Controllable Synthesis, Characterization, and Catalytic Properties of

Article pubs.acs.org/crystal

Controllable Synthesis, Characterization, and Catalytic Properties of Three Inorganic−Organic Hybrid Copper Vanadates in the Highly Selective Oxidation of Sulfides and Alcohols Jikun Li,†,‡ Xianqiang Huang,† Song Yang,† Yanqing Xu,*,† and Changwen Hu*,† †

Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Key Laboratory of Photoelectronic/Electrophotonic, School of Chemistry, Beijing Institute of Technology, Beijing 100081, P. R. China ‡ College of Chemistry and Chemical Engineering, Taishan University, Tai’an 271021, Shandong P. R. China S Supporting Information *

ABSTRACT: Three novel inorganic−organic hybrid copper vanadates α[Cu(mIM)4]V2O6 (1), β-[Cu(mIM)4]V2O6 (2), and [Cu(mIM)2)](VO3)2 (3) (mIM = 1-methylimidazole) have been synthesized by rationally controlling of the hydrothermal conditions and fully characterized by singlecrystal XRD, powder XRD, elemental analyses, TGA, and FT-IR spectroscopy. Interestingly, compounds 1 and 2 were isolated as geometric isomers by tuning the solvothermal reaction temperature. Because of the different coordination modes between the tetradentate [V4O12]4− and [Cu(mIM)4]2+ subunits, the supramolecular structures of the two isomers show a 3D framework with an interpenetrating diamond topology for 1 and a 2D network for 2, respectively. Compound 3 was obtained by tuning the reaction temperature and the ratio of mIM/H2O, which showed interesting left- and right-handed helixes with an identical pitch of ca. 5.388 Å in its 2D network structure. As heterogeneous catalysts, compounds 1−3 exhibit excellent catalytic performance in the oxidation of sulfides with H2O2 as oxidant. Among them, the catalytic activity of 1 (conv. up to 98.7%, sele. up to 100%) outperforms others and can be reused without losing its activity. The activity of 1 is also investigated in the oxidation of various alcohols, and excellent results (conv. up to 98.5%, sele. up to 100%) are obtained.

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INTRODUCTION Inorganic−organic hybrid materials have recently become an area of great interest, especially those hybrid materials combining both metal-O clusters and organic ligands, not only because of their intriguing varieties of structures but also owing to their potential applications in many fields, such as catalysis, hydrogen storage, molecular adsorption, magnetism, and photochemistry.1−5 Polyoxometallates (POMs) are familiar inorganic building blocks for their prominent catalytic properties along with their chemical stability, including oxidative stability, thermal stability, and hydrolytic stability.6−16 As a subclass of POMs, vanadates are a promising candidate to construct POMs-based hybrid materials due to their unprecedented structural diversity, mixed valency, and variable coordination geometries.17 Hybrid vanadates with first-row transition metals show very rich structural diversities.18−25 Among them, V-O clusters exhibit a range of different dimensional structures due to various polyhedral fragments built of VOn units (n = 4, 5, 6).26 The structure of [VxOy]n− ranges from monomeric [VO4]3− units to polymeric [V2O7]4−, [V4O12]4−, and [V6O18]6−, etc., from [VO3]nn− chains to [VxOy]n− networks.27 Further, the hybrid vanadates are expected to exhibit improved properties and functions by the synergetic interaction between the metal− © XXXX American Chemical Society

organic subunits and the vanadium oxide. Therefore, the rationally controlling synthesis of hybrid vanadates is essential but remains challenging. Because of the enormous diversity, it is difficult to draw straight correlations between the obtained crystal structures of hybrid materials and the initial synthetic conditions. However, it is possible to partially control the synthesis of hybrid vanadates by adjusting the initial pH value, concentration, temperature, solvents, etc., for the formation of the VxOy is sensitive to these factors. For example, a recent research reported by the Arriortua group of the Ni(II)/Bpe/VxOy and Ni(II)/4,4′-Bpy/VxOy systems revealed the effect of the initial concentration and stoichiometry on the vanadium coordination environment and the vanadates subunits of the final 3D crystal structures.28 Their excellent work prompts us to explore the possibility to synthesize hybrid vanadates with potential applications by tuning the initial reaction conditions. It is well-known that vanadium-based materials are active heterogeneous catalysts for several reactions but remain unexplored to a great degree.29 To our knowledge, only a Received: January 20, 2015 Revised: February 22, 2015

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DOI: 10.1021/acs.cgd.5b00086 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Table 1. Crystal Data and Structure Refinement for Compounds 1−3 formula Mr crystal system space group temperature (K) a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalc.(g cm−3) F(000) R1 [I > 2σ(I)] wR2 [I > 2σ(I)] R1 (all data) wR2 (all data) GOOF CCDC No.

