Copper(ii)-containing tungstotellurates(vi): syntheses, structures and their catalytic performances in selective oxidation of thioethers

We report the syntheses and structures of two new copper(ii)-containing tungstotellurates(vi) Na12[TeVI2W8O38Cu2(H2O)2]·7H2O (Te2W8Cu2) and Na6[TeVIW6O24Cu(NH2CH2CO2)2]·6H2O (TeW6Cu). The two compounds were synthesized by a simple one-pot method and characterized by single-crystal X-ray diffraction (XRD), powder XRD, FT-IR spectroscopy, elemental analysis, and thermogravimetric analysis in the solid state. Furthermore, their catalytic properties for the selective oxidation of thioethers were also studied systematically. The catalytic experiment results indicate that the tungstotellurate(vi) Te2W8Cu2 is an effective heterogeneous catalyst for the selective oxidation of thioethers to sulfoxides or sulfones by an H2O2 oxidant at room temperature. Under the ambient conditions, Te2W8Cu2 can convert 99% of methyl(phenyl)sulfane to sulfoxides or sulfones with 96% or 99% selectivity, respectively, and the utilization rate of H2O2 is up to 80%. Furthermore, Te2W8Cu2 as a heterogeneous catalyst is stable in the reaction and could be reused at least five cycles with conserved activity.


Introduction
The selective oxidation of thioethers into the corresponding sulfoxides is a very important process in organic synthesis chemistry, because the sulfoxides are a type of essential building block in pharmaceuticals, agrochemicals, and other ne chemicals. 1 Many signicant methods have been developed for the synthesis of sulfoxides. 2,3 Traditionally, the process was carried out with a wide variety of oxidant agents, such as peracids, dioxiranes, NaIO 4 , MnO 2 , CrO 3 , SeO 2 , and PhIO. 4 However, these oxidation reactions relied upon strong or environmentally-unfriendly oxidants, some of which are hazardous or toxic. Therefore, developing an environmentally benign method and using easily available oxidants in thioethers oxidation is attractive and needs to be further explored. H 2 O 2 can be used as oxidant in the oxidation of thioethers because it's a green oxidant and water is the only by-product. 5 However, how to improve the utilization ratio and reduce the consumption of H 2 O 2 with good catalytic selectivity in the oxidation reaction of thioethers still remains a challenge. In recent years, some polyoxometalates (POMs) were used as effective catalysts for the oxidation of thioethers using H 2 O 2 as oxidant. [6][7][8] In those reported procedures, low conversion of thioethers and the service life and reusability of the POM catalysts also need to be further improved. Consequently, based on the above reported results and our previous work on the oxidation of sulphides, 9 We believe that more attractive catalytic systems can be developed along this line to further increase the utility of these thioethers oxidation reactions by using some new stable and recyclable POM catalysts.
As we all know, POMs are a large class of discrete anionic metal-oxygen clusters of early transition metals with high oxidation states, such as Mo, W, V, Nb, Ta, 10-13 which present abundant structural diversity, favourable electrical carrier properties and great potentials in medicine, catalysis and material science. 14 [15][16][17] It is well-known that the heterogroups play a key role in the structures and properties of these POMs, especially the redox properties, which is very important to their redox catalytic activities. 18 Since heteropolytungstates are usually synthesized in acidic media by the condensation of molybdate or tungstate with relevant heteroanions, 19 24 Recently, we reported the selfassembly of a series Ln(III)-containing tungstotellurates(VI) with interesting photoluminescence properties. 25 In order to develop new stable and recyclable POM catalysts for selective oxidation of thioethers, in this work, we prepared two new copper(II)-containing tungstotellurates(VI) by a simple one-pot synthetic method. The two tungstotellurates(VI) Na 12

Experimental
Materials and methods Na 6 TeW 6 O 24 $22H 2 O (TeW 6 ) was prepared according to the published procedure and characterized by IR spectroscopy. 26 All reagents were purchased from commercial sources, and used without further purication. Elemental analyses were determined by inductively coupled plasma mass spectrometry (ICP-MS) with PerkinElmer NexlON 350X spectrometer and the Elemental Analyser (C, H, N). FT-IR spectra (KBr pellets) were recorded with a Nicolet 170SX-FT/IR spectrometer. Thermogravimetric analyses were carried out with a TG-DTA 6200 device at a heating rate of 10 C min À1 under nitrogen atmosphere. Powder X-ray diffraction (PXRD) data were recorded on a Bruker D8 instrument equipped with graphite-monochromatized Cu Ka radiation (l ¼ 0.154060 nm; scan speed ¼ 8 min À1 ; 2q ¼ 5-50 ) at room temperature.

