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Synthesis, structure, and characterization of a new sandwich-type arsenotungstocerate, [As2W18Ce3O71(H2O)3]12−
Journal of Molecular Structure 688 (2004) 33–39 www.elsevier.com/locate/molstruc
Synthesis, structure, and characterization of a new sandwich-type arsenotungstocerate, [As2W18Ce3O71(H2O)3]122 M.H. Alizadeh*, H. Eshtiagh-Hosseini, R. Khoshnavazi Department of Chemistry, Ferdowsi University, P.O. Box 1436, Mashhad 91775, Iran Received 7 July 2003; revised 22 August 2003; accepted 22 August 2003
Abstract The rational synthesis of the new sandwich-type arsenotungstocerate [As2W18Ce3O71(H2O)3]122 is reported for the first time by reaction of the trivacant lacunary species A-a-[AsW9O34]92 with appropriate CeIV. The single crystal structure analysis was carried out on K7(H3O)5[As2W18Ce3O71(H2O)3]·9H2O; H39As2Ce3K7O88W18; (2) which crystallizes in triclinic system, space group P1 with ˚ , b ¼ 17:638ð7Þ A ˚ , c ¼ 19:448ð8Þ A ˚ , a ¼ 73:643ð7Þ8; b ¼ 88:799ð7Þ8; g ¼ 88:078ð7Þ8 and Z ¼ 2: The anion consists on a ¼ 11:615ð5Þ A 92 two lacunary A-a-[AsW9O34] Keggin moieties linked via a (H2OCeO)3 belt leading to a sandwich-type structure. Each cerium atom adopts tri-capped trigonal-prismatic coordination achieved by two terminal oxygen of an edge shared paired of WO6 octahedra to each 92 A-a-AsW9O92 34 moiety and two oxygen from the belt and the cap by one m3 2 O (As, W2) to each A-a-AsW9O34 moiety and one external water ligand. The Ce – O bond lengths average in CeO6 group, Ce – O(As, W2) and Ce – O(nW) are 2.300(9), 2.887(3) and ˚ , respectively. The acid/base titration curve reveals that the anion has two different titrable protons. 2.682(5) A q 2003 Elsevier B.V. All rights reserved. Keywords: Arsenotungstocerate; Crystal structure; Sandwich-type; Tri-caped trigonal-prismatic coordination; Acid/base titration
1. Introduction Interest in the chemistry of polyoxometalates has increased enormously in past decade. This can probably be due to the richness of their structural and electronic properties and also to the broader availability of tools such as X-ray single crystal structural analysis and 183W NMR spectroscopy [1 –3]. In 1973, Weakley et al. [4] established a novel structural class of heteropolyanions by self-assembly synthesis the [P2W18Co4O68(H2O)2]102 anion. In 1986, Evans et al. [5] synthesized the [X2W18M4O68(H2O)2]102 (X ¼ As, P; M ¼ Co, Zn) anions by the same way and characterized their structures by X-ray single crystal analysis and 183W NMR spectroscopy. In 1981, however, Finke and Droege [6] and recently Wang et al. [7] reported the synthesis of the anions and other members of these families, [P2W18M4O68(H2O)2]102 (M ¼ Co, Cu, Zn) and [As2W18M4O68(H2O)2]102 (M ¼ Co, Cu, Mn, * Corresponding author. Fax: þ 98-5118438032. E-mail address: [email protected] (M.H. Alizadeh). 0022-2860/$ - see front matter q 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2003.08.021
Ni, Fe, Zn, Cd), by reaction of trivacant lacunary B-[PW9O34]92 and B-[AsW9O34]92 respectively. At present, these categories of heteropolyanions are known as Keggin-type sandwich-type polytungstates. In other hand, Knoth et al. [8,9] synthesized another sandwich-type polytungstates, [P2W18M3O68(H2O)3]122 (M ¼ Co, Cu, Fe, Ni, Mn, Pd or Zn) and [P2W18Ce3O71(H2O)2]122 and characterized their structures by single crystal analysis and 183W NMR spectroscopy. Continuing our synthesis and catalytic behavior studies on polyoxometalates [10 – 13], we explored the first sandwich-type arsenotungstates [As2W18(UO2)3O68]122, [MAs2W18(UO2)2O68]132 (M ¼ NH4, K) [14] and [Na2As2W18(UO2)2O68]122 [15] in which three or two dioxouranate groups are sandwiched between two A-a[AsW9O34]92 Keggin moieties by ‘self-assembly’ reaction of the starting material. Here, we report the synthesis of arsenotungstocerate(IV), [As2W18Ce3O71(H2O)3]122 in which a belt containing three oxygen atoms alternating with three cerium (nine-coordinate) atoms is sandwiched between two A-a-[AsW9O34]92 Keggin moieties. Its synthesis, spectroscopic characterization and single crystal
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M.H. Alizadeh et al. / Journal of Molecular Structure 688 (2004) 33–39
analysis are reported. This is the first rational synthesis in which a sandwich-type arsenotungstate is synthesized by reaction of the ligand (A-a-[AsW9O34]92) and metal cation.
