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Grafting Transition Metal-Organic Fragments onto W/Ta Mixed-Addendum Nanoclusters for Broad-Spectrum-Driven Photocatalysis [ChemPlusChem](ChemPlusChem Via Acquire Media NewsEdge) Two new transition-metal (TM)-containing polytantalotungstates, CsNa2 H[Cu(bpy)(H2 O)3]3 {[Cu(bpy)2]2 [Cu(bpy)(H2 O)2]3 - [Ta4 O6 (SiW9 Ta3 O40)4]}.17H2 O (1) and K4 Na4 H4 [Ta4 O6 (SiW9 Ta3 O40)4] [Cu(apy)(H2 O)2]4.42H2 O (2) (bpy=2,2'-bipyridine, apy=3-aminopyridine), have been synthesised under hydrothermal conditions. Both compounds 1 and 2 were determined and characterised by single-crystal X-ray diffraction analysis, thermogravimetric analysis, IR spectroscopy, UV/Vis spectroscopy and elemental analysis. Compounds 1 and 2 contain W/Ta mixed-addendum nanoclusters decorated by TM-organic fragments. Compounds 1 and 2 are the first TM-containing polytantalotungstates promise a more diverse set of structures of the polytantalotungstate family. The obtained materials can harvest a wide spectrum of solar light, from UV to near-infrared (NIR) wavelength. Photocatalytic study revealed that compounds 1 and 2 exhibited UV- and visible-light-driven photocatalytic water splitting activity. Compound 1 could also be used as a catalyst for the photocatalytic decomposition of 2,4-dichlorophenol in water with NIR-light irradiation. This is the first NIR photocatalyst obtained in polyoxometalate chemistry. Keywords : heterogeneous catalysis · photocatalytic properties · polyoxometalates · structure elucidation · water splitting Introduction Polyoxometalates (POMs) are a wide family of discrete nanometric metal oxide clusters with various applications in biology, catalysis, materials science and so forth.[1-4] POMs are mostly constructed from corner- and edge-shared MO6 (M=Mo6+, W6+, Nb5+, Ta5+) octahedra, and they undergo fast, reversible and stepwise multiple electron-transfer reactions without changing their structures. It is generally appreciated that POM clusters usually exhibit semiconductor-like photochemical behaviours owing to analogous electronic characteristics (bandgap transition for semiconductors and HOMO-LUMO transition for POMs). In POMs, the electrons could be promoted from HOMO to LUMO as from valence band to conduction band in metal oxide semiconductors inducing oxygen-to-metal charge transfer under irradiation. As is well known, great progress has been achieved in the development of photocatalysts, largely by the synthesis of a number of metal oxides and sulfides, since the discovery of photocatalytic splitting of water on TiO2 electrodes in 1972.[5-10] Compared with inorganic solid materials, molecular catalysts are more amenable to mechanistic studies and fine-tuning by using synthetic chemistry to optimise their performances.[11] Accordingly, POMs could be regarded as an attractive candidate to study the relationship between photocatalytic activity and inorganic cluster structures.[ 12, 13] In this field, the photocatalytic H2 evolution activity of POMs has been studied and showed that most of them are active under UV irradiation. For example, the Keggin-type and Wells- Dawson-type polyoxotungstates [XW12O40]n (X=P, Si, Fe, Co or H; n=3, 4, 5 and 6) and [P2W18O64]6, and a sandwich polyoxotungstate [(GeW9Ti3O37)2O3], exhibit photocatalytic water reduction activity under UV irradiation.[14] Recently, great progress has been achieved in the synthetic chemistry of polyoxoniobates,[ 15, 16] and several polyoxoniobates were selected as efficient photocatalysts for water reduction.[17] Compared with the polyoxotungstate and polyoxoniobate chemistry, very limited information is available in the literature on the polyoxotantalate photocatalysts, because of the restricted conditions in synthesis of Ta-containing compounds. However, tantalate and tantalum oxide have been regarded as efficient photocatalysts for water reduction all the time.[18] Accordingly, the synthesis of novel polyoxotantalates has attracted great attention with respect to their good photocatalytic performance in water splitting. Recently, lacunary polyoxotungstate was used as the inorganic polydentate ligand to isolate two high-nuclear tantalate clusters, which were further confirmed to be an efficient photocatalyst for water reduction under UV irradiation.