1,720,997 research outputs found
Optically Active Polyoxotungstates Bearing Chiral Organophosphonate Substituents
No full textDivacant Keggin-type polyoxotungstates [γ-XW10O36]8– with X = Si or Ge, were functionalized with chiral phosphoryl groups. The hybrid compounds [(R*PO)2(γ-XW10O36)]4– with R = N-protected aminoalkyl groups or O-protected amino acid derivatives, were isolated. The solution characterization of the products was performed by different techniques: 183W, 31P, 13C, and 1H NMR spectroscopy, electrospray ionization mass spectrometry, UV/Vis spectroscopy, and circular dichroism (CD). The experimental data confirm the covalent grafting of the organic moieties onto the polyanionic surface. A chirality transfer, from the pendant organic arm to the inorganic framework is apparent from CD studies. Multiple Cotton effects were observed in the region of charge-transfer transitions pertaining to W–O bonds. Furthermore, the 183W NMR spectra are consistent with the expected C2 symmetry, resulting from introduction of two organic stereocenters. The title complexes were used in the presence of hydrogen peroxide to perform the oxidation of methyl p-tolyl sulfide. Implications for the design of enantioselective catalysts based on these derivatives are discussed
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
Electrochemical Asymmetric Radical Functionalization of Aldehydes Enabled by a Redox Shuttle
Full text linkAminocatalysis is a well-established tool that enables the production of enantioenriched compounds under mild conditions. Its versatility is underscored by its seamless integration with various synthetic approaches. While the combination of aminocatalysis with metal catalysis, photochemistry, and stoichiometric oxidants has been extensively explored, its synergy with electrochemical activation remains largely unexplored. Herein, we present the successful merger of electrochemistry and aminocatalysis to perform SOMO-type transformations, expanding the toolkit for asymmetric electrochemical synthesis. The methodology harnesses electricity to drive the oxidation of catalytically generated enamines, which ultimately partake in enantioselective radical processes, leading to α-alkylated aldehydes. Crucially, mechanistic studies highlight how this electrochemical strategy is enabled by the use of a redox shuttle, 4,4′-dimethoxybiphenyl, to prevent catalyst degradation and furnishing the coveted compounds in good yield and high enantioselectivity
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
Mechanistic Insights into Light-Activated Catalysis for Water Oxidation
No full texthe development of catalysts for water oxidation to oxygen has been the subject of intense investigation in the last decade. In parallel to the search for high catalytic performance, many works have focused on the mechanistic analysis of the process. In this perspective, the oxidation of water through light-assisted cycles composed of an electron acceptor (EA), a photosensitizer (PS), and a water oxidation catalyst (WOC) can provide insightful and complementary information with respect to the use of chemical oxidants or to electrochemical techniques. In this minireview, we discuss the mechanistic aspects of the EA/PS/WOC photoactivated cycles, and in particular: (i) the general elementary steps; (ii) the required features and the nature of the PS employed; (iii) the electron transfer processes and kinetics from the WOC to PS+ (hole scavenging); (iv) the detrimental quenching of the PS by the WOC and the alterna-tive mechanistic routes; (v) the identification of WOC intermedi-ates and, finally, (vi) the transposition of the above processes into a dye-sensitized photoanode embedding a WOC
Chelating di(N-heterocyclic carbene) complexes of iridium(III): Structural analysis, electrochemical characterisation and catalytic oxidation of water
No full textThe synthesis of novel iridium(III) complexes of general formula [IrClCp∗(diNHC)](PF6), with chelating di(N-heterocyclic carbene) ligands (diNHC) is reported. The obtained complexes differ for the wingtip substituents at the nitrogen atom of the heterocycles (Me, t-Bu, Cy, n-oct), the linker between the carbene donors (-CH2-, -CH2CH2-, -CH2CH2CH2-) and for the type of heterocyclic rings (imidazole and benzimidazole). The new complexes have been characterised in solution with NMR spectroscopy, ESI-MS spectrometry and cyclic voltammetry and, in the solid state, by single crystal X-ray structure analysis. These techniques allow to evaluate the influence of the dicarbene ligand on the electronic properties of the metal centre. The complexes have been successfully tested as water oxidation catalysts using Ce(IV) as chemical oxidant
Quinacridone dyes: versatile molecules and materials for photo- and photoelectrochemical processes
No full textThe renaissance of photochemistry and the explosion of photo- and photoelectro-catalysis open new opportunities in organic photocatalyst design and applications towards solar fuels and sustainable organic reactivity. In this perspective, we discuss the relevant case of quinacridone (QA) dyes: these have long been known to the scientific community, but their application in photocatalysis is recent and still explored in a limited way. This is somehow surprising given that QA is a cheap and readily available organic pigment, and in front of the appealing properties of QA derivatives, including intense absorption in the visible region, balanced redox properties making them suitable for both oxidative and reductive photochemistry, and versatility to several operative conditions. We will discuss recent examples of photo- and photoelectrochemical processes taking advantage of QA dyes, from solution photocatalysis to photoactive materials and devices (nanoparticles, covalent organic frameworks, photoelectrodes); the target applications include water splitting, carbon dioxide reduction, and organic transformations. We aim to show the potential of organic photocatalyst design and implementation, and to inspire the readers with new opportunities in this field.We discuss the versatility of quinacridone pigment in photocatalysis, embracing water splitting, carbon dioxide reduction and organic synthesis. Applications include homogeneous catalysis, nanoparticles, covalent organic frameworks, photoelectrodes
Light-Driven Water Oxidation with the Ir-blue Catalyst and the Ru(bpy)32+/S2O82- Cycle: Photogeneration of Active Dimers, Electron-Transfer Kinetics, and Light Synchronization for Oxygen Evolution with High Quantum Efficiency
No full textLight-driven water oxidation is achieved with the Ru(bpy)32+/S2O82− cycle employing the highly active Ir-blue water oxidation catalyst, namely, an IrIV,IV2(pyalc)2 μ-oxo-dimer [pyalc = 2- (2′-pyridyl)-2-propanoate]. Ir-blue is readily formed by stepwise oxidation of the monomeric Ir(III) precursor 1 by the photogenerated Ru(bpy)33+, with a quantum yield φ of up to 0.10. Transient absorption spectroscopy and kinetic evidence point to a stepwise mechanism, where the primary event occurs via a fast photoinduced electron transfer from 1 to Ru(bpy)33+, leading to the Ir(IV) monomer I1 (k1 ∼ 108 M−1 s−1). The competent Ir-blue catalyst is then obtained from I1 upon photooxidative loss of the Cp* ligand and dimerization. The Ir-blue catalyst is active in the Ru(bpy)32+/S2O82− light-driven water oxidation cycle, where it undergoes two fast photoinduced electron transfers to Ru(bpy)33+ [with kIr‐blue = (3.00 ± 0.02) × 108 M−1 s−1 for the primary event, outperforming iridium oxide nanoparticles by ca. 2 orders of magnitude], leading to a IrV,V2 steady-state intermediate involved in O−O bond formation. The quantum yield for oxygen evolution depends on the photon flux, showing a saturation regime and reaching an impressive value of φ(O2) = 0.32 ± 0.01 (corresponding to a quantum efficiency of 64 ± 2%) at low irradiation intensity. This result highlights the key requirement of orchestrating the rate of the photochemical events with dark catalytic turnover
- …