2. Experimental 2.1. Chemical and apparatus All chemicals were regent grade and used without further purification. Infrared spectra were recorded as KBr disks on a Buck 500 Scientific spectrophotometer. The electronic spectra were obtained with a Shimadzu UV-160A UV-visible Recording Spectrophotometer. 2.2. Compounds preparation 2.2.1. A-a-Na8H[AsW9O34]·x H2O This preparation was done according to literature, [7] unless we used Na2HAsO4·7H2O instead of H3AsO4. First Na2WO4·2H2O (29.7 g, 90 mmol) were dissolved in 40 ml of water with stirring. Then Na2HAsO4·7H2O (3.15 g, 10 mmol) and, after dissolving, glacial acetic acid (7.0 ml) were added. After a few second the solution became cloudy and precipitation was completed after 5 min. The solid was collected by suction and air-dried. This product is suggested to be predominantly A-a-Na8H[AsW9O34] [Fig. 1]. 2.2.2. K9NH4H2[As2W18Ce3O71(H2O)3]·22H2O (1) Ceric ammonium sulfate, (NH4)4Ce(SO4)4·2H2O (12.0 g, 14 mmol) was added to slurry of Na8H[AsW9O34]·x H2O (30 g, 11 mmol) in 200 ml water. The solution became clear after a few minutes. The solution was filtered off and then refluxed for 2 h. Addition of solid KCl (20 g) led to a pale yellow product, which was filtered off after 15 min and recrystallized at least twice from hot water. The light yellow crystals were obtained overnight. Yield 10.0 g (31.3%) Anal. Calc.: As, 2.57; Ce, 7.20; K, 6.02; W, 56.64; H2O, 6.90; NH4, 0.31. Found: As, 2.25; Ce, 6.50; K, 5.83; W, 55.85; H2O, 6.80%. 2.2.3. K7H5[As2W18Ce3O71(H2O)3]·14H2O (2) Five grams of 1 were dissolved in 20 ml warm water and passed through a column containing Potassium salt of strongly acidic ion exchange resin. Two-three grams of KCl was added to the effluent; the precipitated solid was refluxed in water for 1 h. The product was obtained overnight, which filtered off and re crystallized from warm water. The yellow crystals suitable for X-ray crystallography were obtained overnight. Yield 3.2 g (64%). Anal. Calc.: As, 2.62; Ce, 7.36; K, 6.84; W, 57.91; H2O, 4.41%. Found: As, 2.34; Ce, 7.12; K, 6.50; W, 57.45; H2O, 4.30%.
Fig. 1. IR spectra (metal-oxygen stretching region) of (a) Na8HAsW9O34·x H2O (b) 4 (c) 3 (d) 2 (e) 1.
