Influence of Transition Metal Coordination Nature on the Assembly of

DOI: 10.1021/cg1006742

Influence of Transition Metal Coordination Nature on the Assembly of Multinuclear Subunits in Polyoxometalates-Based Compounds

2010, Vol. 10 4786–4794

Xiu-li Wang,* Hai-liang Hu, and Ai-xiang Tian Faculty of Chemistry and Chemical Engineering, Bohai University, Jinzhou 121000, P. R. China Received May 20, 2010; Revised Manuscript Received September 14, 2010

ABSTRACT: Four inorganic-organic hybrids based on Keggin-type polyoxometalate (POM), [Ag3(bmte)2(PMo12O40)] (1), [CuI3(bmte)3(PMo12O40)] (2), [CuII(bmte)3(HPMo12O40)] (3), and [Ni(bmte)3(HPMo12O40)] (4) (bmte = 1,2-bis(1-methyl-5mercapto-1,2,3,4-tetrazole)ethane), have been synthesized under hydrothermal conditions by tuning the coordination nature of transition metal (TM) ions. The transformation of coordination modes of TM ions has a crucial influence on the multinuclear structures of this series. In compound 1, binuclear subunits [Ag(1)2(bmte)2]2þ occur, which is connected by PMo12O403(PMo12) anions to construct a one-dimensional (1D) chain. Furthermore, these chains are linked by Ag(2) ions acting as fuses to construct a two-dimensional (2D) gridlike sheet. Compound 2 shows trinuclear clusters, [CuI3(bmte)3]3þ, which are also connected by PMo12 anions to construct a 1D chain. These two neighboring chains arrange like a “zipper”. Compounds 3 and 4 are isostructural. In compound 3, the metal-organic subunit is a 2D sheet constructed by a series of three-membered circles. The polyoxoanions are sandwiched by these two sheets like a “hamburger”. The influence of the TM coordination nature on the assembly of multinuclear subunits in POM-based compounds is discussed. Introduction Polyoxometalates (POMs), as a unique class of discretemolecular metal-oxo clusters, exhibit structural diversity and a multitude of physical and chemical properties, such as catalytic activity, magnetism, photochemical activity, and reversible redox behavior.1 Up to now, owing to their many merits, POMs have attracted more attention. In this field, a remarkable branch is to modify the polyoxoanions with metal-organic subunits, which can be grafted onto polyoxoanions to construct new structures with one-, two-, or three-dimensional (1D, 2D or 3D) frameworks.2 Thus, the design of different modifiers of metal-organic subunits becomes an important task, especially the choice of proper organic ligands. Recently, flexible organonitrogen ligands, which are favorable for assembling novel structures, have become a popular field for modifying POMs. The flexible bis(imidazole) and bis(triazole) ligands are commonly used, and the Wang,3 Su4, and Peng5 groups have reported many POM-based compounds modified by this series of ligands. In our previous work, we also introduced the flexible bis(bipyridyl) ligand into the POM system, aiming for construction of new topology frameworks.6 In this work, from the synthetic strategy point of view, we improve upon these flexible bis(bipyridyl), bis(imidazole) and bis(triazole) ligands from two aspects: (i) increasing potential coordination sites; and (ii) continuing to intensify the flexibility. Thus, an optimal organic ligand 1,2-bis(1-methyl-5-mercapto-1,2,3,4-tetrazole)ethane (bmte) has been selected, which can achieve our expected strategy (Scheme 1). This ligand exhibits more advantages. For example, the bmte molecule has more potential coordination sites, six N donors and two S atoms, which possess a strong coordination ability. The introduction of S atoms to a flexible organic ligand intensifies the molecular flexibility, which can bend or distort with ease to construct novel frameworks.7 *Corresponding author. Tel.: þ86-416-3400158. E-mail: wangxiuli@bhu. edu.cn. pubs.acs.org/crystal

