Half-Open Hollow Cages of Pentadecavanadate and

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Half-Open Hollow Cages of Pentadecavanadate and Hexadecavanadate Compounds with Large -O-V-O-V- Windows Lian Chen, Feilong Jiang, Mingyan Wu, Ning Li, Wentao Xu, Chunfeng Yan, Chengyang Yue, and Maochun Hong*

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 11 4092–4099

Key Laboratory of Optoelectronic Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ReceiVed May 21, 2008; ReVised Manuscript ReceiVed July 10, 2008

ABSTRACT: Five new polyoxovanadate-based cluster compounds, (Et4N)4[HV15O39(acac)Cl] · 3CH3CN (1), (Et4N)5[V16O40X] (X ) Br 2a; X ) Cl 2b), (Me2NH2)8[H2V16O42(ClO4)] · 4H2O (3), and (Me2NH2)2(Et4N)2[H2V10O28] (4), having different types of vanadium oxide shells are isolated in organic media. Compounds 1-3 are hollow structures with different encapsulated anions in their centers. The surfaces of four compounds are constituted by vanadium and oxygen atoms, showing large 10-membered -O-V-O-V- rings and/or eight-membered -O-V-O-V- rings, which distinguish vanadium-oxide clusters from the common hollow clusters built up with closed surfaces. Introduction Polyoxometalates (POM) constitute a fascinating class of inorganic systems since they are unapproachable in structural diversity and wide-ranging applications such as catalysis, coatings, pigments, smart materials, luminescence, electrochemistry, biochemistry, and medicine.1 Polyoxovanadates, or vanadium oxide clusters, are a prominent subclass of POMs which, as compared to polyoxomolybdates and polyoxotungstates, are relatively underinvestigated.2 They are of current interest mainly due to their relevance to catalysis and biochemical systems, their variable geometries, and their redox properties.3 In recent years, a number of polyoxovanadate clusters exhibiting diverse topologies and interesting structural and electronic properties such as [V3O9]3-, [V5O14]3-, [V8O14]4-, [V10O28]6- [V12O32]4-, [V13O34]3-, [V14O36]4-, [V15O36]5-, [V15O42]9-, [H2V16O39]7-, [V16O38]7-, [V17O42]4-, [V18O42]12-, [V19O49]9- and [V34O82]10-have been reported.4-18 Compared to the compact structures, the hollow vanadium oxide clusters are more attractive since they can act as host shells for a variety of neutral or anionic guest species which may exert templating effects on the electronic and framework structures of the host metal-oxide shells. Much work has been done over the years to synthesize the frameworks of various hollow vanadium oxide shells with or without encapsulated guests.6,8,10,11,13,14 Nevertheless, most of these clusters exhibit closed spherelike structures, but open or half-open hollow frameworks, which may offer a path for the guests to exchange with other ions, are really scarce.6,8,10 On the other hand, despite the fast development of polyoxovanadate systems, nonaqueous synthetic methodology is still underinvestigated, which may further the research in the assembly and construction of new vanadate clusters containing different compositions and frameworks. We currently focus our interest on the investigation of a new synthetic approach, employing nonaqueous media and the organic solvent-soluble VO2+ derivatives as VV sources. In the course of our ongoing research,10 five polyoxovanadate cluster compounds containing totally different types of vanadium oxide frameworks (Et4N)4[HV15O39(acac)Cl] · 3CH3CN (1), (Et4N)5[V16O40X] (X ) Br 2a; X ) Cl 2b), (Me2NH2)8[H2V16O42(ClO4)] · 4H2O * Author to whom correspondence should be addressed. Tel: 86-59183792460. Fax: 86-591-83714946. E-mail: [email protected].

(3), and (Me2NH2)2(Et4N)2[H2V10O28] (4) have been prepared. Four of the vanadium oxide shells (1-3) in these fully oxidized (VV) or mixed-valence (VVVIV) vanadium compounds possess novel frameworks which have never been encountered before. Although the four clusters are quite different in composition, encapsulated guests, assemblies and symmetries, all of them exhibit half-open frameworks with large windows on the surfaces of the clusters. In this paper, we will describe the syntheses and characterization of compounds 1-4 involving complete single crystal X-ray diffraction analyses, FT-IR spectroscopies, elemental analyses, electron paramagnetic resonance (EPR), X-ray powder diffraction, and thermogravimetric analyses. Experimental Section Materials and Physical Techniques. All chemical reagents were purchased from commercial sources and used as received. The IR spectrum was recorded on a Perkin-Elmer Spectrum One FT-IR spectrometer using the KBr pellet technique in the range of 4000-400 cm-1. Elemental analyses of C, H and N were carried out using an Elemental Vario EL III microanalyzer. The polycrystalline powder EPR spectra were recorded with a Bruker ER-420 spectrometer employing X-band radiation and a cylindrical cavity with 100 kHz magnetic field modulations. TG (thermal gravimetric) analyses were performed with a heating rate of 15 °C min-1 using a NETZSCH STA449C simultaneous TG-DSC instrument and the powder X-ray diffraction data were taken on a Rigaku DMAX2500 diffractometer. Synthesis of (Et4N)4[HV15O39(acac)Cl] · 3CH3CN (1). To the solution of VO2(acac) (91 mg, 0.5 mmol)19 in acetonitrile (25 mL), Et4NCl · H2O (92 mg, 0.5 mmol) was added with stirring at room temperature. About 30 min later the dark-brown mixture solution slowly changed to reddish-brown clear. The reddish-brown filtrate was kept evaporating at room temperature for crystallization. The prism, reddishbrown crystals of 1 suitable for X-ray diffraction were obtained in a few days. The yield was 62.3% (45 mg). Anal. Calcd for C43H97Cl1N7O41V15: C, 23.82; H, 4.51; N, 4.52%. Found: C, 23.46; H, 4.59; N, 4.54%. IR (KBr pellet): 3445(br, w), 2980(w), 1529(w), 1483(m), 1392(m), 1172(m), 989(vs, sh), 851(vs), 744(s, sh), and 639(m) cm-1. Synthesis of (Et4N)5[V16O40Br] (2a). To the solution of VO2(acac) (91 mg, 0.5 mmol) in acetonitrile/DMF (5:1, 30 mL), Et3N (0.03 mL, 0.21 mmol) was added with stirring at room temperature. About 30 min later Et4NBr · H2O (114 mg, 0.5 mmol) was then added. The reddish-brown mixture solution slowly changed to light brown. After filtration of the sample, the filtrate was kept evaporating at room temperature for crystallization. About 30 days later, the prism, darkgreen crystals of 2a suitable for X-ray diffraction were obtained. The

