Synthesis, Characterization, and Crystal Structures of Double-Cubane

The [β-SiW8O31] unit in the two compounds and the [x-SiW9O34] unit in 2 were reported ... polyoxometalates K12[(SiW8O31)2Ni7(H2O)4(OH)6]·23H2O (1) a...
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Synthesis, Characterization, and Crystal Structures of Double-Cubane-Substituted and Asymmetric Penta-Ni-Substituted Dimeric Polyoxometalates Zhiming Zhang, Enbo Wang,* Yanfei Qi, Yangguang Li, Baodong Mao, and Zhongmin Su*

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 7 1305-1311

Key Laboratory of Polyoxometalate Science of Ministry of Education, Department of Chemistry, Northeast Normal UniVersity, Ren Min Street No. 5268, Changchun, Jilin 130024, P. R. China ReceiVed NoVember 30, 2006; ReVised Manuscript ReceiVed May 9, 2007

ABSTRACT: Two novel multi-Ni-substituted polyoxometalates K12[(SiW8O31)2Ni7(H2O)4(OH)6]‚23H2O (1) and K6Na18[H2{Ni5(H2O)5(OH)3(x-SiW9O34)(β-SiW8O31)}2]‚39H2O (2) have been successfully synthesized by routine synthetic reactions in aqueous solution. Single-crystal X-ray analysis was carried out on the two compounds (1 and 2), which both crystallize in the triclinic system. For 1, space group P1h, a ) 11.442(2) Å, b ) 11.905(2) Å, c ) 17.169(3) Å, R ) 107.62(3)°, β ) 91.67(3)°, γ ) 102.37(3)°, V ) 2166.1(8) Å3, Z ) 1. For 2, space group P1h, a ) 18.435(4) Å, b ) 25.095(5) Å, c ) 25.095(5) Å, R ) 81.44(3)°, β ) 73.87(3)°, γ ) 85.31(3)°, V ) 9624(3) Å3, Z ) 2. Compound 1 represents the first example of the double-cubane Ni7O8-clustersubstituted polyoxometalates, and the [β-SiW8O31] unit in 1 was scarcely reported previously. Compound 2 exhibits a novel asymmetric dimeric structure, consisting of [β-Ni3SiW9O37(H2O)3] and [γ-SiW8Ni2O34(H2O)2] units linked by three µ3-OH groups. In the [β-Ni3SiW9O37(H2O)3] half unit, this fashion of the tri-Ni-substituted was scarcely reported, and in the [γ-SiW8Ni2O34(H2O)2] units, the mode of the di-Ni-substituted is also observed scarcely in the polyoxometalates chemistry. Introduction Over the past decades, polyoxometalates (POMs) have been attracting extensive interest in many fields because of their abundant topological properties and their great potential for application in catalysis, photochemistry, ion exchange, electrochromism, and magnetism.1,2 The evolution of POMs chemistry is dependent upon the synthesis of novel polyoxoanions possessing unique structures and properties; however, design and synthesis of such compounds remains a challenge. The transition-metal-substituted POMs (TMSPs), obtained by reaction of lacunary polyoxoanions with various transition metals,3 exhibit interesting catalysis, magnetic, and electrochemical properties. Among them, a great subclass is the sandwich-type polyoxoanions,4-6 accommodating lots of paramagnetic transition metal cations between two lacunary polyoxoanions. These compounds mostly belong to the well-known Weakley ([M4(H2O)2(XW9O34)2]n- and [M4(X2W15O56)2]n-), Herve´ ([M3(H2O)3(R-XW9O33)2]n- (X ) AsIII, SbIII, SeIV, TeIV, BiIII)), Krebs ([M2(H2O)6(WO2)2(β-XW9O33)2]n-) and Knoth type ([M3(A-XW9O34)2]n- (X ) P and Si)) sandwich structures, and the number of transition-metal substitution among the reported examples is mostly less than five. In addition, the dimeric compounds are commonly composed of two identical lacunary polyoxoanion fragments. However, asymmetric dimeric structures, which are composed of two different lacunary polyoxoanion fragments, have so far been unexplored but are attracting increased attention recently. Until now, only a few dimeric compounds with asymmetric features have been reported.7-9 The divacant polyoxoanion [γ-SiW10O36]8-, a metastable polyoxoanion, was often studied recently. When dissolved in the aqueous solution, the polyoxoanion [γ-SiW10O36]8- usually undergoes an isomerization course that has been well studied by Te´ze` et al.10 As a result, the reaction of such a polyoxoanion * To whom correspondence should be addressed. E-mail: wangenbo@ public.cc.jl.cn, [email protected] (E.W.); [email protected] (Z.S.). Fax: 86-431-85098787.

