Browsing by Author "Mendes, Ricardo F."
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- 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.
- Host−guest complexes of cyclopentadienyl iron dicarbonyl (CpFe(CO)2) CO-releasing molecules with Cucurbit[7]urilPublication . Monteiro, Rodrigo P.; Calhau, Isabel B.; Gomes, Ana C.; Mendes, Ricardo F.; Paz, Filipe A. Almeida; D. Lopes, André; Silva, José Paulo da; Romão, Carlos C.; Gonçalves, Isabel S.; Pillinger, MartynIron(II) cyclopentadienyl carbonyl complexes are promising as CO-releasing molecules (CORMs) for therapeutic applications. In common with other metallodrugs, the practical application of Fe-CORMs may require their conjugation with biocompatible carriers to improve their bioavailability and protect them from premature degradation. Here, we show that the CO-releasing properties of the complexes [CpFe(CO)2Cl] (1) and [CpFe(CO)2CH2CONH2] (2) are retained when noncovalently encapsulated within cucurbit[7]uril (CB7), a well-established drug-enhancing excipient. The inclusion compounds were characterized in the solid-state by single-crystal and powder XRD, ATR-IR spectroscopy, Raman spectroscopy, TGA, and 13C{1H} CP MAS NMR. In the crystal structure of 2@CB7, there are two crystallographically independent [2@CB7] binary complexes that differ in the orientation of the guest molecules inside the CB cavity. High-resolution ESI-MS and 1H NMR studies verified the formation and stability of 1:1 2@CB7 inclusion complexes in an aqueous solution. In a physiological buffer, complex 2 is stable in the dark, but releases ca. 1.4 equiv of CO when irradiated with low-power cold white light, with a half-life (t 1/2) of 19.2 +/- 1.9 min. The photodecarbonylation behavior of the complexes is largely maintained in the inclusion compounds, with t 1/2 of 10.0 +/- 0.6 and 21.1 +/- 1.9 min for encapsulated 1 and 2.
- 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, structure and antileishmanial evaluation of endoperoxide–pyrazole hybridsPublication . Amado, Patrícia S. M.; Costa, Inês C. C.; Paixão, José A.; Mendes, Ricardo F.; Cortes, Sofia; Cristiano, Maria L.Leishmaniases are among the most impacting neglected tropical diseases. In attempts to repurpose antimalarial drugs or candidates, it was found that selected 1,2,4-trioxanes, 1,2,4,5-tetraoxanes, and pyrazole-containing chemotypes demonstrated activity against Leishmania parasites. This study reports the synthesis and structure of trioxolane–pyrazole (OZ1, OZ2) and tetraoxane–pyrazole (T1, T2) hybrids obtained from the reaction of 3(5)-aminopyrazole with endoperoxide-containing building blocks. Interestingly, only the endocyclic amine of 3(5)-aminopyrazole was found to act as nucleophile for amide coupling. However, the fate of the reaction was influenced by prototropic tautomerism of the pyrazole heterocycle, yielding 3- and 5-aminopyrazole containing hybrids which were characterized by different techniques, including X-ray crystallography. The compounds were evaluated for in vitro antileishmanial activity against promastigotes of L. tropica and L. infantum, and for cytotoxicity against THP-1 cells. Selected compounds were also evaluated against intramacrophage amastigote forms of L. infantum. Trioxolane–pyrazole hybrids OZ1 and OZ2 exhibited some activity against Leishmania promastigotes, while tetraoxane–pyrazole hybrids proved inactive, most likely due to solubility issues. Eight salt forms, specifically tosylate, mesylate, and hydrochloride salts, were then prepared to improve the solubility of the corresponding peroxide hybrids and were uniformly tested. Biological evaluations in promastigotes showed that the compound OZ1•HCl was the most active against both strains of Leishmania. Such finding was corroborated by the results obtained in assessments of the L. infantum amastigote susceptibility. It is noteworthy that the salt forms of the endoperoxide–pyrazole hybrids displayed a broader spectrum of action, showing activity in both strains of Leishmania. Our preliminary biological findings encourage further optimization of peroxide–pyrazole hybrids to identify a promising antileishmanial lead.