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- C-scorpionate rhenium complexes and their application as catalysts in Baeyer-Villiger oxidation of ketonesPublication . Martins, Luisa; Pombeiro, ArmandoThis work concerns recent advances in the synthesis of tris(pyrazol-1-yl)-methane rhenium complexes and their application as catalysts for the industrially significant Baeyer-Villiger oxidative functionalization of ketones.
- Supported C-Scorpionate Vanadium(IV) Complexes as Reusable Catalysts for Xylene OxidationPublication . Wang, Jiawei; Martins, Luisa; Da Costa Ribeiro, Ana Paula; Carabineiro, Sonia Alexandra Correia; Figueiredo, José L.; Pombeiro, ArmandoC-Scorpionate vanadium(IV) [VOxCl3-x{k(3)-RC(pz)(3)}] [pz = pyrazol-1-yl; x=0, R = SO3 (1); x = 1, R= CH2OH (2) or CH2OSO2Me (3)] complexes supported on functionalized carbon nanotubes (CNTs) are the first V-scorpionate catalysts used so far for the neat oxidation of o-, m- or p-xylene, with TBHP (70% aqueous solution), to the corresponding toluic acids (main products), tolualdehydes and methylbenzyl alcohols. Remarkably, a p-toluic acid yield of 43% (73% selectivity, TON = 1.34 V 10(3)) was obtained with 2@CNT in a simple microwave-assisted mild oxidation procedure, using a very low catalyst charge (3.2 x 10(-2) mol% vs. substrate). Further, this occurred in the absence of any bromine source, what is significant towards the development of a greener and more sustainable process for oxidation of xylenes. Moreover, reuse of catalysts with preservation of their activity was found for up to six consecutive cycles. The effects of reaction parameters, such as reaction time, temperature, amount of catalyst or type of heating source, on the performance of the above catalytic systems are reported and discussed.
- Trits(Pyrazol-1-YL)methane metal complexes for catalytic mild oxidative functionalizations of alkanes, alkanes and ketonesPublication . Martins, Luisa; Pombeiro, ArmandoThis work concerns recent advances (since 2005) in the oxidative functionalization of alkanes, alkenes and ketones, under mild conditions, catalyzed by homoscorpionate tris(pyrazol-1-yl)methane metal complexes. The main types of such homogeneous or supported catalysts are classified, and the critical analysis of the most efficient catalytic systems in the different reactions is presented. These reactions include the mild oxidation of alkanes (typically cyclohexane as a model substrate) with hydrogen peroxide (into alkyl hydroperoxides, alcohols, and ketones), the hydrocarboxylation of gaseous alkanes (with carbon monoxide and potassium peroxodisulfate) into the corresponding Cn+1 carboxylic acids, as well as the epoxidation of alkenes and the Baeyer-Villiger oxidation of linear and cyclic ketones with hydrogen peroxide into the corresponding esters and lactones. Effects of various reaction parameters are highlighted and the preferable requirements for a prospective homogeneous or supported C-scorpionate-M-based catalyst in oxidative transformations of those substrates are identified. (C) 2014 Elsevier B.V. All rights reserved.
- Catalytic performance of Fe(II)-scorpionate complexes towards cyclohexane oxidation in organic, ionic liquid and/or supercritical CO2 media: a comparative studyPublication . Da Costa Ribeiro, Ana Paula; Martins, Luisa; Alegria, Elisabete; Matias, Inês A. S.; Duarte, Tiago A. G.; Pombeiro, ArmandoThe catalytic activity of the iron(II) C-scorpionate complexes [FeCl2{HC(pz)3}] 1 (pz = pyrazol-1-yl) and [FeCl2{HOCH2C(pz)3}] 2, and of their precursor FeCl2·2H2O 3, towards cyclohexane oxidation with tert-butyl hydroperoxide was evaluated and compared in different media: acetonitrile, ionic liquids (1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6], and 1-butyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate, [bmim][FAP]), supercritical carbon dioxide (scCO2), and scCO2/[bmim][X] (X = PF6 or FAP) mixtures. The use of such alternative solvents led to efficient and selective protocols for the oxidation of cyclohexane. Moreover, tuning the alcohol/ketone selectivity was possible by choosing the suitable solvent.
