Tuning the Dimensionality of the Coordination Polymer Based on

Tuning the Dimensionality of the Coordination Polymer Based on Polyoxometalate by Changing the Spacer Length of Ligands Ai-xiang Tian, Jun Ying, Jun Peng,* Jing-quan Sha, Hai-jun Pang, Peng-peng Zhang, Yuan Chen, Min Zhu, and Zhong-min Su

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 10 3717–3724

Key Laboratory of Polyoxometalate Science of Ministry of Education, Faculty of Chemistry, Northeast Normal UniVersity, Changchun, Jilin 130024, P.R. China ReceiVed April 6, 2008; ReVised Manuscript ReceiVed June 5, 2008

ABSTRACT: Through tuning the spacer length of flexible bis(triazole) ligands, three Keggin anion-based coordination polymers with different dimensionalities, [Cu4(H2O)4(bte)2(HPMoVI10MoV2O40)] · 2H2O (1) (bte ) 1,2-bis(1,2,4-triazol-1-yl)ethane), [Cu(btb)][Cu2(btb)2(PMo12O40)] (2) (btb ) 1,4-bis(1,2,4-triazol-1-y1)butane), and [Cu5(btx)4(PMoVI10MoV2O40)] (3) (btx ) 1,6-bis(1,2,4triazol-1-y1)hexane), were synthesized and structurally characterized. Compound 1 exhibits a ladder-like chain, in which the polyoxometalate (POM) anions act as the “middle rails” of the ladder. The bte ligand with shortest spacer length acts as not only the bridging linker but also the chelator, which terminates the dimensional extension. Compound 2 exhibits a 2D POM-based framework, containing POM/Cu/btb grid-like layers. In compound 3, there exist three sets of (123)(121)2 2D Cu-btx frameworks to generate a 3-fold interpenetrating structure, into which the hexadentate POM anions are inserted to construct a 3D structure. The structural analyses reveal that the spacer length of flexible bis(triazole) ligands has influence on the dimensionality of these compounds. Introduction Recently, the design and synthesis of coordination polymers, with backbones constructed from metal ions as connectors and ligands as linkers, has been an area of rapid growth.1-3 In this field, a remarkable branch of the metal/flexible ligand system has attracted extensive attention because the flexibility and conformational freedom of such ligands provide the possibility to construct unprecedented frameworks with tailored properties and functions,4 especially the flexible and dynamic crystal transformation property to specific guest molecules that opens up a new field in porous coordination materials.5 On the other hand, polyoxometalates (POMs), as a unique class of metaloxide clusters, possess abundant structural diversity and versatile physical and chemical properties, such as catalytic activity, photochemical activity, and reversible redox behavior.6-8 Up to now, there are many hybrids emerging composed of POMs associated with various metal (especially Cu)-rigid organic complexes.9 The combination of POMs with metal/flexible ligands frameworks to construct novel inorganic-organic compounds can bring the merits of some aspects together such as aggregating the extensive properties of POMs and the flexibility and functions of such metal-organic frameworks (MOFs). However, there are a few reports on POMs-based flexible frameworks, usually utilizing the bis(imidazole)10 or bis(bipyridyl)11 ligands. In our previous work, we introduced the flexible 1,1′-(1,4-butanediyl)bis(imidazole) (bbi) ligand into the polyoxovanadate systems,12 aiming at construction of flexible frameworks with active sites of polyanions. However, the bbi molecule has only two N donors as coordination sites that limit its coordination ability. Therefore, we choose the derivatives of bis(triazole) to improve the coordination capacity of the flexible ligands. The ligands of bis(triazole) have two more advantages than the bbi ligand:13,14 The 1,2,4-triazole group in the ligands unites the coordination geometry of both imidazoles and pyrazoles to provide more potential coordination sites; it can donate four N-donor atoms, and the coordination capacity will be enhanced consequently. * To whom correspondence should be addressed. E-mail: [email protected]. Phone: +86 43185099667; Fax: +86 43185098768.

