Hydrothermal Synthesis, Crystal Structure, and Catalytic Potential of a One-Dimensional Molybdenum Oxide/Bipyridinedicarboxylate Hybrid

The reaction of MoO₃, 2,2′-bipyridine-5,5-dicarboxylic acid (H₂bpdc), water, and dimethylformamide in the mole ratio 1:1:1730:130 at 150 °C for 3 days in a rotating Teflon-lined digestion bomb leads to the isolation of the molybdenum oxide/bipyridinedicarboxylate hybrid material (DMA)­[MoO₃(Hbpdc)]·<i>n</i>H₂O (<b>1</b>) (DMA = dimethylammonium). Compound <b>1</b> was characterized by scanning electron microscopy, FT-IR and ¹³C­{¹H} CP MAS NMR spectroscopies, and elemental and thermogravimetric analyses. The solid state structure of <b>1</b> was solved and refined through Rietveld analysis of high resolution synchrotron X-ray powder diffraction data in conjunction with information derived from the above techniques. The material, crystallizing in the noncentrosymmetric monoclinic space group <i>P</i>c, is composed of an anionic one-dimensional organic–inorganic hybrid polymer, _∞¹[MoO₃(Hbpdc)]⁻, formed by corner-sharing distorted {MoO₄N₂} octahedra, which cocrystallizes with charge-balancing DMA⁺ cations and one water molecule per metal center. In the crystal structure of <b>1</b>, the close packing of individual anionic polymers, DMA⁺ cations, and water molecules is mediated by a series of supramolecular contacts, namely strong (O–H···O, N⁺–H···O^–) and weak (C–H···O) hydrogen bonding interactions, and π–π contacts involving adjacent coordinated Hbpdc^– ligands. The catalytic potential of <b>1</b> was investigated in the epoxidation reactions of the bioderived olefins methyl oleate (Ole) and dl-limonene (Lim) using <i>tert</i>-butylhydroperoxide (TBHP) as the oxygen donor and 1,2-dichloroethane (DCE) or (trifluoromethyl)­benzene (BTF) as cosolvent, at 55 or 75 °C. Under these conditions, <b>1</b> acts as a source of active soluble species, leading to epoxide yields of up to 98% for methyl 9,10-epoxystearate (BTF, 75 °C, 100% conversion of Ole) and 89% for 1,2-epoxy-<i>p</i>-menth-8-ene (DCE, 55 °C, 95% conversion of Lim). Catalytic systems employing the ionic liquid 1-butyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide as solvent could be effectively recycled

Hydrothermal Synthesis, Crystal Structure, and Catalytic Potential of a One-Dimensional Molybdenum Oxide/Bipyridinedicarboxylate Hybrid

The reaction of MoO₃, 2,2′-bipyridine-5,5-dicarboxylic acid (H₂bpdc), water, and dimethylformamide in the mole ratio 1:1:1730:130 at 150 °C for 3 days in a rotating Teflon-lined digestion bomb leads to the isolation of the molybdenum oxide/bipyridinedicarboxylate hybrid material (DMA)­[MoO₃(Hbpdc)]·<i>n</i>H₂O (<b>1</b>) (DMA = dimethylammonium). Compound <b>1</b> was characterized by scanning electron microscopy, FT-IR and ¹³C­{¹H} CP MAS NMR spectroscopies, and elemental and thermogravimetric analyses. The solid state structure of <b>1</b> was solved and refined through Rietveld analysis of high resolution synchrotron X-ray powder diffraction data in conjunction with information derived from the above techniques. The material, crystallizing in the noncentrosymmetric monoclinic space group <i>P</i>c, is composed of an anionic one-dimensional organic–inorganic hybrid polymer, _∞¹[MoO₃(Hbpdc)]⁻, formed by corner-sharing distorted {MoO₄N₂} octahedra, which cocrystallizes with charge-balancing DMA⁺ cations and one water molecule per metal center. In the crystal structure of <b>1</b>, the close packing of individual anionic polymers, DMA⁺ cations, and water molecules is mediated by a series of supramolecular contacts, namely strong (O–H···O, N⁺–H···O^–) and weak (C–H···O) hydrogen bonding interactions, and π–π contacts involving adjacent coordinated Hbpdc^– ligands. The catalytic potential of <b>1</b> was investigated in the epoxidation reactions of the bioderived olefins methyl oleate (Ole) and dl-limonene (Lim) using <i>tert</i>-butylhydroperoxide (TBHP) as the oxygen donor and 1,2-dichloroethane (DCE) or (trifluoromethyl)­benzene (BTF) as cosolvent, at 55 or 75 °C. Under these conditions, <b>1</b> acts as a source of active soluble species, leading to epoxide yields of up to 98% for methyl 9,10-epoxystearate (BTF, 75 °C, 100% conversion of Ole) and 89% for 1,2-epoxy-<i>p</i>-menth-8-ene (DCE, 55 °C, 95% conversion of Lim). Catalytic systems employing the ionic liquid 1-butyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide as solvent could be effectively recycled

