English

• 0.   Introduction

  The inorganic-organic hybrid materials, which combine inorganic and organic constituents, have been studied and have a broad scope of applications in catalysis, magnetism, photochemistry, and biomedicine^[1-3]. Thus, the preparation of functionalized inorganic-organic hybrid materials is of great significance. The key procedure in the preparation of such combined materials is the selection of eligible organic and inorganic constituents. Polyoxometalates (POMs) are a fascinating class of inorganic cluster materials possessing enormous structures and multifaceted applications. Because of their excellent performances, POMs have been researched extensively as precursors for the construction of hybrid materials^[4-7]. Polyoxovanadates (POVs), as a special subclass of POMs, have aroused great interest owing to their electromagnetism, redox activity, and medical chemistry^[8-10]. Additionally, the V—O fragments possess the potency to bind transition metals to form functional hybrids using their terminal oxygen atoms. These hybrid materials have been considered as oxidation catalysts to catalyze a variety of organic substrates. The materials not merely overcome the troubles of the easy aggregation and hard recovery of POMs but also enhance the stabilization and recyclability of catalysts^[11-13].

  Olefins epoxidation is a kind of important industrial catalytic reaction, and its epoxidation products are a kind of important organic intermediates, that have important applications in the fine chemical industry, petrochemical industry, polymer materials, and pharmaceutical synthesis^[14-16]. In recent years, green chemistry has attracted great attention from researchers, hydrogen peroxide has high reactive oxygen content and its product is water, therefore, the reaction system using hydrogen peroxide as an oxidant has been widely studied. For the past few decades, varieties of POMs have been generally used as effective catalysts for the epoxidation of olefins. Up to now, many inorganic-organic hybrid POV materials have been applied to the selective oxidation of sulfides and alcohols and have shown efficient catalytic performance^[17-20]. However, the investigation of hybrid POVs for olefin epoxidation catalysis is still rare^[21-23]. In addition, based on previous literature reports, cobalt-containing compounds show excellent catalytic effect and high selectivity of epoxidation products in various olefin epoxidation reactions and are potential epoxidation catalysts^[24-26]. The combination of POVs and Co-complex may cooperatively interact giving rise to synergistic effects to enhance the catalytic activity.

  On considerations of the above content, to investigate the oxidation catalytic performance of POVs, we successfully synthesized an inorganic-organic hybrid cobalt vanadate, [Co(pIM)V₂O₆] (1) (pIM=2-(2-Pyridyl)imidazole), by reacting CoCl₂·6H₂O with NaVO₃ and pIM under hydrothermal conditions. The compound exhibited the 2D network composed of VO₄ tetrahedra and CoO₃N₂ square pyramid via both edge- and corner-sharing. As a catalyst for epoxidation, the conditions of epoxidation of olefin were optimized, and the reusability of the catalyst was also studied.

  1.   Experimental

  1.1   Materials and methods

  The chemicals for the experiments were commercially sourced and no additional purifying was performed. Elemental analyses of Co and V were confirmed by PLASMASPEC (I) ICP atomic emission spectrometer, and the contents of C, H, and N were analyzed by a PerkinElmer 2400 CHN elemental analyzer. Powder X-ray diffraction (PXRD) was implemented on a Rigaku D/MAX-3 instrument and the radiation of Cu Kα (λ=0.154 2 nm) at 298 K and X-ray 40 kV/30 mA over a 2θ range of 5°-50°. The Fourier transform infrared (IR) spectra were collected using KBr pellets on an Alpha Centaurt FTIR system, implementing from 4 000 cm^-1 to 400 cm^-1. Thermogravimetric (TG) analysis was determined with the Perkin-Elmer TGA7 apparatus with a heating speed of 10 ℃·min^-1 in an atmosphere of N₂. The catalytic reaction process was monitored and evaluated by the GC-2014 (Shimadzu) system with biphenyl as an internal standard substrate. The collection of magnetic susceptibility data was used a SQUID magnetometer (Quantum Design, MPMS-5) with an external magnetic field of 1 000 Oe and a temperature region of 2 to 300 K.

  1.2   Synthesis of compound 1

  CoCl₂·6H₂O (0.24 g, 1.0 mmol), NaVO₃ (0.12 g, 1.0 mmol) and pIM (0.15 g, 1.0 mmol) were added to 10 mL distilled water, and the reaction solution was adjusted to pH 4.2 with 1 mol·L^-1 HCl in the stirring process. The reaction solution was stirred for 15 min and then transferred to a 23 mL stainless reactor. The stainless reactor was placed in the oven at 170 ℃ for three days and then decreased to ambient temperature at a rate of 10 ℃·h^-1. Blocky crystals were collected by filtration, washing, and dried at ambient temperatures. Yield: 22.2% (V-based). Anal. Calcd.(%) for C₈H₇N₃ O₆CoV₂: C 24.3; H 1.7; N 10.6; Co 14.9; V 25.8; Found(%): C 23.9; H 1.8; N 10.9; Co 15.4; V 25.2.

