1,721,013 research outputs found
Microwave-assisted fast cyclohexane oxygenation catalyzed by iron-substituted polyoxotungstates
No full textUnder microwave irradiation, iron-substituted
polyoxotungstates (Fe-POMs) catalyze cyclohexane
oxygenation to A/K oil with 90–95% selectivity,
unmatched turnover frequencies (40–400 h1),
and multi-turnover regime (>1000 TON). Such a
rapid reaction protocol allowed the screening of
so-called inorganic Fe-synzymes with 1 – 4 nuclearity,
including a family of Krebs-type isostructural complexes.
Product distribution, kinetic analysis, mechanistic
probes and fitting calculations, are consistent
with a radical chain oxidation propagated by Fe-catalysed
decomposition of organic peroxides which ultimately
depends on the redox-properties and structural
arrangement of the iron moiety within the
POM cage
Bio-inspired oxidations with polyoxometalate catalysts
No full textTransition metal substituted polyoxometalates (TMS-POM) provide a redox-active metal center, (Fe, Ru, Mn or else), with a totally inorganic ligand system, featuring rigid polydentate binding sites, high electron-acceptor character, extreme robustness and interesting structural and coordination properties, in some cases, mimicking the coordination geometry of natural oxygenase enzymes. Their synthesis and reactivity can be promoted by microwave (MW) induced dielectric heating. In particular, MW irradiation induces a very efficient and selective hydrothermal synthesis of the diamagnetic, air-stable [RuII(DMSO)PW11O39]5−. Its catalytic activity has been screened with shunt oxidants like NaIO4 and KHSO5 and PyCl2NO. Under dioxygen and MW irradiation, [RuII(DMSO)PW11O39]5− catalyzes the oxidation of DMSO to DMSO2 in water. FeIII-POMs with nuclearities 1–4, namely [α-Fe(H2O)SiW11O39]5− (FeSiW11), [γ-Fe2(H2O)2SiW10O38]6− (Fe2SiW10), [α-Fe3(H2O)3SiW9O37]7− (Fe3SiW9) and [β-Fe4(H2O)10(XW9O33)2]n− (Fe4X2W18, X = SeIV, TeIV; AsIII, SbIII; n = 4, 6) were also found to catalyze the MW assisted cyclohexane oxygenation with high turnover frequencies. Preliminary results indicate that the Krebs-type Fe polyoxotungstates can also promote the oxidative cleavage of 3,5-ditert-butylcatechol with molecular oxygen
Aerobic oxidation of cis-cyclooctene by iron-substituted polyoxotungstates: Evidence for a metal initiated auto-oxidation mechanism
No full textCatalytic aerobic epoxidation is a key process for new sustainable systems in the field of hydrocarbon functionalization. The screening of selected Fe-POM catalysts for the aerobic epoxidation has been performed using cis-cyclooctene as model substrate. Our results support a ubiquitous radical chain epoxidation with selectivity up to 66%, TOF in the range 4–50 h−1 and total turnover number >7000. FT-IR spectroscopy has been used to assess the stability of the various POM-based catalysts with respect to analogue organic porphyrine ligands
Chiral Strandberg-type molybdates [(RPO3)(2)Mo5O15](2-) as molecular gelators: Self-assembled fibrillar nanostructures with enhanced optical activity
No full textTwisted fibers with a helical structure are formed by self-assembly of chiral polyoxomolybdates (POMs) [(R*PO3)2Mo5O15]2− (R=CH3CH(NH3), CH3CH(CH3)CH(NH3)) through H-bonding. High-resolution TEM images of single fibers show an ordered substructure with parallel rows about 2 nm in width, in agreement with the packing of the H-bonded POM subunits in the X-ray structur
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Peroxo-Zr/Hf-Containing Undecatungstosilicates and -Germanates
No full textA family of dimeric, peroxo-containing heteropolytungstates, [M2(O2)2(XW11O39)2]12− [M = Zr4+, X = Si (1), Ge (2); M = Hf4+, X = Si (3)], have been synthesized by reacting ZrCl4/HfCl4 with the respective monolacunary Keggin precursor [XW11O39]8− (X = Si, Ge) in an aqueous acidic medium (pH 4.8). The isostructural polyanions 1−3 are composed of two (XW11O39) Keggin units encapsulating a central diperoxo−dimetal fragment {M2(O2)2}4+ (M = Zr4+, Hf4+). Cyclic voltammetry and exhaustive electrolysis studies indicate fast reductive release of the peroxo ligands upon reduction of 1−3. Stoichiometric oxo-transfer studies from 1−3 to the substrate l-methionine were performed, and the reactions were monitored by 1H NMR
Iron-Substituted Polyoxotungstates as Inorganic Synzymes: Evidence for a Biomimetic Pathway in the Catalytic Oxygenation of Catechols
No full textConvergent structural, spectroscopic, and mechanistic evidence is presented on the bio-inspired catalysis by FeIII-substituted polyoxotungstates [Fe4(H2O)10(β-XW9O33)2]n− (X=AsIII, SbIII, SeIV, TeIV) in promoting the aerobic cleavage of catechols (see figure). These polyanions display two of the four iron sites with three external and substitution-labile coordination positions typical of non-heme dioxygenase enzymes
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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