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71.
Elza Nelkenbaum 《Journal of organometallic chemistry》2005,690(9):2297-2305
The neutral octahedral nickel complex (PhC(NSiMe3)NC(Ph)CHSiMe3)Ni(acac)(TMEDA) (7), has been synthesized and characterized including X-ray diffraction analysis. The complex was formed by the reaction of Ni(acac)2(TMEDA) with the lithium salt of the corresponding β-diketiminate ligand. The formation of the benzamidinate motif from the corresponding β-diketiminate is a consequence of a retro-Brook isomerization that is operative only at the nickel centre. A plausible mechanism for the metal mediated isomerization is proposed. When complex 7 was activated with MAO it showed a good catalytic activity for the addition polymerization of norbornene. Furthermore, this catalytic system has been found to oligomerize ethylene to a mixture of butenes and hexenes with a high turnover frequency, η = 29,300 h−1, when the reaction is performed in dichloromethane. 相似文献
72.
Volkis V Nelkenbaum E Lisovskii A Hasson G Semiat R Kapon M Botoshansky M Eishen Y Eisen MS 《Journal of the American Chemical Society》2003,125(8):2179-2194
The synthesis and structural X-ray diffraction studies for some benzamidinate ligations and several group 4 benzamidinate complexes are presented. The use of the cis-octahedral C(2)-symmetry compounds was studied to shed light on the conceptual applicability of these complexes as potential catalysts for the stereoregular polymerization of propylene. We demonstrate that the stereoregular polymerization of propylene catalyzed by early-transition metal octahedral benzamidinate complexes, activated with either MAO or B(C(6)F(5))(3) as cocatalysts, can be modulated by pressure (from atactic to isotactic through elastomers). The different effects in the polymerization process such as the nature of solvent or cocatalyst, temperature, pressure, molar ratio catalyst:cocatalyst, and the relationship between the symmetry of the complex and the polymer microstructure have been investigated. When the complex [4-CH(3)-C(6)H(4)C(NTMS)(2)](2)ZrMe(2) (9) was activated with MAO, it was found to be a good catalyst for the polymerization of propylene, at atmospheric pressure, producing an oily polymer resembling an atactic polypropylene. Being activated with B(C(6)F(5))(3), complex 9 produces a highly isotactic (mmmm = 98%) product. Likewise, when the polymerization of propylene was performed with complex 9 and MAO at high pressure (liquid propylene), a highly stereoregular polymer was also obtained. Larger activities and stereoregularities were achieved by performing the reaction in CH(2)Cl(2) as compared to toluene. Contrary to complex 9, at atmospheric pressure the complex [4-CH(3)-C(6)H(4)C(NTMS)(2)](2)TiMe(2) (10) is not active either in CH(2)Cl(2) or in toluene. At high pressure, complex 10 produces elastomeric polypropylene. Activities of the isolobal complexes [C(6)H(4)C(NTMS)(2)](2)ZrMe(2) (11) and [C(6)H(4)C(NTMS)(2)](2)TiMe(2) (12) were found to be larger than those of complexes 9 and 10, respectively. Contrary to the structures of the elastomeric polypropylenes described in the literature, the obtained elastomers are characterized by frequent alternation of the isotactic domains with stereodefects. The stereoregular errors are formed by the intramolecular epimerization of the growing chain at the last inserted unit. The epimerization reaction was corroborated through the isomerization of alkenes. 相似文献
73.
In this paper we introduce new approximation operators for univariate set-valued functions with general compact images in
Rn. We adapt linear approximation methods for real-valued functions by replacing linear combinations of numbers with new metric
linear combinations of finite sequences of compact sets, thus obtaining "metric analogues" of these operators for set-valued
functions. The new metric linear combination extends the binary metric average of Artstein to several sets and admits any
real coefficients. Approximation estimates for the metric analogue operators are derived. As examples we study metric Bernstein
operators, metric Schoenberg operators, and metric polynomial interpolants. 相似文献
74.