  1.3   X-ray crystallography

  A regular block single crystal was selected to be wrapped with vaseline and encapsulated in a fine glass tube of appropriate size. Crystal data were obtained using a Bruker Smart-CCD diffractometer with monochromated Mo Kα radiation (λ=0.071 07 nm) at room temperature. Structure determination was fulfilled by direct methods using the SHELXS-2014 crystallographic program via the Olex 2 platform^[27], and succedent atom refinement was accomplished using full-matrix least-squares procedures. In the process of refinement, all the non-hydrogen atoms in the structure were refined anisotropically. The H atoms on the C and N atoms were arranged geometrically. Table 1 summarizes the crystallology information of 1 and its refinement results.

  Table 1

  

  Table 1.  Crystallographic data of 1 and corresponding structural refinements

  下载: 导出CSV

  Parameter	1		Parameter	1
  Formula	C8H7N3O6CoV2		Crystal size / mm	0.21×0.15×0.14
  Formula weight	401.98		Z	2
  Crystal system	Triclinic		Dc / (g·cm-3)	2.256
  Space group	P1		μ / mm-1	2.961
  a / nm	0.767 3(5)		θ range / (°)	2.16-25.00
  b / nm	0.847 7(5)		Reflection collected	4 349
  c / nm	1.034 3(5)		F(000)	394
  α / (°)	112.425(5)		Rint	0.015 7
  β / (°)	92.074(5)		GOF	1.053
  γ / (°)	105.599(5)		R1 [I > 2σ(I)]a	0.025 4
  V / nm3	0.591 7(6)		wR2 (all data)b	0.069 9
  ${ }^{\mathrm{a}} R_1=\sum\left\|F_{\mathrm{o}}|-| F_{\mathrm{c}}\right\| / \sum\left|F_{\mathrm{o}}\right|; { }^{\mathrm{b}} w R_2=\left\{\sum\left[w\left(F_{\mathrm{o}}{ }^2-F_{\mathrm{c}}{ }^2\right)^2\right] / \sum\left[w\left(F_{\mathrm{o}}{ }^2\right)^2\right]\right\}^{1 / 2}$.

  2.   Results and discussion

  2.1   Synthesis and structure

  X-ray single-crystal diffraction reveals the crystallization of 1 in the triclinic P1 space group. The structure contains a crystallographically independent Co²⁺, a [V₂O₆]^2- unit, and a pIM ligand. In this structure, Co²⁺ coordinates with three O atoms and two N atoms from the ligand to form a twisted CoO₃N₂ square pyramid configuration. There are two crystallographically different vanadium atoms: V1 and V2, both vanadiums adopt a distorted tetrahedral coordination pattern. Where, V1 coordinates with two bridging O atoms from two VO₄, one bridging O from CoO₃N₂ and the end O atoms of its own VO₄, V2 coordinates with two bridged O atoms from CoO₃N₂ and two bridged O atoms from VO₄ (Fig. 1a). The average bond length of V—O is 0.172 9 nm, and those of Co—O and Co—N is 0.199 2 and 0.213 2 nm. The valence states of V and Co are determined to be +5 and +2 respectively through bond-valence sum calculations. An interesting feature of the structure is that the VO₄ tetrahedra and the CoO₃N₂ tetragonal cone are connected by sharing O atoms to form a ternary ring system containing two five-membered rings and one six-membered ring (Fig. 1b). The five-membered ring includes four VO₄ tetrahedra and one CoO₃N₂ tetragonal cone, while the six-membered ring includes four VO₄ tetrahedra and two CoO₃N₂ tetragonal cones. These ternary rings are further pointed and coplanar to form a 2D layer network (Fig. 1c).

  Figure 1

  

  Figure 1.  (a) Coordination for the Co and V in 1; (b) Ternary ring system containing two five-membered rings and one six-membered ring; (c) 2D layer formed by ternary rings

  下载: 全尺寸图片 幻灯片

  2.2   IR spectra, PXRD and TG analysis

  The IR spectra of 1 were studied in a range of 4 000-400 cm^-1 using a KBr disc (Fig. 2a). The absorption peaks at 978 and 928 cm^-1 are attributed to the vibrations of ν_as(V—O—V) and the absorption peaks at 841 and 646 cm^-1 are assigned to the vibrations of ν_as(V—O—Co). The absorption peaks at 963, 882, and 835 cm^-1 belong to V=O_t (O_terminal) vibration, the region from 1 621 to 1 308 cm^-1 corresponds to the ligand C—C and C—N stretching vibration^[28-30]. To further check the repeatability and purity of the crystal, the recovered crystalline samples were crushed as a fine powder for PXRD analysis. Compared with the crystal structure, the experimental PXRD patterns of the samples were in good agreement with the crystal simulation results, indicating that the bulk powders were pure phase (Fig. 2b). The TG test of 1 was conducted in N₂ atmosphere at a heating of 10 ℃·min^-1. The TG curve exhibited a sustained weight loss of 34.2% (calculated value 33.8%) between 335 ℃ and 775 ℃, corresponding to the loss of pIM (Fig. 3).

