pH-Dependent Assembly of 1D to 3D Octamolybdate Hybrid Materials

ARTICLE pubs.acs.org/crystal

pH-Dependent Assembly of 1D to 3D Octamolybdate Hybrid Materials Based on a New Flexible Bis-[(pyridyl)-benzimidazole] Ligand Hai-Yan Liu,†,‡ Hua Wu,† Jin Yang,*,† Ying-Ying Liu,† Bo Liu,† Yun-Yu Liu,† and Jian-Fang Ma*,† †

Key Lab of Polyoxometalate Science, Department of Chemistry, Northeast Normal University, Changchun 130024, People’s Republic of China ‡ Department of Chemistry and Pharmaceutical Engineering, Suihua University, Suihua 152061, People’s Republic of China

bS Supporting Information ABSTRACT: Four octamolybdate hybrid materials based on a new flexible multidentate ligand, namely, H4L(γ-Mo8O26) 3 2H2O (1), [Cu(H2L)(β-Mo8O26)(H2O)2] 3 3H2O (2), [Cu(H2L)(γ-Mo8O26)] 3 3H2O (3), and Cu(HL)2(β-Mo8O26) (4), where L is 1,10 -(1,4butanediyl)bis[2-(4-pyridyl)benzimidazole], have been synthesized at different pH values under hydrothermal conditions. Compound 1, which is hydrothermally prepared at pH ≈ 1, exhibits a rare one-dimensional (1D) molybdenum oxide chain. Compounds 2 and 3 are hydrothermally obtained at pH ≈ 2.5. In 2, CuII cations are bridged by H2L2þ ligands and (βMo8O26)4
anions to form an interesting two-dimensional (2D) layer. In 3, the H2L2þ ligands and (γ-Mo8O26)4
clusters bridge adjacent CuII cations to form a 3D framwork. When the pH value is adjusted to 3.5, compound 4 is obtained. Because the HLþ cation acts as a monodentate ligand, compound 4 only exhibits a 1D chain structure. The structural diversities of 1
4 reveal that the pH value of the reaction system plays a crucial role in the assembly of POM-based metal
organic frameworks (MOFs). In addition, the electrochemical property of compound 4 has also been investigated in 1 M H2SO4 aqueous solution.

’ INTRODUCTION Design and assembly of organic
inorganic hybrid materials based on polyoxometalates (POMs) are currently of great interest in the field of coordination chemistry and crystal engineering because they can provide versatile architectures and potential applications in catalysis, electrochemistry, biochemistry, photochemistry and magnetism.1
5 Among the different types of POMs, the polyoxomolybdate anions, because of its diverse electronic, photochemical, and catalytic properties, have been viewed as ideal inorganic building blocks to construct POM-supported metal
organic frameworks (MOFs).6 Octamolybdate {Mo8}, as an important category of isopolymolybdate clusters, has eight isomers, R, β, γ, δ, ε, ζ, η, and θ.7 These isomers can not only provide a large number of terminal and bridging O atoms as multidentate inorganic ligands but also exhibit a wide variety of structural motifs with different sizes and topologies. Meanwhile, {Mo8} clusters can be easily made under hydrothermal conditions from (NH4)6Mo7O24 with controlled pH values.8 Also, the pH values of the reaction system play a key role in the assembly of POM-based MOFs.9 For example, alterations in pH values may lead to structural changes of POM itself. Moreover, the pH values can affect protonation degree of the organic ligand, resulting in different structures.8b,10 On the other hand, the selection of the organic ligand is extremely important because changing the structure of the organic ligand can control and adjust the topologies of MOFs. In this regard, the flexible N-containing polydentate ligands are excellent candidates for the construction of novel intriguing structures and topologies, because they have multiple coordination sites and can r 2011 American Chemical Society

