Loading...
138 results
Search Results
Now showing 1 - 10 of 138
- Vanadium complexes: recent progress in oxidation catalysisPublication . Sutradhar, Manas; Martins, Luisa; Guedes Da Silva, M. Fátima C.; Pombeiro, ArmandoOxidovanadium complexes and, to a less extent, some non-oxido ones, are widely used as catalysts or catalyst precursors for various oxidative catalytic reactions by H2O2, (BuOOH)-Bu-t or O-2 under mild conditions. Oxidation reactions (oxidation of alkanes and alcohols, epoxidation of alkenes and allylic alcohols, oxidative bromination, sulfoxidation and oxidative Strecker reactions) of organic compounds are the most relevant ones and are reviewed considering the recent advances in the last five years (2010-2014). The main types of both homogeneous and supported vanadium catalysts and the most efficient catalytic systems in the different reactions are presented and compared. The proposed mechanisms of various catalytic oxidation processes are also outlined. (C) 2015 Elsevier B.V. All rights reserved.
- Fe(III) complexes in cyclohexane oxidation: comparison of catalytic activities under different energy stimuliPublication . Roy Barman, Tannistha; Sutradhar, Manas; Alegria, Elisabete; Silva, M. Fátima C. Guedes Da; Pombeiro, ArmandoIn this study, the mononuclear Fe(III) complex [Fe(HL)(NO3)(H2O)2]NO3 (1) derived from N0 -acetylpyrazine-2-carbohydrazide (H2L) was synthesized and characterized by several physicochemical methods, e.g., elemental analysis, infrared (IR) spectroscopy, electrospray ionization mass spectrometry (ESI-MS), and single crystal X-ray diffraction analysis. The catalytic performances of 1 and the previously reported complexes [Fe(HL)Cl2] (2) and [Fe(HL)Cl(µ-OMe)]2 (3) towards the peroxidative oxidation of cyclohexane under three different energy stimuli (microwave irradiation, ultrasound, and conventional heating) were compared. 1-3 displayed homogeneous catalytic activity, leading to the formation of cyclohexanol and cyclohexanone as final products, with a high selectivity for the alcohol (up to 95%). Complex 1 exhibited the highest catalytic activity, with a total product yield of 38% (cyclohexanol + cyclohexanone) under optimized microwave-assisted conditions.
- The influence of multiwalled carbon nanotubes and graphene oxide additives on the catalytic activity of 3d metal catalysts towards 1-phenylethanol oxidationPublication . Da Costa Ribeiro, Ana Paula; Fontolana, Emmanuele; Alegria, Elisabete; Kopylovich, Maximilian; Bertani, Roberta; Pombeiro, Armando3d metal (Cu, Fe, Co, V) containing composite catalysts for the solvent-free microwave-assisted trans-formation of 1-phenylethanol to acetophenone with tert-butyl hydroperoxide (TBHP) as oxidant wereprepared by ball milling. The influence of multiwalled carbon nanotubes (CNTs) and graphene oxide (GO)additives on the catalytic activity of the catalysts was studied. CNTs or GO were mixed by ball millingwith the metal salts (CoCl2), oxides (CuO, Fe2O3, V2O5) or binary systems (Fe2O3-CoCl2, CoCl2-V2O5, CuO-Fe2O3). For CoCl2-based catalytic systems, addition of small amounts (0.1–5%) of CNTs or GO leads tosignificant improvement in catalytic activity, e.g. 1% of the CNTs additive allows to rise yields from 28to 77%, under the same catalytic conditions. The CoCl2-5%CNTs composite is the most active among thestudied ones with 85% yield and TON of 43 after 1 h
- Microwave-assisted peroxidative oxidation of toluene and 1-phenylethanol with monomeric keto and polymeric enol aroylhydrazone Cu(II) complexesPublication . Sutradhar, Manas; Alegria, Elisabete; Roy Barman, Tannistha; Scorcelletti, Francesco; Guedes Da Silva, M. Fátima C.; Fatima, C.; Pombeiro, ArmandoCu(II) complexes in two different tautomeric forms (keto and enol) derived from the aroylhydrazone Schiff base 2-hydroxy(2-hydroxybenzylidene)benzohydrazide (H2L) have been synthesized and characterized by elemental analysis, IR spectroscopy, ESI-MS and single crystal X-ray crystallography. While the compound with the enol form of the ligand exists as the 1D polymer [Cu(kappa KNOO',2 kappa O',3 kappa O"-L)](n) (1), that with the keto form of the ligand occurs as the monomer [Cu(KNOO'-HL)CI(CH3OH)] (2). Both complexes act as good catalysts for the microwave-assisted peroxidative oxidation of toluene and 1-phenylethanol with tert-butyl hydroperoxide. Complex 2 exhibits the highest activity in both reactions, leading selectively to,a maximum product yield of 39 and 92%, respectively.
