1,720,966 research outputs found
Functional molecular hybrids based on polyoxometalates: catalytic and biological studies
Full text linkPolyoxometalates (POMs) are molecular and nano-dimensional, multi-metal oxides, which have found applications in catalysis, materials science, and nano-medicine. Their general formula are: a) [MmOy]p-; b) [XxMmOy]q-, where M is the main transition metal constituent of the POM (M = Mo, V, W), O is the oxygen atom and X can be a non-metal atom, such as P, Si, As, Sb, or a different transition metal. These species are characterized by a remarkable variety, since their properties depend on elemental composition, structure, and associated counterion. In addition, it is possible to synthesize vacant POMs derivatives, with surface defects. These last structures feature coordinatively unsaturated, terminal oxygen atoms, whose nucleophilicity can be exploited to foster reactions with electrophilic organic moieties to give O-X-R bonds, where X = As, P, Si, Sn, and R = organic residue. In this way, several organic-inorganic hybrid complexes can be obtained. Thus, the merging of organic domains with POM nano-scaffolds can be exploited to design new functional molecules and materials. In particular, POM-appended organic/organometallic moieties are instrumental for advanced catalytic applications and can direct the supramolecular organization of the hybrid molecules towards extended functional nanostructures.
In this thesis, we report the synthesis of hybrid POM derivatives containing luminescent chromophores (dansyl, pyrene and fluorescein), grafted as silane (with general formula (nBu4N)4[(R-Si)2O(gamma-SiW10O36)]) or chiral phosphonate (with general formula (nBu4N)3Na2[(R*PO)2(alfa-A-PW9O34)]) derivatives, with unique spectroscopic features. The use of tetrabutylammonium (nBu4N+) as counterion promotes the solubilization of the POM in CH3CN. The resulting hybrids have been characterized at the solid state and in solution by a combination of techniques (multinuclear NMR, FT-IR, ESI-MS). The characterization suggested a bis-substitution: the inorganic POM framework provides a molecular nanosurface where two molecules of the same fluorophore are anchored in a tweezer-type arrangement. Optical and chiroptical properties of the hybrid derivatives have been investigated. In particular, the fluorescence spectroscopy of the fluorophore-tagged POMs has been exploited for sensing applications towards metal ions (Cu and Pb ions) and organic molecules. A promising potential for applications in many different fields such as sensing, catalysis, nanoelectronics, and photochemical conversion of solar energy is foreseen for these luminescent systems.
Moreover, due to the potential applications of POMs in medicine (many POMs exhibit antiviral, antitumoral and antibiotic activity), their association with organic domains may be also of interest to improve targeting and delivery strategies. Luminescent hybrid POMs, in particular, may couple the bio-imaging diagnostic potential with innovative therapy protocols. Thus, the assembly behavior and the stability of the luminescent hybrid POMs in physiological conditions have been investigated by means of dynamic light scattering (DLS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The analysis showed the formation of spherical aggregates with a broad size distribution for these derivatives. Moreover, since the fluorescence of these compounds is maintained under physiological conditions, they were tracked in the cells, showing their localization in different subcellular regions. An unprecedented uptake into nucleosomes and mitochondria has been highlighted. The cytotoxicity of hybrid POMs has also been determined, showing a reduced toxicity at low doses. These results are expected to pave the way to the use of suitable POMs both as drugs and as nanodimensional scaffolds for organic drugs. Furthermore, since the preparation of hybrid derivatives could be a powerful strategy for the introduction of molecular recognition sites and the enhancement of biocompatibility, studies on the interaction between POM and hybrid POMs with important biological macromolecules (ferritin and avidin) were explored. The binding of two different kind of inorganic POMs containig Ru(IV) (RuPOM) and Eu(III) (EuPOM) respectively, on the ferritin (Ftn) has been indeed higlighted by a combined investigation, involving DLS, zeta-potential measurements, ITC (isothermal titration calorimetry), fluorimetry, CD (circular dichroism) and TEM. In particular, the complete quenching of the Trp residues luminescence, upon addition of >24 eqs of POM, likely due to energy transfer between the two domains, have shown that the POMs can be successfully attached to the Ftn by means of ionic interactions. In addition, the integrity of the protein has been estabilished by DLS and CD analysis. Moreover, a biological hybrid POM containing biotin moieties was synthesized and its interaction capability with avidin was investigated. Such study allowed to investigate the confinement of hybrid POMs in correspondence of specific protein binding sites and to design bioconjugated systems with molecular recognition properties, to be exploited in targeting therapies.
A last work, developed during a short scientific mission in Dublin, in collaboration with the research group of Prof. Martin Albrecht, is also presented. We have studied a strategy for the synthesis of a POM-appended N-heterocyclic carbene (NHC) iridium complex. To this end, imidazolium moieties have been successfully grafted on the defect site of a divacant Keggin polyanion. The hybrid POM synthesized was tested in catalyzing hydrogen transfer reactions. The reduction of benzophenone to diphenyl methanol in iPrOH as solvent and hydrogen donor was used as a model reaction for probing the catalytic activity of the iridium(I) complex. In particular, full conversion after 120 min has been obtained using tBuOK as base
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
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
Transfer Hydrogenation Catalysis by a N-Heterocyclic Carbene (NHC) Iridium Complex on a Polyoxometalate Platform
Full text linkA divacant Keggin polyanion has been decorated with a N-heterocyclic carbene (NHC) iridium(I) organometallic complex to provide a molecular model of an Ir-based supported catalyst. The characterization of the hybrid compound has been performed by multinuclear NMR spectroscopy, infrared spectroscopy, cyclic voltammetry, and mass spectroscopy, and the results are in agreement with a bisfunctionalization of the polyoxometalate scaffold. The resulting supported homogeneous complex has been successfully used to catalyze the transfer hydrogenation from iPrOH to benzophenone [with a turnover number (TON) of 680 and a turnover frequency (TOF) of up to 540 h–1].European Research CouncilEuropean Science Foundatio
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
- …