adopt more versatile conformations. Based on above consideration, we synthesized a new flexible N-containing polydentate ligand 1,10 -(1,4butanediyl)bis[2-(4-pyridyl)benzimidazole] (L) (Scheme 1), which contains both pyridyl groups and benzimidazole groups. To the best of our knowledge, the coordination chemistry of the flexible 1,10 -(1,4butanediyl)bis[2-(4-pyridyl)benzimidazole] ligand has not been investigated.11 The selection of the L ligand is based on the following considerations: (i) the flexible nature of
CH2
spacers allows the ligand to bend and rotate freely when coordinating to metal centers so as to conform to the coordination geometries of metal ions; (ii) the large (pyridyl)benzimidazole rings not only can provide potential supramolecular recognition sites for π
π stacking interactions, but also act as hydrogen bond acceptors and donors to assemble supramolecular structures. In this work, we successfully synthesized four POM-based MOFs in the reaction system of octamolybdate, L ligand, and CuII cation, namely, H4L(γ-Mo8O26) 3 2H2O (1), [Cu(H2L) (β-Mo8O26)(H2O)2] 3 3H2O (2), [Cu(H2L)(γ-Mo8O26)] 3 3 H2O (3), and Cu(HL)2(β-Mo8O26) (4). Their structural diversities show that the pH value of the reaction system played a key role in the structural self-assembled process. In addition, the electrochemical property of compound 4 has also been investigated in 1 M H2SO4 aqueous solution. Received: January 22, 2011 Revised: May 10, 2011 Published: May 27, 2011 2920

dx.doi.org/10.1021/cg200095c | Cryst. Growth Des. 2011, 11, 2920–2927

Crystal Growth & Design Scheme 1. Molecular Structure of Multidentate L Ligand

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Table 1. Crystal Data and Structure Refinements for Compounds 1
4

’ EXPERIMENTAL SECTION Materials and Methods. Chemicals were purchased from commercial sources and used without further purification. The 2-(4-pyridyl)benzimidazole was synthesized according to the literature method.12 Elemental analyses were carried out with a Carlo Erba 1106 elemental analyzer, and the FT-IR spectra were recorded from KBr pellets in the range of 4000
400 cm
1 on a Mattson Alpha-Centauri spectrometer. The UV
vis-NIR absorption spectroscopies were collected on finely ground samples with a Cary 500 spectrophotometer equipped with a 110 mm diameter inte-grating sphere. X-ray powder diffraction (XRPD) patterns were measured on a Siemens D5005 diffractometer. X-ray photoelectron spectroscopy (XPS) analyses were performed on an ESCALABMKII spectrometer with an Al
KR (1486.6 eV) achromatic X-ray source. Thermogravimetric analysis (TGA) was performed on a Perkin-Elmer TG-7 analyzer heated from 25 to 600 °C under nitrogen gas. Electrochemical measurements were performed with a CHI660b electrochemical workstation. A conventional threeelectrode system was used. Ag/AgCl (3 M KCl) electrode was used as a reference electrode, and a Pt wire as a counter electrode. Chemically bulk-modified carbon-paste electrodes (CPEs) were used as the working electrodes. Synthesis of 1,10 -(1,4-Butanediyl)bis[2-(4-pyridyl)benzimidazole] (L). A mixture of 2-(4-pyridyl)benzimidazole (7.8 g, 40 mmol), and