Vladimir A. Burilov Alina A. Artemenko Ramilya I. Garipova Rezeda R. Amirova Aigul M. Fatykhova Julia A. Borisova Diana A. Mironova Elza D. Sultanova Vladimir G. Evtugyn Svetlana E. Solovieva Igor S. Antipin 《Molecules (Basel, Switzerland)》2022,27(8)
New fluorescent systems for photocatalysis, sensors, labeling, etc., are in great demand. Amphiphilic ones are of special interest since they can form functional colloidal systems that can be used in aqueous solutions. A new macrocycle platform for click chemistry and its adduct with o-propargylfluoresceine was synthesized and characterized using modern physical techniques. Nanosized solid lipid nanoparticles (SLNs) from the calixarene—fluoresceine adduct were synthesized through the solvent injection technique and well-characterized in the solution and in solid state using light-scattering and microscopy methods. The maximum fluorescence intensity of the SLNs was found to be in the pH range from 7 to 10. The Förster resonance energy transfer (FRET) efficiency from SLNs to rhodamine 6g was found to be 97.8%. Finally, pure SLNs and the FRET system SLNs—Rh6G were tested in model photocatalytic ipso oxidative hydroxylation of phenylboronic acid under blue LED light. The SLNs—Rh6G system was found to be the best, giving an almost qualitative phenol yield, which was shown by HPLC-UV analysis. 相似文献
75.
Stefania Garzoli Anna Baldisserotto Elisa Andreotti Federico Pepi Vanja Tadić 《Natural product research》2018,32(9):1056-1061
A study on essential oil fractions of the Western Balkan endemic Sideritis romana L. subsp. purpurea (Tal. ex Benth.) Heywood collected in Montenegro is reported. The 24-h systematic steam distillation extraction procedure was performed. The gas chromatographic/mass spectrometric (GC/MS) analysis of the fractions showed γ-elemene and spathulenol as two main constituents, revealing a new chemotype of this plant species. Although varying in the content of these two main compounds, which makes the fractions quite different between each other, evaluation of the anti-Candida activity showed the lack of any significant efficacy. 相似文献
76.
Moônica M. Bastos Lúcia M. U. Mayer Elza C. S. Figueira Marcio Soares Núbia Boechat Warner B. Kover 《Journal of heterocyclic chemistry》2008,45(4):969-973
77.
This preliminary study was designed to determine the extent to which the carbon isotope ratio in four species of lichens was influenced by such features as humidity, rainfall, radioactivity, and air quality. The sampling sites were selected to be at a great distance from any pollution. At the time of sampling, field data (temperature, relative humidity, average monthly precipitation, and radioactivity) were recorded. delta(13)C in whole lichen specimens were determined using standard mass spectrometric techniques with a standard deviation of+/-0.3 per thousand. We have found a weak but negative correlation between delta(13)C and relative humidity, and a positive correlation between delta(13)C and average monthly precipitation at the studied sites. The effects were minor, of the order of 1.5 per thousand for all the lichens. We have examined the correlation between (137)Cs activity concentration of the studied lichens and delta(13)C of these lichens, and we have suggested that fixation of radiocesium does not alter photosynthesis rate. There was a delta(13)C variation with the altitude gradient with less negative delta(13)C values at higher altitude. It is possible that this difference is caused by the ozone and occurred at the high elevations. 相似文献
78.
Gianna Reginato Barbara Di Credico Daniele Andreotti Anna Mingardi Alfredo Paio Daniele Donati Bernardo Pezzati Alessandro Mordini 《Tetrahedron: Asymmetry》2010,21(2):191-194
New enantiomerically enriched 1,4,5-piperazines and 1,4,5-dihydro-2H-pyrazines have been prepared by reduction of the corresponding 2-oxo-piperazines. Selective reduction can be achieved by careful control of the reaction conditions using LiAlH4. Notably the two nitrogen atoms of the final compounds are orthogonally protected. 相似文献
79.
In this work univariate set-valued functions (SVFs, multifunctions) with 1D compact sets as images are considered. For such a continuous SFV of bounded variation (CBV multifunction), we show that the boundaries of its graph are continuous, and inherit the continuity properties of the SVF. Based on these results we introduce a special class of representations of CBV multifunctions with a finite number of ‘holes’ in their graphs. Each such representation is a finite union of SVFs with compact convex images having boundaries with continuity properties as those of the represented SVF. With the help of these representations, positive linear operators are adapted to SVFs. For specific positive approximation operators error estimates are obtained in terms of the continuity properties of the approximated multifunction. 相似文献
80.
Elisa Marenna Carmela Aruta Elza Bontempi Antonio Cassinese Paolo Colombi Laura E. Depero Pasquale Pernice Antonio Aronne 《Journal of Sol-Gel Science and Technology》2009,49(1):106-111
The morphological and electrical characterization of transparent nanostructured LiNbO3–SiO2 thin films synthesized by a novel sol–gel route is reported. Films annealed at different temperatures exhibit different size
of the nanocrystals, as demonstrated by Atomic Force Microscopy and Glancing Incidence X-ray diffraction. The dc electrical
measurements performed on planar devices reveal electrical bistability. A clear relationship between the electrical bistability
and the size of LiNbO3 nanocrystals embedded in the matrix is observed. 相似文献