with transition-metal cations can lead to unprecedented POMs structures, such as the cyclic tetrameric polyoxotungstate [{βTi2SiW10O39}4]24-;11 trimeric Zr-substituted [Zr6O2(OH)4(H2O)3(β-SiW10O37)3]14- ,12 Cu-substituted [{SiW8O31Cu3(OH)(H2O)2(N3)}3(N3)]19-,13 and the satellite-shaped polyoxotungstate [Co6(H2O)30{Co9Cl2(OH)3(H2O)9(β-SiW8O31)3}]5-;19 the dimeric silicotungstates [{SiM2W9O34(H2O)}2]12- (M ) Mn, Cu, Zn)14 and [{γ-SiTi2W10O36(OH)2}2(µ-O)2]8-;15 and the monomeric polyoxoanions.16 The charming structures of the POMs have attracted much attention on this field. But, the reactive mechanism of the [γ-SiW10O36]8- reaction with transition-metal cations has not been clear, and the systematic structural design of the novel polyoxoanions derived from [γ-SiW10O36]8- has rarely been carried out. Here, we report the synthesis, characterization and crystal structures of the hepta-Ni-substituted dimeric POMs K12[(SiW8O31)2Ni7(H2O)4(OH)6]‚23H2O (1) and the asymmetric penta-Ni-substituted dimeric POMs K6Na18[H2{Ni5(H2O)5(OH)3(x-SiW9O34)(β-SiW8O31)}2]‚39H2O (2). Experimental Section Materials and Methods. All chemicals were commercially purchased and used without further purification. K8[γ-SiW10O36] was synthesized according to the literature10 and characterized by IR spectrum. Elemental analyses (H) were performed on a Perkin-Elmer 2400 CHN elemental analyzer; Si, W, Ni, Na, and K were analyzed on a PLASMA-SPEC(I) ICP atomic emission spectrometer. The EPR spectra are recorded on a Japanese JES-FE3AX spectrometer at 77 K. IR spectra were recorded in the range 400-4000 cm-1 on an Alpha Centaurt FT/IR spectrophotometer using KBr pellets. Electrochemical measurements were carried out on a CHI 600 electrochemical workstation at room temperature (25-30 °C) under a nitrogen atmosphere. Synthesis of 1. In a typical synthesis procedure for 1, K8[γ-SiW10O36] (0.5 g) was dissolved in 10 mL of distilled water with stirring. Next, 1 mL of NiCl2 solution (1 M) was added dropwise with vigorously stirring, and then 10 g KCl was added to the solution. The mixture was heated for half an hour at 80 °C. After the mixture was cooled to room temperature, 40 mL of distilled water was added to the green solution. At this point, 2 M Na2CO3 was used to adjust the pH value to 9.5, and the solution was heated for 1 h at the same temperature.

10.1021/cg060868m CCC: $37.00 © 2007 American Chemical Society Published on Web 06/08/2007

1306 Crystal Growth & Design, Vol. 7, No. 7, 2007

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Table 1. Crystal Data and Structure Refinement for 1 and 2

empirical formula formula mass T (K) wavelength (Å) cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (g cm-3) µ (mm-1) F000 data/restraints/params GOF on F2 R1a [I > 2σ(I)] wR2b largest diff. peak and hole (e Å-3) a

1

2

H60K12Ni7O95Si2W16 5458.43 293(2) 0.71073 triclinic P1h 11.442(2) 11.905(2) 17.169(3) 107.62(3) 91.67(3) 102.37(3) 2166.1(8) 1 4.185 23.357 2456 7559/210/581 1.020 0.0865 0.1921 3.158 and -2.994

H106K6Na18Ni10O185Si4W34 10665.63 293(2) 0.71073 triclinic P1h 18.435(4) 21.920(4) 25.095(5) 81.44(3) 73.87(3) 85.31(3) 9624(3) 2 3.680 21.483 9500 31397/690/2450 1.066 0.0675 0.1684 2.392 and -2.640

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

The mixture was cooled to room temperature and then filtered. The filtrate was slowly evaporated at room temperature slow for a month, resulting in a green crystalline product (yield: 8%). Anal. Found (%): K, 8.46; H, 1.03; Ni, 7.59; Si, 0.99; W, 53.6. Calcd for 1: K, 8.60; H, 1.11; Ni, 7.53; Si, 1.03; W, 53.9. IR (KBr pellet): νmax/cm-1 933 (SiOa) and 914 (W ) Od), 860 (W-Ob-W), 806 (W-Oc-W), 767 (WOc-W), 691 (W-Oc-W). Synthesis of 2. In a typical synthesis procedure for 2, K8[γ-SiW10O36] (0.5 g) was dissolved in 10 mL of distilled water with stirring. Next, 1 mL of NiCl2 solution (1M) was added dropwise with vigorously stirring, and the mixture was heated for half an hour at 95 °C. After the solution was cooled to room temperature, 5.5 g of KCl and 40 mL of distilled water were added to the green solution. At this point, 2 M Na2CO3 was used to adjust the pH value to 8.8 and heated for 1 h at the same temperature. The mixture was then cooled to room temperature and filtered. The filtrate was slowly evaporated at room temperature for 3 weeks, resulting in a green crystalline product (yield: 32%). Anal. Found (%): K, 2.32; Na, 3.11; H, 1.16; Ni, 5.33; Si, 0.96; W, 59.5. Calcd for 2: K, 2.20; Na, 3.88; H, 1.00; Ni, 5.50; Si, 1.05; W, 58.6. IR (KBr pellet): νmax/cm-1 980 (W ) Od) and 941 (Si-Oa), 876 (WOb-W), 774 (W-Oc-W), 704 (W-Oc-W). X-ray Crystallography. Single-crystal X-ray data for 1 and 2 were collected on a Rigaku R-AXIS RAPID IP diffractometer equipped with a normal focus 18 KW sealed tube X-ray source (Mo-Ka radiation, λ ) 0.71073 Å) operating at 50 KV and 200 mA. Data processing was accomplished with the RAXWISH processing program. A numerical absorption correction was applied. The structures were solved by direct methods and refined by full-matrix least-squares on F2 using the SHELXL 97 software.17 All the non-hydrogen atoms were refined anisotropically. Further details of the X-ray structural analysis are given in Table 1. Selected bond lengths and angles are listed in Table 2.

