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Special Issue "Molybdenum-Catalyzed Oxidation Reactions"

A special issue of Molecules (ISSN 1420-3049). This special issue belongs to the section "Organometallic Chemistry".

Deadline for manuscript submissions: closed (15 March 2019)

Special Issue Editors

Guest Editor
Prof. Agustín Galindo del Pozo

Departamento de Química Inorgánica, Facultad de Química, Universidad de Sevilla, Aptdo 1203, 41071 Sevilla, Spain
Website | E-Mail
Interests: Coordination chemistry; Homogeneous catalysis; Green solvents; Oxidation reactions; Molybdenum; Vanadium; Copper; DFT.
Guest Editor
Prof. Francisco J. Montilla Ramos

Departamento de Química Inorgánica, Facultad de Química, Universidad de Sevilla, Aptdo 1203, 41071 Sevilla, Spain.
Website | E-Mail
Interests: Coordination chemistry; Homogeneous catalysis; Green solvents; Oxidation reactions; Molybdenum; Vanadium; Copper; DFT.

Special Issue Information

Dear Colleagues,

Homogeneous metal-catalyzed oxidation reactions play an important role in chemistry, and, over the last few decades, there has been an increasing demand for chemoselective, mild, and green processes. This fact has produced remarkable developments in the field and now a large number of metal complexes (based, amongst others, on rhenium, manganese, copper, vanadium, iron, etc., metals) are able to catalyze oxidation reactions of a broad spectrum of organic substrates. The substitution of classic stoichiometric oxidants for catalytic reactions employing more environmentally friendly oxidants (air, molecular oxygen, hydrogen peroxide, etc.) is now a requirement of modern oxidation chemistry. The choice of greener reactants is imposed for sustainable, economic and environmental strategies and thus the selected green oxidant should be compatible with the catalyst. In general, molybdenum complexes fulfill this requirement and their facile preparation, stability and low cost would make them attractive alternatives to less accessible complexes of rarer metals. This Special Issue will compile novel results on the use of molybdenum complexes as catalysts in oxidation reactions (epoxidation, sulfoxidation and others).

Prof. Agustín Galindo del Pozo
Prof. Francisco J. Montilla Ramos
Guest Editors

Manuscript Submission Information

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Keywords

  • Molybdenum
  • Homogeneous Catalysis
  • Oxidation
  • Epoxidation
  • Sulfoxidation
  • Green oxidants
  • Green Solvents

Published Papers (5 papers)

