(CH2)n - American Chemical Society

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Inserting -(CH2)n- (n = 2, 3, 4) Spacers into the Reactant Mercaptomethyltetrazole Ligand for Tuning the Multinuclear AgI Clusters in Keggin-Based Compounds Xiu-Li Wang,* Qiang Gao, Ai-Xiang Tian, and Guo-Cheng Liu Department of Chemistry, Bohai University, Jinzhou 121000, P. R. China S Supporting Information *

ABSTRACT: Four SiMo12O404− (SiMo12)-based compounds, namely, [Ag 6 Cl 2 (mmt) 4 (H 4 SiMo 1 2 O 4 0 )(H 2 O) 2 ] (1), [Ag4(bmte)2(H2O)2(SiMo12O40)] (2), [Ag4(bmtr)2(H2O)2(SiMo12O40)] (3), and [Ag4(bmtb)3(SiMo12O40)] (4) (mmt = 1-methyl-5-mercapto1,2,3,4-tetrazole, bmte = 1,2-bis(1-methyl-5-mercapto-1,2,3,4tetrazole)ethane, bmtr = 1,3-bis(1-methyl-5-mercapto-1,2,3,4tetrazole)propane, bmtb = 1,4-bis(1-methyl-5-mercapto-1,2,3,4tetrazole)butane), have been synthesized under hydrothermal conditions. Single crystal X-ray diffraction analyses reveal that insertion of -(CH2)n- spacers into the reactant mmt ligand plays important roles in constructing multinuclear AgI clusters in the title compounds and tuning the formation of different multinuclear AgI clusters. In compound 1, the mmt ligands link AgI ions forming a three-dimensional non-multinuclear selfpenetrating framework with large dimension channels occupied by SiMo 12 polyanions. In 2, a tetranuclear [Ag4(bmte)2(H2O)2]4+ and SiMo12 anions arrange alternately forming a one-dimensional (1D) chain. The structure of compound 3 is similar to that of 2, except for different coordination modes of AgI ions in tetranuclear clusters and SiMo12 polyanions owing to the longer -(CH2)3- alkyl skeleton of bmtr ligand. Compound 4 exhibits a two-dimensional grid layer formed by a 1D AgI “ribbon” based on binuclear AgI clusters and bridging bmtb ligands with the longest -(CH2)4- alkyl skeleton. The SiMo12 polyanions as tetradentate inorganic linkages reside in the grids. The influences of -(CH2)n- spacers on forming and tuning different multinuclear AgI clusters have been discussed. Furthermore, the photochemical catalysis and electrochemical properties of the title compounds have been studied.

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INTRODUCTION As an outstanding class of inorganic metal-oxide clusters, polyoxometalates (POMs) possess abundant structural diversity and versatile physical and chemical properties, such as catalytic, photochemical, and electrochemical activity.1 These merits of POMs allow them to receive more attention in recent years.2 Parallel to the rapid progress of POMs, a remarkable branch is to modify the polyanions with various transition-metal complexes (TMCs), aiming for designing and constructing one-, two-, or three-dimensional (1D, 2D, or 3D) frameworks with novel topologies and specific properties.3 To our knowledge, reports on the introduction of multinuclear clusters as special TMCs bearing organic ligands are very limited in this branch.4 Furthermore, the limited reports on these compounds are almost based on the introduction of multinuclear CuI clusters. Only a limited number of complexation of AgI multinuclear clusters has been reported to construct POM hybrid complexes. AgI ions have a potential to form such a multinuclear cluster hybrid POM complex because they have a strong coordination ability with organic ligands with various coordination modes and have been known to form many types of coordination polymers of multinuclear AgI clusters,5 and © 2012 American Chemical Society

have been investigated even though there are no POM units in the complexes.6 Therefore, we aim to establish new multinuclear AgI hybrid complexes of the POM system. Viewing from the reports, whether multinuclear clusters can be formed in POM-based compounds almost rests on the selection of proper organic ligands. Furthermore, the organonitrogen ligands with adjacent coordination sites (N donors) conduce to form transition metal (TM) multinuclear clusters with ease, such as triazole, tetrazole, and pyrazole.7 In our previous work, we have selected flexible bis(tetrazole)-based thioethers ligands, which own two sets of adjacent 3-N donors, to construct multinuclear clusters, aiming for modifying POMs. As a result, a series of POM-based multinuclear CuI clusters have been obtained as expected.8 In this work, considering the similar merits of AgI ions to CuI ions, we try to extend the synthetic strategy to design multinuclear AgI clusters in POM’s field. First, we choose 1-methyl-5-mercapto-1,2,3,4-tetrazole (mmt, Chart S1, Supporting Information) as an organic moiety, Received: January 3, 2012 Revised: April 14, 2012 Published: April 17, 2012 2346

dx.doi.org/10.1021/cg300009a | Cryst. Growth Des. 2012, 12, 2346−2354

Crystal Growth & Design

Article

Table 1. Crystal Data and Structure Refinement Parameters for Compounds 1−4 formula fw cryst syst space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z Dc (g m−3) Rint GOF R1a [I > 2σ(I)] wR2b (all data) a

