Using Flexible and Rigid Organic Ligands to Tune Topology

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DOI: 10.1021/cg900806k

Using Flexible and Rigid Organic Ligands to Tune Topology Structures Based on Keggin Polyoxometalates

2010, Vol. 10 1104–1110

Ai-xiang Tian,†,‡ Jun Ying,†,‡ Jun Peng,*,† Jing-quan Sha,† Zhong-min Su,*,† Hai-jun Pang,† Peng-peng Zhang,† Yuan Chen,† Min Zhu,† and Yan Shen† †

Key Laboratory of Polyoxometalate Science of Ministry of Education, Faculty of Chemistry, Northeast Normal University, Changchun, Jilin 130024, P. R. China, and ‡Faculty of Chemistry and Chemical Engineering, Bohai University, Jinzhou 121000, P. R. China Received July 14, 2009; Revised Manuscript Received January 18, 2010

ABSTRACT: Three inorganic-organic hybrids based on polyoxometalate (POM), [Cu6(bbtz)6(HPM12O40)] 3 2H2O (M = Mo, 1; W, 2) and [Cu6(trz)2(bbtz)2(SiW12O40)] (3) (trz=1-H-1,2,4-triazole, bbtz=1,4-bis(1,2,4-triazol-1-ylmethyl)benzene), were synthesized under hydrothermal conditions, and structurally characterized. Through the use of the flexible ligand bbtz, isostructural compounds 1 and 2 with interpenetrating structures were obtained. In compound 1, ladder-like chains exist, in which the PMo12 anions act as “middle rails”. These chains are linked by wave-like [Cu(bbtz)]nnþ lines to construct a threedimensional (3D) framework. Two such frameworks penetrate each other to construct a 2-fold interpenetrating structure. By introducing the rigid ligand trz, compound 3 with an un-interpenetrating structure is obtained. In compound 3, two-dimensional (2D) (63)2 metal organic framework (MOF) layers exist, which are linked by Keggin anions to construct a 3D (44 3 62)(63)2 framework. The differences between these compounds should be ascribed to the introduction of the rigid molecule trz that plays a role in restraining the formation of interpenetrating structures.

Introduction Polyoxometalates (POMs), made up of early transition metals, are a rich family of metal-oxygen clusters and have obtained extensive attention due to not only the abundant structural diversity, but also versatile physical and chemical properties, such as catalytic activity, magnetism, photochemical activity, and electrical chemistry.1 In this field, a remarkable branch is to design and synthesize new POM-based compounds modified by different transition-metal coordination polymers (TMCs),2 through using the coordination ability of the polyoxoanions. With the introduction of the TMCs, these polyoxoanions serve as inorganic linkages through providing terminal and/or bridging oxygen atoms. The choice of a proper organic moiety becomes important work, which can construct a series of abundant TMC modifiers. The N-donor organic ligands with strong coordination capacity3 have been extensively utilized for constructing these TMCs-modified POM-based compounds. Initially, the rigid organic ligands with N-donors become the preferred choice, such as pyrazine,4 imidazole,5 triazole,6 2,20 -bipyridine,7 and 4,40 -bipyridine,8 which have merits of tiny conformational transformation and controllable structural tune for final products. However, by using these rigid ligands, it is difficult to obtain structures with high dimensional and intriguing interpenetrating topologies. Thus, the rigid N-donor ligands are usually called “terminator” for assembly of interpenetrating structures. Further, another kind of organic ligands, the flexible ligands, such as 1,3-bis(4pyridyl)propane,9 also attract interest, as they exhibit flexibility and conformational freedom which allow them to conform to the coordination environment of the metal ions and polyoxoanions, in favor of the construction of high *Corresponding author. Tel.: þ86 43185099667; fax: þ86 43185098768; e-mail: [email protected] (J.P.), [email protected] (Z.-M.S.). pubs.acs.org/crystal