10.1021/cg800534v CCC: $40.75  2008 American Chemical Society Published on Web 09/25/2008

Penta- and Hexadecavanadate Compounds

Crystal Growth & Design, Vol. 8, No. 11, 2008 4093

Table 1. Crystallographic Data for the Compounds 1-4 compound

1

2a

2b

3

4

formula formula weight crystal size (mm) crystal system space group a (Å) b (Å) c (Å) β (°) V (Å3) Z Dc (Mg · m-3) µ (mm-1) F (000) T (K) λ (Mo KR) (Å) reflns collected reflns unique parameters goodness-of-fit on F2 R1, wR2 (I > 2σ(I))a R1, wR2 (all data)b max, min ∆F (e · Å-3)

C43H97Cl1N7O41V15 2167.83 0.24 × 0.11 × 0.04 monoclinic P21/c 24.871(7) 13.213(3) 25.489(7) 103.474(3) 8145(4) 4 1.768 1.753 4376 293(2) 0.71073 62827 18465 1003 1.014 0.0691, 0.1302 0.1118, 0.1505 0.883 and -0.589

C40H100Br1N5O40V16 2186.20 0.17 × 0.04 × 0.03 orthorhombic I222 13.130(6) 14.346(6) 21.161(9) 90 3896(3) 2 1.822 2.364 2196 293(3) 0.71073 15720 4545 179 1.057 0.0775, 0.1896 0.1312, 0.2200 0.870 and -0.824

C40H100Cl1N5O40V16 2141.74 0.25 × 0.20 × 0.20 orthorhombic I222 14.298(7) 20.828(10) 12.990(7) 90 3868(3) 2 1.839 1.954 2160 293(2) 0.71073 11996 3361 169 1.079 0.0788, 0.1819 0.0883, 0.1884 0.810 and -0.902

C16H66Cl1N8O46V16 1957.26 0.20 × 0.20 × 0.15 orthorhombic P42/nnm 13.8424(5) 13.8424(5) 16.4013(8) 90 3142.7(2) 2 2.068 2.403 1942 293(2) 0.71073 22832 1904 109 1.051 0.643, 0.1535 0.646, 0.1537 1.338 and -1.089

C20H58N4O28V10 1312.10 0.30 × 0.25 × 0.15 monoclinic P21/c 22.342(5) 17.134(4) 11.673(3) 90.149(3) 4468.2(18) 4 1.950 2.082 2640 293(2) 0.71073 32458 10215 574 1.010 0.0420, 0.0869 0.0488, 0.0903 1.465 and -0.639

a

R ) Σ(|Fo| - |Fc|)/Σ(|Fo|. b wR ) {Σ(|w[(Fo2 - Fc2)2]/Σ(|w[(Fo2)2]}1/2.

