Three Hybrid Organic−Inorganic Assemblies Based on Different

Aug 31, 2009 - Three new extended arsenatomolybdates constructed from different As−Mo−O cluster units and Cu-based complex bridges were obtained b...
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DOI: 10.1021/cg9006718

Three Hybrid Organic-Inorganic Assemblies Based on Different Arsenatomolybdates and CuII-Organic Units

2009, Vol. 9 5206–5212

Lili Li,† Bin Liu,† Ganglin Xue,*,† Huaiming Hu,† Feng Fu,‡ and Jiwu Wang‡ † Key Laboratory of Synthetic and Natural Functional Molecule Chemistry (Ministry of Education), Shaanxi Key Laboratory of Physico-Inorganic Chemistry, Department of Chemistry, Northwest University, Xi’an, 710069, China, and ‡Shaanxi Key Laboratory of Chemical Reaction Engineering, Yanan University, Yan’an, Shaanxi, China 716000

Received June 16, 2009; Revised Manuscript Received August 5, 2009

ABSTRACT: Three new extended arsenatomolybdates constructed from different As-Mo-O cluster units and Cu-based complex bridges, namely, (As6CuMo6O30){[Cu(imi)4]3[As6CuMo6O30]}2 3 6H2O (1), [Cu(enMe)2]3[As3Mo3O15]2 3 2H2O (2), and (NH4)10{Cu(H2O)4}[AsMo6O21(OAc)3]2 3 12H2O (3) (imi = imidazole, enMe = 1,2-propane diamine, OAc = acetate), were obtained by modulating the organic ligands in the conventional solution and similar conditions. Their structures were determined by single-crystal X-ray diffraction analysis and were further characterized by elemental analysis, IR, fluorescent spectroscopy, and TG analysis. The structure of 1 is constructed from [As6CuMo6O30]4- anionic clusters and [Cu(imi)4]2þ complex subunits via Cu-O interactions to form a two-dimensional (2D) framework with 436 topology structure. Compound 2 is constructed from [As3Mo3O15]3- anionic clusters and [Cu(enMe)2]2þ complex subunits via Cu-O interactions to form a 2D honeycomb framework with 63 topology structure. Compound 3 is based on the unprecedented functionalized polyanions [AsMo6O21(OAc)3]6-; two of them are bridged by a {Cu(H2O)42þ complex to form a dimeric polyanion {Cu(H2O)4[AsMo6O21(OAc)3]2}10-, and the dimeric polyanions are further connected via hydrogen bonds to three-dimensional supramolecular structure. Compound 1 has an intense emission at 399 nm assigned to O2p to Mo4d charge transfer and may be an excellent candidate for potential solid-state photofunctional material.

Introduction The heteropolymolybdates are of great interest in solidstate material chemistry, because of their unusual structural chemistry and properties that make them attractive for applications in catalysis, materials science, photochemistry and electrochemistry.1,2 In this field, arsenatomolybdates show a great variety of structures and properties owing to the redoxactive nature of both molybdenum and arsenic; therefore, polymolybdates could be molecularly fine-tuned and provide potential new types of catalyst systems, as well as interesting functionalized materials with other properties.3 So far a number of discrete arsenatomolybdates have been synthesized and structurally characterized, most of them are based on Keggin-type polyanions, such as, [HnAsVMoVnMoVI12-nO40]3- (n = 0, 1, 2, and 4),4-6 bicapped Keggintype polyanions [AsIII2AsVMo12O40]5-,7 monovacant and trivacant Keggin polyanions, [AsVMo11O39]7- and [AsVMo9O34]9-,8,9 monocapped trivacant Keggin-type polyanions [AsIIIAsVMoVI9O34]6- and [HAsIIIAsVMoVMoVI8O34]6-,10,11 and those containing trivacant Keggin units, [(AsIIIOH)3(MoO3)3(AsIIIMo9O33)]7-, [(AsIIIOH)6(MoO3)2(O2Mo-OMoO2)2(AsIIIMo9O33)2]10-, and [(AsIIIOH)4(AsIIIO)2(O2MoO-MoO2)2(AsIIIMo9O33)2]8-.6,12 Also 2:18 and 2:17 Dawsontype polyanions, [AsV2Mo18O62]6- and [AsV2Mo17O61]10-,13,14 and sandwich-like polyanions [M2(AsMo7O27)2]n- (M = Cr3þ or Cu2þ) reported recently by our group,15 and other types of polyanions, [AsVMo8O30]7- and [AsVMo8O28(OH)2]5-,16 [AsV2Mo6O26]6-,17 [H4AsV4Mo12O50]4-,18 [(HAsVO4)4(Mo4O10)]4-,19 [AsIII6MoV4O20(OH)2]4-,20 [AsIII3MoVI3O15]3-,21 and *To whom correspondence should be addressed: E-mail: xglin707@163. com. pubs.acs.org/crystal

