Loading...
3 results
Search Results
Now showing 1 - 3 of 3
- Highly active and selective supported rhenium catalysts for aerobic oxidation of n-Hexane and n-HeptanePublication . Mishra, Gopal; Alegria, Elisabete; Pombeiro, Armando; Martins, LuisaA series of derivative C-scorpionate rhenium complexes, i.e., [ReCl2{NNC(O)C6H5} (Hpz)(PPh3)2] (A) (where Hpz is pyrazole), [ReCl2{NNC(O)C6H5}(Hpz)2(PPh3)] (B), [ReClF {NNC(O)C6H5}(Hpz)2(PPh3)] (C), and their precursor [ReOCl3(PPh3)2] (D), immobilized on 3-aminopropyl-functionalized silica have been prepared and used for neat O2 oxidation of n-hexane and n-heptane mainly to the corresponding alcohols and, in lower amounts, ketones. The supported catalyst C, with fluoro- and diazenido-ligands, exhibits the highest activity for both alkanes (overall turnover numbers (TONs) up to 3.8 _ 103 and 2.5 _ 103, for n-hexane and n-heptane, respectively) and can be reused in consecutive catalytic cycles. Improved conversion was observed after addition of hetero-carboxylate co-catalysts. A free-radical-based mechanism is proposed to explain the product formation.
- Synthesis and structural characterization of iron complexes with 2,2,2-tris(1-pyrazolyl)ethanol ligands: Application in the peroxidative oxidation of cyclohexane under mild conditionsPublication . Silva, Telma F. S.; Guedes Da Silva, M. Fátima C.; Mishra, Gopal; Martins, Luisa; Pombeiro, ArmandoThe reactions of FeCl2 center dot 2H(2)O and 2,2,2-tris(1-pyrazolyl) ethanol HOCH2C(pz)(3) (1) (pz = pyrazolyl) afford [Fe{HOCH2C(pz)(3)}(2)][FeCl4]Cl (2), [Fe{HOCH2C(pz)(3)}(2)](2)[Fe2OCl6](Cl)(2)center dot 4H(2)O (3 center dot 4H(2)O), [Fe{HOCH2C(pz)(3)}(2)] [FeCl{HOCH2C(pz)(3)}(H2O)(2)](2)(Cl)(4) (4) or [Fe{HOCH2C(pz)(3)}(2)]Cl-2 (5), depending on the experimental conditions. Compounds 1-5 were isolated as air-stable crystalline solids and fully characterized, including (1-4) by single-crystal X-ray diffraction analyses. The latter technique revealed strong intermolecular H-bonds involving the OH group of the scorpionate 2 and 3 giving rise to 1D chains which, in 3, are further expanded to a 2D network with intercalated infinite and almost plane chains of H-interacting water molecules. In 4, intermolecular pi center dot center dot center dot pi interactions involving the pyrazolyl rings are relevant. Complexes 2-5 display a high solubility in water (S-25 degrees C ca. 10-12 mg mL(-1)), a favourable feature towards their application as catalysts (or catalyst precursors) for the peroxidative oxidation of cyclo-hexane to cyclohexanol and cyclohexanone, with aqueous H2O2/MeCN, at room temperature (TON values up to ca. 385). (C) 2011 Elsevier B. V. All rights reserved.
- Cyclohexane oxidation with dioxygen catalyzed by supported pyrazole rhenium complexesPublication . Mishra, Gopal S.; Alegria, Elisabete; Martins, Luisa; Fraústo Da Silva, João; Pombeiro, ArmandoThe pyrazole complexes [ReCl2{N2C(O)Ph}(Hpz)(PPh3)2] 2 (Hpz = pyrazole), [ReCl2{N2C(O)Ph}(Hpz)2(PPh3)] 3 and [ReClF{N2C(O)Ph}(Hpz)2(PPh3)] 4, and their precursor [ReOCl3(PPh3)2] 1, immobilized on 3-aminopropyl functionalized silica, catalyze the cyclohexane oxidation with dioxygen, to cyclohexanol and cyclohexanone (the main product), in the absence of solvent and additives and under relatively mild conditions. Complex 4, whose synthesis and characterization are reported herein for the first time, provides the best activity (ca. 16% overall conversion towards the ketone and alcohol, at the O2 pressure of 19 atm, at 150 °C, 8 h reaction time). The reaction is further promoted by pyrazinecarboxylic acid. TGA analysis shows that the supported complexes are stable up to ca. 200 °C. The use of radical traps supports the involvement of a free-radical mechanism via carbon- and oxygen-centred radicals. The effects of various factors were studied towards the optimization of the processes. Complex 4 also catalyzes the oxidation of other cycloalkanes to the corresponding cycloalkanols and cycloalkanones.