yield was 61.5% (42 mg). Anal. Calcd for C40H100Br1N5O40V16: C, 21.97; H 4.61; N 3.20%. Found: C, 22.06; H, 4.61; N, 3.22%. IR (KBr pellet): 3438(br, m), 2982(w), 1479(sh), 1384(sh), 1170(sh, m), 988(sh, vs), 848(sh, s), 785(m), 760(m), 652(m) and 588(m) cm-1. Synthesis of (Et4N)5[V16O40Cl] (2b). Complex 2b was prepared in a manner similar to that described for 2a except that Et4NBr · H2O was replaced by Et4NCl · H2O. Anal. Calcd for C40H100Cl1N5O40V16: C, 22.43; H, 4.71; N, 3.27%. Found: C, 22.47; H, 4.73; N, 3.28%. IR (KBr pellet): 3436(br, m), 2980(w), 1481(sh), 1385(sh), 1172(sh, m), 984(sh, vs), 846(sh, s), 783(m), 755(m), 656(m) and 590(m) cm-1. Synthesis of (Me2NH2)8[H2V16O42(ClO4)] (3). To the solution of VO2(acac) (274 mg, 1.5 mmol) in methanol/DMF (5:1, 30 mL), the tetramethylthiuram disulfide (361 mg, 1.5 mmol) and the Et4NClO4 · H2O (124 mg, 0.5 mmol) was added with stirring at room temperature. The reddish-brown mixture solution slowly changed to dark-green over 24 h. After filtration, the dark-green crystalline product precipitated from the filtrate was recrystallized by DMF (N,Ndimethylformamide). The solution was allowed to evaporate slowly at room temperature for a few weeks to form the pyramidal, dark green crystals of 3 suitable for X-ray diffraction. The yield was 66.5% (122 mg). Anal. Calcd for C16H66Cl1N8O46V16: C, 9.82; H, 3.40; N, 5.73%. Found: C, 9.89; H, 3.37; N, 5.69%. IR(KBr pellet): 3435(br, m), 2998(br, m), 2770(m), 2410(m), 1585(br, m), 1463(sh, m), 1432(sh, m), 1246(w), 1114(sh, m), 1022(w), 957(sh, vs), 789(sh, s), 625(sh, w), 576(sh, vs) and 510(m) cm-1. Synthesis of (Me2NH2)2(Et4N)2[H2V10O28] (4). Complex 4 was a byproduct that could be found in the syntheses of complex 3 and tetradecavanadate compound [Et4N]5[V14O36Cl].10 The yield was ca. 8.4%. Anal. Calcd for C20H58N4O28V10: C 18.31; H 4.46; N 4.27%. Found: C, 18.30; H, 4.48; N, 4.23%. IR (KBr pellet): 3344 (br, s), 3110 (br, vs), 2979 (vs, sh), 2929(vs), 2527(s), 1460(m), 1222(w), 1199(w), 1132(w), 1055(s), 976(s, sh), 943(vs), 823(vs), 751(s), 655(s) and 576(s) cm-1. Crystallographic Studies. Intensity data were collected on a Rigaku mercury CCD diffractometer with graphite-monochromated Mo KR (λ ) 0.71073 Å) radiation by using the ω-2θ scan method at room temperature. The structure was solved with direct methods and refined on F2 with full-matrix least-squares methods using SHELXS-97 and SHELXL-97 programs, respectively.20,21 All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were added in the riding model and refined isotropically with C-H ) 0.93 Å, N-H ) 0.86 Å. The crystallographic data are summarized in Table 1, and the selected bond lengths are listed in Table 2.

Results and Discussion Syntheses. The five compounds exhibit totally different polyoxovanadate clusters. It seems that the frameworks of the

clusters are mainly affected by the amount of the solvents, the amount of Lewis base, and/or the shape of the template anions. Interestingly, the formation of compound 3 is enabled by the addition of the second component, tetramethylthiuram disulfide, which actually did not participate in the structure construction. An attempt to synthesize 3 in the absence of tetramethylthiuram disulfide was likely unsuccessful. It seems that the tetramethylthiuram disulfide might, as a Lewis base, play an important role in the formation of the [H2V16O42]7- cluster. Furthermore, the redox reaction might occur between tetramethylthiuram disulfide and VO2(acac), since the mix-valent final product 3 was yielded from the pentavalent vanadate source VO2(acac). Single-crystal X-ray analysis reveals that the crystal structure of compound 3 contains the dimethylammonium cations (Me2NH2+). It is doubtful at first whether dimethylammonium cations really exist in the final product since no dimethylammonium was added in the reaction. Nevertheless, their bond distances and angles are quite reasonable, as compared with those in other related compounds.22,23 The dimethylammonium cations were also found in compound 4, a byproduct isolated from the reaction system of 3. The mass spectrum of compound 3 shows a strong peak with m/z (mass to charge ratio) of 46, corresponding to the single charged dimethylammonium cations. The result proves the existence of the dimethylammonium cations. This phenomenon is not without precedence,24,25 and, according to the literature,24-26 it is considered that these dimethylammonium cations might be generated through the decarbonylation of DMF in the acidic circumstance (Scheme 1). The main difference of the decarbonylation between their cases and ours is that the former only occurred upon heating or refluxing while the latter significantly took place at room temperature, where the vanadium(IV/V) intermediate in the reaction system may as the catalyst play an important role.27 Compounds 1, 2a, 2b, and 4 are dissolvable in some of organic solvents such as acetonitrile and DMF, while compound 3 does not dissolve in most organic solvents except in water. Structural Description. The five compounds were all synthesized from the same starting materials [VO2(acac)] in the organic media system; however, the different solvents, Lewis acidity and anion templates make the three polyoxovanadate

4094 Crystal Growth & Design, Vol. 8, No. 11, 2008

Chen et al.

Table 2. Selected Bond Distances (Å) for the Compounds 1-4a V(1)-O(1) V(1)-O(3) V(1)-O(5) V(1)-O(12) V(1)-O(10) V(2)-O(2) V(2)-O(6) V(2)-O(4) V(2)-O(9) V(2)-O(11) V(3)-O(7) V(3)-O(4) V(3)-O(3) V(3)-O(13) V(3)-O(14) V(4)-O(8) V(4)-O(5) V(4)-O(6) V(4)-O(16) V(4)-O(17) V(5)-O(21) V(5)-O(10) V(5)-O(14) V(5)-O(30) V(5)-O(27) V(6)-O(15) V(6)-O(9) V(6)-O(23) V(6)-O(13) V(6)-O(28) V(7)-O(22) V(7)-O(27) V(7)-O(28) V(7)-O(13) V(7)-O(14) V(8)-O(18) V(8)-O(11) V(8)-O(17) V(1)-O(1) V(1)-O(10)#1 V(1)-O(2)#2 V(1)-O(3) V(1)-O(2) V(2)-O(9) V(2)-O(6)#3 V(2)-O(7) V(2)-O(8) V(2)-O(10) V(3)-O(5) V(3)-O(7) V(3)-O(6) V(1)-O(6) V(1)-O(5) V(1)-O(1) V(1)-O(2) V(1)-O(1)#1 V(2)-O(9) V(2)-O(4) V(2)-O(10) V(2)-O(2)#1 V(2)-O(1) V(3)-O(8) V(3)-O(10)#2 V(3)-O(4)