NaOH (1.68 g, 42 mmol) in DMSO (20 mL) was stirred at 60 °C for 1 h, and then 1,4-dibromobutane (4.32 g, 20 mmol) was added. The mixture was cooled to room temperature being stirring at 60 °C for 12 h, and then poured into 400 mL of ice water. A yellow solid of L formed immediately, which was isolated by filtration in 72% yield after drying in air. Elemental analyses calcd for C28H24N6 (444.53): C, 75.65; H, 5.44; N, 18.91. Found: C, 75.73; H, 5,38; N, 18.87. IR (solid KBr pellet, cm
1): 3373(s), 3048(s), 2934(s), 2872(s), 1930(m), 1635(s), 1604(s), 1553(s), 1520(s), 1451(s), 1416(s), 1392(s), 1365(s), 1326(s), 1283(s), 1252(s), 1214(s), 1183(s), 1153(s), 1063(m), 1000(m), 959(m), 827(s), 741(v), 683(s), 621(m), 594(s), 556(s), 505(s), 426(m). Synthesis of H4L(γ-Mo8O26) 3 2H2O (1). A mixture of (NH4)6Mo7O24 3 4H2O (0.124 g, 0.1 mmol), L (0.044 g, 0.1 mmol), Cu(NO3)2 3 3H2O (0.072 g, 0.3 mmol), and H2O (12 mL) was adjusted to pH ≈ 1 with HNO3 (1M) and NaOH (1M), stirred for 0.5 h, and then transferred and sealed in a 18 mL Teflon-lined stainless steel container, which was heated at 170 °C for 72 h and then cooled to room temperature at a rate of 10 °C 3 h
1. Yellow crystals of 1 were collected in 48% yield based on Mo. Elemental analyses calcd for C28H32Mo8N6O28 (1668.12): C, 20.16; H, 1.93; N, 5.04. Found: C, 20.09; H, 1.99; N, 5.11. IR (solid KBr pellet, cm
1): 3491(s), 3192(m), 3064(s), 1636(s), 1480(s), 1376(s), 1323(s), 1228(s), 1172(m), 1145(s), 948(s), 911(v), 843(s), 802(s), 663(s), 522(s).

Syntheses of [Cu(H2L)(β-Mo8O26)(H2O)2] 3 3H2O (2) and [Cu(H2L)(γ-Mo8O26)] 3 3H2O (3),. The synthetic method of 2 and

3 was similar to that used for the preparation of 1, except that the pH value was adjusted to about 2.5. Blue crystals of 2 (yield 15% based on

1

2

empirical formula

C28H32Mo8N6O28

C28H36CuMo8N6O31

fw crystal size [mm]

1668.12 0.20  0.16  0.12

1783.69 0.18  0.14  0.08

crystal system

triclinic

monoclinic

space group

Pi

C2/c

a [Å ]

9.6482(4)

10.398(5)

b [Å ]

9.8674(4)

35.826(5)

c [Å ]

14.1812(7)

13.418(5)

R [deg]

96.766(4)

90

β [deg] γ [deg]

102.504(4) 116.635(4)

108.416(5) 90

volume [Å3 ]

1141.92(9)

4742(3)

Z

1

4

Dc (g/cm3)

2.426

2.498

GOF

0.921

0.953

reflns collected/unique

9312/5221

15695/5997

Rint

0.0317

0.0250

R1 [I > 2σ(I)] R2 (all data)

0.0279 0.0788

0.0273 0.0657

largest residuals [e Å
3]

0.846/
0.993

0.476/
0.745

3

4

empirical formula

C28H32CuMo8N6O29

C56H50CuMo8N12O26

fw

1747.66

2138.14

crystal size [mm]

0.18  0.16  0.08

0.16  0.16  0.12

crystal system

triclinic

triclinic

space group a [Å ]

Pi 7.914(5)

Pi 10.9471(3)

b [Å ]

10.423(5)

11.5905(4)

c [Å ]

13.808(5)

14.5109(5)

R [deg]

91.308(5)

104.956(3)

β [deg]

96.509(5)

94.749(3)

γ [deg]

100.792(5)

110.967(3)

volume [Å3 ]

1110.5(10)

1627.46(9)

Z Dc (g/cm3)

1 2.613

1 2.182

GOF

0.965

0.903

reflns collected/unique

9363/5483

13427/7499

Rint

0.0222

0.0362

R1 [I > 2σ(I)]

0.0280

0.0429

R2 (all data)

0.0699

0.1082

largest residuals [e Å
3]