Published on Web 08/31/2009

[AsIII6MMo6O30]n- (M = Co2þ, Cu2þ or Mo)21-23 have been reported. During the past few years, the modification of the surface of polyoxometalate (POM) clusters has attracted much more attention and mighty endeavors have been devoted to prepare functionalized POM-based materials via incorporation or coordination of organic moieties with success in some aspects.24,25 Recently, Zubieta et al. reported the systemic study of molybdenum organoarsonates under hydrothermal synthesis conditions,26 and in addition several assemblies of molybdenum arsenate fragments as inorganic building blocks hybrid materials have also been reported, such as [{Cu(en)2}2Mo4As6O20(OH)2] 3 2H2O,27 [AsMo6O21L3]3- (L = amino acids),28 [{Cu(I)(imi)2}3As3Mo3O15] 3 H2O,29 [HAsIIIAsVMoVMoVI8O34{Co(C5H5N)2(H2O)3}]4-,9 (4,4-bipy)[Zn(4,4-bipy)2(H2O)2]2[As6ZnMo6O30] 3 7H2O, [Zn(phen)2(H2O)]2[As6ZnMo6O30] 3 4H2O, and [Zn(2,2bipy)2(H2O)]2[As6ZnMo6O30] 3 4H2O.30 However, in contrast to the rich information on discrete arsenatomolybdates, functionalized arsenatomolybdates remain relatively undeveloped. In this paper, we report three new extended functionalized arsenatomolybdates, (As6CuMo6O30){[Cu(imi)4]3[As6CuMo6O30]}2 3 6H2O (1), [Cu(enMe)2]3[As3Mo3O15]2 3 2H2O (2), and (NH4)10{Cu(H2O)4}[AsMo6O21(OAc)3]2 3 12H2O (3). Experimental Section All chemicals were commercially purchased and used without further purification. Elemental analyses (C, H, and N) were performed on a Perkin-Elmer 2400 CHN elemental analyzer; As, Mo and Cu were analyzed on an IRIS Advantage ICP atomic emission spectrometer. IR spectra were recorded in the range of 4004000 cm-1 on an EQUINOX55 FT/IR spectrophotometer using KBr pellets. Thermogravimetric-differential scanning calorimetry (TG-DSC) analyses were performed on a NETZSCH STA 449C TGA instrument in flowing N2 with a heating rate of 10 °C 3 min-1. r 2009 American Chemical Society