Published on Web 10/06/2010

In some reported coordination polymers, the adjacent coordination sites (N donors) in organonitrogen ligands can induce the formation of transition metal (TM) multinuclear clusters with ease.8 In the bmte ligand, there are three adjacent N donors in each (1-methyl-1,2,3,4-tetrazole) group, which may induce the construction of multinuclear clusters as expected. Thus, in this work, we chose the POM/TM/bmte system to design and synthesize compounds with multinuclear clusters. First, considering many reports on multi-Ag clusters,9 we chose the Agþ ion as the reactant, and then a POM-based compound with binuclear subunits was obtained. Second, we selected Cuþ ions with a coordination nature similar to Agþ10 to further approve our idea. And a new compound with trinuclear clusters was obtained. Finally, we tried to use another ion, which exhibits a coordination nature different from Agþ and Cuþ ions, to verify whether different coordination natures of TM ions have an influence on the construction of multinuclear structures. Thus, Cu2þ ion was selected, which usually exhibits five- or six-coordianted modes. And a mononuclear structure was obtained. The selection of Ni2þ ion with a coordination mode similar to Cu2þ induces the formation of an isostructural mononuclear compound (Scheme 2), which can further validate our synthetic strategy. Herein, we report two new compounds with multinuclear structures, [Ag3(bmte)2(PMo12O40)] (1) and [CuI3(bmte)3(PMo12O40)] (2) (bmte = 1,2-bis(1-methyl-5-mercapto-1,2,3, 4-tetrazole)ethane). Further, by changing TM ions with different coordination natures, two isostructural compounds with mononuclear clusters [CuII(bmte)3(HPMo12O40)] (3) and [Ni(bmte)3(HPMo12O40)] (4) were obtained. The influence of the coordination nature of TM ions on the assembly of multimuclear subunits is discussed. Experimental Section Materials and General Methods. All reagents and solvents for syntheses were purchased from commercial sources and were used as received. Elemental analyses (C, H, and N) were performed on a Perkin-Elmer 2400 CHN elemental analyzer. The IR spectra were r 2010 American Chemical Society

Article Scheme 1. The Organic Synthon 1,2-Bis(1-methyl-5-mercapto1,2,3,4-tetrazole)ethane (bmte) for Constructing the Title Compounds 1-4

Scheme 2. Preparation Routes for Compounds 1-4

obtained on an Alpha Centaurt FT/IR spectrometer with KBr pellet in the 400-4000 cm-1 region. The thermal gravimetric analyses (TGA) were carried out in N2 on a Perkin-Elmer DTA 1700 differential thermal analyzer with a rate of 10.0 °C/min. 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 (CPEs) were used as the working electrodes. Synthesis of [Ag3(bmte)2(PMo12O40)] (1). A mixture of H3[PMo12O40] 3 13H2O (0.15 g, 0.07 mmol), AgNO3 (0.2 g, 1.2 mmol), and bmte (0.057 g, 0.22 mmol) was dissolved in 10 mL of distilled water at room temperature. When the pH of the mixture was adjusted to about 3.8 with 1.0 mol 3 L-1 HNO3, the suspension was put into a Teflon-lined autoclave and kept under autogenous pressure at 160 °C for 3 days. After slow cooling to room temperature, dark red block crystals were filtered and washed with distilled water (25% yield based on Mo). Anal. Calcd for C12H20Ag3N16S4PMo12O40 (2662): C 5.41, H 0.76, N 8.41%. Found: C 5.37, H 0.79, N 8.36%. IR (solid KBr pellet, cm-1): 3740 (w), 3616 (w), 1693 (m), 1645 (w), 1555 (w), 1460 (m), 1415 (m), 1307 (w), 1217 (w), 1180 (m), 1056 (s), 959 (s), 885 (s), 801(s). Synthesis of [CuI3(bmte)3(PMo12O40)] (2). A mixture of H3[PMo12O40] 3 13H2O (0.15 g, 0.07 mmol), Cu(CH3COO)2 3 2H2O (0.13 g, 0.6 mmol), and bmte (0.13 g, 0.5 mmol) was dissolved in 10 mL of distilled water at room temperature. When the pH of the mixture was adjusted to about 4.0 with 1.0 mol 3 L-1 HNO3, the suspension was put into a Teflon-lined autoclave and kept under autogenous pressure at 160 °C for 3 days. Dark red crystals were filtered and washed with distilled water (40% yield based on Mo). Anal. Calcd for C18H30Cu3N24S6PMo12O40 (2787): C 7.75, H 1.08, N 12.05%. Found: C 7.79, H 1.04, N 12.11%. IR (solid KBr pellet, cm-1): 3733 (m), 3615 (w), 1691 (m), 1645 (w), 1548 (m), 1508 (m), 1463 (m), 1417 (w), 1316 (w), 1271 (w), 1185 (m), 1054 (s), 966 (s), 876(s), 797 (s). Synthesis of [CuII(bmte)3(HPMo12O40)] (3). A mixture of H3[PMo12O40] 3 13H2O (0.06 g, 0.03 mmol), Cu(CH3COO)2 3 2H2O (0.13 g, 0.6 mmol), and bmte (0.013 g, 0.05 mmol) was dissolved in 10 mL of distilled water at room temperature. When the pH of the mixture was adjusted to about 4.0 with 1.0 mol 3 L-1 HNO3, the suspension was put into a Teflon-lined autoclave and kept under autogenous pressure at 160 °C for 3 days. Green crystals were filtered and washed with distilled water (30% yield based on Mo). Anal. Calcd for C18H31CuN24S6PMo12O40 (2662): C 8.11, H 1.16, N 12.62%. Found: C 8.15, H 1.14, N 12.57%. IR (solid KBr pellet, cm-1): 3465 (m), 1711 (m), 1627 (m), 1465 (s), 1413 (s), 1296 (m), 1250 (w), 1217 (w), 1178 (m), 1063 (s), 966 (s), 883 (s), 799 (s).