[19] In this field, it is of great interest but a challenging project to develop novel polyoxotantalate photocatalysts with visible- and nearinfrared (NIR)-light activity for more efficient use of solar energy. As is well known, the introduction of transition-metal (TM) cations into the POM frameworks was regarded as an efficient method to obtain visible-light-active photocatalysts for water splitting.[20, 21] Herein, the TM-organic fragments were firstly grafted on a W/Ta mixed-addendum nanocluster resulting in two UV-, visible- and NIR-light-active molecular photocatalytic materials CsNa2H[Cu(bpy)(H2O)3]3{[Cu(bpy)2]2[Cu(bpy)- (H2O)2]3[Ta4O6(SiW9Ta3O40)4]}.17H2O (1) and K4Na4H4- [Ta4O6(SiW9Ta3O40)4][Cu(apy)(H2O)2]4.42H2O (2) (bpy=2,2'-bipyridine, apy=3-aminopyridine). To the best of our knowledge, compounds 1 and 2 represent the first wide-spectrum POM photocatalysts that could harvest solar light from the UV to NIR region. Results and Discussion Synthesis and crystal structures Compounds 1 and 2 were both synthesised through self-as- sembly processes of [SiW9(TaO2)3O37]7^, CuCl2·2H2O and bpy (for 1) or apy (for 2) under hydrothermal conditions, which has been proved to be a useful method to isolate stable photoca- talysts. Further study showed that compounds 1 and 2 are not only broad-spectrum active photocatalysts, but also very stable in a wide pH range of 0.3-10.0 and could be readily recycled for reuse as well. Single-crystal X-ray diffraction analyses re- vealed that compound 1 crystallises in the C2/c space group and contains a tetrameric W/Ta mixed-addendum nanocluster {Si4Ta16W36O166} surrounded by eight Cu-bpy fragments (Fig- ure 1 b, Figures S1-S4 in the Supporting Information). In the tetrameric cluster, there are four trivacant a-Keggin units resulting from removal of three [WO6] octahedra from the saturated a-Keggin structure, and the vacant sites are filled with three TaO6 octahedra resulting in the {Ta3SiW9} unit (Figure 2). The four {Ta3SiW9} units are fused together by a cen- tral {Ta4} unit into the {Ta16} cluster-containing nanocluster, which could also be regarded as a {Ta16} cluster surrounded by four trivacant POM units. The W^O bond lengths are in the range of 1.657(19)-2.408(17) ^, and the O-W-O angles are in the range of 71.4(7)-173.5(9)8. The Ta^O bond lengths are in the range of 1.796(16)-2.313(17) ^, and the O-Ta-O angles range from 73.1(6) to 176.7(6)8. In compound 1, the Cu centres can be divided into two groups according to their different co- ordination environments. The first group is composed of the Cu2 and Cu4 atoms, which are coordinated by two nitrogen atoms from one bpy ligand, two bridging oxygen atoms [W^ (m2-O)^Ta] from two {SiW9Ta3} units and two aqua ligands (Fig- ures S1 and S2), whereas the other Cu atoms connect with four nitrogen atoms from two bpy ligands and one bridging oxygen atom [W^(m2-O)^W] from one {SiW9Ta3} unit (Figures S3 and S4). The bond lengths of Cu^N are in the range of 1.90(3)-2.06(3) ^ and Cu^O ranges from 1.93(2) to 2.26(7) ^. In the structure of 1, there are eight Cu-bpy fragments around the W/Ta mixed-addendum nanocluster. In compound 2, the structure of the central W/Ta mixed-addendum nanocluster is similar to that in compound 1, and consists of a {Ta16} cluster coordinated by four trivacant {SiW9O34} units (Figure 1 c). The tetrameric nanocluster is surrounded by four Cu-apy frag- ments. In this structure, the Cu centre is coordinated by one nitrogen atom from one apy ligand, one bridging oxygen atom [W^(m2-O)^W] from a {SiW9O34} unit and two aqua li- gands. As shown in Figure S6, each {SiW9O34} unit is decorated with one Cu-apy fragment. To be different from compound 1, there is no copper atom linked with the {Ta16} cluster through the Ta^O^Cu bridging modes. The bond valence sum yields the following results : W6 + ,Ta5 +,Si4 + and Cu2 + . UV/Vis analysis As the metal oxide cluster, POMs always absorbed UV light, as the band gaps of oxides are usually wider than 3.0 eV.