Results and Discussions Synthesis. In this paper, the hepta-Ni-substituted dimeric POMs K12[(SiW8O31)2Ni7(H2O)4(OH)6]‚23H2O (1) and the asymmetric penta-Ni-substituted dimeric POMs K6Na18[H2{Ni5(H2O)5(OH)3(x-SiW9O34)(β-SiW8O31)}2]‚39H2O (2) are all obtained by reaction of NiCl2 with [γ-SiW10O36]8- in an aqueous solution. The polyoxoanion [γ-SiW10O36]8- usually undergoes an isomerization course in the aqueous solution and decomposes at a certain pH value.10 We have systemically studied the reaction of [γ-SiW10O36]8- with Ni2+ and obtained three mutiNi-substituted polyoxoanions [Ni7(OH)4(H2O)(CO3)2(HCO3)(A-R-SiW9O34)(β-SiW10O37)]15- (3), [{Ni6(H2O)4(µ2-H2O)4(µ3-OH)2}(x-SiW9O34)2]10- (4), and [{β-SiNi2W10O36(OH)2(H2O)}2]12- (5).9 Here, compounds 1 and 2 were also separated

from the same starting materials of [γ-SiW10O36]8-and NiCl2. By plenty of parallel experiments, it was found that the syntheses of these compounds are affected by the pH value of the reaction media. At pH 6, we obtained the tetra-Ni-substituted [{βSiNi2W10O36(OH)2(H2O)}2]12-, which has been synthesized by Kortz et al. in a different way.18 At 80 °C in the pH 9.5 aqueous solution, we obtained the light green single crystal of 1, composed of two [β-SiW8O31]10- units. When the pH value was adjusted to 8.2 -t the same temperature, compound 4 constructed from two [x-SiW9O34]10- units was gained. Further research of the experimental conditions leads us to believe that the asymmetric dimeric polyoxoanions composed of the [β-SiW8O31]10- and [x-SiW9O34]10- units could be obtained in the pH region between 8.2 and 9.5 as evidence of the successful achievement of compound 2 at pH 8.8. Moreover, we also found that K8[γ-SiW10O36] with the Ni2+ reaction system was sensitive to reaction temperature. The different crystals of compounds 3 and 4 were formed at 80 and 95 °C at the same pH value, respectively.9 Descriptions of Crystal Structures. Single-crystal X-ray diffraction analysis reveals that compound 1 consists of two [β-SiW8O31] units connected by a double-cubane Ni7O8 cluster (Figure 1a). Two [β-SiW8O31] units in 1, which were observed very recently in the Co and Cu-substituted POMs,8,13,19 each share eight oxygen atoms, including two oxygen atoms on Si, with the central Ni7O8 cluster to constitute the hitherto unknown dimeric structure. In the Ni7O8 cluster, the arrangement of the seven Ni2+ centers is never reported in the POMs chemistry. There are two types of Ni2+ ions in the Ni7O8, two Ni3O13 triplets and the bridging NiO6 octahedron. Alternatively, the Ni7O8 cluster also could be regarded as a double-cubane structure (Figure 2a). In each cubane unit, Ni1, Ni2 ,and Ni3 constitute a triplet and the Ni4 caps the triplet, which was observed by Kortz et al. in 1999.20 Polyoxoanions 1 were connected together by the K+ ions to constitute a 3D structure (see the Supporting Information, Figure S9). Compound 2 exhibits an asymmetric dimeric structure, composed of [x-SiW9O34]10- and [β-SiW8O31]10- units, linked via a penta-Ni-cluster (Figure 1b). Alternatively, the structure of 2 can also be regarded as a dimer consisting of the tri-Nisubstituted β-Keggin-type polyoxoanions [β-Ni3SiW9O37(H2O)3] and the di-Ni-substituted γ-structure [γ-SiW8Ni2O34(H2O)2]