Compound 1 1.593(4) V(8)-O(24) 1.823(4) V(8)-O(26) 1.835(4) V(9)-O(19) 1.935(4) V(9)-O(12) 1.939(4) V(9)-O(16) 1.582(4) V(9)-O(31) 1.815(4) V(9)-O(25) 1.831(4) V(10)-O(20) 1.917(4) V(10)-O(25) 1.934(4) V(10)-O(26) 1.591(4) V(10)-O(17) 1.788(4) V(10)-O(16) 1.798(4) V(11)-O(29) 1.984(4) V(11)-O(31) 1.987(4) V(11)-O(30) 1.597(4) V(11)-O(40) 1.784(4) V(11)-O(41) 1.807(4) V(12)-O(33) 1.983(4) V(12)-O(32) 1.989(4) V(12)-O(38) 1.591(4) V(12)-O(27) 1.700(4) V(12)-O(30) 1.906(4) V(13)-O(34) 1.928(4) V(13)-O(39) 2.037(4) V(13)-O(32) 1.595(4) V(13)-O(23) 1.718(4) V(13)-O(28) 1.901(4) V(14)-O(36) 1.922(4) V(14)-O(35) 2.049(4) V(14)-O(38) 1.594(4) V(14)-O(25) 1.827(4) V(14)-O(31) 1.830(4) V(15)-O(37) 1.895(4) V(15)-O(39) 1.900(4) V(15)-O(35) 1.600(4) V(15)-O(24) 1.699(4) V(15)-O(26) 1.909(4) Compound 2a 1.576(8) V(3)-O(3) 1.759(9) V(3)-O(2) 1.861(9) V(4)-O(4) 1.893(8) V(4)-O(8) 1.963(9) V(4)-O(3) 1.590(8) V(4)-O(8)#1 1.802(9) V(4)-O(10)#1 1.856(9) O(2)-V(1)#2 1.970(9) O(6)-V(2)#3 1.987(9) O(8)-V(4)#1 1.573(9) O(10)-V(1)#1 1.767(8) O(10)-V(4)#1 1.811(9) Compound 2b 1.567(7) V(3)-O(5) 1.772(8) V(3)-O(3) 1.865(7) V(4)-O(7) 1.894(7) V(4)-O(3)#3 1.968(8) V(4)-O(3) 1.597(7) V(4)-O(2) 1.787(8) V(4)-V(4)#3 1.794(7) V(4)-O(5) 1.949(7) O(1)-V(1)#1 1.974(7) O(2)-V(2)#1 1.592(7) O(3)-V(4)#3 1.812(8) O(10)-V(3)#2 1.814(8)

1.920(5) 2.042(4) 1.599(4) 1.699(4) 1.908(4) 1.920(4) 2.044(4) 1.585(4) 1.829(4) 1.830(4) 1.905(4) 1.912(4) 1.579(4) 1.820(4) 1.823(4) 1.957(4) 1.977(4) 1.597(4) 1.742(4) 1.848(4) 1.953(4) 2.020(4) 1.588(4) 1.800(4) 1.850(4) 1.888(4) 1.955(4) 1.590(4) 1.754(4) 1.833(4) 1.954(4) 2.033(4) 1.578(4) 1.793(4) 1.848(4) 1.920(5) 1.976(4) 1.988(8) 2.000(8) 1.589(10) 1.720(10) 1.894(9) 1.929(10) 2.047(10) 1.861(9) 1.802(9) 1.929(10) 1.759(9) 2.047(10) 1.933(8) 1.956(8) 1.534(12) 1.571(9) 1.835(9) 1.990(8) 2.140(7) 2.195(9) 1.968(8) 1.949(7) 1.571(9) 1.812(8)

V(1)-O(4) V(1)-O(3) V(1)-O(6) V(1)-O(5) V(1)-O(5)#1 V(2)-O(7) V(2)-O(8) V(2)-O(5)#1 V(2)-O(5)#2 V(2)-O(6) V(3)-O(1) V(3)-O(2) V(3)-O(3)#4 V(3)-O(3) V(1)-O(1) V(1)-O(12)#1 V(1)-O(10)#1 V(1)-O(3) V(1)-O(2) V(1)-O(9)#1 N(1)-C(7) N(1)-C(5) N(1)-C(3) N(1)-C(1) C(1)-C(2) V(2)-O(2) V(2)-O(5) V(2)-O(4) V(2)-O(6) V(2)-O(9)#1 V(2)-O(9) N(2)-C(9) N(2)-C(11) N(2)-C(13) N(2)-C(15) V(3)-O(14) V(3)-O(13) V(3)-O(12) V(3)-O(7)#1 V(3)-O(5) V(3)-O(9) O(3)-V(5) N(3)-C(17) N(3)-C(18) C(3)-C(4) V(4)-O(11) V(4)-O(10) V(4)-O(13) V(4)-O(4)#1 V(4)-O(6) V(4)-O(9) O(4)-V(4)#1 O(4)-V(5)#1 N(4)-C(19) N(4)-C(20) V(5)-O(8) V(5)-O(6) V(5)-O(7) V(5)-O(4)#1 V(5)-O(9)#1 C(5)-C(6) V(6)-O(27)

