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Hydrothermal synthesis, crystal structure and characterization of [Zn(H2O)4]2 [H2As6V15O42(H2O)]·2H2O
Journal of Molecular Structure 660 (2003) 131–137 www.elsevier.com/locate/molstruc
Hydrothermal synthesis, crystal structure and characterization of [Zn(H2O)4]2 [H2As6V15O42(H2O)]·2H2O Xiao-Bing Cuia,b, Ji-Qing Xub,*, Lan Dingb, Hong Dingb, Ling Yeb, Guo-Yu Yanga,c a
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China b College of Chemistry and State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun, Jilin 130023, China. c State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing, Jiangsu 210093, China. Received 19 June 2003; revised 20 August 2003; accepted 20 August 2003
Abstract The compound [Zn(H2O)4]2[H2As6V15O42(H2O)]·2H2O (1) has been synthesized and characterized by elemental analysis, IR, ESR, magnetic measurement, third-order nonlinear property study and single crystal X-ray diffraction analysis. The ˚ , c ¼ 33.970(7) A ˚ , g ¼ 1208, V ¼ 4278.8(12) A ˚ 3, compound 1 crystallizes in trigonal space group R3; a ¼ b ¼ 12.0601(17) A Z ¼ 3 and R1ðwR2Þ ¼ 0.0512 (0.1171). The crystal structure is constructed from [H2As6V15O42(H2O)]42 anions and [Zn(H2O)4]2þ cations linked through hydrogen bonds into a network. The [H2As6V15O42(H2O)]62 cluster consists of 15 VO5 square pyramids linked by three As2O5 handle-like units. q 2003 Elsevier B.V. All rights reserved. Keywords: Hydrothermal synthesis; Polyoxometalate; Arsenic–vanadium cluster; Arsenic; Vanadium; Cluster; Hydrogen bond; Crystal structure
1. Introduction The chemistry of polyoxometalates continues to attract interests as a result of their realized and potential applications in fields as diverse as catalysis, analysis, biochemistry, medicine and material science [1 – 3]. Although the synthesis of such materials by rational designs remains elusive, one of the strategies used in the synthesis of polyoxometalates is to employ a specific cation as inducing * Corresponding author. Tel./fax: þ 86-591-3710051. E-mail addresses: [email protected] (G.Y. Yang), xjq@ mail.jlu.edu.cn (J.Q. Xu). 0022-2860/$ - see front matter q 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2003.08.014
agent. Practicing this strategy results in the successful synthesis of a great deal of structures. In the past few years, an important subclass of polyoxometalates is the family of arsenic – vanadium clusters with arsenic and vanadium in low oxidation states. The arsenic –vanadium clusters can be divided into two different groups based on the numbers of the As and V atoms: As8V14 clusters and As6V15 clusters. The first case is represented by anions [As8V14O42(X)]62 (X ¼ SO22 or SO22 3 4 ) [4] and compound [N(Me)4]4 [As8V14O42(H2O)0.5 [5]. The second group is exemplified by compounds K6 [As6V15O42(H2O)]·8H2O [6] and K6 [As6V15O42 (H2O)]·6H2O [7]. Here we present the synthesis and structural investigations of
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a new mixed As/V polyoxometalate, [Zn(H2O)4]2 [H2As6V15O42 (H2O)]·2H2O (1), which is the first As/V polyoxometalate containing neither alkali metals nor organic components.
2. Experimental 2.1. General procedures All reagents were purchased commercially and used without further purification. The content of vanadium, zinc and arsenic element were determined by a Perkin–Elmer Optima 3300DV spectrometer. The infrared spectrum was obtained on an Perkin–Elmer spectrophotometer in the 200 – 4000 cm21 region with pressed KBr pellet. Determination of electron spin resonance (ESR) carried out on a Bruker ER 200D-SRC diffractometer. Variable-temperature magnetic susceptibility measurements for 1 were performed on a MPMS-XL magnetometer in 2–300 K. 2.2. Hydrothermal synthesis Compound 1 was synthesized hydrothermally in 53% yield (based on As). A mixture of As2O3 (0.39 g), V2O5 (0.36 g), H2C2O4·2H2O (0.25 g), en (0.12 g), Zn(Ac)2·2H2O (0.44 g), and distilled water (21 ml) in a molar ratio of 1:1:1:1:1:600 was stirred for ca. 10 min and then transferred to a Teflon-lined reactor and heated at 160 8C for three days. H2C2O4·2H2O and en were used as reducing agents and were necessary to maintain the pH value of the reaction system. After cooling to room temperature, black block crystals were isolated. Anal. Calcd. For H24As6O53V15Zn2: As, 20.32%; V, 34.53%, Zn, 5.97%. Found: As, 20.28%; V, 34.47%, Zn, 5.90%. 