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- Pyrazole and trispyrazolylmethane rhenium complexes as catalysts for ethane and cyclohexane oxidationsPublication . Alegria, Elisabete; Kirillova, Marina V.; Martins, Luisa; 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 the tris(pyrazolyl)methane compounds [ReCl2(HCpz3)(PPh3)][BF4] 5 (pz = pyrazolyl), [ReCl3{HC(pz)3}] 7, [ReOCl2{SO3C(pz)3}(PPh3)] 8 and [ReO3{SO3C(pz)3}] 9, and their precursors [ReCl2{η2-N,O-N2C(O)Ph}(PPh3)2] 1 and [ReOCl3(PPh3)2] 6, act as selective catalysts (or catalyst precursors), in a single-pot process, for the oxidation of ethane, in the presence of potassium peroxodisulfate K2S2O8, in trifluoroacetic acid (TFA), to give acetic acid, in a remarkable yield (up to ca. 40%) and under mild conditions (in some cases carboxylation can also occur to give propionic acid, but in a much lower yield). The catalytic peroxidative oxidation of ethane to acetaldehyde and of cyclohexane to cyclohexanone and cyclohexanol by an aqueous solution of H2O2 at room temperature is also achieved by using most of those catalyst precursors. The effects of a variety of factors were studied towards the optimization of the processes which are shown to proceed via both C-centered and O-centered radical mechanisms.
- 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.
- 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.
- Synthesis, characterization and redox behaviour of benzoyldiazenido- and oxorhenium complexes bearing N,N- and S,S-type ligandsPublication . Kirillov, Alexander M.; Guedes Da Silva, M. Fátima C.; Martins, Luisa; Sousa, Catarina; Pombeiro, Armando; Alegria, ElisabeteThe reactions of [ReCl2{eta(2)-N2C(O)Ph}(PPh3)(2)](1) with 2-aminopyrimidine (H(2)Npyrm), 2,2'-bipyridine (bpy) and tetraethylthiuram disulfide (tds), in MeOH upon reflux, lead to the new eta(1)-(benzoyldiazenido)-rhenium(III) complexes [ReCl{eta(1)-N2C(O)Ph}(HNpyrm)(PPh3)(2)](2)and [ReCl2{eta(1)-N2C(O)Ph}(bpy)(PPh3)] (3), and the known oxo(diethyldithiocarbamato)dirhenium(v)complex [Re2O2(mu O){Et2NC(S)S}(4)](4), respectively. The Et2NC(S)S ligands in 4 result from S-S bond rupture of tds molecules. The obtained compounds have been characterized by IR, H-1, P-31{H-1} and C-13{H-1} NMR spectroscopies, FAB(+)-MS, elemental and single-crystal X-ray diffraction (for 2 and 4)analyses. Complex 2 represents the first structurally characterized Re compound derived from 2-aminopyrimidine. Besides, the redox behaviour of 2-4 in CH2Cl2 solution has been studied by cyclic voltammetry, and the Lever electrochemical ligand parameter (E-L)has been estimated, for the first time, for HNpyrm. The electrochemical results are discussed in terms of electronic properties of the Re centres and the ligands.
- Half‐sandwich scorpionate vanadium, iron and copper complexes: synthesis and application in the catalytic peroxidative oxidation of cyclohexane under mild conditionsPublication . Silva, Telma F. S.; Alegria, Elisabete; Martins, Luisa; Pombeiro, ArmandoThe new half‐sandwich scorpionate complexes [hydridotris(1‐pyrazolyl)methane]vanadium trichloride, ([VCl3{HC(pz)3}] 1 (pz=pyrazolyl), [tris(1‐pyrazolyl)methanesulfonate]vanadium trichloride, [VCl3{SO3C(pz)3}] 2 , [hydrido(1‐pyrazolyl)methane]iron dichloride, [FeCl2{HC(pz)3}] 3 , lithium [tris(1‐pyrazolyl)methanesulfonate]dichloroferrate, Li[FeCl2{SO3C(pz)3}] 4 , and [tris(1‐pyrazolyl)methanesulfonate]copper chloride, [CuCl{SO3C(pz)3}] 5 were synthesized, characterized and shown to act, as well as the related (tripyrazolylmethane)copper dichloride, [CuCl2{HC(pz)3}] 6 , as selective catalysts (or catalyst precursors) for the peroxidative oxidation (with hydrogen peroxide) of cyclohexane to cyclohexanol and cyclohexanone, under mild conditions (at room temperature and using an aqueous solution of H2O2). The iron complexes are the most active ones (reaching TON values up to ca. 690), the effects of a variety of factors are reported and the reaction is shown to proceed via both C‐ and O‐centred radical mechanisms, conceivably involving a metal‐based oxidant.
- C-scorpionate iron(II) complexes as highly selective catalysts for the hydrocarboxylation of cyclohexanePublication . Matias, Inês; Da Costa Ribeiro, Ana Paula; Alegria, Elisabete; Pombeiro, Armando; Martins, LuisaThe C-scorpionate iron(II) complexes [FeCl2{kappa(3)-HC(pz)(3)}](1) (pz = pyrazol-1-yl), Li[FeCl2{kappa(3)-SO3C(pz)(3)}] (2) and the new [FeCl2{kappa(3)-HOCH2C(pz)(3)}] (3) act as catalysts for the hydrocarboxylation of cyclohexane (with CO and H2O) to cyclohexanecarboxylic acid in a remarkable yield (up to 60%) and under mild conditions. The catalysts selectivity for the acid can be tuned by CO pressure, catalyst amount and reaction temperature. Complex [FeCl2{kappa(3)-HC(pz)(3)}] (1) provides the most effective catalyst requiring low CO pressure. This work constitutes an unprecedented use of iron complexes as catalysts for the hydrocarboxylation of alkanes.
- 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.