An Octanuclear Molybdenum(VI) Complex Containing Coordinatively Bound 4,4′-di-<i>tert</i>-Butyl-2,2′-Bipyridine, [Mo₈O₂₂(OH)₄(di-<i>t</i>Bu-bipy)₄]: Synthesis, Structure, and Catalytic Epoxidation of Bio-Derived Olefins

The reaction of [MoO₂Cl₂(di-<i>t</i>Bu-bipy)] (<b>1</b>) (di-<i>t</i>Bu-bipy = 4,4′-di-<i>tert</i>-butyl-2,2′-bipyridine) with water at 100–120 °C in a Teflon-lined stainless steel autoclave, in an open reflux system, or in a microwave synthesis system gave the octanuclear complex [Mo₈O₂₂(OH)₄(di-<i>t</i>Bu-bipy)₄] (<b>2</b>) as a microcrystalline powder in good yields. Single crystals of <b>2</b> suitable for X-ray diffraction were obtained by the reaction of MoO₃ and di-<i>t</i>Bu-bipy in water at 160 °C for 3 days. The molecular structure of <b>2</b> comprises a purely inorganic core, Mo₄O₈(μ₃-OH)₂(μ₂-O)₂, attached to two peripheral oxo-bridged binuclear units, Mo₂O₄(μ₂-O)₂(OH)­(di-<i>t</i>Bu-bipy)₂. The inorganic core is composed of a unique assembly of four {MoO₅} distorted square pyramids connected to each other <i>via</i> edge-sharing. Overall, the octanuclear complex adopts a highly distorted form strongly resembling an “S”-shaped molecular unit. Complex <b>2</b> was applied in the catalytic epoxidation of the biorenewable olefins DL-limonene (Lim) and methyl oleate (Ole), using <i>tert</i>-butylhydroperoxide (TBHP) as an oxygen donor, under mild reaction conditions (55 °C, air). The reactions of Lim and Ole gave the respective epoxide monomers in fairly high selectivities at high conversions (89% 1,2-epoxy-<i>p</i>-menth-8-ene selectivity at 96% Lim conversion; 99% methyl 9,10-epoxystearate selectivity at 94% Ole conversion, reached within 24 h reaction). Iodometric titrations revealed no measurable “non-productive” decomposition of TBHP

An Octanuclear Molybdenum(VI) Complex Containing Coordinatively Bound 4,4′-di-<i>tert</i>-Butyl-2,2′-Bipyridine, [Mo₈O₂₂(OH)₄(di-<i>t</i>Bu-bipy)₄]: Synthesis, Structure, and Catalytic Epoxidation of Bio-Derived Olefins

The reaction of [MoO₂Cl₂(di-<i>t</i>Bu-bipy)] (<b>1</b>) (di-<i>t</i>Bu-bipy = 4,4′-di-<i>tert</i>-butyl-2,2′-bipyridine) with water at 100–120 °C in a Teflon-lined stainless steel autoclave, in an open reflux system, or in a microwave synthesis system gave the octanuclear complex [Mo₈O₂₂(OH)₄(di-<i>t</i>Bu-bipy)₄] (<b>2</b>) as a microcrystalline powder in good yields. Single crystals of <b>2</b> suitable for X-ray diffraction were obtained by the reaction of MoO₃ and di-<i>t</i>Bu-bipy in water at 160 °C for 3 days. The molecular structure of <b>2</b> comprises a purely inorganic core, Mo₄O₈(μ₃-OH)₂(μ₂-O)₂, attached to two peripheral oxo-bridged binuclear units, Mo₂O₄(μ₂-O)₂(OH)­(di-<i>t</i>Bu-bipy)₂. The inorganic core is composed of a unique assembly of four {MoO₅} distorted square pyramids connected to each other <i>via</i> edge-sharing. Overall, the octanuclear complex adopts a highly distorted form strongly resembling an “S”-shaped molecular unit. Complex <b>2</b> was applied in the catalytic epoxidation of the biorenewable olefins DL-limonene (Lim) and methyl oleate (Ole), using <i>tert</i>-butylhydroperoxide (TBHP) as an oxygen donor, under mild reaction conditions (55 °C, air). The reactions of Lim and Ole gave the respective epoxide monomers in fairly high selectivities at high conversions (89% 1,2-epoxy-<i>p</i>-menth-8-ene selectivity at 96% Lim conversion; 99% methyl 9,10-epoxystearate selectivity at 94% Ole conversion, reached within 24 h reaction). Iodometric titrations revealed no measurable “non-productive” decomposition of TBHP