1

2

3

C16H24CuN8O6V2 589.85 tetragonal P4(2)/n 296(2) 13.106(3) 13.106(3) 13.634(6) 90 90 90 2341.8(13) 4 1.673 1196 0.0421 0.1193 0.0596 0.1278 1.055 932116

C16H24CuN8O6V2 589.85 monoclinic P2(1)/n 296(2) 10.8254(3) 16.3599(4) 12.9484(3) 90 95.073(1) 90 2484.21(1) 4 1.715 1196 0.0266 0.067 0.0331 0.0703 1.019 1008975

C8H12CuN4O6V2 425.64 monoclinic P2(1)/c 296(2) 7.7433(13) 5.3882(9) 17.039(3) 90 102.523(2) 90 694.0(2) 2 2.037 422 0.0273 0.1322 0.0330 0.1587 1.034 932117

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few researches focused on the catalytic properties of inorganic− organic hybrid vanadates in the field of the oxidation sulfides, the cyanosilylation reaction of aldehydes with trimethylsilyl cyanide, the decomposition of methylene blue, splitting of water, and degradation of pollutants.29 The hybrid vanadates used in these catalytic reactions are the family of M(HAep)2(VO3)4 (M = Co, Ni, Cu; Aep = 1-(2-aminoethl)piperazine) compounds,30 [{CoNi(H2O)2(Bpe)2}(V4O12)]·4H2O·Bpe,31 [{Ag(Bpy)}4 (V4 O12 )]·2H2O, [{Ag(Dpa)} 4(V4O12)]·4H2O and {Ag4(2-Pzc)2}(V2O6) (Bpy = 4,4′-bipyridine; Dpa = 1,2bis(4-pyridyl)-ethane; Pzc = pyrazinecarboxylate),32 [Ag(Bbi)][{Ag(Bbi)} 4 {Ag 3 (V 4 O 12 ) 2 }]·2H 2 O (Bbi = 1,4-bis(Nimidazolyl)butane),33 and Mn(Bpy)(V4O11)(Bpy).34 From the above-mentioned point of view, there is much room for expanding the catalytic application of inorganic−organic hybrid vanadates. Herein, to continue our solvothermal synthesis of polyoxovanadates (POVs),35 we select 1-methylimidazole (mIM) as ligand/solvent and POV precursors to synthesize the inorganic−organic hybrid vanadates. Fortunately, by the rational controlling of the hydrothermal conditions, three novel inorganic−organic hybrid copper vanadates α-[Cu(mIM)4]V2O6 (1), β-[Cu(mIM)4]V2O6 (2), and [Cu(mIM)2](VO3)2 (3) were synthesized and fully characterized by singlecrystal X-ray diffraction (SXRD), powder X-ray diffraction (PXRD), elemental analyses, TGA, and FT-IR spectroscopy. Among them, α-[Cu(mIM)4]V2O6 (1) and β-[Cu(mIM)4]V2O6 (2) were isolated as a pair of isomers showing 3D interpenetrating frameworks and 2D network structures, respectively. Further, the catalytic activity of 1 as heterogeneous catalyst was probed in the oxidation of various sulfides and alcohols using H2O2 as oxidant. The hybrid copper vanadate 1 showed high activity and selectivity in the oxidation reactions toward corresponding sulfoxides and aldehydes. To our knowledge, it is the first time that inorganic−organic hybrid vanadate was used as catalyst in the conversion of alcohols to corresponding aldehydes.

EXPERIMENTAL SECTION

Materials and Methods. All reagents and solvents were commercially obtained and used without further purification. All syntheses were carried out in 20 mL Teflon-lined stainless steel containers under autogenous pressure. The mixed reactants were stirred at room temperature for 120 min before heating. The metal content of the compounds 1−3 was measured by inductively coupled plasma (ICP) on a JY-ULTIMA2 analyzer. The FT-IR spectra were recorded from KBr pellets in the range of 4000−400 cm−1 on a Nicolet 170 SXFT/IR spectrometer. The PXRD of all samples was collected on a Bruker D8 X-ray diffractometer equipped with graphite monochromatized Cu Kα radiation (λ = 0.15418 Å). Elemental (C, H, and N) analyses were performed on a PerkinElmer 2400 II analyzer. The TGA was performed on a DTG-60AH instrument under a N2 atmosphere with a heating rate of 10 °C/min. After the catalytic reaction was completed, the resulting mixture was analyzed by GC-MS and GC using naphthalene as internal standard substrate. The GC analyses were performed on a Shimadzu GC-2014C with an FID detector equipped with an Rtx-1701 Sil capillary column. The GC-MS spectra were recorded on an Agilent 7890A-5975C at an ionization voltage of 1200 V. Atomic absorption analysis was measured by an inductively coupled plasma spectrometer (ICP) on an ICP-6000 analyzer. Synthesis of α-[Cu(mIM)4]V2O6 (1). A mixture of CuCl2·2H2O (0.1705 g, 1.0 mmol), NH4VO3 (0.1404 g, 1.2 mmol), mIM (2 mL), and H2O (12 mL) was heated at about 160 °C for 72 h. After the mixture cooled to room temperature, dark blue crystals were isolated from the mixture. Yield: 58% (based on NH4VO3); Anal. Calcd (found) for C16H24N8CuV2O6 (%): C, 32.58 (32.35); H, 4.10 (4.32); N, 19.00 (18.75); Cu, 10.77 (10.55); V, 17.27 (17.02). Synthesis of β-[Cu(mIM)4]V2O6 (2). A mixture of CuCl2·2H2O (0.1705 g, 1.0 mmol), NH4VO3 (0.1404 g, 1.2 mmol), mIM (2 mL), and H2O (12 mL) was heated at about 120 °C for 72 h. After the mixture cooled to room temperature, dark blue crystals were isolated from the mixture. Yield: 55% (based on NH4VO3); Anal. Calcd (found) for C16H24N8CuV2O6 (%): C, 32.58 (32.37); H, 4.10 (4.35); N, 19.00 (18.78); Cu, 10.77 (10.62); V, 17.27 (17.12). Synthesis of [Cu(mIM)2](VO3)2 (3). A mixture of CuCl2·2H2O (0.1705 g, 1.0 mmol), NH4VO3 (0.1404 g, 1.2 mmol), mIM (1 mL), and H2O (13 mL) was heated at 120 °C for 72 h. After the mixture cooled to room temperature, pale blue block crystals were isolated from the mixture. Yield: 65% (based on NH4VO3); Anal. Calcd B