1

2

3

4

C8H20Ag6Cl2Mo12N16O42S4Si 3039 monoclinic P21/c 9.939(5) 14.021(5) 20.341(5) 90 107.325(2) 90 2706.0(2) 2 3.719 0.021 1.033 0.0397 0.1282

C12H24Ag4Mo12N16O42S4Si 2803 triclinic P1̅ 11.722(5) 11.788(5) 13.251(5) 98.789(5) 115.721(5) 109.598(5) 1454.8(10) 1 3.195 0.023 1.005 0.0595 0.1757

C14H28Ag4Mo12N16O42S4Si 2831 triclinic P1̅ 11.655(5) 11.968(5) 13.259(5) 112.989(5) 99.356(5) 109.907(5) 1504.6(11) 1 3.121 0.028 1.021 0.0583 0.1616

C24H42Ag4Mo12N24O40S6Si 3110 monoclinic P21/c 12.9556(6) 21.2285(10) 16.6606(6) 90 128.810(2) 90 3570.5(3) 2 2.893 0.026 1.186 0.0941 0.2489

R1 = Σ||Fo| − |Fc||/Σ|Fo|. bwR2 = Σ[w(Fo2 −Fc2)2]/Σ[w(Fo2)2]1/2.

[Ag4(bmtb)3(SiMo12O40)] (4) (mmt = 1-methyl-5-mercapto1,2,3,4-tetrazole, bmte = 1,2-bis(1-methyl-5-mercapto-1,2,3,4tetrazole) ethane, bmtr = 1,3-bis(1-methyl-5-mercapto-1,2,3,4tetrazole) propane, bmtb = 1,4-bis(1-methyl-5-mercapto1,2,3,4-tetrazole) butane) have been obtained. Whether expected multinuclear AgI clusters can be obtained by inserting -(CH2)n- (n = 2, 3, 4) spacers into the reactant mmt ligand has been discussed. The effect of different spacer lengths of ligands on the formation of various multinuclear AgI clusters has also been studied. The photochemical catalysis and electrochemical properties of the title compounds are reported.

which has adjacent 3-N donors and a pendent S atom. Unfortunately, the mmt ligand itself is unable to form a multinuclear cluster and it only acts as a linker for polyanions through a mononuclear AgI complex. The high coordination ability of the S atom to AgI ions may cause the formation of a non-multinuclear structure. So, the synthetic strategy must be improved by introducing a functional group to S for the purpose to restrict the coordination ability of the S atom for multinuclear-cluster formation. Thus, we introduced -(CH2)nalkyl chains on the mmt ligand at the S atom through covalent bonds which can increase the steric hindrance to restrict the coordination ability of the ligand. Fortunately, a tetranuclear AgI cluster has been constructed as expected. The formation of another similar tetranuclear AgI cluster further conforms the rationality of our synthetic strategy by introducing -(CH2)3alkyl to mmt, which has a similar length to a -(CH2)2- spacer. In order to enrich experimental data, a longer -(CH2)4- spacer was introduced and a binuclear AgI cluster has been built, which are further connected to form a AgI “ribbon”. Therefore, inserting -(CH2)n- (n = 2, 3, 4) spacers into the reactant mercaptomethyltetrazole ligand is an effective strategy for constructing the multinuclear AgI clusters in POM-based compounds. In addition, changing the spacer length of ligands can tune the formation of different multinuclear AgI clusters. Many of the reported POM-based compounds modified by TMCs are synthesized under hydrothermal conditions, which can be affected by subtle changes, such as reactants, pH value, and reaction time.9 The introduction of flexible organic ligands to POMs systems under hydrothermal conditions becomes a popular field.10 The influences of the -(CH2)n- spacer lengths of flexible ligands on structural dimensionality and frameworks of corresponding compounds have been discussed.10a,11 However, up to now, the effect of -(CH2)n- spacers on the formation of various multinuclear clusters in the POM field has not been not reported. Thus, it becomes an attractive branch with a hope to obtain informative examples for designable syntheses with hydrothermal techniques. As a result, four new SiMo12-based AgI coordination polymers, [Ag 6 Cl 2 (mmt) 4 (H 4 SiMo 12 O 4 0 )(H 2 O) 2 ](1), [Ag4(bmte)2(H2O)2(SiMo12O40)] (2), [Ag4(bmtr)2(H2O)2(SiMo12O40)] (3),

