Synthesis and Structural Elucidation of Triazolylmolybdenum(VI) Oxide Hybrids and Their Behavior as Oxidation Catalysts

When the Study of the Post-Synthetic Modification Method on a 1D Spin Crossover Coordination Polymer Highlights its Catalytic Activity

We are interested in studying the catalytic activity of the spin crossover (SCO) complex ([Fe(NH2trz)3](NO3)2). In this work, we demonstrate that, by adapting the experimental conditions, we can switch from a quantitative post-synthetic modification (PSM) reaction to the use of this complex as a catalyst for the formation of imine from 4-amino-1,2,4-triazole. During the catalytic reaction, the iron complex undergoes two different PSM reactions: the first is the action of the aldehyde on the NH2 groups present on the complex, whereas the second PSM reaction occurs between the imine complex and aminotriazole, leading back to the starting complex. These two PSM reactions are at least partially involved in the catalytic mechanism. Furthermore, the combination of these two PSM reactions enables us to modulate the particle size and shape of the final amine complex without altering its excellent SCO properties. This result is of interest in the field of heterogeneous catalysis, where particle size has a strong influence on the catalytic activity, and for the proper integration in devices for different applications.

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-Olefins

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.

Synthesis and structural characterization of a new heterometallicmolybdate coordination polymer based on a µ3-bridging amino alcohol

The reaction of Na2MoO4·2H2O with 2-amino-2-(hydroxymethyl)propane-1,3-diol (LH) in water at room temperature results in the formation of the heterometallic coordination polymer [Mo2O6L2(Na2(H2O)4)]·2H2O 1 (L = 2-amino-3-hydroxy-2-(hydroxymethyl)propan-1-olato). The structure of 1 consists of a neutral (Mo2O6) unit located on an inversion center. The Mo atoms exhibit hexa-coordination and are bonded to two terminal and two bridging oxido ligands, an alkoxide oxygen and the amine N atoms of an anionic ligand L– resulting in the formation of an edge-sharing {Mo2O8N2} bioctahedron. The Na+ cations of a centrosymmetric bis(μ2-aqua)-bridged (Na2(H2O)4)2+ unit are penta-coordinated and bonded to two symmetry related L– ligands via the oxygen atoms of their OH groups. The µ3-bridging tetradentate binding mode of L– results in the formation of a two-dimensional heterometallic coordination polymer. The constituents of 1 viz. (Mo2O6), (L)–, (Na2(H2O)4)2+ and lattice water molecules are interlinked with the aid of three varieties of hydrogen bonding interactions. The corresponding tungstate reported recently has been obtained through a similar synthetic protocol and is isostructural.

Isolated molybdenum-based microporous POMs for selective adsorption of gases

Dodecanuclear, icosanuclear and octanuclear porous MOF-like POMs materials [MoV12O12(μ2-O)4(μ3-O)12(Htrz)4(trz)4]∙nH2O (n = 22, 1; n = 92, 2; Htrz = 1H-1,2,3-triazole), [MoV8O8(μ2-O)12(Htrz)8]½∙[MoV12O12(μ2-O)4(μ3-O)12(Htrz)4(trz)4]∙44H2O (3), and [MoV8O8(μ2-O)12(Htrz)8]·62H2O (4) have been obtained and well...

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 Olefins

There 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.

Heterogeneous catalysis with an organic–inorganic hybrid based on MoO3 chains decorated with 2,2′-biimidazole ligands

Polymeric [MoO₃(2,2′-biimidazole)]·H₂O outperforms other one-dimensional MoO₃-ligand hybrid materials as a heterogeneous and recyclable catalyst for (bio)olefin epoxidation and sulfoxidation.

