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  • Open access
  • 57 Reads
Synthesis and characterization of a nanocomposite containing a copper-based MOF and nickel ferrite

Recently, Metal-organic frameworks (MOFs) have been developed rapidly as a new class of functional inorganic-organic hybrid materials. They are crystalline microporous materials which include metals at the center and organic ligands as linkers. These ligands create an open porous three-dimensional structures. MOFs have particular properties like, high pore volume and surface area, unsaturated metal sites textural properties and so on. With these properties we can use them in many applications including separation, photocatalysis, gas storage, adsorption, electrochemistry, fluorescence, sensing, etc. [1]

HKUST-1, is a copper-based MOF characterized by a 3D system of square-shaped pore, that draws a great attention owning to it can be synthesized with commercially available reagents and has high surface area, accessible coordinatively unsaturated sites (CUS) and high chemical stability.

On the other hand, the post-synthesis modifications (PSMs) of MOF have been demonstrated to be a crucial strategy for advanced functions. In particular, the incorporation of magnetic nanoparticles (especially nanocrystalline spinel ferrites with the common formula MFe2O4 (M = Ni, Zn, Mn, Co, Mg, etc.) into MOFs has been investigated because the product of nanocomposites can be transferred via an external magnetic field. This function is very significant in the adsorption of heavy metal ions or dyes or it can be utilized for drug delivery or magnetic resonance imaging. [2] Nickel ferrite powder, is one of the very important ferrite materials that has been considered for many applications such as high density magnetic storage media, MRI contrast agents, and so on. [3] In this research, a nanocomposite of HKUST-1/NiFe2O4 with core-shell structure was prepared and characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and Field Emission Scanning Electron Microscopy (FESEM) methods.

  • Open access
  • 56 Reads
Tuning of conditions for light-driven characterization of poly(phenylacetylene)s

Poly(phenylacetylene)s are a family of dynamic helical polymers where the elongation and the helical sense of the double helix can be easily tuned in a controlled way using the proper external stimuli. Since there is a direct relationship between the secondary structure of the polymer and its potential applications, the structural elucidation of these polymers is nowadays a challenge in polymer chemistry.

These polymers are characterised using the combination of different techniques such as CD, UV-Vis, Raman, DSC… However, the interpretation of the data is time consuming and in occasions difficult, madding necessary appeal to AFM. Due these obstacles, we focus on the light-driven cyclization of PPAs as a fast and easy-going alternative approach to decipher the configuration of the polyene backbone.

Several parameters should be considered to achieve acceptable values of reproducibility and repeatability on this technique. On this work, we evaluate the influence of the relation between volume/concentration of sample and photoabsorption on PPAs when monitoring results by circular dichroism, resulting in and easy and controlled way to tune the light-driven characterization of PPAs.

  • Open access
  • 112 Reads
Inlfuence of the secondary structure of PPAs in the formation of Silver Nanoparticles

In the last years, the preparation of novel nanocomposites based on polymers and metal nanoparticles (AuNPs and AgNPs) has attracted the attention of the scientific community. This is due to their potential applications in fields such as sensing, medical therapy or catalysis, among others.

In literature, several examples for the preparation of nanocomposites can be found where the organic coating is composed by biomacromolecules with helical structure such as DNA or proteins but nanocomposites with synthetic helical polymers as coating remain still unexplored. For this reason, poly(phenylacetylene)s (PPAs), which are a type of synthetic helical polymers, are an excellent candidates to act as organic coatings for metal nanoparticles. PPAs present the ability of modulate its secondary structure —helical sense and elongation— in the presence of different external stimuli (e.g.: temperature, metal ions, light, etc.).

In this work, the preparation of novel nanocomposites based on PPA-AgNPs is reported. For that, meta- and orto- substituted PPAs derived from (R)-a-methoxy-a-phenylacetic acid as poendat group (poly-1 and poly-2 respectively) were selected as organic coating for the silver nanoparticles. The differences in the elongation and dynamic behaviour between poly-1 and poly-2 allowed to study the influence of the secondary structure in the formation of poly-1-AgNPs and poly-2-AgNPs nanocomposites.

  • Open access
  • 63 Reads
Influence of the Substitution Pattern Effect in Achiral and Rigid Spacers over the Folding of Helical Poly(phenylacetylene)s

The design, synthesis and structure elucidation of helical polymers with a predominant helical sense have been actively studied during the last years due to the different applications that these materials can present. The helical sense of a polymer can be determined by the chirality of the pendants and is directly related to the distance from the chiral centre to the backbone. In order to achieve the best folding this distance between the polymeric backbone and the chiral centre should be small, so the closer is the pendant to the backbone the better is the helical control. Nevertheless, few examples have demonstrated that the helical control is also possible when a spacer is placed between the pendant and the backbone. Related to this, our research group has proved that is possible to have chiral teleinduction when the chiral centre is separated form the backbone through a flexible spacer. Recently, we have also demonstrated that the transmission of information is also possible when a long rigid and achiral spacer is employed observing that, although the chiral centre is placed in a remote position, its chirality is transmitted to the polyene backbone through a previous organization into a helical fashion of the rigid spacer.

