Modern Analytical Tools essential for Studying Olefin Polymerization

As an alternative to inorganic supports, emulsions and organic carriers were developed for metallocene-catalyzed polyolefin synthesis in the last years. Oil-in-oil emulsions based on a perfluorinated solvent provide the possibility to synthesize polymer particles on the nanometer length scale, while the latex particles consisting of polystyrene with different surface functionalities yield particles on the micrometer range. To obtain a deeper insight to the reaction course of both techniques and to the product morphology and fragmentation behavior of the organic carriers, substantial information concerning the kinetics of these reactions is crucial. Thus, standard analytical methods were combined with real-time video microscopy and laser scanning fluorescence microscopy (LSCFM) of perylene labeled particles for improving these methods for polyolefin synthesis.     

Elusive Zintl Ions [μ‐HSi4]3− and [Si5]2− in Liquid Ammonia: Protonation States,Sites, and Bonding Situation Evaluated by NMR and Theory

The existence of [μ‐HSi₄]^3? in liquid ammonia solutions is confirmed by ¹H and ²⁹Si NMR experiments. Both NMR and quantum chemical calculations reveal that the H atom bridges two Si atoms of the [Si₄]^4? cluster, contrary to the expectation that it is located at one vertex Si of the tetrahedron. The calculations also indicate that in the formation of [μ‐HSi₄]^3?, protonation is driven by a high charge density and an increase of electron delocalization compared to [Si₄]^4?. Additionally, [Si₅]^2? was detected for the first time and characterized by NMR. Calculations show that it is resistant to protonation, owing to a strong charge delocalization, which is significantly reduced upon protonation. Thus, our methods reveal three silicides in liquid ammonia: unprotonated [Si₅]^2?, terminally protonated [HSi₉]^3?, and bridge‐protonated [μ‐HSi₄]^3?. The protonation trend can be roughly predicted by the difference in charge delocalization between the parent compound and the product, which can be finely tuned by the presence of counter ions in solution.     

Information theory for evaluating environmental classification systems

Environmental pollution data are often ranked in rule-based classification systems. These environmental data are separated in predetermined classes of a classification system for a better and smarter characterization of the state of pollution. Often the measured values are transformed, e.g. in pseudocolor maps, and can then be presented in maps. For some environmental compartments different classification systems for evaluating environmental loadings are used. Because of the dissimilarity of the various classification systems direct visual comparison is difficult. However, by means of information theory an objective comparison of these various classification systems based on their information content enables a decision to be made about which system is the most informative for objective assessment of the state of pollution. By means of the new measure multiple medium information content (multiple entropy) objective and simultaneous comparison of all channels (in an environmental classification system: pollutants) of each classification system is now possible. Furthermore the development of the state of pollution over the whole investigation period can be detected by means of information theory. On the basis of the conditions of the established rule-based systems the use of information theory enables definition of new ranges of classes in order to reach the optimum of information during conversion into the environmental classification system.     

Gold electrodes wired for coupling with the deeply buried active site of Arthrobacter globiformis amine oxidase

Diethylaniline-terminated oligo(phenyl-ethynyl)-thiol (DEA-OPE-SH) wires on Au-bead electrodes facilitate electron tunneling to and from the deeply buried topaquinone (TPQ) cofactor in Arthrobacter globiformis amine oxidase (AGAO). Reversible cyclic voltammograms were observed when AGAO was adsorbed onto this DEA-OPE-SAu surface: the 2e-/2H+ reduction potential is -140 mV versus SCE.     

Thermal Rearrangement of Nitrone and Nitrile Oxide Cycloadducts to Bicyclopropylidene.(1) Synthesis of 3-Spirocyclopropane-4-pyridone and Furo[2,3-c]pyridine Derivatives

Bicyclopropylidene smoothly undergoes 1,3-dipolar cycloadditions to nitrones or nitrile oxides under different reaction conditions. The strained bisspirocyclopropanated isoxazolidines obtained from nitrones easily rearrange upon heating with selective opening of one of the two spirofused cyclopropane rings. This process produces 4-pyridone, 7-indolizinone, and 2-quinolizinone derivatives containing a spirocyclopropane moiety in the alpha-position to the carbonyl group in good yields. The same sequence of cycloaddition and rearrangement can be achieved in a "one-pot" operation with considerable benefit for the reaction yield. Bisspirocyclopropaneisoxazolines obtained from nitrile oxides are more stable than their saturated counterparts and rearrange only at higher temperature less chemoselectively. Opening of both spiro-fused cyclopropyl rings followed by aromatization produces interesting 2-substituted dihydrofuro[2,3-c]pyridine derivatives.     