Article Excitation and emission spectra were obtained on a Hitachi F4500 spectrofluorometer equipped with a 450W xenon lamp as the excitation source. Synthesis of (As6CuMo6O30){[Cu(imi)4]3[As6CuMo6O30]}2 3 6H2O (1). The synthesis of compound 1 was accomplished by adding a solution of As2O3 (0.48 g, 2.4 mmol) dissolving in 6 M hydrochloric acid (6 mL) to a solution of (NH4)6Mo7O24 3 4H2O (0.85 g, 0.7 mmol) dissolving in H2O (10 mL). Then 0.35 g (2.0 mmol) of CuCl2 3 2H2O was added and the pH was adjusted to 6.0 by 3 M ammonium hydroxide. The resulting suspension was heated to about 90 °C, and a clear solution formed. Solid imidazole (0.32 g, 4.8 mmol) was added to the hot solution and the pH was adjusted to 6.0 by adding 6 M hydrochloric acid. Then the solution was heated for an additional 30 min. The hot solution was filtered and allowed to stand in a closed container at 5 °C. At this point the pH was 6.2. The following day cyan crystals started to form, and after 12 h 0.85 g (yield 47% based on Mo) of crystalline product was isolated. Anal. found (calcd) for C72H108N48As18Cu9Mo18O96: Mo, 26.0 (25.3); As 20.4 (19.7); Cu, 8.8 (8.4); N 9.6 (9.8), C 12.2 (12.7), H, 1.52 (1.59). FT-IR (KBr, cm-1): 1632 (m), 1531 (m), 1498 (m), 1439 (m), 1329 (m), 1261 (m), 1176 (w), 1110 (w), 1072 (s), 929 (s), 901 (s), 809 (s), 762 (m), 668 (s), 651 (s). Synthesis of [Cu(enMe)2]3[As3Mo3O15]2 3 2H2O (2). The synthesis of compound 2 was accomplished by adding a solution of As2O3 (0.48 g, 2.4 mmol) dissolving in 6 M hydrochloric acid (6 mL) to a solution of (NH4)6Mo7O24 3 4H2O (0.85 g, 0.7 mmol) dissolving in H2O (10 mL). Then 0.35 g (2.0 mmol) of CuCl2 3 2H2O was added and the pH was adjusted to 6.0 by 3 M ammonium hydroxide. The resulting suspension was heated to about 90 °C, and a clear solution formed. 1,2-Propane diamine (0.37 g, 4.8 mmol) was added to the hot solution, and the pH was adjusted to 6.0 by 6 M hydrochloric acid. Then the solution was heated for an additional 30 min. The hot solution was filtered and allowed to stand in a closed container at 5 °C. At this point the pH was 6.5. The following day purple crystals started to form, and after 1 h 0.90 g (yield 51% based on Mo) of crystalline product was isolated. Anal. found (calcd) for C18H64N12As6Cu3Mo6O32: Mo, 26.3 (26.4); As, 20.2 (20.7); Cu, 8.8 (8.8); N, 7.4 1(7.72);C, 9.85 (9.93); H,3.01 (2.96). FT-IR (KBr, cm-1): 1631 (s), 1601 (s), 1459 (w), 1383 (w), 1350 (m), 1065 (m), 916 (m), 889 (s), 803 (s), 735 (s). Synthesis of (NH4)10{Cu(H2O)4}[AsMo6O21(OAc)3]2 3 12H2O (3). The synthesis of compound 3 was accomplished by adding a solution of As2O3 (0.48 g, 2.4 mmol) dissolving in 6 M hydrochloric acid (6 mL) to a solution of (NH4)6Mo7O24 3 4H2O (0.85 g, 0.7 mmol) dissolving in H2O (10 mL). Then 0.35 g (2.0 mmol) of CuCl2 3 2H2O was added and the pH was adjusted to 6.0 by 3 M ammonium hydroxide. The resulting suspension was heated to about 90 °C, and a clear solution formed. Acetic acid (0.28 g, 4.8 mmol) was added to the hot solution, and the pH was adjusted to 6.0 by 3 M ammonium hydroxide. Then the solution was heated for an additional 30 min. The hot solution was filtered and allowed to stand in a closed container at 5 °C. At this point the pH was 6.1. The following day blue crystals started to form, and after 3 h 0.60 g (yield 52% based on Mo) of crystalline product was isolated. Anal. found (calcd) for C12H90N10As2CuMo12O70: Mo 40.0, (40.3); As, 5.4 (5.2); Cu, 2.4 (2.3); N, 5.12 (4.89); C, 5.20 (5.04); H, 3.26 (3.17). FT-IR (KBr, cm-1): 1257 (m), 1025 (w), 886 (s), 767 (m), 661 (s), 618 (m). X-ray Crystallography. Intensity data were collected on a Rigaku BRUKER SMART APEX II CCD diffractometer with Mo KR monochromated radiation (λ = 0.71073 A˚) at 293 K. Empirical absorption correction was applied. The structures of 1-3 were solved by the direct method and refined by the Full-matrix leastsquares on F2 using the SHELXTL-97 software.31 All of the nonhydrogen atoms were refined anisotropically. The organic hydrogen atoms were generated geometrically; those attached to other water molecules were not located. A summary of the crystallographic data and structure refinement for compounds 1-3 is given in Table 1. Crystallographic data for the structural analysis of 1-3 have been deposited with the Cambridge Crystallographic Data Centre, CCDC Nos.: 729938 for C72H108N48As18Cu9Mo18O96 (1), 729939 for C18H64N12As6Cu3Mo6O32 (2) and 729940 for C12H90N10As2CuMo12O70 (3).

