Inspired by nature: light driven organometallic catalysis by heterooligonuclear Ru(II) complexes

In view of diminishing resources and an ever increasing demand for energy attention has been directed towards the development of new photocatalytic systems modelling natural photosynthesis. Recent investigations have shown that supramolecular devices consisting of photosynthetic reaction centres and catalyst metals--connected via bridging ligands--are capable of performing light driven catalytic reactions such as hydrogen production, reduction of CO(2), and conversion reactions of olefins.     

Pressure-driven phase transitions in NaBH4: theory and experiments

We have investigated pressure-induced structural transitions in NaBH4 through density-functional theory calculations combined with X-ray and neutron diffraction experiments. Our calculations confirm that the cubic phase is stable up to 5.4 GPa and an orthorhombic phase occurs above 8.9 GPa, as observed in X-ray diffraction experiments. Both the calculations and X-ray diffraction measurements identify an intermediate tetragonal phase that appears between 6 and 8 GPa; that is, between the cubic and orthorhombic phases. This result is also confirmed by high-pressure neutron diffraction experiments performed on NaBD4. Our calculations and X-ray diffraction measurements show that the space group of the orthorhombic phase above 8.9 GPa is Pnma and the orthorhombic phase remains stable up to 30 GPa. The calculated equations of state are in excellent agreement with experiments.     

Epitaxial growth of single-crystalline Al(2)O(3) films on Cr(2)O(3)(0001)

Thin, crystallographically oriented single-crystalline Al2O3 films can be grown epitaxially on Cr2O3(0001) by codeposition of Al vapor and O2 at a substrate temperature of 825 K. The properties and growth of these films were monitored by Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), low-energy ion scattering (LEIS), and X-ray photoelectron spectroscopy (XPS). Two routes of preparation were investigated: (i) stepwise growth by alternating deposition of Al at room temperature and subsequent exposure to O2 at elevated temperatures; (ii) codeposition of Al and O2 at T > 800 K. The first route was consistently found to result in the growth of a complex interfacial oxide followed by the growth of polycrystalline Al2O3. The second mode of preparation provided homogeneous and ordered, probably (0001)-oriented, films of Al2O3 that maintained a LEED pattern up to a thickness around 10 A. The surface sensitive Cr MVV Auger transition at 34 eV was completely attenuated once the Al2O3 layer had reached a thickness of 6 A, pointing to film homogeneity at an early stage. This was confirmed by the absence of a significant Cr signal in LEIS spectra.     

Interactions of novel, nonhemolytic surfactants with phospholipid vesicles

PEG-12-acyloxystearates constitute a novel class of pharmaceutical solubilizers and are synthesized from polyethylene glycol and 12-hydroxystearic acid, which has been esterified with a second acyl chain. The hemolytic activity of these surfactants decreases drastically with increasing pendant acyloxy chain length, and surfactants with an acyloxy chain of 14 carbon atoms or more are essentially nonhemolytic. In this paper, the interactions of PEG-12-acyloxystearates (acyloxy chain lengths ranging from 8 to 16 carbon atoms) with phosphatidylcholine vesicles, used as a model system for erythrocyte membranes, were studied in search of an explanation for the large variations in hemolytic activity. Surfactant-induced alterations of membrane permeability were investigated by studying the leakage of vesicle-entrapped calcein. It was found that all of the surfactants within the series interact with the vesicle membranes and cause slow leakage at elevated surfactant concentrations, but with large variations in leakage kinetics. The initial leakage rate decreases rapidly with increasing pendant acyloxy chain length. After prolonged incubation, on the other hand, the leakage is not a simple function of acyloxy chain length. The effect of the surfactants on membrane integrity was also investigated by turbidity measurements and cryo-transmission electron microscopy. At a surfactant/lipid molar ratio of 0.4, the vesicle membranes are saturated with surfactant. When the surfactant/lipid molar ratio is further increased, the vesicle membranes are progressively solubilized into mixed micelles. The rate of this process decreases strongly with increasing acyloxy chain length. When comparing the results of the different experiments, it can be concluded that there is no membrane permeabilization below saturation of the vesicle membranes. The large variations in the kinetics suggest that several steps are involved in the mechanism of leakage induced by PEG-12-acyloxystearates and that their relative rates vary with acyloxy chain length. The slow kinetics may in part be explained by the low critical micelle concentrations (CMCs) exhibited by the surfactants. The CMCs were found to be in the range of 0.003-0.025 microM.     

