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- 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.
- 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.
- 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.
- New copper(II) tetramer with arylhydrazone of barbituric acid and its catalytic activity in the oxidation of cyclic C5–C8 alkanesPublication . Sutradhar, Manas; Alegria, Elisabete; Roy Barman, Tannistha; Guedes Da Silva, M. Fátima C.; Mahmudov, Kamran T.; Guseynov, Firuddin I.; Pombeiro, Armando2-(2-(2,4,6-Trioxotetrahydropyrimidin-5(2H)-ylidene)hydrazinyl)terephthalic acid (H5L) was synthesized by azocoupling of diazonium salts of 2-aminoterephthalic acid with barbituric acid and characterized by IR, 1H and 13C NMR spectroscopies and ESI-MS analysis. In solution and solid state, H5L exists in the hydrazone form. The reaction of H5L with copper(II) nitrate hydrate in methanol affords the tetramer species [Cu2(μ2-1κNOO′:2κO′O″-H3L)2]2 (1) where carboxylate groups bridge two Cu(II) centers in a bridging chelate μ2-(O,O′,O′) fashion, giving rise to a cyclic Cu4O4 core. A rare and intense intermolecular non-covalent interaction of the type Ocarboxylic(lone pair)⋯π(pyrimidine), with O⋯centroid distance of 2.705 Å, was found in 1. In addition, medium intense N H⋯O and O H⋯O contacts expand the structure to a 3D network. Compound 1 behaves as efficient catalyst in the mild oxidation, by aqueous H2O2 in acidic MeCN/H2O media, of different cyclic C5–C8 alkanes to produce the corresponding alcohols and ketones. Overall product yields up to ca. 25% (based on alkane) were achieved.
- Trinuclear Cu-II structural isomers coordination, magnetism, electrochemistry and catalytic activity towards the oxidation of alkanesPublication . Sutradhar, Manas; Martins, Luisa; Guedes Da Silva, M. Fátima C.; Mahmudov, Kamran; Liu, Cai-Ming; Pombeiro, ArmandoThe reaction of the Schiff base (3,5-di-tert-butyl-2-hydroxybenzylidene)-2-hydroxybenzohydrazide (H3L) with copper(II) nitrate, acetate or metaborate has led to the isomeric complexes [Cu-3(L)(2)(MeOH)(4)] (1), [Cu-3(L)(2)(MeOH)(2)]2MeOH (2) and [Cu-3(L)(2)(MeOH)(4)] (3), respectively, in which the ligand L exhibits dianionic (HL2-, in 1) or trianionic (L3-, in 2 and 3) pentadentate 1O,O,N:2N,O chelation modes. Complexes 1-3 were characterized by elemental analysis, IR spectroscopy, single-crystal X-ray crystallography, electrochemical methods and variable-temperature magnetic susceptibility measurements, which indicated that the intratrimer antiferromagnetic coupling is strong in the three complexes and that there exists very weak ferromagnetic intermolecular interactions in 1 but weak antiferromagnetic intermolecular interactions in both 2 and 3. Electrochemical experiments showed that in complexes 1-3 the Cu-II ions can be reduced, in distinct steps, to Cu-I and Cu-0. All the complexes act as efficient catalyst precursors under mild conditions for the peroxidative oxidation of cyclohexane to cyclohexyl hydroperoxide, cyclohexanol and cyclohexanone, leading to overall yields (based on the alkane) of up to 31% (TON = 1.55x10(3)) after 6 h in the presence of pyrazinecarboxylic acid.
- Aroylhydrazone Cu(II) complexes in keto form: structural characterization and catalytic activity towards cyclohexane oxidationPublication . Sutradhar, Manas; Alegria, Elisabete; Guedes Da Silva, M. Fátima C.; Martins, Luisa; Pombeiro, ArmandoThe reaction of the Schiff base (3,5-di-tert-butyl-2-hydroxybenzylidene)-2-hydroxybenzohydrazide (H3L) with a copper(II) salt of a base of a strong acid, i.e., nitrate, chloride or sulphate, yielded the mononuclear complexes [Cu(H2L)(NO3)(H2O)] (1), [Cu(H2L)Cl]center dot 2MeOH (2) and the binuclear complex [{Cu(H2L)}(2)(mu-SO4)]center dot 2MeOH (3), respectively, with H2L- in the keto form. Compounds 1-3 were characterized by elemental analysis, Infrared (IR) spectroscopy, Electrospray Ionisation-Mass Spectrometry (ESI-MS) and single crystal X-ray crystallography. All compounds act as efficient catalysts towards the peroxidative oxidation of cyclohexane to cyclohexyl hydroperoxide, cyclohexanol and cyclohexanone, under mild conditions. In the presence of an acid promoter, overall yields (based on the alkane) up to 25% and a turnover number (TON) of 250 (TOF of 42 h(-1)) after 6 h, were achieved.