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Synthesis of [Ni(bmte)3(HPMo12O40)] (4). Compound 4 was prepared similarly to compound 1, except that Ni(NO3)2 (0.091 g, 0.5 mmol) was used instead of AgNO3. Jade-green block crystals were filtered and washed with distilled water (40% yield based on Mo). Anal. Calcd for C18H31NiN24S6PMo12O40 (2656): C 8.14, H 1.18, N 12.65%. Found: C 8.09, H 1.23, N 12.61%. IR (solid KBr pellet, cm-1): 3726 (w), 3589 (w), 1693 (w), 1639 (w), 1562 (w), 1508 (w), 1460 (s), 1413 (s), 1296 (m), 1255(m), 1215 (w), 1178 (s), 1056 (s), 966 (m), 871 (s), 804 (s). Preparations of 1-, 2-, 3-, 4-CPEs. Compound 1 modified carbon paste electrode (1-CPE) was fabricated as follows: 90 mg of graphite powder and 8 mg of 1 were mixed and ground together by an agate mortar and pestle to achieve a uniform mixture, and then 0.1 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. In a similar manner, 2-, 3-, and 4-CPEs were made with compounds 2, 3, and 4. X-ray Crystallographic Study. X-ray diffraction analysis data for compounds 1-4 were collected with a Bruker Smart Apex CCD diffractometer with Mo-KR (λ = 0.71073 A˚) at 293 K. The structures were solved by direct methods and refined on F2 by full-matrix least-squares methods using the SHELXTL package.11 For the compounds, all the hydrogen atoms attached to carbon atoms were generated geometrically. A summary of the crystallographic data and structural determination for them is provided in Table 1. Selected bond lengths and angles of the four compounds are listed in Table S1 (Supporting Information). Crystallographic data for the structures reported in this paper were deposited in the Cambridge Crystallographic Data Center with CCDC No. 773485 for 1, 773486 for 2, 785989 for 3, and 773487 for 4.