Intramolecular hydroamination/cyclisation of aminoallenes mediated by a neutral zirconocene catalyst: a computational mechanistic study

The complete catalytic cycle for the intramolecular hydroamination/cyclisation (IHC) of 4,5-hexadien-1-ylamine (1) by a prototypical [ZrCp(2)Me(2)] precatalyst (2) has been scrutinized by employing a reliable DFT method. The present study conducted by means of a detailed computational characterisation of structural and energetic aspects of alternative pathways for all of the relevant elementary steps complements the mechanistic insights revealed from experimental results. The operative mechanism entails an initial transformation of precatalyst 2 into the thermodynamically prevalent, but dormant, bis(amido)-Zr compound in the presence of aminoallene 1. This complex undergoes a reversible, rate-determining alpha-elimination of 1 to form the imidoallene-Zr complex. The substrate-free form, which contains a chelating imidoallene functionality, is the catalytically active species and is rapidly transformed into azazirconacyclobutane intermediates through a [2+2] cycloaddition reaction. This highly facile process does not proceed regioselectively because the alternative pathways for the formation of five- and six-membered azacycles have comparable probabilities. Degradation of cyclobutane intermediates by following the most feasible pathway occurs through protonolysis of the metallacycle moiety and subsequent proton transfer from the Zr-NHR moiety onto the azacycle. The five-membered allylamine is generated through protonation at carbon atom C(6) followed by alpha-hydrogen elimination, whereas protonolysis of the cyclobutane moiety at the Zr-N bond followed by proton transfer onto carbon atom C(5) is the dominant route for the six-membered product. Of the two consecutive proton transfer steps, the second one determines the overall kinetics of the entire protonation sequence. This process is predicted to be substantially slower than the cycloaddition reaction. The factors that regulate the composition of the cycloamine products have been elucidated.     

Single Crystal Refinement of the Incommensurately Modulated Mn0.55Ta0.45O1.7, an Oxygen Deficient Fluorite Type Compound

A structural investigation of the incommensurately ordered compound Mn_0.55Ta_0.45O_1.7 has been carried out, using single‐crystal X‐ray diffraction data. The basic structure is related to the fluorite type with a monoclinic distortion. Electron diffraction (ED) patterns for various crystal orientations were used for determination of unit cell, incommensurate modulation vector and superspace group. The unit cell parameters and the length and direction of the incommensurate vector were refined using X‐ray powder diffraction (XRPD) data. P2/m was chosen as the three‐dimensional space group for a subcell with parameters a = 3.5005(2) Å (√2/2 · a_f), b = 3.5730(2) (√2/2 · a_f), c = 5.0015(2) Å (a_f) and β = 91.677(7)°. From the systematic absences hklm: k + m ≠ 2n, the four‐dimensional superspace group was determined to be B : P2/m(α 0 c) with determined modulation vector components α = –0.1833(1) and γ = 0.3582(2). A total of 511 unique reflections (79 basic fluorite type, 268 first‐order and 164 second‐order satellites) were used in the structure refinement, which resulted in weighted R‐values of 5.6% for the fluorite type sublattice reflections, 5.5% for the first‐order satellites and 7.4% for the second‐order satellites. The Mn and Ta atoms were found to be both positionally and occupationally modulated and the oxygen atoms to be highly disordered. The structure is the first fluorite‐related modulated structure of an oxide that has been determined from single‐crystal data. The structural relation to the previously characterised disordered cubic phase Mn_0.6Ta_0.4O_1.65 and the diffuse scattering exhibited by it are discussed in the article.     

Conductivity in polyacetylene. VI. Semiconductor—Metal transition of alkali-doped polymer

A local model is set up for the conductivity in alkali-doped polyacetylene (PA) based on results of ab initio and semiempirical calculations. At low doping levels, solitons and polarons appear naturally in the (nondegenerate) ground state. Alkali atoms donate their valence electrons to neutral solitons, which have the highest electron affinity in the PA structure. Due to the high polarizability of the PA chains, there is a charge buildup on a few carbon atoms close to the alkali ion. At the same time, a new soliton, screened from the alkali ion, is formed some distance away from the latter. This solition may migrate through the PA polymer partly by hopping for one chain segment to another (E_a ≥ 0.15 eV) and partly by soliton motion. In the calculation of the spectra, we used geometry-optimized structures and configuration interaction (i.e., taking into account electron-lattice interaction and explicit Coulomb correlation) and obtained good agreement with experimental spectra. As the concentration of alkali is increased, absorption occurs at energies below 1 eV. At higher doping levels, corresponding to a few mol%, the electrons delocalize over many alkali spacings and the trapping capability of the polymer decreases the conductivity becomes bandlike. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 655–665, 1997     

Perhydroxylated 1,7-Dioxaspiro[5.5]undecanes (“Spiro Sugars”): Synthesis,Stereochemistry, and Structure

As spiro sugars is an apt way of considering perhydroxylated 1,7-dioxaspiro[5.5]undecanes–a class of compounds which has not been found in nature up to now. The crystal structure of such a spiroacetal, in which the two pyran rings show the β-D -manno configuration, is depicted. Note that the all-trans arrangement of C-6, C,-5, O_pyr, C_spiro, O_pyr^′, C-5′, and C-6′ does not allow any of the stereoelectronic effects that are typical of carbohydrates.