Compound 3 1.605(4) V(3)-O(8)#5 1.729(4) Cl(1)-O(9) 1.9073(16) Cl(1)-O(9)#5 1.925(4) Cl(1)-O(9)#2 2.032(4) Cl(1)-O(9)#6 1.598(6) O(2)-V(3)#6 1.724(5) O(2)-H(2) 1.881(4) O(5)-V(2)#5 1.881(4) O(5)-V(1)#1 2.031(5) O(6)-V(1)#3 1.602(5) O(8)-V(3)#2 1.7786(15) N(1)-C(1)#7 1.925(4) N(1)-C(1) 1.925(4) Compound 4 1.614(3) V(6)-O(26) 1.792(3) V(6)-O(25) 1.842(2) V(6)-O(21) 1.953(2) V(6)-O(20) 2.065(2) V(6)-O(19) 2.298(2) V(7)-O(16) 1.494(5) V(7)-O(20) 1.499(6) V(7)-O(28) 1.510(6) V(7)-O(18) 1.525(6) V(7)-O(19) 1.483(7) V(7)-O(19)#2 1.680(2) O(7)-V(3)#1 1.689(2) C(7)-C(8) 1.937(2) V(8)-O(15) 1.944(2) V(8)-O(25)#2 2.103(2) V(8)-O(23)#2 2.128(2) V(8)-O(17) 1.451(7) V(8)-O(16) 1.486(6) V(8)-O(19)#2 1.536(6) V(9)-O(22) 1.547(7) V(9)-O(17) 1.592(3) V(9)-O(18) 1.787(2) V(9)-O(21)#2 1.859(3) V(9)-O(28)#2 2.003(3) V(9)-O(19)#2 2.019(2) O(9)-V(2)#1 2.322(2) O(9)-V(5)#1 1.767(2) O(9)-V(1)#1 1.467(6) C(9)-C(10) 1.476(6) V(10)-O(24) 1.562(9) V(10)-O(23) 1.600(2) V(10)-O(26) 1.813(2) V(10)-O(28)#2 1.870(2) V(10)-O(18) 1.961(2) V(10)-O(19) 2.000(2) O(10)-V(1)#1 2.302(2) C(11)-C(12) 1.961(2) O(12)-V(1)#1 2.054(2) C(13)-C(14) 1.472(6) C(15)-C(16) 1.479(7) O(19)-V(7)#2 1.601(2) O(19)-V(9)#2 1.913(2) O(19)-V(8)#2 1.953(3) O(21)-V(9)#2 2.054(2) O(23)-V(8)#2 2.240(2) O(25)-V(8)#2 1.557(9) O(28)-V(10)#2 1.602(2) O(28)-V(9)#2

1.979(5) 1.418(6) 1.418(6) 1.418(6) 1.418(6) 1.7786(15) 0.9(9) 1.881(4) 2.032(4) 1.9073(16) 1.979(5) 1.505(11) 1.505(11) 1.783(2) 1.847(2) 1.991(2) 2.022(2) 2.321(2) 1.682(2) 1.692(2) 1.933(2) 1.940(2) 2.111(2) 2.117(2) 2.003(3) 1.547(7) 1.610(2) 1.800(2) 1.834(2) 1.951(2) 2.056(2) 2.293(2) 1.601(2) 1.763(2) 1.912(2) 1.943(2) 2.061(2) 2.242(2) 2.103(2) 2.240(2) 2.298(2) 1.524(11) 1.598(2) 1.811(2) 1.878(2) 1.970(2) 1.996(2) 2.296(2) 1.842(2) 1.485(7) 1.792(3) 1.454(7) 1.522(8) 2.117(2) 2.242(2) 2.293(2) 1.943(2) 1.834(2) 1.800(2) 1.970(2) 2.061(2)

a Symmetry transformations used to generate equivalent atoms for compound 2a: #1, x, -y, -z + 1, #2, -x + 1, -y, z, #3 -x + 1, y, -z + 1; for compound 2b: #1, x,-y,-z + 1, #2, -x + 1, -y, z, #3, -x + 1, y, -z + 1; for compound 3: #1, x, -y + 1/2, -z+1/2; #2, y + 1/2, -x + 1, -z + 1/ 2; #3, -y + 1, -x + 1, z; #4, y + 1/2, x - 1/2, z; #5, -y + 1, x - 1/2, -z + 1/2; #6, -x + 3/2, -y+1/2, z; #7, y, x, -z + 1; for compound 4: #1, -x + 1,-y + 1, -z + 1, #2, -x, -y + 1, -z + 2.

clusters present different frameworks which all have not encountered before. (Et4N)4[HV15O39(acac)Cl] · 3CH3CN (1). Single-crystal Xray analysis reveals that compound 1 is composed of a

{HV15O39(acac)} cage, a chloride anion, four Et4N+ cations and three acetonitrile molecules. The structure of the hollow anionic [HV15O39(acac)]3- cage in compound 1 is shown in Figure 1.

Penta- and Hexadecavanadate Compounds

Figure 1. A view of the anion cluster {HV15O39(acac)} with the encapsulated chloride anion in the crystals of 1 showing the atom labeling. Thermal ellipsoids are drawn at 30% probability.

