Browsing by Author "Almeida Paz, Filipe A."
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- 4,4 '-Di-tert-butyl-2,2 '-bipyridinePublication . Amarante, Tatiana R.; Figueiredo, Sonia; Lopes, Andre D.; Gonçalves, Isabel S.; Almeida Paz, Filipe A.In the title compound, C18H24N2, the molecular unit adopts a trans conformation around the central C-C bond [N-C-C-N torsion angle of 179.2(3)degrees], with the two aromatic rings almost coplanar [dihedral angle of only 0.70 (4)degrees]. The crystal packing is driven by co-operative contacts involving weak C H center dot center dot center dot N and C-H center dot center dot center dot pi interactions, and also the need to fill effectively the available space.
- Dichlorodioxomolybdenum(VI) complexes bearing oxygen-donor ligands as olefin epoxidation catalystsPublication . Oliveira, Tânia S. M.; Gomes, Ana C.; Lopes, Andre D.; Lourenço, J. P.; Almeida Paz, Filipe A.; Pillinger, Martyn; Gonçalves, Isabel S.Treatment of the solvent adduct [MoO2Cl2(THF)2] with either 2 equivalents of N,N-dimethylbenzamide (DMB) or 1 equivalent of N,N'-diethyloxamide (DEO) gave the dioxomolybdenum(vi) complexes [MoO2Cl2(DMB)2] () and [MoO2Cl2(DEO)] (). The molecular structures of and were determined by single-crystal X-ray diffraction. Both complexes present a distorted octahedral geometry and adopt the cis-oxo, trans-Cl, cis-L configuration typical of complexes of the type [MoO2X2(L)n], with either the monodentate DMB or bidentate DEO oxygen-donor ligands occupying the equatorial positions trans to the oxo groups. The complexes were applied as homogeneous catalysts for the epoxidation of olefins, namely cis-cyclooctene (Cy), 1-octene, trans-2-octene, α-pinene and (R)-(+)-limonene, using tert-butylhydroperoxide (TBHP) as oxidant. In the epoxidation of Cy at 55 °C, the desired epoxide was the only product and turnover frequencies in the range of ca. 3150-3200 mol molMo(-1) h(-1) could be reached. The catalytic production of cyclooctene oxide was investigated in detail, varying either the reaction temperature or the cosolvent. Complexes and were also applied in liquid-liquid biphasic catalytic epoxidation reactions by using an ionic liquid of the type [C4mim][X] (C4mim = 1-n-butyl-3-methylimidazolium; X = NTf2, BF4 or PF6] as a solvent to immobilise the metal catalysts. Recycling for multiple catalytic runs was achieved without loss of activity.
- 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
- Tripyridinium cis-tetrachloridodioxidomolybdate(VI) chloridePublication . Fernandes, José A.; Gomes, Ana C.; Figueiredo, Sonia; Gago, Sandra; Ribeiro-Claro, Paulo J. A.; Gonçalves, Isabel S.; Almeida Paz, Filipe A.In the title compound, (C(5)H(6)N)(3)[MoCl(4)O(2)]Cl, the pyridinium cations are N-H center dot center dot center dot Cl hydrogen bonded to the anionic [MoCl(4)O(2)](2-) complexes and to the two crystallographically independent chloride anions (located on C2 axes). The Mo(6+) centre adopts a highly distorted octahedral geometry, being surrounded by four chloride and two terminal oxide groups. The oxide ligands are mutually cis.
- 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.