Published on Web 02/03/2010

dimensional and interpenetrating structures with intriguing topologies.10-13 Furthermore, these flexible ligands help to exert the coordination capacity and modes of the transition metals and polyoxoanions, which can help obtain high connective compounds with ease. If the rigid N-donor ligand was introduced into these potential interpenetrating systems, they might exert their terminal function for assembly of the uninterpenetrating structures. However, the study on this aspect is scarce. In this work, we choose both flexible and rigid organic ligands (Scheme 1) to tune the assembly of the interpenetrating and un-interpenetrating structures, and expect to verify the “terminator” role of the rigid ligand. This strategy may be effective in crystal engineering in the preparation of interpenetrating and un-inperpenetrating POM-based hybrid solid materials. Herein, we report two new compounds with a interpenetrating structure, [Cu6(bbtz)6(HPM12O40)] 3 2H2O (M = Mo, 1; W, 2; bbtz = 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene), by using the flexible ligand bbtz. Further, we introduce the rigid ligand trz into the Keggin-CuI-bbtz system, expecting to inhibit the interpenetrating framework, and a 3D compound [Cu6(trz)2(bbtz)2(SiW12O40)] (3) (trz = 1-H1,2,4-triazole) is obtained. The roles of the flexible and rigid organic ligands in assembly of interpenetrating and un-interpenetrating structures are 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 obtained on an Alpha Centaurt FT/IR spectrometer with KBr pellet in the 400-4000 cm-1 region. X-ray photoelectron spectroscopy (XPS) analyses were performed on a VG ESCALAB MK II spectrometer with an MgKR (1253.6 eV) achromatic X-ray source. The vacuum inside the analysis chamber was maintained at 6.2  10-6 Pa during analysis. Thermal gravimetric analyses r 2010 American Chemical Society

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Scheme 1. Organic Synthons bbtz and trz for Tuning the Topology of the Title Compounds 1-3

Table 1. Crystal Data and Structure Refinements for Compounds 1-3 1

2

3

C72H77Cu6N36- C72H77Cu6N36- C28H28Cu6N18O42PMo12 O42PW12 O40SiW12 Fw 3682 4737 3872 crystal system triclinic triclinic monoclinic P1 P21/c space group P1 a (A˚) 13.9986(16) 13.9881(8) 11.382(4) b (A˚) 14.3641(17) 14.3592(8) 22.063(7) c (A˚) 15.346(2) 15.3319(9) 13.442(4) 88.164(2) 76.4150(10) R (o) o 62.988(2) 62.9530(10) 113.458(5) β( ) 72.394(2) 72.6040(10) γ (o) 2600.1(6) 2599.5(3) 3096.7(17) V (A˚3) Z 1 1 2 -3 2.349 3.023 4.153 Dc (g 3 cm ) 2.709 14.527 24.322 μ (mm-1) F(000) 1785 2169 3436 0.0521 0.0767 final R1a, wR2b 0.0427 [I > 2σ(I)] 0.1090 0.1394 0.1345 0.0596, 0.1089, final R1a, wR2b 0.0561, (all data) 0.1187 0.1444 0.1462 1.003 1.088 1.094 GOF on F2 P P P P a R1 = Fo| - |Fc / |Fo|. b wR2 = { [w(Fo2 - Fc2)2]/ [w(Fo2)2]}1/2. )

formula

)