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Research

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Open AccessArticle Organic Salts and Merrifield Resin Supported [PM12O40]3− (M = Mo or W) as Catalysts for Adipic Acid Synthesis
Molecules 2019, 24(4), 783; https://doi.org/10.3390/molecules24040783
Received: 1 February 2019 / Revised: 15 February 2019 / Accepted: 15 February 2019 / Published: 21 February 2019
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Abstract
Adipic acid (AA) was obtained by catalyzed oxidation of cyclohexene, epoxycyclohexane, or cyclohexanediol under organic solvent-free conditions using aqueous hydrogen peroxide (30%) as an oxidizing agent and molybdenum- or tungsten-based Keggin polyoxometalates (POMs) surrounded by organic cations or ionically supported on functionalized Merrifield [...] Read more.
Adipic acid (AA) was obtained by catalyzed oxidation of cyclohexene, epoxycyclohexane, or cyclohexanediol under organic solvent-free conditions using aqueous hydrogen peroxide (30%) as an oxidizing agent and molybdenum- or tungsten-based Keggin polyoxometalates (POMs) surrounded by organic cations or ionically supported on functionalized Merrifield resins. Operating under these environmentally friendly, greener conditions and with low catalyst loading (0.025% for the molecular salts and 0.001–0.007% for the supported POMs), AA could be produced in interesting yields. Full article
(This article belongs to the Special Issue Molybdenum-Catalyzed Oxidation Reactions)
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Open AccessArticle New Molybdenum(II) Complexes with α-Diimine Ligands: Synthesis, Structure, and Catalytic Activity in Olefin Epoxidation
Molecules 2019, 24(3), 578; https://doi.org/10.3390/molecules24030578
Received: 9 January 2019 / Revised: 30 January 2019 / Accepted: 1 February 2019 / Published: 6 February 2019
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Abstract
Three new complexes [Mo(η3-C3H5)Br(CO)2{iPrN=C(R)C5H4N}], where R = H (IMP = N-isopropyl 2-iminomethylpyridine), Me, and Ph, were synthesized and characterized, and were fluxional in solution. The most interesting feature [...] Read more.
Three new complexes [Mo(η3-C3H5)Br(CO)2{iPrN=C(R)C5H4N}], where R = H (IMP = N-isopropyl 2-iminomethylpyridine), Me, and Ph, were synthesized and characterized, and were fluxional in solution. The most interesting feature was the presence, in the crystal structure of the IMP derivative, of the two main isomers (allyl and carbonyls exo), namely the equatorial isomer with the Br trans to the allyl and the equatorial with the Br trans to one carbonyl, the position trans to the allyl being occupied by the imine nitrogen atom. For the R = Me complex, the less common axial isomer was observed in the crystal. These complexes were immobilized in MCM-41 (MCM), following functionalization of the diimine ligands with Si(OEt)3, in order to study the catalytic activity in olefin epoxidation of similar complexes as homogeneous and heterogeneous catalysts. FTIR, 13C- and 29Si-NMR, elemental analysis, and adsorption isotherms showed that the complexes were covalently bound to the MCM walls. The epoxidation activity was very good in both catalysts for the cis-cyclooctene and cis-hex-3-en-1-ol, but modest for the other substrates tested, and no relevant differences were found between the complexes and the Mo-containing materials as catalysts. Full article
(This article belongs to the Special Issue Molybdenum-Catalyzed Oxidation Reactions)
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Open AccessArticle A Comparative Study of Molybdenum Carbonyl and Oxomolybdenum Derivatives Bearing 1,2,3-Triazole or 1,2,4-Triazole in Catalytic Olefin Epoxidation
Molecules 2019, 24(1), 105; https://doi.org/10.3390/molecules24010105
Received: 15 November 2018 / Revised: 21 December 2018 / Accepted: 23 December 2018 / Published: 28 December 2018
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Abstract
The molybdenum(0)-carbonyl-triazole complexes [Mo(CO)3(L)3] [L = 1,2,3-triazole (1,2,3-trz) or 1,2,4-triazole (1,2,4-trz)] have been prepared and examined as precursors to molybdenum(VI) oxide catalysts for the epoxidation of cis-cyclooctene. Reaction of the carbonyl complexes with the oxidant tert-butyl hydroperoxide [...] Read more.
The molybdenum(0)-carbonyl-triazole complexes [Mo(CO)3(L)3] [L = 1,2,3-triazole (1,2,3-trz) or 1,2,4-triazole (1,2,4-trz)] have been prepared and examined as precursors to molybdenum(VI) oxide catalysts for the epoxidation of cis-cyclooctene. Reaction of the carbonyl complexes with the oxidant tert-butyl hydroperoxide (TBHP) (either separately or in situ) gives oxomolybdenum(VI) hybrid materials that are proposed to possess one-dimensional polymeric structures in which adjacent oxo-bridged dioxomolybdenum(VI) moieties are further linked by bidentate bridging triazole (trz) ligands. A pronounced ligand influence on catalytic performance was found and the best result (quantitative epoxide yield within 1 h at 70 °C) was obtained with the 1,2,3-triazole oxomolybdenum(VI) hybrid. Both molybdenum oxide-triazole compounds displayed superior catalytic performance in comparison with the known hybrid materials [MoO3(trz)0.5], which have different structures based on organic-inorganic perovskite-like layers. With aqueous H2O2 as the oxidant instead of TBHP, all compounds were completely soluble and active. A pronounced ligand influence on catalytic performance was only found for the hybrids [MoO3(trz)0.5], and only the 1,2,4-trz compound displayed reaction-induced self-precipitation behavior. An insight into the type of solution species that may be involved in the catalytic processes with these compounds was obtained by separately treating [MoO3(1,2,4-trz)0.5] with excess H2O2, which led to the crystallization of the complex (NH4)1.8(H3O)0.2[Mo2O2(μ2-O)(O2)4(1,2,4-trz)]·H2O. The single-crystal X-ray investigation of this complex reveals an oxo-bridged dinuclear structure with oxodiperoxo groups being further linked by a single triazole bridge. Full article
(This article belongs to the Special Issue Molybdenum-Catalyzed Oxidation Reactions)
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Open AccessFeature PaperArticle Molybdenum-Catalyzed Enantioselective Sulfoxidation Controlled by a Nonclassical Hydrogen Bond between Coordinated Chiral Imidazolium-Based Dicarboxylate and Peroxido Ligands
Molecules 2018, 23(7), 1595; https://doi.org/10.3390/molecules23071595
Received: 22 May 2018 / Revised: 25 June 2018 / Accepted: 28 June 2018 / Published: 30 June 2018
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Abstract
Chiral alkyl aryl sulfoxides were obtained by molybdenum-catalyzed oxidation of alkyl aryl sulfides with hydrogen peroxide as oxidant in mild conditions with high yields and moderate enantioselectivities. The asymmetry is generated by the use of imidazolium-based dicarboxylic compounds, HLR. The in-situ-generated [...] Read more.
Chiral alkyl aryl sulfoxides were obtained by molybdenum-catalyzed oxidation of alkyl aryl sulfides with hydrogen peroxide as oxidant in mild conditions with high yields and moderate enantioselectivities. The asymmetry is generated by the use of imidazolium-based dicarboxylic compounds, HLR. The in-situ-generated catalyst, a mixture of aqueous [Mo(O)(O2)2(H2O)n] with HLR as chirality inductors, in the presence of [PPh4]Br, was identified as the anionic binuclear complex [PPh4]{[Mo(O)(O2)2(H2O)]2(μ-LR)}, according to spectroscopic data and Density Functional Theory (DFT) calculations. A nonclassical hydrogen bond between one C–H bond of the alkyl R group of coordinated (LR) and one oxygen atom of the peroxido ligand was identified as the interaction responsible for the asymmetry in the process. Additionally, the step that governs the enantioselectivity was theoretically analyzed by locating the transition states of the oxido-transfer to PhMeS of model complexes [Mo(O)(O2)2(H2O)(κ1-O-LR)] (R = H, iPr). The ∆∆G is ca. 0 kcal∙mol−1 for R = H, racemic sulfoxide, meanwhile for chiral species the ∆∆G of ca. 2 kcal∙mol−1 favors the formation of (R)-sulfoxide. Full article
(This article belongs to the Special Issue Molybdenum-Catalyzed Oxidation Reactions)
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Review

