Palladium‐, Platin‐ und Dieisenkomplexe mit Isocyanacetat: Ringschluß, säureinduzierte Ringöffnung,Diprotonierung

Palladium, Platinum, and Diiron Complexes with Isocyanoacetate: Ring Closure, Acid‐Induced Ring Opening, Diprotonation Substitution by isocyanoacetate (CNCH₂CO₂^?) of one chloro ligand in trans‐[MCl₂(PPh₃)₂] (M = Pd, Pt) results in the Δ²‐oxazolin‐5‐on‐2‐ato complexes 4a , b , i.e. immediate cyclization occurs in contact with these metal(II) species. In contrast, the open‐chain form of the functional isocyanide is retained in [K(18‐crown‐6][Fe₂Cp₂(CNCH₂CO₂)(CO)₃] ( 16 ) in which it occupies a terminal position. Protonation (alkylation) of the platinum complex 4b proceeds with ring cleavage and formation of isocyano acetic acid 11 (ethyl isocyanoacetate 12 ) stabilized by metal ion coordination. Protonation of 16 requires two equivalents of acid to yield the aminocarbyne‐bridged complex [{μ‐C=N(H)CH₂CO₂H}Fe₂Cp₂(CO)₃](BF₄) ( 17 ) as the only isolable product. Here isocyanoacetate displays a third kind of reactivity pattern in addition to that at Pd^II/Pt^II and that at Cr⁰/W⁰ where the primary species [M(CO)₅CNCH₂CO₂]^? and [M(CO)₅CNCH₂CO₂H] proved to be the most stable. All of the proposed structures are substantiated by analytical and the usual spectroscopic (IR, NMR{¹H, ¹³C, ³¹P}, FAB‐MS) data, that of 4b also by an X‐ray structure determination which reveals a practically perpendicular arrangement of the coordination and the ring plane, and a long C2‐O bond as the predetermined breaking point of the heterocycle.     

Chemical modification of colloidal masks for nanolithography

A method is presented to tune the holes in colloidal masks used for nanolithography. Using a simple wet-chemical method, a thin layer of silica is grown on masks of silica particles. The size of the holes is controlled by the amount of tetraethoxysilane (TEOS) added. More accurate tuning of the hole size is possible in the presence of a calibrated seed dispersion of silica colloids. We demonstrate modified masks that were used to create arrays of metal nanoparticles with a size ranging from 400 nm, for unmodified masks, down to tens of nanometers. The method is easy-to-use, fast, and inexpensive.     

Rhenium‐Dicarbonyl‐Nitrosyl‐Komplexe mit Imidazol

Rhenium Dicarbonyl‐Nitrosyl Complexes with Imidazole Different rhenium‐dicarbonyl‐nitrosyl complexes with imidazole (Im) as monodentate ligand have been synthesized and characterized, starting from [NEt₄][ReCl₃(CO)₂(NO)] and [ReCl(μ?Cl)(CO)₂(NO)]₂. Whereas the complexes [ReCl₂(Im)(CO)₂(NO)] and [ReCl(Im)₂(CO)₂(NO)]⁺ were achieved in high yields, the complex [Re(Im)₃(CO)₂(NO)]²⁺ with three imidazole ligands could only be isolated after complete removal of all halide ions (with AgBF₄) in low yield. The synthesis of a corresponding ^99mTc‐dicarbonyl‐nitrosyl complex with imidazole opens a new perspective for such compounds as potential radiopharmaceuticals and alternatives to the already established ^99mTc‐tricarbonyl complexes.     