2.3. X-Ray crystallography The crystal structure of the compound 1 was determined by single-crystal X-ray diffraction. Crystal data: H24As6O53V15Zn2, M ¼ 2216.55, ˚, trigonal, R3; a ¼ b ¼ 12.0601(17) A c¼ ˚ ˚ 33.970(7) A, g ¼ 1208, V ¼ 4278.8(12) A3, Z ¼ 3, Dc ¼ 2.581 g cm3, Fð000Þ ¼ 3153; A black crystal (dimensions 0.344 £ 0.317 £ 0.249 mm) was carefully chosen and mounted on a glass fiber for
data collection. The crystal data were collected on a Rigaku RAXIS-RAPID diffractometer with Mo Ka radiation at 293 K. A total of 1890 reflections were measured. The structure solution and refinement were carried out using SHELXL 97. The structure was solved using direct methods and all of the non-hydrogen atoms were located from the initial solution or from subsequent electron density difference maps during the initial stage of the refinement. All of the non-hydrogen atoms except Ow7 in the structure were refined using anisotropic thermal displacement parameters. O(8)
Table 1 Crystal data and structure refinement for 1 Empirical formula Formula weight Temperature Wavelength Crystal system
H24 As6 O53 V15 Zn2 2216.55 293(2) K ˚ 0.71073 A Trigonal
Space group ˚) a (A ˚) b (A ˚) c (A að8Þ bð8Þ gð8Þ
R3 12.0601(17) 12.0601(17) 33.970(7) 90 90 120
˚ 3) Volume (A Z rcalc (mg m23) Absorption efficient (mm21) Fð000Þ Crystal size (mm £ mm) u Range (8)
4278.8(12) 3 2.581 6.715 3153 0.344 £ 0.317 £ 0.249 4.08– 27.48
Limiting indices
211 # h # 15 211 # k # 13 243 # l # 11
Reflections collected Reflections unique Completeness to u ¼ 27:48 Refinement method Data/restraints/parameters Goodness-of-fit on F 2
1890 1650 ðrint ¼ 0:058Þ 86.2% Full-matrix least-squares on F 2 1890/7/239 1.017
Final R indices ½I . 2sðIÞ
R1 ¼ 0:0512 wR2 ¼ 0:1171
R indices (all data)
R1 ¼ 0:0569 wR2 ¼ 0:1203 0.00
Absolute structure parameter
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Table 2 Atomic coordinates ( £ 104) and equivalent isotropic displacement parameters ðA2 £ 103 Þ for compound 1
As(1) As(2) V(1) V(2) V(3) V(4) V(5) Zn(1) Zn(2) O(1) O(2) O(3) O(4) O(5) O(6) O(7)
X
Y
Z
UðeqÞ
23476(1) 1047(1) 22826(2) 2380(2) 21436(2) 21870(2) 2869(2) 3333 2333 22638(10) 114(9) 21250(10) 777(9) 2680(10) 2287(9) 23708(8)
2544(1) 5152(1) 927(2) 4000(2) 3603(2) 5310(2) 2129(2) 6667 3333 251(10) 1635(9) 6112(10) 4251(10) 3709(11) 5050(8) 4430(9)
3168(1) 2614(1) 2248(1) 3539(1) 2005(1) 3782(1) 2888(1) 4190(1) 4959(1) 1962(3) 2881(3) 4176(3) 3825(3) 1593(3) 2358(3) 3843(3)
36(1) 35(1) 29(1) 30(1) 29(1) 31(1) 33(1) 77(1) 48(1) 46(2) 46(2) 43(2) 41(2) 49(2) 35(2) 37(2)
O(8) O(9) O(10) O(11) O(12) O(13) O(14) O(15) OW1 OW2 OW3 OW4 OW5 OW6 OW7
x
y
z
UðeqÞ
1505(11) 324(17) 24002(9) 21431(8) 23116(9) 21235(9) 22607(8) 180(8) 23333 0 1666(14) 3333 24707(17) 23333 23870(120)
6599(11) 5703(18) 352(9) 2184(8) 2114(8) 2441(9) 569(8) 4046(9) 3333 0 6470(17) 6667 1616(14) 3333 22010(110)
2898(4) 2890(6) 3439(3) 3418(3) 1957(3) 2359(2) 2797(3) 2987(3) 2850(30) 4107(6) 4394(4) 3627(5) 4741(5) 5518(5) 2080(30)
25(2) 17(3) 36(2) 35(2) 34(2) 33(2) 34(2) 39(2) 190(30) 51(4) 81(4) 77(7) 91(5) 46(4) 190(40)
UðeqÞ is defined as one third of the trace of the orthogonalized Uij tensor.
is disordered over two positions O(8) and O(9) with occupancy factors 0.64 and 0.36, respectively. No hydrogen atoms were added. The absolute structure was determined and in accordance with the selected setting (Flack x parameter ¼ 0.00) [8]. The crystallographic data are given in Table 1. Atomic positional parameters and isotropic temperature factors are given in Table 2. Selected bond lengths are listed in Table 3.