Coordination polymers of Cu-bis(triazole) based on the Wells-Dawson-type POM have been studied by us to discuss the influences of flexible bis(triazole) ligands and metal ion coordination geometries on the high-dimensional structures.15 In this work, we choose the classical Keggin-type POM ([PMo12O40]3-) as the alternative inorganic building block, and three flexible bis(triazole) ligands of bte, btb, and btx as organic units with different -(CH2)n- (n ) 2, 4, and 6) spacers. The selection of the three ligands is based on the following considerations: (i) These ligands can act as bridging or chelating ligands depending on the ligand length. (ii) These ligands have different flexibility and conformational freedom, endowing different capacities of spatial extension. These characteristics can induce the construction of different attractive frameworks. We have observed that the different spacer lengths of rigid organic ligands such as bipyridine and bis(4-pyridyl)ethylene can significantly affect the modification and assembly of the POMs with Cu-N coordination polymeric chains.16 In the present work, we want to study the influence of the spacer length of flexible bis(triazole) ligands. Furthermore, we choose Cu ion as a second metal to construct coordination polymers which have smart and diverse coordination modes. Herein, we report three new POM-based coordination polymers, [Cu4(H2O)4(bte)2(HPMoVI10MoV2O40)] · 2H2O (1) (bte ) 1,2-bis(1,2,4-triazol-1-yl)ethane), [Cu(btb)][Cu2(btb)2(PMo12O40)] (2) (btb ) 1,4-bis(1,2,4-triazol-1-y1)butane), and [Cu5(btx)4(PMoVI10MoV2O40)] (3) (btx ) 1,6-bis(1,2,4-triazol-1y1)hexane) (Scheme 1), with dimensionalities rising from 1D to 3D. The influence of the spacer length of the flexible bis(triazole) ligands on the dimensionality of these compounds 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 PerkinElmer 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. The thermogravimetric analyses (TGA) were carried out in N2 on a Perkin-Elmer DTA 1700 differential thermal

10.1021/cg800353y CCC: $40.75  2008 American Chemical Society Published on Web 08/23/2008

3718 Crystal Growth & Design, Vol. 8, No. 10, 2008

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Scheme 1. Experimental Routes for Compounds 1-3

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

formula Fw T (K) space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dc (g · cm-3) µ (mm-1) F(000) final R1a, wR2b [I > 2σ(I)] final R1a, wR2b (all data) GOF on F2 a

1

2

3

C12H29Cu4Mo12N12O46P 2513.85 293(2) P1j 11.1348(4) 11.3764(4) 11.8118(4) 77.3010(10) 63.0580(10) 86.5580(10) 1299.88(8) 1 3.195 4.54 1175 0.0355 0.0863 0.0386 0.0884 1.038

C24H36Cu3Mo12N18O40P 2589.53 293(2) P21/c 12.4830(7) 21.2207(12) 11.9177(7) 90 96.9940(10) 90 3133.5(3) 2 2.745 3.44 2464 0.0433 0.0864 0.0626 0.0945 1.024

C40H64Cu5Mo12N24O40P 3021.06 293(2) P21/c 13.569(3) 24.746(5) 11.501(3) 90 97.175(3) 90 3831.5(15) 2 2.619 3.371 2912 0.0657 0.1558 0.1019 0.1801 1.019

R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) (∑[w(Fo2 s Fc2)2]/∑[w(Fo2)2])1/2.