X-ray crystallography
Crystal data for all compounds were collected at 150 K on a Bruker APEX 2 DUO CCD single-crystal diffractometer equipped with a sealed Mo tube and a graphite monochromator (l ¼ 0.71073 A). The selected crystals were selected by examination under mineral oil using a polarising microscope and mounted in a Hampton cryoloop with oil and placed within one minute under a stream of cold N 2 . The structure solution and renement were carried out by the SHELXTL program package (Bruker), and all structures were solved by direct methods and rened by the full-matrix least-squares method (Sw(|F o | 2 À |F c | 2 ) 2 ). 27 The hydrogen atoms of waters were not incorporated in the renements, and all other atoms were rened with anisotropic thermal parameters. The crystal data and structure renement details for the two compounds are discussed and summarized in Table S1. † The crystallographic data have been deposited to the Cambridge Crystallographic Data Centre (CCDC) as entries CCDC-1991106, CCDC-1991107.

Catalytic oxidation of thioethers
The selective oxidation of thioethers using different catalysts were carried out as follows. Thioethers (0.5 mmol) and 30% H 2 O 2 (0.6 mmol or 1.25 mmol) were dissolved in EtOH/MeCN (1 ml) in a pressure tube (10 ml), then added catalyst (0.17 mol%) with a sealing cap at room atmosphere. The reaction mixture was vigorously stirred until the completion as indicated by GC. The reaction mixture was ltered through a short pad of celite and quenched with water. Then the mixture was extracted with dichloromethane (10 ml Â 3), dried over MgSO 4 , and evaporated under reduced pressure to afford the crude product, which was further puried by ash chromatography on silica gel with n-hexane/EtOAc to obtain the corresponding sulfoxides or sulfones. In the recycle experiments, the catalyst was ltered aer the reaction and washed with dichloromethane and dried under vacuum at 50 C and used for the next cycle.

Results and discussion
Synthesis and structure description As described in the synthetic procedures above, the two copper(II)-containing tungstotellurates(VI) Te 2 W 8 Cu 2 and TeW 6 Cu were prepared by simple one-pot reaction with a time-resolved process in which the hydrated salts of the two compounds were isolated successively from the same mother liquor as bulkpure materials. The crystals of Te 2 W 8 Cu 2 were isolated rstly and then the crystals of TeW 6 Cu. We noticed that the glycine ligand plays a very important role in the synthesis of the two compounds although it is absent in the nal structure of Te 2 W 8 Cu 2 . Moreover, heteropolytungstates are usually formed in condensation reactions of tungstate oxoanions with relevant heteroanions in an acid medium, but the two copper(II)-containing tungstotellurates(VI) were obtained straightly in the reaction solution without addition of appropriate acid and the pH value is 8.9, which means the two compounds can be constructed in the presence of Te(OH) 6 which can provide enough protons during the condensation reactions.
