Browsing by Author "Neves, Patrícia"
Now showing 1 - 7 of 7
Results Per Page
Sort Options
- A 5-(2-Pyridyl) tetrazolate complex of Molybdenum(VI), its structure, and transformation to a Molybdenum Oxide-Based Hybrid Heterogeneous Catalyst for the Epoxidation of OlefinsPublication . Nunes, Martinique S.; Gomes, Diana M.; Gomes, Ana C.; Neves, Patrícia; Mendes, Ricardo F.; Paz, Filipe A. Almeida; Lopes, A. D.; Valente, Anabela A.; Gonçalves, Isabel S.; Pillinger, MartynThere is a considerable practical interest in discovering new ways to obtain organomolybdenum heterogeneous catalysts for olefin epoxidation that are easier to recover and reuse and display enhanced productivity. In this study, the complex salt (H2pytz)[MoO2Cl2 (pytz)] (1) (Hpytz = 5-(2-pyridyl)tetrazole) has been prepared, structurally characterized, and employed as a precursor for the hydrolysis-based synthesis of a microcrystalline molybdenum oxide/organic hybrid material formulated as [MoO3 (Hpytz)] (2). In addition to single-crystal X-ray diffraction (for 1), compounds 1 and 2 were characterized by FT-IR and Raman spectroscopies, solid-state 13C{1H} cross-polarization (CP) magic-angle spinning (MAS) NMR, and scanning electron microscopy (SEM). Compounds 1 and 2 were evaluated as olefin epoxidation catalysts using the model reaction of cis-cyclooctene (Cy8) with tert-butyl hydroperoxide (TBHP), at 70 ◦C, which gave 100% epoxide selectivity up to 100% conversion. While 1 behaved as a homogeneous catalyst, hybrid 2 behaved as a heterogeneous catalyst and could be recovered for recycling without showing structural degradation or loss of catalytic performance over consecutive reaction cycles. The substrate scope was broadened to monoterpene DL-limonene (Lim) and biobased unsaturated fatty acid methyl esters, methyl oleate (MeOle), and methyl linoleate (MeLin), which gave predominantly epoxide products.
- Catalytic epoxidation and sulfoxidation activity of a dioxomolybdenum(VI) complex bearing a chiral tetradentate oxazoline ligandPublication . Neves, Patrícia; Gago, Sandra; Pereira, Cláudia C. L.; Figueiredo, Sónia; Lemos, Americo; Lopes, Andre D.; Gonçalves, Isabel S.; Pillinger, Martyn; Silva, Carlos M.; Valente, Anabela A.A dioxomolybdenum(VI) complex bearing a tetradentate anionic N,O oxazoline ligand with four stereocenters has been studied as a catalyst in the liquid-phase epoxidation of 17 different aliphatic and aromatic olefins(including prochiral, racemate or pure enantiomers) using tert-butyl hydroperoxide as the oxidant. Epoxide selectivities of up to 100% and variable epoxide yields (3–100% within 24 h) were obtained. Although the complex generally exhibited low or no chiral induction ability, diastereoselectivity was significant in some cases (in the reaction of limonene, for example). Kinetic studies and recycling tests with the substrates cis-cyclooctene and trans-b-methylstyrene showed that the catalyst is stable and reusable, and recycling is facilitated by immobilization of the complex in a room temperature ionic liquid. Preliminary results show that the complex may have a broad substrate scope, not only for olefin epoxidation, but also for the dehydrogenation of alcohols to carbonyl compounds and the sulfoxidation of sulfides to sulfoxides.
- Inclusion complexes of cucurbit[n]urils (n = 7, 8) with η5 -cyclopentadienyl methyl tricarbonyl molybdenum(II) and their use in epoxidation catalysisPublication . Neves, Patrícia; Gomes, Ana C.; Monteiro, Rodrigo P.; Santos, Mirela J.; Valente, Anabela A.; D. Lopes, André; Gonçalves, Isabel S.; Pillinger, MartynThere are very few known examples of supramolecular compounds comprising molybdenum species hosted inside the portals/cavities of cucurbit[n]urils (CBn). In this work, CB7 and CB8 macrocycles have been studied as hosts for the carbonyl complex [CpMo(CO)(3)Me] (1) (Cp = eta(5)-C5H5). Compounds were isolated in the solid state and characterized as genuine 1:1 inclusion complexes (1@CBn) by elemental and thermogravimetric analyses, powder X-ray diffraction, scanning electron microscopy, C-13{H-1} cross-polarization magic-angle spinning NMR, FT-IR, Raman, and diffuse reflectance UV-Vis spectroscopies. The host-guest structures can act as supramolecular precatalysts for olefin epoxidation. Based on the model reaction of cis-cyclooctene with hydroperoxide oxidants (tert-butylhydroperoxide or hydrogen peroxide), the structural features of 1@CBn as well as the operating conditions influence the catalytic process. The metal species in 1@CBn undergo oxidative decarbonylation in situ, giving oxidized metal species that are catalytically active for olefin epoxidation. The type of oxidant and solvent influences the catalytic activity and stability. 1@CB8 was more stable than 1@CB7 with regard to catalyst recycling and reuse. Based on the substrate scope investigation, for relatively large olefins, such as the fatty acid methyl ester methyl oleate, the size of the macrocyclic host may be a determining factor for catalytic activity.