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Scheme 1. Controlling Syntheses of Compounds 1−3

Figure 1. (a) The 1D zigzag chain structure in 1. (b) Polyhedral view of the 2D network in 1. (c) The 3D framework of 1. (d) The interpenetrating diamond topological structure of 1. Color code: [V4O12] cluster, green; [CuN4O2] octahedron, sky blue. (found) for C8H12N4CuV2O6 (%): C, 22.57 (22.32); H, 2.84 (4.98); N, 13.16 (12.95); Cu, 14.93 (14.73); V, 23.94 (23.85). General Procedure for Compound 1 Catalyzed Oxidation of Sulfides and Alcohols. Sulfides oxidation: Compound 1 (5.0 mg, 8.48 μmol), sulfide (0.25 mmol), H2O2 (9.4 mg, 0.275 mmol), and methanol (1 mL) were added to a glass tube; then the catalytic reaction was performed on a Wattecs Parallel Reactor at 40 °C for 4 h. Alcohols oxidation: Compound 1 (6.0 mg, 10.17 μmol), alcohol (0.25 mmol), H2O2 (34.1 mg, 1 mmol), and chlorobenzene (1 mL) were added to a glass tube; then the catalytic reaction was performed on a Wattecs Parallel Reactor at 120 °C for 8 h. After the reaction was completed, the resulting mixture was analyzed by GC-MS and GC. Crystal Determination. Crystal data for all compounds were collected at 298(2) K on a Bruker APEX−II CCD detector with graphite monochromatic Mo Kα radiation (λ = 0.71073 Å). Crystals were mounted on a glass fiber and coated with oil. All absorption corrections were applied using a multiscan technique. The reflections collected were integrated and scaled using the APEX 2 software package.36,37 All structures were solved by the direct method and refined by full-matrix least-squares on F2 using the SHELXTL program package (Bruker).36,37 All non-hydrogen atoms were located by direct methods and were refined anisotropically. All hydrogen atoms were placed at calculated position and refined as riding models. The crystallographic data have been deposited with the Cambridge Crystallographic Data Centre (CCDC) as CCDC-932116 (1), CCDC-1008975 (2), and CCDC-932117 (3). The detailed crystallo-

graphic data are summarized in Table 1. For the selected bond lengths and bond angles, please see Table S1 (Supporting Information).

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RESULTS AND DISCUSSION Syntheses. As vanadates can exhibit different dimensional structures, in order to explore the control toward the forming of the various architectures of hybrid vanadates with potential applications, we chose the simple vanadium source NH4VO3 as the precursors to react with CuCl2·2H2O and mIM under hydrothermal/solvothermal conditions. By controlling the reaction temperature, compounds α-[Cu(mIM)4]V2O6 (1) and β-[Cu(mIM)4]V2O6 (2) with isomeric structures were isolated, respectively, as shown in Scheme 1. Interestingly, the different coordination modes between [Cu(mIM)4]2+ and [V4O12]4− subunits lead to the 3D interpenetration framework structure of 1 and the 2D network structure of 2. Since mIM may serve as coordination ligand, organic base and solvent, compound [Cu(mIM)2](VO3)2 (3) with a 2D network structure is formed by the linkage of [VO3]nn− chains and [Cu(mIM)2]2+ subunits, which can be prepared by reducing the ratio of mIM and H2O to 1/13 with other synthetic conditions remaining unchanged with 2. Therefore, the ratio of mIM/H2O is an important factor in the synthetic procedure of compounds 2 and 3. C