[22] Thus, to extend the photosensitivity of the photoactive oxides into the visible-light region, an efficient solution is to change the valence band position of the photocatalysts.[23] In experiments, it was found that the powders of Cs3K3.5H0.5[SiW9- (TaO2)3O37]·9H2O and {Si4Ta16W36O166} are colourless and have no visible-light absorption (Figure 3 b). We propose that visi- ble-light-active photocatalysts may be achieved by introducing TM cations into the POM frameworks. In plenty of experiments, the Cu-pyridine (py) fragments were successfully grafted on W/ Ta mixed-addendum nanoclusters, which resulted in the first TM-containing polytantalotungstates. UV/Vis diffuse reflectance spectra of the two compounds all display a similar range of ab- sorption bands in the UV region at around 200 to 364 nm (Fig- ure 3 b), which correspond to the O!W transitions. Com- pounds 1 and 2 also exhibit well-developed bands in the range of 625-1400 nm owing to the d-d transitions of the oc- tahedrally coordinated, trigonal bipyramidal coordinated high- spin CuII ions for 1 and square-planar coordinated CuII for 2.[24] The broad peaks at 455 nm for 1 and 462 nm for 2 were as- signed to the O !Cu charge-transfer transitions.[25] To explore the conductivity of the title compounds, the measurement of diffuse reflectivity for a powder sample was used to obtain its band gap Eg, which agrees well with that obtained by absorp- tion measurement from a single crystal.[26] The Kubelka-Munk function F versus E plots for the title compounds are shown in Figure S8, in which a steep absorption edge is displayed. The Eg can be assessed as 3.00 eV for 1 and 3.18 eV for 2, and the band gap of [Si(TaO2)3W9O40] is 3.25 eV (Figure 3 a). It is worth noting that the LUMO levels of compounds 1 and 2 are ob- tained in 0.5m CH3COOK+ CH3COOH buffer solution (pH 4.7) during electrochemical tests (Figure S9).[27] It was found that the LUMO levels of compound 1 (^^0.26 vs. NHE) and 2 (^ ^0.44 vs. NHE) are lower than those of the Ta 5d orbital and [*Ru(bpy)3]2+.[28] Photocatalytic water splitting under UV and visible-light ir- radiation To determine the photocatalytic activity of the title com- pounds, 1 and 2 were used as catalysts, Pt as co-catalyst and methanol as sacrificial electron donor to explore their potential use for photocatalytic water splitting with the irradiation of a 300 W Xe lamp. As shown in Figure 4 a, H2 was evolved con- tinuously in 4 hours at a rate of about 275 mmolh^1 g^1 for 1 (black line) and 262 m mol h^ 1 g ^ 1 for 2 (red line). Further, the Cu-py fragments were grafted on the POMs, which resulted in a broad absorption in the visible region. So, the photocatalytic activity of 1 was measured under visible-light irradiation (l > 400 nm). Compound 1 was used as the photosensitiser and catalyst, Pt as H2 evolution co-catalyst and triethylamine (TEA) as a sacrificial electron donor ; the H2 evolution rate was 3.6 mmolh^1 g^1. If compound 1 or TEA is absent, no H2 is de- tected (Figure 4 b). Under the same conditions, when {Ta4O6(SiW9Ta3O40)4} was used to replace compound 1,noH2 could be detected. Further, the dye cation [Ru(bpy)3]2 + was in- troduced into this photocatalytic system of 1 to sensitise the TM-containing polytantalotungstate. It was found that the H2 evolution was observed at a rate of about 10.5 mmolh^1g^1 (Figure 4 b). In the visible-light-driven photocatalytic system of 2,noH2 could be detected in the absence of the dye cation [Ru(bpy)3]2 + , which might result from the different linking modes between Ta5d and Cu 3d centres in these two com- pounds. In compound 1, the Cu centre was connected with the {Ta16} cluster directly through the Ta^O^Cu bridges ; how- ever, the Cu centre was coordinated by the Keggin units {SiW9O34} through the W^O^Cu bonds in 2. As a result, the band gap of 1 (3.00 eV) is lower than that of 2 (3.18 eV). When [Ru(bpy)3]2 + was introduced into the photocatalytic system of 2,H2 evolution was detected at a rate of about 12.9 mmolh^1 g^1 (Figure 4 b). These results reveal that grafting the TM-organic fragments on the W/Ta mixed-addendum nanocluster is an efficient method for adjusting its band gap. The NIR photocatalytic process For efficient use of solar energy, exploration of IR