Single-crystal X-ray diffraction analysis reveals that Te 2 W 8 Cu 2 is a di-copper-containing tungstotellurate(VI) which crystallizes in the triclinic lattice with space group P 1. As shown in Fig. 1a For compound TeW 6 Cu, X-ray diffraction analysis reveals that it consists of one dimensional organic-inorganic hybrid polymeric chains having zig-zag architecture, which crystallizes in the triclinic lattice with space group P 1 in form of sodium salts. As shown in Fig. 1b Fig. S1b. †

Catalysis
Selective oxidation of thioethers to the versatile utility of sulfoxides and sulfones is considered to be a topical interest in organic synthesis. As mentioned above, POMs can be used as effective catalysts for the oxidation of thioethers using H 2 O 2 as oxidant. In this work, in order to further develop a more attractive catalytic system for the selective oxidation of thioethers, we selected three different tungstotellurates(VI), the classic Anderson-Evans type POM TeW 6 , the organic-inorganic hybrid complex TeW 6 Cu, and the di-copper-containing POM Te 2 W 8 Cu 2 , as heterogeneous catalysts for this reaction and optimized the reaction conditions systematically. Moreover, we also veried that the Te 2 W 8 Cu 2 is an effective heterogeneous catalyst with good conversion and selectivity for the oxidation of different thioethers to corresponding sulfoxides or sulfones at room temperature.
In order to investigate the catalytic performance and effects of the different catalysts for selective oxidation of thioethers, Te 2 W 8 Cu 2 , TeW 6 Cu, TeW 6 , CuO and Cu(OAc) 2 were used as catalysts, and the catalytic reaction experiments of the oxidation of thioethers were investigated systematically by using H 2 O 2 as oxidant under different conditions. At the beginning of our studies, in order to nd out the best catalyst and optimize the reaction conditions for this catalytic system, methyl phenyl sulde (1a) was employed as the model substrate for the oxidation reaction. The catalytic study results were summarized in Table 1. Notably, when the reaction was carried out in the absence of catalysts, only trace of product methyl phenyl sulfoxide (2a) was observed, and no methyl phenyl sulfone (3a) was observed, and the intact substrate 1a can be recovered (Table 1, entry 1). The copper(II) salts and oxides, such as Cu(OAc) 2 and CuO were rst evaluated in the reaction (1a to 2a), although the selectivity of 2a was 99%, while the conversion of 1a were 19% and 31%, respectively. Considering the Anderson-Evans type TeW 6 is the basic structure of TeW 6 Cu, we also tested the activity of TeW 6 in the model reaction (1a to 2a). The results show that the conversion of 1a was 88% and the selectivity of 2a was 84%. Continuing experiments, the activity of TeW 6 Cu and Te 2 W 8 Cu 2 were also examined in the model reaction, as shown in Table 1. Interestingly, according to the results of the catalysts screening, the nal conversion of 1a and selectivity of 2a and 3a indicate that Te 2 W 8 Cu 2 is the best catalyst for the both selective oxidation reactions of 1a to 2a and 1a to 3a (Table 1, Fig. 2a, and their kinetic studies indicated that if the reaction was carried out using Te 2 W 8 Cu 2 as catalyst, the reaction proceeded much faster than using other catalysts, which means the Te 2 W 8 Cu 2 present best activity for this reaction. The initial velocity order of the different catalysts is Te 2 W 8 Cu 2 > TeW 6 Cu > TeW 6 > CuO > Cu(OAc) 2 > no catalyst (41.70 > 23.18 > 18.88 > 17.50 > 6.79 > 1.17, conversion moles per unit time). There are two possible reasons may explain these results. Firstly, many other POMs were used as effective catalyst for the oxidation of thioethers due to their excellent redox properties, and the tungstotellurates(VI) Te 2 W 8 Cu 2 , TeW 6 Cu, TeW 6 with {Te VI O 6 } heterogroup can also present interesting oxidation catalytic activity which is much better than the simple cupric salts and cupric oxides for the selective oxidation of thioethers. Sencondly, by introducing the Cu II ions into the structures of the tungstotellurates(VI) Te 2 W 8 Cu 2 and TeW 6 Cu, the Cu II -containing tungstotellurates(VI) are more active than the simple Anderson-Evans type tungstotellurates(VI) TeW 