Double-Cubane- and Multi-Ni-Substituted Polyoxometalates

Crystal Growth & Design, Vol. 7, No. 7, 2007 1307

Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1and 2a 1 Ni(1)-O(10) Ni(1)-O(34) Ni(1)-O(28) Ni(1)-Ni(4) Ni(2)-O(34)#7 Ni(2)-O(15) Ni(2)-O(31) Ni(3)-O(30) Ni(3)-O(15) Ni(3)-O(36) Ni(4)-O(30) Ni(4)-O(34) Ni(4)-O(15) O(10)-Ni(1)-O(30)#7 O(30)#7-Ni(1)-O(34) O(30)#7-Ni(1)-O(36)#7 O(10)-Ni(1)-O(28) O(34)-Ni(1)-O(28) O(10)-Ni(1)-O(22) O(34)-Ni(1)-O(22) O(28)-Ni(1)-O(22) O(14)-Ni(2)-O(36) O(14)-Ni(2)-O(15) O(36)-Ni(2)-O(15) O(34)#7-Ni(2)-O(20) O(15)-Ni(2)-O(20) O(34)#7-Ni(2)-O(31) O(15)-Ni(2)-O(31) O(13)-Ni(3)-O(30) O(30)-Ni(3)-O(19) O(30)-Ni(3)-O(15) O(13)-Ni(3)-O(33) O(19)-Ni(3)-O(33) O(13)-Ni(3)-O(36) O(19)-Ni(3)-O(36) O(33)-Ni(3)-O(36) O(30)-Ni(4)-O(34) O(30)-Ni(4)-O(34)#7 O(34)-Ni(4)-O(34)#7 O(30)#7-Ni(4)-O(15) O(34)#7-Ni(4)-O(15) O(30)#7-Ni(4)-O(15)#7 O(34)#7-Ni(4)-O(15)#7

2.02(2) 2.05(3) 2.08(3) 2.895(5) 2.01(3) 2.09(2) 2.12(3) 2.03(3) 2.09(2) 2.12(3) 2.02(2) 2.04(3) 2.14(2) 173.8(10) 88.5(10) 81.9(11) 90.1(10) 174.4(12) 80.4(9) 93.3(10) 81.7(10) 93.1(12) 91.1(10) 85.8(11) 92.0(12) 88.1(9) 93.9(11) 174.0(10) 171.1(11) 96.3(10) 89.2(9) 87.6(13) 90.7(11) 91.6(12) 173.6(13) 94.8(14) 91.1(11) 88.9(11) 180.000(5) 91.8(10) 87.1(10) 88.2(10) 92.9(10)

Ni(1)-O(30)#7 Ni(1)-O(36)#7 Ni(1)-O(22) Ni(2)-O(14) Ni(2)-O(36) Ni(2)-O(20) Ni(3)-O(13) Ni(3)-O(19) Ni(3)-O(33) Ni(4)-O(15)#7 Ni(4)-O(30)#7 Ni(4)-O(34)#7 O(10)-Ni(1)-O(34) O(10)-Ni(1)-O(36)#7 O(34)-Ni(1)-O(36)#7 O(30)#7-Ni(1)-O(28) O(36)#7-Ni(1)-O(28) O(30)#7-Ni(1)-O(22) O(36)#7-Ni(1)-O(22) O(14)-Ni(2)-O(34)#7 O(34)#7-Ni(2)-O(36) O(34)#7-Ni(2)-O(15) O(14)-Ni(2)-O(20) O(36)-Ni(2)-O(20) O(14)-Ni(2)-O(31) O(36)-Ni(2)-O(31) O(20)-Ni(2)-O(31) O(13)-Ni(3)-O(19) O(13)-Ni(3)-O(15) O(19)-Ni(3)-O(15) O(30)-Ni(3)-O(33) O(15)-Ni(3)-O(33) O(30)-Ni(3)-O(36) O(15)-Ni(3)-O(36) O(30)-Ni(4)-O(30)#7 O(30)#7-Ni(4)-O(34) O(30)#7-Ni(4)-O(34)#7 O(30)-Ni(4)-O(15) O(34)-Ni(4)-O(15) O(30)-Ni(4)-O(15)#7 O(34)-Ni(4)-O(15)#7 O(15)-Ni(4)-O(15)#7

2.03(2) 2.05(3) 2.18(2) 1.99(3) 2.02(4) 2.10(3) 2.02(3) 2.08(2) 2.11(4) 2.14(2) 2.02(2) 2.04(3) 91.6(10) 104.2(11) 79.9(14) 89.3(10) 104.9(13) 93.5(9) 171.8(13) 174.8(12) 81.7(12) 89.1(10) 93.2(11) 171.3(12) 85.4(10) 89.5(12) 97.0(11) 92.0(10) 93.7(11) 91.3(9) 89.3(12) 177.6(11) 80.4(11) 83.2(12) 180.000(8) 88.9(11) 91.1(11) 88.2(10) 92.9(10) 91.8(10) 87.1(10) 180.000(7)