  Figure 2

  

  Figure 2.  (a) IR spectra of 1 after each catalytic cycle; (b) PXRD patterns of recovered 1

  下载: 全尺寸图片 幻灯片

  Figure 3

  

  Figure 3.  TG curve of 1 measured from 30 to 800 ℃ under N₂ atmosphere

  下载: 全尺寸图片 幻灯片

  2.3   Epoxidation of olefins

  The catalytic epoxidation of olefin is affected by many factors, such as the dosage of the catalyst, temperature, reaction time, and the amount of oxidant. Therefore, we need to find the best reaction conditions to improve the conversion and selectivity of the product. Under gentle conditions, the olefins were oxidized in acetonitrile (CH₃CN) with 1 as a heterogeneous catalyst and H₂O₂ as an oxidant. An initiatory study on the oxidation of the cyclooctene to cyclooctane epoxide was selected to explore the catalytic activity of 1 in CH₃CN at 60 ℃. Under the above conditions, the dosage of oxidant and catalyst was determined through controlled experiments. As shown in Fig. 4a, the conversion of the epoxidation increased from the beginning 84.8% (0.01 mmol catalyst) to 98.6% (0.04 mmol catalyst). When the dosage of the catalyst added up to 0.07 mmol, the conversion remained nearly constant, suggesting that the appropriate dosage of the catalyst was only 0.04 mmol. Then, we examined the conversion for different dosages of oxidant. The conversion increased from 77.2% to 98.6% with the increase in the amount of oxidant from 0.5 mmol to 1.5 mmol (Fig. 4b), however, the conversion did not improve significantly with further increase of H₂O₂ dosage. According to the above test results, the most reasonable conditions for catalytic oxidation of cyclooctene are available 0.04 mmol catalyst and 1.5 mmol oxidant. So, we got optimum reaction conditions using 1 as the catalyst (0.04 mmol) and 30% H₂O₂ as the oxidant (1.5 mmol) in CH₃CN at 60 ℃ (Scheme 1). As reflected in Table 2, 1 could availably catalyze cyclooctene to cyclooctane epoxide with the conversion of 98.6% and selectivity of 99.2% after 8 h of reaction, which was comparable to the previously reported POVs-based hybrids, such as [Zn(pIM)₃]₂V₄O₁₂·H₂O, [Zn(ipIM)₃]₂V₄O₁₂, and [Co(eIM)₃]₂V₄O₁₂·H₂O^[23].

  Figure 4

  

  Figure 4.  Effect of (a) catalyst and (b) oxidant factors for cyclooctene oxidation; (c) Thermal filtration experiment; (d) Recycling of catalyst for oxidation of cyclooctene

  下载: 全尺寸图片 幻灯片

  Scheme 1

  

  Scheme 1.  Epoxidation of cyclooctene catalyzed by 1

  下载: 全尺寸图片 幻灯片

  Table 2

  

  Table 2.  Epoxidation of cyclooctene with different catalysts^a

  下载: 导出CSV

  Catalyst	Conversion / %	Selectivity / %	Reaction system
  1	98.6	99.2	Heterogeneous
  Without catalyst	6.9	94.6
  (n-Bu4N)4[V4O12]b	65.5	62.9	Homogeneous
  CoCl2·6H2Oc	23.2	91.7	Homogeneous
  a Reaction conditions: cyclooctene (1 mmol), 1 (0.04 mmol), H2O2 (1.5 mmol), CH3CN (5 mL), 60 ℃, 8 h; b V content equivalent to that of 1; c Co content equivalent to that of 1.

  To further explore the role of the Co-complex and vanadium-oxygen anion in the catalytic reaction, contrast experiments were carried out, and the CoCl₂·6H₂O, (n-Bu₄N)₄[V₄O₁₂] were also used as a catalyst to explore catalytic activity. When CoCl₂·6H₂O was used as a catalyst, the cyclooctene conversion was very low, while(n-Bu₄N)₄[V₄O₁₂] produced a different result: the cyclooctene conversion achieved 65.5% (Table 2). From the above results, we could conclude that the combination of Co²⁺ and V—O cluster by the complexation may cause a positive synergistic catalysis and significantly increase catalytic activity, this was similar to the previous report^[31]. Besides, the catalytic activity might be also related to the unsaturated coordination sites of the Co²⁺, which could interact with the substrate to facilitate chemical reactions. Again, when the reaction was carried out without catalyst, only 6.9% conversion was observed, which indicated that the catalyst was vital for the reaction. According to the above catalytic results and literature reports^[32-33], a possible epoxidation mechanism was suggested using 1 as a catalyst (Scheme 2). Primary, the coordinatively unsaturated Co²⁺ in the structure as Lewis acidic centers available activated the olefin substrate, which not only pi-electron delocalization to the metal center but also shortened the distance between substrate and the peroxovanadium groups, the four-coordinated V⁵⁺ simultaneously reacted with H₂O₂ to generate active peroxovanadium groups, then, the O atom in peroxovanadium nucleophilic attacked the olefin double bond forming the epoxidation products and the catalytic cycle completed.