6 , because the Cu II centers can affect their redox properties and offer the new active sites for the catalysts during the catalytic reaction. More importantly, the structure and the coordination environment of Cu II in Te 2 W 8 Cu 2 and TeW 6 Cu are different, which can also affect their catalytic activity for selective oxidation of thioethers. As proved by single crystal analyses, for Te 2 W 8 Cu 2 , there are two Cu II centers in the structure, and each Cu II center is ve-coordinated square-pyramidal coordination sphere, which make it benecial to active the substrates (sulde and H 2 O 2 ). While in TeW 6 Cu, the Cu II center is six-coordinated with an octahedra coordination sphere, and the Cu II center is  coordination-saturated, resulting in strong coordination of the ligand to Cu. The results make the Cu II in TeW 6 Cu is less active to the substrates. So the activity of Te 2 W 8 Cu 2 is higher than TeW 6 Cu. Based on the above results, Te 2 W 8 Cu 2 can be proved as the best optimal catalyst for the oxidation reaction under this conditions. Moreover, the heterogeneous nature of Te 2 W 8 Cu 2 was also investigated by a leaching experiment. As shown in Fig. 3, when the conversion of 1a reached to about 38% at 0.5 h, the catalyst was ltered off and the ltrate continued to react under the same conditions for 6 h, the reaction almost stopped and the conversion of 1a increased slightly due to the transformation of 1a itself without catalyst under such reaction conditions. The result revealed that the catalyst of Te 2 W 8 Cu 2 is a heterogeneous catalyst in this reaction and stable without leaching under the optimized conditions. As far as we know, the mechanism of catalytic oxidation reaction using H 2 O 2 as oxidant usually shows two different pathways, the radical pathway and the peroxo species pathways. To further understand the mechanism of our system, the oxidation of thioethers using H 2 O 2 as oxidant and Te 2 W 8 Cu 2 as catalyst, radical trap experiments on the oxidation of methyl(phenyl)sulfane were employed (ESI, Table S3 †). The reaction was carried out under the optimized conditions, aer adding Ph 2 NH as oxygen-radical scavenger, 1,4-benzoquinone as superoxide (cO 2À /cO 2 H scavenger) and tert-butyl alcohol as a hydroxyl radical scavenger, 28 no obvious changes on the conversion and selectivity were observed (ESI, Table S3 †). The results indicate that the mechanism of the reaction is not a radical pathway, which is consistent with previous literature that POMs tend to form peroxo-metal species in the presence of H 2 O 2 . 29 So, we proposed that an active peroxo species formed during the reaction by using H 2 O 2 as oxidant and Te 2 W 8 Cu 2 as catalyst, although it has not been tested in this work.
According to the study of the catalytic performance of different catalysts for selective oxidation of thioethers, we noticed that the tungstotellurates(VI) Te 2 W 8 Cu 2 , TeW 6 Cu and TeW 6 shown good catalytic potentials for this system, we think the {Te VI O 6 } heterogroup may sufficiently affect the redox property of the tungstotellurate(VI) catalysts which might be good for their catalytic activity of selective oxidation of thioethers. Moreover, the copper-containing tungstotellurates(VI) Te 2 W 8 Cu 2 and TeW 6 Cu present better catalytic activity than the classic Anderson-Evans type tungstotellurates(VI) TeW 6 , which means introducing Cu II centers in to the POM based complexes can improve the catalytic activity of tungstotellurates(VI), especially the Te 2 W 8 Cu 2 with two Cu II centers. Because the heterogeneous catalyst Te 2 W 8 Cu 2 shows best catalytic activities for the selective oxidation of thioethers (Table 1), we studied systematically the catalytic performance of selective oxidation of different thioethers to corresponding sulfoxides using H 2 O 2 (30%) (1.2 equiv.) as oxidant. A variety of organic sulfur compounds were subjected to this highly chemoselective catalytic system. As shown in Table 2, all of the desired sulfoxides products were obtained in moderate to good yield. Surprisingly, electron-donating as well as electronwithdrawing substituted phenyl ring thioanisole gave the desired sulfoxides (2a-2f) in high yields with good selectivities and high H 2 O 2 utilization rate. Ethyl phenyl sulde 1g can be converted into ethyl phenyl sulfoxide 2g with 99% conversion and 93% selectivity. The sterically more hindered substrate diphenylsulfane 1h were also investigated in this catalytic system. Fortunately, the corresponding product 3h was obtained in moderate yield. It is worth noting that less reactive dibutylsulfane 1i could be efficiently oxidized into the desired sulfoxides 2i in high yields with 98% selectivity.  