Arene-metal complexes: Preparation and reactions of h6-arene-Cr(CO)2(diphos) and analogs

Reaction of 1,2-bis(triphenylphosphino)ethane (diphos) with h⁶-arenetricarbonylchromium(0) complexes results in replacement of one CO by diphos to give h⁶-arene(diphos)dicarbonylchromium(0) (e.g., complex 4) and μ-diphosbis(h⁶-arene)tetracarbonyldichromium(0). Comparison of rate of arene exchange for h⁶-benzenetricarbonylchromium, h⁶-benzene(triphenylphosphine)dicarbonylchromium, and complex 4 showed that: (1) replacement of CO by a phosphine retards arene exchange and (2) the effect of diphos in stabilizing an h⁴-benzene ligand in complex 4 is not sufficient to allow rapid arene exchange. Nucleophilic substitution for hydrogen was observed for the first time in a phosphine-substituted arene-chromium complex.     

Polymerisation von α-Methylstyrol durch Bestrahlung mit Elektronen

Zusammenfassung Es wird die durch Elektronenbestrahlung ausgelöste Substanzpolymerisation von-Methylstyrol beschrieben. Auf Grund der Untersuchungen des Polymerisationsverlaufes und der Taktizitäten der Polymeren mit Hilfe der magnetischen Kernresonanz wird ein kationischer Mechanismus angenommen.

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Summary The polymerization in bulk of-methylstyrene induced by irradiation with electrons is studied. From investigations of the polymerization and polymer tacticities by nuclear magnetic resonance it is concluded that the polymerization takes place by a kationic mechanism.

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Wir danken Herrn cand. phys.Klaus Kolbe für die Taktizitätsbestimmungen.     

Trace element analysis of geological glasses by laser plasma ionization mass spectrometry (LIMS): A comparison with other multielement and microanalytical methods

Laser plasma ionization mass spectrometry (LIMS) is used in our laboratory as an in-situ microanalytical method for the investigation of solids, especially of rocks and minerals. To demonstrate the accuracy of this method we have analyzed homogeneous geological glass samples. The results are compared with data obtained from other analytical techniques. The performance of the LIMS method for geochemical investigations is discussed. Received: 2 December 1996 / Revised: 21 February 1997 / Accepted: 26 February 1997     

DFB laser diodes in the wavelength range from 760 nm to 2.5 μm

We present a novel device technology to produce DFB laser diodes which are suitable for tunable diode laser spectroscopy. The new technological approach employs lateral metal distributed feedback (DFB) gratings in close proximity to the laser ridge which results in single mode emission with high spectral purity and output powers as required for most spectroscopic applications. Over the entire wavelength range from the visible (760 nm) up to the near-infrared (2.5 microm) single mode emission can be obtained for devices based on different semiconductor systems such as GaAs, InP and GaSb. Typical side mode suppression ratios are better than 35 dB for cw-room temperature operation and narrow linewidths ensure high spectroscopic resolution.     

Tailoring Large Pores of Porphyrin Networks on Ag(111) by Metal–Organic Coordination

The engineering of nanoarchitectures to achieve tailored properties relevant for macroscopic devices is a key motivation of organometallic surface science. To this end, understanding the role of molecular functionalities in structure formation and adatom coordination is of great importance. In this study, the differences in formation of Cu‐mediated metal–organic coordination networks based on two pyridyl‐ and cyano‐bearing free‐base porphyrins on Ag(111) are elucidated by use of low‐temperature scanning tunneling microscopy (STM). Distinct coordination networks evolve via different pathways upon codeposition of Cu adatoms. The cyano‐terminated module directly forms 2D porous networks featuring fourfold‐coordinated Cu nodes. By contrast, the pyridyl species engage in twofold coordination with Cu and a fully reticulated 2D network featuring a pore size exceeding 3 nm² only evolves via an intermediate structure based on 1D coordination chains. The STM data and complementary Monte Carlo simulations reveal that these distinct network architectures originate from spatial constraints at the coordination centers. Cu adatoms are also shown to form two‐ and fourfold monoatomic coordination nodes with monotopic nitrogen‐terminated linkers on the very same metal substrate—a versatility that is not achieved by other 3d transition metal centers but consistent with 3D coordination chemistry. This study discloses how specific molecular functionalities can be applied to tailor coordination architectures and highlights the potential of Cu as coordination center in such low‐dimensional structures on surfaces.     

Surface‐Assisted Cyclodehydrogenation; Break the Symmetry,Enhance the Selectivity

Selectivity in chemical reactions is a major objective in industrial processes to minimize spurious byproducts and to save scarce resources. In homogeneous catalysis the most important factor which determines selectivity is structural symmetry. However, a transfer of the symmetry concept to heterogeneous catalysis still requires a detailed comprehension of the underlying processes. Here, we investigate a ring‐closing reaction in surface‐confined meso‐substituted porphyrin molecules by scanning tunneling microscopy, temperature‐programmed desorption, and computational modeling. The identification of reaction intermediates enables us to analyze the reaction pathway and to conclude that the symmetry of the porphyrin core is of pivotal importance regarding product yields.     

Supramolecular architectures of cellulose derivatives

Several cellulose derivatives belong to a special class of polymers called hairy-rod macromolecules which are used to generate well-defined supramolecular architectures by the Langmuir-Blodgett (LB) technique. In particular trimethylsilyl cellulose (TMSC) forms monomolecular films on the Langmuir-trough and is transferred onto hydrophobic substrates with a constant transfer ratio, as it does not undergo chemical changes in the film-building process. Silylated celluloses was regenerated which represents a convenient method for the generation of homogeneous ultrathin films with hydrophilic surfaces. The adsorption of polymers and dyes as well as biomolecules onto regenerated and modified cellulose LB films have been studied. In addition, chemical reactions, such as cycloaddition, desilylation and crosslinking reactions within single monolayers have been performed.     

Synthese und dynamisches verhalten des 1-pentamethylcyclopentadienyl-2,3,4,5,6-pentamethyl-2,3,4,5,6-pentacarba-nido-hexaboran-kations

In the reaction of pentamethylcyclopentadienyldichloroborane with pentamethylcyclopentadienyllithium bis(pentamethylcyclopentadienyl)chloroborane is formed, from which the 1-pentamethylcyclopentadienyl-2,3,4,5,6-pentamethyl-2,3,4,5,6-pentacarba-nido-hexaborane cation can be synthesized by halogen abstraction. Investigations concerning the dynamic behaviour of the cluster are described.