  Scheme 2

  

  Scheme 2.  Proposed mechanism of catalytic epoxidation procedure

  下载: 全尺寸图片 幻灯片

  To support the heterogeneous nature of the catalyst, a hot filtering test was conducted during the cyclooctene epoxidation. The solid catalyst 1 was separated from the reaction system after 2 h of reaction, and the filter was kept reacting for another 6 h with this understanding. The obtained filter was monitored by gas chromatography (GC) analysis, and the conversion was almost immobile (38.2%), which was significantly lower than the value in the presence of 1 (Fig. 4c). The result confirmed the heterogeneous nature of the reaction system. Due to the excellent catalytic properties, 1 was chosen to test the cycling stability in heterogeneous systems. After the reaction was completed, the catalyst could be recovered easily from the reaction system through filtering and further reused in the subsequent epoxidation reaction. 1 could be recirculated at least four times without significant reduction in activity (Fig. 4d). The combination of IR and PXRD patterns (Fig. 2) before and after catalysis certified that the structure and crystallization remained unaltered after the circular reactions, which indicated excellent cycling stability of the catalysts.

  Subsequently, various olefins were selected to estimate the catalytic universality of the catalyst. As shown in Table 3, the catalytic action of cyclohexene was examined under the same conditions, the slightly lower conversion achieved 94.2% within 6 h (entry 2), and the epoxidation yield was also slightly lower than that of cyclooctene (entry 1). So, the catalyst exerted excellent activity on cycloolefins.

  Table 3

  

  Table 3.  Oxidation of various olefins catalyzed by 1 using H₂O₂ oxidant

  下载: 导出CSV

  Entry	Substrate	Product	Time / h	Conversion / %	Selectivity / %
  1			8	98.6	99.2
  2			6	94.2	96.8
  3			8	89.8	92.4
  4			10	91.1	89.6
  5			10	72.3	79.9
  6			9	86.1	78.6
  7			9	69.6	96.8
  8			6	89.5	90.3
  9			6	82.6	77.5
  10			7	67.8	71.6
  11			7	65.4	81.8

  However, for aromatic olefins, the effect of the catalyst was lower than that for cycloolefins. Under optimal contexts, the conversion of styrene was 89.8% with a narrowly satisfying selectivity of 92.4% for 8 h (entry 3). Middling catalytic activities for the oxidation of p-methylstyrene (conversion 91.1%, selectivity 89.6%) and o-methylstyrene (conversion 72.3%, selectivity 79.9%) (entries 4-5) were given after 10 h of catalytic reactions. Compared with styrene and p-methylstyrene, a relatively lower catalytic was observed for the electron-deficient p-chlorostyrene (conversion 86.1%, selectivity 78.6%) (entry 6) for 9 h. The catalytic activity of multi-substituted and large steric hindrance substances was also studied, trans-stilbene afforded obvious reduced activity with 69.6% conversion for 9 h, probably due to a larger steric resistance containing diphenyl groups (entry 7). The reaction of 2, 5-dimethylstyrene exhibited 89.5% conversion and 90.3% selectivity for 6 h (entry 8), compared with 2, 5-dimethylstyrene, 3, 4-dichlorostyrene resulted in a relatively lower reduced activity with 82.6% conversion and 77.5% selectivity at the same time (entry 9). As for aliphatic linear olefins, the 1-hexene was transformed into the corresponding epoxide with 67.8% conversion and 71.6% selectivity for 7 h, and the reaction of 1-octene afforded 65.4% conversion and 81.8% selectivity for 7 h (entries 10-11). The above results show that the nature of the substrates is an important element affecting the epoxidation, and the catalytic oxidation of the circular substrates is more effective than that of the aromatic and linear substrates during the epoxidation process^[34].

  2.4   Magnetic measurements

  The variable temperature magnetic susceptibility (χ_M) of 1 was conducted with a field intensity of 1 kOe. Fig. 5a showed the χ_MT plot against T at temperatures variable between 2-300 K. When T was 300 K, the χ_MT value was 2.27 emu·K·mol^-1, which was slightly higher than the theoretical spin-only value of 1.875 emu·K·mol^-1 for the high-spin d⁷ Co²⁺ ions (S=3/2, g=2.0), implying the existence of an orbital angular momentum contribution^[35-37]. Gradually lowered the temperature, the χ_MT value decreased gently to 2.12 emu·K·mol^-1 at 45 K, the curvilinear relationship between χ_MT and T implied the intramolecular antiferromagnetic coupling among the Co²⁺ centers. Then the χ_MT value dramatically descended to a minimum of 0.66 emu·K·mol^-1 at 2 K, this might be attributed to the integrated action of the magnetic anisotropy and spin-orbit coupling of Co²⁺ as well as the antiferromagnetic interactions^[38-40]. As displayed in Fig. 5b, the linear fitting of the χ_M^-1 vs T kept to the Curie-Weiss law between 300-10 K, with the Curie constant C of 2.26 emu·K·mol^-1 and the Weiss constant θ of -2.22 K, farther notarizing that the antiferromagnetic effect present in 1.