3. Results and discussion 3.1. Structure of the compound 1 The single crystal analysis reveals that the compound 1 contains discrete [H2As6V15O42(H2O)]42 polyoxoanions, [Zn(H2O)4]2þ cations and two water molecules of crystallization. The [H2As6V15O42 (H2O)]42 anion consists of 15 distorted VO5 square
Table 3 ˚ ) for compound 1 Selected bond lengths (A As(1)– O(14) As(1)– O(10) As(1)– O(8)#1 As(1)– O(9)#1 As(2)– O(9) As(2)– O(15) As(2)– O(6) As(2)– O(8) V(1)–O(1) V(1)–O(12) V(1)–O(13) V(1)–O(14) V(1)–O(6)#1 V(2)–O(4)
1.754(9) 1.763(8) 1.777(13) 1.798(19) 1.630(19) 1.755(10) 1.779(9) 1.821(12) 1.627(10) 1.909(9) 1.911(10) 1.963(10) 1.984(9) 1.599(9)
V(2)–O(7)#1 V(2)–O(11) V(2)–O(15) V(2)–O(10)#2 V(3)–O(5) V(3)–O(12) V(3)–O(13) V(3)–O(12)#2 V(3)–O(6) V(4)–O(3) V(4)–O(7) V(4)–O(7)#1 V(4)–O(11)#2 V(4)–O(10)#2
1.938(9) 1.948(9) 1.985(10) 1.998(9) 1.639(10) 1.928(9) 1.952(8) 1.957(9) 1.996(9) 1.601(8) 1.931(9) 1.948(9) 1.962(10) 2.033(9)
V(5)– O(2) V(5)– O(13) V(5)– O(11) V(5)– O(14) V(5)– O(15) Zn(1)–OW4 Zn(1)–OW3 Zn(1)–OW3#3 Zn(1)–OW3#4 Zn(2)–OW6 Zn(2)–OW5#2 Zn(2)–OW5 Zn(2)–OW5#1
1.571(9) 1.933(8) 1.935(9) 2.022(9) 2.033(9) 1.910(18) 2.026(15) 2.026(16) 2.026(15) 1.899(19) 2.037(13) 2.037(13) 2.037(13)
Symmetry transformations used to generate equivalent atoms: #1 2x þ y 2 1; 2x; z; #2 2y; x 2 y þ 1; z; #3 2x þ y; 2x þ 1; z; #4 2y þ 1; x 2 y þ 1; z:
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pyramids and 6 AsO3 triangular units, with a water molecular at its center, as shown in Fig. 1. The polyoxoanion lies on a crystallographic three-fold axis, which passes through the central water molecular Ow(6). All the V atoms with square-pyramidal environments show apical vanadyl O atoms in the V – ˚ and the V –O O bond distances 1.571(9) –1.639(10) A bonds corresponding to the square-pyramidal basal ˚ , The As –O planes vary from 1.909(9) to 2.033(9) A bonds of the AsO3 units are in the range of 1.630(19) – ˚ , The 15 VO5 square pyramids are linked 1.821(12) A with one another through edges, and with AsO3 groups through vertices. Two AsO3 groups are joined together by an oxygen bridge, forming a handle-like As2O5 moiety. The 15 VO5 square pyramids linked through edges are connected to As2O5 units by sharing O atoms, and form a ball-like structure. It is worth noting that the structure 1 is an extensive three-dimensional network constructed from [Zn(H2O)4]2þ cations and [H2As6V15O42(H2O)]42 anions through hydrogen bonds which exhibit between the water molecules of the [Zn(H2O)4]2þ cations and the terminal oxygen atoms of the clusters (Fig. 2). The tetrahedral environments of the [Zn(H2O)4]2þ cations are completed by four oxygen atoms of water molecules with Zn – O distances ˚ and angles ranging from 1.899(19) to 2.037(13) A varying from 107.6 (5) to 111.3(5)8. The isolated
Fig. 1. The ball-and-stick representation of the [H2As6V15O42(H2O)]42 clusters of the compound 1.
Fig. 2. View of crystal packing of compound 1 along b-axis.
[Zn(H2O)4]2þ cations interact with the clusters through hydrogen bonds: OW4· · ·O4a ða : 1 2 y; ˚ , OW3· · ·O12b ðb : 1 þ x 2 y;zÞ : 3.077(13) A 20:33333 2 x þ y; 0:33333 2 x; 0:33333 þ zÞ : ˚ , OW4· · ·O4: 3.076(9) A ˚ , OW5· · ·O7c 3.059(17) A ˚ ðc : 2y; 1 þ x 2 y; zÞ : 3.067(20) A, OW5· · ·O1d ðd : 20:33333 2 y; 0:33333 þ x 2 y; 0:33333 þ zÞ : ˚. 3.066(24) A The assignment of the oxidation states for the V and As atoms is consistent with the electric charge and confirmed by bond valence sum calculations [9]. The BVS values for the crystallographic independent vanadium atoms V(1) –V(5) are 4.15, 4.10, 3.99, 4.09 and 4.15, respectively, indicating that the oxidation state for each V atom is þ 4. While the calculated valence sum for the crystallographic independent arsenic atoms As(1) and As(2) are 3.18 and 3.27, respectively, indicating the oxidation state of each As atom is þ 3. To our knowledge, no example of {As6V15O42} cluster containing mixedvalence vanadium atoms has been reported. In four
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known compounds containing {As6V15O42} cluster unit such as K6[As6V15O42(H2O)]·8H2O [6], K6[As6V15O42(H2O)]·6H2O [7], [Co(en)3][{Co(en)2}2As6V15O42]·4H2O [12] and [Ni(en)3]2[Ni(en)2(H2O)2] [As6V15O42]·4H2O [13], the oxidation states of all the vanadium atoms were þ 4. In the structure 1, assuming the valence of As, V, and O to be þ 3, þ 4, and 2 2, respectively, the cluster anion of [As6V15O42(H2O)] creates a net negative charge of 2 6. The presence of two [Zn(H2O)4] would account for the þ 4 charge arising from the Zinc cation. The excess negative charge of 2 2 is then balanced by the two hydrogen atoms, as given in the formula of [H2As6V15O42(H2O)] [Zn(H2O)4]2·2H2O. According to the related compounds reported previously, i.e. Na0.5H4.5 [As3Mo12O40]·4H2O [14], (Me3NH)4K2[H14Mo16 V O52].8H2O [15], K3[H12 –(AsO)2(AsO4)VIV 6 V6 O36]· 12H2O [16], (Me3NH)4(NH4)[H4PV14O42] [17], (C2 H5NO2)3.5[H4SiMo12O40]·8.5H2O [18], KH3Fe2Mo2 O10 [19], [Cu(en)2]3 [{Cu(en)2}2(H2W12 O42)]·12H2O [20], [(CH3)4N]4Na2H[a-PW11 O39]]·8H2O [21], etc. the hydrogen atoms attached to the polyoxometalate anion should be reasonable. In addition, comparing with the starting vanadium source of the above mentioned four compounds of K6 [As 6V15O42 (H2O)]·8H2O [6] (I), K6 [As6V15O42(H2O)]·6H2O [7] (II), [Co(en)3][{Co(en)2}2As6V15O42]·4H2O [12] (III) and [Ni(en) 3] 2[Ni(en) 2(H 2O) 2] [As 6V 15 O42]·4H2O [13] (IV), KVO3 for I and V2O5 for II – IV, respectively. The reducing agent is KSCN for I, and en for II – IV, respectively. The title compound was also prepared from V2O5 and used both en and H2C2O4·2H2O as reducing agents. As shown in table below, the reducing ability of the H2C2O4·2H2O is stronger than that of KSCN. So the possibility of V5þ in title compound can be excluded. Equation for half-reaction
E8 ðVÞ
(NCS)2 þ 2e2 ¼ 2NCS2 2CO2(g) þ 2Hþ þ 2e2 ¼ H2C2O4
0.77 2 0.49
135
Fig. 3. The ESR spectrum of the compound 1.