New Coordination Polymers of Zinc(II), Copper(II) and Cadmium(II) with 1,3-Bis(1,2,4-triazol-4-yl)adamantane

The new coordination polymers (CPs) [Zn(tr2ad)Cl2]n, {[Cu(tr2ad)Cl]Cl∙4H2O}n, [Cd2(tr2ad)Cl4]n, {[Cu(tr2ad)(NO3)](NO3)}n and {[Cd(tr2ad)(NO3)](NO3)∙H2O}n were obtained in the form of air- and moisture-stable microcrystalline powders by the solvothermal reactions of zinc(II), copper(II) and cadmium(II) chlorides or nitrates with the ligand 1,3-bis(1,2,4-triazol-4-yl)adamantane (tr2ad). Investigation of the thermal behaviour assessed the thermal stability of these CPs, with [Cd2(tr2ad)Cl4]n starting to decompose only around 365 °C. As retrieved by powder X-ray diffraction, while [Zn(tr2ad)Cl2]n features 1-D chains along which the metal centre shows a tetrahedral geometry and the spacer is exo-bidentate, the other CPs contain 2-D double-layers in which the metal ions possess an octahedral stereochemistry and the linker is exo-tetradentate. A comparative structural analysis involving known coordination compounds containing the tr2ad ligand enabled us to disclose (i) the versatility of the ligand, as far as the coordination modes are concerned; (ii) the variability in crystal structure dimensionality, ranging from 1-D to 3-D; (iii) the fact that, to the best of our knowledge, [Zn(tr2ad)Cl2]n is the first ZnII-based CP containing the tr2ad spacer.

Simple Hybrids Based on Mo or W Oxides and Diamines: Structure Determination and Catalytic Properties

Abstract

Crystalline hybrid catalysts based on molybdenum or tungsten oxide and aliphatic diamines were synthesized via simple, eco-friendly reproducible methodologies, starting from commercially available and relatively inexpensive organic and inorganic precursors, and using water as solvent under mild conditions. The crystal structures of the obtained fine powdered solids were solved ab initio from powder X-ray diffraction data. The type of organic component (1,2-diaminoethane, 1,2-diaminopropane, 1,3-diaminopropane) may play a structure-directing role. On the other hand, different metals (M = Mo, W) may lead to isostructural one-dimensional hybrids of the type MO3(L) with the same bidentate diamine ligand L. The prepared catalysts were investigated for the liquid phase oxidation of saturated and unsaturated hydrocarbons (cis-cyclooctene, cyclooctane), using different types of oxidants (O2, H2O2, tert-butyl hydroperoxide). Differences in catalytic performances associated with distinct structural features were investigated.

Graphic Abstract

Solvothermally-derived MoO3-benzotriazole hybrid structures for nanocontainer depot systems

Layered MoO₃ containing intercalated benzotriazole (up to 40 mol%) shows much promise as a container system for corrosion protection.

A Molybdenum Trioxide Hybrid Decorated by 3-(1,2,4-Triazol-4-yl)adamantane-1-carboxylic Acid: A Promising Reaction-Induced Self-Separating (RISS) Catalyst

3-(1,2,4-Triazol-4-yl)adamantane-1-carboxylic acid (tradcH), a heterobifunctional organic ligand in which carboxylic acid and 1,2,4-triazole groups are united through a rigid 1,3-adamantanediyl spacer, was employed for the synthesis of a Mo^VI oxide organic hybrid. The ligand crystallized from water as tradcH·H₂O (1), possessing a two-dimensional hydrogen-bonding network, and from ethanol as a cyclic molecular solvate with the composition (tradcH)₃·2EtOH (2). Treatment of tradcH with MoO₃ under hydrothermal conditions afforded a new Mo trioxide hybrid, [MoO₃(tradcH)]·H₂O (3), which was structurally characterized. In 3, the molybdenum atoms form a polymeric zigzag chain of {μ₂-O-MoO₂}_n which is supported by double triazole bridges, while the carboxylic acid termini are left uncoordinated. The coordination environment of the Mo centers appears as distorted cis-{MoN₂O₄} octahedra. The hybrid exhibits high thermal stability (up to 270 °C) and was employed for a relatively broad scope of catalytic oxidation reactions in the liquid phase. Its catalytic behavior may be compared to a reversible mutation, featuring the best sides of homogeneous and heterogeneous catalysis. The original solid material converts into soluble active species, and the latter revert to the original material upon completion of the catalytic reaction, precipitating and allowing straightforward catalyst separation/reuse (like a heterogeneous catalyst). This catalyst was explored for a chemical reaction scope covering sulfoxidation, oxidative alcohol dehydrogenation, aldehyde oxidation, and olefin epoxidation, using hydrogen peroxide as an eco-friendly oxidant that gives water as a coproduct.