  Figure 5

  

  Figure 5.  (a) Temperature reliance of χ_M and χ_MT for 1; (b) Temperature reliance of χ_M^-1 for 1

  下载: 全尺寸图片 幻灯片

  3.   Conclusions

  A cobalt-vanadates architecture was hydrothermally prepared, containing cobalt nodes and V—O sheets. The compound was used as the catalyst for the olefins epoxidation and hydrogen peroxide was used as the oxidant. The catalytic results show that the compound has excellent epoxidation catalytic performance under optimized conditions, and can be recycled many times. The studies of other potential catalytic reactions using the compound were ongoing. Besides, magnetic measurements reveal the antiferromagnetical interactions between the Co²⁺ ions.

  
  Acknowledgments: This work was supported by the Basic Research Project Fund of Shanxi Province (Grant No.202203021222296), the Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (Grants No.2023L254, 2022L425), the Foundation of Shanxi Datong University (Grants No.2017-B-04, 2019-B-11, 2022Q24), the Key Research and Development Project of Datong (Grant No.2023003).
• 1. [1]

     Yang H, Dai K, Zhang J F, Dawson G. Inorganic-organic hybrid photocatalysts: Syntheses, mechanisms, and applications[J]. Chin. J. Catal., 2022, 43(8):  2111-2140. doi: 10.1016/S1872-2067(22)64096-8

  2. [2]

     Lu B B, Xing Z X, Bao Y S, Ye F, Fu Y. Selective luminescent sensing of teflubenzuron and oxyfluorfen by a resorcin[4]arene-based metal-organic framework[J]. Chem. Eng. J., 2023, 452:  139234. doi: 10.1016/j.cej.2022.139234

  3. [3]

     李季坤, 赵帅恒, 胡长文. 多酸基主客体框架材料POMs@MOFs(COFs)[J]. 无机化学学报, 2019,35,(11): 1934-1956. LI J K, ZHAO S H, HU C W. Polyoxometalate-based host-guest framework materials POMs@MOFs(COFs)[J]. Chinese J. Inorg. Chem., 2019, 35(11):  1934-1956.

  4. [4]

     Seemann K M, Bauer A, Kindervater J, Meyer M, Besson C, Luysberg M, Durkin P, Pyckhout-Hintzen W, Budisa N, Georgii R, Schneider C M, Kögerler P. Polyoxometalate-stabilized, water dispersible Fe₂Pt magnetic nanoparticles[J]. Nanoscale, 2013, 5(6):  2511-2519. doi: 10.1039/c3nr33374d

  5. [5]

     郭婷婷, 安燕燕, 赵丹, 闫娟枝. 多酸导向的间苯二酚杯[4]芳烃[Co₈]配位笼的组装及电化学性质[J]. 无机化学学报, 2023,39,(9): 1791-1799. GUO T T, AN Y Y, ZHAO D, YAN J Z. Polyoxometalate-directing calix[4]resorcinarene-based giant[Co₈]coordination cage: Self-assembly and electrochemical performance[J]. Chinese J. Inorg. Chem., 2023, 39(9):  1791-1799.

  6. [6]

     Peng J B, Zhang Q C, Kong X J, Ren Y P, Long L S, Huang R B, Zheng L S, Zheng Z P. A 48-metal cluster exhibiting a large magnetocaloric effect[J]. Angew. Chem. Int. Ed., 2011, 50(45):  10649-10652. doi: 10.1002/anie.201105147

  7. [7]

     Pradeep C P, Misdrahi M F, Li F Y, Zhang J, Xu L, Long D L, Liu T B, Cronin L. Synthesis of modular "inorganic-organic-inorganic" polyoxometalates and their assembly into vesicles[J]. Angew. Chem. Int. Ed., 2009, 48(44):  8309-8313. doi: 10.1002/anie.200903070

  8. [8]

     Li N, Yue C Y, Yang H, Song J, Li J W, Pan Q L, Jiang S, Zhao J G, Yu D. Oxidative desulfurization and magnetic properties of a mixed-valence cobalt vanadate[J]. Polyhedron, 2022, 226:  116077. doi: 10.1016/j.poly.2022.116077

  9. [9]

     Hu N, Du J, Ma Y Y, Cui W J, Yu B R, Han Z G, Li Y G. Unravelling the role of polyoxovanadates in electrocatalytic water oxidation reaction: Active species or precursors[J]. Appl. Surf. Sci., 2021, 540:  148306. doi: 10.1016/j.apsusc.2020.148306

  10. [10]

      应俊, 张宝月, 田爱香. 三核和四核金属-有机簇修饰的四个多酸化合物的结构、选择性光催化和汞离子传感性能[J]. 无机化学学报, 2020,36,(10): 1831-1844. YING J, ZHANG B Y, TIAN A X. Four keggin compounds modified by tri-and tetra-nuclear metal-organic clusters: Structures, selective photocatalytic and Hg²⁺ recognition characteristics[J]. Chinese J. Inorg. Chem., 2020, 36(10):  1831-1844.