2.2.4. K12[As2W18Ce3O71(H2O)3]·20H2O (3) One gram of 1 was dissolved in 10 ml water. The pH was raised to 8.14 from 4.45 with 0.5 M KOH. The yellow color of the solution faded predominately; addition of solid KCl (0.7 g) led to a cream-colored precipitation, which was re crystallized from water. Cream-colored crystalline powder was obtained overnight. Yield: 0.5 g (50%) Anal. Calc.: As, 2.54; Ce, 7.13; K, 7.95; W, 56.10; H2O, 6.10%. Found: As, 2.24; Ce, 6.78; K, 8.10; W, 55.60; H2O, 5.80%. 2.2.5. (NH4)10H2[As2W18Ce3O71(H2O)3]·27H2O (4) The synthetic procedure of 1 was followed using NH4Cl (25 g) instead of KCl. Yield: 8.9 g (28%). Anal. Calc.: As, 2.61; Ce, 7.33; W, 57.68; (cation content þ H2O), 12.54%. Found: As, 2.39; Ce, 7.00; W, 57.00; (cation content þ H2O), 12.34%. 2.3. X-ray crystallography A light yellow prism of 2 with dimensions 0.11 £ 0.15 £ 0.18 mm3 was used for data collection at 110(2) K on a Bruker SMART 1000 CCD single crystal diffractometer with graphite-monochromated Mo Ka Of the 40,871 unique reflections radiation ðl ¼ 0:71073 AÞ:
M.H. Alizadeh et al. / Journal of Molecular Structure 688 (2004) 33–39
ð2umax ¼ 50:068Þ; 13,446 reflections ðRint ¼ 0:0797Þ were considered observed ðI . 2sðIÞÞ: The final cycle of refinement converged at R ¼ 0:055 and Rw ¼ 0:1297 ðI . 2sðIÞÞ: Crystallographic data and structure refinement parameters are listed in Table 1, atomic coordinates in Table 2 and selected atomic distances and angles in Table 3. Intensities were corrected for Lorenz-polarization effects, and the SADABS program [16] was applied for absorption. The structure was refined with SHELEX -97 [17] by full-matrix Least-squares of F2. In the final differences map the deepest hole and the highest peak were 2 2.996 and ˚ 23 respectively. The unit cell 2 is shown in [Fig. 2]. 6.144 e A
3. Result and discussion 3.1. Structure The novel arsenotungstocerate [As2W18Ce3O71(H2O)3]122 consists of two lacunary A-a-[AsW9O34]92 Keggin moieties linked via a belt containing three oxygen atoms alternating with three cerium atoms leading to a sandwich-type structure (Fig. 3). Although the synthetic procedure is similar to that phosphorous analogue [9], [P2W18Ce3O71(H2O)2]122 however, the structures have some different. The crystal structure of [P2W18Ce3O71(H2O)2]122 reveals that three cerium atoms have dissimilar coordination (two seven- and one six-) and only two cerium atoms have external water ligands. But crystal structure of [As2W18Ce3O71(H2O)3]122 shows that Table 1 Crystal data and structure refinement for 2 Empirical formula Formula weight Temperature (K) ˚) Wavelength (A Crystal system Space group Unit cell dimensions ˚) a (A ˚) b (A ˚) c (A a (8) b (8) g (8) Volume Z Density Absorption coefficient F(000) Crystal size Reflections collected Goodness-of-fit on F 2 Final R (I . 2s(I)) R indices (all data)