Scheme 1. The Probable Formation of the Dimethylammonium Cation

In the cage, the vanadium atoms are all five-coordinated with the VO5 square pyramid coordination geometries. As is common in polyoxovanadate cages, the axial position of the each VO5 square pyramid is occupied by a double bond terminal oxygen atom with the V-O bond distances in the range of 1.578(4)1.600(4) Å, and four single bond bridge oxygen atoms occupy the basal positions with V-O bond distances in the range of 1.699(4)-2.049(4) Å. It is notable that V11 is different from the other vanadium atoms in the basal oxygen atoms it connected. In the vanadium oxide clusters, usually, the basal positions of VO5 square pyramid are occupied by separate oxygen atoms which often exist in the forms of µ2-bridging and µ3-bridging oxygen atoms with protonation or nonprotonation. However, the basal oxygen atoms of V11 are composed of two µ3-bridging oxygen atoms (O30 and O31) and two oxygen atoms (O40 and O41) of the terminal coordinated acetylacetone molecule, which is unprecedented in the reported literature. The distances of V11-O40 and V11-O41 are 1.957(4) and 1.977(4) Å, respectively, which means the two oxygen atoms are both single bonded with V11. These distorted VO5 square pyramids constitute the novel half-open pentadecavanadate shell configuration by sharing vertices and edges through 14 µ2-oxygen and 10 µ3-oxygen atoms. Encapsulated in the open cavity of the negatively charged cluster is a chloride anion. There is no covalent interaction of this chloride ion with the atoms in vanadium oxide framework, as the average V · · · Cl distance is ca. 3.672 Å. The pentadecavanadate cluster is a highly irregular cage whose only symmetry element is a symmetry plane. Unlike most known polyoxovanadate clusters that exhibit closed sphere-like structures, this {HV15O39(acac)} shell contains large openings on its surface. As shown in Figure 2a (in which the half-transparent green sphere presents the large space in the hollow cluster), the four large openings are shown on the surface of the cluster. In the front of the half-transparent green sphere, both 10-membered ring and 8-membered ring openings consisting of V and O atoms

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stand by sharing edge V15-O39-V13. The irregular 10membered ring opening (V15-O24-V8-O11-V2-O9-V6O23-V13-O39) has the approximate shape of the openings we found in the floral basket-shape tetradecavanadate {V14O36} cluster10 (Figure 2b,c). The eight-membered ring (V14-O35V15-O39-V13-O32-V12-O38) has the crown shape as Figure 2b shows. Four vanadium atoms (V12, V13, V14, V15) lie almost in the same plane with mean deviations from the best plane 0.0087 Å and constitute an approximate square figure with V-V-V angles close to 90°. So does the other plane of four oxygen atoms (O32, O39, O35, O38) with mean deviations from the best plane being 0.0864 Å and a dihedral angle of the two planes 1.1°. The longest distances measured from the diametrically opposed atoms in the two openings are 4.8956(9) Å (V13-V14 for the eight-membered ring opening) and 5.9528(11) Å (V1-O39 for the 10-membered ring opening), respectively. The other eight-membered ring opening exhibiting the same crown-like shape described above, shows in back of the half-transparent ball, sharing vertices (V1) with the longest distances (V1-V2) 4.996 and (V5-V9) 4.826 Å. These large openings which may offer a path for the guests to exchange with other ions make the cluster more open than the most hollow polyoxovanadate clusters. Like the {V14O36} shell, this kind of clusters can be viewed as half-open frameworks presenting the structural singularity, which would be of the particular interest to the polyoxovanadate community. The bond valence calculations28 show that all the vanadium atoms in compound 1 are in the state of +5, while the valence of bridging oxygen atoms are in the range 1.6-2.0 except 1.564 for O(23) and 1.484 for O(24), indicating that protonation may occur in the position near the above two oxygen atoms. Crystal analysis shows that a hydrogen atom is located between O23 and O24. The distance of O24-H24, O23-H24 and O23-O24 are 0.853, 1.748, and 2.585 Å, respectively, which means that O24 is a protonated oxygen atom and hydrogen bonded with the O23 through the H24. Thus, the reasonable composition of compound 1 is (Et4N)4[HVV15O39(acac)Cl] · 3CH3CN, which is entirely consistent with that calculated from charge balance. (Et4N)5[V16O40Br] (2a) and (Et4N)5[V16O40Cl] (2b). Compounds 2a and 2b possess the same frameworks, except their inclusion of different guest anions (the bromide anion for 2a and the chloride anion for 2b). Herein, 2b is selected to represent their structures. Compound 2b is composed of a {V16O40} cage, a chloride anion, and five tetraethylamine cations (Et4N+). The space group of the compound is I222. The structure of the hollow anionic hexadecavanadate cage in compound 2b, projected approximately down the a-axis of the monoclinic unit cell, is shown in Figure 3. In the cage, the 16 vanadium atoms are all five-coordinated with the VO5 square pyramid coordination geometries. These distorted VO5 square pyramids constitute the {V16O40} shell configuration by sharing vertices and edges through 8 µ2-oxygen and 16 µ3-oxygen atoms. Among the 16 vanadium atoms in the hexadecavanadate cluster, four are independent and divided into two groups according to their coordination with different bridging oxygen atoms (µ2-oxygen or µ3-oxygen atoms). Two of these four atoms (V1 and V4) each link to four µ3-oxygen atoms, while the other two vanadium atoms (V2 and V3) each link to two µ2-oxygen atom and two µ3-oxygen atoms. The V-O distances for the terminal oxygen atoms all lie within the narrow range of 1.534(12) to 1.597(7) Å. Like those in compound 1, similar large eight-membered ring openings consisting of V and O atoms with a crown-like shape are shown on the surface of cluster 2b. The longest distances measured from the diametrically opposed atoms in