Discrimination of active palladium sites in catalytic liquid-phase oxidation of benzyl alcohol

Knowledge of the structure of active sites is a prerequisite for the rational design of solid catalysts. Using site-selective blocking by CO and isotope labeling combined with in situ attenuated total reflection infrared (ATR-IR) spectroscopy, we were able to discriminate the different sites involved in the liquid-phase oxidation of benzyl alcohol on Pd/Al(2)O(3). The main reaction, that is, the oxidative dehydrogenation of the alcohol to the corresponding aldehyde, showed only little dependence on structure and occurred on all exposed Pd faces, whereas the undesired product decarbonylation occurred preferentially on hollow sites on (111) Pd faces. This explains why specific blocking of the latter sites, as realized in the industrially used Pd-Bi/Al(2)O(3) catalysts, leads to improved catalytic performance.     

Time-lapse STM studies of diastereomeric cinchona alkaloids on platinum metals

The adsorption of cinchonidine (CD) and cinchonine (CN) on Pt(111) and Pd(111) single crystals has been investigated by means of scanning tunneling microscopy (STM) in an ultrahigh vacuum system. In time-lapse series the mobilities of different adsorption species have been determined on a single molecule basis and with varying hydrogen background pressures in the system. The diastereomeric cinchona alkaloids, CD and CN, which are widely used as chiral modifiers of platinum group metals in catalytic enantioselective hydrogenation, showed similar adsorption modes and diffusion behavior on Pt(111), except that the flatly adsorbed CN molecules which were free (not in a dimer/cluster) were significantly more mobile than their CD analogues. CD adsorbed on Pd(111) showed similar adsorption modes as observed on Pt(111) but at considerably higher mobility of the flatly absorbed species already in the low-pressure region. The observed adsorption behaviors are discussed in the context of independent ATR-IR measurements and theoretical calculations. Special emphasis is put on the nonlinear effect observed in hydrogenation reactions with CD/CN mixtures. Our observations corroborate that this effect is mainly a consequence of the different adsorption strengths of CD and CN on Pt.     

In situ extended X-ray absorption fine structure study during selective alcohol oxidation over Pd/Al2O3 in supercritical carbon dioxide

High-pressure in situ X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data are reported during the selective oxidation of benzyl alcohol to benzaldehyde in supercritical carbon dioxide over a Pd/Al(2)O(3) catalyst (shell impregnated). For this purpose, a continuous-flow system with a spectroscopic cell suitable for in situ X-ray absorption studies on heterogeneous catalysts up to 200 degrees C and 200 bar has been developed. Due to the high contribution of the dense fluid to the overall X-ray absorption, high stability of the process pressure is mandatory, particularly when recording EXAFS spectra. According to EXAFS and XANES results, the palladium particles were fully reduced after exposure to benzyl alcohol in scCO(2). In contrast to Pd-catalyzed liquid-phase oxidation, a higher oxygen tolerance of the catalyst was observed. Palladium was partially oxidized on the surface under typical reaction conditions (0.9 mol % benzyl alcohol/0.5 mol % O(2) in carbon dioxide), which gradually increased when the concentration of oxygen in the feed was raised. Both XANES and EXAFS data uncovered that palladium is mainly oxidized on the surface or within the outermost layers. These results are in accordance with simulations of the XANES data using the FEFF8.20 code (program for ab initio calculations on multiple scattering XAS) and EXAFS data fitting/simulation.     

Safety and Quality Aspects of Acrylic Monomers

Summary : Acrylic monomers are important intermediates for the chemical industry. Especially acrylic acid (AA) is the basis for various reactions, such as polymerizations and esterifications and is, therefore, responsible for high product diversity. Spontaneous polymerization is a safety problem during the transportation and storage of acrylic monomers. In the production process, polymerization leads to blockages in the apparatus. For the prevention of these issues, special stabilizer systems are used such as hydroquinone monomethyl ether (MeHQ)/oxygen and phenothiazine (PTZ). The reactions of these stabilizer systems are not well understood at the moment. Therefore a lot of expertise and experience are necessary to guarantee safe handling. In this paper some methods for the investigation of stability related reaction kinetics are presented. A better comprehension of the mechanism of the polymerization inhibition is generated by the kinetic simulation with these data.     