light, which occupies more than 50 % of the solar light, will be a promising method. In past decades, plenty of effort has been paid to uti- lising the UV and visible regions of sunlight, but studies aimed at the exploration of NIR light are rather rare. In this work, the photocatalytic decomposition of 2,4-dichlorophenol (2,4-DCP) in aqueous solution with compound 1 was probed under NIR light. The study revealed that compound 1 could be used for photocatalytic decomposition of 2,4-DCP in aqueous solution with NIR-light irradiation. During the NIR photocatalytic pro- cess, a 300 W Xe lamp with a glass filter (l >715 nm) was used to obtain the NIR spectrum, and the photocatalytic experi- ments were performed in the temperature range of 20-25 8C. As shown in Figure 5 a, the major absorption band of 2,4-DCP occurs at 284 nm and its degradation rate is 63 % for 1 (Fig- ure 5 b). In these heterogeneous photocatalytic systems, com- pounds 1 and 2 can be easily recycled by simple filtration. Powder X-ray diffraction (PXRD) after the photocatalytic meas- urements revealed that both 1 and 2 retained their crystallinity after the photocatalytic experiments (Figure 6 and Figure 7). Conclusion Copper-organic fragments have been successfully grafted on the {Ta4O6(SiW9Ta3O40)4} cluster, which enabled the materials to harvest a wide spectrum of solar light, from UV to the near-in- frared (NIR) region. Photocatalytic studies revealed that com- pounds 1 and 2 exhibited UV- and visible-light-driven photoca- talytic water splitting activity, and they could also be used as catalysts for the photocatalytic decomposition of 2,4-dichloro- phenol in water with NIR-light irradiation. The successful syn- thesis of transition-metal-containing high-nuclear W/Ta clusters not only provides sparse examples of tantalate chemistry, but also suggests a new route for adjusting the band gap of the high-nuclear tantalate cluster compounds. In this field, other functional metal-organic groups might be suitable to function- alise the high-nuclear tantalate cluster, such as the photoactive cobaloximes and dye molecules. Further study might supply useful information on coupling different photoactive compo- nents into the same photocatalytic system for overall water splitting. Studies in this field are ongoing in our group. Experimental Section Materials and methods Cs3K3.5H0.5[SiW9(TaO2)3O37]·9H2O was synthesised according to the procedure described in the literature,[18, 19] and the powder product was used without further purification. The material was character- ised by IR spectroscopy and elemental analysis. All other reagents were purchased commercially and used without further purifica- tion. Elemental analyses (C, H and N) were performed on a Perki- nElmer 2400 CHN elemental analyser. W, Ta, K, Cs, Na and Cu were determined by a Leeman inductively coupled plasma spectrometer. IR spectra were obtained on an Alpha Centaurt FTIR spectrometer in the 400-4000 cm^1 region with a KBr pellet. Thermogravimetric analysis (TGA) was performed on a PerkinElmer TGA7 instrument under flowing N2 with a heating rate of 108C min^1. All electro- chemical measurements were performed on a CHI 660 electro- chemical workstation at room temperature (25-30 8C). UV/Vis ab- sorption spectra were recorded on a 752 PC UV/Vis spectropho- tometer and a Varian Cary 500 scan UV/Vis NIR spectrophotometer. The PXRD patterns were recorded on a Rigaku D/Max-2500 diffrac- tometer. Synthesis of 1 A mixture of Cs3K3.5H0.5[SiW9(TaO2)3O37]·9H2O (0.15 g), CuCl2·2H2O (0.12 g), 2,2'-bpy (0.014 g) and H2O (10 mL) was sealed in a Teflon- lined autoclave and heated at 170 8C for 3 days, followed by slow cooling to room temperature with a cooling rate of 2 8Ch^1. After being washed with water several times, crystals of 1 were collected as blue block crystals with a yield of about 46 % (based on W). Ele- mental analysis calcd (%): Cu 3.36, Cs 0.88, Ta 19.17, W 43.81, Si 0.74, Na 0.30, C 7.95, H 0.97, N 1.85 ; found : Cu 3.05, Cs 0.73, Ta 18.92, W 43.48, Si 0.66, Na 0.36, C 8.12, H 1.13, N 1.68. Synthesis of 2 A mixture of Cs3K3.5H0.5[SiW9- (TaO2)3O37]·9H2O (0.15 g), CuCl2· 2H2O (0.12 g), 2-aminopyridine (0.008 g) and