Sulfur-containing palladacycles as catalyst precursors for the Heck reaction

[formula: see text] The air, water, and highly thermally stable sulfur-containing palladacycles, mainly derived from the ortho-palladation of benzylic thioethers, are exceptional catalyst precursors for the Heck reaction. The reaction can be performed with aryl lodides, bromides, and chlorides, with acrylic esters and styrene, leading to turnover numbers up to 1,850,000.     

Preparation and characterization of d tungsten compounds with amino acid derivatized diimine ligands and preparation of dipeptide derivatives using peptide coupling agents

Attempts to prepare dipeptide compounds from organometallic N-substituted amino acids are reported. Condensation of pyridine-2-carboxaldehyde, α-amino acids (H-l-Ala-OH or H-l-Asp-OH) and W(CO)₄(pip)₂ leads to formation of W(CO)₄(pyca-Et) (1) (pyca refers to the α-diimine fragment, C₅H₄NCHN) following decarboxylation of one or two equivalents of CO₂. This decarboxylation does not occur for β-alanine or GABA (γ-aminobutyric acid). Coupling of [Hpip][W(CO)₄(pyca-β-Ala-O)] (2) or [Hpip][W(CO)₄(pyca-GABA-O)] (3) to amino acid esters, H-l-Ala-OEt, or H-l-Val-OMe, using the standard 1,3-dicyclohexylcarbodiimide (DCC), 1-hydroxybenzotriazole (HOBt) procedure produced four new dipeptide compounds, 4-7. The reactions proceed in good yield and compounds were characterized spectroscopically. The dipeptide complex, W(CO)₄(pyca-Ala-Ala-OMe) (8), was prepared by reaction of W(CO)₄(pip)₂ with H-l-Ala-l-Ala-OMe and pyridine-2-carboxaldehyde. In addition the molecular structure of W(CO)₄(pyca-β-Ala-Val-OMe) (5) is reported.     

Scaling laws in aeolian sand transport

We report on wind tunnel measurements on saltating particles in a turbulent boundary layer and provide evidence that over an erodible bed the particle velocity in the saltation layer and the saltation length are almost invariant with the wind strength, whereas over a nonerodible bed these quantities vary significantly with the air friction speed. It results that the particle transport rate over an erodible bed does not exhibit a cubic dependence with the air friction speed, as predicted by Bagnold, but a quadratic one. This contrasts with saltation over a nonerodible bed where the cubic Bagnold scaling holds. Our findings emphasize the crucial role of the boundary conditions at the bed and may have important practical consequences for aeolian sand transport in a natural environment.     

Generic variables in acyclic cluster algebras

Let Q be an acyclic quiver. We introduce the notion of generic variables for the coefficient-free acyclic cluster algebra A(Q). We prove that the set G(Q) of generic variables contains naturally the set M(Q) of cluster monomials in A(Q) and that these two sets coincide if and only if Q is a Dynkin quiver. We establish multiplicative properties of these generic variables analogous to multiplicative properties of Lusztig’s dual semicanonical basis. This allows to compute explicitly the generic variables when Q is a quiver of affine type. When Q is the Kronecker quiver, the set G(Q) is a Z-basis of A(Q) and this basis is compared to Sherman-Zelevinsky and Caldero-Zelevinsky bases.     

High-speed stereoscopic PIV study of rotating instabilities in a radial vaneless diffuser

This paper presents an experimental analysis of the unsteady phenomena developing in a vaneless diffuser of a radial flow pump. Partial flow operating conditions were investigated using 2D/3C high repetition rate PIV, coupled with unsteady pressure transducers. Pressure measurements were acquired on the shroud wall of the vaneless diffuser and on the suction pipe of the pump, whereas PIV flow fields were determined on three different heights in the hub to shroud direction, inside the diffuser. The classical Fourier analysis was applied to both pressure signals to identify the spectral characteristics of the developing instabilities, and the high-order spectral analysis was exploited to investigate possible non-linear interaction mechanisms between different unsteady structures. A dedicated PIV averaging procedure was developed and applied to the PIV flow fields so as to capture and visualize the topology of the spectrally identified phenomena. The influence of these phenomena on the diffuser efficiency was also investigated.     

On a category of cluster algebras

We introduce a category of cluster algebras with fixed initial seeds. This category has countable coproducts, which can be constructed combinatorially, but no products. We characterise isomorphisms and monomorphisms in this category and provide combinatorial methods for constructing special classes of monomorphisms and epimorphisms. In the case of cluster algebras from surfaces, we describe interactions between this category and the geometry of the surfaces.     

The Crystal Structure of Ba2CuAlF9: a New Structure Type in Copper Fluoride Chemistry

Ba₂CuAlF₉ is monoclinic: a = 5.374(2) Å, b = 7.312(2) Å, c = 9.371(3) Å, β = 90.20(1)°, Z = 2, space group P2₁/c (n° 14). The crystal structure was solved from X-ray single crystal data using 1071 unique reflections (900 with F_o/σ(F_o) > 4, R factor = 0.075). It is built up from infinite isolated cis chains of [MF₆] mixed occupied fluorine octahedra sharing each, one edge and one vertex (M is randomly Cu or Al). An analogous kind of linkage was already observed for two other compounds from the ternary system BaF₂/CuF₂/AlF₃. Close structural relationships exist between the cationic subnetworks of γ-BaAlF₅ and Ba₂CuAlF₉.     

The influence of the transition metal and the heteroatomic-bridge on the action of metallocene/methyl aluminoxane catalysts in ethylene polymerization and on the properties of the polymer

This work reports a comparative study of the catalytic behaviour for a series of metallocenes derived from Ti, Zr, Hf and Nb, which after activation with methylaluminoxane can polymerize ethylene. Results show that the Zr metallocene with a  (CH₃)₂Si Bridge presents the highest activity, and the metallocenes based on Hf and Nb do not show any significant activity under the tested conditions.