analyzer with a rate of 10.00 °C/min. X-ray photoelectron spectroscopy (XPS) analyses were performed on a VG ESCALAB MK II spectrometer with a Mg KR (1253.6 eV) achromatic X-ray source. The vacuum inside the analysis chamber was maintained at 6.2 × 10-6 Pa during analysis. The X-ray powder diffraction (XRD) patterns were recorded on a Siemens D5005 diffractometer with Cu KR (λ)1.5418 Å) 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 [Cu4(H2O)4(bte)2(HPMoVI10MoV2O40)] · 2H2O (1). A mixture of H3[PMo12O40] · 13H2O (0.3 g, 0.15 mmol), Cu(CH3COO)2 · 2H2O (0.12 g, 0.55 mmol), bte (0.04 g, 0.24 mmol) was dissolved in 10 mL of distilled water at room temperature. When the pH value of the mixture was adjusted to about 3.9 with 1.0 mol L-1 NaOH, the suspension was put into a Teflon-lined autoclave and kept under autogenous pressure at 160 °C for 5 days. After slow cooling to room temperature (final pH ) 3.1), black block crystals were filtered and washed with distilled water (43% yield based on Mo). Anal. Calcd for C12H29Cu4Mo12N12O46P (2513.85): C 5.73, H 1.15, N 6.68. Found: C 5.69, H 1.17, N 6.65. IR (solid KBr pellet, cm-1): 3435 (s), 3118 (w), 1712 (w), 1616 (m), 1539 (m), 1428 (w), 1376 (w), 1260 (m), 1220 (w), 1141 (m), 1057 (s), 955 (s), 850(w), 777(s), 645(w). Synthesis of [Cu(btb)][Cu2(btb)2(PMo12O40)] (2). Compound 2 was prepared similarly to compound 1, but the btb ligand was used instead of bte ligand (0.05 g, 0.26 mmol). The final pH was 3.2. Dark red block crystals were filtered and washed with distilled water (43% yield based on Mo). Anal. Calcd for C24H36Cu3Mo12N18O40P (2589.53): C 11.12, H 1.4, N 9.73. Found: C 11.08, H 1.42, N 9.69. IR (solid KBr pellet, cm-1): 3436 (s), 3116 (w), 1629 (m), 1523 (m), 1434 (w), 1384 (m), 1274 (m), 1209 (w), 1130 (m), 1057 (s), 956 (s), 861(w), 779 (s), 620(w), 563(w). Synthesis of [Cu5(btx)4(PMoVI10MoV2O40)] (3). Compound 3 was prepared similarly to compound 2, but the btx ligand was used instead of btb ligand (0.06 g, 0.27 mmol). The final pH was 3.0. Black block

crystals were filtered and washed with distilled water (32% yield based on Mo). Anal. Calcd for C40H64Cu5Mo12N24O40P (3021.06): C 15.89, H 2.12, N 11.12. Found: C 15.84, H 2.15, N 11.07. IR (solid KBr pellet, cm-1): 3437 (s), 3087 (w), 2923 (w), 2854 (w), 1630 (s), 1525 (s), 1437 (w), 1363 (w), 1276 (s), 1214 (w), 1130 (m), 1053 (s), 931 (s), 851(w), 785 (s), 647(w), 583(w). Preparations of 1-, 2-, and 3-CPEs. The 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 with an agate mortar and pestle to achieve a uniform mixture and then was added 0.1 mL of Nujol 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 Å) at 293 K. The structures were solved by direct methods and refined on F2 by full-matrix leastsquares methods using the SHELXTL package.17 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 Tables S1-S3 (Supporting Information). Crystallographic data for the structures reported in this paper have been deposited in the Cambridge Crystallographic Data Center with CCDC Nos. 682173 for 1, 682174 for 2, and 682175 for 3.

Results and Discussion Synthesis. Compounds 1-3 were synthesized under hydrothermal conditions. The synthetic conditions for 1-3 are the same, except for using bis(triazole) ligands with different spacer lengths. Therefore, it deserves mention that the organic bis-

Keggin Anion-Based Coordination Polymers

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Table 2. Coordination Sites of POM, Coordination Numbers and Modes of L (L ) bte, btb and btx) and Copper Ions, and Schematic POM-based Frameworks in Compounds 1-3

(triazole) ligands of the starting materials have a key role in the structural control of the self-assembly process, which will be discussed in detail later. Furthermore, the oxidation state of copper changes from a reactant CuII ion to a resultant CuI ion. The main reason may be that the organonitrogen species generally act not only as ligands but also as reductants under hydrothermal conditions. Such a phenomenon is often observed in the hydrothermal reaction system containing N-donor ligand and CuII.14b,15,18 Description of the Crystal Structures. The [PMo12O40]3(abbreviated to PMo12) anion is the inorganic building block in compounds 1-3. The P-O and Mo-O bond distances are in the normal ranges.19 To present clearly the crystal structures, all the coordination sites of PMo12, the coordination numbers and modes of L (L ) bte, btb and btx) and copper ions, and the schematic POM-based frameworks in compounds 1-3 are summarized in Table 2. Crystal Structure of Compound 1. Crystal structure analysis reveals that compound 1 consists of four CuI ions, two bte ligands, one PMo12 anion, and six water molecules (Figure 1). The valence sum calculations20 and XPS spectra show that two Mo atoms are in the +V oxidation state, and all the Cu atoms