the paramagnetic signal of V4þ at 273 K with g ¼ 1:949; which is consistent with the result of valence sum calculations of compound 1. 3.3. Magnetic properties of the compound 1 The temperature dependence susceptibilities were measured in the temperature range 2 – 300 K for compound 1. Fig. 4 shows the magnetic behavior of 1 in the form of xT vs T and x21 m vs T plots. The product xT; where x is the molar magnetic susceptibility in terms of the unit formula, continuously decreased as the temperature is lowered, indicating the presence of the antiferromagnetic exchange
3.2. IR spectrum and ESR spectrum The IR spectrum of 1 exhibits intense bands at 933 and 719 cm21 attributed to y(V – O). The ESR spectrum of compound 1 (Fig. 3) shows
Fig. 4. Plots of the experimental temperature dependences of xm T (A) and x21 m (L) for compound 1.
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interactions. The room temperature value ðmeff ¼ 4:28 mb Þ is smaller than the expected value for the 15 uncoupled S ¼ 1=2 spins V4þ atoms (assuming the g ¼ 2:0 for V4þ, meff ¼ 6.71 mb Þ; indicating antiferomagnetic coupling. These antiferromagnetic coupling interactions are related to electron delocalisation of poly-nucleus systems. Previous studies indicated that electron delocalisation can favor spin pairing [10], suggesting the [H2As6V15O42(H2O)]42 clusters of the compound 1 may be antiferromagnetic coupled. Unfortunately, it is too difficult to fit the experimental magnetic data of this poly-nucleus spin system using a suitable theoretical model [11], more detailed magnetic analyses were not performed for the compound 1. The magnetic data of the sample 1 obey the Curie –Weiss law in the temperature region, and the fitting in the range 190– 300K gives values C ¼ 5.00 emu mol21 K and u ¼ 2 370 K, characteristic of an overall antiferromagnetic interaction. 3.4. The third-order NLO properties of the title compound The third-order nonlinear optical (NLO) property of the compound 1 was investigated at 532 nm with 8 ns pulses produced by a Q-switched frequencydoubled Nd:YAG laser in 6.76 £ 1025 mol/dm3 DMF solution for 1, and revealed by using a Z-scan technique. The cell being selected to place the sample is 1-mm-thick quartz one. The Z-scan result for the compound 1 is shown in Fig. 5, where the open circles were the Z-scan data measured without the aperture and the filled circles were the results obtained from the division of the closed aperture Z-scan data by the open aperture Z-scan data. The solid curve is the theoretical fit. A reasonably good fit between the experimental data and the theoretical curve was obtained, which suggests that the experimental obtained NLO effects are effectively third-order in nature. The effective a2 value of 1.1 £ 10211 mW21 and n2 value of 2 1.7 £ 10219 esu ða2 and n2 are effective third-order NLO absorptive and refractive coefficients, respectively) in 6.76 £ 1025 mol/dm3 DMF solution, were derived for the sample from the theoretical curve. In accordance with the observed a2 and n2 ; the modulus of the effective third-order
Fig. 5. Z-scan data of 6.76 £ 1025 mol dm23 of compound 1. (a) Collected under the open aperture configuration showing NLO absorption (the solid curve is a theoretical fit); (b) obtained by dividing the normalized Z-scan data obtained under the closed aperture configuration by the normalized Z-scan data in (a).
susceptibility x can be calculated by Eq. (1): lxð3Þ l ¼ ½ð9 £ 108 n20 10 cla2 =8p2 Þ2 þ ðn0 cn2 =80p2 Þ1=2 ð1Þ Where l is wavelength of the laser light, n0 is the linear refractive index of the simple ðn0 can be replaced by the one of the solvent if the concentration of the sample is very small during calculation), 10 and c are the permittivity and speed of light in vacuo, respectively. For the compound 1, the xð3Þ value was calculated to be 3.86 £ 10213 esu. The corresponding modulus of the hyperpolarizabilities g of 2.87 £ 10230 esu was
X.-B. Cui et al. / Journal of Molecular Structure 660 (2003) 131–137
got from lgl ¼ lxð3Þ l=NF 4 ðF 4 ¼ ½ðn2 þ 2Þ=34 ; n is the linear refractive index of the solvent), where N is the molecular number density of the compound in the sample and F 4 is the local Lorents field correction factor. From the discussion above, we can reasonably state that the compound 1 exhibits strong nonlinear absorption and self-defocusing performance.
