Wilfried Assenmacher

Efficient Air‐Stable Organometallic Low‐Molecular‐Mass Gelators for Ionic Liquids: Synthesis, Aggregation and Application of Pyridine‐Bridged Bis(benzimidazolylidene)–Palladium Complexes

Novel pincer-type, pyridine-bridged bis(benzimidazolylidene)-palladium complexes 5-7 were synthesised from cheap commercial precursors under microwave assistance. Although simple in structure, carbene complexes 5a,b are efficient low-molecular-mass metallogelators. They gelate not only a broad variety of protic and aprotic organic solvents, but also different types of customary ionic liquids (such as imidazolium, pyridinium, pyrazolidinium, piperidinium and ammonium salts) at concentrations as low as 0.5 mg mL(-1). The morphologies of the resulting 3D gel networks composed from long and thin fibres were studied by TEM and light microscopy for a selection of organic and ionic liquids. The achiral gelators are able to induce the formation of helical fibres. The thermal stability of the gel samples increases with the gelator concentration as demonstrated by thermoreversible DSC studies. Temperature-dependent NMR and X-ray diffraction studies, as well as comparisons with pincer complex analogues bearing shorter alkyl chains, suggest that the 3D networks responsible for gelation are based on non-covalent interactions, such as pi-stacking, van der Waals interactions, and hydrogen and metal-metal bonding. Ionic liquids and gels obtained from them and 5a,b display comparable high conductivities, which characterises pyridine-bridged bis(benzimidazolylidene)-palladium pincer complexes as air-stable metallo gelators that efficiently immobilise ionic liquids in low gelator concentration indicating--beyond catalysis--their potential applications in electrochemical devices.

Structural characterization of amorphous ceramics in the system Si–B–N–(C) by means of transmission electron microscopy methods

Amorphous ceramics with the chemical composition Si 3 B 3 N 7 and SiBN 3 C were produced from single-source molecular precursors by polymerization and pyrolysis. The powder and fiber materials were investigated by means of energy filtering transmission electron microscopy. The intensity of elastically scattered electrons is recorded to calculate the pair distribution function of these ceramics. In the pair distribution function of Si 3 B 3 N 7 three significant maxima at 0.144, 0.172, and 0.291 nm are clearly resolved and are assigned to the pair distances B–N, Si–N, and Si–Si (N–N), respectively, by comparison to crystalline materials. The predominant structural units of the ceramic are trigonal planar BN 3 and tetrahedral SiN 4 groups, which are close to their regular symmetry. The overall pair distribution function of SiBN 3 C is very similar to that of Si 3 B 3 N 7 ; however, the maxima are broadened due to the incorporation of carbon into the network. High-resolution mapping of the elements Si, B, N, and C with electron spectroscopic imaging reveals a homogeneous distribution on a subnanometer scale without precipitation or separation of, for example, carbon-rich clusters. Similarly, elemental mapping of Si 3 B 3 N 7 reveals a random distribution of the elements Si, B, and N at the same scale. Both new ceramics consist of an amorphous network with bonds and coordinations as preformed in the precursor.

Novel Graphitic Spheres: Raman Spectroscopy at High Pressures

Novel graphitic spheres with a shell structure of curved graphene layers were discovered as a side product of fullerene synthesis. The Raman spectra of these carbon spheres with typical sizes of 20–500 μm have been found to be similar to that of micro-crystalline graphite. The silent low-frequency B1g(1) phonon of graphite, however, becomes Raman active in the C-spheres. Raman spectroscopy of the C-spheres under high pressures up to 2.4 GPa shows that their vibrational properties are very similar to those of graphite, despite the curvature of the graphene sheets. We have studied the effect of high pressure on the D* mode near 2700 cm—1 which is present in all graphitic materials and shows a peculiar dependence of its frequency on the exciting laser wavelength.

Solution-Based Synthesis of GeTe Octahedra at Low Temperature

GeTe octahedra were prepared by reaction of equimolar amounts of GeCl2·dioxane and Te(SiEt3)2 in oleylamine, whereas a slight excess of the Te precursor yielded GeTe octahedra decorated with elemental Te nanowires, which can be removed by washing with TOP. The mechanism of the GeTe formation is strongly influenced by the solvent. The expected elimination of Et3SiCl (dehalosilylation) only occurred in aprotic solvents, whereas Te(SiEt3)2 was found to react with primary and secondary amines with formation of silylamines. Temperature-dependent studies on the reaction in oleylamine showed that crystalline GeTe particles are formed at temperatures higher than 140 °C. XRD, SAED, and HRTEM studies proved the formation of rhombohedral GeTe nanoparticles. These findings were confirmed by a single-crystal and powder X-ray analysis. The rhombohedral structure modification was found, and the structure was solved in the acentric space group R3m.