- A Molybdenum(VI) Complex of 5-(2-pyridyl-1-oxide)tetrazole: synthesis, structure, and transformation into a MoO3-Based hybrid catalyst for the epoxidation of Bio-OlefinsPublication . Nunes, Martinique S.; Gomes, Diana M.; Gomes, Ana C.; Neves, Patrícia; Mendes, Ricardo F.; Paz, Filipe A. Almeida; Lopes, Andre D.; Pillinger, Martyn; Valente, Anabela A.; Gonçalves, Isabel S.The discovery of heterogeneous catalysts synthesized in easy, sustainable ways for the valorization of olefins derived from renewable biomass is attractive from environmental, sustainability, and economic viewpoints. Here, an organic–inorganic hybrid catalyst formulated as [MoO3 (Hpto)]·H2O (2), where Hpto = 5-(2-pyridyl-1-oxide)tetrazole, was prepared by a hydrolysis– condensation reaction of the complex [MoO2Cl2 (Hpto)]·THF (1). The characterization of 1 and 2 by FT-IR and Raman spectroscopies, as well as 13C solid-state NMR, suggests that the bidentate N,O-coordination of Hpto in 1 (forming a six-membered chelate ring, confirmed by X-ray crystallography) is maintained in 2, with the ligand coordinated to a molybdenum oxide substructure. Catalytic studies suggested that 2 is a rare case of a molybdenum oxide/organic hybrid that acts as a stable solid catalyst for olefin epoxidation with tert-butyl hydroperoxide. The catalyst was effective for converting biobased olefins, namely fatty acid methyl esters (methyl oleate, methyl linoleate, methyl linolenate, and methyl ricinoleate) and the terpene limonene, leading predominantly to the corresponding epoxide products with yields in the range of 85–100% after 24 h at 70 ◦C. The versatility of catalyst 2 was shown by its effectiveness for the oxidation of sulfides into sulfoxides and sulfones, at 35 ◦C (quantitative yield of sulfoxide plus sulfone, at 24 h; sulfone yields in the range of 77–86%). To the best of our knowledge, 2 is the first molybdenum catalyst reported for methyl linolenate epoxidation, and the first of the family [MoO3 (L)x] studied for methyl ricinoleate epoxidation.
- Molybdenum(VI) complexes with ligands derived from 5-(2-pyridyl)-2H-tetrazole as catalysts for the epoxidation of olefinsPublication . Nunes, Martinique S.; Gomes, Ana C.; Neves, Patrícia; Mendes, Ricardo F.; Almeida Paz, Filipe A.; Lopes, André; Pillinger, Martyn; Gonçalves, Isabel S.; Valente, Anabela A.The development of effective catalytic epoxidation processes that are an alternative to stoichiometric non-selective oxidation routes is important to meet environmental sustainability goals. In this work, molybdenum (VI) compounds bearing 5-(2-pyridyl)-2H-tetrazole derivatives as organic components, namely the ionic and neutral mononuclear complexes (H2ptz)[MoO2Cl2(ptz)] (1) and [MoO2Cl2(tBu-ptz)] (2), and the new Lindqvist-type polyoxometalate (POM) [tBu-Hptz]2[Mo6O19] (3), where Hptz = 5-(2-pyridyl)tetrazole and tBu-ptz = 2-tert- butyl-5-(2-pyridyl)- 2H-tetrazole, were studied as epoxidation catalysts using readily available and relatively ecofriendly hydroperoxide oxidants, namely hydrogen peroxide and tert-butyl hydroperoxide (TBHP). The pre-pared catalysts were very active. For example, 100% cis-cyclooctene conversion and 100% epoxide selectivity were reached at 1 h for 1 and 3, and 10 min for 2 (with TBHP). Catalytic and characterization studies indicated that the mononuclear complexes suffered chemical transformations under the reaction conditions, whereas 3 was structurally stable. This POM acted as a homogeneous catalyst and could be recycled by employing an ionic liquid solvent. The POM can be synthesized from 2 under different conditions, including those used in the catalytic process. Moreover, 3 was an effective epoxidation catalyst for a biobased substrate scope that included fatty acid methyl esters and the terpene dl-limonene.