0.786/
1.074

1.162/
2.149

Mo) and green crystals of 3 (yield 35% based on Mo) were collected. Elemental analyses calcd for 2 C28H36CuMo8N6O31 (1783.69): C, 18.85; H, 2.03; N, 4.71. Found: C, 18.77; H, 1.98; N, 4.79. Elemental analyses calcd for 3 C28H32CuMo8N6O29 (1747.66): C, 19.24; H, 1.85; N, 4.81. Found: C, 19.31; H, 1.81; N, 4.75. IR (solid KBr pellet, cm
1): 3442(w), 1617(m), 1548(m), 1416(m), 1324(m), 1217(w), 1061(w), 946(s), 901(s), 846(s), 702(s), 664(s), 550(s) for 2; 3442(w), 1615(m), 1513(w), 1447(m), 1417(m), 1324(m), 1219(w), 1059(w), 945(s), 899(s), 853(s), 660(s), 549(s) for 3. Synthesis of Cu(HL)2(β-Mo8O26) (4). Compound 4 was prepared similar to 1, except that the pH value of the system was adjusted to 2921

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Crystal Growth & Design about 3.5 with HNO3 (1M) and NaOH (1M). Green crystals suitable for X-ray analyses were obtained in 43% yield based on Mo. Elemental analyses calcd for C56H50CuMo8N12O26 (2138.14): C, 31.46; H, 2.36; N, 7.86. Found: C, 31. 33; H, 2.48; N, 7.97. IR (solid KBr pellet, cm
1): 3443(w), 1615(m), 1550(w), 1447(m), 1417(m), 1324(m), 1216(w), 1059(w), 944(s), 898(s), 853(s), 659(s), 548(s). X-ray Crystallography. Single-crystal X-ray diffraction data for L and 1
4 were recorded on an Oxford Diffraction Gemini R CCD with graphite-monochromated Mo KR radiation (λ = 0.71073 Å) at 293 K. The structures were solved with the direct method of SHELXS-9713 and refined with full-matrix least-squares techniques using the SHELXL-97 program14

Scheme 2. Experimental Conditions for the Syntheses of Compounds 1
4

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within WINGX.15 The non-hydrogen atoms of the complexes were refined with anisotropic temperature parameters. The hydrogen atoms attached to carbons were generated geometrically. H atoms bonded to O and N atoms were located in a difference Fourier map and refined with isotropic displacement parameters. The detailed crystallographic data and structure refinement parameters for 1
4 are summarized in Table 1.

’ RESULTS AND DISCUSSION Selected bond distances and angles for compounds 1
4 are listed in Table S1 (see the Supporting Information). Crystallographic data and drawing for L ligand are given in the Supporting Information. Syntheses of Compounds 1
4. In the process of hydrothermal synthesis, several factors can influence the formation of crystal phases, such as initial reactants, molar ratio, pH value, reaction time, temperature, etc. In this work, parallel experiments show that the initial reactants and the pH values of the reaction system are crucial for formation of the compounds. Compounds 1-4 could only be obtained in the special pH values of 1 for 1, 2.5 for 2 and 3, and 3.5 for 4 (Scheme 2). When the pH value is lower or higher than that special value, the expected crystals could not be obtained. In addition, the nature of the CuII salt is crucial for the formation of the compounds. We tried to replace Cu(NO3)2 3 3H2O with CuCl2 3 2H2O or Cu(CH3COO)2 3 2H2O in the syntheses of compounds, but no suitable crystals for singlecrystal X-ray diffraction were obtained. Structure of H4L(γ-Mo8O26) 3 2H2O (1). Complex 1 was hydrothermally obtained at acid condition (pH ≈ 1). Single-crystal X-ray diffraction analysis reveals that there is half a (γ-Mo8O26)4
anion

Figure 1. (a) Asymmetric unit of compound 1 (30% probability displacement ellipsoids). (b) The 1D molybdenum oxide chain of 1. (c) The 2D supramolecular layer formed via hydrogen bonds in 1. (d) The 3D supramolecular structure formed via hydrogen bonds in 1. 2922

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Crystal Growth & Design

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Figure 3. (a) Coordination environment of the CuII center in 3 (at 30% probability level). (b) View of the 3D framework in 3.