Herein we describe another family of polymers bearing an achiral rigid spacer between the pendant and the backbone, evaluating the effect that the aromatic substitution pattern of the rigid spacer has over the final folding of the polymer.

  • Open access
  • 54 Reads
Diplodomica II. Repugnatorial secretion of Ecuadorian endemic millipede Rhinocricus sp. (Diplopoda, Spirobolida, Rhinocricidae) from Orellana Province, Amazonia, Ecuador

Millipedes (Myriapoda: Diplopoda), with more than 80.000 species, represent one of oldest and evolutivelly successful classes of invertebrates. The order Spirobolida is dominantly tropical in its distribution and diversification and the family Rhinocricidae is wide distributed in the Ecuadorian edaphic fauna inhabiting in the Amazonia region. Their repugnatorial secretions constitute a source of biologically active metabolites with a great molecular diversity. The millipede collected were stimulated mechanically and an ejected fluid was subjected to structural analysis. The spectral (FTIR, NMR-1H/13C) and chromatographic-mass data (GC-MS) analysis of the defensive-repugnatorial secretion of the endemic millipede Rhinocricus sp., inhabiting in the Ama-zonian region of Orellana, Republic of Ecuador, and chemical evidence (chromogenic reactions: KI starch paper and rodanine test in aqueous ammonia) as well, reveled the quinonoide composition of the secretion. Was identified 2-methyl-3-methoxy-1,4-benzoquinone as an active major component. It can be ejaculated from dorsolateral ozo-pores in the event of strong mechanical irritation, disturbance and predator attack in natural conditions. The defensive secretion showed an interesting biocide effect vs. human pathogens at concentrations in the interval of 7.25 μg/mL and 34.4 μg/mL for Fonsecae pedrosoi and Candida albicans respectively. 2-methyl-3-methoxy-1,4-benzoquinone is one of the most prevalent active substance in the defensive secretions in the orders Spirobolida and Spirostreptida, independently of geographical distribution of taxa. This is the first report related to chemical composition of repugnatorial secretion of Ecuadorian Rhinocricidae from Amazonian region. The results concerning biological action and its detailed mechanism of action (repellent, microbiocidal, virucidal and insecticidal effects) will be reported soon.

  • Open access
  • 60 Reads
Evaluation of reactivity (pKa) of substituted aromatic diamines, monomers for polyamides synthesis, via Nuclear Magnetic Resonance, NMR-1H
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Determining the relationship between properties of aromatic polyamides and the structure of monomers allows modeling and designing the synthesis of polymers with special properties, from dialysis membranes, ion exchange systems, to thermo-resistant materials and impact protection devices. A singular dependency is observed when investigating aromatic diamines-type monomers, specifically their basicity, which constitutes one of the fundamental criteria for evaluating their reactivities or reaction capacity under catalytic conditions to generate special polymers. Nuclear Magnetic Resonance (250MHz NMR-1H, in CaCl2-added dimethylformamide) is a very useful and versatile analytical tool for such purposes. The direct dependence between the magnitude of the chemical shifts (d, ppm) of the protons of the amino groups (NH2-) and the pKa values ​​is demonstrated in the case of substituted aromatic diamines substituted with ionogenic functional groups (-COOH, -SO3H). Based on the obtained equation (pka = 13.1512 – 1.67039 x d (ppm)), and on the experimental data of the values ​​of d, ppm, the basicity of the primary amino groups, which characterize the reactivity of the monomers at the initial moment of the interphasic poly-condensation, can be calculated and selectively modify some properties of polyamides to optimize their rational use and effectiveness. The similar conside-ration for correlating chemical shift and net charge on nitrogen atom was taking into account and an obtained equation also correlates significantly (chemical shift (d, ppm) = -7.0593 – (37.57628 x φ). Aromatic diaminas were synthetized according established protocols and statistical-molecular studies were developed using Hyperchem pack-2005 and Mopac-6.

  • Open access
  • 67 Reads
Stable anionic sigma-complexes of highly electrophilic aromatics and C-nucleophiles: synthesis and oxidation.

