Olefin Isomerization by Iridium Pincer Catalysts. Experimental Evidence for an η(3)-Allyl Pathway and an Unconventional Mechanism Predicted by DFT Calculations

The isomerization of olefins by complexes of the pincer-ligated iridium species ((tBu)PCP)Ir ((tBu)PCP = κ(3)-C(6)H(3)-2,6-(CH(2)P(t)Bu(2))(2)) and ((tBu)POCOP)Ir ((tBu)POCOP = κ(3)-C(6)H(3)-2,6-(OP(t)Bu(2))(2)) has been investigated by computational and experimental methods. The corresponding dihydrides, (pincer)IrH(2), are known to hydrogenate olefins via initial Ir-H addition across the double bond. Such an addition is also the initial step in the mechanism most widely proposed for olefin isomerization (the "hydride addition pathway"); however, the results of kinetics experiments and DFT calculations (using both M06 and PBE functionals) indicate that this is not the operative pathway for isomerization in this case. Instead, (pincer)Ir(η(2)-olefin) species undergo isomerization via the formation of (pincer)Ir(η(3)-allyl)(H) intermediates; one example of such a species, ((tBu)POCOP)Ir(η(3)-propenyl)(H), was independently generated, spectroscopically characterized, and observed to convert to ((tBu)POCOP)Ir(η(2)-propene). Surprisingly, the DFT calculations indicate that the conversion of the η(2)-olefin complex to the η(3)-allyl hydride takes place via initial dissociation of the Ir-olefin π-bond to give a σ-complex of the allylic C-H bond; this intermediate then undergoes C-H bond oxidative cleavage to give an iridium η(1)-allyl hydride which "closes" to give the η(3)-allyl hydride. Subsequently, the η(3)-allyl group "opens" in the opposite sense to give a new η(1)-allyl (thus completing what is formally a 1,3 shift of Ir), which undergoes C-H elimination and π-coordination to give a coordinated olefin that has undergone double-bond migration.     

Anchoring of a liquid crystal on a photoaligning layer with varying surface morphology

We report the studies of the photoalignment properties of fluorinated polyvinylcinnamate (PVCN-F) films. The influence of film preparation and UV light irradiation conditions on anchoring properties were investigated. We have studied surface director reorientation in a magnetic field for various exposures and surface conditions. The results obtained show that both the preparation conditions and exposure time determine surface morphology, which in turn affect the value of anchoring energy. For small concentration of the PVCN-F in a spincoated solution the surface of the aligning film is rather smooth, and the value of anchoring energy W_φ = 10^-5 J m^-2 is presumably determined by a physico-chemical interaction. Higher concentration of the PVCN-F results in rough irregularities on the surface, and the value of the anchoring value increases due to a predominant contribution of the Berreman-type anchoring up to 7 X 10^-5 J m^-2.     

Oxidation of alkanes and alcohols with hydrogen peroxide catalyzed by complex Os3(CO)10(µ‐H)2

Trinuclear carbonyl hydride cluster, Os₃(CO)₁₀(µ‐H)₂, catalyzes oxidation of cyclooctane to cyclooctyl hydroperoxide by hydrogen peroxide in acetonitrile solution. The hydroperoxide partly decomposes in the course of the reaction to afford cyclooctanone and cyclooctanol. Selectivity parameters obtained in oxidations of various linear and branched alkanes as well as kinetic features of the reaction indicated that the alkane oxidation occurs with the participation of hydroxyl radicals. A similar mechanism operates in transformation of benzene into phenol and styrene into benzaldehyde. The system also oxidizes 1‐phenylethanol to acetophenone. The kinetic study led to a conclusion that oxidation of alcohols does not involve hydroxyl radicals as main oxidizing species and apparently proceeds with the participation of osmyl species, ‘Os?O’. Copyright © 2010 John Wiley & Sons, Ltd.     

Determination of the constants of affinity of FeCl3, CuCl2, and ZnCl2 for a nitrogen-containing organosilane bonded on Al2O3-cellulose acetate hybrid material surface from ethanol solution

This work describes the preparation and characterization of a cellulose acetate fiber coated with Al(2)O(3), resulting in the organic-inorganic hybrid Cel/Al(2)O(3). Furthermore, the hybrid was modified by attaching organofunctional groups by reaction with the precursor reagents (RO)(3)Si(CH(2))(3)L (L=NH(2), NH(CH(2))(2)NH(2), NH(CH(2))(2)NH(CH(2))(2)NH(2), and N(2)C(3)H(3) (imidazole)), resulting in Cel/Al(2)O(3)/Si(CH(2))(3)NH(2) (1), Cel/Al(2)O(3)/Si(CH(2))(3)NH(CH(2))(2)NH(2) (2), Cel/Al(2)O(3)/Si(CH(2))(3)NH(CH(2))(2)NH(CH(2))(2)NH(2) (3), and Cel/Al(2)O(3)/Si(CH(2))(3)N(2)C(3)H(3) (4). The amounts of attached organofunctional groups were (in mmol per gram of the material) 1=1.90, 2=1.89, 3=1.66, and 4=1.35. The isotherms of adsorption of FeCl(3), CuCl(2), and ZnCl(2) by Cel/Al(2)O(3)/Si(CH(2))(3)L from ethanol solutions were obtained at 298 K. Accurate estimates of the specific sorption capacities and the heteregeneous stability constants of the immobilized metal complexes were determined with the aid of several computational procedures. It is shown that the sorptional capacities are much less than the concentrations of the attached organofunctional groups. As all sorption isotherms are fitted properly with the Langmuir isotherm equation, the effects of the energetic heterogeneity and the lateral interactions do not affect the chemisorption equilibria. The heterogeneous stability constants of the immobilized complexes are fairly high, which provides efficient removal of the metal ions from solutions by the hybrid materials.     