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Open AccessReview From the Eukaryotic Molybdenum Cofactor Biosynthesis to the Moonlighting Enzyme mARC
Molecules 2018, 23(12), 3287; https://doi.org/10.3390/molecules23123287
Received: 30 October 2018 / Revised: 23 November 2018 / Accepted: 5 December 2018 / Published: 11 December 2018
Cited by 1 | PDF Full-text (1908 KB) | HTML Full-text | XML Full-text
Abstract
All eukaryotic molybdenum (Mo) enzymes contain in their active site a Mo Cofactor (Moco), which is formed by a tricyclic pyranopterin with a dithiolene chelating the Mo atom. Here, the eukaryotic Moco biosynthetic pathway and the eukaryotic Moco enzymes are overviewed, including nitrate [...] Read more.
All eukaryotic molybdenum (Mo) enzymes contain in their active site a Mo Cofactor (Moco), which is formed by a tricyclic pyranopterin with a dithiolene chelating the Mo atom. Here, the eukaryotic Moco biosynthetic pathway and the eukaryotic Moco enzymes are overviewed, including nitrate reductase (NR), sulfite oxidase, xanthine oxidoreductase, aldehyde oxidase, and the last one discovered, the moonlighting enzyme mitochondrial Amidoxime Reducing Component (mARC). The mARC enzymes catalyze the reduction of hydroxylated compounds, mostly N-hydroxylated (NHC), but as well of nitrite to nitric oxide, a second messenger. mARC shows a broad spectrum of NHC as substrates, some are prodrugs containing an amidoxime structure, some are mutagens, such as 6-hydroxylaminepurine and some others, which most probably will be discovered soon. Interestingly, all known mARC need the reducing power supplied by different partners. For the NHC reduction, mARC uses cytochrome b5 and cytochrome b5 reductase, however for the nitrite reduction, plant mARC uses NR. Despite the functional importance of mARC enzymatic reactions, the structural mechanism of its Moco-mediated catalysis is starting to be revealed. We propose and compare the mARC catalytic mechanism of nitrite versus NHC reduction. By using the recently resolved structure of a prokaryotic MOSC enzyme, from the mARC protein family, we have modeled an in silico three-dimensional structure of a eukaryotic homologue. Full article
(This article belongs to the Special Issue Molybdenum-Catalyzed Oxidation Reactions)
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