(TGA) were carried out in N2 on a Perkin-Elmer DTA 1700 differential thermal analyzer with a rate 10.00 °C/min. The X-ray powder diffraction (XRPD) patterns were recorded on a Siemens D5005 diffractometer with Cu KR (λ=1.5418 A˚) radiation. Electrochemical measurements were performed with a CHI 660b electrochemical workstation. A conventional three-electrode system was used. Ag/AgCl (3 M KCl) electrode was used as a reference electrode, and a Pt wire as a counter electrode. Chemically bulkmodified carbon-paste electrodes (CPEs) were used as the working electrodes. Synthesis of [Cu6(bbtz)6(HPMo12O40)] 3 2H2O (1). A mixture of H3[PMo12O40] 3 13H2O (0.16 g, 0.075 mmol), Cu(CH3COO)2 3 2H2O (0.26 g, 1.2 mmol), and bbtz (0.048 g, 0.2 mmol) was dissolved in 10 mL of distilled water at room temperature. When the pH of the mixture was adjusted to about 4.2 with 1.0 mol 3 L-1 HCl, 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, black block crystals were filtered and washed with distilled water (40% yield based on Mo). Anal. Calcd for C72H77Cu6N36O42PMo12 (3682): C 23.47, H 2.09, N 13.69%. Found: C 23.41, H 2.12, N 13.60%. IR (solid KBr pellet, cm-1): 3431 (s), 3100 (w), 2923 (w), 1633 (m), 1525 (m), 1419 (w), 1386 (m), 1288 (w), 1199 (w), 1132 (m), 1054 (s), 950 (s), 850 (w), 765(s). Synthesis of [Cu6(bbtz)6(HPW12O40)] 3 2H2O (2). Compound 2 was prepared similarly to compound 1, except that H3[PW12O40] 3 12H2O (0.22 g, 0.07 mmol) was used instead of H3[PMo12O40] 3 13H2O. Black block crystals were filtered and washed with distilled water (40% yield based on W). Anal. Calcd for C72H77Cu6N36O42PW12 (4737): C 18.24, H 1.63, N 10.64%. Found: C 18.19, H 1.59, N 10.68%. IR (solid KBr pellet, cm-1): 3431 (s), 3116 (w), 2918 (w), 1637 (m), 1531 (s), 1425 (w), 1378 (w), 1299 (m), 1206 (w), 1138 (m), 1097 (s), 1056 (s), 960 (s), 883 (s), 808 (s). Synthesis of [Cu6(trz)2(bbtz)2(SiW12O40)] (3). Compound 3 was prepared similarly to compound 1, except that trz (0.014 g, 0.2 mmol) was added additionally, and H4[SiW12O40] 3 14H2O was used instead of H3[PMo12O40] 3 13H2O. Orange block crystals were filtered and washed with distilled water (25% yield based on W). Anal. Calcd for C28H28Cu6N18O40SiW12 (3872): C 8.68, H 0.72, N 6.51%. Found: C 8.72, H 0.69, N 6.56%. IR (solid KBr pellet, cm-1): 3434 (s), 3128 (w), 1633 (m), 1515 (w), 1427 (w), 1283 (m), 1213 (w), 1132 (m), 975 (m), 921 (s), 871 (w), 792 (s). Preparations of 1-, 2-, 3-CPEs. Compound 1 modified CPE (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 1.5-mm inner diameter, and the tube surface was wiped with paper. Electrical contact was established with a copper rod through the back of the electrode. In a similar manner, 2- and 3-CPEs were made with compounds 2 and 3. X-ray Crystallographic Study. X-ray diffraction analysis data for compounds 1-3 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.14 For the compounds, all the hydrogen atoms attached to carbon atoms were generated geometrically, while the hydrogen atoms attached to water molecules were not located but were included in the structure factor calculations. A summary of the crystallographic data and structural determination for them is provided in Table 1. Selected bond lengths and angles of the three compounds are listed in Table S1 (Supporting Information). Crystallographic data for the

structures reported in this paper have been deposited in the Cambridge Crystallographic Data Center with CCDC No. 694259 for 1, 694260 for 2, and 699888 for 3.

Results and Discussion Synthesis. Compounds 1-3 were synthesized under hydrothermal conditions. The copper changes from a reactant CuII ion to a resultant CuI ion. The main reason may be that the organonitrogen species generally act as not only ligands but also reductants under hydrothermal conditions. Such a phenomenon is often observed in the hydrothermal reaction system containing N-donor ligand-CuII-POM systems.15 Crystal Structure of Compound 1. Crystal structure analysis reveals that compound 1 consists of six CuI ions, six bbtz ligands, one PMo12 anion, and two water molecules (Figure 1). The valence sum calculations16 and XPS spectra show that 4 out of 12 Mo atoms are in þV oxidation state, and all the Cu atoms are in þI oxidation state. Similar to the case of [Ag2(3atrz)2]2[(HPMoVI10MoV2O40)],6a to balance the charge of the compound, a proton is added and then 1 is formulated as [Cu6(bbtz)6(HPMoVI8MoV4O40)] 3 2H2O. In compound 1, there are three crystallographically independent copper ions (Cu1, Cu2, and Cu3). The Cu1 ion adopts a linear geometry, coordinated by two N atoms from two bbtz ligands, with Cu-N distances of 1.874(4) and 1.880(5) A˚, and Cu-N-Cu angle of 176.8(3)°. Both the Cu2 and Cu3 ions are three-coordinated by two N atoms from two bbtz ligands and one O atom from one PMo12 anion (one bridging O atom for Cu2 and one terminal O atom for Cu3), in a slightly distorted T-type coordination mode. The bond distances and angles around the Cu2 and

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Figure 1. Stick/polyhedral view of the asymmetric unit of 1. The hydrogen atoms and crystal water molecules are omitted for clarity.