H39As2Ce3K7O88W18 5600.51 110(2) 0.71073 Triclinic P1 11.615(5) 17.638(7) 19.448(8) 73.643(7) 88.799(7) 88.078(7) ˚ 3821(3) A 2 4.868 mg m23 30.089 mm21 4896 0.11 £ 0.15 £ 0.18 mm3 40,871 1.057 R1 ¼ 0:0555 wR2 ¼ 0.1297 R1 ¼ 0:0808 wR2 ¼ 0.1403
R1 ¼ SlF0 l 2 lFc ll=SF0 l; wR2 ¼ ½S½wðF02 2 Fc2 Þ2 =S½wðF02 Þ2 1=2 :
35
Table 2 Atomic coordinates ( £ 104) and equivalent isotropic displacement ˚ 2 £ 103) for 2 parameter (A Atom
x
y
z
U(eq)
W(1) W(2) W(3) W(4) W(5) W(6) W(7) W(8) W(9) W(10) W(11) W(12) W(13) W(14) W(15) W(16) W(17) W(18) Ce(1) Ce(2) Ce(3) As(1) As(2) O(1) O(2) O(3) O(4) O(5) O(6) O(7) O(8) O(9) O(10) O(11) O(12) O(13) O(14) O(15) O(16)
7455(1) 7636(1) 5022(1) 6764(1) 9397(1) 9650(1) 7253(1) 4341(1) 4103(1) 8454(1) 8691(1) 6042(1) 7308(1) 9925(1) 10,177(1) 7790(1) 4862(1) 4646(1) 8085(1) 8320(1) 5116(1) 6925(2) 7460(2) 7981(13) 6075(11) 5938(12) 6736(13) 8738(13) 8934(11) 7068(12) 4702(11) 4527(11) 8417(13) 9773(11) 8874(12) 5708(11) 3435(12) 5339(12) 7079(13)
21289(1) 2552(1) 2685(1) 33(1) 157(1) 997(1) 1596(1) 1445(1) 746(1) 5637(1) 6359(1) 6259(1) 3642(1) 3739(1) 4543(1) 5181(1) 5061(1) 4331(1) 2020(1) 3001(1) 2836(1) 833(1) 4419(1) 21345(9) 2891(10) 21486(8) 2794(8) 2698(8) 221(9) 439(8) 305(8) 2255(8) 2243(9) 624(8) 1309(9) 1631(9) 1036(10) 407(8) 962(9)
3087(1) 1270(1) 2126(1) 4226(1) 3428(1) 1404(1) 392(1) 1321(1) 3122(1) 3727(1) 1914(1) 2715(1) 4561(1) 3730(1) 1708(1) 707(1) 1622(1) 3438(1) 3716(1) 1336(1) 2394(1) 2313(1) 2634(1) 2156(8) 1404(9) 2798(8) 3704(8) 3089(9) 1451(8) 701(8) 1455(7) 2851(8) 4244(8) 2417(7) 482(8) 739(8) 2180(8) 3779(8) 4404(9)
30(1) 30(1) 28(1) 24(1) 25(1) 24(1) 24(1) 23(1) 24(1) 23(1) 22(1) 26(1) 20(1) 21(1) 21(1) 21(1) 22(1) 20(1) 24(1) 25(1) 25(1) 15(1) 14(1) 40(4) 37(4) 32(4) 30(4) 36(4) 28(3) 28(3) 23(3) 26(3) 35(4) 22(3) 33(4) 31(4) 36(4) 29(3) 35(4)
O(17) O(18) O(19) O(20) O(21) O(22) O(23) O(24) O(25) O(26) O(27) O(28) O(29) O(30) O(31) O(32) O(33) O(34) O(35) O(36)
9470(12) 9752(11) 7492(12) 4296(11) 4077(11) 7883(13) 8111(13) 3875(13) 6345(13) 10,659(12) 11,016(12) 7052(12) 3305(12) 2923(13) 6784(11) 7583(13) 7715(12) 5582(10) 9080(11) 7222(13)
1124(8) 2041(9) 2588(9) 2427(8) 1769(9) 22216(9) 21003(9) 21223(9) 2568(8) 2295(9) 794(9) 1746(9) 1546(9) 375(10) 2101(8) 770(8) 1366(9) 1265(8) 6365(8) 6852(8)
3604(8) 29(3) 1343(7) 27(3) 411(7) 29(3) 1459(7) 23(3) 3121(7) 29(3) 3665(9) 39(4) 635(9) 37(4) 1999(8) 35(4) 5046(8) 33(4) 3763(8) 32(4) 1118(9) 34(4) 2500(8) 33(4) 660(8) 34(4) 3646(8) 35(4) 2211(8) 24(3) 3101(7) 29(3) 1628(7) 28(3) 2294(7) 19(3) 2890(8) 29(3) 2069(7) 32(4) (continued on next page)
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M.H. Alizadeh et al. / Journal of Molecular Structure 688 (2004) 33–39 Table 3 ˚ ), angles (8) and valence bond values for 2 Selected bond distances (A
Table 2 (continued) Atom
x
y
z
U(eq)
O(37) O(38) O(39) O(40) O(41) O(42) O(43) O(44) O(45) O(46) O(47) O(48) O(49) O(50) O(51) O(52) O(53) O(54) O(55) O(56) O(57) O(58) O(59) O(60) O(61) O(62) O(63) O(64) O(65) O(66) O(67) O(68) O(69) O(70) O(71) O(1W) O(2W) O(3W)