2 Ni(1)-O(30) Ni(1)-O(14) Ni(1)-O(11) Ni(2)-O(39) Ni(2)-O(15) Ni(2)-O(3) Ni(3)-O(30) Ni(3)-O(12) Ni(3)-O(89) Ni(4)-O(6) Ni(4)-O(51) Ni(4)-O(104) Ni(5)-O(65) Ni(5)-O(14) Ni(5)-O(2) Ni(6)-O(1) Ni(6)-O(20) Ni(6)-O(73) Ni(7)-O(38) Ni(7)-O(91) Ni(7)-O(114) Ni(8)-O(31) Ni(8)-O(57) Ni(8)-O(82) Ni(9)-O(25) Ni(9)-O(83) Ni(9)-O(70) Ni(10)-O(115) Ni(10)-O(130) Ni(10)-O(144) O(30)-Ni(1)-O(3) O(3)-Ni(1)-O(14)

2.020(19) 2.06(2) 2.11(2) 2.02(2) 2.05(2) 2.125(16) 2.01(2) 2.06(2) 2.101(19) 1.979(19) 2.02(2) 2.118(19) 2.015(16) 2.033(17) 2.09(2) 1.979(19) 2.046(18) 2.082(19) 1.99(2) 2.05(2) 2.10(2) 2.05(2) 2.030(18) 2.09(2) 2.00(2) 2.06(2) 2.105(17) 1.98(2) 2.05(2) 2.12(3) 85.6(8) 94.3(8)

Ni(1)-O(3) Ni(1)-O(18) Ni(1)-O(19) Ni(2)-O(30) Ni(2)-O(120) Ni(2)-O(95) Ni(3)-O(8) Ni(3)-O(15) Ni(3)-O(14) Ni(4)-O(31) Ni(4)-O(41) Ni(4)-O(70) Ni(5)-O(50) Ni(5)-O(45) Ni(5)-O(73) Ni(6)-O(3) Ni(6)-O(92) Ni(6)-O(45) Ni(7)-O(57) Ni(7)-O(135) Ni(7)-O(31) Ni(8)-O(99) Ni(8)-O(83) Ni(8)-O(35) Ni(9)-O(62) Ni(9)-O(112) Ni(9)-O(104) Ni(10)-O(57) Ni(10)-O(91) Ni(10)-O(83) O(30)-Ni(1)-O(14) O(30)-Ni(1)-O(18)

2.042(19) 2.102(18) 2.145(16) 2.03(2) 2.09(3) 2.140(17) 2.05(2) 2.09(2) 2.178(18) 2.02(2) 2.07(2) 2.125(19) 2.02(2) 2.08(2) 2.13(2) 2.032(16) 2.05(2) 2.111(18) 2.00(2) 2.09(2) 2.13(2) 2.07(2) 2.048(19) 2.179(19) 2.03(2) 2.08(2) 2.109(18) 1.99(2) 2.06(2) 2.13(2) 84.2(8) 97.0(7)

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Table 2 (Continued) O(3)-Ni(1)-O(18) O(30)-Ni(1)-O(11) O(14)-Ni(1)-O(11) O(30)-Ni(1)-O(19) O(14)-Ni(1)-O(19) O(11)-Ni(1)-O(19) O(39)-Ni(2)-O(15) O(39)-Ni(2)-O(120) O(15)-Ni(2)-O(120) O(30)-Ni(2)-O(3) O(120)-Ni(2)-O(3) O(30)-Ni(2)-O(95) O(120)-Ni(2)-O(95) O(30)-Ni(3)-O(8) O(8)-Ni(3)-O(12) O(8)-Ni(3)-O(15) O(30)-Ni(3)-O(89) O(12)-Ni(3)-O(89) O(30)-Ni(3)-O(14) O(12)-Ni(3)-O(14) O(89)-Ni(3)-O(14) O(6)-Ni(4)-O(51) O(6)-Ni(4)-O(41) O(51)-Ni(4)-O(41) O(31)-Ni(4)-O(104) O(41)-Ni(4)-O(104) O(31)-Ni(4)-O(70) O(41)-Ni(4)-O(70) O(65)-Ni(5)-O(50) O(50)-Ni(5)-O(14) O(50)-Ni(5)-O(45) O(65)-Ni(5)-O(2) O(14)-Ni(5)-O(2) O(65)-Ni(5)-O(73) O(14)-Ni(5)-O(73) O(2)-Ni(5)-O(73) O(1)-Ni(6)-O(20) O(1)-Ni(6)-O(92) O(20)-Ni(6)-O(92) O(3)-Ni(6)-O(73) O(92)-Ni(6)-O(73) O(3)-Ni(6)-O(45) O(92)-Ni(6)-O(45) O(38)-Ni(7)-O(57) O(57)-Ni(7)-O(91) O(57)-Ni(7)-O(135) O(38)-Ni(7)-O(114) O(91)-Ni(7)-O(114) O(38)-Ni(7)-O(31) O(91)-Ni(7)-O(31) O(114)-Ni(7)-O(31) O(57)-Ni(8)-O(31) O(57)-Ni(8)-O(99) O(31)-Ni(8)-O(99) O(83)-Ni(8)-O(82) O(99)-Ni(8)-O(82) O(83)-Ni(8)-O(35) O(99)-Ni(8)-O(35) O(25)-Ni(9)-O(62) O(62)-Ni(9)-O(83) O(62)-Ni(9)-O(112) O(25)-Ni(9)-O(70) O(83)-Ni(9)-O(70) O(25)-Ni(9)-O(104) O(83)-Ni(9)-O(104) O(70)-Ni(9)-O(104) O(115)-Ni(10)-O(130) O(115)-Ni(10)-O(91) O(130)-Ni(10)-O(91) O(57)-Ni(10)-O(144) O(91)-Ni(10)-O(144) O(57)-Ni(10)-O(83) O(91)-Ni(10)-O(83)