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

are in the +I oxidation state. Similar to the case of [Ag2(3atrz)2]2[(HPMoVI10MoV2O40)],21 to balance the charge of the compound, a proton is added, then 1 is formulated as [Cu4(H2O)4(bte)2(HPMoVI10MoV2O40)] · 2H2O. In compound 1, there are two crystallographically independent copper ions (Cu1 and Cu2). The Cu1 ion is three-coordinated by two N atoms from two bte ligands and one O atom from one PMo12 anion in a slightly distorted T-type coordination mode. The bond distances and angles around the Cu1 are 1.890(4) and 1.889(4) Å for Cu-N, 2.70(6) Å for Cu-O, 171.9(2)° for N-Cu-N, and 87.75-96.10° for N-Cu-O. The Cu2 ion is four-coordinated by two N atoms from one bte ligand and two water molecules in a “seesaw” style. The bond distances and angles around the Cu2 are 1.952(5) and 2.043(6) Å for Cu-N, 2.034(6) and 2.248(7) Å for Cu-Ow, 108.5(2)° for N-Cu-N, and 93.5(3)° for Ow-Cu-Ow. These bond distances

Figure 2. (a) Ladder-like chain in compound 1, with PMo12 anions as “middle rails”. (b) Schematic view of the ladder-like chain. Color code: green ball, PMo12 anion; blue ball, CuI ion.


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Figure 3. (a) Ladder-like chains are connected by the weak interactions between Cu1 ions and PMo12 anions in 1; the blue broken line represents the weak interaction. Color code: green ball, PMo12 anion; blue ball, CuI ion. (b) The “stair”-like supramolecular layer seen along the given direction.

Figure 4. Stick/polyhedral view of the two crystallographically distinct motifs in 2. The hydrogen atoms are omitted for clarity.

and angles are comparable to those in the three- and fourcoordinated CuI compounds.12,22 A notable feature in the structure of 1 is that the bte ligand acts as both a bridging linker and a chelator. On one hand, in a bte molecule, it utilizes the two apical N atoms to bridge two Cu1 ions, as a bidentate linker, to extend the dimensionality of the structure. On the other hand, the remaining two N atoms chelate to one Cu2 ion (Figure S1, Supporting Information), which terminates the dimensional extension. This dual coordination mode of the bte molecule is expected as it has the shortest -(CH2)2- spacer of the three ligands. The combination of the two coordination modes of the bte ligand results in the formation of 1D ladder-like chains in compound 1. As shown in Figure 2, the Cu/bte chains act as two “sides” of the ladder, while the PMo12 anions act as “middle rails” through their two terminal O atoms coordinating with Cu1 ions. The ladder is robust owing to the seven-member chelate cycle of Cu2/bte. Otherwise, the ladder might be “soft” without the chelate cycle (Figure S2, Supporting Information). Furthermore, the Cu1 atoms have weak interactions with the O16 atoms of the PMo12 anions (Cu1 · · · O16: 2.91 Å), which connect the 1D ladder chains into a 2D supramolecular layer (Figure 3a). Interestingly, the layers look like “stairs” (Figure 3b). Crystal Structure of Compound 2. Compound 2 consists of three CuI ions, three btb ligands, and one PMo12 anion (Figure 4). The valence sum calculations and XPS spectra show that all the Mo atoms are in the +VI oxidation state and the Cu atoms are in the +I oxidation state. In compound 2, the Cu1 ion adopts the linear geometry, coordinated by two N atoms from two btb ligands, with Cu-N bond distances of 1.853(7) and 1.860(6) Å and Cu-N-Cu angle of 175.3(3)°. The Cu2 ion is four-coordinated in a square-planar geometry by two N atoms from two btb ligands and two O atoms from two PMo12 anions. The bond distances and angles around the Cu2 ion are 1.871(7) Å for Cu-N, 2.615(6) Å for Cu-O, 180° for N-Cu-N and O-Cu-O. The bond distances and angles are comparable to those in the similar CuI coordination modes.15,16,23