Furthermore, we also explored its catalytic activity for selective oxidation of different thioethers to corresponding sulfones using H 2 O 2 (30%) (2.5 equiv.). As shown in Table 3, a broad array of organic sulfur compounds worked well to afford the corresponding sulfones in good to excellent yields (Table 3, entries 1-9). It is worth noting that sterically hindered substrate diphenylsulfane 1h and less reactive dibutylsulfane 1i gave the corresponding sulfones 3h and 3i in 71% and 99% selectivity, respectively. Interestingly, we noticed that the molar ratio of H 2 O 2 and Te 2 W 8 Cu 2 plays a very important role for the selectivity of thioethers to corresponding sulfoxides and sulfones. This results are much better than other catalytic system using H 2 O 2 as oxidant, and in this work we have developed a good method to improve the utilization ratio and reduce the consumption of H 2 O 2 with good catalytic selectivity in the oxidation reaction of thioethers by using Te 2 W 8 Cu 2 as catalyst. It is interesting to see that different oxidized products can be performed in high selectivity by using different solvents. The reason could be that the hydrogen bond and interaction between the reactants and solvents are different in ethanol and acetonitrile, which could solvate H 2 O 2 effectively and thereby reduce its availability at the surface of catalyst.
As mentioned above, we have investigated the heterogeneous nature of Te 2 W 8 Cu 2 by a leaching experiment, which reveals that the catalyst is a heterogeneous catalyst in this reaction and stable without leaching under the optimized condition. Therefore, in order to the service life and reusability of the catalyst, we also evaluated the recyclability and stability of Te 2 W 8 Cu 2 for the selective oxidation of thioethers. As shown in Fig. 4, the catalytic reactivity of Te 2 W 8 Cu 2 remains without obviously decreasing the conversion and selectivity aer 5 cycles, which means the catalyst is recyclable and reusable aer the reaction. The results of comparison of FT-IR spectra of the catalyst before and aer the recycle reactions also conrms that the structure of Te 2 W 8 Cu 2 is stable without decomposition in the reaction (ESI, Fig. S5 †). This results indicate that the structure of Te 2 W 8 Cu 2 is very favourable for this catalytic system. The recyclability and stability of TeW 6 and TeW 6 Cu were also evaluated (ESI, Fig. S6 and S7 †). Interestingly, the stability order of the catalyst is Te 2 W 8 Cu 2 > TeW 6 Cu > TeW 6. The results indicate that the new catalyst becomes more stable than POMs when the Cu II centers were introduced into the POMs.

Conclusions
In summary, we have synthesized two new copper-containing tungstotellurates(VI) Te 2 W 8 Cu 2 and TeW 6 Cu by a simple one pot reaction which were characterized by single-crystal X-ray diffraction(XRD), powder XRD, FT-IR spectroscopy, elemental analysis, and thermogravimetric analysis in solid state. The catalytic properties of the tungstotellurates(VI) for the selective oxidation of thioethers were also studied systematically. The compound Te 2 W 8 Cu 2 showed very high catalytic activity for the selective oxidation of thioethers to sulfoxides (conversion: up to 99%, selectivity: up to 98%) and sulfones (conversion: up to 99%, selectivity: up to 98%) by H 2 O 2 under ambient conditions. In addition, the Te 2 W 8 Cu 2 catalyst was stable in the environmentally benign catalytic system and could be reused at least ve cycles without a signicant loss of reactivity on the model reaction. This work offer a new strategy for the selective oxidation of thioethers by using tungstotellurates(VI) as heterogeneous catalyst, and the utilization ratio of the oxidant H 2 O 2 is up to about 80%.