Figure 2. (a) Top: The ladder-like chain in compound 1, with PMo12 anions as “middle rails”. Bottom: Schematic view of the ladder-like chain. Color code: green ball: PMo12 anion, blue ball: CuI ion. (b) Top: The [Cu3(bbtz)]þ chain in 1. Bottom: Schematic view of this chain.

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Cu3 are 1.874(4)-1.894(5) A˚ for Cu-N, 2.663(4) and 2.630(11) A˚ for Cu-O, 173.5(2) and 173.4(2)o for N-Cu-N and 86.74-95.11° for N-Cu-O. These bond distances and angles are comparable to those in the two- and threecoordinated CuI compounds.4a,13,17 The bbtz ligand in 1 acts as a bidentate ligand, exhibiting a single trans conformation like a “Z”, as shown in Figure S1a (Supporting Information). In compound 1, two motifs exist: (i) Two kinds of coordination modes of Cu1 and Cu2 ions result in the formation of 1D ladder-like chain. As shown in Figure 2a, the [Cu(bbtz)]þ chains, composed of Cu1 and Cu2 ions, act as two sides of the ladder, while the PMo12 anions act as “middle rails”. But the anions only connect with Cu2 ions through two bridging O atoms. This connection mode induces to form big voids. (ii) There also exhibits a wave-like [Cu3(bbtz)]þ line in compound 1 (Figure 2b). Furthermore, the ladder-like chains are parallel with each other without any connection. The [Cu3(bbtz)]þ lines just act as the linkage to connect these ladder-like chains through Cu3-O19 bonds. Therefore, a 3D framework is constructed through the connections of ladderlike chains and wave-like lines (Figure 3a). However, this single 3D framework has big voids, and thus a 2-fold interpenetrating structure is naturally generated, as shown in Figure 3b, to stabilize the whole structure. Crystal Structure of Compound 2. Compound 2 is isostructural with compound 1. The valence sum calculations and XPS spectra show that 4 out of 12 W atoms are in þV oxidation state, and all the Cu atoms are in þI oxidation state. Crystal Structure of Compound 3. Single crystal X-ray structural analysis reveals that compound 3 is constructed from one SiW12 anion, six CuI ions, two trz molecules, and two bbtz molecules (Figure 4). The valence sum calculations and XPS spectra show that all W atoms are in þVI oxidation state, and Cu atoms in þI oxidation state. There are three crystallographically independent CuI centers in compound 3. The Cu1 ion is four-coordinated by two O atoms from two SiW12 anions and two N atoms from one trz ligand and one bbtz ligand in a “seesaw” style. The bond distances and angles around the copper ion are 1.90(2)-1.94(2) A˚ for Cu-N, 2.50(1)-2.73(6) A˚ for Cu-O, 161.2(9)° for N-Cu-N, 82.2(4)-110.0° for N-Cu-O, and 110.2(3)° for O-Cu-O, comparable to those in the four-coordinated CuI complexes.13,17 The Cu2 and Cu3 atoms adopt the linear geometry, coordinated by two N atoms from one trz ligand and one bbtz ligand. The bond distances and angles around

Figure 3. (a) The ladder-like chains (blue) are linked by wave-like lines (orange) in 1. Color code: green ball: PMo12 anion. (b) Schematic view of the 2-fold interpenetrating framework of 1.