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Structural Description. Bond valence sum calculations show that all vanadium atoms are in +V oxidation states, and copper atoms are in +II oxidation states in 1−3 (Table S2, Supporting Information). This result is consistent with structural analyses and charge balance. Structure Description of Compound 1. The SXRD analysis reveals that 1 consists of one Cu2+ cation, one [V2O6]2− anion cluster, and mIM ligands (Scheme 1a). The Cu atom is six-coordination in an octahedral geometry, defined by four nitrogen atoms from mIM ligands and two oxygen atoms from two [VO4] tetrahedra. In the [V2O6]2− cluster, there is only one crystallographically independent V atom. The V atom possesses a distorted tetrahedron achieved by a terminal oxygen atom, three bridging oxygen atoms from two [VO4] tetrahedral, and one [CuN4O2] octahedron. Compound 1 is a novel hybrid compound based on POVs that displays an interesting 3D interpenetrating framework. Such an intriguing structure can be described in the following three steps: first, the adjacent [VO4] tetrahedra are connected by the corner-sharing oxygen atoms to form [V4O12]4− clusters; then two alternate [VO4] tetrahedra of each [V4O12]4− cluster are linked by two Cu[(mIM)4]2+ subunits, giving rise to a 1D zigzag chain structure along the crystallographic c-axis (Figure 1a). The [V4O12]4− cluster in 1 serves as a tetradentate ligand, and in the second step, each [V4O12]4− donates its third [VO4] tetrahedra to coordinate to a Cu[(mIM)4]2+ subunit by one terminal O atom to connect the adjacent 1D zigzag chains into a 2D network parallel to the [111] plane (Figure 1b). At last, as shown in Figure 1c, a 3D interpenetrating-framework is formed by the coordination of the forth [VO4] tetrahedra of each [V4O12]4− to other Cu[(mIM)4]2+ ions. In order to better understand the structure of compound 1, the topological analysis of the 3D framework has been performed by considering each [V4O12]4− cluster as a 4-connected node; thus the structure displays 2-fold interpenetrating diamond topology with a Schläfli symbol of {66} (Figure 1d). For the hybrid vanadates, the use of bridging ligands allows the combination of the vanadium oxide subunits with secondary metal−organic subnets to give rise to various dimensional crystal architectures.29 Especially, the hybrid vanadates with 3D frameworks are usually the extension of the 1D or 2D {M(L)n}mz+ structures with the connection of the bridging ligands. For example, most of the reported [V4O12]4− containing inorganic−organic hybrid vanadates with 3D frameworks are formed by the combination of [V4O12]4− and the secondary metal−ligand subunit.29 In 1, however, the 3D interpenetrating framework was constructed by the mere inorganic skeleton [Cu2V4O12]n without the participation of the bridging ligands. The mIM here only serves as a chelating ligand to complete the coordination environment of the Cu2+ center and stabilize the crystal structure of 1. Such a type of 3D interpenetrating framework is rare in the [V4O12]4− containing inorganic−organic hybrid vanadates. Structure Description of Compound 2. Interestingly, by lowering the hydrothermal temperature with other conditions remaining unchanged, compound 2 (Scheme 1b) was isolated as a geometric isomer of compound 1. Although 1 and 2 have the same chemical formula, the coordination mode between the [Cu(mIM)4]2+ cations and the [V4O12]4− anions is very different. As in 1, the [V4O12]4− subunits in 2 can also be regarded as a tetradentate ligand and each [VO4] tetrahedra coordinates with a Cu2+ ion via one of the terminal O atoms. However, the connectivity patterns of the [Cu4N16V4O12]

skeleton in 1 and 2 are different. In order to distinguish them, hereafter the [Cu4N16V4O12] skeletons in 1 and 2 are labeled as α1-[Cu4N16V4O12] and β1-[Cu4N16V4O12], respectively (Figure 2). In the α1-[Cu4N16V4O12] skeleton, the alternate [VO4]

Figure 2. Coordination environment of [V4O12] subunits in 1 (α1[Cu4N16V4O12]) and 2 (β1-[Cu4N16V4O12]).

tetrahedra of the [V4O12]4− subunit coordinate to two [Cu(mIM)4]2+ ions by the terminal O atoms on the same side of the pseudo-plane formed by the [V4O12]4−. In the β1[Cu4N16V4O12] skeleton, however, the alternate [VO4] tetrahedra coordinate to two [Cu(mIM)4]2+ ions on the different side of the [V4O12]4− pseudo-plane. It is worth mentioning that the formation of the two isomers (compounds 1 and 2) can be controlled by the hydrothermal reaction temperature only. In addition, the different coordination modes between the [V4O12]4− subunit and the [Cu(mIM)4]2+ ions result in the different supramolecular structures; only a 2D network is formed, as shown in Figure 3, instead of the 3D framework of 1.

Figure 3. Polyhedral view of the 2D network in β-[Cu(mIM)4]V2O6 (2).

Structure Description of Compound 3. When we reduce the ratio of mIM and H2O (ligand/solvent) and keep other synthetic conditions the same with 2, compound 3 was isolated when the reaction mixture was down to room temperature. According to the SXRD and element analysis, the structural formula of compound 3 is [Cu(mIM)2](VO3)2, in which there is only one crystallographically independent vanadium atom and it is in a tetrahedral coordination environment, as shown in D



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1, 805 and 665 cm−1 for 2, 836, 760, and 631 cm−1 for 3 can be assigned to the bridging V-O-Cu vibrations. Catalytic Activities of Hybrid Cu-vanadates. Selective oxidation of organic compounds, such as sulfides, alcohols, aromatic and aliphatic hydrocarbons, etc., is one of the most vital transformations for upgrading raw starting materials into high value added products. In these reactions, vanadium peroxides are known as very effective oxidants, where the terminal oxidant is normal, environmentally friendly hydrogen peroxide or alkylhydroperoxide. Because of the importance of sulfoxides as intermediates of biologic molecules,41 chiral auxiliaries,42 and oxo-transfer reagents,43,44 the high selective oxidation of sulfides to obtain sulfoxides is more desirable. Using hydrogen peroxide as an oxidant offers significant environmental and economic benefits over traditional stoichiometric oxidants. Our initial oxidation studies focused on the case of oxidation of sulfides, with the aim to understand the catalytic property of the Cu-vanadates (compounds 1−3) for developing a new oxidation system in organic chemistry. The oxidation of methyl phenyl sulfide was selected as a model reaction to evaluate the catalytic activities of compounds 1−3 (Scheme 2). Before the reaction, all catalysts were

Scheme 1c. Each [VO4] tetrahedra is linked to other two tetrahedra through two of their vertex, forming [VO3]nn− chains along the crystallographic b-axis. The adjacent [VO3]nn− chains were further connected together by [Cu(mIM)2]2+ subunits, giving rise to a 2D network, parallel to the [100] plane (Figure 4a). The Cu atom, residing in a square planar environment, is coordinated by two O atoms derived from two [VO3]nn− chains and two N atoms from the mIM ligands.