7027(12) 7549(11) 9576(11) 9800(11) 7923(11) 5524(11) 5371(12) 8981(12) 10,247(11) 9455(12) 6210(10) 3994(11) 5850(11) 7304(12 9719(11) 10,022(11) 7799(12) 4576(12) 4412(12) 9054(13) 9453(12) 5041(13) 7015(13) 11,255(12) 11,589(11 7662(12) 3886(11) 3543(11) 7612(12) 7983(11) 8162(11) 6058(11) 8703(13) 6584(12) 6404(12) 9196(13) 9653(13) 2816(12)
6298(9) 4836(8) 4886(7) 5552(7) 6006(8) 5923(8) 5366(9) 3620(7) 4048(8) 5130(8) 5145(8) 4886(8) 3908(8) 2624(8) 2706(8) 3609(8) 4206(8) 4089(8) 3380(8) 5983(8) 7157(9) 6987(8) 3597(8) 3728(9) 4730(8) 5807(7) 5638(8) 4438(8) 5344(8) 3756(7) 4355(7) 4256(7) 2536(8) 2979(9) 2348(8) 1473(10) 3387(9) 3175(9)
3499(8) 4258(7) 3623(7) 1967(7) 1223(7) 1941(7) 3320(8) 4590(7) 2739(7) 815(7) 1035(7) 2507(7) 4123(7 4566(7) 3792(7) 1528(8) 568(7) 1582(7) 3327(7) 4384(8) 1432(9) 2734(9) 5466(8) 4087(8) 1532(7) 2147(8) 1019(7) 4017(7) 2724(7) 3382(7) 1896(7) 2559(7) 2540(7) 1695(9) 3267(8) 4924(8) 136(8) 2316(8)
32(4) 22(3) 21(3) 21(3) 22(3) 25(3) 31(4) 24(3) 24(3) 25(3) 21(3) 23(3) 21(3) 27(3) 22(3) 25(3) 25(3) 29(4) 26(3) 31(4) 38(4) 35(4) 31(4) 31(4) 23(3) 32(4) 25(3) 22(3) 24(3) 19(3) 18(3) 20(3) 30(4) 36(4) 29(4) 39(4) 36(4) 31(4)
Valence bond values
the cerium atoms have similar coordination and each cerium atom also has an external water ligand. Each cerium atom adopts tri-capped trigonal-prismatic coordination, the local 3-fold axis of the CeO6 group lying into the Ce3O3 plane. Trigonal-prismatic achieved by two the terminal oxygen of an edge shared paired of WO6 octahedra to each A-a-[AsW9O34]92 moiety and two oxygen from the belt. The cap achieved by one m3-O (As, W2) to each A-a-[AsW9O34]92 moiety and one external water ligand. The algorithm derived by Brown and Altermatt [18] from bond lengths stored in the Inorganic Crystal Database indicates that O(As, W2) atoms, oxygen atoms linked to one As and two W atoms, have valance sums in the range (1.773 – 1.877) average 1.818(4), and presumably therefore have residual bonding capacity. By contrast the valance sums for O(As, W3), which are linked to one As and three W atoms, are 2.002(2). The valance bond sums average for As, Ce and W are 4.868 (1), 3.932(1), and 6.078(1) respectively. The Ce –O
W 2 Od W– Ob W– Oc W– Od(Ce) W– O(As,W3) W– O(As,W2) As–O Ce –Od Ce –Obelt Ce –O(As,W2) Ce –O(nW) Ce1· · ·Ce2 Ce1· · ·Ce3 Ce2· · ·Ce3 Ce1 – Ce2 – Ce3 Ce2 – Ce1 – Ce3 Ce1 – Ce3 – Ce2 O32 –Ce1 –O66 O71 –Ce1 –O(1W) O71 –Ce1 –O17 O71 –Ce1 –O16 O71 –Ce1 –O32 O17 –Ce1 –O51 O17 –Ce1 –O50 O17 –Ce1 –O66 Ce1 – O71 –Ce3 Ce2 – O70 –Ce3 Ce2 – O69 –Ce1
1.724(2) 1.924(6) 1.938(2) 1.806(2) 2.386(2) 2.377(2) 1.695(2) 2.345(4) 2.212(9) 2.887(3) 2.682(5) 4.468(3) 4.303(2) 4.193(2) 59.49(3) 57.08(3) 63.44(4) 141.1(4) 143.7(5) 134.2(6) 81.0(5) 76.4(5) 79.8(5) 140.8(5) 134.0(4) 156.3(8) 160.8(9) 150.6(7)
1.687(9) 0.983(1) 0.941(4) 1.353(7) 0.286(3) 0.290(2) 1.217(7) 0.505(6) 0.727(19) 0.116(1) 0.203(3)
Od, Ob and Oc show terminal, corner shared and edge shared oxygen atoms, respectively.