Results and Discussion Synthesis. In this work, we aim at discussing the influence of the coordination nature of TM ions on the assembly of multinuclear subunits in POM-based compounds. From the crystal engineering point of view, we would select another TM ion having a coordination nature similar to Agþ ion to confirm our idea after binuclear compound 1 was obtained. Thus, we selected Cuþ ions, and then CuCl was used as reactant. However, there were no crystals with good quality for the X-ray crystallographic experiment, which may be due to the low solubility of CuCl. In some reports, the copper ions changes from a reactant Cu2þ ion to a resultant Cuþ ion, which is often observed in the hydrothermal reaction system containing organonitrogen-CuII-POM system. The organonitrogen species generally play a role of not only ligands but also reductants under hydrothermal conditions.3a,12 Therefore, we chose this synthetic strategy and increased the quantum of bmte aiming at reduction of reactant Cu2þ ions to resultant Cuþ ions. We tried ratios of bmte/Cu2þ of 1/12, 1/10, 1/6, and 1/1.2. Fortunately, when the ratio is higher than 1/10, we realized our plan and the þI oxidation state of copper ion was obtained in dark red crystals of compound 2. However, while the ratio is lower than 1/10, green crystals of compound 3 were obtained with the oxidation state of copper ion keeping þII. For synthesis of compound 1, we also tried the ratios of bmte/Agþ of 1/12, 1/9, 1/5, and 1/3, and can always obtain the crystals of compound 1. However, when the ratio of bmte/Agþ was adjusted to 1/5, the yield was higher than other ratios. The results indicate that the bmte may play a role of not only ligand but also reductant in the synthesis of compound 2. Crystal Structure of Compound 1. Crystal structure analysis reveals that compound 1 consists of three Agþ ions, two bmte ligands and one [PMo12O40]3- (abbreviated as PMo12)

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Table 1. Crystal Data and Structure Refinements for Compounds 1-4 formula Fw crystal system space group a (A˚) b (A˚) c (A˚) R (o) β (o) γ (o) V (A˚3) Z Dc (g 3 cm-3) μ (mm-1) F(000) final R1a, wR2b [I>2σ(I)] final R1a, wR2b (all data) GOF on F2

2

3

4

C18H30Cu3N24S6 PMo12 O40 2787 monoclinic C2/c 18.9871(7) 21.7357(8) 15.7400(6)

109.803(5)

96.3660(10)

2745.4(18) 2 3.221 3.976 2496 0.0330 0.0624 0.0405 0.0650 1.042

6455.8(4) 4 2.868 3.540 5312 0.0229 0.0528 0.0248 0.0536 1.106

C18H31CuN24S6 PMo12O40 2662 hexagonal R3 11.6599(6) 11.6599(6) 41.664(4) 90.00 90.00 120.00 4905.5(6) 3 2.702 2.861 3810 0.0326 0.0944 0.0358 0.0965 0.983

C18H31NiN24S6 PMo12O40 2656 hexagonal R3 11.6660(6) 11.6660(6) 41.825(4) 90.00 90.00 120.00 4929.5(6) 3 2.685 2.810 3807 0.0354 0.0934 0.0437 0.0976 0.989

)

R1 = Σ Fo| - |Fc /Σ|Fo|. b wR2={Σ[w(Fo2 - Fc2)2]/Σ [w(Fo2)2]}1/2. )

a

1 C12H20Ag3N16S4 PMo12O40 2662 monoclinic P21/n 9.609(5) 21.569(5) 14.079(5)

Figure 1. Ball-stick/polyhedral view of the asymmetric unit of 1. The hydrogen atoms are omitted for clarity.

anion (Figure 1). The valence sum calculations13 show that all the Mo atoms are in þVI oxidation state. In compound 1, there are two crystallographically independent argentum ions (Ag1 and Ag2), which exhibit two different coordination modes. For example, the Ag1 ion is four-coordinated by one terminal O atom from one PMo12 anion and three N atoms from two bmte ligands in a tetrahedron style. The bond distances and angles around the Ag1 ion are 2.276(5)-2.298(5) A˚ for Ag-N, 2.622(5) A˚ for Ag-O, and 114.02(19)-131.33(19)o for N-Ag-N, which are comparable to those in the four-coordinated Agþ complexes.14 The Ag2 ion adopts the linear geometry, coordinated by two N atoms from two bmte ligands. The bond distance and angle around the Ag2 ions are 2.142(6) A˚ for Ag-N and 180.00(14)o for N-Ag-N. In compound 1, the bmte ligand with six potential coordination N-donors exhibits a new coordination mode. In the bmte molecule, four N donors (N2, N3, N6 and N7) in the contraposition of two -CH3 have been used to fuse two Ag1 and link one Ag2 ion (Figure S1, Supporting Information). The N1 and N8 linked with -CH3 are not coordinated, which may be due to the big steric hindrance of -CH3. Thus, the bmte in 1 acts as not only the bridging ligand to link Ag2 but also chelate ligand to fuse Ag1, leading to the formation of a binuclear Ag1 subunit [Ag(1)2(bmte)2]2þ, as