Figure 5. (a) Top: The infinite wave-like chain in compound 2 (top). Bottom: The chains parallel with each other along the a axis. (b) Left: Stick/polyhedral view of the layer. Right: Binodal (2,4)-connected (61)(64 · 81 · 101) net of 2. (c) The 1D chains and 2D layers arrange alternately to form a 3D supramolecular framework; blue broken lines represent the weak interaction. Color code: green ball, PMo12 anion; blue ball, CuI ion.

The noticeable feature is that compound 2 consists of two crystallographically distinct motifs: a 1D polymeric [Cu(btb)]+∞ chain and a 2D [Cu2(btb)2PMo12O40]-∞ layer. In the first motif, the btb molecule as a bidentate ligand utilizes the two apical N atoms to link the CuI ions to construct an infinite wave-like chain (Figure 5a top). These chains are parallel with each other along the a axis (Figure 5a bottom). In the second motif, each PMo12 anion is covalently bonded to two copper ions via two bridging O atoms, and each copper ion is connected with two PMo12 anions and two adjacent copper ions. Therefore, the 2D structure can be rationalized as a binodal (2,4)-connected net with Schla¨fli symbol (61)(64 · 81 · 101) (Figure 5b). The 2D net contains uniform square rings made of [Cu2(btb)2PMo12O40] units. The adjacent layers do not overlap each other but have a


Figure 6. Stick/polyhedral view of the asymmetric unit of 3. The hydrogen atoms and crystal water molecules are omitted for clarity.

little packing offset (Figure S3, Supporting Information). This kind of packing mode may decrease the molecular repulsion and stabilize the whole structure. Furthermore, the 1D polymeric [Cu(btb)]+∞ chains and 2D layers arrange alternately; then the 1D chains link the adjacent layers via the weak interactions (Cu1 · · · O4, 2.86 Å, and Cu1 · · · O15, 2.90 Å) to form a 3D supramolecular framework (Figure 5c). Crystal Structure of Compound 3. Compound 3 consists of five CuI ions, four btx ligands, and one PMo12 anion (Figure 6). The valence sum calculations and XPS spectra show that the two molybdenum atoms are in the +V oxidation state and the Cu atoms are in the +I oxidation state. There are three types of coordination modes for the CuI centers: (i) The Cu1 ion is three-coordinated in a T-type geometry, coordinated by two N atoms from two btx ligands and one O atom from one PMo12 anion. The bond distances and angles around the Cu1 ion are 1.889(7) and 1.905(7) Å for Cu-N, 2.50(1) Å for Cu-O, 169.43° for N-Cu-N, 91.42(4) and 97.31(3)° for N-Cu-O. (ii) The Cu2 ion is fourcoordinated in a “seesaw” geometry, coordinated by two N atoms from two btx ligands and two O atoms from two PMo12 anions. The bond distances and angles around the Cu2 ion are 1.873(9) and 1.911(10) Å for Cu-N, 2.429(7) and 2.533(7) Å for Cu-O, 166.55° for Cu-N, and 87.2(9)-100.1(3)° for N-Cu-O. (iii) The Cu3 ion is two-coordinated in a linear geometry, coordinated by two N atoms from two btx ligands (Cu-N: 1.867(10) Å, N-Cu-N: 180.0(2)°). The bond distances and angles are comparable to those in 1 and 2. It is the coordination diversity of the CuI ion that makes possible the coexistence of the three coordination geometries. Additionally, the coordination sites of the PMo12 anion are six, which is more than that in 1 and 2, by using four symmetrical terminal O atoms and two symmetrical bridging O atoms. The btx ligand in 3 exhibits flexible coordination modes. It acts as not only a bidentate linkage, using two apical N atoms, but also a tridentate linkage in an unusual asymmetrical coordination mode to bridge the CuI ions. In compound 3, Cu1, Cu2, and Cu3 ions are viewed as two connecting nodes respectively, with the adjacent Cu-Cu distances of 5.762 Å (Cu1-Cu1), 8.060 Å (Cu1-Cu2), and 13.694 Å (Cu1-Cu3). Thus, a (123)(121)2 Cu-btx layer is formed with a “grid” style, as shown in Figure 7a. The big void of the grid induces the formation of the interpenetrating structure to stabilize the compound. Interestingly, there exist three sets of (123)(121)2 2D nets, which generate a 3-fold interpenetrating wave-like structure, as shown in Figure 7b. The PMo12 anion, acting as a hexadentate ligand, offers four terminal and two bridging O atoms to link the adjacent two interpenetrating nets. Thus, the three interpenetrating Cu-btx nets are united by the