- Diethydithiocarbamate complexes with metals used as food suplements show diferente effects in cancer cellsPublication . Sedlacek, Jindrich; Martins, Luisa; Danek, Petr; Pombeiro, Armando; Cvek, BorisDiethyldithiocarbamate (ditiocarb), a metabolite of the old anti-alcoholic drug disulfiram (Antabuse), forms proteasome-inhibiting metal complexes with copper or zinc that suppress cancer cells both in vitro and in vivo. The drug has been used in a clinical trial (NCT00742911) along with copper gluconate as a dietary supplement in patients with cancer spreading to the liver. In this study, we demonstrate the effect of synthetic complexes of disulfiram with four various metals (Mn, Fe, Cr and Cu) used as food supplements. These complexes may be spontaneously formed in the blood during the use of disulfiram with divalent metals and thus may suppress the growth of cancer in vivo. The cytotoxic effect of the compounds and the compounds' ability to inhibit the cellular proteasome were tested in the osteosarcoma cell line U2OS. After 48 h, copper and manganese complexes exhibited cytotoxic effect on the cell line, in sharp contrast to both iron and chromium complexes. (C) 2014 Faculty of Health and Social Studies, University of South Bohemia in Ceske Budejovice. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved.
- Ultrasound and radiation-induced catalytic oxidation of 1-phenylethanol to acetophenone with iron-containing particulate catalystsPublication . Soliman, Mohamed Mostafa Aboelhassan; Kopylovich, Maximilian N.; Alegria, Elisabete; Da Costa Ribeiro, Ana Paula; Ferraria, Ana Maria; Rego, Ana; Correia, Luís M. M.; Saraiva, Marta S.; Pombeiro, ArmandoIron-containingparticulatecatalystsof0.1–1 µmsizewerepreparedbywetandball-milling procedures from common salts and characterized by FTIR, TGA, UV-Vis, PXRD, FEG-SEM, and XPS analyses. It was found that when the wet method was used, semi-spherical magnetic nanoparticles were formed, whereas the mechanochemical method resulted in the formation of nonmagnetic microscale needles and rectangles. Catalytic activity of the prepared materials in the oxidation of 1-phenylethanol to acetophenone was assessed under conventional heating, microwave (MW) irradiation, ultrasound (US), and oscillating magnetic field of high frequency (induction heating). In general, the catalysts obtained by wet methods exhibit lower activities, whereas the materials prepared by ball milling afford better acetophenone yields (up to 83%). A significant increase in yield (up to 4 times) was observed under the induction heating if compared to conventional heating. The study demonstrated that MW, US irradiations, and induction heating may have great potential as alternative ways to activate the catalytic system for alcohol oxidation. The possibility of the synthesized material to be magnetically recoverable has been also verified.
- Trends in properties of para-substituted 3-(phenylhydrazo)pentane-2,4-dionesPublication . Kopylovich, Maximilian; Mahmudov, Kamran; Guedes Da Silva, M. Fátima C.; Martins, Luisa; Kuznetsov, Maxim L.; Silva, Telma F. S.; Fraústo Da Silva, João; Pombeiro, ArmandoTrends between the Hammett's sigma(p) and related normal sigma(n)(p), inductive sigma(I), resonance sigma(R), negative sigma(-)(p) and positive sigma(+)(p) polar conjugation and Taft's sigma(o)(p) substituent constants and the N-H center dot center dot center dot O distance, delta(N-H) NMR chemical shift, oxidation potential (E-p/2(ox), measured in this study by cyclic voltammetry (CV)) and thermodynamic parameters (pK, Delta G(0), Delta H-0 and Delta S-0) of the dissociation process of unsubstituted 3-(phenylhydrazo)pentane-2,4-dione (HL1) and its para-substituted chloro (HL2), carboxy (HL3), fluoro (HL4) and nitro (HL5) derivatives were recognized. The best fits were found for sigma(p) and/or sigma(-)(p) in the cases of d(N center dot center dot center dot O), delta(N-H) and E-p/2(ox), showing the importance of resonance and conjugation effects in such properties, whereas for the above thermodynamic properties the inductive effects (sigma(I)) are dominant. HL2 exists in the hydrazo form in DMSO solution and in the solid state and contains an intramolecular H-bond with the N center dot center dot center dot O distance of 2.588(3)angstrom. It was also established that the dissociation process of HL1-5 is non-spontaneous, endothermic and entropically unfavourable, and that the increase in the inductive effect (sigma(I)) of para-substitutents (-H < -Cl < -COOH < -F < -NO2) leads to the corresponding growth of the N center dot center dot center dot O distance and decrease of the pK and of the changes of Gibbs free energy, of enthalpy and of entropy for the HL1-5 acid dissociation process. The electrochemical behaviour of HL1-5 was interpreted using theoretical calculations at the DFT/HF hybrid level, namely in terms of HOMO and LUMO compositions, and of reactivities induced by anodic and cathodic electron-transfers. Copyright (C) 2010 John Wiley & Sons, Ltd.