shown in Figure 2 (top). The PMo12 anions act as bidentate inorganic linkages to connect these binuclear subunits, and an Ag1-contained chain is obtained (Figure 2). These chains are parallel to each other and are further connected by Ag2 atoms coordinating with N7, which forms a 2D gridlike layer (Figure 3). Its topology structure is shown in Figure S2, Supporting Information. The adjacent layers are staggerpacked that may decrease the molecular repulsion and stabilize the whole structure (Figure S3, Supporting Information). Furthermore, these layers also can be viewed as the metal-organic chain arranging alternately with PMo12 anions (Figure S4, Supporting Information). Crystal Structure of Compound 2. Crystal structure analysis reveals that compound 2 consists of three Cuþ ions, three bmte ligands, and one PMo12 anion (Figure 4). The valence sum calculations13 show that all the Mo atoms are in the þVI oxidation state and all the Cu atoms are in the þI oxidation state. In compound 2, there are two crystallographically independent copper ions (Cu1 and Cu2), which exhibit similar coordination modes. The Cu1 ion is four-coordinated by one terminal O atom from one PMo12 anion and three N atoms from three bmte ligands in a “seesaw” style. The bond distances and angles around the Cu1 ion are 1.942(3)-2.031(3) A˚ for Cu-N, 2.286(3) A˚ for Cu-O, and 105.14(12)-125.29(13)o for N-Cu-N. The Cu2 ion is also four-coordinated by four N atoms from three bmte ligands. The bond distances and angles around the Cu2 ion are 2.009(3) and 2.040(3) A˚ for Cu-N and 98.31(12)-138.00 (13)o for N-Cu-N. In compound 2, two kinds of coordination modes of bmte exist. The N1-bmte provides three N-donors (N1, N2, and N6) acting as a tridentate ligand to fuse three copper ions (Figure 5b). Furthermore, the N9-bmte can use its four N-donors (two N9 atoms and two N10 atoms) to coordinate with these three copper ions (Figure 5a,c). The N9-bmte plays the role of fuse and further stabilizes the trinuclear cluster. The N9-bmte acts as not only a bridging ligand to link two Cu1 atoms but also a chelate ligand to fuse a Cu2 atom (Figure 5c). This trinucler cluster is the structural character of compound 2. In compound 2, the PMo12 anions act as bidentate inorganic linkages, providing two asymmetrical terminal oxygen

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Figure 2. The 1D Ag1-contained chain in compound 1, in which the binuclear subunits are linked by PMo12 anions. (Top): The binuclear subunit [Ag(1)2(bmte)2]2þ.

Figure 3. The Ag1-chains are connected by Ag2 atoms to form a 2D layer in compound 1.


atoms, to link the trinuclear clusters. Each trinuclear cluster uses two Cu1 atoms to connect the PMo12 anions. Thus, a 1D chain is formed with the PMo12 anions and trinuclear cluster arranging alternately, as shown in Figure 6. Two adjacent chains arrange like a “zipper” (Figure S5, Supporting Information). Crystal Structure of Compound 3. Crystal structure analysis reveals that compound 3 consists of one Cu2þ ion, three

bmte ligands, and one PMo12 anion (Figure 7). The valence sum calculations13 show that all the Mo atoms are in the þVI oxidation state and all the Cu atoms are in the þII oxidation state. To balance the charge of the compound, a proton is added, similar to the case of [Ag2(3atrz)2]2[(HPMo12O40)].9d Then 3 is formulated as [Cu(bmte)3(HPMo12O40)]. In compound 3, there is only one crystallographically independent Cu2þ ion (Cu1), which exhibits a single coordination mode. The Cu1 is six-coordinated by six N atoms from six bmte ligands. The bond distance and angles around the Cu1 ion are 2.131(6) A˚ for Cu-N and 88.8(2)-179.998(2)o for N-Cu-N. The bmte in compound 3 also shows a single coordination mode, acting as bidentate ligand to link two Cu1 ions through two apical N atoms. Thus, a 2D layer is obtained, consisting of three-membered circles, as shown in Figure 8. In compound 3, the PMo12 anions are sandwiched by two layers described above (Figure 9a), just like a “hamburger” (Figure 9b). One PMo12 anion is covered by two opposite triangular circles in these two layers (Figure S6a, Supporting Information). It also can be viewed that one triangular circle can support one PMo12 anion (Figure S6b, Supporting Information). Crystal Structure of Compound 4. Compound 4 is isostructural with compound 3. The valence sum calculations13