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EXPERIMENTAL SECTION

Materials and Methods. All reagents and solvents for syntheses were purchased without further purification. The organic ligands bmte, bmtr, and bmtb were prepared according to literature methods.12 The infrared (IR) spectra were obtained on a Magana FT-IR 560 spectrometer with KBr pellet in the 400−4000 cm−1 region. Elemental analyses (C, H, and N) were performed on a Perkin-Elmer 2400 CHN elemental analyzer. Powder X-ray diffraction patterns were recorded on a Bruker D8 X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å). Thermal gravimetric analyses (TGA) were carried out in N2 on a Pyris-Diamond thermal analyzer with a rate of 10 °C·min−1. Electrochemical measurements were performed with a CHI 440 electrochemical workstation. A conventional three-electrode system was used. A SCE was used as a reference electrode, and a Pt wire was used as a counter electrode. Chemically bulk-modified carbon paste electrodes were used as the working electrodes. UV−vis absorption spectra were obtained using a SP-1900 UV−vis spectrophotometer. Preparation of [Ag6Cl2(mmt)4(H4SiMo12O40)(H2O)2] (1). A mixture of AgNO3 (0.2 g, 1.2 mmol), mmt (0.035 g, 0.3 mmol), and H4SiMo12O40·26H2O (0.16 g, 0.07 mmol) was dissolved in 10 mL of distilled water and stirred for 30 min in air. When the pH of the mixture was adjusted to about 1.5 with 1 M HCl, the suspension was transferred into a Teflon-lined autoclave and kept under autogenous pressure at 160 °C for 3 days. After slow cooling to room temperature, black and block crystals of 1 were obtained in 23% yield (based on Mo). Elemental analysis (%) calcd for C8H20Ag6Cl2Mo12N16O42S4Si: C 3.16, H 0.65, N 7.38; Found: C 3.20, H 0.61, N 7.40. IR (KBr pellet, cm−1): 3067(w), 2920(w), 1515(m), 1360(m), 1308(w), 945(s), 893(s), 790(s), 689(s). Preparation of [Ag4(bmte)2(H2O)2(SiMo12O40)] (2). Compound 2 was prepared similarly to 1, except that bmte (0.057 g, 0.22 mmol) was used instead of mmt. Yellow and block crystals of 2 were obtained 2347



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in 28% yield (based on Mo). Elemental analysis (%) calcd for C12H24Ag4Mo12N16O42S4Si: C 5.14, H 0.86, N 7.99; Found: C 5.12, H 0.87, N 8.00. IR (KBr pellet, cm−1): 3736(w), 3612(w), 1685(m), 1550(w), 1415(m), 952(s), 896(s), 795(s), 697(s). Preparation of [Ag4(bmtr)2(H2O)2(SiMo12O40)] (3). Compound 3 was prepared similarly to 1, except that bmtr (0.06 g, 0.22 mmol) was used instead of mmt. Yellow and block crystals of 3 were obtained in 25% yield (based on Mo). Elemental analysis (%) calcd for C14H28Ag4Mo12N16O42S4Si: C 5.93, H 0.99, N 7.91; Found: C 5.95, H 1.01, N 7.95. IR (KBr pellet, cm−1): 3741(w), 3614(w), 1680(m), 1546(w), 1415(m), 950(s), 900(s), 791(s), 695(s). Preparation of [Ag4(bmtb)3(SiMo12O40)] (4). Compound 4 was prepared similarly to 1, except that bmtb (0.063 g, 0.22 mmol) was used instead of mmt. Yellow and block crystals of 4 were obtained in 31% yield (based on Mo). Elemental analysis (%) calcd for C24H42Ag4Mo12N24O40S6Si: C 9.26, H 1.35, N 10.80; Found: C 9.30, H 1.33, N 10.78. IR (KBr pellet, cm−1):3745(w), 3615(w), 1675(w), 1540(m), 1420(m), 948(s), 892(s), 793(s), 689(s). Single Crystal Structure Determination. Crystal structure determinations for compounds 1−4 were performed on a Bruker Smart Apex 1000 CCD diffractometer with Mo Kα radiation (λ = 0.71069 Å) by ω and θ scan mode at room temperature. The structures were solved by direct methods and refined on F2 by fullmatrix least-squares using the SHELXL package.13 All non-hydrogen atoms were refined anisotropically, all the H atoms attached to carbon atoms were generated geometrically while the H atoms attached to water molecules were not located but were included in the structure factor calculations. The crystal data and structure refinements for the title compounds are summarized in Table 1. Selected bond distances and angles for the four compounds are listed in Table S1 (Supporting Information). The CCDC reference numbers are 852618−852621 for the title compounds. Preparation of 1−, 2−, 3−, and 4−CPEs. The compound 2 bulk-modified carbon paste electrode (2−CPE) was fabricated as follows: 0.03 g of compound 2 and 0.5 g of graphite powder were mixed and grounded together by an agate mortar and pestle to achieve a uniform mixture, and then 0.18 mL of Nujol was added with stirring. The homogenized mixture was packed into a glass tube with a 3 mm inner diameter, and the tube surface was wiped with weighing paper. Electrical contact was established with a copper rod through the back of the electrode. The same procedure was used for preparation of bare CPE, 1−, 3−, and 4−CPEs.