Acknowledgements This work was supported by The State Key Basic Research Program of China (No. 001CB108906), NSF of China (No. 20271021, 20271050), the Ministry of Finance of China and the Talents Program of Chinese Academy of Sciences.
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Hydrothermal synthesis, crystal structure and characterization of [Zn(H2O)4]2 [H2As6V15O42(H2O)]·2H2O Xiao-Bing Cuia,b, Ji-Qing Xub,*, Lan Dingb, Hong Dingb, Ling Yeb, Guo-Yu Yanga,c a
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China b College of Chemistry and State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun, Jilin 130023, China. c State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing, Jiangsu 210093, China. Received 19 June 2003; revised 20 August 2003; accepted 20 August 2003
Abstract The compound [Zn(H2O)4]2[H2As6V15O42(H2O)]·2H2O (1) has been synthesized and characterized by elemental analysis, IR, ESR, magnetic measurement, third-order nonlinear property study and single crystal X-ray diffraction analysis. The ˚ , c ¼ 33.970(7) A ˚ , g ¼ 1208, V ¼ 4278.8(12) A ˚ 3, compound 1 crystallizes in trigonal space group R3; a ¼ b ¼ 12.0601(17) A Z ¼ 3 and R1ðwR2Þ ¼ 0.0512 (0.1171). The crystal structure is constructed from [H2As6V15O42(H2O)]42 anions and [Zn(H2O)4]2þ cations linked through hydrogen bonds into a network. The [H2As6V15O42(H2O)]62 cluster consists of 15 VO5 square pyramids linked by three As2O5 handle-like units. q 2003 Elsevier B.V. All rights reserved. Keywords: Hydrothermal synthesis; Polyoxometalate; Arsenic–vanadium cluster; Arsenic; Vanadium; Cluster; Hydrogen bond; Crystal structure
1. Introduction The chemistry of polyoxometalates continues to attract interests as a result of their realized and potential applications in fields as diverse as catalysis, analysis, biochemistry, medicine and material science [1 – 3]. Although the synthesis of such materials by rational designs remains elusive, one of the strategies used in the synthesis of polyoxometalates is to employ a specific cation as inducing * Corresponding author. Tel./fax: þ 86-591-3710051. E-mail addresses: [email protected] (G.Y. Yang), xjq@ mail.jlu.edu.cn (J.Q. Xu). 0022-2860/$ - see front matter q 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2003.08.014
agent. Practicing this strategy results in the successful synthesis of a great deal of structures. In the past few years, an important subclass of polyoxometalates is the family of arsenic – vanadium clusters with arsenic and vanadium in low oxidation states. The arsenic –vanadium clusters can be divided into two different groups based on the numbers of the As and V atoms: As8V14 clusters and As6V15 clusters. The first case is represented by anions [As8V14O42(X)]62 (X ¼ SO22 or SO22 3 4 ) [4] and compound [N(Me)4]4 [As8V14O42(H2O)0.5 [5]. The second group is exemplified by compounds K6 [As6V15O42(H2O)]·8H2O [6] and K6 [As6V15O42 (H2O)]·6H2O [7]. Here we present the synthesis and structural investigations of
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a new mixed As/V polyoxometalate, [Zn(H2O)4]2 [H2As6V15O42 (H2O)]·2H2O (1), which is the first As/V polyoxometalate containing neither alkali metals nor organic components.
2. Experimental 2.1. General procedures All reagents were purchased commercially and used without further purification. The content of vanadium, zinc and arsenic element were determined by a Perkin–Elmer Optima 3300DV spectrometer. The infrared spectrum was obtained on an Perkin–Elmer spectrophotometer in the 200 – 4000 cm21 region with pressed KBr pellet. Determination of electron spin resonance (ESR) carried out on a Bruker ER 200D-SRC diffractometer. Variable-temperature magnetic susceptibility measurements for 1 were performed on a MPMS-XL magnetometer in 2–300 K. 2.2. Hydrothermal synthesis Compound 1 was synthesized hydrothermally in 53% yield (based on As). A mixture of As2O3 (0.39 g), V2O5 (0.36 g), H2C2O4·2H2O (0.25 g), en (0.12 g), Zn(Ac)2·2H2O (0.44 g), and distilled water (21 ml) in a molar ratio of 1:1:1:1:1:600 was stirred for ca. 10 min and then transferred to a Teflon-lined reactor and heated at 160 8C for three days. H2C2O4·2H2O and en were used as reducing agents and were necessary to maintain the pH value of the reaction system. After cooling to room temperature, black block crystals were isolated. Anal. Calcd. For H24As6O53V15Zn2: As, 20.32%; V, 34.53%, Zn, 5.97%. Found: As, 20.28%; V, 34.47%, Zn, 5.90%. 