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Table 1. Summary of Crystallographic Data for the Structures of Compounds 1-3 1 C72H108N48As18Cu9Mo18O96 6829.34 hexagonal P3 22.7883(16) 22.7883(16) 9.3807(10) 90 90 120 4218.8(8) 1 293(2) 2.688 6.010 20751 4889 R(int) = 0.0574 parameters 394 GOF 0.960 0.0383 R1a (I > 2σ(I)) wR2b (I > 2σ(I)) 0.0866 0.0638 R1a (all data) 0.0928 wR2b (all data) 1.491, -1.093 diff peak and hole, e A-3

empirical formula M, g mol-1 cryst syst space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V, A˚3 Z temp (K) dcalcl, g cm-3 μ, cm-1 total reflns indep reflns

a

2

3

C18H64N12As6Cu3Mo6O32 2176.59 monoclinic C2/m 28.82(3) 16.154(15) 7.899(7) 90 90.53(2) 90 3677(6) 2 293(2) 1.966 4.590 9222 3384 R(int) = 0.1765 189 0.812 0.0796 0.1951 0.1649 0.2280 1.360, -1.907

C12H90N10As2CuMo12O70 2859.54 triclinic P1 10.7383(17) 11.9618(18) 18.087(3) 71.024(2) 88.929(3) 67.634(2) 2017.0(5) 1 293(2) 2.354 2.989 10178 7034 R(int) = 0.0249 488 0.945 0.0375 0.1032 0.0548 0.1181 1.297, -0.906

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

Results and Discussion Description of the Structures. (As6CuMo6O30){[Cu(imi)4]3[As6CuMo6O30]}2 3 6H2O (1). The single-crystal X-ray analysis reveals that compound 1 is constructed from two crystallographic asymmetric [As6CuMo6O30]4- (1a) anionic clusters and [Cu(imi)4]2þ complex subunits via Cu-O bridges (see Figure 1) to form a 2D framework with 436 topology structure (Figure 2). The anion [As6CuMo6O30]4- (1a) is derived from the well-known A-type Anderson anion [CuO6Mo6O18]10-, in which a central {CuO6} octahedron is coordinated with six {MoO6} octahedra hexagonally arranged by sharing their edges in a plane. The cyclic As3O6 trimers are capped on opposite faces of the Anderson-type anion plane. Each As3O6 group consists of three AsO3 pyramids linked in a triangular arrangement by sharing corners and bonded to the central CuO6 octahedron and two MoO6 octahedra via μ3-oxo groups. This polyoxoanion architecture was reported for the first time by Jeannin et al. for the cobalt(II)-containing arsenomolybdate [Co(H2O)6]K2[As6CoMo6O30].20 The two crystallographic asymmetric [As6CuMo6O30]4- (named them as 1a(1) and 1a(2)) have different coordination environments. 1a(1) acts as a tridentate ligand coordinating to three copper imidazole complexes through the terminal oxygen atoms of three nonadjacent MoO6 octahedra, and the copper imidazole complexes all located at the same side of the Anderson-type anion plane, and 1a(2) acts as a hexadentate ligand coordinating to six copper imidazole complexes through the terminal oxygen atoms of six MoO6 octahedra and the adjacent copper imidazole complexes located at the opposite faces of the Anderson-type anion plane. Furthermore, 1a(1) and 1a(2) are linked each other to form an unusual novel 2D layered framework with 436 topology structure in which 1a(1) clusters serve as three-connected nodes and 1a(2) clusters serve as sixconnected nodes. Coordination sphere of crystallographically unique CuII in the copper imidazole complex is defined by two

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Figure 1. (a) ORTEP drawing of 1a(1) with thermal ellipsoids at 30% probability. (b) Two crystallographic asymmetric [As6CuMo6O30]4(1a) anionic clusters with different coordination environments connected by [Cu(imi)4]2þ complex bridges.

