behavior of Anionic Oligomeric and Polymeric Ag₂Au₂ Clusters

Mixtures of silver salts AgX (X = NO₃, CF₃CO₂, CF₃SO₃) with M­[Au­(C₆F₅)₂] (M = NBu₄, PPh₄) gave respectively the ionic mixed-metal clusters [M₂{(C₆F₅)₄Au₂Ag₂X₂}]_<i>n</i> (<b>1</b>, X = NO₃; <b>a</b>, M = NBu₄; <b>b</b>, M = PPh₄) and [M­{(C₆F₅)₄Au₂Ag₂X}]_<i>n</i>, (<b>2a</b>,<b>b</b>, X = CF₃CO₂; <b>3a</b>,<b>b</b>, X = CF₃SO₃). The degree of aggregation <i>n</i> of these cluster compounds depends strongly on the method of isolation (solvent evaporation or precipitation); for example, recrystallization of <b>1a</b> gave a crystalline salt of the tetraanion [(C₆F₅)₄Au₂Ag₂X₂]₂^4– as well as the polymer [(NBu₄)₂{(C₆F₅)₄Au₂Ag₂(NO₃)₂}]_<i>n</i>. The aurophilic Au···Au interactions strongly influence the photoemission wavelength. The anion X has remarkably little effect on the luminescence color but strongly influences the conformation of the polyanionic chains, leading to a variety of solid-state structures, from well-defined dimers (<b>1a</b>^<b>1</b>) to linear (<b>1b</b>) and curved (<b>1a</b>^<b>2</b>, <b>2a</b>) polymeric chain aggregates

Reactivity of “Ligand-Free” Au⁺: C–H and C–C Activation versus π Coordination

[(Me₂S)­Au]⁺ cations, generated from (Me₂S)­AuCl and AgSbF₆ in dichloromethane at 0–20 °C, serve as sources of solvated Au⁺ (alongside unreactive [Au­(SMe₂)₂]⁺), which reacts with the methyl-substituted arenes C₆Me_6–<i>n</i>H_<i>n</i> (n = 0–2) with C–H bond cleavage to give the sulfonium salts [C₆Me_5–<i>n</i>H_<i>n</i>CH₂SMe₂]⁺. There was no evidence for arene π coordination to Au⁺ or for the formation of σ-bonded Au–benzyl species. Surprisingly, the reaction of Au⁺ with CH₂Ar₂ leads to C–C bond cleavage (Ar = 2,4,6-C₆H₂Me₃). The reactions are highly selective for benzylic C–H and C–C bonds, whereas metalation of the arene ring is not observed

Ultrafast Charge Dynamics in Dispersions of Monolayer MoS₂ Nanosheets

Ultrafast charge dynamics in dispersions of MoS₂ nanosheets in <i>N</i>-methyl-2-pyrrolidone are reported. Samples were prepared by the ultrasonication-assisted exfoliation of MoS₂ powder, resulting in nanosheets that were predominantly monolayer and had an average sheet size of 32.4 ± 0.1 nm. These dispersions were characterized using absorption and photoluminescence spectroscopy, transient photoluminescence measurements, and atomic force microscopy before the ultrafast charge dynamics were studied via transient absorption spectroscopy. The transient absorption spectra exhibited bleach peaks and photoinduced absorption peaks in spectral regions corresponding to both the A and B excitons of MoS₂. The growth and decay of the features in the B exciton region were determined largely by the dynamics of the exciton population, while the features in the A exciton region depend on the dynamics of both excitons and trions

Reactivity of “Ligand-Free” Au⁺: C–H and C–C Activation versus π Coordination

[(Me₂S)­Au]⁺ cations, generated from (Me₂S)­AuCl and AgSbF₆ in dichloromethane at 0–20 °C, serve as sources of solvated Au⁺ (alongside unreactive [Au­(SMe₂)₂]⁺), which reacts with the methyl-substituted arenes C₆Me_6–<i>n</i>H_<i>n</i> (n = 0–2) with C–H bond cleavage to give the sulfonium salts [C₆Me_5–<i>n</i>H_<i>n</i>CH₂SMe₂]⁺. There was no evidence for arene π coordination to Au⁺ or for the formation of σ-bonded Au–benzyl species. Surprisingly, the reaction of Au⁺ with CH₂Ar₂ leads to C–C bond cleavage (Ar = 2,4,6-C₆H₂Me₃). The reactions are highly selective for benzylic C–H and C–C bonds, whereas metalation of the arene ring is not observed

Tribology of Copper Metal Matrix Composites Reinforced with Fluorinated Graphene Oxide Nanosheets: Implications for Solid Lubricants in Mechanical Switches

The potential for the use of copper coatings on steel switching mechanisms is abundant owing to the high conductivities and corrosion resistance that they impart on the engineered assemblies. However, applications of these coatings on such moving parts are limited due to their poor tribological properties; tendencies to generate high friction and susceptibility to degradative wear. In this study, we have fabricated a fluorinated graphene oxide–copper metal matrix composite (FGO-CMMC) on an AISI 52100 bearing steel substrate by a simple electrodeposition process in water. The FGO-CMMC coatings exhibited excellent lubrication performance under pin-on-disk (PoD) tribological sliding at 1N load, which reduced CoF by 63 and 69%, compared to the GO-CMMC and pure copper coatings that were also prepared. Furthermore, FGO-CMMC achieved low friction and low wear at higher sliding loads. The lubrication enhancement of the FGO-CMMCs is attributed to the tribochemical reaction of FGO with the AISI 52100 steel counterface initiated by the sliding load. The formation of an asymmetric tribofilm structure on the sliding track is critical; the performance of the FGO/Cu tribofilm formed in the track is boosted by the continued fluorination of the counterface surface during PoD sliding, passivating the tribosystem from adhesion-driven breakdown. The FGO-CMMC and GO-CMMC coatings also provide increased corrosion protection reaching 94.2 and 91.6% compared to the bare steel substrate, allowing for the preservation of the long-term low-friction performance of the coating. Other influences include the improved interlaminar shear strength of the FGO-containing composite. The excellent lubrication performance of the copper matrix composite coatings facilitated by FGO incorporation makes it a promising solid lubricant candidate for use in mechanical engineering applications