bond lengths in CeO 6 group, Ce – O(As, W 2 ) and ˚ average Ce – O(nW) are in the range 2.117 –2.400(14) A ˚ ˚ ˚ and 2.300(9) A, 2.865(14) –2.944(13) A average 2.887(3) A ˚ ˚ 2.624(15) – 2.716(14) A average 2.682(5) A respectively (Table 3). Structural different of [As2W18Ce3O71(H2O)3]122 and phosphorous analogue can be attributed to different of bond lengths of As – O and P –O in AsO4 and PO4 in the compounds, respectively. The bond lengths average of ˚ ) is about 11% longer than that of the As – O (1.688(2) A ˚ P – O (1.499(3) A) [9]. Bond length longer in AsO4 makes available condition for coordination of m3-O (As, W2) to cerium atoms. 3.2. IR spectra IR spectra of [As2W18Ce3O71(H2O)3]122 with that of A-a-[AsW9O34]92 are shown in Fig. 4. As expected, IR spectra of Keggin-like sandwich-type of [X2W18M4O68(H2O)2]n2 (X ¼ As, P, Si; M ¼ Co, Cu, Mn, Ni, Fe, Zn, Cd) and those of [P2W18M3O68(H2O)3]122 (M ¼ Co, Cu, Fe, Ni, Mn, Pd, Zn) are similar to those of Keggin and the trivacant lacunary structures, respectively [8,10 –11,19]. IR spectra of [As2W18Ce3O71(H2O)3]122 anion are very
M.H. Alizadeh et al. / Journal of Molecular Structure 688 (2004) 33–39
37
Fig. 2. Unit cell of 2.
similar to that of A-a-[AsW9O34]92. The following points can be drawn from of IR spectra: (i)
All of the characteristic vibrational frequencies increased compared with those of A-a-AsW9O92 34 ,
which is attributed to the decrease of negative charges per the A-a-AsW9O92 34 moiety in the anion [20]. (ii) The asymmetry stretching vibration frequency of W –Oc –W in the anion is more up shifted (40 cm21) and also its intensity amplified, which are attributed to
Fig. 3. Structure of 2.
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M.H. Alizadeh et al. / Journal of Molecular Structure 688 (2004) 33–39
Fig. 4. UV-visible spectrum of 2 (1 £ 1025 M). Inset: absorption spectrum of 2 (1.3 £ 1023 M).
the present of cerium cation (directly bound to the terminal Oxygen of an edge shared pair of WO6) and duplication of A-a-[AsW9O34]92 in the anion. (iii) The vibrational frequency of As – O bonds overlaps those W –Ob – W bonds [21]. 3.3. Electronic spectra The UV-visible spectra of 2 have three absorption bonds (Fig. 5). Two the higher energy bonds at ca. 208 and 260 nm are attributed to charge transfer ðOc =Ob Þ ! W and Od ! W respectively [1]. The bond at 359 nm with 1 ¼ 1750 M 21 cm12 is attributed to charge transfer O ! Ce. 3.4. Acid/base titration The acid/base titration curves of 1 and 2 show two break points at 1.0 and 2.0 equivalent of added base. It shows that the anion has two different titrable protons. Whereas the lacunary A-a-[AsW9O34]92anion, [MAs2W18U2O72]132
(M ¼ K, NH4) (pH 7) and [As2W18U3O74]122 (pH 5) [14] anions are unstable in solution, 1 and 2 have an unprecedented range of pH stability. The anions appear to be stable between pH 2.5 and 9. When 2 is titrated with NaOH up to pH 11.2, large amount of the anion can be isolated as 3 by precipitating with KCl and then re crystallization.
4. Conclusion A new example of polyoxometalate complexes incorporating nine-coordinate (tri-capped trigonal-prismatic) hetero atoms has been prepared from the reaction of A-a-Na8H[AsW9O34] with (NH4)4Ce(SO4)4. X-Ray single crystal analysis of K7(H3O)5[As2W18Ce3O71(H2O)3]·9H2O confirms the structure and shows that it has some different with that phosphorous analogue, K9(H3O)3[P2W18Ce3O71(H2O)2], in which is attributed to longer As –O bond in compared to P– O bond.
Acknowledgements We thank Prof. I. David Brown, Department of Physics and Astronomy McMaster University Hamilton, for his advice on valance bond calculation.
References
Fig. 5. Potentiometric titration of a 3.57 £ 1023 M solution of 2.
[1] M.T. Pope, Heteropoly and Isopoly Oxometalates, Springer, Berlin, 1983. [2] M.T. Pope, A. Mu¨ller, Int. Ed. Engl. 30 (1991) 34. [3] Hill, C., Guest Ed. Polyoxomeralates, Chem. Rev. 98, 1. [4] T.J.R. Weakley, H.T. Evans, J.S. Showell, G.F. Tourne´, C.M. Tourne´, J. Chem. Soc., Chem. Commun. (1973) 139. [5] H.T. Evans, C.M. Tourne´, G.F. Tourne´, T.J.R. Weakley, J. Chem. Soc., Dalton Trans. (1986) 2699. [6] R.G. Finke, M.W. Droege, J. Am. Chem. Soc. 103 (1981) 1587.