Figure 2. (a) Coordination environment of the CuII center in 2 (at 30% probability level). (b) View of 2D layer in 2. (c) View of the 3D supramolecular structure formed via π
π interactions in 2.

lying about an inversion center, half a H4L4þ cation lying about another inversion center, and one water molecule in the asymmetric unit (Figure 1a). The H4L4þ ligand is fully protonated to balance the charges of the (γ-Mo8O26)4
anion. The γ-octamolybdate has Ci symmetry and is composed of six {MoO6} octahedra interlinked along edges and two {MoO5} trigonal bipyramids each sharing two edges with the octahedra.16 Usually, attack at the five-coordinate molybdenum centers often leads to polymeric structures that are constructed from (γ-Mo8O26)4
-like monomer units and connected via oxide bridges. Thus, in 1, the (γ-Mo8O26)4
anions are linked together through two shared corners to form a 1D molybdenum oxide chain along the a-axis (Figure 1b). To the best of our knowledge, the octamolybdate chain is rarely observed in the Mo8-based hybrid compounds.17 These molybdenum oxide chains are connected by H4L4þ cations through hydrogen bonds between bridging oxygen atoms from the (γ-Mo8O26)4
anions and the protonated nitrogen atoms from benzimidazole rings to form a 2D supramolecular layer (Figure 1c). In addition, the 2D

layers are linked by the hydrogen bonds among water molecule, oxygen atoms of octamolybdate, and the protonated nitrogen atoms of pyridyl rings to generate a 3D supramolecular structure (Figure 1d). The geometric parameters of the hydrogen bonds are summarized in Supporting Information Table S2. Sturcture of [Cu(H2L)(β-Mo8O26)(H2O)2] 3 3H2O (2). When we increase the pH value of the reaction system to 2.5, a structurally different compound 2 is produced. Single-crystal X-ray analysis shows that compound 2 is a unique 2D network. The asymmetric unit of 2 contains one Cu1 cation, half a H2L2þ cation, half a (β-Mo8O26)4
anion, and one coordinated water molecule (Figure 2a). All molecules lie in independent inversion centers. Each CuII cation is six-coordinated by two nitrogen atoms of the pyridine groups from two H2L2þ ligands, two terminal oxygen atoms from two (β-Mo8O26)4
anions, and two water molecules, showing a distorted octahedral geometry. The Cu
N bond distance is 2.303(2) Å, and the Cu
O bond lengths are 1.925(2) and 2.494(2) Å, respectively. Surprisingly, the N atoms of the bezimidazole rings from H2L2þ ligand are fully protonated, while the N atoms of the pyridine rings are not protonated and coordinate to the CuII cations. Thereby, the H2L2þ ligand acts as a bivalent cation in 2. As shown in Figure 2a, the typical (βMo8O26)4
has a Mo6O6 ring (six edge-shared MoO6 octahedra) capped on each side by two MoO6 octahedra and possesses four sorts of O atoms: μ5-O, μ3-O, μ2-O and the terminal O atoms (Ot). The H2L2þ ligand adopts a cis-conformation and serves as a bidentate bridging ligand. CuII cations are bridged by the H2L2þ 2923

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Crystal Growth & Design

Figure 4. (a) Coordination environment of the CuII center in 4 (at 30% probability level). (b) View of the 1D chain in 4. (c) View of the 2D supramolecular layer formed via hydrogen bonds in 4.