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Figure 2. (a) The structure of pentadecavanadate cluster {HV15O39(acac)} in compound 1. The green half-transparent sphere represents the large space in the center of the hollow cluster. (b) The 10-membered ring and crown-like eight-membered ring openings showing on the surface of the cluster. (c) The irregular 10-membered rings exhibit a similar shape with the openings of the basket-shape tetradecavanadate {V14O36} cluster.

Figure 3. The structure of hexadecavanadate cluster {V16O40} in compound 2b with the encapsulated chloride anion. The green halftransparent sphere represents the large space in the center of the hollow cluster.

Figure 4. The structure of hexadecavanadate cluster {H2V16O42} in compound 3 with the encapsulated perchlorate anion. The green halftransparent sphere represents a large space in the center of the hollow cluster.

the two openings are 4.9143(32) Å (V3-V3#2). The negatively charged {V16O40} hosts a chloride anion at its center. There is no covalent interaction of this guest ion with the atoms in the vanadium oxide shell since the average distance V · · · Cl distance is ca. 3.5865 Å. (Me2NH2)8[H2V16O42(ClO4)] (3). Single-crystal X-ray analysis reveals that compound 3 exhibits a hollow anionic hexadecavanadate cage {H2V16O42} with a perchlorate anion located in the center of the cluster. The space group of the compound is P42/nnm. The structure of the hollow anionic hexadecavanadate cage in compound 3, projected approximately down the c-axis of the monoclinic unit cell, is shown in Figure 4. In the cage, the 16 vanadium atoms are all five-coordinated with the VO5 square pyramid coordination geometries. These distorted VO5 square pyramids constitute the {H2V16O42} shell configuration by sharing vertices and edges through 14 µ2oxygen and 12 µ3-oxygen atoms. Among the 16 vanadium atoms in the hexadecavanadate cluster, three are independent, and divided into two groups according to their coordination with different bridging oxygen atoms (µ2-oxygen or µ3-oxygen atoms). One of these atoms (V3) is bonded by four µ3-oxygen atoms with V-O distances ranging from 1.7786(15) to 1.979(5)

Å. The other two vanadium atoms (V1 and V2) link to one µ2-oxygen atom [V-O ) 1.729(4) and 1.724(5) Å] and three µ3-oxygen atoms [V-O ) 1.881(4) to 2.032(4) Å]. The V-O distances for the terminal oxygen atoms all lie within the narrow range of 1.598(6) to 1.605(4) Å. Although cluster 3 is quite different with the {HV15O39(acac)} shell in compound 1 in composition, assembly, and symmetry, it possesses large openings and exhibits a half-open framework too. Four eightmembered ring openings consisting of V and O atoms are shown on the surface of the cluster, and they are symmetrically equivalent. As Figure 4 shows, the openings have a shape approximate to the eight-membered rings in cluster of compounds 1 and 2. The longest distances measured from the diametrically opposed atoms in the openings are 4.9720(12) Å (V1-V3#6). The negatively charged {H2V16O42} hosts a perchlorate anion at its center. There is no covalent interaction of this guest ion with the atoms in the vanadium oxide shell since the distance of the closest vanadium atom (V3) to the oxygen atom of the perchlorate anion (O9) is ca. 2.579 Å. Although inclusion of an anionic guest into an anionic molecular oxide host is not a strange phenomenon for the vanadium oxide shell with hollow

Penta- and Hexadecavanadate Compounds

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Figure 6. A view of the two-dimensional hydrogen-bonded structure of 4. O · · · O hydrogen bonds are shown as red broken lines and N · · · O hydrogen bonds are shown as blue broken lines (Et4N+ cations have been omitted for clarity).

Figure 5. The hydrogen bonding interactions in 3 and a view of the three-dimensional supramolecular framework of the compound.

structures, encapsulating a perchlorate anion, which is larger as compared to the chloride ion, in the polyoxovanadate cluster is quite rare. The negatively charged {H2V16O42} shell is surrounded and charge balanced by the dimethylammonium cations which are hydrogen bonded to the host shell to generate a three-dimensional supramolecular framework. As shown in Figure 5a, each {H2V16O42} cluster is connected with eight dimethylammonium cations in four directions by the outprojecting µ2-oxygen atoms (O8) of clusters hydrogen bonding to the nitrogen atoms (N1) of cations. The neighboring clusters are linked by a couple of (CH3)2NH2+ cations (Figure 5b) with the O · · · N distances of ca. 2.898 Å. Thus, the hydrogen bonding interactions extend compound 3 into a three-dimensional inorganic-organic hybrid supramolecular structure (Figure 5c) showing one-dimensional channels, which may be occupied by the remaining disordered (CH3)2NH2+ cations. Although the vanadium oxide shells in compounds 2a, 2b and 3 are all hexadecavanadate clusters, they are different in many aspects: (1) Compositions: although the number of vanadium atoms in three clusters are the same, those of the oxygen atoms are not. Thus, the clusters are constituted by different compositions with {H2V16O42} for 3 and {V16O40} for 2a and 2b. (2) Encapsulated anions: the three vanadium oxide shells with hollow structures all encapsulate the anions in the centers of the host shells, yet their encapsulated anions are distinguishable. A bromide anion, chloride anion, and perchlorate anion act as the included guests for compounds 2a, 2b and 3, respectively. (3) Symmetry: the clusters show different symmetries with D2d for 3 and D2 for 2a and 2b. (Me2NH2)2(Et4N)2[H2V10O28] (4). The asymmetric unit in compound 4 contains two half-decavanadate clusters, with the second half of each being generated by a center of symmetry, two Et4N+ cations, and two dimethylammonium cations. The decavanadate anion consists of an aggregation of 10 edgesharing VO6 octahedra. The bond lengths and angles of the {V10O28} unit show a similar trend to those found in the literature.7 Bond valence calculations show that all the V atoms of (I) are in the +5 state, while the valence of the bridging O