PMo12 anions to construct a 3D POM coordination polymer structure (Figure 8). Influence of the Flexible Bis(triazole) Ligands with Different Spacer Lengths on the Dimensionalities of the Coordination Polymers. In this work, we select three kinds of flexible bis(triazole) ligands, bte, btb, and btx, intending to observe their effect on the assembly of the POM coordination polymers by alteration of the spacer lengths of the organic ligands. The three compounds are all based on PMo12 anions and CuI ions, and the sole difference rests on the -(CH2)n- (n ) 2, 4, and 6) spacers of bis(triazole) ligands, which leads to the distinct POM-based frameworks of 1D chains in 1, 2D layers in 2, and a 3D interpenetrating structure in 3. In compound 1, the short -(CH2)2- spacer length makes it possible for the bte molecule to provide two N atoms to chelate one Cu2 ion; otherwise, they might link more Cu ions to extend the dimensionality. The formation of the seven-member chelate cycle shortens the distance of the two apical N atoms (about 7.37 Å, see Figure S4a, Supporting Information). The remaining apical N atoms then coordinate to two Cu1 ions, leading to a 1D ladder-like chain in 1. It is worth mentioning that the PMo12 anion in 1 uses two symmetrical terminal oxygen atoms (about 10.42 Å for the O · · · O distance) rather than bridging oxygen atoms to coordinate to Cu ion. Theoretically, the bridging oxygen atoms are more basic and easier to coordinate to metal ions.24 A possible reason is the presence of the chelate [Cu(H2O)2]+ ion. If two opposite bridging oxygen atoms (about 7.28 Å for the O · · · O distance) were used to connect with Cu1 ions, the “rail” length of the ladder-like chain would be shortened so as to increase the repulsion between the PMo12 anion and the coordinated water molecule of the chelated [Cu(H2O)2]+ ions, making the structure unstable. In compound 2, the btb molecule with a -(CH2)4- spacer is too long to form a stable Cu(btb)+ nine-member chelate cycle. Therefore, the btb molecule acts as only a bidentate bridging ligand leading to a 2D POM-based grid. In the grid layer, the btb molecule adopts the “S” conformation (Figure S4b, Supporting Information) and the PMo12 anion uses two symmetrical bridging oxygen atoms to coordinate to Cu ions, which is different from 1. Both the “S” conformation of the btb molecule and the use of bridging oxygen atoms as linkages can reduce the dimension of the grid to avoid formation of a big void. In compound 3, the btx molecule, with a -(CH2)6- spacer, is the longest one compared with the other two ligands. It therefore has more flexibility and conformational freedom. This feature allows the btx molecule to conform sufficiently to the coordination geometries of the metal ions and the environment of the POM template. The distance of the two apical N atoms increases beyond 9.2 Å in 3 (Figure S4c, Supporting Information), and the dimension of the grid in a (123)(121)2 Cu-btx layer extends to about 11.5 Å × 40.18 Å, much larger than those in 1 (ca. 11.14 Å × 14.62 Å) and 2 (ca. 11.92 Å × 12.43 Å). Therefore, to increase the stability of the whole structure, the formation of a tightly packed 3D interpenetrating framework becomes a trend. In summary, along with the length increase of the -(CH2)nspacer, the flexibility of the organic ligands improves gradually, and the effect that the POM template exerts is significant. The average POM · · · POM distances (measured by P · · · P distance) are about 11.1 Å in a 1D chain for 1, 12.2 Å in a 2D layer for 2, and 12.9 Å in a 3D framework for 3. The trend with increasing dispersion of the POM anions is that of decreasing molecular densities, that is, 3.195 g · cm-3 for 1, 2.745 g · cm-3 for 2, and 2.619 g · cm-3 for 3.