Incorporation of chromium carbenes in a silica matrix by sol-gel processing: application to aminolysis of alkoxycarbene complexes

Chromium carbene complexes have been immobilized in a silica matrix by sol-gel processing based on the polycondensation of (trialkoxy)silylethylphosphane ligands and tetraalkoxysilanes. The microstructure of the material obtained depends on the gelation conditions. In situ gelation of alkoxy- or aminocarbene complexes with tetramethoxysilane (TMOS) affords mesoporous materials with a homogeneous distribution of the metal complex. The metal carbene moiety is accessible for small substrates as demonstrated for the aminolysis of the incorporated methoxy(phenyl)carbene complex which slows down with increasing bulk of the amine.

Growth of GaSb whiskers by thermal decomposition of a single source precursor

Thermal decomposition reactions of the Lewis acid–base adducts t-Bu3Ga–Sb(t-Bu)31 and t-Bu3Ga–Sb(i-Pr)32 were investigated at different temperatures. Both adducts lead to the formation of crystalline GaSb particles in the temperature range of 275–450 °C, proving their potential to serve as single source precursors for the preparation of GaSb. In contrast to 1, 2 tends to form crystalline, highly-oriented GaSb whiskers under the pyrolysis conditions. Detailed temperature-dependent studies clearly reveal the strong influence of the decomposition conditions on the whisker growth. While at temperatures between 275 and 375 °C single GaSb needles preferably were formed, higher temperatures (400–450 °C) lead to the formation of crystalline GaSb dendrites. As-prepared carbon-free, cubic GaSb whiskers were characterized in detail by SEM, TEM, electron diffraction, EDX and EEL spectroscopy.

Zur struktur sterisch gehinderter Zinn-Elementverbindungen: Kristallstruktur der Zinn-Schwefel-Stickstoff-Ringe [(tBu2Sn)2N2S2] und [(tBu2Sn)N2S2]2

The structures of two new tin heterocycles, tBU2SnNSNSn(tBu2)S (2a) and [tBu2SnSN]2 (3a), were determined by single-crystal X-ray analysis. Both compounds contain almost planar ring systems. Compound 2a contains a six-membered ring (space group P21/n (No. 14); Z = 4); 3a consists of two identical five-membered ring systems, which are associated via N → Sn pentacoordination to give a four-membered tin-nitrogen ring (space group P1(No. 2); Z = 2).

Synthesis of Bi2Te3 and (BixSb1−x)2Te3 nanoparticles using the novel IL [C4mim]3[Bi3I12]

The novel Bi-containing reactive ionic liquid [C4mim]3[Bi3I12], which was synthesized in quantitative yield by equimolar reaction of BiI3 and [C4mim]I, was used as a novel Bi-source for the ionothermal synthesis of Bi2Te3 nanoparticles by reaction with (Et3Si)2Te in the ionic liquid [C4mim]I. The solid state structure of [C4mim]3[Bi3I12] was determined by single crystal X-ray diffraction. In addition, the ionothermal synthesis of the single source precursor (Et2Sb)2Te and [C4mim]3[Bi3I12] yielded the ternary (BixSb1-x)2Te3 (x = 0.25, 0.5, 0.75) nanoparticles. The chemical composition and phase purity of the tetradymite-type materials were determined by EDX and XRD and the surface composition of the nanoparticles was further investigated by IR and XPS. In addition, the morphology of the nanoparticles was investigated by SEM and TEM.

Temperature-controlled synthesis of gallium antimonide nanoparticles in solution

Abstract Reactions of GaCl3 and Sb(SiMe3)3 in a 1:1 molar ratio in different solvents (pentane, hexane, toluene, and xylene) are described. The resulting GaSb was characterized by electron and X-ray diffraction studies, energy-dispersive X-ray (EDS) analysis, and transmission electron micrography (TEM) studies. The particle size and chemical composition was controlled by the solvent. The size of the GaSb nanoparticles varied between 10 and 50 nm.

Onion-like marbles and bats: new morphological forms of carbon

New spherical and bat-like carbon objects up to 1 mm in diameter and 3 mm in length have been produced in a radio-frequency furnace by evaporation and deposition of pure carbon. The objects form without the use of any catalyst. They have been characterised by scanning and high-resolution transmission electron microscopy, energy-dispersive microanalysis, X-ray diffraction, Raman, infrared and electron energy loss spectroscopy, magnetic, thermogravimetric and micro-hardness measurements. The objects consist of nearly perfect concentric shells, scaling down to nanometer dimensions in the centre. Their properties reveal similarities to both graphite and carbon nanotubes.