Figure 1. Ball-stick/polyhedral view of the molecule structure of 1. All H atoms are omitted for clarity.

are 2.211(7)−2.331(7) Å for Ag−N, 2.453(2)−2.705(2) Å for Ag−S, 3.018(2) Å for Ag−Cl and 2.536(7)−2.873(2) Å for Ag−O. Herein, the distances of some Ag−O bonds are longer, which can be considered as coordination interaction according to the previous literature reports.15 Notably, the mmt ligand in compound 1 exhibits an interesting coordination mode, providing two N and one S donors to link four AgI ions. However, the left N2 and N6 donors are noncoordinated, which maybe induced by hindrance of the -CH3 group. Furthermore, the introduction of S atom for linking AgI ions in POM’s field is seldom reported. Owing to the abundant potential coordination sites scattering in different directions of the organic mmt ligand, the metal− organic section in 1 constructed from AgI ions and mmt molecules shows the extended structure. Although the -CH3 group shows the influence on the coordination ability of N2 and N4 donors, the steric hindrance of methyl has not hindered the dimensional expansion of 1. First, through the linkage of S2 and N5 atoms of the mmt ligands, mmt and Ag1 ions connect with each other alternately to form a 1D helical chain. The adjacent helical chains are connected by Ag2-mmt subunits through Ag−N bonds, leading to the formation of 2D wavy layer (Figure 2). Further, Ag3 ions connect the adjacent 2D wavy layer forming a 3D framework, in which large dimension channels exist running along the a axis. The channels are occupied by the nanosized SiMo12 polyanions (Figure 3). The most fascinating structural feature of 1 is that each SiMo12 is coordinated by 12 AgI ions in the 3D host framework, which represents the highest connected number of the keggin-type POMs to date. As shown in Figure 4, considering the [Ag12Cl2S6O2N16C8H12] fragments (Figure S1, Supporting Information) as 8-connected nodes and SiMo12 anions as 6connected nodes respectively, the total topology of 1 can be considered as a (6, 8)-connected 3D framework with (316·452·516·67) (38·47) topology when analyzed by the OLEX program.16 The topology framework exhibits a self-penetrating character (Figure 4). Structural Description of 2. Single crystal X-ray analysis shows that compound 2 consists of four AgI ions, two bmte ligands, two coordinating water molecules, and one SiMo12 anion (Figure 5). The valence sum calculations14 show that all the Mo atoms are in +VI oxidation state. In compound 2, the bmte ligand was utilized, in which -(CH2)2- spacer was inserted into two mmt molecules and the S atoms were covalently linked by C atoms. Furthermore, the

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RESULTS AND DISCUSSION Structural Description of 1. Crystal structure analysis reveals that compound 1 is constructed from one SiMo12 anion, six AgI ions, four mmt ligands, two Cl− ions, and two water molecules. The valence sum calculations14 show that all the Mo atoms are in +VI oxidation state. Compound 1 is obtained as black crystals different from 2−4 that may be attributable to the anionic mercapto group. In order to balance the charge of the compound, four protons are added to SiMo12, and then 1 is formulated as [Ag6Cl2(mmt)4(H4SiMo12O40)(H2O)2]. It is worth mentioning that the mmt ligand is first utilized in the construction of POM-based hybrid compounds. As shown in Figure 1, there are three crystallographically independent AgI ions, which exhibit three types of coordination geometries in compound 1. The Ag1 ion adopts sixcoordinated triprism geometry of [AgN2O3S] by two N atoms of different mmt ligands, three O atoms of the SiMo12 polyanion, and one S atom from one mmt ligand. The Ag2 ion is six-coordinated by two O atoms of SiMo12 cluster, one water molecule, one N atom, and one S atom from different mmt and one Cl− ion, showing a distorted octahedron pyramid arrangement. The Ag3 ion is four-coordinated by two S atoms and one N atom from two mmt ligands and one O atom from one SiMo12 anion. The bond distances around AgI ions 2348



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Figure 2. The view of the 2D wavy layer in 1 constructed from 1D helical chains and Ag2-mmt subunits.