2.3. X-Ray crystallography The crystal structure of the compound 1 was determined by single-crystal X-ray diffraction. Crystal data: H24As6O53V15Zn2, M ¼ 2216.55, ˚, trigonal, R3; a ¼ b ¼ 12.0601(17) A c¼ ˚ ˚ 33.970(7) A, g ¼ 1208, V ¼ 4278.8(12) A3, Z ¼ 3, Dc ¼ 2.581 g cm3, Fð000Þ ¼ 3153; A black crystal (dimensions 0.344 £ 0.317 £ 0.249 mm) was carefully chosen and mounted on a glass fiber for
data collection. The crystal data were collected on a Rigaku RAXIS-RAPID diffractometer with Mo Ka radiation at 293 K. A total of 1890 reflections were measured. The structure solution and refinement were carried out using SHELXL 97. The structure was solved using direct methods and all of the non-hydrogen atoms were located from the initial solution or from subsequent electron density difference maps during the initial stage of the refinement. All of the non-hydrogen atoms except Ow7 in the structure were refined using anisotropic thermal displacement parameters. O(8)
Table 1 Crystal data and structure refinement for 1 Empirical formula Formula weight Temperature Wavelength Crystal system
H24 As6 O53 V15 Zn2 2216.55 293(2) K ˚ 0.71073 A Trigonal
Space group ˚) a (A ˚) b (A ˚) c (A að8Þ bð8Þ gð8Þ
R3 12.0601(17) 12.0601(17) 33.970(7) 90 90 120
˚ 3) Volume (A Z rcalc (mg m23) Absorption efficient (mm21) Fð000Þ Crystal size (mm £ mm) u Range (8)
4278.8(12) 3 2.581 6.715 3153 0.344 £ 0.317 £ 0.249 4.08– 27.48
Limiting indices
211 # h # 15 211 # k # 13 243 # l # 11
Reflections collected Reflections unique Completeness to u ¼ 27:48 Refinement method Data/restraints/parameters Goodness-of-fit on F 2
1890 1650 ðrint ¼ 0:058Þ 86.2% Full-matrix least-squares on F 2 1890/7/239 1.017
Final R indices ½I . 2sðIÞ
R1 ¼ 0:0512 wR2 ¼ 0:1171
R indices (all data)
R1 ¼ 0:0569 wR2 ¼ 0:1203 0.00
Absolute structure parameter
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Table 2 Atomic coordinates ( £ 104) and equivalent isotropic displacement parameters ðA2 £ 103 Þ for compound 1
As(1) As(2) V(1) V(2) V(3) V(4) V(5) Zn(1) Zn(2) O(1) O(2) O(3) O(4) O(5) O(6) O(7)
X
Y
Z
UðeqÞ
23476(1) 1047(1) 22826(2) 2380(2) 21436(2) 21870(2) 2869(2) 3333 2333 22638(10) 114(9) 21250(10) 777(9) 2680(10) 2287(9) 23708(8)
2544(1) 5152(1) 927(2) 4000(2) 3603(2) 5310(2) 2129(2) 6667 3333 251(10) 1635(9) 6112(10) 4251(10) 3709(11) 5050(8) 4430(9)
3168(1) 2614(1) 2248(1) 3539(1) 2005(1) 3782(1) 2888(1) 4190(1) 4959(1) 1962(3) 2881(3) 4176(3) 3825(3) 1593(3) 2358(3) 3843(3)
36(1) 35(1) 29(1) 30(1) 29(1) 31(1) 33(1) 77(1) 48(1) 46(2) 46(2) 43(2) 41(2) 49(2) 35(2) 37(2)
O(8) O(9) O(10) O(11) O(12) O(13) O(14) O(15) OW1 OW2 OW3 OW4 OW5 OW6 OW7
x
y
z
UðeqÞ
1505(11) 324(17) 24002(9) 21431(8) 23116(9) 21235(9) 22607(8) 180(8) 23333 0 1666(14) 3333 24707(17) 23333 23870(120)
6599(11) 5703(18) 352(9) 2184(8) 2114(8) 2441(9) 569(8) 4046(9) 3333 0 6470(17) 6667 1616(14) 3333 22010(110)
2898(4) 2890(6) 3439(3) 3418(3) 1957(3) 2359(2) 2797(3) 2987(3) 2850(30) 4107(6) 4394(4) 3627(5) 4741(5) 5518(5) 2080(30)
25(2) 17(3) 36(2) 35(2) 34(2) 33(2) 34(2) 39(2) 190(30) 51(4) 81(4) 77(7) 91(5) 46(4) 190(40)
UðeqÞ is defined as one third of the trace of the orthogonalized Uij tensor.
is disordered over two positions O(8) and O(9) with occupancy factors 0.64 and 0.36, respectively. No hydrogen atoms were added. The absolute structure was determined and in accordance with the selected setting (Flack x parameter ¼ 0.00) [8]. The crystallographic data are given in Table 1. Atomic positional parameters and isotropic temperature factors are given in Table 2. Selected bond lengths are listed in Table 3.
