High-Spin and Reactive Fe13 Cluster with Exposed Metal Sites

Atomically defined large metal clusters have applications in new reaction development and preparation of materials with tailored properties. Expanding the synthetic toolbox for reactive high nuclearity metal complexes, we report a new class of Fe clusters, Tp*₄W₄Fe₁₃S₁₂ , displaying a Fe₁₃ core with M−M bonds that has precedent only in main group and late metal chemistry. M₁₃ clusters with closed shell electron configurations can show significant stability and have been classified as superatoms. In contrast, Tp*₄W₄Fe₁₃S₁₂ displays a large spin ground state of S=13. This compound performs small molecule activations involving the transfer of up to 12 electrons resulting in significant cluster rearrangements.     

Series representations for densities functions of a family of distributions—Application to sums of independent random variables

Series representations for several density functions are obtained as mixtures of generalized gamma distributions with discrete mass probability weights, by using the exponential expansion and the binomial theorem. Based on these results, approximations based on mixtures of generalized gamma distributions are proposed to approximate the distribution of the sum of independent random variables, which may not be identically distributed. The applicability of the proposed approximations are illustrated for the sum of independent Rayleigh random variables, the sum of independent gamma random variables, and the sum of independent Weibull random variables. Numerical studies are presented to assess the precision of these approximations.     

Selective ethylene oligomerization: Recent advances in chromium catalysis and mechanistic investigations

Selective production of linear α-olefins is of significant commercial interest. Recently discovered catalytic systems based on titanium, tantalum, and chromium show remarkable selectivity and productivity for the oligomerization of ethylene to 1-hexene or 1-octene. Chromium-based catalysts are the most selective and active and show the highest structural diversity. This paper discusses the most recent advances in chromium chemistry related to selective olefin oligomerization. Aspects regarding ligand design, catalyst generation, selectivity for different products, and reaction mechanism are presented. Isotopic labeling protocols designed to distinguish between various mechanisms of catalysis are reviewed.     

Mechanisch-chemische Reaktionen in künstlichen organischen Fasern

Zusammenfassung Die beim Verstrecken sowie Mahlen von künstlichen organischen Fasermaterialien (Polyester-, Polyamid- und Viskosefasern) als Folge homolytischer Kettenspaltungen auftretende Freiradikalbildung wurde untersucht.

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Mechanochemical reactions in synthetic organic fibres

The formation of free radicals due to homolytic chain fission occurring in the course of stretching and grinding of synthetic organic fibres (polyester, polyamide, and viscose fibres) has been investigated by means of ESR spectroscopy.

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Mit 3 Abbildungen

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Die Elektronspinresonanz-Messungen wurden im Chemischen Institut der Sir George Williams University, Montreal, Canada, durchgeführt.     

Arene C?H Amination at Nickel in Terphenyl–Diphosphine Complexes with Labile Metal–Arene Interactions

The meta‐terphenyl diphosphine, m‐P₂, 1 , was utilized to support Ni centers in the oxidation states 0, I, and II. A series of complexes bearing different substituents or ligands at Ni was prepared to investigate the dependence of metal–arene interactions on oxidation state and substitution at the metal center. Complex (m‐P₂)Ni ( 2 ) shows strong Ni⁰–arene interactions involving the central arene ring of the terphenyl ligand both in solution and the solid state. These interactions are significantly less pronounced in Ni⁰ complexes bearing L‐type ligands ( 2‐L : L=CH₃CN, CO, Ph₂CN₂), Ni^IX complexes ( 3‐X : X=Cl, BF₄, N₃, N₃B(C₆F₅)₃), and [(m‐P₂)Ni^IICl₂] ( 4 ). Complex 2 reacts with substrates, such as diphenyldiazoalkane, sulfur ylides (Ph₂S?CH₂), organoazides (RN₃: R=para‐C₆H₄OMe, para‐C₆H₄CF₃, 1‐adamantyl), and N₂O with the locus of observed reactivity dependent on the nature of the substrate. These reactions led to isolation of an η¹‐diphenyldiazoalkane adduct ( 2‐Ph₂CN₂ ), methylidene insertion into a Ni? P bond followed by rearrangement of a nickel‐bound phosphorus ylide ( 5 ) to a benzylphosphine ( 6) , Staudinger oxidation of the phosphine arms, and metal‐mediated nitrene insertion into an arene C? H bond of 1 , all derived from the same compound ( 2 ). Hydrogen‐atom abstraction from a Ni^I–amide ( 9 ) and the resulting nitrene transfer supports the viability of Ni–imide intermediates in the reaction of 1 with 1‐azido‐arenes.     

Specific Features of Strain Dependences of Loss Modulus in Highly Stretched Elastomers

The loss modulus E″ in elastomer strips stretched by a factor of λ is studied theoretically. A small oscillating deformation is applied to these strips in the transition frequency zone between the glassy and rubbery states. The range of λ under study ranges up to ultimate extensions close to fracture. A model of E″/λ dependence in the transition frequency zone is suggested that considers the possible distortion in the local chain structure in the ultimate extension range. It is found that E″ in the range of moderate and high λ increases owing to the finite extensibility of the chains. However, upon further extension up to ultimate values, E″ decreases and a maximum appears on the E″/λ curves. These features are also confirmed experimentally.

     

Über trilinear verwandte Felder als Raumbilder

Monatshefte für Mathematik -