Figure 4. (a) The 2D network of 3. (b) Stereoview of the left- and right-handed helical chains in 3. Color code: V, green; O, red; [CuN2O2] square plane, pale blue.

Scheme 2. Oxidation of Methyl Phenyl Sulfide Using Cuvanadates as Catalysts under the Preliminary Optimization Conditions

Interestingly, compared with recently reported 2D vanadates based on V-O chains such as [Zn(pyim)]2V4O12,38 [Zn2(bbi)2(V4O12)],39 [{Cu(bipy)(en)}{Cu(bipy)(H2O)}{VO3}4]n,40 and M(HAep)2(VO3)4 (M = Co, Ni, Cu),30 the V-O chains in compound 3 show a pair of left- and right-handed helixes with an identical pitch of ca. 5.388 Å along the b-axis connected by [Cu(mIM)2]2+ subunits (Figure 4b). Powder XRD and TGA Analyses. To further study the purity and repeatability of all the crystals, the bulk products were crushed to obtain a fine powder suitable for powder XRD analysis. The powder XRD detection allowed us to confirm that the bulk powder was a single phase material. All peaks are perfectly indexed on the basis of the simulated parameters from single crystal analysis. As shown in Figure S1 (Supporting Information), the diffraction peak positions of the experimental powder XRD patterns of 1−3 are in agreement with the simulated patterns, which indicate that the structures of bulk powders are the same with those of single crystals and the phase purity is satisfactory. The TGA analyses show that all compounds are stable under 150 °C and meet the conditions of the catalytic reactions (Figure S2, Supporting Information). FT-IR Spectra. The IR spectra of compounds 1−3 display similar characteristic vibration patterns mainly resulting from the mIM ligands and vanadate groups. As shown in Figure S3 (Supporting Information), characteristic bands in the range of 1500−1600 cm−1 can be attributed to the CC and CN stretch modes of the mIM. Peaks associated with CC−H vibrational modes can be clearly observed at 3113 cm−1 for 1, 3115 cm−1 for 2, and 3135 cm−1 for 3, respectively. The characteristic absorption bands of the methyl organic group appear at 1100−1290 and 2700−2900 cm−1. Bands below 1000 cm−1 are principally assigned to inorganic groups vibrational modes and two types of oxygen atoms: Ot (terminal oxygen atoms), and Ob (bridging oxygen atoms). Specifically, the terminal VOt stretching bands appear at 923 cm−1 for 1, 937 cm−1 for 2, and 967 cm−1 for 3. Bands at 795 and 661 cm−1 for

activated by stirring them with H2O2 (mole ratio, ncatalyst/nH2O2 = 1:2) in methanol at 40 °C for 30 min. Then, they were separated from the solvent and dried for use. The methyl phenyl sulfide oxidation catalyzed by Cu-vanadates (mole ratio, ncatalyst/nsubstrate = 3.4%) was performed in methanol at 40 °C for 4 h. The results are illustrated in Table 2. As shown in Table 2, Table 2. Conversion of Methyl Phenyl Sulfide to Methyl Phenyl Sulfoxide with Different Catalystsa catalyst

conv. (%)

sele. (%)b

reaction system

1 2 3 [Cu(mIM)4(H2O)2]Cl2c [VIV2(mIM)8]VV4O14d

98.7 95.8 92.5 28.6 >99

100 100 98.5 100 95.5

heterogeneous heterogeneous heterogeneous homogeneous homogeneous

a

Reaction conditions: methyl phenyl sulfide (0.25 mmol, 1 equiv), catalyst (8.48 μmol, 3.4 mol %), H2O2 (1.1 equiv), methanol (1 mL), 40 °C, 4 h. bSelectivity to sulfoxides. The byproduct was sulfone. cCu content equivalent to 1. dV content equivalent to 1.

compounds 1−3 all can efficiently catalyze the oxidation of methyl phenyl sulfide to corresponding sulfoxide with the conversion of 98.7−92.5% and selectivity of 98.5−100%. Notably, compound 1 showed higher activity than the other two compounds under optimized conditions. Indeed, using compound 1 as catalyst, without any further optimization, the conversion of methyl phenyl sulfide reaches to 98.7% with methyl phenyl sulfoxide as the only product. Given the fact that no channels exist in compounds 1−3, the different catalytic E