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the copper ions are 1.83(2)-1.89(2) A˚ for Cu-N and 169.4(9)-173.8(9)° for N-Cu-N. The feature in the structure of 3 relies on the coexistent rigid and flexible organic ligands trz and bbtz. Notably, the 1,2,4-triazole and its flexible derivatives acting as organic ligands have more advantages:10,18,19 The 1,2,4-triazole group, with three coordination N atoms, integrates the coordination geometry of both imidazoles and pyrazoles to provide more potential coordination sites. Thus, this kind of ligand exhibits strong coordination capacity, acting as bridging ligands to construct new structures. In compound 3, trz and bbtz ligands exert their expected coordination capacities, as shown in Figure S2, Supporting Information. The trz ligand utilizes all the coordination sites to link three CuI centers, acting as a tridentate ligand. The bbtz ligand has four potential coordination sites. However, owing to the phenyl-ring with big steric hindrance, the bbtz molecule utilizes three of the four coordination sites, instead of all four sites, to coordinate with three CuI centers, acting as a tridentate linkage in an unusual asymmetrical coordination mode. The bbtz molecule in 3 exhibits a single syn conformation like a “U”, which is different from that of compound 1, as shown in Figure S1(b), Supporting Information. According to the valence sum calculations and XPS spectra, the charge of the polyoxoanion should be -4. Because the organic ligand bbtz is neutral, and the trz ligand is negatively charged ([trz]-, Figure S2, Supporting Information) due to coordination with three copper atoms, compound 3 is formulated as [Cu6(trz)2(bbtz)2(SiW12O40)]. In compound 3, the CuI centers are connected by the tridentate trz and bbtz ligands to construct a 2D layer (Figure 5 left). This layer is composed of hexanuclear circuits. To simplify this layer, both the [Cu3(trz)] subunit and the bbtz ligand can be viewed as three-connecting nodes (Figure 5 middle). Thus, this layer is composed of pentagons (Figure 5 right). Furthermore, in the layer, the trz molecule

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and the phenyl-ring of the bbtz molecule have π 3 3 3 π stacking interactions, which can stabilize the layer (Figure S3, Supporting Information). From the topological point of view, the SiW12 anions act as four-connected linkages to link the adjacent layers through using Cu1-O4/5 bonds to construct a 3D framework with a Schl€ afli symbol of (44 3 62)(63)2, as shown in Figure 6. The inorganic anionic layers and the Cu-trz/bbtz layers alternately stack in the -ABAB- style. The Roles of Flexible bbtz and Rigid trz Organic Ligands for Tuning the Assembly of the Interpenetrating and UnInterpenetrating Structures. Compounds 1-3 were all based on the Keggin anion/CuI/bbtz system, except for introducing extra rigid ligand trz into compound 3 (Scheme 2). Compounds 1 and 2 exhibit 2-fold interpenetrating structures, while compound 3 shows an un-interpenetrating 3D framework. As expected, the bbtz ligand exerts its flexibility to construct 2-fold interpenetrating structures in compounds 1 and 2. Whereas the character of trz is the conformation stability, so-called “rigidness”, this nature endows it a “terminator” role to inhibit the formation of interpenetrating structure. From a crystal engineering point of view, it is a challenge to introduce an extra rigid ligand to a flexible ligand system. In this work, the small rigid ligand trz is introduced, and a 3D framework with a Schl€ afli symbol of (44 3 62)(63)2 in compound 3 is obtained as expected. The trz acts as tridentate ligand, fastening tightly to the Keggin anion, CuI and bbtz, and terminating the interpenetrating tendency. The introduction of trz induces a compact packing mode and makes the framework shrink, which can be supported by the increasing molecular densities from 3.0 g 3 cm-3 for compounds 1 and 2 to 4.1 g 3 cm-3 for compound 3.

Figure 6. The Cu-trz-bbtz layers (in ellipse) are linked by the fourconnected linkages SiW12 anions to construct a 3D framework of 3.

Scheme 2. Preparation Routes for Compounds 1-3

Figure 4. Ball/stick and polyhedral view of the asymmetric unit of 3. The hydrogen atoms are omitted for clarity.

Figure 5. Left: The Cu-trz-bbtz layer in compound 3. Middle: Both the [Cu3(trz)] subunit and the bbtz ligand are viewed as three-connected nodes. Right: The simplified layer composed of pentagons.

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Figure 7. The cyclic voltammograms of the 1-, 2-, and 3-CPE in 1 M H2SO4 at different scan rates (from inner to outer: 40, 60, 80, 100, 120, 140, 160, 180, 200, 220, 240, and 300 mV 3 s-1 for 1, 40, 80, 120, 160, 200, 240, 280, and 320 mV 3 s-1 for 2, and 20, 40, 60, 80, 100, 120, 140, 160, 180, and 200 mV 3 s-1 for 3, respectively).