- Cd(II) coordination compounds as heterogeneous catalysts for microwave-assisted peroxidative oxidation of toluene and 1-phenylethanolPublication . Sutradhar, Manas; Roy Barman, Tannistha; Alegria, Elisabete; Lapa, Hugo M.; Guedes Da Silva, M. Fátima C.; Pombeiro, ArmandoThe aroylhydrazone Schiff bases 2-hydroxy(2-hydroxybenzylidene)benzohydrazide (H2L1) and 2-amino(2hydroxybenzylidene)benzohydrazide (H2L2) have been used to synthesize the new Cd(II) coordination polymer [Cd(m-1kONO0:2kO:3kO00-L1)(DMF)]n (1) and the dinuclear complex [Cd(1kONO0:2kO-HL2)(kOO0Ac)]2 2DMF (2). Both 1 and 2 have been characterized by elemental analysis, FT-IR and single crystal X-ray diffraction analysis. The catalytic performances of 1 and 2 were evaluated towards the microwave (MW) assisted peroxidative oxidation (with tert-butylhydroperoxide, TBHP) of toluene and 1-phenylethanol under heterogeneous conditions. The MW-assisted peroxidative oxidation of toluene by a Cd(II) catalytic system is reported for the first time. At 50 1C, the 2/TBHP/MW catalytic system shows good activity for toluene oxidation with a total (benzaldehyde + benzyl alcohol) product yield of 49% in 1 h, in the absence of any additives. In the case of neat solvent-free peroxidative oxidation of 1-phenylethanol the 1/TBHP/MW catalytic system resulted in 24% total yield of acetophenone under optimized conditions. After the catalytic reactions, both 1 and 2 were successfully recovered and reused.
- Dinuclear Mn (II,II) complexes: magnetic properties and microwave assisted oxidation of alcoholsPublication . Sutradhar, Manas; Martins, Luisa; Guedes Da Silva, M. Fátima C.; Alegria, Elisabete; Liu, Cai-Ming; Pombeiro, ArmandoA series of six new mixed-ligand dinuclear Mn(II, II) complexes of three different hydrazone Schiff bases (H3L1, H3L2 and H3L3), derived from condensation of the aromatic acid hydrazides benzohydrazide, 2-aminobenzohydrazide or 2-hydroxybenzohydrazide, with 2,3-dihydroxy benzaldehyde, respectively, is reported. Reactions of Mn(NO3)(2) center dot 4H(2)O with the H3L1-3 compounds, in the presence of pyridine (1 : 1 : 1 mole ratio), in methanol at room temperature, yield [Mn(H2L1)(py)(H2O)](2)(NO3)(2) center dot 2H(2)O (1 center dot 2H(2)O), [Mn(H2L2)(py)(CH3OH)](2)(NO3)(2) center dot 4H(2)O (2 center dot 4H(2)O) and [Mn(H2L3)(py)(H2O)](2)(NO3)(2) (3) respectively, whereas the use of excess pyridine yields complexes with two axially coordinated pyridine molecules at each Mn(II) centre, viz. [Mn(H2L1)(py)(2)] 2(NO3)(2) center dot H2O (4 center dot H2O), [Mn(H2L2)(py) H-O (6 center dot 2CH(3)OH), respectively. In all the complexes, the (H2L1-3)-ligand coordinates in the keto form. Complexes 1 center dot 2H(2)O, 2 center dot 4H(2)O, 4 center dot H2O, 5 center dot 2H(2)O and 6 center dot 2CH(3)OH are characterized by single crystal X-ray diffraction analysis. The complexes 1, 2 and 6, having different coordination environments, have been selected for variable temperature magnetic susceptibility measurements to examine the nature of magnetic interaction between magnetically coupled Mn(II) centres and also for exploration of the catalytic activity towards microwave assisted oxidation of alcohols. A yield of 81% (acetophenone) is obtained using a maximum of 0.4% molar ratio of catalyst relative to the substrate in the presence of TEMPO and in aqueous basic solution, under mild conditions.
- Copper(II) and cobalt(II) tetrazole-saccharinate complexes as effective catalysts for oxidation of secondary alcoholsPublication . Frija, Luís; Alegria, Elisabete; Sutradhar, Manas; Cristiano, Maria De Lurdes; Ismael, Amin; Kopylovich, Maximilian; Pombeiro, ArmandoMononuclear Cu(II) and Co(II) complexes comprising 2-methyltetrazole-saccharinate bidentate N,N-chelating ligand have been synthesized for the first time and tested as homogeneous catalysts for oxidation of secondary alcohols in a solvent-free and microwave assisted protocol using aqueous tert-butyl hydroperoxide (TBHP) as oxidant. The developed catalytic system exhibits broad functional group compatibility, allowing efficient and selective conversion of a variety of secondary alcohols, including allylic ones, into the corresponding ketones. With typical 0.2 mol% content of the catalyst and under 20–50 W microwave irradiation, most reactions are complete within 10 min, presenting TONs up to 5.5 × 102 and TOFs up to 1.1 × 104 h−1. No additives and co-oxidants have been used, while TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxyl) acts as inhibitor in most cases. A plausible reaction mechanism involving the new catalytic systems is outlined.
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