Figure 2. (a) Polyhedral representation of the connections of the [CuAs6Mo6O30]4þ via {Cu(imi)4}2þ groups. (b) Schematic presentation of 2D (436) topology structure.

trans terminal oxygen atoms from 1a(1) and 1a(2) (Cu(3)-O(15) = 2.418(4) A˚ and Cu(3)-O(4) = 2.860(4) A˚) and four nitrogen atoms from four imidozole ligands (Cu(3)-N(1) = 2.011(6) A˚, Cu(3)-N(3) = 2.004(6) A˚, Cu(3)-N(5) = 1.999(6) A˚, Cu(3)-N(7) = 2.014(6) A˚). Among the layer, the significant difference in MoO-Cu bond distances is remarkable, and the 1a(2) cluster bonding with six [Cu(imi)4]2þ complexes has a longer MoO-Cu bond

(Cu(3)-O(4) = 2.860(4) A˚) and the 1a(1) cluster bonding with three [Cu(imi)4]2þ complexes has a shorter MoO-Cu bond (Cu(3)-O(15) = 2.418(4) A˚) because of adjacent copper complexes experience or do not experience the imidazole-imidazole steric interaction. [Cu(enMe)2]3[As3Mo3O15]2 3 2H2O (2). Compound 2 is constructed from [As3Mo3O15]3- anionic clusters (2a) and [Cu(enMe)2]2þ complex subunits (see Figure 3) via Cu-O interactions to form 2D honeycomb framework with 63 topology structure (Figure 4). The molybdenum arsenate fragment [As3Mo3O15]3- contains a triplet of edge-sharing MoO6 octahedra stabilized by a linear As3O75- triarsenate which is capped on Mo3O13 with four μ2-O atoms and one μ4-O atom.20 Thus, the Mo-O bond lengths fall into three classes: Mo-Ot = 1.715(11)-1.751(11) A˚, Mo-O(μ2) = 1.812(10)-2.212(10) A˚, and Mo-O(μ4) = 2.294(10)-2.421(13) A˚. And the As-O bond lengths fall into two classes: As-O(μ2) = 1.805(10)-1.829(11) A˚ and As-O(μ4) = 1.786(13) A˚. In the structure of compound 2, 2a acts as a tridentate ligand coordinating to three copper enMe complexes through the terminal oxygen atoms of three MoO6 octahedra. Two crystallographic asymmetric copper atoms

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Figure 3. (a) ORTEP drawing of 2 with thermal ellipsoids at 30% probability. (b) Polyhedral and ball-stick representation of 2.

Figure 4. (a) Representation of the connections of the [As3Mo3O15]3- units in 2 via {Cu(enMe)2}2þ bridges with large 18-membered rings. (b) Schematic presentation of 2D (63) topology structure with 2a as nodes.

exist in 2. Both Cu(1) and Cu(2) are six-coordinate, defined by two trans terminal oxygen atoms from two adjacent clusters (Cu(1)-O(5) = 2.543(14) A˚ and Cu(2)-O(1) = 2.635(14) A˚) and four nitrogen atoms from two enMe ligands (Cu(1)-N(2) = 1.992(15) A˚, Cu(1)-N(3) = 2.048(15) A˚, and Cu(2)-N(1) = 1.967(14) A˚). Cu(1)-enMe complexes link [As3Mo3O15]3- building blocks together to form infinite one-dimensional (1D) chains along the b axis, which are further joined by Cu(2)-enMe subunits to form an unusual 63 topology 2D honeycomb network. And the adjacent layers are further linked to each other via hydrogen bonding (N1 3 3 3 O9 (3.038 A˚)) to form a 3D supramolecular network with 1D channels along the c axis. The channels show honeycomb shape with dimensions (corresponding to the longest interacage 2a-2a distances) of about 20.6  20.8 A˚2 with guest water molecules residing in the channels. (NH4)10{Cu(H2O)4}[AsMo6O21(OAc)3]2 3 12H2O (3). Compound 3 is based on the unprecedented acetate functionalized polyanions [AsMo6O21(OAc)3]6- (3a); two of them are bridged by a {Cu(H2O)42þ complex to form a dimeric polyanion {Cu(H2O)4[AsMo6O21(OAc)3]2}10- (3b) (Figure 5). Polyoxoanion 3a consists of a heteroatom As(III) surrounded by a ring of six MoO6 octahedra which alternately share edges

