Highly Selective Dimerization and Trimerization of Isobutene to Linearly Linked Products by Using Nickel Catalysts

The unique linear linkage of isobutene to generate highly valuable C₈ precursors for plasticizers is feasible by using special nickel catalysts. (4‐Cyclooctene‐1‐yl)(1,1,1,5,5,5‐hexafluoro‐2,4‐acetylacetonato)nickel and aluminum‐alkyl‐activated nickel acetylacetonates produce isobutene dimers with high selectivities of up to 95 %. Moreover, selectivity for the head‐to‐head products (2,5‐dimethylhexenes) is remarkably high at up to 99 %. Additionally, novel C₁₂ isobutene trimers are also formed with a very high selectivity of up to 99 % for the linear linkage. The trimer structure (2,5,8‐trimethylnonenes) reflects the stepwise characteristic of the reaction mechanism. Pathways of insertion and activation and the deactivation processes of the catalyst are discussed in detail.     

Synthesis and Ambiphilic Reactivity of Metalated Diorgano-Phosphonite Boranes

Unprecedented metalated phosphonite boranes were prepared from PH-substituted precursors and silyl amides. Although potassium derivatives were thermally stable and could even be isolated and structurally characterised, lithiated analogues proved to be unstable towards self-condensation under cleavage of LiOR at ambient temperature. Reaction studies revealed that the metalated phosphonite boranes exhibit ambiphilic character. Their synthetic potential as nucleophilic building blocks was demonstrated in the synthesis of the first stannylated phosphonite representing a new structural motif in phosphine chemistry.     

Terminal Ligand and Packing Effects on Slow Relaxation in an Isostructural Set of [Dy(H2dapp)X2]+ Single Molecule Magnets**

We report three structurally related single ion Dy compounds using the pentadentate ligand 2,6-bis((E)-1-(2-(pyridin-2-yl)-hydrazineylidene)ethyl)pyridine (H₂dapp) [Dy(H₂dapp)(NO₃)₂]NO₃ ( 1 ), [Dy(H₂dapp)(OAc)₂]Cl ( 2 ) and [Dy(H₂dapp)(NO₃)₂]Cl_0.92(NO₃)_0.08 ( 3 ). The (H₂dapp) occupies a helical twisted pentagonal equatorial arrangement with two anionic ligands in the axial positions. Further influence on the electronic and magnetic structure is provided by a closely associated counterion interacting with the central N−H group of the (H₂dapp). The slow relaxation of the magnetisation shows that the anionic acetates give the greatest slowing down of the magnetisation reversal. Further influence on the relaxation properties of compounds 1 and 2 is the presence of short nitrate-nitrate intermolecular ligand contact opening further lattice relaxation pathways.     

Nanoscale Organolanthanum Clusters: Nuclearity-Directing Role of Cyclopentadienyl and Halogenido Ligands

Tetramethylaluminato/halogenido(X) ligand exchange reactions in half-sandwich complexes [Cp^RLa(AlMe₄)₂] are feasible in non-coordinating solvents and provide access to large coordination clusters of the type [Cp^RLaX₂]_x. Incomplete exchange reactions generate the hexalanthanum clusters [Cp^R₆La₆X₈(AlMe₄)₄] (Cp^R=Cp*=C₅Me₅, X=I; Cp^R=Cp′=C₅H₄SiMe₃, X=Br, I). Treatment of [Cp*La(AlMe₄)₂] with two equivalents Me₃SiI gave the nonalanthanum cluster [Cp*LaI₂]₉, while the exhaustive reaction of [Cp′La(AlMe₄)₂] with the halogenido transfer reagents Me₃GeX and Me₃SiX (X=I, Br, Cl) produced a series of monocyclopentadienyl rare-earth-metal clusters with distinct nuclearity. Depending on the halogenido ion size the homometallic clusters [Cp′LaCl₂]₁₀ and [Cp′LaX₂]₁₂ (X=Br, I) could be isolated, whereas different crystallization techniques led to the aggregation of clusters of distinct structural motifs, including the desilylated cyclopentadienyl-bridged cluster [(μ-Cp)₂Cp′₈La₈I₁₄] and the heteroaluminato derivative [Cp′₁₀La₁₀Br₁₈(AlBr₂Me₂)₂]. The use of the Cp′ ancillary ligand facilitates cluster characterization by means of NMR spectroscopy.     

Defined carrageenan derivatives as angiogenesis inhibitors

Three carrageenans derived from red algae and three partially modified carrageenans were tested for their ability to inhibit the formation of new blood capillaries on the chorionallantois membrane of the hen egg. It could be clearly demonstrated that they were better angiogenesis inhibitors than the standard compound suramin.     

Immunoassay methods for paralytic shellfish poisoning toxins

The current status of immunochemical techniques for analysis of paralytic shellfish poisoning (PSP) toxins is summarized. Important aspects regarding production of the biological reagents necessary for immunochemical methods, the characteristics of polyclonal and monoclonal antibodies against saxitoxin and neosaxitoxin, and the importance of test sensitivity and specificity are discussed. Applications of immunochemical techniques for PSP toxins include microtiter plate enzyme immunoasays and enzyme-linked immunofiltration assays for toxin detection, and immunoaffinity chromatography (IAC) for sample extract cleanup. A major advantage of enzyme immunoassay (EIA) is simplicity and rapidity of the test procedure, and higher sensitivity than other methods. However, quantitative agreement between EIA and mouse bioassay is dependent on antibody specificity and the toxin profile in the shellfish; thus, both over- and underestimation of total toxicity may occur. For screening purposes, however, EIAs offer major advantages over the mouse bioassay, which is criticized in Europe because of animal welfare. A major application of antibodies against PSP toxins is their use for extract cleanup by IAC, which gives highly purified extracts, thereby enhancing determination of PSP toxins by conventional physicochemical methods such as liquid chromatography. IAC can also be used to isolate PSP toxins for preparation of analytical standard solutions.     

Oligomers as initiators of free radical polymerization

Methyl methacrylate, benzyl methacrylate, butyl methacrylate, and methacrylic acid can be polymerized by 1, 2, 2-tetraphenyl-1, 2-diphenoxyethane (TPPA), 1, 1, 2, 2-tetraphenyl-1, 2-bis-(trimethylsiloxy) ethane (TPSA), and 1, 1, 2, 2-tetraphenyl-1, 2-dicyanoethane (TPCA) (1–3). The polymerization with these initiators is characterized by three steps; in the first period oligomers from MMA and initiator radicals are formed by primary radical termination.