ligands and (β-Mo8O26)4
anions to form a 2D layer structure (Figure 2b).18 These layers are connected by π
π interactions between benzimidazole rings with centroid
centroid distance of 3.39 Å, yielding a 3D supramolecular structure (Figure 2c). Structure of [Cu(H2L)(γ-Mo8O26)] 3 3H2O (3). Compound 3 was obtained at the same pH value as 2. As shown in Figure 3a, the asymmetric unit of 3 has one CuII cation, half a H2L2þ ligand, and half a (γ-Mo8O26)4
anion. All molecules lie in independent inversion centers. CuII center shows an octahedral coordination geometry [CuO4N2], completed by four terminal oxygen atoms from four (γ-Mo8O26)4
anions (Cu(1)
O(12) = 2.056(3), Cu(1)
O(9) = 2.384(2) Å), and two nitrogen atoms of the pyridine rings from two H2L2þ ligands (Cu(1)
N(1) = 1.951(3) Å). Notably, in 3, the N atoms of the bezimidazole rings are also fully protonated. Thereby, the H2L2þ ligand in 3 also acts as a bivalent cation. The H2L2þ ligands and (γ-Mo8O26)4
clusters bridge the adjacent CuII cations to form a 3D framwork structure (Figure 3b).19 Structure of Cu(HL)2(β-Mo8O26) (4). Compound 4 is obtained at the pH ≈ 3.5. The asymmetric unit of 4 consists of one CuII cation lying on independent inversion center, one HLþ

ARTICLE

cation in a general position, and half a (β-Mo8O26)4
anion lying on another inversion center. The (Mo8O26)4
cluster, which is built from eight distorted MoO6 edge-shared octahedra, is a typical β-octamolybdate. As shown in Figure 4a, Cu1 cation is six-coordinated by two nitrogen atoms of pyridine groups from two HLþ ligands and four terminal oxygen atoms from two (βMo8O26)4
anions, showing a distorted octahedral coordination geometry. The Cu
N bond distance is 2.025(6) Å, and the Cu
O bond lengths are 2.002(4) and 2.363(4) Å, respectively. The Cu
N and Cu
O distances are quite similar to the normal Cu
N and Cu
O distances. Interestingly, in 4, the N atoms of the pyridine rings from HLþ ligand are partly protonated, while the N atoms of the benzimidazole rings are not protonated. The HLþ ligand acts as a monovalent cation. Unexpectedly, the N atoms of the benzimidazole rings do not coordinate to CuII cations, which may be due to the weak coordination ability of the benzimidazole groups and the large steric hindrance of the (βMo8O26)4
anion. As shown in Figure 4b, each (β-Mo8O26)4
anion with the bidentate chelating mode coordinates to two Cu1 cations, and the Cu1 cations are bridged by (β-Mo8O26)4
anions to form a 1D chain along the a-axis. Surprisingly, the HLþ ligand acts as a monodentate ligand, attaching to the chain of (β-Mo8O26)4
anions and CuII ions. Adjacent chains further interact via hydrogen bonds between the N atoms of the benzimidszole rings and the protonated N atoms of the pyridine rings to form a 2D supramolecular structure (Figure 4c). The corresponding hydrogen-bond parameters were listed in Supporting Information Table S2. Influence of the pH Value on the Structures of Compounds 1
4. Compounds 1
4 were synthesized under the same reaction conditions, except for the pH values of the reactions. Their structural differences ambiguously indicate that the assembly process is pH-dependent. On the basis of the structures of 1
4, it was found that the influence of the pH value on the structures is in fact based on the protonated extents of the L ligand. For example, at pH ≈ 1, the N atoms of the L ligand is completely protonated, restricting themselvies to coordinating to CuII cation, so that a 1D molybdenum oxide chain forms in 1. With the increase of the pH value (≈ 2.5), compounds 2 and 3 can be obtained. The N atoms of benzimidszole rings from the L ligand are completely protonated, while the ones of pyridine rings from the L ligand are not protonated. In 2 and 3, the N atoms from pyridine rings can coordinate to CuII cations to generate 2D and 3D structures, respectively. When the pH value is adjusted to 3.5, compound 4 is obtained. In 4, the N atoms of the pyridine rings from L ligand are partly protonated, while the N atoms of the benzimidazole rings are not protonated. Compound 4 only exhibits a 1D chain structure. The result may be explained by the weak coordination ability of the benzimidazole group. The above-mentioned results show that the reaction pH value has an important influence on the structures of the assembly process since it determines the N atoms of the L ligand being protonated or coordinating to the metal ion. Influence of the Conformations of the L Ligand on the Structures of Compounds 2
4. Usually, the L ligand can show cis- and trans-conformations (Figure 5). In 2, the L ligand shows cis-conformation, while in 3 the L ligand gives a trans-conformation. Although compounds 2 and 3 contain the same H2L2þ ligands, their structures are entirely different. Compound 2 shows a unique 2D structure, whereas compound 3 displays a 3D framework structure. The structural changes may be explained by the different conformations of the H2L2þ ligands. In 2, the 2924