88.3(8) 98.0(8) 90.2(8) 173.0(7) 101.2(7) 77.7(7) 94.2(8) 88.7(9) 175.8(9) 83.2(7) 85.9(8) 93.0(8) 93.8(8) 174.1(8) 89.6(8) 93.5(8) 95.8(8) 86.6(8) 81.6(7) 89.5(8) 175.1(8) 95.6(9) 91.3(8) 89.3(8) 96.4(8) 170.3(9) 89.5(8) 90.9(7) 93.7(7) 93.5(8) 176.9(8) 90.8(7) 92.2(7) 78.6(7) 98.2(7) 169.3(7) 97.2(8) 89.9(8) 89.6(8) 97.9(7) 169.2(7) 89.5(7) 91.7(8) 176.9(8) 82.5(8) 95.1(9) 92.1(9) 90.1(9) 94.4(8) 90.9(8) 173.3(9) 85.5(8) 98.3(8) 173.1(8) 176.6(9) 86.2(8) 101.6(8) 77.4(8) 94.0(8) 93.7(8) 88.6(8) 86.8(8) 85.4(8) 80.8(8) 96.4(8) 86.1(7) 94.4(9) 91.0(9) 89.9(9) 94.9(10) 176.4(10) 82.3(7) 94.5(8)

O(14)-Ni(1)-O(18) O(3)-Ni(1)-O(11) O(18)-Ni(1)-O(11) O(3)-Ni(1)-O(19) O(18)-Ni(1)-O(19) O(39)-Ni(2)-O(30) O(30)-Ni(2)-O(15) O(30)-Ni(2)-O(120) O(39)-Ni(2)-O(3) O(15)-Ni(2)-O(3) O(39)-Ni(2)-O(95) O(15)-Ni(2)-O(95) O(3)-Ni(2)-O(95) O(30)-Ni(3)-O(12) O(30)-Ni(3)-O(15) O(12)-Ni(3)-O(15) O(8)-Ni(3)-O(89) O(15)-Ni(3)-O(89) O(8)-Ni(3)-O(14) O(15)-Ni(3)-O(14) O(6)-Ni(4)-O(31) O(31)-Ni(4)-O(51) O(31)-Ni(4)-O(41) O(6)-Ni(4)-O(104) O(51)-Ni(4)-O(104) O(6)-Ni(4)-O(70) O(51)-Ni(4)-O(70) O(104)-Ni(4)-O(70) O(65)-Ni(5)-O(14) O(65)-Ni(5)-O(45) O(14)-Ni(5)-O(45) O(50)-Ni(5)-O(2) O(45)-Ni(5)-O(2) O(50)-Ni(5)-O(73) O(45)-Ni(5)-O(73) O(1)-Ni(6)-O(3) O(3)-Ni(6)-O(20) O(3)-Ni(6)-O(92) O(1)-Ni(6)-O(73) O(20)-Ni(6)-O(73) O(1)-Ni(6)-O(45) O(20)-Ni(6)-O(45) O(73)-Ni(6)-O(45) O(38)-Ni(7)-O(91) O(38)-Ni(7)-O(135) O(91)-Ni(7)-O(135) O(57)-Ni(7)-O(114) O(135)-Ni(7)-O(114) O(57)-Ni(7)-O(31) O(135)-Ni(7)-O(31) O(57)-Ni(8)-O(83) O(83)-Ni(8)-O(31) O(83)-Ni(8)-O(99) O(57)-Ni(8)-O(82) O(31)-Ni(8)-O(82) O(57)-Ni(8)-O(35) O(31)-Ni(8)-O(35) O(82)-Ni(8)-O(35) O(25)-Ni(9)-O(83) O(25)-Ni(9)-O(112) O(83)-Ni(9)-O(112) O(62)-Ni(9)-O(70) O(112)-Ni(9)-O(70) O(62)-Ni(9)-O(104) O(112)-Ni(9)-O(104) O(115)-Ni(10)-O(57) O(57)-Ni(10)-O(130) O(57)-Ni(10)-O(91) O(115)-Ni(10)-O(144) O(130)-Ni(10)-O(144) O(115)-Ni(10)-O(83) O(130)-Ni(10)-O(83) O(144)-Ni(10)-O(83)