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

A large family of bifunctional 1,2,4-triazole molecular tectons (tr) has been explored for engineering molybdenum­(VI) oxide hybrid solids. Specifically, tr ligands bearing auxiliary basic or acidic groups were of the type amine, pyrazole, 1<i>H</i>-tetrazole, and 1,2,4-triazole. The organically templated molybdenum­(VI) oxide solids with the general compositions [MoO₃(tr)], [Mo₂O₆(tr)], and [Mo₂O₆(tr)­(H₂O)₂] were prepared under mild hydrothermal conditions or by refluxing in water. Their crystal structures consist of zigzag chains, ribbons, or helixes of alternating <i>cis</i>-{MoO₄N₂} or {MoO₅N} polyhedra stapled by short [N–N]-tr bridges that for bitriazole ligands convert the motifs into 2D or 3D frameworks. The high thermal (235–350 °C) and chemical stability observed for the materials makes them promising for catalytic applications. The molybdenum­(VI) oxide hybrids were successfully explored as versatile oxidation catalysts with <i>tert</i>-butyl hydroperoxide (TBHP) or aqueous H₂O₂ as an oxygen source, at 70 °C. Catalytic performances were influenced by the different acidic–basic properties and steric hindrances of coordinating organic ligands as well as the structural dimensionality of the hybrid

Synthesis, Structural Elucidation, and Application of a Pyrazolylpyridine–Molybdenum Oxide Composite as a Heterogeneous Catalyst for Olefin Epoxidation

The reaction of [MoO₂Cl₂(pypzEA)] (<b>1</b>) (pypzEA = ethyl­[3-(pyridin-2-yl)-1<i>H</i>-pyrazol-1-yl]­acetate) with water in a Teflon-lined stainless steel autoclave (100 °C) or in an open reflux system leads to the isolation of the molybdenum oxide/pyrazolylpyridine composite material [Mo₂O₆(HpypzA)] (<b>2</b>; HpypzA = [3-(pyridinium-2-yl)-1<i>H</i>-pyrazol-1-yl]­acetate). The solid state structure of <b>2</b> was solved through single crystal and powder X-ray diffraction analyses in conjunction with information derived from FT-IR and ¹³C CP MAS NMR spectroscopies and elemental analyses. In the asymmetric unit of <b>2</b>, two crystallographically distinct Mo⁶⁺ centers are bridged by a <i>syn</i>,<i>syn</i>-carboxylate group of HpypzA. The periodic repetition of these units along the <i>a</i> axis of the unit cell leads to the formation of a one-dimensional composite polymer, _∞¹[Mo₂O₆(HpypzA)]. The outstretched pyrazolylpyridine groups of adjacent polymers interdigitate to form a zipper-like motif, generating strong onset π–π contacts between adjacent rings of coordinated HpypzA molecules. The composite oxide <b>2</b> is a stable heterogeneous catalyst for liquid-phase olefin epoxidation

Triazolyl, Imidazolyl, and Carboxylic Acid Moieties in the Design of Molybdenum Trioxide Hybrids: Photophysical and Catalytic Behavior