3. Results and discussion 3.1. Structure of the compound 1 The single crystal analysis reveals that the compound 1 contains discrete [H2As6V15O42(H2O)]42 polyoxoanions, [Zn(H2O)4]2þ cations and two water molecules of crystallization. The [H2As6V15O42 (H2O)]42 anion consists of 15 distorted VO5 square
Table 3 ˚ ) for compound 1 Selected bond lengths (A As(1)– O(14) As(1)– O(10) As(1)– O(8)#1 As(1)– O(9)#1 As(2)– O(9) As(2)– O(15) As(2)– O(6) As(2)– O(8) V(1)–O(1) V(1)–O(12) V(1)–O(13) V(1)–O(14) V(1)–O(6)#1 V(2)–O(4)
1.754(9) 1.763(8) 1.777(13) 1.798(19) 1.630(19) 1.755(10) 1.779(9) 1.821(12) 1.627(10) 1.909(9) 1.911(10) 1.963(10) 1.984(9) 1.599(9)
V(2)–O(7)#1 V(2)–O(11) V(2)–O(15) V(2)–O(10)#2 V(3)–O(5) V(3)–O(12) V(3)–O(13) V(3)–O(12)#2 V(3)–O(6) V(4)–O(3) V(4)–O(7) V(4)–O(7)#1 V(4)–O(11)#2 V(4)–O(10)#2
1.938(9) 1.948(9) 1.985(10) 1.998(9) 1.639(10) 1.928(9) 1.952(8) 1.957(9) 1.996(9) 1.601(8) 1.931(9) 1.948(9) 1.962(10) 2.033(9)
V(5)– O(2) V(5)– O(13) V(5)– O(11) V(5)– O(14) V(5)– O(15) Zn(1)–OW4 Zn(1)–OW3 Zn(1)–OW3#3 Zn(1)–OW3#4 Zn(2)–OW6 Zn(2)–OW5#2 Zn(2)–OW5 Zn(2)–OW5#1
1.571(9) 1.933(8) 1.935(9) 2.022(9) 2.033(9) 1.910(18) 2.026(15) 2.026(16) 2.026(15) 1.899(19) 2.037(13) 2.037(13) 2.037(13)
Symmetry transformations used to generate equivalent atoms: #1 2x þ y 2 1; 2x; z; #2 2y; x 2 y þ 1; z; #3 2x þ y; 2x þ 1; z; #4 2y þ 1; x 2 y þ 1; z:
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pyramids and 6 AsO3 triangular units, with a water molecular at its center, as shown in Fig. 1. The polyoxoanion lies on a crystallographic three-fold axis, which passes through the central water molecular Ow(6). All the V atoms with square-pyramidal environments show apical vanadyl O atoms in the V – ˚ and the V –O O bond distances 1.571(9) –1.639(10) A bonds corresponding to the square-pyramidal basal ˚ , The As –O planes vary from 1.909(9) to 2.033(9) A bonds of the AsO3 units are in the range of 1.630(19) – ˚ , The 15 VO5 square pyramids are linked 1.821(12) A with one another through edges, and with AsO3 groups through vertices. Two AsO3 groups are joined together by an oxygen bridge, forming a handle-like As2O5 moiety. The 15 VO5 square pyramids linked through edges are connected to As2O5 units by sharing O atoms, and form a ball-like structure. It is worth noting that the structure 1 is an extensive three-dimensional network constructed from [Zn(H2O)4]2þ cations and [H2As6V15O42(H2O)]42 anions through hydrogen bonds which exhibit between the water molecules of the [Zn(H2O)4]2þ cations and the terminal oxygen atoms of the clusters (Fig. 2). The tetrahedral environments of the [Zn(H2O)4]2þ cations are completed by four oxygen atoms of water molecules with Zn – O distances ˚ and angles ranging from 1.899(19) to 2.037(13) A varying from 107.6 (5) to 111.3(5)8. The isolated
Fig. 1. The ball-and-stick representation of the [H2As6V15O42(H2O)]42 clusters of the compound 1.
Fig. 2. View of crystal packing of compound 1 along b-axis.
[Zn(H2O)4]2þ cations interact with the clusters through hydrogen bonds: OW4· · ·O4a ða : 1 2 y; ˚ , OW3· · ·O12b ðb : 1 þ x 2 y;zÞ : 3.077(13) A 20:33333 2 x þ y; 0:33333 2 x; 0:33333 þ zÞ : ˚ , OW4· · ·O4: 3.076(9) A ˚ , OW5· · ·O7c 3.059(17) A ˚ ðc : 2y; 1 þ x 2 y; zÞ : 3.067(20) A, OW5· · ·O1d ðd : 20:33333 2 y; 0:33333 þ x 2 y; 0:33333 þ zÞ : ˚. 3.066(24) A The assignment of the oxidation states for the V and As atoms is consistent with the electric charge and confirmed by bond valence sum calculations [9]. The BVS values for the crystallographic independent vanadium atoms V(1) –V(5) are 4.15, 4.10, 3.99, 4.09 and 4.15, respectively, indicating that the oxidation state for each V atom is þ 4. While the calculated valence sum for the crystallographic independent arsenic atoms As(1) and As(2) are 3.18 and 3.27, respectively, indicating the oxidation state of each As atom is þ 3. To our knowledge, no example of {As6V15O42} cluster containing mixedvalence vanadium atoms has been reported. In four
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known compounds containing {As6V15O42} cluster unit such as K6[As6V15O42(H2O)]·8H2O [6], K6[As6V15O42(H2O)]·6H2O [7], [Co(en)3][{Co(en)2}2As6V15O42]·4H2O [12] and [Ni(en)3]2[Ni(en)2(H2O)2] [As6V15O42]·4H2O [13], the oxidation states of all the vanadium atoms were þ 4. In the structure 1, assuming the valence of As, V, and O to be þ 3, þ 4, and 2 2, respectively, the cluster anion of [As6V15O42(H2O)] creates a net negative charge of 2 6. The presence of two [Zn(H2O)4] would account for the þ 4 charge arising from the Zinc cation. The excess negative charge of 2 2 is then balanced by the two hydrogen atoms, as given in the formula of [H2As6V15O42(H2O)] [Zn(H2O)4]2·2H2O. According to the related compounds reported previously, i.e. Na0.5H4.5 [As3Mo12O40]·4H2O [14], (Me3NH)4K2[H14Mo16 V O52].8H2O [15], K3[H12 –(AsO)2(AsO4)VIV 6 V6 O36]· 12H2O [16], (Me3NH)4(NH4)[H4PV14O42] [17], (C2 H5NO2)3.5[H4SiMo12O40]·8.5H2O [18], KH3Fe2Mo2 O10 [19], [Cu(en)2]3 [{Cu(en)2}2(H2W12 O42)]·12H2O [20], [(CH3)4N]4Na2H[a-PW11 O39]]·8H2O [21], etc. the hydrogen atoms attached to the polyoxometalate anion should be reasonable. In addition, comparing with the starting vanadium source of the above mentioned four compounds of K6 [As 6V15O42 (H2O)]·8H2O [6] (I), K6 [As6V15O42(H2O)]·6H2O [7] (II), [Co(en)3][{Co(en)2}2As6V15O42]·4H2O [12] (III) and [Ni(en) 3] 2[Ni(en) 2(H 2O) 2] [As 6V 15 O42]·4H2O [13] (IV), KVO3 for I and V2O5 for II – IV, respectively. The reducing agent is KSCN for I, and en for II – IV, respectively. The title compound was also prepared from V2O5 and used both en and H2C2O4·2H2O as reducing agents. As shown in table below, the reducing ability of the H2C2O4·2H2O is stronger than that of KSCN. So the possibility of V5þ in title compound can be excluded. Equation for half-reaction
E8 ðVÞ
(NCS)2 þ 2e2 ¼ 2NCS2 2CO2(g) þ 2Hþ þ 2e2 ¼ H2C2O4
0.77 2 0.49
135
Fig. 3. The ESR spectrum of the compound 1.