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Figure 5. (a) The N9-bmte in 2 acts as tetra-dentate to fuse the trinuclear cluster. (b) Two N1-bmte molecules act as tridentate ligands to fuse three copper ions. (c) The trinuclear cluster in compound 2.

Figure 6. The 1D chain in compound 2 with the PMo12 anions and trinuclear clusters arranging alternately.


show that all the Mo atoms are in the þVI oxidation state. In compound 4, the bond distance and angles around the Ni1 ion are 2.095(5) A˚ for Ni-N and 88.25(18)-179.997(1)o for N-Ni-N. The Choice of bmte Ligand Aiming at Assembly of Multinuclear Subunits. The choice of organic ligand bmte is a character of this paper, which have some merits for assembly of multinuclear subunits. First, the bmte is a N-rich molecule, containing six potential coordination N-donors, which can enhance the coordination ability. Furthermore, in each (1-methyl-1,2,3,4-tetrazole) group, three N-donors arrange adjacently, which can induce the formation of a multinuclear cluster with ease. The coordination modes may be similar to

Figure 8. The 2D layer in compound 3 containing three-membered circles.

pyrazole, which have two adjacent N-donors and is in favor of constructing multinuclear clusters frequently.8,9d Second, the introduction of the S atoms also can promote assembly of multinuclear subunits, which can bend or distort with ease and further intensify the molecular flexibility. Recently, the choice of flexible ligands has become a popular field in the construction of POM-based compounds. However, in reported work, the spacers are usually -(CH2)n-, which have less flexibility than S-containing spacers. And this is really the case in compounds 1-4 (Table 2). For example, in compound 1, the angles are 103° and 104° for C-S-C, smaller than 112° and 115° for S-C-C. Furthermore, in

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Figure 9. The PMo12 anions are sandwiched by two layers (a) in compound 3, like a “hamburger” (b). Table 2. C-S-C and S-C-C Angles in Compounds 1-4 and Chelate Modes in 1 and 2

reported flexible ligands, only the ligands with spacers of -(CH2)2- and -(CH2)3- can act as chelate ligands to fuse metal ions.5b,c However, in compounds 1 and 2, though bmte has a longer spacer (-S-(CH2)2-S-), the introduction of S atoms intensifies the flexibility of bmte and can make it also act as a chelate ligand (Table 2). The more flexible the bmte is, the more chances the bmte can fuse the metal ions, which can induce the formation of multinuclear structures. Thus, considering these merits, bmte is the preferred ligand for the assembly of multinuclear subunits. Influence of Coordination Natures of TM Ions on Assembly of Multinuclear Subunits. Compounds 1-4 are all based on the PMo12/bmte system and synthesized under similar conditions, except for using different TM ions. These four compounds exhibit distinct structures. Thus, it deserves to be mentioned that different coordination natures of TM ions play a key role in the structural control of the self-assembly process, especially for constructing multinuclear subunits. In this work, the synthetic strategy for assembling multinuclear subunits contains four steps: First, considering many reported multinuclear structures based on Agþ ion, we chose Agþ as the reactant, which usually shows two-, three-, and four-coordinated modes.10b,15 And then, a new compound (1) with binuclear subunits was obtained as expected. The Agþ ions exhibit two- and four-coordinated modes. In this