Figure 3. The 3D framework with the large dimension channels running along the a axis in 1 and the SiMo12 polyanions occupy the channels.


Figure 4. Schematic view of the (6, 8)-connected framework in compound 1. Color code: green balls, SiMo12 anions; pink balls, [Ag12Cl2S6O2N16C8H12] fragments.

are 2.354(1)−2.777(9) Å for Ag−O, 2.358(1)−2.429(1) Å for Ag−N, and 72.9(4)−105.0 (5)° for N−Ag−O, 83.9(9)− 153.0(4)° for O−Ag−O, 125.9(4)° for N−Ag−N, which are comparable to those in the AgI compounds.17 The Ag2 ion is six-coordinated by μ4-O12, μ4-O12#1, μ3-O3 and μ4-O7 of SiMo12 polyanions, two N atoms from one bmte ligand. The bond distances and angles around the Ag2 ion are 2.152(1)− 2.183(1) Å for Ag−N, 2.712(1)−2.812(1) Å for Ag−O, and 144.9(4)° for N−Ag−N, 58.9(0)−134.4(3)° for O−Ag−O. Intriguingly, four AgI ions and two coordinating O23 atoms

steric hindrance reduces the coordination ability of S atoms with Ag ions. Then, the left three N donors conduce to make the Ag ions be converged. Fortunately, a tetranuclear cluster [Ag4(bmte)2(H2O)2]4+ was constructed (Figure 6a), in which the AgI ions show two different coordination modes. The Ag1 ion is five-coordinated by μ4-O12 and μ4-O7 from two SiMo12 polyanions, two N atoms from two bmte ligands, and one water molecule. The bond distances and angles around the Ag1 ion 2349



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anion has not been reported. The bond distances and angles around the AgI ions are 2.162(1)−2.442(1) Å for Ag−N, 2.426(1)−2.864(3) Å for Ag−O, and 120.7(4)−161.3(4)° for N−Ag−N. Similar to the structure of 2, the SiMo12 polyanions and tetranuclear [Ag4(bmtr)2(H2O)2]4+ clusters arrange alternately forming a 1D chain (Figure S4, Supporting Information). Structural Description of 4. When bmtb with four -(CH2)- spacers was used in compound 4, a 2D network was obtained. Crystal structure analysis reveals that compound 4 is constructed from 1D AgI ribbon based on binuclear AgI clusters, SiMo12 anions and bmtb ligands. As shown in Figure 8, there are two crystallographically independent AgI ions (Ag1

Figure 6. (a) The tetranuclear AgI subunit in 2. (b) Eight-connected SiMo12 polyanion. (c) View of the 1D chain in 2.

from water molecules are in the same plane in one [Ag4(bmte)2(H2O)2]4+ cluster, which is not commonly observed in POM’s field. The formation of tetranuclear cluster in 2 has proven amply that the synthetic strategy of inserting -(CH2)2- spacer is rational for preparing multinuclear clusters. Another interesting structural feature for compound 2 is that the SiMo12 polyanion shows an entirely new coordination mode. As shown in Figure 6b, the SiMo12 polyanion acts as an eight-connected inorganic linkage, providing six O atoms for linking eight AgI ions, in which O12 atom acts as a μ4-O coordinating with three AgI ions, O3 and O7 atoms act as a μ3O and μ4-O coordinating with one AgI ion and two AgI ions, respectively. To the best of our knowledge, this uncommon coordination pattern has never been observed. Furthermore, the SiMo12 polyanions and [Ag4(bmte)2(H2O)2]4+ clusters arrange alternately forming a 1D chain (Figure 6c). Structural Description of 3. When bmte ligand was replaced by bmtr with three -(CH2)- spacers, compound 3 was obtained. Crystal structure analysis reveals that compound 3 shows a structure similar to that of 2 except for some different coordination modes of AgI ions and SiMo12 polyanions. Four AgI ions, two water molecules, and two bmtr ligands also construct a tetranuclear cluster [Ag4(bmte)2(H2O) 2]4+ (Figure 7). However, in 3, the Ag1 ion is four-coordinated by two O atoms (μ4-O9, μ4-O15) from two SiMo12 and two N atoms (N1, N5) from one bmtr. Meanwhile, the Ag2 ion shows an uncommon hepta-coordination mode, coordinating with four O atoms of SiMo12 polyanions (μ3-O7, μ4-O9, μ4-O15, and μ2O17), one water molecule and two N atoms from two bmtr. The angle between the plane of tetranuclear cluster [Ag4(bmtr)2(H2O)2]4+ and the central line of SiMo12 polyanion is about 36.08°, which is larger than that in 2 (Figure S2, Supporting Information) and induces to the different coordination mode of the SiMo12 polyanion (Figure S3, Supporting Information). The Ag2 connects with three O atoms from the same SiMo12 polyanion. In 3, the SiMo12 polyanion also acts as an eight-connected inorganic linkage but provides eight O atoms for linking eight AgI ions, which is different from that of 2. To our knowledge, this kind of uncommon octadentate coordination mode of SiMo12 poly-