- C-scorpionate Au(III) complexes as pre-catalysts for industrially significant toluene oxidation and benzaldehyde esterification reactionsPublication . Lapa, Hugo; Guedes Da Silva, M. Fátima C.; Pombeiro, Armando; Alegria, Elisabete; Martins, LuisaThe new Au(III) complex [AuCl2(Tpms)] (1) and the previously reported [AuCl2(Tpm)]Cl (2), bearing the potentially tridentate ligands tris(1-pyrazolyl)methanesulfonate (SO3C(C3H3N2)(3)(-), Tpms) or hydrotris(1-pyrazolyl) methane (HC(C3H3N2)(3), Tpm), respectively, were synthesized in water at room temperature and characterized using NMR and IR spectroscopy. The molecular structure of 1 was authenticated by single crystal X-ray diffraction analysis. The catalytic performance of the Au(III) complexes was tested, for the first time, in toluene and benzyl alcohol oxidation reactions. The oxidative esterification of benzaldehyde, by-product of toluene oxidation, was further explored. In order to optimize the catalytic systems, the influence of parameters such as temperature, reaction time, amount of pre-catalyst and the presence of additives was evaluated. In the peroxidative (by H2O2 or t-BuOOH) oxidation reactions, a maximum total yield (benzylic alcohol and benzaldehyde) of 8% for toluene oxidation with pre-catalyst 1 (6 h, 80 degrees C, H2O2 30% aq. sol.) and a maximum total yield (benzaldehyde and benzoic acid) of 43% for benzyl alcohol oxidation with pre-catalyst 2 (24 h, 80 degrees C, t-BuOOH 70% aq. sol.) with a selectivity of 72% for benzaldehyde, were obtained. The esterification of benzaldehyde yielded, in the presence of 1, a maximum of 27% and 48% of methyl benzoate, at room temperature and 80 degrees C, respectively, and with a selectivity of 78% for methyl benzoate.
- New trendy magnetic C-scorpionate iron catalyst and its performance towards cyclohexane oxidationPublication . Da Costa Ribeiro, Ana Paula; Matias, Inês; Alegria, Elisabete; Ferraria, Ana Maria; Rego, Ana; Pombeiro, Armando; Martins, LuisaFor the first time, a magnetic C-scorpionate catalyst was prepared from the iron(II) complex [FeCl2{_3-HC(pz)3}] (pz = pyrazol-1-yl) and ferrite, using the sustainable mechanochemical synthetic procedure. Its catalytic activity for the cyclohexane oxidation with tert-butyl hydroperoxide (TBHP) was evaluated in different conditions, namely under microwave irradiation and under the effect of an external magnetic field. The use of such magnetic conditions significantly shifted the catalyst alcohol/ketone selectivity, thus revealing a promising, easy new protocol for tuning selectivity in important catalytic processes.
- Vanadium C-scorpionate supported on mesoporous aptes-functionalized SBA-15 as catalyst for the peroxidative oxidation of benzyl alcoholPublication . Correia, Luís M. M.; Soliman, Mohamed Mostafa Aboelhassan; Granadeiro, Carlos; Balula, Salete; Martins, Luisa; Pombeiro, Armando; Alegria, ElisabeteThe neutral trichloro[hydrotris(1-pyrazolyl)methane]vanadium(III) [VCl3(Tpm)] (Tpm = HC(pz)(3); pz = pyrazolyl) C-scorpionate complex was immobilized on amine-functionalized mesoporous silica (aptesSBA-15) via an impregnation method forming the [VCl3(Tpm)]@aptesSBA-15 composite. The immobilization of the vanadium compound was confirmed by several characterization techniques, namely SEM/EDS, powder XRD, FT-IR/ATR, ICP and BET surface area analysis, revealing the successful incorporation of the complex, and confirming the structural and morphological preservation of the porous support and the vanadium complex. The vanadium composite was tested as heterogeneous catalyst for the peroxidative oxidation of benzyl alcohol under mild conditions and its catalytic performance was compared to that of the analogous homogeneous [VCl3(Tpm)] complex. The catalytic studies were extended to other substrates. The effect of various parameters, such as amount and type of oxidant, catalyst and additives, temperature and reaction time were investigated allowing to reach overall yields of ca. 60% and turnover numbers (TONs) up to ca. 7.6 x 10(3). The results obtained demonstrated the higher performance of the heterogeneous catalyst using much less [VCl3(Tpm)] complex under a solvent-free system. Furthermore, consecutive reaction cycles could be performed, showing its recycling capacity. Structural stability was also investigated, indicating the viability of the vanadium C-scorpionate composite as catalyst for other oxidative reactions with high industrial interest.
- C-homoscorpionate oxidation catalysts—electrochemical and catalytic activityPublication . Martins, LuisaA survey of the electrochemical properties of homoscorpionate tris(pyrazol-1-yl)methane complexes is presented. The relationship between structural features and catalytic efficiency toward the oxidative functionalization of inexpensive and abundant raw-materials to added-value products is also addressed.