  11. [11]

      Tian H R, Zhang Z, Liu S M, Dang T Y, Li Z, Lu Y, Liu S X. A highly stable polyoxovanadate-based Cu(Ⅰ)-MOF for the carboxylative cyclization of CO₂ with propargylic alcohols at room temperature[J]. Green Chem., 2020, 22(21):  7513-7520. doi: 10.1039/D0GC02812F

  12. [12]

      Huang X X, Zhou Z, Qin L, Zhang D P, Wang H N, Wang S N, Yang L. Structural regulation of two polyoxometalate-based metal-organic frameworks for the heterogeneous catalysis of quinazolinones[J]. Inorg. Chem., 2023, 62(14):  5565-5575. doi: 10.1021/acs.inorgchem.3c00055

  13. [13]

      Huo Y, Huo Z Y, Ma P T, Wang J P, Niu J Y. Polyoxotungstate incorporating organotriphosphonate ligands: Synthesis, characterization, and catalytic for alkene epoxidation[J]. Inorg. Chem., 2015, 54(2):  406-408. doi: 10.1021/ic502404m

  14. [14]

      Huber S, Cokoja M, Kuehn F E. Historical landmarks of the application of molecular transition metal catalysts for olefin epoxidation[J]. J. Organomet. Chem., 2014, 751:  25-32. doi: 10.1016/j.jorganchem.2013.07.016

  15. [15]

      Sen R, Saha D, Mal D, Brandão P, Rogez G, Lin Z. Synthesis, structural aspects and catalytic performance of a tetrahedral cobalt phosphonate framework[J]. Eur. J. Inorg. Chem., 2013, :  5020-5026.

  16. [16]

      Taghiyar H, Yadollahi B. New perspective to catalytic epoxidation of olefins by Keplerate containing Keggin polyoxometalates[J]. Polyhedron, 2018, 156:  98-104. doi: 10.1016/j.poly.2018.09.015

  17. [17]

      Xiao W R, Li S J, Zhao Y, Ma Y B, Li N, Zhang J, Chen X N. Multinuclear transition metal-containing polyoxometalates constructed from Nb/W mixed-addendum precursors: Synthesis, structures and catalytic performance[J]. Dalton Trans., 2021, 50(25):  8690-8695. doi: 10.1039/D1DT00924A

  18. [18]

      Zhang J L, Zhao Q X, Cheng M Y, Xuan W M, Liu Y. Polyoxovanadate-based metal-organic frameworks consisted of open vanadium sites for selective catalytic oxidation of sulfides[J]. Tungsten, 2023, 5(2):  261-269. doi: 10.1007/s42864-022-00199-6

  19. [19]

      Wang X, Zhang T, Li Y H, Lin J F, Li H, Wang X L. In situ ligand-transformation-involved synthesis of inorganic-organic hybrid polyoxovanadates as efficient heterogeneous catalysts for the selective oxidation of sulfides[J]. Inorg. Chem., 2020, 59(23):  17583-17590. doi: 10.1021/acs.inorgchem.0c02798

  20. [20]

      李季坤, 胡长文. 多钒氧簇化学研究进展[J]. 无机化学学报, 2015,31,(9): 1705-1725. LI J K, HU C W. Progress in polyoxovanadate chemistry[J]. Chinese J. Inorg. Chem., 2015, 31(9):  1705-1725.

  21. [21]

      Wang S, Liu Y W, Zhang Z, Li X H, Tian H R, Yan T T, Zhang X, Liu S M, Sun X W, Xu L, Luo F, Liu S X. One-step template-free fabrication of ultrathin mixed-valence polyoxovanadate-incorporated metal-organic framework nanosheets for highly efficient selective oxidation catalysis in air[J]. ACS Appl. Mater. Interfaces, 2019, 11(13):  12786-12796. doi: 10.1021/acsami.9b00908

  22. [22]

      Li J, Zhang D, Chi Y N, Hu C W. Catalytic application of polyoxovanadates in the selective oxidation of organic molecules[J]. Polyoxometalates, 2022, 1(2):  9140012. doi: 10.26599/POM.2022.9140012

  23. [23]

      Niu Y H, Yang S, Li J K, Xu Y Q, Hu C W. Design and synthesis of hybrid solids based on the tetravanadate core toward improved catalytic properties[J]. Chin. Chem. Lett., 2016, 27(5):  649-654. doi: 10.1016/j.cclet.2016.01.007

  24. [24]

      Patil M V, Yadav M K, Jasra R V. Catalytic epoxidation of α-pinene with molecular oxygen using cobalt(Ⅱ)-exchanged zeolite Y-based heterogeneous catalysts[J]. J. Mol. Catal. A-Chem., 2007, 277(1/2):  72-80.