- Heterogenisation of a c-scorpionate feII complex on carbon materials for cyclohexane oxidation with hydrogen peroxidePublication . Martins, Luisa; Almeida, M. Peixoto de; Carabineiro, S. A. C.; Figueiredo, J. L.; Pombeiro, ArmandoThe hydrotris(pyrazol-1-yl)methane iron(II) complex [FeCl2{h3-HC(pz)3}] (pz=pyrazol-1-yl) (1) was immobilized on three diferente carbon materials (activated carbon, carbon xerogel and multi-walled carbon nanotubes) with three different surface treatments (original, treated with nitric acid, and treated with nitric acid followed by sodium hydroxide) to produce active, selective and recyclable catalysts. The heterogenisation process was more efficient for carbon nanotubes treated with nitric acid and sodium hydroxide. An outstanding improved catalytic performance of complex 1 upon heterogenisation on carbon nanotubes treated with nitric acid and sodium hydroxide (turnover numbers up to 5.6_103 and overall yield of 21 %), relative to the homogeneous system, was achieved for the single-pot peroxidative oxidation of cyclohexane to the cyclohexanone and cyclohexanol mixture. The heterogenised systems allowed their easy recovery and reuse, at least for five consecutive cycles, maintaining 96% of the initial activity and concomitante rather high selectivity to cyclohexanol and cyclohexanone.
- Copper(II) and iron(III) complexes with arylhydrazone of ethyl 2-cyanoacetate or formazan ligands as catalysts for oxidation of alcoholsPublication . Martins, Nuno; Mahmudov, Kamran T.; Guedes Da Silva, M. Fátima C.; Martins, Luisa; Pombeiro, ArmandoThe aquasoluble [Cu(1 kappa N,O-2:2 kappa O-HL1)(S)](2) [S = CH3OH (1), (CH3)(2)NCHO (2)] and [Cu(kappa N-HL1)(en)(2)]center dot CH3OH center dot H2O (3) Cu-II complexes were prepared by the reaction of Cu-II nitrate hydrate with the new ligand (E/Z)-4-(2-(1-cyano-2-ethoxy-2-oxoethylidene)hydrazinyl)-3-hydroxybenzoic acid (H3L1), in the presence (for 3) or absence (for 1 and 2) of ethylenediamine (en), while the Fe-III complex [Fe(kappa N-3-HL2)(2)] (4) was isolated by treatment of iron(III) chloride hexahydrate with the new ligand (1E,1E)-N',2-di(1H-1,2,4-triazol-3-yl)diazenecarbohydrazonoyl cyanide (H3L2), and characterized by elemental analysis, IR spectroscopy and single crystal X-ray diffraction. Cooperative E,Z -> E isomerization of H3L1, induced by coordination and ionic interactions, occurs upon interaction with Cu-II in the presence of en. Complexes 1-4 act as catalyst precursors for the solvent-free microwave (MW) assisted selective oxidation of primary or secondary alcohols and diols to the corresponding aldehydes, ketones and diketones, respectively, with yields in the 5-99% range (TONs up to 4.96 x 10(2)) after 60 min of MW irradiation at 120 degrees C. The influence of temperature, time and organic radicals was studied and also the regioselective oxidation of the catalytic systems involving the primary and secondary alcohols.
- Efficient solvent-free friedel-crafts benzoylation and acylation of m-xylene catalyzed by N-acetylpyrazine-2-carbohydrazide-Fe(III)-chloro complexesPublication . Roy Barman, Tannistha; Sutradhar, Manas; Alegria, Elisabete; Guedes Da Silva, M. Fátima C.; Kuznetsov, Maxim L.; Pombeiro, ArmandoReaction of N-acetylpyrazine-2-carbohydrazide (H2L) with anhydrous Fe(II) or Fe(III) chloride in CH3CN or in MeOH leads to the mononuclear [Fe(kNN’O-HL)Cl2] (1) or binuclear [Fe(kNN’O-HL) Cl(μ-OMe)]2 (2) Fe(III) complex, respectively. These complexes are characterized by elemental analysis, ESI-MS, IR spectroscopy, single-crystal X-ray crystallography, electrochemical techniques and DFT calculations. The theoretical calculations indicate that the three single-electron sequential reductions of 1 are centred at the metal, at the pyrazine group and at both, respectively. The catalytic activity of 1 and 2 was screened towards Friedel-Crafts benzoylation and acylation of m-xylene. The effects of reaction parameters, such as catalyst amount, reaction time and temperature, were studied and, under optimal conditions, 96% yield of 2,4-dimethylbenzophenone and 20% yield of 2,4-dimethylacetophenone were obtained, respectively. Complex 1 exhibited the highest activity in both reactions. The structural and electrochemical properties were supported by theoretical calculations and the importance of the Lewis acid character of the catalyst in the promotion of this catalytic reaction is discussed.