177.3(8) 174.5(7) 87.2(8) 98.3(7) 77.4(7) 176.8(7) 83.9(8) 93.1(9) 94.3(7) 90.9(7) 89.6(8) 89.2(7) 176.1(8) 93.9(8) 83.4(8) 174.9(8) 89.2(7) 89.3(8) 93.7(7) 94.4(8) 89.4(9) 174.7(8) 92.5(8) 92.6(7) 81.5(8) 177.6(9) 85.5(8) 85.4(7) 172.2(8) 86.1(7) 86.5(8) 89.2(8) 93.9(7) 92.4(8) 84.5(7) 89.0(7) 173.5(8) 92.4(7) 93.5(8) 79.7(7) 177.8(8) 84.3(8) 85.2(7) 94.7(9) 87.6(9) 175.8(8) 89.4(9) 93.4(9) 84.2(7) 85.4(8) 83.5(8) 95.6(8) 90.6(8) 96.0(8) 87.7(8) 173.2(9) 98.3(8) 78.6(8) 171.9(7) 91.1(8) 91.6(8) 178.2(8) 93.0(8) 92.4(8) 171.9(9) 171.3(9) 91.3(9) 82.4(8) 91.9(10) 87.7(10) 92.6(8) 171.7(9) 87.5(9)

a Symmetry transformations used to generate equivalent atoms: for 1:#1 -x + 2, -y + 1, -z; #2 -x + 2, -y + 2, -z; #3 x, y + 1, z; #4 x + 1, y, z; #5 x - 1, y, z; #6 -x + 3, -y + 2, -z + 1; #7 -x + 2, -y + 2, -z + 1; #8 x, y - 1, z; #9 -x + 1, -y + 1, -z; #10 x - 1, y - 1, z; #11 x + 1, y + 1, z; for 2: #1 -x + 1, -y + 1, -z; #2 x + 1, y, z; #3 -x, -y, -z + 1; #4 x, y - 1, z; #5 x - 1, y, z; #6 -x, -y + 1, -z + 1; #7 x, y + 1, z; #8 -x, -y + 1, -z.

Double-Cubane- and Multi-Ni-Substituted Polyoxometalates

Figure 1. (a) Polyhedral representation of polyoxoanion 1. (b) Polyhedral representation of polyoxoanion 2. The color codes are following: WO6 octahedra, red; central SiO4 tetrahedra, yellow; NiO6 octahedra, green; O, red spheres.

Figure 2. (a) Ball and stick representation of the central double-cubane Ni7O8 cluster in 1. (b) Ball and stick representation of the central Ni5 cluster in 2.

linked by three µ3-OH groups. In the [β-Ni3SiW9O37(H2O)3] unit, three nickel ions are distributed to two M3O13 triplets. Ni1 ion is in the triplet Ni1-W16-W6, and the Ni5 and Ni6 ions are in the adjacent triplet (Ni5-Ni6-W10), corner-sharing with the Ni1 ion. To the best of our knowledge, the fashion of the tri-Ni-substituted was reported for the second time in the POMs chemistry.9 In the [γ-SiW8Ni2O34(H2O)2] unit, two nickel ions replace two edge-sharing WO6 octahedra of the [γ-SiW10O36] unit to consist of the [γ-SiW8Ni2O34(H2O)2] fragment. This is the first time observed in the Ni-substituted polyoxoanions. Such an asymmetric dimeric structure, composed of [β-Ni3SiW9O37(H2O)3] and the γ-structure [γ-SiW8Ni2O34(H2O)2] units, is unprecedented in POM chemistry. Up until now, only a few examples of the asymmetric dimeric structure have been reported: the mixed-valence heteropoly and isopoly brown species [H4BWIV3WVI17O66]10- and [H6WIV3WVI17O66]11-,7 the 3-Co-substituted dimeric [Co3(B-R-SiW9O33(OH))(B-R-SiW8O29(OH)2)]11-,8 and the 7-Ni-substituted [Ni7(OH)4(H2O)(CO3)2(HCO3)(A-R-SiW9O34)(β-SiW10O37)]15-.9 The former two are constructed from only the [B-R-XW9O34]10- and [β-W8O30]12units, and the latter two, polyoxoanion [Co3(B-R-SiW9O33(OH))(B-R-SiW8O29(OH)2)]11-, are composed of [B-R-SiW9O33(OH)]9- and [B-β-SiW8O29(OH)2]8- units linked by three Co ions; polyoxoanion [Ni7(OH)4(H2O)(CO3)2(HCO3)(A-R-SiW9O34)(β-SiW10O37)]15- is constructed from the [A-R-SiW9O34]10- and [β-SiW10O37]10- units, linked by a hepta-Ni-cluster and three carbonates. The combination fashion and the structures of them

Crystal Growth & Design, Vol. 7, No. 7, 2007 1309

Figure 3. Cyclic voltammograms of complex 1 in the pH 5 (0.4 M CH3COONa + CH3COOH) buffer solution at different scan rates (from inner to outer: 5, 10, 20, 50, 100, and 200 mV s-1). The working electrode was glassy carbon; the reference electrode was Ag/AgCl.