and corners, and furthermore three acetates are each bonded to two edge-sharing Mo centers by their carboxylate groups on the same side of the ring. The central heteroatom, located slightly above the plane of six molybdenum atoms, is coordinated to three μ3-oxo groups which lead to a trigonal-pyramidal coordination geometry. The lone pair electrons of As atom and the acetic acids are on the same side of the ring. The center copper atom in the dimeric polyanion is sixcoordinate, defined by two trans terminal oxygen atoms from two adjacent clusters (Cu(1)-O(10) = 2.585(4) A˚) and four oxygen atoms from four water molecule (Cu(1)-O(112) = 2.018(8) A˚ and Cu(1)-O(113) = 2.070(8) A˚). Polyanions 3b are further connected via hydrogen bonds (MoO 3 3 3 OW(101) (2.742-2.758 A˚), MoO 3 3 3 OW(103) (2.845-3.025 A˚), MoO 3 3 3 OW(105) (2.873-2.974 A˚), MoO 3 3 3 OW(108) (2.995 A˚), and MoO 3 3 3 OW(110) (2.742-2.758 A˚) (Figure S1, Supporting Information) to form 3D structure. The structure of polyanion {Cu(H2O)4[AsMo6O21(OAc)3]2}10- (3b) is somewhat similar to that of [AsMo6O21(O2CCH2NH3)3]3- reported by Ulrich Kortz in 2002.28 The main differences are (1) the polyanion [AsMo6O21(OAc)3]6in compound 3 was functioned by acetate and the polyanion [AsMo6O21(O2CCH2NH3)3]3- functioned by amino acid; (2) the polyanion compound 3b is a dimer linked by a copper complex, and [AsMo6O21(O2CCH2NH3)3]3- is a discrete ion. Synthesis. Reaction of a solution of As2O3 with (NH4)6Mo7O24 3 4H2O, Cu2þ and different organic ligand (imidazole, 1,2-propane diamine or acetic acid, respectively) at 90 °C and in pH = 6.0-7.0 aqueous solution, we obtained three functionalized arsenatomolybdates, (CuAs6Mo6O30){[Cu(imi)4]3[As6CuMo6O30]}2 3 6H2O (1), [Cu(enMe)2]3[As3Mo3O15]2 3 2H2O (2), and (NH4)10{Cu(H2O)4}[AsMo6O21(OAc)3]2 3 12H2O (3). It is interesting that three functionalized arsenatomolybdates are based on three different arsenatomolybdate fragments and were isolated in a similar condition except different organic ligands, which means that the system of molybdenum-arsenite mixtures involves many species mainly related to pH, the stability of polyanion,

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Figure 5. (a) ORTEP drawing of 3a with thermal ellipsoids at 30% probability. (b) Polyhedral and ball-stick representation of the polyoxoanion 3a.

Scheme 1. The Formation Conditions of the Relative Arsenatomolybdates

temperature, metal ions, and organic ligands. In our case, the organic ligand plays a crucial role in the isolation of title compounds. Besides, as known, the pH value of the solution is also an important factor for the formation of title compounds. At lower pH values, the ligand groups tend to be protonated and fail to bond to the metal ions. After plenty of parallel experiments, it was found that adjustment of the pH of the mixture to 6.0-7.0 is crucial for the synthesis of all the compounds. In the absence of ligand, we obtained a sandwich-like cluster anion [As2Cu2Mo14O54]14-.15 When Cu2þ was replaced by Ni2þ, we obtained the nickel(II)-containing arsenomolybdate (NH4)2Na2NiAs6Mo6O30 3 6H2O32 (4) and its polyanion has the same structure as that of [CuAs6Mo6O30]4-. Scheme 1 shows the

formation conditions of the relative arsenatomolybdates, briefly. Thermal Analyses. According to the TG-DSC curve of compound 1 (Figure S1, Supporting Information), we can deduce that the thermal decomposition process of the compound is approximately divided into two steps. There is a continuous decrease among 35-163-479 °C; the weight loss of 25.7% is comparable with the calculated value of 25.5%, corresponding to the loss of six lattice water molecules and 24 molecules of imidazole. The second weight loss of 16.7% is between 479 and 750 °C accompanying a remarkable exothermal peak at 533 °C in the DSC curve due to most of the As2O3 escaping.

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emissions at about 400 nm should be assigned to O2p to Mo4d charge transfer. These observations indicate that the photoluminescence properties of POMs could be adjusted by changing their structures and compositions, and compound 1 may be excellent candidates for potential solidstate photofunctional material, since compound 1 is thermally stable and insoluble in common polar and nonpolar solvents. Conclusions

Figure 6. Luminescent spectra of compounds 1-4.