Figure 7. EPR spectrum of compound 2a at room temperature.

atoms are in the range 1.6-2.0, except 1.249 for O(7) and 1.288 for O(21), indicating that the cluster is a diprotonated core and protonation occurrs at O(7) and O(21). The main feature of 4 is its interesting two-dimensional structure exhibiting two different kinds of hydrogen bonds: OsH · · · O between two decavanadate clusters and NsH · · · O between dimethylammonium cations and decavanadate clusters. As shown in Figure 6, the terminal O atoms of the decavanadate clusters, O(1) and O(15), are strongly hydrogen bonded with the protonated atoms O(7) and O(21) with the distances of OsH · · · O being 2.714(3) and 2.624(3) Å respectively. As most of the multipledimensional frameworks are built by polyoxovanadate clusters and organic cations, a two-dimensional layer constructed by hydrogen bonds directly between clusters is rare. To strengthen the layer, the N atoms of dimethylammonium cations are also hydrogen bonded with the µ2-O atoms of neighboring clusters. EPR, IR, XRD and TGA. X-band electron paramagnetic resonance (EPA) spectra of the compounds were recorded to further investigate the valence state of the three polyoxovanadates. The compounds 1 and 4 are EPR silent both in the solution state and solid state, which may suggest that all vanadium atoms in the clusters are pentavalent. The results are in good agreement with the assignments calculated from charge balance and bond valence sum calculations. The EPR spectrum of compound 2a at room temperature is shown in Figure 7. The compound 2a in the solid state gives the paramagnetic signal with g ) 1.9577 at 298 K, indicating the existence of V4+.10,29

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diffraction analyses have been done to characterize the compounds. In addition, this work also introduces a new synthetic approach for producing different types of polyoxovanadate clusters under mild conditions, which may further the exploration of vanadate clusters. Acknowledgment. This work was supported by the grants of 973 Program (2006CB932900), National Nature Science Foundation of China and Nature Science Foundation of Fujian Province, Young Scientist Funds of Fujian Province (No. 2007F3111). Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.

References Figure 8. TG curves of the compounds 1, 2a and 3 measured under flowing air.

The EPR response of 2b and 3 are similar to 2a, with g ) 1.9585 and 1.9583 at 298 K, respectively, showing that compounds 2a, 2b, and 3 are all mixed-valent compounds. The FT-IR spectrum of the five compounds all shows characteristic vibrational features similar to other polyoxovanadates reported.30 Symmetric and asymmetric stretching of different kinds of V-O bonds is observed: The peaks in the range of 957-1022 cm-1 are ascribed to terminal V-O bonds; strong bands at 744-854 and 510-656 cm-1 are assigned to the antisymmetric stretching vibrations of V-O-V features. The homogeneities of compounds 1-3 are confirmed by X-ray powder diffraction analyses (XRD, see Figure S1, Supporting Information). Their peaks are in good agreement with those calculated from X-ray single-crystal diffraction data. Thermal gravimetric (TG) data for the compounds under flowing air are shown in Figure 8. The result shows that decomposition of 1 starts at ca. 100 °C with several overlapping steps that correspond to the loss of acetonitrile molecules, tetraethylamine cations, acetylacetone groups and chloride anions. The maximum mass loss up to 395 °C is 60.52%, and the residue could be mixed-valence vanadium oxides V3O7/ V6O13 based on calculations, suggesting that a part of vanadium(V) atoms are deoxidized by NH3 generated during the thermal composition.31 Similar thermal decomposition behaviors have also been observed and reported in other polyoxovanadate compounds.7,31,32 Then, the above-mentioned mixed-valence vanadium oxides are reoxidized giving the final product at 465 °C with a total mass loss of 62.88%, which is in agreement with the expected value 62.92% when the residue corresponds to V2O5. Compounds 2a and 3 exhibit thermal gravimetric behavior similar to 1, with the starting decomposition temperature at 180 °C and 188 °C, respectively. The remaining residues are both in V2O5 phase with the total mass loss of 66.08% (calculated mass loss of 66.55%) for 2a and 73.43% (calculated mass loss of 74.34%) for 3. Conclusion In summary, we have described a series of polyoxovanadate compounds synthesized in organic media at room temperature. These clusters present novel hollow structures which all exhibit large openings on the surfaces of the clusters. FT-IR spectroscopies, elemental analyses, bond valence sum calculations, electron paramagnetic resonance, X-ray powder diffractions, thermogravimetric analyses and complete single crystal X-ray

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