Ag-based dimer (Table 3), two kinds of circles exist, sixmembered and nine-membered ones. The formation of circles can stabilize the structures, especially the sixmembered circle. Second, for further proving the synthetic strategy for assembling multinuclear structures, Cuþ ions were introduced, which shows a coordination nature similar to Agþ ions.16 The similarity of the coordination nature results in the formation of a trinuclear structure in 2. The Cuþ ions exhibit a single four-coordinated mode in 2. The trinuclear cluster also has two kinds of circles, one ninemembered circle and two six-membered circles (Table 3). The successful syntheses of compounds 1 and 2 show that Cuþ and Agþ ions with similar coordination natures induce the construction of multinuclear structures, which have synergetic effects with bmte ligands. Third, a TM ion having a different coordination nature from Cuþ and Agþ ions would be selected to verify whether a multinuclear structure could be obtained. Therefore, Cu2þ ions were used, which usually exhibit five- or six-coordinated modes.17 However, compound 3 with mononuclear structure was obtained, which verifies that the different coordination natures of TM ions play a key role in the structural control of self-assembling multinuclear subunits. In compound 3, the Cu2þ ions exhibit a single six-coordinated mode, which is different from Cuþ and Agþ ions. If compound 3 also exhibits multinuclear

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Table 3. Coordination Nature of TM ions and Steric States of CH3- Groups (in Black Ellipse) in 1-4 and the Circles (in Green Ellipse) in Multinuclear Subunits in 1 and 2

clusters and the Cu2þ ions still keep the six-coordinated mode, the bmte molecules will become congested and the structure will become unstable. Thus, compound 3 adopts a stable 2D layer structure, consisting of three-membered circles. Fourth, we selected the Ni2þ ion, with a coordination mode similar to Cu2þ, to further validate if another nonmultinuclear struture also can be formed. At last, an isostructural compound 4 was obtained, which further validates our synthetic strategy. Therefore, in this work, different coordination natures of TM ions play a key role in the structural control of assembling multinuclear subunits. Furthermore, in these four compounds, the steric states of CH3- groups in bmte ligand are interesting. In order to reduce the steric hindrance, the CH3- groups are all at the edge of multinuclear cluster or circles, as shown in Table 3. FT-IR Spectra. The IR spectra of compounds 1-4 are shown in Figure S7 (Supporting Information). In the spectra of 1-4, characteristic bands at 959, 885, 801, and 1056 cm-1 for 1, 966, 876, 797, and 1054 cm-1 for 2, 966, 883, 799, and 1063 cm-1 for 3, 966, 871, 804, and 1056 cm-1 for 4 are attributed to ν(Mo-Ot), ν(Mo-Ob-Mo), ν(Mo-Oc-Mo), and ν(P-O), respectively. Bands in the regions of 1693-1180 cm-1 for 1, 1691-1185 cm-1 region for 2, 1711-1178 cm-1 region for 3, and 1693-1171 cm-1 for 4 are attributed to the bmte ligands, respectively. Thermogravimetric Analyses (TGA). TGA experiments were performed under N2 atmosphere with a heating rate of 10 °C 3 min-1 in the temperature range of 30-700 °C, shown in Figure S8, Supporting Information. In the TG curves, the weight loss of 21.72% (calc. 22.05%) for

compound 1, 29.42% (calc. 29.47%) for compound 2, 32.58% (calc. 32.08%) for 3, and 32.2% (calc. 32.15%) for compound 4 from 200 to 600 °C corresponds to the loss of bmte molecules. Differential thermal analyses (DTA) give the starting decomposition temperatures (DT) of compounds 1-4, 293 °C for 1, 284 °C for 2, 243 °C for 3 and 294 °C for 4, which are all higher than some TM coordination polymers with similar flexible ligands.18 Thus, one can find that these metal-(flexible ligand) coordination polymers based on POMs are more thermally stable than the pure metal-organic frameworks. Voltammetric Behavior of 1-CPE in Aqueous Electrolyte and Electrocatalytic Activity. Although compounds 1-4 are all based on the PMo12/bmte system, they contain different TM ions and exhibit different structures. To check whether some structural effects on their redox properties exist, cyclic voltammetry measurements are carried out in 1 M H2SO4 aqueous solution. The bulk-modified CPE becomes the optimal choice to study their electrochemical properties, since these compounds are insoluble in water and common organic solvents. The results show that there are only some slight potential shifts of the redox peaks (Figure S9, Supporting Information), which shows almost no structural effect on the redox property. Thus, the 1-CPE has been taken as an example to study their electrochemical properties. The cyclic voltammograms for 1-CPE in 1 M H2SO4 aqueous solution at different scan rates are presented in Figure 10. Three reversible redox peaks appear in the potential range of 700 to -110 mV; the half-wave potentials E1/2 = (Epa þ Epc)/2 are þ331 (I-I0 ), þ193 (II-II0 ), -29 (III-III0 )