and Ag2) in compound 4. The Ag1 is penta-coordinated by three N atoms (N1, N6, and N7) from different bmtb ligands and two O atoms (μ3-O14 and μ3-O15) from one SiMo12 polyanion with the bond lengths of 2.288(2)−2.503(2) Å for Ag−N and 2.506(1)−2.510(2) Å for Ag−O, showing a distorted quadrangular pyramid coordination mode. The Ag2 ion adopts a four-coordinated trigonal-pyramidal geometry of [AgN4] by four N atoms from four bmtb ligands with Ag−N distances ranging from 2.232(2) to 2.509(2) Å. Intriguingly, the N atoms coordinated with the same AgI ion are from different bmtb ligands and the chelating coordination style of tetrazolefunctionalized thioethers ligands disappear in compound 4, which is different from that in 2 and 3 and probably due to the 2350



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Figure 9. (a) The binuclear [Ag2(bmtb)]2+ clusters (green circle) are connected by bmtb to form a 1D chain. (b) The bridging bmtb ligand linking four AgI ions. (c) The view of 2D grid layer in 4 formed by organic-metal ribbon and bridging bmtb ligands.

properties from a new topological design. For the anionic POM, cationic metal complexes are able to bridge the large POM anion as countercation. By using mmt ligands which has multiple-coordination sites from adjacent N donor groups and additional S coordination site, it can bridge POM units and form a 3D framework as linker units like in 1. To further expand the design of those hybrid materials, we planned to combine the large anionic POM unit with a multinuclear AgI cluster. The combination of multinuclear metal clusters and POM is fundamental importance of new topological structure, because multinuclear cluster tend to have larger cationic charges per unit and it is a good combination with the large anionic charges of POM units: the combination of highly positive charged metal cluster and highly negative charged POM fit well to form a novel topological structure. For the design of such a molecule, restriction of the coordination ability on S atom in mmt ligand is necessary. The steric compactness of mmt ligands make the multinuclear cluster unstable because mmt has four possible coordination sites, and if all mmt sites are occupied by metal cations to form a multinuclear cluster, steric congestion of too small mmt and metal cations makes the whole cluster unstable. In other words, mmt ligands are too small for the multinuclear cluster formation. Thus, we designed the modified mmt ligands by introducing -(CH2)n- spacers (n = 2, 3, 4) through S which interconnect two mmt ligands. By the introduction of the methylene chains, the steric hindrance of S is increased, so that the mmt ligands is now big enough to support a multimetal cluster frameworks, and it is free from the steric congestion of original mmt ligands when it forms multinuclear clusters. In the course of our strategic synthesis by varying the length of the methylene chains, we successfully synthesized several multinuclear silver clusters. Influence of -(CH2)n- (n = 2, 3, 4) Spacer Lengths for AgI Cluster Formation. Ligand design study by interconnecting two mmt ligands with systematically varied lengths of methylene spacer has revealed an important ligand effect for the silver cluster formation on the hybrid POM system. The original mmt which owns three adjacent nitrogen donors can bridge two silver units which can be regarded as an edge of a cluster. Further coordination of metal cations to the edge may destabilize the charge balance and eventually it is a disadvantage for cluster formation. We designed a modified mmt ligand by

longer -(CH2)4- spacers of bmtb. As a result, a binuclear subunit [Ag2(bmtb)2]2+ has been obtained (Figure 9a). As shown in Figure 9a, the [Ag2(bmtb)]2+ clusters connect with each other forming a 1D chain. Furthermore, the bridging bmtb ligands (Figure 9b) link the adjacent 1D chains extending the molecular architecture of compound 4 into a 2D network with square grids (Figure 9c). The nanosized SiMo 12 polyanions occupy the grids (Figure 10). Although the longer

Figure 10. The view of 2D network of 4 and nanosized SiMo12 anions occupy the grids.

spacer -(CH2)4- of bmtb and its bridging coordination mode results in the formation of binuclear cluster different from that in 2 and 3, it is still indicates that inserting spacers into mmt is a valid route for construction of multinuclear AgI clusters. Furthermore, it is noteworthy that a SiMo12 polyanion offers four μ3-O atoms to coordinate with two Ag1 ions (Figure S5, Supporting Information). The similar coordination style is reported in [{Ag(bpy)}2 {Ag4(bpy) 6}{PMo 11VO40 }][{Ag(bpy)}2{PMo11VO40}] by You’s group.5c Ligand Design for Multinuclear AgI Cluster and POM Hybrid Complexes. Design and synthesis of metal clusterPOM hybrid complexes are attractive subjects aiming for novel 2351