  25. [25]

      Tonigold M, Lu Y, Mavrandonakis A, Plus A, Staudt R, Möllmer J, Sauer J, Volkmer D. Pyrazolate-based cobalt(Ⅱ)-containing metal-organic frameworks in heterogeneous catalytic oxidation reactions: Elucidating the role of entatic states for biomimetic oxidation processes[J]. Chem.-Eur. J., 2011, 17(31):  8671-8695. doi: 10.1002/chem.201003173

  26. [26]

      Zhou W L, Peng J, Zhang Z Y, Shi Z Y, Khana S U, Liu H S. Assembly of hybrids based on polyoxotungstates and Co-tris (imidazolyl) complexes with bifunctional electrocatalytic activities[J]. RSC Adv., 2015, 5(45):  35753-35759. doi: 10.1039/C5RA01165E

  27. [27]

      Dolomanov O V, Bourhis L J, Gildea R J, Howard J A K, Puschmann H. OLEX2: A complete structure solution, refinement and analysis program[J]. J. Appl. Crystallogr., 2009, 42(2):  339-341. doi: 10.1107/S0021889808042726

  28. [28]

      Hu Y, Luo F, Dong F F. Design synthesis and photocatalytic activity of a novel lilac-like silver-vanadate hybrid solid based on dicyclic rings of[V₄O₁₂]^4- with {Ag₇}⁷⁺ cluster[J]. Chem. Commun., 2011, 47(2):  761-763. doi: 10.1039/C0CC02965C

  29. [29]

      Wu S J, Yang X H, Hu J F, Ma H W, Lin Z G, Hu C W. Synthesis, structure and characterization of three different dimension inorganic-organic hybrid vanadates: [Co₂(mIM)₅(H₂O)₂]V₄O₁₂, [Ni₂(mIM)₇(H₂O)]V₄O₁₂·H₂O and[Cd(eIM)₂(H₂O)]V₂O₆[J]. CrystEngComm, 2015, 17(7):  1625-1630. doi: 10.1039/C4CE02335H

  30. [30]

      Larrea E S, Mesa J L, Pizarro J L, Rodríguez-Fernández J, Arriortua M I, Rojo T. Mild hydrothermal synthesis and structural determination of two layered, structurally related inorganic-organic hybrid vanadates with nickel(Ⅱ) and tris (2-aminoethyl) amine[J]. Eur. J. Org. Chem., 2009, 24:  3607-3612.

  31. [31]

      Li J K, Huang X Q, Yang S, Xu Y Q, Hu C W. Controllable synthesis, characterization, and catalytic properties of three inorganic-organic hybrid copper vanadates in the highly selective oxidation of sulfides and alcohols[J]. Cryst. Growth Des., 2015, 15(4):  1907-1914. doi: 10.1021/acs.cgd.5b00086

  32. [32]

      Tian H R, Zhang Z, Liu S M, Dang T Y, Li X H, Lu Y, Liu S X. A novel polyoxovanadate-based Co-MOF: Highly efficient and selective oxidation of a mustard gas simulant by two-site synergetic catalysis[J]. J. Mater. Chem. A, 2020, 8(25):  12398-12405. doi: 10.1039/D0TA00537A

  33. [33]

      Han Q X, Li W W, Wang S G, He J C, Du W, Li M X. Asymmetric cascade catalysis with chiral polyoxometalate-based frameworks: Sequential direct aldol and epoxidation reactions[J]. ChemCatChem, 2017, 9(10):  1801-1807. doi: 10.1002/cctc.201700160

  34. [34]

      Solé-Daura A, Zhang T, Fouilloux H, Robert C, Thomas C M, Chamoreau L M, Carbó J J, Proust A, Guillemot G, Poblet J M. Catalyst design for alkene epoxidation by molecular analogues of heterogeneous titanium-silicalite catalysts[J]. ACS Catal., 2020, 10(8):  4737-4750. doi: 10.1021/acscatal.9b05147

  35. [35]

      Clemente-Juan J M, Coronado E, Gaita-Ariño A. Magnetic polyoxometalates: From molecular magnetism to molecular spintronics and quantum computing[J]. Chem. Soc. Rev., 2012, 41(22):  7464-7478. doi: 10.1039/c2cs35205b

  36. [36]

      Xu J H, Guo L Y, Su H F, Gao X, Wu X F, Wang W G, Tung C H, Sun D. Heptanuclear Co₅^ⅡCo₂^Ⅲ cluster as efficient water oxidation catalyst[J]. Inorg. Chem., 2017, 56(3):  1591-1598. doi: 10.1021/acs.inorgchem.6b02698

  37. [37]

      Sasaki S, Yonesato K, Mizuno N, Yamaguchi K, Suzuki K. Ring-shaped polyoxometalates possessing multiple 3d metal cation sites: [{M₂(OH₂)₂}₂{M(OH₂)₂}₄P₈W₄₈O₁₇₆(OCH₃)₈]^16- (M=Mn, Co, Ni, Cu, Zn)[J]. Inorg. Chem., 2019, 58(12):  7722-7729. doi: 10.1021/acs.inorgchem.9b00061

  38. [38]

      李芬芳, 何婧. 基于含双吡唑的四羧酸配体构筑的Fe(Ⅱ)/Co(Ⅱ)同构配合物的合成、晶体结构及磁性质[J]. 无机化学学报, 2022,38,(11): 2259-2266. LI F F, HE J. Synthesis, structural and magnetic characterization of Fe(Ⅱ)/Co(Ⅱ) isomorphous complexes based on a dipyrazole-containing tetracarboxylate ligand[J]. Chinese J. Inorg. Chem., 2022, 38(11):  2259-2266.