M.H. Alizadeh et al. / Journal of Molecular Structure 688 (2004) 33–39 [7] L.H. Bi, R.D. Hung, J. Peng, E.B. Wang, Y.H. Wang, C.W. Hu, J. Chem. Soc., Dalton Trans. (2001) 121. [8] W.H. Knoth, P.J. Domaille, R.D. Farlee, Organometallics 4 (1) (1985) 62. [9] W.H. Knoth, P.J. Domaille, R.L. Harlow, Inorg. Chem. 25 (1986) 1577. [10] M.H. Alizadeh, H. Razavi, F. Mohammadi Zonoz, M.R. Mohammadi, Polyhedron 22 (2003) 933. [11] M.H. Alizadeh, H. Razavi, F.F. Bamoharram, K. Daneshvar, J. Mol. Catal. A Chemical (2003) in press. [12] M.H. Alizadeh, H. Razavi, F.F. Bamoharram, M. Hassanzadeh, R. Khoshnavazi, F. Mohammadi Zonoz, Kinetic Catal. (2003) 4. [13] M.H. Alizadeh, F. Mohammadi Zonoz, J. Mol. Struct. (2003) in press.
39
[14] M.H. Alizadeh, H. Eshtiagh-Hosseini, R. Khoshnavazi, M.T. Pope, Inorg. Chem., submitted for publication. [15] R. Khoshnavazi. PhD Thesis. Ferdowsi University, Mashhad, 2003. [16] G.M. Sheldrick, SADABS v. 2.01, Bruker/Siemens Area Detector Absorption Correction Program, Bruker AXS, Madison, WI, USA, 1998. [17] G.M. Sheldrick, SHELEX v. 5.10, Structure Determination software Suit, Bruker AXS, Madison, WI, USA, 1998. [18] I.D. Brown, D. Altermatt, Acta Crystallogr. Sect. B 41 (1985) 244. [19] R.G. Finke, M.W. Droege, P.J. Domaille, Inorg. Chem. 26 (1987 3886. [20] R. Contant, R. Thouvenot, Can. J. Chem. 69 (1991) 1498. [21] Rocchiccioli-Deltcheff, M. Fournier, R. Franck, Inorg. Chem. 22 (1983) 207.
Synthesis, structure, and characterization of a new sandwich-type arsenotungstocerate, [As2W18Ce3O71(H2O)3]122 M.H. Alizadeh*, H. Eshtiagh-Hosseini, R. Khoshnavazi Department of Chemistry, Ferdowsi University, P.O. Box 1436, Mashhad 91775, Iran Received 7 July 2003; revised 22 August 2003; accepted 22 August 2003
Abstract The rational synthesis of the new sandwich-type arsenotungstocerate [As2W18Ce3O71(H2O)3]122 is reported for the first time by reaction of the trivacant lacunary species A-a-[AsW9O34]92 with appropriate CeIV. The single crystal structure analysis was carried out on K7(H3O)5[As2W18Ce3O71(H2O)3]·9H2O; H39As2Ce3K7O88W18; (2) which crystallizes in triclinic system, space group P1 with ˚ , b ¼ 17:638ð7Þ A ˚ , c ¼ 19:448ð8Þ A ˚ , a ¼ 73:643ð7Þ8; b ¼ 88:799ð7Þ8; g ¼ 88:078ð7Þ8 and Z ¼ 2: The anion consists on a ¼ 11:615ð5Þ A 92 two lacunary A-a-[AsW9O34] Keggin moieties linked via a (H2OCeO)3 belt leading to a sandwich-type structure. Each cerium atom adopts tri-capped trigonal-prismatic coordination achieved by two terminal oxygen of an edge shared paired of WO6 octahedra to each 92 A-a-AsW9O92 34 moiety and two oxygen from the belt and the cap by one m3 2 O (As, W2) to each A-a-AsW9O34 moiety and one external water ligand. The Ce – O bond lengths average in CeO6 group, Ce – O(As, W2) and Ce – O(nW) are 2.300(9), 2.887(3) and ˚ , respectively. The acid/base titration curve reveals that the anion has two different titrable protons. 2.682(5) A q 2003 Elsevier B.V. All rights reserved. Keywords: Arsenotungstocerate; Crystal structure; Sandwich-type; Tri-caped trigonal-prismatic coordination; Acid/base titration