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Crystal Growth & Design

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Figure 5. (a) cis-Conformation of L ligand in 2. (b) The trans-conformation of L ligand in 3. (c) The distorted cis-conformation of L ligand in 4.

Figure 7. Cclic voltammograms of the 4-CPE in 1 M H2SO4 at different scan rates (from inner to outer: 100, 150, 200, 250, 300, 350, 400, 450, and 500 mV 3 s
1).

Figure 6. TGA curves of 1
4.

H2L2þ ligands coordinate to CuII ions in cis-conformations (Figure 5a) and produce meso-helical chains. These chains are linked by the (β-Mo8O26)4
anions to generate a 2D layer. Since the H2L2þ ligands in 3 adopt trans-conformations (Figure 5b), and they bridge the adjacent inorganic layers to form a 3D framwork structure. In 4, the HLþ ligand adopts a distorted cisconformation (Figure 5c), acting as a monodentate ligand. Thus, compound 4 show a 1D chain structure. Owing to the distorted cis-conformation of the HLþ ligand, the chains in 4 are connected by hydrogen bonds between N atoms of benzimidszole groups and protonated pyridine groups to generate a 3D supramolecular framework. UV
vis-NIR Spectra, XPS, and XRPD Analyses. The UV
visNIR spectra for 1
4 are shown in Supporting Information Figure S2 in the range of 200
800 nm. Two characteristic absorption peaks can be detected at 237 and 345 nm for 1, 233 and 322 nm for 2, 235 and 305 nm for 3, and 238 and 307 nm for 4, respectively. These lower energy bands are attributed to the O f Mo ligand-tometal charge transfers in the polyanions for 1
4. XPS was performed to identify the oxidation states of Mo and Cu in compounds 2
4. The XPS spectra show two peaks at 232.7 and 235.8 eV for 2, 232.4 and 235.5 eV for 3, and 232.2 and 235.3 eV for 4, which are attributed to Mo6þ (3d5/2) and Mo6þ (3d3/2), respectively.20 Two peaks at 933.8 and 954.2 eV for 2,

934.1 and 953.7 eV for 3, and 934.1 and 954.3 eV for 4 are attributed to Cu2þ (2p3/2) and Cu2þ (2p1/2),21 respectively (Figure S3 in the Supporting Information). These results further confirm the valencies of Mo and Cu atoms in 2
4. Figure S4 (see the Supporting Information) presents the XRPD patterns for compounds 1
4. The diffraction peaks of both simulated and experimental patterns match well in relevant positions, indicating thus the phase purities of the compounds 1
4. Thermal Analysis. To characterize the compounds more fully in terms of thermal stability, their thermal behaviors were studied by TGA. The experiments were performed under a N2 atmosphere with a heating rate of 10 °C/min in temperatures ranging from room temperature to 600 °C (Figure 6). Compound 1 lost its two lattice water molecules (exptl, 2.05%; calcd, 2.15%) from 25 to 110 °C, and then decomposition occurs at 276 °C. For compound 2, there are two distinct weight loss procedures in the range of 25
210 °C, corresponding to the loss of three lattice water and two coordinated water molecules per formula unit (exptl, 5.12%; calcd, 5.05%). The remaining structure was broken down above 303 °C, and decomposition does not end until heating to 600 °C. For 3, the TG data shows an initial loss of 2.91% (calcd: 3.09%) from room temperature to 100 °C, representing a loss of the lattice water molecules. The residue remains intact until it is heated 310 °C, and then mass loss occurs in a consecutive step and does not stop until heating to 600 °C. The decomposition of the anhydrous compound 4 occurs at 265 °C. Voltammetric Behavior of 4-CPE in Aqueous Electrolyte. The compound 4 has been taken as an example to study electrochemical property. The 4-modified carbon paste electrode (4-CPE) was fabricated according to the literature.22 Figure 7 shows the typical cyclic voltammogram behaviors at potential range 2925