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Figure 11. (a) Cyclic voltammograms of 2−CPE in a 1 M H2SO4 solution at different scan rates (from inner to outer: 60, 80, 100, 120, 150, 200, 250, 300, 350, 400, 450, and 500 mV/s). (b) Cyclic voltammograms of 2−CPE in a 1 M H2SO4 solution containing 0.0−8.0 mM KNO2 and a bare CPE in a 4.0 mM KNO2 + 1 M H2SO4 solution. Scan rate: 120 mV/s.

good. The difference in reflection intensities between the simulated and experimental patterns is due to the different orientation of the crystals in the powder samples. Electrochemical Behavior of the 2−CPE in Aqueous Electrolyte. Among those important properties of POMs, the ability to undergo reversible multielectron redox processes endows them with great attraction in electrochemical and electrocatalytic research.18 To study the redox property of the title compounds, the compounds 1−4 bulk-modified carbon paste electrodes (CPEs) were fabricated as the working electrode owing to insolubility of the POM-based inorganic− organic hybrids prepared by hydrothermal reaction in water and poor solubility in common organic solvents. The redox properties of the title compounds were investigated in 1 M H2SO4 aqueous solution. Considering that there are only some slight potential shifts of the redox peaks among the title compounds (Figure S10, Supporting Information), the 2−CPE has been taken as an example to study their electrochemical properties. Figure 11a shows the cyclic voltammetric behaviors of 2− CPE in a 1 M H2SO4 solution. There are three reversible redox peaks in the potential range of 800 to −150 mV; the mean peak potentials E1/2 = (Epa + Epc)/2 are 306, 188, −18 mV (scan rate: 200 mV/s), attributable to the SiMo12 polyanions.19 However, the oxidation peak of the Ag centers is not observed in the potential range of 800 to −150 mV. This phenomenon was also observed in the similar SiMo12/Ag system, such as [Ag4(Hfcz)2(SiMo12O40)] (Hfcz = 2-(2,4-difluorophenyl)-1,3di(1H-1,2,4-triazol-1-yl)propan-2-ol).19c As can be seen from Figure S11, Supporting Information, with increasing of the scan rates, the cathodic peak potentials shift to the negative direction and the corresponding anodic peak potentials shift to the positive direction. The peak currents are proportional to the scan rates, indicating that the redox process of 2−CPE is surface-controlled. It is well-known that direct electroreduction of nitrite requires a large overpotential at most electrode surfaces, and no obvious response was observed at a bare CPE in the potential range of 800 to −150 mV in 1 M H2SO4 aqueous solution containing 4 mM KNO2. However, the 2−CPE has good electrocatalytic activity toward the reduction of nitrite. At the 2−CPE, with the addition of nitrite, all the reduction peak currents increase markedly while the corresponding oxidation peak currents decrease gradually (Figure 11b), which indicates

inserting a methylene chain between two mmt ligands. Each mmt unit can bridge adjacent silver cations to form a corner of a trinuclear silver cluster and the -(CH2)n- chain support the trinuclear silver unit from the outside of the corner keeping “L” shape. To evaluate the best -(CH2)n- length for the cluster formation, we examined three types of chain lengths from two to four -CH2- units. The results indicate -(CH2)2- and -(CH2)3are best suited for the silver tetranuclear cluster formation in square geometry. The modified ligands support the opposite corners of the square cluster. Both compounds, 2 and 3, show a similar silver square platform with POM bindings, but the coordination modes of silver atoms were different by the chain lengths (Figure S6, Supporting Information). The longest -(CH2)4- ligand, bmtb, exhibited the formation of a binuclear silver cluster and no further aggregation of silver cations was observed, yet an interesting AgI ribbon network was formed in 4 (Figure S7, Supporting Information). In summary, the best length for cluster formation in hybrid POM complexes was two or three methylene chains in the current ligand system. It gives tetranuclear silver clusters with square geometry and the modified ligand binds three silver atoms to support a corner of the square. The longest -(CH2)4- chain hinder the formation of a larger cluster and it exhibits silver ribbon formation with binuclear moiety of silver cluster. Thermal Gravimetric Analyses. To examine the thermal stabilities of the title compounds, the decomposition behaviors were examined by thermogravimetric (TG) analyses in flowing N2 with a heating rate of 10 °C·min−1 in the temperature range of 40−800 °C, as shown in Figure S8, Supporting Information. All TG curves of compounds 1, 2, and 3 show two distinct weight loss steps. The first weight loss steps occur before 150 °C, which corresponds to the loss of water molecules, 1.25% (calcd. 1.19%) in 1, 1.45% (calcd. 1.28%) in 2 and 1.42% (calcd. 1.27%) in 3. The second weight loss steps in 200−400 °C correspond to the loss of organic ligands, 15.46% (calcd. 15.16%) in 1, 18.94% (calcd. 18.41%) in 2 and 18.62% (calcd. 19.20%) in 3. The TG curve of compound 4 exhibits only one weight loss step, which corresponds to the loss of bmtb molecules 28.02% (calcd. 27.60%). Powder X-ray Diffraction. Figure S9 (Supporting Information) presents the powder X-ray diffraction patterns for compounds 1−4. The diffraction peaks of both simulated and experimental patterns match well in positions, thus indicating that the phase purities of the compounds 1−4 are 2352