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The generality of 1 for sulfoxidation was examined on various sulfides with different electronic and steric effects under optimized conditions. As shown in Table 4, no obvious

activity may be ascribed to the different supramolecular structures. As heterogeneous catalyst, compound 1 with a 3D interpenetrating framework may result in more exposure of the active sites than the 2D network structures in 2 and 3. To probe the role of each active center of compound 1 in the selective oxidation of methyl phenyl sulfide, we further studied the activity of [Cu(mIM)4(H2O)2]Cl245 (Figure S4) and [VIV2O2(mIM)8]VV4O1235 (Figure S5) (Supporting Information). As shown in Table 2, the low conversion of methyl phenyl sulfide (28.6%) and prominent selectivity toward sulfoxide (100%) were achieved when using [Cu(mIM)4(H2O)2]Cl2 as catalyst, and using [VIV2O2(mIM)8]VV4O12 as catalyst; however, the conversion of methyl phenyl sulfide reached to 99% with barely satisfactory selectivity (95.5%). From the above results, we can conclude that the combination of [Cu(mIM)4]2+ and the vanadate anion cluster can produce the heterogeneous catalyst (Cu-vanadates) with satisfactory conversion and excellent selectivity in the catalytic oxidation of methyl phenyl sulfide (Table 2). Further, we speculate that the cation part in compounds 1−3 possibly contributes to enhance the selectivity and the anion part contributes to increase the conversion of the substrates.46−50 Generally speaking, the cooperation between cations and anions in all Cu-vanadates plays some synergistic role in the catalytic sulfonation process, which can improve the catalytic performance. Reutilization is one of the greatest advantages of heterogeneous catalysts and can also provide useful information about the catalyst stability along the catalytic cycle. For the excellent performance, compound 1 is selected to examine the long-term stability in a heterogeneous system. After the completion of the reaction, the catalyst can easily be separated from the reaction mixture by filtration and further reused directly in the subsequent oxidation reactions. The catalyst was recycled for at least 3 runs and still maintained its activity and selectivity (Table 3). The IR spectra (Figure S6) and PXRD (Figure S7)

Table 4. Results of Selective Oxidation of Various Sulfides Catalyzed by 1 Using H2O2 as Oxidanta

a Reaction conditions: sulfide (0.25 mmol, 1 equiv), catalyst (8.48 μmol, 3.4 mol %), H2O2 (1.1 equiv), methanol (1 mL), 40 °C, 4 h. b Selectivity to sulfoxides. The byproduct was sulfone. cReaction time, 10 h. dHydrolysis products of thiol and alcohol were found and confirmed by GC-MS. eUsing urea hydrogen peroxide (UHP) instead of H2O2 aqueous solution as oxidant.

Table 3. Reutilization Data for Oxidation of Methyl Phenyl Sulfide with H2O2 over Catalyst 1a cycle

conv. (%)

sele. (%)

1 2 3

98.7 96.5 96.2

100 100 100

decrease of the catalytic activity is observed when electron donor or acceptor groups with less steric hindrance are introduced in the monophenyl sulfides (Table 4, entries 2−4). Regarding sulfoxidation of diphenylsulfide and dibenzyl sulfide, due to their large steric hindrance, a drastic decrease of the catalytic activity is observed with excellent selectivity to the sulfoxides (Table 4, entries 5, 6). It was found that the prolonged reaction time could not efficiently increase the conversion of diphenylsulfide and dibenzyl sulfide, but sacrificed the selectivity to sulfoxide (Table 4, entries 5, 6). When the aqueous solution of H2O2 was used as the oxidant of dipropyl sulfide, the hydrolysis of the substrate was observed and the selectivity of dipropyl sulfoxide was lowered to 56.8%. Therefore, we use urea hydrogen peroxide (UHP) instead of H2O2, and dipropyl sulfide was converted to dipropyl sulfoxide with high selectivity (Table 4, entry 7). The excellent stability, high activity and selectivity for the transformation of sulfides to sulfoxides of catalyst 1 prompted us to explore the selective oxidation of other organic compounds. Here, alcohols were selected to investigate the selective oxidation properties using 1 as catalyst. The exploratory experiments started using benzyl alcohol as the model substrate. Under optimized conditions, benzyl alcohol

a

Conversion and selectivity toward the sulfoxide formation results under the optimized reaction conditions.

(Supporting Information) of 1 after the catalytic reactions indicate that no significant changes were shown by comparing with those of the freshly obtained catalysts. To check the heterogeneity of 1, a hot filtration experiment (separation of catalyst at the reaction temperature) was carried out. When 1 was filtrated off after 30 min (conv. 38.5%, sele. of sulfoxide 100%) and the supernatant was then allowed to react under the same conditions, after 3.5 h, a slight increase in the conversion, up to 45.8%, was detected, which was obviously lower than in the presence of 1 (98.7%) (Figure S8, Supporting Information). The results further indicate that 1 has good stability during the oxidation reaction. The heterogeneity of the reaction was also probed by a leaching test with 1 after the third reaction cycle. Atomic absorption analysis of the liquid phase after separation of the catalyst showed that there was no leaching of the V ions from the catalyst to the reaction mixture (S9, Supporting Information). F



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selectivity in the oxidation of sulfides. Specifically, compound 1 can convert sulfides to corresponding sulfoxides efficiently and can be reused by filtration without loss of its activity. The catalytic reaction can also be expanded to the oxidation of various alcohols with high efficiency using compound 1 as catalyst, and this is the first time that inorganic−organic hybrid vanadates are used as catalysts in oxidation of these substrates. The rational controlling synthesis of other hybrid vanadates and the use of them in other catalytic reactions are in progress.