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Figure 7. (a) Stick (left) and schematic (right) views of the (123)(121)2 Cu-btx layer in compound 3. (b) Schematic views of the 3-fold interpenetrating network of the MOF in 3 along the a (left) and c (right) axes, respectively.

Figure 8. In compound 3, the three interpenetrating Cu-btx networks are linked by PMo12 anions (green balls) to construct a 3D framework. The blue, green and orange grid layers represent a three interpenetrating Cu-btx net viewing along the c axis.

FT-IR Spectra, XPS Spectra, and Powder X-ray Diffractions. The IR spectra of compounds 1-3 are shown in Figure S5 (Supporting Information). In the spectra of 1-3, characteristic bands at 954, 850, 777, and 1057 cm-1 for 1, 956, 861, 779, and 1056 cm-1 for 2, and 931, 851, 784, and 1053 cm-1 for 3, are attributed to to ν(Mo-Ot), ν(Mo-Ob-Mo), ν(Mo-Oc-Mo) and ν(P-O), respectively. Bands in the regions of 1616-1141 cm-1 for 1, 1629-1130 cm-1 region for 2, and 1630-1130 cm-1 for 3, are attributed to the bte, btb, and btx ligands, respectively. The XPS spectra show two overlapped peaks at 232.5 and 231.4 eV for 1, 232.5 and 231.5 eV for 3 are attributed to Mo3d6+ and Mo3d5+, respectively, while two peaks at 232.4 and 235.6 eV for 2 are attributed to Mo6+(3d5/2) and Mo6+(3d3/ 16 2). Two peaks at 933.8 and 953.8 eV for 1, 934.2 and 953.7 eV for 2, and 934.5 and 954.5 eV for 3 are attributed to Cu+(2p3/2) and Cu+(2p5/2) (Figure S6, Supporting Information).18b All these results further confirm the valence sum calculations and the structural analyses. Figure S7 (Supporting Information) presents the powder X-ray diffraction patterns for compounds 1-3. The diffraction peaks

Figure 9. Cyclic voltammograms of the 1-CPE in 1 M H2SO4 at different scan rates (from inner to outer: 60, 100, 140, 180, 220, 260, 300, 340, 380, and 420 mV · s-1).

of both simulated and experimental patterns match well in relevant positions, thus indicating that the phase purities of the compounds 1-3 are good. The difference in reflection intensities between the simulated and the experimental patterns is due to the different orientation of the crystals in the powder samples. Thermogravimetric Analyses. The thermogravimetry (TG) experiments were performed under a N2 atmosphere with a heating rate of 10 °C · min-1 in the temperature range of 25-600 °C, shown in Figure S8 (Supporting Information). The TG curve of compound 1 shows two distinct weight loss steps. The first weight loss step below 170 °C corresponds to the loss of water molecules, 4.1% (calc.4.3%). The second weight loss step is ascribed to the loss of organic bte molecules, 14.39% (calc.14.3%). The TG curves of compounds 2 and 3 also exhibit two weight loss steps, which is ascribed to the loss of organic btb molecules, 23.78% (calc.24.08%) in 2, and btx molecules, 29.49% (calc.29.66%) in 3. Voltammetric Behavior of 1-CPE in aqueous electrolyte and Electrocatalytic Activity. Compounds 1-3 are insoluble in water and common organic solvents. Thus, the bulk-modified



Figure 10. (a) Cyclic voltammograms of a bare CPE in 1 M H2SO4 containing 0 (a) and 1.5 (b) mM NaBrO3; the 1-CPE in 1 M H2SO4 containing 0 (c); 1.5 (d); 3 (e); 4.5 (f) mM NaBrO3. (b) Cyclic voltammograms of a bare CPE in 1 M H2SO4 containing 0 (a) and 2 (b) mM NaNO2; the 1-CPE in 1 M H2SO4 containing 0 (c); 2 (d); 4 (e); 6 (f) mM NaNO2. Scan rate: 80 mV · s-1.