DNA triangles and self-assembled hexagonal tilings

We have designed and constructed DNA complexes in the form of triangles. We have created hexagonal planar tilings from these triangles via self-assembly. Unlike previously reported structures self-assembled from DNA, our structures appear to involve bending of double helices. Bending helices may be a useful design option in the creation of self-assembled DNA structures. It has been suggested that DNA self-assembly may lead to novel materials and efficient computational devices.     

Effect of charge density fluctuations within a droplet on dielectric polarization of ionic microemulsions

A statistical model of the dielectric polarization of ionic water-in-oil microemulsions is proposed. The model makes it possible to describe the effect of temperature and dispersed phase content on the static dielectric permittivity behavior of the microemulsions at a region far below percolation. With the help of this model, the microemulsions formed with the surfactant, sodium bis(2-ethylhexyl) sulfosuccinate (AOT), have been analyzed. The studied systems are considered to consist of nanometer-sized spherical non-interacting water droplets of equal size with negatively charged head groups , staying at the interface and positive counterions Na⁺, distributed in the electrical diffuse double layer of the droplet interior. It can be conjectured that two different mechanisms, that provide an increase of the static dielectric permittivity as a function of temperature, may take place. These may be attributed either to the aggregation of droplets or the temperature growth of polarizability of non-interacting and therefore non-aggregating droplets dispersed in oil. The results support the hypothesis that the experimental temperature behavior of dielectric polarization far below the percolation region is only due to the polarization of a single droplet and not to an aggregation. The droplet polarizability is proportional to the fluctuation mean-square dipole moment of a droplet. It is shown that this mean-square dipole moment and the corresponding value of the dielectric increment, depend upon the equilibrium distribution of counterions within a diffuse double layer. The density distribution of ions is determined by the degree of the dissociation of the ionic surfactant. The dissociation of the ionic surfactant in the system has been analyzed numerically. The relationship between the constant of dissociation and the experimental dielectric permittivity has been ascertained.     

Assessment of Gaussian-3X theory for chlorinated organic molecules. Enthalpies of formation of chlorobenzenes and predictions for polychlorinated aromatic compounds

The enthalpies of formation of chlorinated methanes, ethanes, ethylenes, phenols, and benzenes have been calculated at the G3X level of theory using the atomization energy procedure and the method of isodesmic reactions. By comparing the most reliable experimental data on chlorinated hydrocarbons recommended by Manion [Manion JA (2002) J Phys Chem Ref Data 31:123] with the G3X results, the accuracy of theoretical enthalpies of formation is estimated as ranging from ±4 to ±10 kJ/mol. Only for hexachloroethane, the difference between the experimental value and G3X result was outside this range and the experimental enthalpy of formation of hexachloroethane was called into question by theory. The G3X enthalpies of formation of all chlorobenzenes agree well with experimental data which were partly reanalyzed using recent experimental data on enthalpies of sublimation. Based on the G3X results, a set of self-consistent experimental data for chlorobenzenes is recommended. The enthalpies of formation of some polychlorinated dibenzo-p-dioxins were estimated using improved enthalpies of formation for chlorobenzenes. The possible inaccuracy of previously estimated values for polychlorinated aromatic compounds is discussed.     

Photoemission and near-edge X-ray absorption fine structure studies of the bacterial surface protein layer of Bacillus sphaericus NCTC 9602

The electronic structure of the regular, two-dimensional bacterial surface protein layer of Bacillus sphaericus NCTC 9602 has been examined by photoemission (PE) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Both the O 1s and the N 1s core-level PE spectra show a single structure, whereas the C 1s core-level spectrum appears manifold, suggesting similar chemical states for each oxygen atom and also for each nitrogen atom, while carbon atoms exhibit a range of chemical environments in the different functional groups of the amino acids. This result is supported by the element-specific NEXAFS spectra of the unoccupied valence electronic states, which exhibit a series of characteristic NEXAFS peaks that can be assigned to particular molecular orbitals of the amino acids by applying a phenomenological building-block model. The relative contributions of the C-O, C-N, and C-C bond originating signals into the C 1s PE spectrum are in good agreement with the number ratios of the corresponding bonds calculated from the known primary structure of the bacterial surface protein. First interpretation of the PE spectrum of the occupied valence states is achieved on the basis of electronic density-of-states calculations performed for small peptides. It was found that mainly the pi clouds of the aromatic rings contribute to both the lowest unoccupied and the highest occupied molecular orbitals.