Therefore, it deserves to be mentioned that the nature of the rigid trz acts as a key role in the structural change from the interpenetration in compounds 1 and 2 to un-interpenetration in 3. Furthermore, in compounds 1 and 2, the ligand bbtz exhibits a single trans conformation like a “Z”, which leads to the farther distance of the two terminal N atoms (about 10.27 A˚). However, in compound 3, the ligand bbtz exhibits a single syn conformation like a “U”. This style induces to the short distance of the two terminal N atoms (about 9 A˚). The “Z” bbtz molecules may hold bigger space, which conduces to an incompact packing mode, just like in 1 and 2. FT-IR and XPS Spectra. The IR spectra of compounds 1-3 are shown in Figure S4, Supporting Information. In the spectrum of 1, the characteristic band at 1054 cm-1 is attributed to ν(P-O), 950 cm-1 is attributed to ν(Mo-Od), 850 cm-1 is attributed to ν(Mo-Ob-Mo), and 765 cm-1 is attributed to ν(Mo-Oc-Mo). In the spectrum of 2, the characteristic band at 1056 cm-1 is attributed to ν(P-O), 960 cm-1 is attributed to ν(W-Od), 883 cm-1 is attributed to ν(W-Ob-W), and 808 cm-1 is attributed to ν(W-Oc-W). In the spectrum of 3, the characteristic band at 921 cm-1 is attributed to ν(Si-O), 975 cm-1 is attributed to ν(W-Od), 871 cm-1 is attributed to ν(W-Ob-W), and 792 cm-1 is attributed to ν(W-Oc-W). The absorption bands at 1525 and 1288 cm-1 for compound 1, 1531 and 1299 cm-1 for compound 2, and 1515 and 1282 cm-1 for compound 3 are assigned to the triazole-ring stretching vibrations in trz and bbtz ligands (ν(CdN) þν(Nd N)).20 The absorption bands at 2847 and 2923 cm-1 for compound 1, 2847 and 2920 cm-1 for compound 2, and 2854 and 2977 cm-1 for compound 3

are assigned to the -CH2- stretching vibrations in bbtz ligands (ν(C-H)). Figure S5, Supporting Information presents the XPS spectra for compounds 1-3. The XPS spectra show two peaks at 933.1 and 952.4 eV for 1, 933 and 952.7 eV for 2, 933.2 and 952.8 eV for 3, attributed to Cuþ(2p3/2) and Cuþ(2p1/2). Two overlapped peaks at 233.7 and 232.2 eV for 1 are attributed to Mo6þ and Mo5þ, respectively, while two peaks at 35.5 and 34.6 eV for 2 are attributed to W6þ and W5þ and a peak at 37.3 eV for 3 are attributed to W6þ. All these results further confirm the valence sum calculations and the structural analyses. Thermogravimetric Analyses. The TGA experiments were performed under N2 atmosphere with a heating rate of 10 °C 3 min-1 in the temperature range of 30-600 °C for compounds 1 and 2, while 30-700 °C for compound 3, shown in Figure S6, Supporting Information. In the TG curves of compounds 1 and 2, the weight loss steps below 300 °C correspond to the loss of water molecules. The weight loss steps in the range of 300-600 °C are ascribed to the loss of organic molecules bbtz, 39.05% (calc. 39.15%) for 1, and 30.45% (calc. 30.67%) for 2. In the TG curve for compound 3, the weight loss of 15.04% (calc. 14.29%) from 300 to 600 °C corresponds to the loss of bbtz molecules. Differential thermal analyses (DTA) give the starting decomposition temperatures (DT) of compounds 1-3, 356 °C for 1, 350 °C for 2, and 390 °C for 3. DT of 3 is 30 °C higher than those of 1 and 2. This would be reasonably explained by their structural features. Both compounds 1 and 2 are isostructural, which are solely constructed by flexible ligand bbtz. Two identical 3D frameworks