Three organic ligands bearing 1,2,4-triazolyl donor moieties, (<i>S</i>)-4-(1-phenylpropyl)-1,2,4-triazole (<i>trethbz</i>), 4-(1,2,4-triazol-4-yl)­benzoic acid (<i>trPhCO</i>₂<i>H</i>), and 3-(1<i>H</i>-imidazol-4-yl)-2-(1,2,4-triazol-4-yl)­propionic acid (<i>trhis</i>), were prepared to evaluate their coordination behavior in the development of molybdenum­(VI) oxide organic hybrids. Four compounds, [Mo₂O₆(<i>trethbz</i>)₂]·H₂O (<b>1</b>), [Mo₄O₁₂(<i>trPhCO</i>₂<i>H</i>)₂]·0.5H₂O (<b>2a</b>), [Mo₄O₁₂(<i>trPhCO</i>₂<i>H</i>)₂]·H₂O (<b>2b</b>), and [Mo₈O₂₅(<i>trhis</i>)₂(<i>trhisH</i>)₂]·2H₂O (<b>3</b>), were synthesized and characterized. The monofunctional <i>tr</i>-ligand resulted in the formation of a zigzag chain [Mo₂O₆(<i>trethbz</i>)₂] built up from <i>cis-</i>{MoO₄N₂} octahedra united through common μ₂-O vertices. Employing the heterodonor ligand with <i>tr/–CO</i>₂<i>H</i> functions afforded either layer or ribbon structures of corner- or edge-sharing {MoO₅N} polyhedra (<b>2a</b> or <b>2b</b>) stapled by <i>tr</i>-links in axial positions, whereas −CO₂H groups remained uncoordinated. The presence of the <i>im-</i>heterocycle as an extra function in <i>trhis</i> facilitated formation of zwitterionic molecules with a protonated imidazolium group (<i>imH</i>^<i>+</i>) and a negatively charged −CO₂^– group, whereas the <i>tr-</i>fragment was left neutral. Under the acidic hydrothermal conditions used, the organic ligand binds to molybdenum atoms either through [N–N]-<i>tr</i> or through both [N–N]-<i>tr</i> and μ₂-CO₂^– units, which occur in protonated bidentate or zwitterionic tetradentate forms (<i>trhisH</i>^<i>+</i> and <i>trhis</i>, respectively). This leads to a new zigzag subtopological motif (<b>3</b>) of negatively charged polyoxomolybdate {Mo₈O₂₅}_<i>n</i>^2<i>n</i>– consisting of corner- and edge-sharing <i>cis-</i>{MoO₄N₂} and {MoO₆} octahedra, while the tetradentate zwitterrionic <i>trhis</i> species connect these chains into a 2D net. Electronic spectra of the compounds showed optical gaps consistent with semiconducting behavior. The compounds were investigated as epoxidation catalysts via the model reactions of achiral and prochiral olefins (<i>cis</i>-cyclooctene and <i>trans</i>-β-methylstyrene) with <i>tert</i>-butylhydroperoxide. The best-performing catalyst (<b>1</b>) was explored for the epoxidation of other olefins, including biomass-derived methyl oleate, methyl linoleate, and prochiral dl-limonene

Synthesis, Structural Elucidation, and Application of a Pyrazolylpyridine–Molybdenum Oxide Composite as a Heterogeneous Catalyst for Olefin Epoxidation

The reaction of [MoO₂Cl₂(pypzEA)] (<b>1</b>) (pypzEA = ethyl­[3-(pyridin-2-yl)-1<i>H</i>-pyrazol-1-yl]­acetate) with water in a Teflon-lined stainless steel autoclave (100 °C) or in an open reflux system leads to the isolation of the molybdenum oxide/pyrazolylpyridine composite material [Mo₂O₆(HpypzA)] (<b>2</b>; HpypzA = [3-(pyridinium-2-yl)-1<i>H</i>-pyrazol-1-yl]­acetate). The solid state structure of <b>2</b> was solved through single crystal and powder X-ray diffraction analyses in conjunction with information derived from FT-IR and ¹³C CP MAS NMR spectroscopies and elemental analyses. In the asymmetric unit of <b>2</b>, two crystallographically distinct Mo⁶⁺ centers are bridged by a <i>syn</i>,<i>syn</i>-carboxylate group of HpypzA. The periodic repetition of these units along the <i>a</i> axis of the unit cell leads to the formation of a one-dimensional composite polymer, _∞¹[Mo₂O₆(HpypzA)]. The outstretched pyrazolylpyridine groups of adjacent polymers interdigitate to form a zipper-like motif, generating strong onset π–π contacts between adjacent rings of coordinated HpypzA molecules. The composite oxide <b>2</b> is a stable heterogeneous catalyst for liquid-phase olefin epoxidation

Triazolyl–Based Copper–Molybdate Hybrids: From Composition Space Diagram to Magnetism and Catalytic Performance