DELAYED EMISSION OF CHLOROPHYLL a AGGREGATES AND RHODAMINE 6G EMBEDDED IN POLYMER MATRIX*

Delayed luminescence (in the microsecond time range) of the chlorophyll (Chl) a“dry” form as well as hydrated dimers located in a polyvinylalcohol film was measured from room temperature down to 8 K. In the same matrix the delayed luminescence of rhodamine 6G (Rhod) was investigated. The delayed emission both of Chl a and Rhod is probably due to the formation and delayed recombination of a radical pair. It seems that this process occurs without participation of triplet states, as it does not reflect their well-known sensitivity to oxygen. The temperature dependence of the delayed luminescence of vanous Chl forms is different. In the region around 678 nm (dry monomer) delayed luminescence needs a thermal activation energy of about 0.03 eV, whereas at 740 nm (wet aggregates) delayed luminescence intensity increases linearly with decreasing temperature. Its assignment as a-type delayed luminescence from the low-lying triplet state can consistently be excluded from both the weak temperature dependence of the delayed fluorescence and its large intensity as compared to the prompt fluorescence. Delayed luminescence of Rhod is almost independent of temperature between 8 K and 300 K. The dependence of delayed luminescence intensity on exciting light intensity is linear at lower intensities and tends to saturation at higher. Therefore the delayed luminescence is not related to exciton annihilation. Positions and intensities of the Chl delayed luminescence bands show that it is not phosphorescence (β-type delayed luminescence). The aggregation of both Chl and Rhod molecules strongly influences delayed luminescence since it differs in several properties if excited in the monomer or in the aggregate absorption range. Every aggregational form of dye emits its characteristic delayed luminescence band.     

Metal/zirconia catalysts for the synthesis of methanol: characterization by vibrational spectroscopy

The activation of carbon dioxide by catalytic hydrogenation has been studied as a route for methanol synthesis. Metal/zirconia catalysts suitable for this reaction have been prepared by (i) activation of amorphous metal alloys [1] or (ii) coprecipitation of amorphous zirconia and metal oxides [2]. Vibrational spectroscopy has been used to obtain information on the catalytic reaction mechanism, by the in situ identification of adsorbed species and intermediates under reaction conditions.The reverse water-gas shift reaction, producing CO from CO₂ and hydrogen, plays a crucial role in the reaction mechanism. This reduction is shown to proceed via surface formate, adsorbed close to the metal/zirconia interface. Over Pd/ZrO₂ and Ni/ZrO₂, formate is reduced to methane without further observable intermediates. Pivotal intermediates on the route to methanol, as observed on Cu/ZrO₂ catalysts, are -bound formaldehyde and surface methylate. Addition of silver as a promoter can result in enhanced selectivities and productivities for methanol formation. The synergy between the two metals becomes evident from the spectroscopic measurements; the most prominent feature of the silver-promoted catalysts is a high concentration of surface formaldehyde, which is either preferentially formed or stabilized by the silver component.     

Herstellung von substituierten 2-Aminooxazol-4-carbonitrilen

Preparation of Substituted 2-Aminooxazole-4-carbonitriles During our synthetic programme to convert 4-isoxazolylthioureas 3 into the corresponding carbodiimides 5 , a side reaction leading to the hitherto unknown 2-aminooxazole-4-carbonitriles 6 was observed. By selecting appropriate reaction conditions, it was possible to improve the yields of the carbodiimides 5 as well as of the novel oxazole-carbonitriles 6 at will, thus allowing the synthesis of 6 to be conducted in very good yields. To overcome the difficulty of isolating unstable carbodiimides, the synthesis of 6 is best carried out in a one-pot procedure. A limited mechanistic study showed that the formation of 6 proceeds via 5 as the only intermediate. The stability of the N?C?N bonds against base attack (depending strongly on both sterical hindrance and electronic-density factors) forms the only limitation of this new synthetic pathway to oxazole-4-carbonitriles.