H2O (10 mL) was sealed in a Teflon-lined autoclave and heated at 170 8C for 3 days, followed by slow cooling to room temperature with a cooling rate of 2 8 Ch^ 1. After being washed with water several times, crystals of 2 were collected as blue block crys- tals with a yield of about 46 % (based on W). Elemental analysis calcd (%): Cu 1.81, K 1.11, Ta 20.58, W 47.05, Si 0.79, Na 0.65, C 1.71, H 0.92, N 0.79 ; found : Cu 2.02, K 1.18, Ta 20.94, W 46.77, Si 0.69, Na 0.72, C 1.87, H 0.83, N 0.85. X-ray crystallography Suitable single crystals of 1 and 2 were selected and mounted onto the end of a thin glass fibre by using Fomblin oil. The crystallographic data were obtained on a Rigaku R-AXIS RAPID IP diffractometer (for 1 ) and Bruker APEX II CCD diffractometer (for 2) with graphite-monochromated MoKa ra- diation (l = 0.71073 ^). The crystal data were solved by the direct method and refined by the full-matrix least-squares method on F2, using the SHELX-97 crystallographic software package.[29] In the re- finement, the restraint command "isor" was employed to restrain the oxygen atoms so as to avoid non-positive definite problems on them. Further details of the X-ray structural analysis are given in Table 1. Selected bond lengths of 1 and 2 are listed in Tables S1 and S2, respectively. CCDC 968384 (1) and 969091 (2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Photocatalytic water splitting under UV irradiation Photocatalytic water splitting was performed as follows : the cata- lyst was prepared by diffusing the appropriate amount of catalyst (50 mg) in a 0.5 m HCl aqueous solution (80 mL, pH 0.3-0.5) con- taining H2PtCl6 (2 mg) and CH3OH (20 mL). The above solution was degassed by purging with N2 gas for 20 min in a flask (300 mL). The reaction was then started by irradiating the solution with a 300 W Xe lamp at room temperature. The amount of H2 was ana- lysed by gas chromatography. Photocatalytic water splitting under visible-light irradiation The photocatalytic water splitting process was similar to that de- scribed above. The catalyst was prepared by diffusing the appro- priate amount of catalyst (100 mg) in a 10 % TEA solution (200 mL, pH 10) containing H2PtCl6 (4 mg) and [Ru(bpy)3]2 + (10 mg). The above solution was degassed by purging with N2 gas for 20 min in a flask (300 mL). Note that there was an induction period of about 1h underthe300WXelampbeforetheexperimentthatmightbe a result of the reduction of H2PtCl6 to Pt nanoparticles. The reac- tion was then started by irradiation of the solution with visible light (l > 400 nm) from a 300 W Xe lamp at room temperature. The NIR photocatalytic process Compound 1 (20 mg) was added to 2,4-DCP solution (50 mL, 20 mg L^1). The obtained suspensions were stirred magnetically in the dark for about 30 min to ensure the equilibrium of the working solution. The solution was then exposed to NIR irradiation from a 300 W Xe lamp with a glass filter ( l > 715 nm), and was kept stir- ring during irradiation at 20-25 8C. The decomposition of 2,4-DCP was detected by using a 752 PC UV/Vis spectrophotometer. Then, the solids were separated by centrifugation. PXRD study indicated that all the materials kept their structural integrity. 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Wasie- lewski, M. G. Kanatzidis, J. Am. Chem. Soc. 2013, 135, 2330- 2337. [29] G. M. Sheldrick, SHELXL97, Program for Crystal Structure Refinement, University of Gçttingen : Gçttingen, Germany, 1997. Received : April 3, 2014 Published online on May 18, 2014 Tian-Zhan Zhang,[a] Shuang Yao,[b] Zhi-Ming Zhang,*[a] Ying Lu,[a] Yang-Guang Li,[a] and En- Bo Wang*[a] [a] T.-Z. Zhang, Dr. Z.-M. Zhang, Y. Lu, Prof. Y.-G. Li, Prof. E.-B. Wang Key Laboratory of Polyoxometalate Science of Ministry of Education Department of Chemistry, Northeast Normal University Renmin Street No. 5268, Changchun, Jilin 130024 (P. R. China) Fax: +86-431-85098787 E-mail: [email protected] [email protected] [b] Dr. S. Yao College of Chemistry and Environmental Engineering Changchun University of Science and Technology, Changchun 130022 (P. R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cplu.201402094. (c) 2014 Blackwell Publishing Ltd. |