- Synthesis and catalytic properties in olefin epoxidation of octahedral dichloridodioxidomolybdenum(VI) complexes bearing N,N-dialkylamide ligands: crystal structure of [Mo2O4(mu(2)-O)Cl-2(dmf)(4)]Publication . Gago, Sandra; Neves, Patrícia; Monteiro, Bernardo; Pessêgo, Márcia; Lopes, Andre D.; Valente, Anabela A.; Almeida Paz, Filipe A.; Pillinger, Martyn; Moreira, José; Silva, Carlos M.; Gonçalves, Isabel S.The catalytic performance of the complexes [MoO2Cl2(L)2][L = N,N-dimethylformamide (dmf), N,N-dimethylacetamide(dma), N,N-dimethylpropionamide (dmpa), N,N-diethylformamide(def) and N,N-diphenylformamide (dpf)] was examined in the epoxidation of cis-cyclooctene with tert-butyl hydroperoxide(tbhp) at 55 °C and in the absence of a cosolvent.The complexes showed high turnover frequencies in the range of 561–577 molmolMo–1h–1, giving the epoxide as the only product in 98% yield after 6 h. The reaction rates decreased significantly in consecutive runs carried out by recharging the reactors with olefin and oxidant. On the basis of the IR spectroscopic characterisation of the solids recovered at the end of the catalytic reactions, the decrease in activity is attributed to the formation of dioxido(μ-oxido)-molybdenum(VI) dimers. Accordingly, the treatment of [MoO2Cl2(dmf)2] with an excess amount of tbhp led to the isolation of [Mo2O4(μ2-O)Cl2(dmf)4], which was characterised by single-crystal X-ray diffraction and found to exhibit a catalytic performance very similar to that found in the second runs for the mononuclear complexes. The kinetics of the reaction of [MoO2Cl2(dmf)2] with tbhp was further examined by UV/Vis spectroscopy, allowing rate constants and activation parameters to be determined. For the dpf adduct, the effect of different solvents on cyclooctene epoxidation and the epoxidation of other olefins, namely, (R)-(+)-limonene, α-pinene and norbornene, were investigated
- Tris(pyrazolyl)methane molybdenum tricarbonyl complexes as catalyst precursors for olefin epoxidationPublication . Gomes, Ana C.; Neves, Patrícia; Figueiredo, Sónia; Fernandes, José A.; Valente, Anabela A.; Almeida Paz, Filipe A.; Pillinger, Martyn; Lopes, Andre D.; Gonçalves, Isabel S.The molybdenum tricarbonyl complexes [Mo(CO)3(HC(3,5-Me2pz)3)] (1) and [Mo(CO)3(HC(pz)3)] (2)(HC(3,5-Me2pz)3 = tris(3,5-dimethyl-1-pyrazolyl)methane, HC(pz)3 = tris(1-pyrazolyl)methane) were obtained in good yields by the microwave-assisted reaction of Mo(CO)6 with the respective organic ligand. Complete oxidative decarbonylation of 1 and 2 was achieved by reaction with excess tert-butylhydroperoxide (TBHP) in 1,2-dichloroethane at 55 ◦C. For complex 1, the (2-oxo)bis[dioxomolybdenum(VI)] hexamolybdate of composition [{MoO2(HC(3,5-Me2pz)3)}2(2- O)][Mo6O19] (3) was obtained in good yield, and its structure was determined by single crystal X-ray diffraction. The compound (4) obtained by oxidative decarbonylation of 2 was not unambiguously identified, but may be chemically analogous to 3. Compounds 1–4 were examined for the first time as homogeneous (pre)catalysts for the epoxidation of olefins with TBHP, using different types of cosolvents at 55 ◦C. During the catalytic reactions 1 and 2 transform in situ into 3 and 4, respectively, and the latter two are fairly stable catalysts. Catalytic tests and characterization studies of the recovered catalysts were carried out in an attempt to understand the kinetic differences observed between the compounds prepared in situ during the catalytic reaction and those prepared prior to the catalytic reaction, from the same precursor complex.