the paramagnetic signal of V4þ at 273 K with g ¼ 1:949; which is consistent with the result of valence sum calculations of compound 1. 3.3. Magnetic properties of the compound 1 The temperature dependence susceptibilities were measured in the temperature range 2 – 300 K for compound 1. Fig. 4 shows the magnetic behavior of 1 in the form of xT vs T and x21 m vs T plots. The product xT; where x is the molar magnetic susceptibility in terms of the unit formula, continuously decreased as the temperature is lowered, indicating the presence of the antiferromagnetic exchange
3.2. IR spectrum and ESR spectrum The IR spectrum of 1 exhibits intense bands at 933 and 719 cm21 attributed to y(V – O). The ESR spectrum of compound 1 (Fig. 3) shows
Fig. 4. Plots of the experimental temperature dependences of xm T (A) and x21 m (L) for compound 1.
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interactions. The room temperature value ðmeff ¼ 4:28 mb Þ is smaller than the expected value for the 15 uncoupled S ¼ 1=2 spins V4þ atoms (assuming the g ¼ 2:0 for V4þ, meff ¼ 6.71 mb Þ; indicating antiferomagnetic coupling. These antiferromagnetic coupling interactions are related to electron delocalisation of poly-nucleus systems. Previous studies indicated that electron delocalisation can favor spin pairing [10], suggesting the [H2As6V15O42(H2O)]42 clusters of the compound 1 may be antiferromagnetic coupled. Unfortunately, it is too difficult to fit the experimental magnetic data of this poly-nucleus spin system using a suitable theoretical model [11], more detailed magnetic analyses were not performed for the compound 1. The magnetic data of the sample 1 obey the Curie –Weiss law in the temperature region, and the fitting in the range 190– 300K gives values C ¼ 5.00 emu mol21 K and u ¼ 2 370 K, characteristic of an overall antiferromagnetic interaction. 3.4. The third-order NLO properties of the title compound The third-order nonlinear optical (NLO) property of the compound 1 was investigated at 532 nm with 8 ns pulses produced by a Q-switched frequencydoubled Nd:YAG laser in 6.76 £ 1025 mol/dm3 DMF solution for 1, and revealed by using a Z-scan technique. The cell being selected to place the sample is 1-mm-thick quartz one. The Z-scan result for the compound 1 is shown in Fig. 5, where the open circles were the Z-scan data measured without the aperture and the filled circles were the results obtained from the division of the closed aperture Z-scan data by the open aperture Z-scan data. The solid curve is the theoretical fit. A reasonably good fit between the experimental data and the theoretical curve was obtained, which suggests that the experimental obtained NLO effects are effectively third-order in nature. The effective a2 value of 1.1 £ 10211 mW21 and n2 value of 2 1.7 £ 10219 esu ða2 and n2 are effective third-order NLO absorptive and refractive coefficients, respectively) in 6.76 £ 1025 mol/dm3 DMF solution, were derived for the sample from the theoretical curve. In accordance with the observed a2 and n2 ; the modulus of the effective third-order
Fig. 5. Z-scan data of 6.76 £ 1025 mol dm23 of compound 1. (a) Collected under the open aperture configuration showing NLO absorption (the solid curve is a theoretical fit); (b) obtained by dividing the normalized Z-scan data obtained under the closed aperture configuration by the normalized Z-scan data in (a).
susceptibility x can be calculated by Eq. (1): lxð3Þ l ¼ ½ð9 £ 108 n20 10 cla2 =8p2 Þ2 þ ðn0 cn2 =80p2 Þ1=2 ð1Þ Where l is wavelength of the laser light, n0 is the linear refractive index of the simple ðn0 can be replaced by the one of the solvent if the concentration of the sample is very small during calculation), 10 and c are the permittivity and speed of light in vacuo, respectively. For the compound 1, the xð3Þ value was calculated to be 3.86 £ 10213 esu. The corresponding modulus of the hyperpolarizabilities g of 2.87 £ 10230 esu was
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got from lgl ¼ lxð3Þ l=NF 4 ðF 4 ¼ ½ðn2 þ 2Þ=34 ; n is the linear refractive index of the solvent), where N is the molecular number density of the compound in the sample and F 4 is the local Lorents field correction factor. From the discussion above, we can reasonably state that the compound 1 exhibits strong nonlinear absorption and self-defocusing performance.
Acknowledgements This work was supported by The State Key Basic Research Program of China (No. 001CB108906), NSF of China (No. 20271021, 20271050), the Ministry of Finance of China and the Talents Program of Chinese Academy of Sciences.
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