can be efficiently oxidized to the only product benzaldehyde by H2O2 with high conversion of 98.5% (Table 5). Table 5. Results of Selective Oxidation of Various Alcohols Catalyzed by 1 Using H2O2 as Oxidanta

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data in CIF format, PXRD for simulated and as-synthesized samples, IR spectra, TG curves, bond valence sum calculations, kinetics of the catalytic reaction, and atomic absorption analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (Y.X.). *E-mail: [email protected] Fax: (+)86 10 68912631. Tel: (+)86 10 68912631 (C.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21173021, 21231002, 21271025, 21276026), 973 Program (2014CB932103), the 111 Project (B07012), the Beijing Higher Education Youth Elite Teacher Project, and the Natural Science Foundation of Shandong Province (ZR2013BL012).

a

Reaction conditions: alcohol (0.25 mmol, 1 equiv), catalyst (10.17 μmol, 4.1 mol %), H2O2 (4 equiv), chlorobenzene (1 mL), 120 °C, 8 h. bSelectivity to aldehydes. The byproducts were corresponding carboxylic acids.

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REFERENCES

(1) Dhakshinamoorthy, A.; Alvaro, M.; Corma, A.; García, H. Dalton Trans. 2011, 40, 6344−6360. (2) Jiang, H. L.; Xu, Q. Chem.Commun. 2011, 47, 3351−3370. (3) Meek, S. T.; Grathouse, J. A.; Allenford, M. Adv. Mater. 2011, 23, 249−267. (4) Liu, Y.; Xuan, W.; Cui, Y. Adv. Mater. 2010, 22, 4112−4135. (5) Rocha, J.; Carlos, L. D.; Paz, F. A. A.; Ananias, D. Chem. Soc. Rev. 2011, 40, 926−940. (6) Dolbecq, A.; Dumas, E.; Mayer, C. R.; Mialane, P. Chem. Rev. 2010, 110, 6009−6048. (7) Kozhevnikov, I. V. Chem. Rev. 1998, 98, 171−198. (8) Hirano, T.; Uehara, K.; Kamata, K.; Mizuno, N. J. Am. Chem. Soc. 2012, 134, 6425−6433. (9) Derat, E.; Kumar, D.; Neumann, R.; Shaik, S. Inorg. Chem. 2006, 45, 8655−8663. (10) Maksimchuk, N. V.; Kovalenko, K. A.; Arzumanov, S. S.; Chesalov, Y. A.; Melgunov, M. S.; Stepanov, A. G.; Fedin, V. P.; Kholdeeva, O. A. Inorg. Chem. 2010, 49, 2920−2930. (11) Han, Q. X.; He, C.; Zhao, M.; Qi, B.; Niu, J. Y.; Duan, C. Y. J. Am. Chem. Soc. 2013, 135, 10186−10189. (12) Kortz, U.; Muller, A.; Slageren, J. V.; Schnack, J.; Dalal, N. S.; Dressel, M. Coord. Chem. Rev. 2009, 253, 2315−2327. (13) Oms, O.; Dolbecq, A.; Mialane, P. Chem. Soc. Rev. 2012, 41, 7497−7536. (14) Miras, H. N.; Yan, J.; Long, D. L.; Cronin, L. Chem. Soc. Rev. 2012, 41, 7403−7430. (15) Zheng, S. T.; Yang, G. Y. Chem. Soc. Rev. 2012, 41, 7623−7646. (16) Long, D. L.; Tsunashima, R.; Cronin, L. Angew. Chem., Int. Ed. 2010, 49, 1736−1758.

In order to explore the applicability of the procedure for a selective oxidation of alcohols to corresponding aldehydes, various aromatic alcohols were investigated according to the protocol (Table 5). It is noteworthy that, without any further optimization, the oxidation of salicylol using 1 proceeded smoothly with a conversion of 97.2% and the satisfactory selectivity (98.8%) for salicylic aldehyde. Under the same reaction conditions, when either electron donating or electron withdrawing groups were introduced onto the aromatic ring of the benzyl alcohol, an obvious decrease of the catalytic activity was observed with conversions between 68.7% and 56.7% (Table 5, entries 3−7) due to the steric hindrance. All the reactions show high selectivity (>95%) toward corresponding aldehydes.

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CONCLUSIONS In summary, we successfully prepared three novel inorganic− organic hybrid copper vanadates, α-[Cu(mIM)4]V2O6 (1), β[Cu(mIM)4]V2O6 (2), and [Cu(mIM)2)](VO3)2 (3), by rationally controlling the hydrothermal synthetic conditions. Among them, 1 and 2 are isolated as a pair of interesting isomers. The different final crystal structures indicate that the reaction temperature and the ratio of mIM/H2O are crucial factors in the synthetic procedure of compounds 1−3. Importantly, all hybrid vanadates show high activity and G



