Winfried P. Kretschmer

Robust Heterogeneous Nickel Catalysts with Tailored Porosity for the Selective Hydrogenolysis of Aryl Ethers

SiC materials with tailored porosity and integrated Ni NPs (Ni@SiC) were synthesized via microphase separation of polycarbosilane‐block‐polyethylene followed by its pyrolysis. Changing the length of organic block allowed the synthesis of micro, meso and hierarchical Ni@SiC materials which were characterized by PXRD, TEM, TGA, and nitrogen physisorption. Selective hydrogenolysis of aryl ethers mimicking the most abundant linkages of lignin was achieved in water avoiding the possible hydrogenation of aromatic rings.

Novel, highly symmetrical halogen-centered polynuclear lanthanide complexes: [Cp6Yb6Cl13](-) and [Cp12Sm12Cl24]

If THF is removed from the complexes [CpLnCl2(thf)3] (Cp=C5H5, Ln=Sm, Yb), halogen-centered polynuclear complexes are formed. [{CpSmCl2}12] displays an icosahedral arrangement of the 12 Sm atoms with 20 Cl atoms in the “outer envelope” and four further Cl atoms within the icosahedron (see structure on the right). The partially desolvated [CpYbCl2]⋅1/3 THF consists of trinuclear cations [Cp3Yb3Cl5(thf)3]+ and octahedral anions [Cp6Yb6Cl13]−.

SiCN Nanofibers with a Diameter Below 100 nm Synthesized via Concerted Block Copolymer Formation, Microphase Separation, and Crosslinking

SiCN fibers with a mean diameter of 50 nm and an aspect ratio of up to 100 are produced in a two-step process by R. Kempe and co-workers. The key step to fabricate the longitudinal and cross-sectional views of the mesofibers shown here is a concerted block-copolymer synthesis, microphase separation, and cross linking at 140 °C followed by pyrolysis at 1100 °C. Inexpensive components like a commercially available silazane and polyethylene are linked. The fibers may find application in electronic devices, as components of ceramic matrix composites, as fiber beds in high-temperature nano-filtering like diesel fine dust removal, or as thermally robust and chemically inert catalyst supports. Furthermore, the SiCN nanofibers introduced on page 984 are a promising alternative to ultrathin carbon fibers, due to their oxidation resistance.

NEUARTIGE, HOCHSYMMETRISCHE, HALOGENZENTRIERTE MEHRKERNKOMPLEXE DER LANTHANOIDE : CP6YB6CL13- UND CP12SM12CL24

Entfernt man thf aus den Komplexen [CpLnCl2(thf)3] (Cp  =  C5H5, Ln  =  Sm, Yb), so werden halogenzentrierte Mehrkernkomplexe gebildet. [{CpSmCl2}12] zeigt eine ikosaedrische Anordnung der 12 Sm-Atome mit 20 Cl-Atomen in der „auseren Hulle” sowie vier weiteren Cl-Atomen innerhalb des Ikosaeders (Strukturbild rechts). Im teilweise desolvatisierten [CpYbCl2]⋅1/3 THF liegen die dreikernigen Kationen [Cp3Yb3Cl5(thf)3]+ und die oktaedrischen Anionen [Cp6Yb6Cl13]− vor.

A broadly tunable synthesis of linear α-olefins

The catalytic synthesis of linear α-olefins from ethylene is a technologically highly important reaction. A synthesis concept allowing the formation of selective products and various linear α-olefin product distributions with one catalyst system is highly desirable. Here, we describe a trimetallic catalyst system (Y–Al–Ni) consisting of a rare earth metal polymerization catalyst which can mediate coordinative chain transfer to triethylaluminum combined with a simultaneously operating nickel β-hydride elimination/transfer catalyst. This nickel catalyst displaces the grown alkyl chains forming linear α-olefins and recycles the aluminum-based chain transfer agent. With one catalyst system, we can synthesize product spectra ranging from selective 1-butene formation to α-olefin distributions centered at 850 gmol−1 with a low polydispersity. The key to this highly flexible linear α-olefin synthesis is the easy tuning of the rates of the Y and Ni catalysis independently of each other. The reaction is substoichiometric or formally catalytic regarding the chain transfer agent.Linear α-olefins are important bulk chemicals annually produced in megaton scale. Here, the authors report a trimetallic catalyst system for the synthesis of 1-butene and a broad range of α-olefins and achieve control over chain length by tuning the reaction rates of the nickel and yttrium catalysts.

Synthesis, structural investigations and ethylene polymerization of titanium complexes with tripodal oxazoline ligands

A series of tripodal nitrogen containing ligands including amidine and aminopyridines with extra oxazoline functionality were synthesized. The corresponding titanium complexes bearing such ligands were synthesized by diethylamine elimination route. Diethylamidotitanium trichloride [Et2NTiCl3] reacts with the functionalized anilines, 2-(4,4-dimethyl-4,5-dihydrooxazol-2-yl)-N-(2-fluorophenyl)-aniline (FOxH) and 2-(4,4-dimethyl-4,5-dihydrooxazol-2-yl)-N-(2-methoxyphenyl)aniline (MeOOxH), the amidine(E)-N’-(2,6-diisopropylphenyl)-N-(2-(4,4-dimethyl-4,5-dihydrooxazol-2-yl)phenyl)benzimidamide ( Am OxH), and the aminopyridines N-(2-(4,4-dimethyl-4,5-dihydro oxazol-2-yl)phenyl)-6-(piperidin-1-yl)pyridin-2-amine (ApOxH) and N-(2-(4,4- dimethyl- 4,5-dihydrooxazol-2-yl)phenyl)-6-(2,4,6-triisopropyl phenyl)pyridin-2-amine (Ap*OxH), under diethylamine elimination to form the corresponding titanium trichlorides [FOxTiCl3], [MeOOxTiCl3], [AmOxTiCl3], [ApOxTiCl3] and [Ap*OxTiCl3] in excellent yields. These compounds were characterized by spectroscopic methods, and X-ray crystal structure analysis (selected complexes). Furthermore, their behavior in ethylene polymerization was explored. The complexes show moderate activity towards ethylene if activated with MAO. The observed PEs were analyzed by HT-GPC and were found to be of low molecular weight for the amidinate AmOxTiCl3 but of very high one (Mp up to 3.5 million g/mol) for the aminopyridinate Ap*OxTiCl3 titanium complex.