CPE becomes the optimal choice to study the electrochemical properties of these compounds which is inexpensive, easy to prepare, and to handle.25 The electrochemical behaviors of the 1-, 2-, and 3-CPE are similar, and the 1-CPE case 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 9. Three reversible redox peaks appear in the potential range of 800 to -100 mV;the half-wave potentials E1/2 ) (Epa + Epc)/2 are +426 (I-I′), +281 (II-II′), and +52 (III-III′) mV (scan rate: 60mV · s-1), respectively. The redox peaks I-I′, II-II′, and III-III′ should be ascribed to the three consecutive two-electron processes of Mo, respectively.14a,11d,26 However, the oxidation peak of the copper centers is not observed in the potential range of 800 to -100 mV. This phenomenon was also observed in the similar PMo12/Cu systems, such as (NH4)[Cu24I10L12][PMoV2MoVI10O40]3 (L ) 4-[3-(1H-1,2,4-triazol-1-yl)propyl]-4H-1,2,4-triazole)14a and [Cu2(H2O)2(bpp)2Cl][PMo12O40] · 20H2O (bpp ) 1,3-bis(4pyridyl)propane).11d The peak potentials change gradually following the scan rates from 60 to 420 mV · s-1: the cathodic peak potentials shift toward the negative direction and the corresponding anodic peak potentials shift to the positive direction with increasing scan rates. The results verify that the redox ability of the parent R-Keggin anion can be maintained in the hybrid solids, which promises an application of this kind of inorganic-organic hybrid materials in electrochemistry. Figure 10a shows cyclic voltammograms for the electrocatalytic reduction of bromate at a bare CPE and the 1-CPE in 1 M H2SO4 aqueous solution. No obvious voltammetric response is observed at a bare CPE in 1 M H2SO4 aqueous solution containing 1.5 mM bromate in the potential range from 800 to -100 mV. However, the 1-CPE displays a good electrocatalytic activity toward the reduction of bromate. At the 1-CPE, with the addition of bromate, the second and the third reduction peak currents increase gradually while the corresponding oxidation peak currents gradually decrease, but the first redox peak remains almost unchanged, which indicates that the four- and six-electron reduced species of PMo12 anions present electrocatalytic activity at 1-CPE for the reduction of bromate. It is noteworthy that the third reduced species show the best electrocatalytic activity, that is, the catalytic activity is enhanced with increasing extent of POM anion reduction. Figure 10b shows the electrocatalytic reduction of nitrite by a 1-CPE in 1

M H2SO4 aqueous solution. It can be clearly seen that with 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 six-electron reduced species of PMo12 anions. Conclusion In this paper, by designing the usage of the bte, btb, and btx ligands with increase of the -(CH2)- chain unit, three inorganic-organic hybrid coordination polymers 1-3 based on PMo12 have been synthesized under hydrothermal conditions. The structural analyses show that the longer the length of the bte, btb, and btx ligands, the higher is the dimensionality for compounds 1-3, that is, the ladder-like chain structure for compound 1, the POM-based 2D net structure for compound 2, and the 3-fold interpenetrating 3D structure for compound 3. This work shows again that tuning the organic spacer length is an effective strategy in crystal engineering of preparing POMbased inorganic-organic hybrid solid materials and that integration of rigid inorganic POMs and flexible metal coordination polymeric matrices can greatly enrich the POM family. Acknowledgment. This work is financially supported by the National Natural Science Foundation of China (20671016), the Program for Changjiang Scholars and Innovative Research Team in University and the Analysis and Testing Foundation of Northeast Normal University. Supporting Information Available: Tables of selected bond lengths and angles for compounds 1-3; IR, TG data, and structural figures of compounds 1-3. This material is available free of charge via the Internet at http://pubs.acs.org.

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CG800353Y

Tuning the Dimensionality of the Coordination Polymer Based on

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