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(17) Hagrman, P. J.; Finn, R. C.; Zubieta, J. Solid State Sci. 2001, 3, 745−774. (18) Kiebach, R.; Näther, C.; Kögerler, P.; Bensch, W. Dalton Trans. 2007, 3221−3233. (19) Wutkowski, A.; Näther, C.; Kögerler, P.; Bensch, W. Inorg. Chem. 2008, 47, 1916−1918. (20) Tong, Y. P.; Luo, G. T.; Zhou, W.; Ng, S. W. Inorg. Chem. Commun. 2010, 13, 1281−1284. (21) Shi, Z.; Feng, S. H.; Zhang, L. R.; Yang, G. Y.; Hua, J. Chem. Mater. 2000, 12, 2930−2935. (22) Khan, M. I.; Nome, R. C.; Deb, S.; McNeely, J. H.; Cage, B.; Doedens, R. J. Cryst. Growth Des. 2009, 9, 2848−2852. (23) de Luis, R. F.; Urtiaga, M. K.; Mesa, J. L.; Vidal, K.; Lezama, L.; Rojo, T.; Arriortua, M. I. Chem. Mater. 2010, 22, 5543−5553. (24) Breen, J. M.; Zhang, L.; Clement, R.; Schmitt, W. Inorg. Chem. 2012, 51, 19−21. (25) de Luis, R. F.; Mesa, J. L.; Urtiage, M. K.; Larrea, E. S.; Rojo, T.; Arriortua, M. I. Inorg. Chem. 2012, 51, 2130−2139. (26) Livage, J. Coord. Chem. Rev. 1998, 178, 999−1018. (27) Muller, A.; Peters, F.; Pope, M. T.; Gatteschi, D. Chem. Rev. 1998, 98, 239−271. (28) de Luis, R. F.; Mesa, J. L.; Urtiage, M. K.; Rojo, T.; Arriortua, M. I. Eur. J. Inorg. Chem. 2009, 2009, 4786−4794. (29) de Luis, R. F.; Orive, J.; Larrea, E. S.; Urtiage, M. K.; Arriortua, M. I. CrystEngComm 2014, 16, 10332−10366. (30) Larrea, E. S.; Mesa, J. L.; Pizarro, J. L.; Iglesias, M.; Rojo, T.; Arriortua, M. I. Dalton Trans. 2011, 40, 12690−12698. (31) de Luis, R. F.; Urtiage, M. K.; Mesa, J. L.; Larrea, E. S.; Rojo, T.; Arriortua, M. I. Inorg. Chem. 2013, 52, 2615−2626. (32) Lin, H.; Maggard, P. A. Inorg. Chem. 2008, 47, 8044−8052. (33) Hu, Y.; Luo, F.; Dong, F. Chem. Commun. 2011, 47, 761−763. (34) Luo, L.; Maggard, P. A. Cryst. Growth Des. 2013, 13, 5282− 5288. (35) Chen, B.; Huang, X.; Wang, B.; Lin, Z.; Hu, J.; Chi, Y.; Hu, C. Chem.Eur. J. 2013, 19, 4408−4413. (36) Sheldrick, G. M. SHELXS-97: Program for Crystal Structure Solution; Göttingen University: Göttingen, Germany, 1997. (37) Sheldrick, G. M. SHELXL-97: Program for Crystal Structure Refinement; Göttingen University: Göttingen, Germany, 1997. (38) Hou, W.; Guo, J.; Xu, X.; Wang, Z.; Zhang, D.; Wan, H.; Song, Y.; Zhu, D.; Xu, Y. Dalton Trans. 2014, 43, 865−871. (39) Qin, J. S.; Du, D. Y.; Li, S. L.; Lan, Y. Q.; Shao, K. Z.; Su, Z. M. CrystEngComm 2011, 13, 779−786. (40) Venegas-Yazigi, D.; Brown, K. A.; Vega, A.; Calvo, R.; Aliaga, C.; Santana, R. C.; Cardoso-Gil, R.; Kniep, R.; Schnelle, W.; Spodine, E. Inorg. Chem. 2011, 50, 11461−11471. (41) Bentley, R. Chem. Soc. Rev. 2005, 34, 609−624. (42) Fernández, I.; Khiar, N. Chem. Rev. 2003, 103, 3651−3705. (43) Khenkin, A. M.; Neumann, R. J. Am. Chem. Soc. 2002, 124, 4198−4199. (44) Figueras, F.; Palomeque, J.; Loridant, S.; Fèche, C.; Essayem, N.; Gelbard, G. J. Catal. 2004, 226, 25−31. (45) Clegg, W. Acta Crystallogr. 1988, C44, 453−461. (46) Neumann, R.; Khenkin, A. M. Chem. Commun. 2006, 2529− 2538. (47) Kirillova, M. V.; Kuznetsov, M. L.; Reis, P. M.; da Silva, J. A. L.; da Silva, J. J. R.; Pombeiro, A. J. L. J. Am. Chem. Soc. 2007, 129, 10531−10545. (48) Kirillova, M. V.; Kuznetsov, M. L.; Kozlov, Y. N.; Shul’pina, L. S.; Kitaygorodskiy, A.; Pombeiro, A. J. L.; Shul’pin, G. B. ACS Catal. 2011, 1, 1511−1520. (49) Kirillova, M. V.; Kuznetsov, M. L.; Romakh, V. B.; Shul’pina, L. S.; da Silva, J. J. R. F.; Pombeiro, A. J. L.; Shul’pin, G. B. J. Catal. 2009, 267, 140−157. (50) Mizuno, N.; Kamata, K. Coord. Chem. Rev. 2011, 255, 2358− 2370.

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