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Crystal Growth & Design from
160 to 800 mV for 4-CPE at different scan rates. The peak potentials change gradually following the scan rates from 100 to 500 mV 3 s
1. It can be clearly seen that three reversible redox peaks appear in 1 M H2SO4 aqueous solution. Redox peaks I
I0 , II
II0 , and III
III0 correspond to three consecutive two-electron processes of Mo.23

’ CONCLUSION In conclusion, we have successfully synthesized four new organic
inorganic hybrid compounds based on different {Mo8} isomers, CuII cations, and flexible L ligands under hydrothermal conditions. These compounds show fascinating 1D, 2D and 3D frameworks with the various topologies. The result shows that the pH values of the reaction system and the conformations of the N-donor ligands play key roles in the structures of the Mo8-based MOFs. ’ ASSOCIATED CONTENT

bS

Supporting Information. X-ray crystallographic files (CIF); selected bond lengths and angles; hydrogen-bond geometries; UV
vis-NIR spectra and XRPD patterns of the compounds 1
4; XPS of the compounds 2
4. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected] (J.Y.); jianfangma@yahoo. com.cn (J.-F.M.). Fax: þ86-431-85098620 (J.-F.M.).

’ ACKNOWLEDGMENT We thank Program for Changjiang Scholars and Innovative Research Teams in Chinese Universities, the National Natural Science Foundation of China (Grant No. 21071028, 21001023), the Science Foundation of Jilin Province (20090137, 20100109), the Fundamental Research Funds for the Central Universities, the Specialized Research Fund for the Doctoral Program of Higher Education, the Science Foundation of Heilongjiang (B201017), the China Postdoctoral Science Foundation (20080431050 and 200801352), the Training Fund of NENU’s Scientific Innovation Project and the Analysis and Testing Foundation of Northeast Normal University for support. ’ REFERENCES (1) (a) Pope, M. T. Heteropoly and Isopoly Oxometalates; Springer: Berlin, 1983. (b) Hagrman, D.; Zubieta, C.; Rose, D. J.; Zubieta, J.; Haushalter, R. C. Angew. Chem., Int. Ed. 1997, 36, 873. (c) Xu, Y.; Xu, J. Q.; Zhang, K. L.; Zhang, Y.; You, X. Z. Chem. Commun. 2000, 6, 153. (d) M€uller, A.; Pope, M. T.; Peters, F.; Gatteschi, D. Chem. Rev. 1998, 98, 239. (e) Xu, L.; Qin, C.; Wang, X. L.; Wei, Y. G.; Wang, E. B. Inorg. Chem. 2003, 42, 7342. (f) Wu, C.-D.; Lu, C.-Z.; Zhuang, H.-H.; Huang, J.-S. J. Am. Chem. Soc. 2002, 124, 3836. (g) Fukaya, K.; Yamase, T. Angew. Chem., Int. Ed. 2003, 42, 654. (2) (a) Pope, M. T.; M€uller, A. Polyoxometalate Chemistry from Topology via Self-Assembly to Applications; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2001. (b) Yamase, T. Chem. Rev. 1998, 98, 307. (c) Chen, L.; Jiang, F.; Lin, Z.; Zhou, Y.; Yue, C.; Hong, M. J. Am. Chem. Soc. 2005, 127, 8588. (d) Xu, L.; Lu, M.; Xu, B.; Wei, Y.; Peng, Z.; Powell, D. R. Angew. Chem., Int. Ed. 2002, 41, 4129. (e) Wang, P.; Wang, X.; Zhu, G. New J. Chem. 2000, 24, 481.

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