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Figure 12. (a) Absorption spectra of the MB solution during the decomposition reaction under UV light irradiation with the use of compound 1. (b) Photocatalytic decomposition of MB solution under UV with the use of four compounds; the black curve is the control experiment without any catalyst and the red curve for SiMo12.

compounds through inserting -(CH2)n- spacers into the reactant mmt ligand. Furthermore, the -(CH2)n- (n = 2, 3, 4) spacers have an influence on formation of different multinuclear AgI clusters. In 2 and 3, although similar tetranuclear AgI clusters have been built, the longer -(CH2)3- spacer makes different coordination modes of Ag ions. The -(CH2)4- spacer, longer than both -(CH2)2- and -(CH2)3-, induce the binuclear AgI clusters in 4. The binuclear AgI clusters are further connected to form a AgI “ribbon”. This work may provide informative examples in exploring multinuclear TM clustermodified POM compounds. Furthermore, our attempts show that reasonable syntheses would be reached in some extent by designing and improving different synthetic strategies.

that the reduction of nitrite is mediated by the reduced species of SiMo12 in compound 2. Furthermore, the electrocatalytic reduction of nitrite at the CPE containing simply SiMo12 (SiMo12−CPE) and 1−, 3−, 4− CPE have been performed (Figure S12, Supporting Information), showing electrocatalytic activity similar to 2−CPE, which proves that the SiMo12 polyoxoanions in compounds are the active agent for reduction of nitrite. Photocatalytic Property. The photocatalytic performance of the title compounds were investigated with photodegradation of the methylene blue (MB) under UV irradiation through a typical process: 150 mg powder of the title compounds was mixed with 10.0 mg·L−1 MB solution 90 mL, magnetically stirred in the dark for 0.5 h to ensure the equilibrium. The mixture was exposed to UV irradiation from a high pressure mercury vapor lamp and kept continuously stirring. 3.0 mL of sample was taken for analysis every 15 min. The photocatalytic property of compound 1 is shown in Figure 12a (Figure S13 for 2, 3, 4, without any catalyst and Figure S14 for simply SiMo12 under Ag+-free conditions). The range of wavelength for the photocatalytic measurements is 500−800 nm. It can be clearly seen that the absorption peak of MB decreased obviously as time goes by for the title compound. Approximately 60% of MB had been decomposed during 75 min. In the same time scope, the dissociation of MB was no more than 10% without any catalyst (Figure 12b) and about 40% for the simply SiMo12 polyanions. This result indicates that there is a synergy between Ag-organic ligands subunits and SiMo12. The title compounds show excellent photocatalytic activity for the degradation of MB.

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ASSOCIATED CONTENT

* Supporting Information S

Table of selected bond lengths and angles for compounds 1−4; structure figures of 2−4 and TG, CV, photochemical catalysis curves. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-416-3400158. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for financial support from the National Natural Science Foundation of China (Nos. 21171025 and 21101015), New Century Excellent Talents in University (NCET-090853), and Natural Science Foundation of Liaoning Province 201102003).

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CONCLUSION Four new compounds based on Keggin polyanions and Scontaining bis(tetrazole) ligands have been successfully obtained under hydrothermal conditions. By using the mmt ligand, a new 3D nonmultinuclear framework (1) has been obtained, in which AgI ions only act as linkers of polyanions and mmt molecules. However, the -(CH2)n- (n = 2, 3, 4) spacers have been introduced to mmt molecules, aiming for increasing the steric hindrance of S donors and forming prospective multinuclear AgI clusters. Fortunately, three POMbased multinuclear AgI clusters (2, 3, and 4) have been constructed as expected. So, it becomes an effective strategy for constructing the multinuclear AgI clusters in POM-based

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REFERENCES

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