Functionalization of arenes provides their chemical diversity and opens a way to valuable substances that are widely used in medicine, pharmaceutics, agriculture, energetic and other areas. In recent years, the functionalization of the CH bond has become an important tool for the implementation of such processes. Nucleophilic substitution of hydrogen (SNH) in arenes has acquired an intensive development as more prospective way of functionalization than classical SNAr processes occurring through ipso-substitution of a nucleofuge. Two general SNH processes are oxidative (ONSH) and vicarious (VNSH) nucleophilic substitution of hydrogen taking place through the generation of sн-adduct. In case of highly electrophilic substrates, the intermediate sigma-adducts can be isolated, identified and their chemical behavior can be studied.

In this work we studied the reactions of dinitrobenzoannulated heterocycles (furazan, thiadiazole, selenadiazole, pyridine) with anionic C-nucleophiles (mono- and diketones, nitroalkanes and related compounds). It was found that these reactions provided stable anionic adducts in high yield. Consecutive oxidation with ammonium cerium (IV) nitrate resulted in re-aromatization with formation of the corresponding substitution products, formally representing C-H-functionalized benzoheterocycles.

Thus, a series of the previously unknown dinitrobenzoazoles and azines with different pharmacophoric fragments in benzene ring were synthesized using sigma adducts as key intermediates of C-H functionalization of π-deficient nitroarenes.

  • Open access
  • 71 Reads
Terminal alkynes hydroamination catalyzed by copper nanoparticles

Nitrogen-containing compounds, such as amines, enamines and imines, are valuable and of great industrial and pharmaceutical relevance. The hydroamination (HA) is a reaction that involve the direct formation of a new C-N bond by addition of an amine to an unsaturated CC bond. HA is one of the most atom efficient processes and can be performed under catalysis of a wide range of metals, being especially versatile the late-transition metals.

In this work we present a study of the reaction of HA of terminal alkynes catalyzed by supported-copper nanoparticles, with very low catalyst loading (< 2 mol%) . Through the methodology of our research group, which involve the addition of the support to a suspension of freshly prepared copper nanoparticles, a wide range of terminal alkynes were regioselectively converted to imines by intermolecular HA. Imines were obtained in moderate to quantitative yields, which were then easily reduced to the corresponding amines. Finally, the heterogeneous and low-cost copper catalyst can be reused in several catalytic cycles with no yield and selectivity decrease.

  • Open access
  • 81 Reads
Fluorescent Calix[4]arene-Oxacyclophane Receptors as Sensors of Transition Metal Cations

Development of sensitive and selective chemosensors for trace detection of metals (e.g. copper(II), lead(II) and mercury(II) ions) rich in nature, either associated to important physiological processes or representing environmental hazards and health issues is an attractive area of current research.

Calixarenes are one of the most widespread scaffolds in host-guest chemistry because of their rigid structures, which make them perfect candidates for complexation studies with ions and neutral molecules through the formation of inclusion complexes.

In this communication we report the chemosensing ability of newly designed bicyclic calix[4]arene-carbazole-polymers (Calix-OCP-PPE-CBZs) and a sole molecular probe (Calix-OCP-2-CBZ) toward metal ions in fluid phase. Evidence for the formation of a ground-state complex between Calix-OCP-PPE-2,7-CBZ and Cu(II) came from UV-Vis spectroscopic titrations.

Two clear isosbestic points appear at 370 and 439 nm indicating the establishment of the binding equilibrium of Cu(II) with Calix-OCP system which was supported by the high binding energy(DE = -1616.6 kJ/mol) obtained by DFT calculations for a model compound with Cu(II), confirming the ligand-to metal charge transfer (LMCT) bands observed in UV-Vis titrations and thus the formation of supramolecular inclusion complexes.

  • Open access
  • 64 Reads
Urea-Functionalized chitosan as efficient and recoverable organocatalyst for the convenient synthesis of pyrimido[4,5-b]quinoline-2,4-dione derivatives
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Urea-Functionalized chitosan (Cs-Pr-Urea) was prepared and characterized using different spectroscopic, microscopic and analytical methods or techniques. The Cs-Pr-Urea modified polymer was used, as a new biopolymeric and efficient bifunctional catalyst, for the synthesis of pyrimido[4,5-b]quinoline-2,4-dione derivatives in aqueous ethanolic media. In this regard, some of the heterocyclic pyrimidine derivatives have been synthesized by through four-component reaction of barbituric acid condensation, aldehyde derivatives, specific amines and dimedone in EtOH/H2O under reflux conditions , efficient and efficient manner, with the reaction. This method has the prominent features such as green solvent, high to quantitative yields of the favorable products, easy product separation, no purification with chromatography column, heterogeneous catalyst and easy recyclability of catalyst. In this methodology, the ideal conditions for the reaction are shown.