  39. [39]

      Gao G G, Xu L, Wang W J, Qu X S, Liu H, Yang Y Y. Cobalt(Ⅱ)/nickel(Ⅱ)-centered Keggin-type heteropolymolybdates: Syntheses, crystal structures, magnetic and electrochemical properties[J]. Inorg. Chem., 2008, 47(7):  2325-2333. doi: 10.1021/ic700797v

  40. [40]

      Guo L Y, Zeng S Y, Jagličić Z, Hu Q D, Wang S X, Wang Z, Sun D. A pyridazine-bridged sandwiched cluster incorporating planar hexanuclear cobalt ring and bivacant phosphotungstate[J]. Inorg. Chem., 2016, 55(17):  9006-9011. doi: 10.1021/acs.inorgchem.6b01468

• 

  Figure 1  (a) Coordination for the Co and V in 1; (b) Ternary ring system containing two five-membered rings and one six-membered ring; (c) 2D layer formed by ternary rings

  下载: 全尺寸图片 幻灯片
  

  Figure 2  (a) IR spectra of 1 after each catalytic cycle; (b) PXRD patterns of recovered 1

  下载: 全尺寸图片 幻灯片
  

  Figure 3  TG curve of 1 measured from 30 to 800 ℃ under N₂ atmosphere

  下载: 全尺寸图片 幻灯片
  

  Figure 4  Effect of (a) catalyst and (b) oxidant factors for cyclooctene oxidation; (c) Thermal filtration experiment; (d) Recycling of catalyst for oxidation of cyclooctene

  下载: 全尺寸图片 幻灯片
  

  Scheme 1  Epoxidation of cyclooctene catalyzed by 1

  下载: 全尺寸图片 幻灯片
  

  Scheme 2  Proposed mechanism of catalytic epoxidation procedure

  下载: 全尺寸图片 幻灯片
  

  Figure 5  (a) Temperature reliance of χ_M and χ_MT for 1; (b) Temperature reliance of χ_M^-1 for 1

  下载: 全尺寸图片 幻灯片

  Table 1.  Crystallographic data of 1 and corresponding structural refinements

  Parameter	1		Parameter	1
  Formula	C8H7N3O6CoV2		Crystal size / mm	0.21×0.15×0.14
  Formula weight	401.98		Z	2
  Crystal system	Triclinic		Dc / (g·cm-3)	2.256
  Space group	P1		μ / mm-1	2.961
  a / nm	0.767 3(5)		θ range / (°)	2.16-25.00
  b / nm	0.847 7(5)		Reflection collected	4 349
  c / nm	1.034 3(5)		F(000)	394
  α / (°)	112.425(5)		Rint	0.015 7
  β / (°)	92.074(5)		GOF	1.053
  γ / (°)	105.599(5)		R1 [I > 2σ(I)]a	0.025 4
  V / nm3	0.591 7(6)		wR2 (all data)b	0.069 9
  ${ }^{\mathrm{a}} R_1=\sum\left\|F_{\mathrm{o}}|-| F_{\mathrm{c}}\right\| / \sum\left|F_{\mathrm{o}}\right|; { }^{\mathrm{b}} w R_2=\left\{\sum\left[w\left(F_{\mathrm{o}}{ }^2-F_{\mathrm{c}}{ }^2\right)^2\right] / \sum\left[w\left(F_{\mathrm{o}}{ }^2\right)^2\right]\right\}^{1 / 2}$.

  下载: 导出CSV

  Table 2.  Epoxidation of cyclooctene with different catalysts^a

  Catalyst	Conversion / %	Selectivity / %	Reaction system
  1	98.6	99.2	Heterogeneous
  Without catalyst	6.9	94.6
  (n-Bu4N)4[V4O12]b	65.5	62.9	Homogeneous
  CoCl2·6H2Oc	23.2	91.7	Homogeneous
  a Reaction conditions: cyclooctene (1 mmol), 1 (0.04 mmol), H2O2 (1.5 mmol), CH3CN (5 mL), 60 ℃, 8 h; b V content equivalent to that of 1; c Co content equivalent to that of 1.

  下载: 导出CSV

  Table 3.  Oxidation of various olefins catalyzed by 1 using H₂O₂ oxidant

  Entry	Substrate	Product	Time / h	Conversion / %	Selectivity / %
  1			8	98.6	99.2
  2			6	94.2	96.8
  3			8	89.8	92.4
  4			10	91.1	89.6
  5			10	72.3	79.9
  6			9	86.1	78.6
  7			9	69.6	96.8
  8			6	89.5	90.3
  9			6	82.6	77.5
  10			7	67.8	71.6
  11			7	65.4	81.8

  下载: 导出CSV
•