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interpenetrate with each other to assemble a 2-fold interpenetrated structure. The formation of an interpenetrated structure may induce the incompact packing mode, that is, lower thermal stability. However, in compound 3, the rigid ligand trz is introduced additionally, which may inhibit formation of an interpenetrated structure. The “terminator” role of trz is exhibited in 3. The trz acting as a tridentate ligand fastens the Keggin anion, CuI and bbtz to construct a compact packing mode, which may induce a higher thermal stability of 3. Cyclic Voltammetry. Redox properties of compounds 1-3 were studied in 1 M H2SO4 aqueous solution (Figure 7). The cyclic voltammograms for 1-CPE at different scan rates are presented in the potential range of þ800 to -150 mV. There exist three reversible redox peaks I-I0 , II-II0 , and III-III0 with the half-wave potentials (E1/2 = (Epa þ Epc)/2) at þ400(I-I0 ), þ253(II-II0 ), þ24(III-III0 ) mV (scan rate: 40 mV 3 s-1), respectively. Redox peaks I-I0 , II-II0 , and III-III0 correspond to three consecutive two-electron processes of Mo centers.19,21 However, the oxidation peak of copper centers is not observed in the scan range of þ800 and -150 mV. This phenomenon was also observed in (NH4)[Cu24I10L12][PMoV2MoVI10O40]3 (L = 4-[3-(1H-1,2,4-triazol-1-yl)propyl]4H-1,2,4-triazole)19 and [Cu2(H2O)2(bpp)2Cl][PMo12O40] 3 20H2O (bpp = 1,3-bis(4-pyridyl)propane).9b In the potential range of þ450 to -650 mV for 2-CPE, there exist three reversible redox peaks II-II0 , III-III0 and IV-IV0 with the half-wave potentials at -68(II-II0 ), -288(III-III0 ), -567(IV-IV0 ) mV for 2-CPE (scan rate: 40 mV 3 s-1). Redox peaks II-II0 and III-III0 correspond to two consecutive oneelectron processes of W centers, while IV-IV0 corresponds to a two-electron process.21 In addition, the irreversible anodic peak I with the potential of þ125 mV for 2-CPE is assigned to the oxidations of the copper centers.13 Although compounds 1 and 2 are isostructural, their CV curves are different (Figure 7), depending on the different electrochemical behaviors of parent PMo12 and PW12 polyoxoanions.21-23 There exists also one irreversible anodic peak I with the potential of þ120 mV assigned to the oxidation of the copper centers for 3-CPE (scan rate: 100 mV 3 s-1). In addition, there exist two reversible redox peaks II-II0 and III-III0 with the half-wave potentials at -414.5 (II-II0 ) and -571 (III-III0 ) mV, respectively, which correspond to two consecutive two-electron processes of W centers.22 As compared to the reported PMo12,21 PW12,22 and SiW1223 systems, the slight potential shifts of the three redox peaks in 1- to 3-CPEs may be related to the combination of Cu-organic ligand complex cations. Furthermore, when the scan rates varied from 40 to 300 mV 3 s-1 for 1-CPE, 40 to 320 mV 3 s-1 for 2-CPE, and 20 to 200 mV 3 s-1 for 3-CPE, the peak potentials change gradually: the cathodic peak potentials shift toward the negative direction and the corresponding anodic peak potentials to the positive direction with increasing scan rates. Conclusion In this paper, two compounds with interpenetrating structures (1 and 2) and a compound with un-interpenetrating structure (3) have been synthesized under hydrothermal conditions, through utilizing the different natures of flexible and rigid organic ligands. Isostructural compounds 1 and 2, based on the flexible bbtz ligand, exhibit 2-fold interpenetrating structures. However, by additionally introducing the rigid trz

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ligand, a 3D (44 3 62)(63)2 framework in compound 3 is obtained. The “terminator” role of rigid trz ligand is embodied in the assembly process of 3. This work may provide informative examples in preparing expected interpenetrating and un-interpenetrating POM-based materials by using the flexible and rigid organic ligand to tune the topological structures. Further study on bigger rigid organic ligands instead of trz is underway. Acknowledgment. This work is financially supported by the National Natural Science Foundation of China (20671016 and 20901031), the Program for Changjiang Scholars and Innovative Research Team in University and Testing Foundation of Northeast Normal University. Supporting Information Available: Tables of selected bond lengths and angles for compound 1-3; IR and structural figures of 1-3. This material is available free of charge via the Internet at http://pubs.acs.org.

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