are all different from that of 2. Also, two equivalent dimeric fragments in 2 are arranged almost reversely parallel to each other and are connected together by Na+ and three K+ ions to constitute a double sandwich-type structure (see the Supporting Information, Figure S10). The linking Na+ ion is coordinated by five O atoms, three of them from two polyoxoanion frameworks with the other two sites occupied by two water molecules. The K1 and K2 ions are in a six-coordinated environment and the K1 is in an eight-coordinated environment. The double sandwich-type structures are linked by the additional Na+ and K+ ions to construct a 3D structure (see the Supporting Information, Figure S11). The bond lengths and the angles of the [β-SiW8O31]10- and [x-SiW9O34]10- units in compounds 1 and 2 are not unusual. Structures a and b in Figure 2 are the ball-and-stick representation of the central double-cubane Ni7O8 cluster in 1 and Ni5 cluster in 2, respectively. The coordination geometry of Ni2+ ions in 1 and 2 is octahedral, and the Ni-O distances fall into the range of 1.99(3)-2.18(2) Å in 1 and 1.979(19)-2.179(19) Å in 2. In the two compounds, bond-valence calculations21 confirm that the terminal oxygen atoms and the bridged oxoygen atoms combining two adjacent nickel ions are all diprotonated; the µ3-oxo linking three nickel ions are monoprotonated.3c Also, the bridging oxogen atoms O13 and O130 (W-O13-W (s ) 1.213076), W-O130-Ni (s ) 1.36597)) are all monoprotonated. The EPR spectra of the two compounds are measured at 77 K. As shown in Figures S12 and S13 of the Supporting Information, the Ni2+ signals are detected with g ) 2.26 for 1 and 2.28 for 2. TG Analyses. To examine the thermal stability of compounds 1 and 2, we carried out thermal gravimetric (TG) analyses for 1 and 2 (see the Supporting Information, Figures S4 and S5). The thermal gravimetric (TG) curve of 1 shows a total weight loss of 9.77% in the range of 34-243 °C (calcd 9.89%), which corresponds to the loss of all noncoordinated and coordinated water molecules. The thermal gravimetric (TG) curve of 2 exhibits a total weight loss of 9.10% in the range of 22-284 °C, which corresponds to the loss of all noncoordinated and coordinated water molecules (8.78%,). There is no weight loss above 284 °C. Electrochemistry. Figure 3 shows the typical cyclic voltammetric behavior of compound 1 in the pH 5 (0.4 M CH3-

1310 Crystal Growth & Design, Vol. 7, No. 7, 2007

Zhang et al.

Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (20371011). Supporting Information Available: X-ray crystallographic files in CIF format, TG curves, IR spectrum, and additional figures for 1 and 2. This material is available free of charge via the Internet at http:// pubs.acs.org.

References

Figure 4. Cyclic voltammograms of complex 1 at the scan rate of 50mV s-1. The working electrode was glassy carbon; the reference electrode was Ag/AgCl.

COONa + CH3COOH) buffer solution at different scan rates. It can be clearly seen that in the potential range -0.4 to -1.0 V, two quasireversible redox peaks appear and the mean peak potentials E1/2 ) (Epa + Epc)/2 are -0.7421 V (I-I′) and -0.5796 V (II-II′) (vs Ag/AgCl), respectively. The two peaks I-I′ and II-II′ correspond to the redox of the WVI in the polyoxoanion framework and the domain where the two waves located at was also observed in the other sandwich-type POMs.6d, -1, the peak currents of the 8 At scan rates lower than 100 mV s two peaks were proportional to the scan rate, which indicates that the redox process of 1 is surface-controlled (see the Supporting Information, Figure S6). With the pH decreasing to 3, the two redox formal potentials all slightly shift to the positive potential direction, noticing that the reduction of 1 is easier than that in the pH 5 buffer solution. These indicate that the reduction of polyoxoanion 1 is accompanied by the evolution of the protons from the solution to the electrode surface to maintain charge neutrality (see Figure 4).22 The electrochemical properties of compound 2 were also detected in the pH 5 (0.4 M CH3COONa + CH3COOH) buffer solution at different scan rates (see the Supporting Information, Figure S7). Two redox peaks appear in the potential range -0.4 to -1.0 V, and the mean peak potentials vs E1/2 ) (Epa + Epc)/2 are -0.7250 V (I-I′) and -0.5692 V (II-II′) (vs Ag/ AgCl) at a scan rate of 100 mV s-1, respectively. When the scan rates are lower than 100 mV s-1, the linear relationships between peak currents and the scan rates for the first W reduction waves in the pH 5 buffer solution provide evidence for a surface-controlled process (see the Supporting Information, Figure S8). Conclusions In summary, we have successfully synthesized two novel multi-Ni-substituted POMs 1 and 2. Compound 1 was composed of two β-SiW8O31 units connected by a double-cubane Ni7O8 cluster that was observed for the first time in POM chemistry. Compound 2 exhibits a novel asymmetric dimeric structure, composed of β-Keggin type [β-Ni3SiW9O37(H2O)3] and γ-structure [γ-SiW8Ni2O34(H2O)2] units. The electrochemical properties of the two compounds were detected in the pH 5 buffer solutions, and two redox couples of the W atoms were detected in the two dimeric polyoxoanions.

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