A Comparative Study of Molybdenum Carbonyl and Oxomolybdenum Derivatives Bearing 1,2,3-Triazole or 1,2,4-Triazoles in Catalytic Olefin Epoxidation

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.

On the Border between Low-Nuclearity and One-Dimensional Solids: A Unique Interplay of 1,2,4-Triazolyl-Based {CuII5(OH)2} Clusters and MoVI-Oxide Matrix

A pentanuclear Cu^II₅-hydroxo cluster possessing an unusual linear-shaped configuration was formed and crystallized under hydrothermal conditions as a result of the unique cooperation of bridging 1,2,4-triazole ligand ( trans-1,4-cyclohexanediyl-4,4'-bi(1,2,4-triazole) ( tr₂ cy)), Mo^VI-oxide, and CuSO₄. This structural motif can be rationalized by assuming in situ generation of {Cu₂Mo₆O₂₂}^4- anions, which represent heteroleptic derivatives of γ-type [Mo₈O₂₆]^4- further interlinked by [Cu₃(OH)₂]⁴⁺ cations through [ N- N] bridges. The framework structure of the resulting compound [Cu₅(OH)₂( tr₂ cy)₂Mo₆O₂₂]·6H₂O (1) is thus built up from neutral heterometallic {Cu₅(OH)₂Mo₆O₂₂} _n layers pillared with tetradentate tr₂ cy. Quantum-chemical calculations demonstrate that the exclusive site of the parent γ-[Mo₈O₂₆]^4- cluster into which Cu^II inserts corresponds with the site that has the lowest defect ("MoO₂ vacancy") formation energy, demonstrating how the local metal-polyoxomolybdate chemistry can express itself in the final crystal structure. Magnetic susceptibility measurements of 1 show strong antiferromagnetic coupling within the Cu₅ chain with exchange parameters J₁ = -500(40) K (-348(28) cm^-1), J₂ = -350(10) K (-243(7) cm^-1) and g = 2.32(2), χ² = 6.5 × 10^-4. Periodic quantum-chemical calculations reproduce the antiferromagnetic character of 1 and connect it with an effective ligand-mediated spin coupling mechanism that comes about from the favorable structural arrangement between the Cu centers and the OH^-, O^2-, and tr₂ cy bridging ligands.

High-yield synthesis and catalytic response of chainlike hybrid materials of the [(MoO3)m(2,2′-bipyridine)n] family

MoO₃/2,2′-bipyridine hybrids have been prepared (hydro)thermally from [MoO₃(2,2′-bipy)] and compared as catalysts in styrene oxide ethanolysis and olefin epoxidation.

Highly dispersed Mo-doped graphite carbon nitride: potential application as oxidation catalyst with hydrogen peroxide

A novel Mo-doped g-C₃N₄ catalyst exhibits high efficiency for selective oxidation of organic substrates with H₂O₂ or TBHP.

A series of organic–inorganic hybrid materials consisting of flexible organic amine modified polyoxomolybdates: synthesis, structures and properties

A series of hybrid materials consisting of Tris[(2-pyridyl)methyl]amine modified polyoxomolybdates were obtained and some complexes have efficient photocatalytic activities to degrade pararosaniline hydrochloride dye molecules.

Redetermination of the crystal structure of 3,5-di-methyl-pyrazolium β-octa-molybdate tetra-hydrate

The title compound, (C5H9N2)4[Mo8O26]·4H2O, was reported previously from a room-temperature data collection from which only the metal atoms could be refined anisotropically [FitzRoy et al. (1989 ▸). Inorg. Chim. Acta, 157, 187-194]. The current redetermination at 180 (2) K models all the non-H atoms with anisotropic displacement parameters and fully describes the supra-molecular N-H⋯O and O-H⋯O hydrogen-bonded network connecting the 3,5-di-methyl-pyrazolium cations, the water mol-ecules of crystallization and the β-octa-molybdate anion. All H atoms involved in the three-dimensional hydrogen-bonding network could be located from difference Fourier maps, with the exception of those of one disordered water mol-ecule, firstly seen in this structural report [refined over two distinct locations with site-occupancy factors of 0.65 (2) and 0.35 (2)]. The complete β-octa-molybdate anion is generated by a crystallographic inversion centre.