The central role of the metal ion for photoactivity: Zn– vs. Ni–Mabiq

Photoredox catalysts are integral components of artificial photosystems, and have recently emerged as powerful tools for catalysing numerous organic reactions. However, the development of inexpensive and efficient earth-abundant photoredox catalysts remains a challenge. We here present the photochemical and photophysical properties of a Ni–Mabiq catalyst ([Ni^II(Mabiq)]OTf (1); Mabiq = 2-4:6-8-bis(3,3,4,4-tetramethyldihydropyrrolo)-10-15-(2,2-biquinazolino)-[15]-1,3,5,8,10,14-hexaene1,3,7,9,11,14-N₆)—and of a Zn-containing analogue ([Zn^II(Mabiq)OTf] (2))—using steady state and time resolved optical spectroscopy, time-dependent density functional theory (TDDFT) calculations, and reactivity studies. The Ni and Zn complexes exhibit similar absorption spectra, but markedly different photochemical properties. These differences arise because the excited states of 2 are ligand-localized, whereas metal-centered states account for the photoactivity of 1. The distinct properties of the Ni and Zn complexes are manifest in their behavior in the photo-driven aza-Henry reaction and oxidative coupling of methoxybenzylamine.

The development of earth-abundant photoredox catalysts remains a challenge. Studies of Ni- and Zn-Mabiq complexes demonstrate how the coordinating metal ion influences the photochemistry, photodynamics and reactivity of photocatalysts.     

Fundamental Aspects of Ceria Supported Au Catalysts Probed by In Situ/Operando Spectroscopy and TAP Reactor Studies

The discovery of the activity of dispersed gold nanoparticles three decades ago paved the way for a new era in catalysis. The unusual behavior of these catalysts sparked many questions about their working mechanism. In particular, Au/CeO₂ proved to be an efficient catalyst in several reactions such as CO oxidation, water gas shift, and CO₂ reduction. Here, by employing findings from operando X-ray absorption spectroscopy at the near and extended Au and Ce L_III energy edges, we focus on the fundamental aspects of highly active Au/CeO₂ catalysts, mainly in the CO oxidation for understanding their complex structure-reactivity relationship. These results were combined with findings from in situ diffuse reflectance FTIR and Raman spectroscopy, highlighting the changes of adlayer and ceria defects. For a comprehensive understanding, the spectroscopic findings will be supplemented by results of the dynamics of O₂ activation obtained from Temporal Analysis of Products (TAP). Merging these results illuminates the complex relationship among the oxidation state, size of the Au nanoparticles, the redox properties of CeO₂ support, and the dynamics of O₂ activation.     

A peptide co-solvent under scrutiny: self-aggregation of 2,2,2-trifluoroethanol

Trifluoroethanol (TFE) and its aggregates are studied via supersonic jet FTIR and Raman spectroscopy as well as by quantum chemistry and simple force field approaches. A multi-slit nozzle is introduced to study collisionally excited clusters. Efforts are made to extract harmonic frequencies from experiment for better comparison to theory. Based on deuteration, the OH stretching anharmonicity changes weakly upon dimerization, but increases for trimers. Among the possible dimer conformations, only an all-gauche, homoconfigurational, compact, OH-F connected structure is observed in an extreme case of chiral discrimination. Quantum tunneling assisted pathways for this surprising helicity synchronization are postulated. The oscillator coupling in hydrogen-bonded trimers is analyzed. Trans conformations of TFE start to become important for trimers and probably persist in the liquid state. Simple force fields can be refined to capture some molecular recognition features of TFE dimer, but their limitations are emphasized.     

Total synthesis of marinomycins A-C and of their monomeric counterparts monomarinomycin A and iso-monomarinomycin A

Marinomycins A-C (1-3), and their monomeric analogues monomarinomycin A (m-1) and iso-monomarinomycin A (m-2), were synthesized by a convergent strategy from key building blocks ketophosphonate 5, aldehyde 6, and dienyl bromide carboxylic acid 7. The first attempt to construct marinomycin A [1, convertible to marinomycins B (2) and C (3) by light] by direct Suzuki-type dimerization/cyclization of boronic acid dienyl bromide 4 led to premature ring closure to afford, after global desilylation, monomarinomycin A (m-1) and iso-monomarinomycin A (m-2) in good yield and only small amounts (< or =2%) of the desired product. A subsequent stepwise approach based on Suzuki-type couplings improved considerably the overall yield of marinomycin A (1), and hence of marinomycins B (2) and C (3). Alternative direct dimerization approaches based on the Stille and Heck coupling reactions also led to monomarinomycins A (m-1 and m-2), but failed to deliver useful amounts of marinomycin A (1).