The multicomponent mixed-metal Cu^II/Mo^VI oxides/1,3-bis­(1,2,4-triazol-4-yl)­adamantane (<i>tr</i>₂<i>ad</i>) system was thoroughly studied employing a compositional diagram approach. The concept allowed us to prepare three layered copper–molybdate hybrid solids [Cu^II₂(<i>tr</i>₂<i>ad</i>)₄]­(Mo₈O₂₆) (<b>1</b>), [Cu₄^II(μ₄-O)­(<i>tr</i>₂<i>ad</i>)₂(MoO₄)₃]·7.5H₂O (<b>2</b>), and [Cu^I₂(<i>tr</i>₂<i>ad</i>)₂]­(Mo₂O₇)·H₂O (<b>3</b>), and to elucidate the relationship between initial reagent concentration/stoichiometry and the stability of the resultant structural motifs. Compounds <b>1</b> and <b>2</b> were found to dominate throughout a wide crystallization range of the concentration triangle, whereas compound <b>3</b> was formed by redox processes in the narrow crystallization area having a high excess of Cu­(OAc)₂·H₂O. Independent experiments carried out with Cu­(OAc)₂ and (NH₄)₆Mo₇O₂₄ in the absence of <i>tr</i>₂<i>ad</i>, under the same conditions, revealed the formation of low-valent and bimetallic oxides, including Cu₂O, MoO₂, Cu­(Mo₃O₁₀)·H₂O, and Cu₃(MoO₄)₂(OH)₂. Compounds <b>1</b> and <b>2</b> show high thermal and chemical stability as examined as catalysts in the epoxidation of <i>cis</i>-cyclooctene and the oxidation of benzyl alcohol (BzOH) with different types of oxidants. The oxidation reaction of BzOH using <i>tert</i>-butyl hydroperoxide (TBHP) as the oxidant, in the presence of <b>1</b> or <b>2</b>, led to benzaldehyde and benzoic acid (PhCO₂H), with the latter being formed in up to 90% yield at 24 h. The results suggest that <b>1</b> and <b>2</b> may be favorable heterogeneous catalysts for the synthesis of PhCO₂H. Whereas compound <b>1</b> only reveals a weak ferromagnetic coupling between neighboring Cu^II centers (<i>J</i> = 0.41 cm^–1), compound <b>2</b> shows distinct intracluster antiferromagnetic exchange interactions (<i>J</i> = −29.9 cm^–1, <i>J</i>′ = −25.7 cm^–1), which consequently results in a diamagnetic ground state

Triazolyl–Based Copper–Molybdate Hybrids: From Composition Space Diagram to Magnetism and Catalytic Performance

The multicomponent mixed-metal Cu^II/Mo^VI oxides/1,3-bis­(1,2,4-triazol-4-yl)­adamantane (<i>tr</i>₂<i>ad</i>) system was thoroughly studied employing a compositional diagram approach. The concept allowed us to prepare three layered copper–molybdate hybrid solids [Cu^II₂(<i>tr</i>₂<i>ad</i>)₄]­(Mo₈O₂₆) (<b>1</b>), [Cu₄^II(μ₄-O)­(<i>tr</i>₂<i>ad</i>)₂(MoO₄)₃]·7.5H₂O (<b>2</b>), and [Cu^I₂(<i>tr</i>₂<i>ad</i>)₂]­(Mo₂O₇)·H₂O (<b>3</b>), and to elucidate the relationship between initial reagent concentration/stoichiometry and the stability of the resultant structural motifs. Compounds <b>1</b> and <b>2</b> were found to dominate throughout a wide crystallization range of the concentration triangle, whereas compound <b>3</b> was formed by redox processes in the narrow crystallization area having a high excess of Cu­(OAc)₂·H₂O. Independent experiments carried out with Cu­(OAc)₂ and (NH₄)₆Mo₇O₂₄ in the absence of <i>tr</i>₂<i>ad</i>, under the same conditions, revealed the formation of low-valent and bimetallic oxides, including Cu₂O, MoO₂, Cu­(Mo₃O₁₀)·H₂O, and Cu₃(MoO₄)₂(OH)₂. Compounds <b>1</b> and <b>2</b> show high thermal and chemical stability as examined as catalysts in the epoxidation of <i>cis</i>-cyclooctene and the oxidation of benzyl alcohol (BzOH) with different types of oxidants. The oxidation reaction of BzOH using <i>tert</i>-butyl hydroperoxide (TBHP) as the oxidant, in the presence of <b>1</b> or <b>2</b>, led to benzaldehyde and benzoic acid (PhCO₂H), with the latter being formed in up to 90% yield at 24 h. The results suggest that <b>1</b> and <b>2</b> may be favorable heterogeneous catalysts for the synthesis of PhCO₂H. Whereas compound <b>1</b> only reveals a weak ferromagnetic coupling between neighboring Cu^II centers (<i>J</i> = 0.41 cm^–1), compound <b>2</b> shows distinct intracluster antiferromagnetic exchange interactions (<i>J</i> = −29.9 cm^–1, <i>J</i>′ = −25.7 cm^–1), which consequently results in a diamagnetic ground state