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activity; that is, the catalytic activity is enhanced with increasing extent of POM anion reduction. Furthermore, the electrocatalytic reduction of nitrite at the CPE containing simply PMo12O403- (PMo12-CPE) has been performed (Figure S10, Supporting Information), showing electrocatalytic activity similar to 1-CPE, which can further prove that the PMo12 polyoxoanions in 1-CPE exhibit the electrocatalytic activity for reduction of nitrite. Conclusion

Figure 10. The cyclic voltammograms of the 1-CPE in 1 M H2SO4 aqueous solution at different scan rates (from inner to outer: 80, 100, 120, 140, 160, 180, 200, 250, 300, 350, 400, 500 mV 3 s-1).

Figure 11. Cyclic voltammograms of a bare CPE in 1 M H2SO4 (a) and the 1-CPE in 1 M H2SO4 containing 0 (b); 1.0 (c); 2.0 (d); 4.0 (e) mM KNO2. Scan rate: 100 mV 3 s-1.

mV (scan rate: 80 mV 3 s-1), respectively. The peak-to-peak separations between the corresponding anodic and cathodic peaks (ΔEp) for the redox peaks I-I0 , II-II0 , and III-III0 are 26, 28, and 36 mV, respectively, which can be ascribed to three consecutive two-electron processes of PMo12, respectively.3b,19 The parent R-Keggin anion maintains its redox ability in the hybrid solids, which can extend the application of these POM-based materials in electrochemistry. The peak potentials change gradually following the scan rates from 80 to 500 mV 3 s-1: the cathodic peak potentials shift toward the negative direction and the corresponding anodic peak potentials to the positive direction with increasing scan rates. Figure 11 shows cyclic voltammograms for the electrocatalytic reduction of nitrite at 1-CPE in 1 M H2SO4 aqueous solution in the potential range from 700 to -110 mV. The 1-CPE displays good electrocatalytic activity toward the reduction of nitrite. At the 1-CPE, with the addition of nitrite, all three reduction peak currents gradually increase while the corresponding oxidation peak currents decrease, suggesting that nitrite is reduced by two-, four-, and sixelectron reduced species of PMo12 anions. It is noteworthy that the third reduced species show the best electrocatalytic

In this paper, four inorganic-organic hybrid compounds have been synthesized under hydrothermal conditions. These four compounds are all based on the PMo12/bmte system, except for choosing different TM ions. Compounds 1 and 2 exhibit binuclear and trinuclear subunits, respectively, which can be because of the similar coordination nature of Cuþ and Agþ ions. When Cu2þ and Ni2þ ions were used, isostructural compounds 3 and 4 with mononuclear subunits were obtained. Both Cu2þ and Ni2þ ions show different coordination natures from Cuþ and Agþ ions. Thus, different coordination natures of TM ions play a key role in the structural control of assembling multinuclear subunits. This work can enrich the POM family and provide informative examples for understanding the influences of various factors step by step on the assembly processes, even in a hydrothermal “black box”. The ligand bmte in compounds 1-4 shows various coordination modes. However, the S atoms have not been utilized for coordinating TM ions, which may induce the formation of more novel structures. Thus, the next work will choose proper synthetic strategies for using the S atoms sufficiently. Further study of new POM-based compounds modified by the derivatives of bmte is underway. Acknowledgment. Financial support of this research by the NCET-09-0853, the National Natural Science Foundation of China (No. 20871022), and Talent-supporting Program Foundation of Education Office of Liaoning Province (No. 2009R03) are greatly acknowledged. Supporting Information Available: Tables of selected bond lengths and angles for compounds 1-4; IR, TG, and CV curves and structural figures of 1-4; crystallographic information files. This information is available free of charge via the Internet at http:// pubs.acs.org/.

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