The TG curve of the compound 2 (Figure S2, Supporting Information) is similar to that of the compound 1; first it gradually loses two lattice water molecules (calcd 1.7%) and six molecules of enMe (calcd 20.4%) among 35-180411 °C, going with two endothermal peaks at around 106 and 238 °C observed in the DSC curve; the total weight loss of 22.3% is consistent with the calculated value of 22.1%. The weight loss of 19.8% from 411 to 750 °C is ascribed to the escaping of most of the As2O3, accompanying a remarkable exothermal peak at around 547 °C on the DSC curve. TG-DSC curve of compound 3 (Figure S3, Supporting Information) indicates that the weight loss of the compound can be divided into three steps. The first weight loss of 9.6% from 30 to 180 °C corresponds to the loss of all 12 lattice water molecules and four coordination molecules (calcd. 10.1%), going with two endothermal peaks at around 99 and 147 °C observed in the DSC curves. The second weight loss of 18.4% between 147 and 368 °C arises from the loss of all 17 ammonia molecules and six HAc molecules (calcd. 18.5%), accompanying endothermal peaks at around 197 °C, and the third weight loss of 8.1% from 368 to 750 °C is ascribed to the escaping of one molecule of As2O3, accompanying a remarkable exothermal peak at around 407 °C on the DSC curve. And the remaining residues correspond to the mixed-metal oxides, CuO 3 12MoO3. The observed total weight loss of 35.4% is comparable with the calculated value of 36.8%. Photoluminescence Spectroscopy. The emission spectra of compounds 1-4 in the solid state at room temperature are depicted in Figure 6. It is unexpected that compound 1 has an intense emission at 399 nm, and weak emissions occur at 405 nm for 2, and 401 nm for 3. To understand the nature of the emission band, the photoluminescence properties of imidazole and ammonium molybdate were analyzed. (NH4)6Mo7O24 3 4H2O has a weak emission at 402 nm and imidazole has a weak emission at 463 nm. Therefore, the

In summary, we successfully synthesized three totally different arsenatomolybdates constructed from [As6CuMo6O30]4-, [As3Mo3O15]3-, and [AsMo6O27]15- clusters as SUBs by modulating the organic ligands in similar reaction conditions. In compounds 1 and 2 polyoxoanions are all act as multidentate ligands for Cu(II) ions and formed a 436 and 63 honeycomb 2D network, respectively, while in compound 3, the discrete dimer polyanion {Cu(H2O)4[AsMo6O21(OAc)3]2}10- was connected via hydrogen bonds to form a 3D structure. This work confirms the utility of conventional open-air solution methods for the synthesis of new polymeric materials with POMs as building blocks and illustrates that the ligand may play an important role in stabilizing the polyoxoanions; for example, imidazole can stabilize [As6CuMo6O30]4-, enMe may stabilize [As3Mo3O15]3-, and the acetate decorated dimer polyanion {Cu(H2O)4[AsMo6O21(OAc)3]2}10- was formed in the presence of acetic acid. And in the absence of organic ligand the sandwich polyanion [As2Cu2Mo14O54]14- is the main product. Furthermore, we obtained the polyanions [As6NiMo6O30]4- by changing Cu2þ into Ni2þ, which demonstrates the kind of secondary transition metal also influences the structure of the cluster. These results show once again the extreme structural versatility in POM chemistry and allow us to expect that a large family of polymeric heteropolymolybdates could be obtained in this way by varying the nature of the secondary transition metal and the organic ligand. We also expect that our work will provide help for the investigation with the potential applications of these compounds since compound 1 has an intense emission at 399 nm in photoluminescence spectroscopy. Acknowledgment. This work was supported by Funded projects of independent innovation of Northwest University Postgraduates (08YZZ41) and the Education Commission of Shaanxi Province (09JK783). Supporting Information Available: X-ray crystallographic reports (CIF) of compounds 1-3; TG-DSC curves of compounds 1-3 (Figure S1-S3). This information is available free of charge via the Internet at http://pubs.acs.org/.

Note Added after ASAP Publication. This article was published ASAP on 08/31/2009 before all author corrections were included. The corrected version was posted on 09/11/ 2009.

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