1. Introduction Interest in the chemistry of polyoxometalates has increased enormously in past decade. This can probably be due to the richness of their structural and electronic properties and also to the broader availability of tools such as X-ray single crystal structural analysis and 183W NMR spectroscopy [1 –3]. In 1973, Weakley et al. [4] established a novel structural class of heteropolyanions by self-assembly synthesis the [P2W18Co4O68(H2O)2]102 anion. In 1986, Evans et al. [5] synthesized the [X2W18M4O68(H2O)2]102 (X ¼ As, P; M ¼ Co, Zn) anions by the same way and characterized their structures by X-ray single crystal analysis and 183W NMR spectroscopy. In 1981, however, Finke and Droege [6] and recently Wang et al. [7] reported the synthesis of the anions and other members of these families, [P2W18M4O68(H2O)2]102 (M ¼ Co, Cu, Zn) and [As2W18M4O68(H2O)2]102 (M ¼ Co, Cu, Mn, * Corresponding author. Fax: þ 98-5118438032. E-mail address: [email protected] (M.H. Alizadeh). 0022-2860/$ - see front matter q 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2003.08.021
Ni, Fe, Zn, Cd), by reaction of trivacant lacunary B-[PW9O34]92 and B-[AsW9O34]92 respectively. At present, these categories of heteropolyanions are known as Keggin-type sandwich-type polytungstates. In other hand, Knoth et al. [8,9] synthesized another sandwich-type polytungstates, [P2W18M3O68(H2O)3]122 (M ¼ Co, Cu, Fe, Ni, Mn, Pd or Zn) and [P2W18Ce3O71(H2O)2]122 and characterized their structures by single crystal analysis and 183W NMR spectroscopy. Continuing our synthesis and catalytic behavior studies on polyoxometalates [10 – 13], we explored the first sandwich-type arsenotungstates [As2W18(UO2)3O68]122, [MAs2W18(UO2)2O68]132 (M ¼ NH4, K) [14] and [Na2As2W18(UO2)2O68]122 [15] in which three or two dioxouranate groups are sandwiched between two A-a[AsW9O34]92 Keggin moieties by ‘self-assembly’ reaction of the starting material. Here, we report the synthesis of arsenotungstocerate(IV), [As2W18Ce3O71(H2O)3]122 in which a belt containing three oxygen atoms alternating with three cerium (nine-coordinate) atoms is sandwiched between two A-a-[AsW9O34]92 Keggin moieties. Its synthesis, spectroscopic characterization and single crystal
34
M.H. Alizadeh et al. / Journal of Molecular Structure 688 (2004) 33–39
analysis are reported. This is the first rational synthesis in which a sandwich-type arsenotungstate is synthesized by reaction of the ligand (A-a-[AsW9O34]92) and metal cation.
2. Experimental 2.1. Chemical and apparatus All chemicals were regent grade and used without further purification. Infrared spectra were recorded as KBr disks on a Buck 500 Scientific spectrophotometer. The electronic spectra were obtained with a Shimadzu UV-160A UV-visible Recording Spectrophotometer. 2.2. Compounds preparation 2.2.1. A-a-Na8H[AsW9O34]·x H2O This preparation was done according to literature, [7] unless we used Na2HAsO4·7H2O instead of H3AsO4. First Na2WO4·2H2O (29.7 g, 90 mmol) were dissolved in 40 ml of water with stirring. Then Na2HAsO4·7H2O (3.15 g, 10 mmol) and, after dissolving, glacial acetic acid (7.0 ml) were added. After a few second the solution became cloudy and precipitation was completed after 5 min. The solid was collected by suction and air-dried. This product is suggested to be predominantly A-a-Na8H[AsW9O34] [Fig. 1]. 2.2.2. K9NH4H2[As2W18Ce3O71(H2O)3]·22H2O (1) Ceric ammonium sulfate, (NH4)4Ce(SO4)4·2H2O (12.0 g, 14 mmol) was added to slurry of Na8H[AsW9O34]·x H2O (30 g, 11 mmol) in 200 ml water. The solution became clear after a few minutes. The solution was filtered off and then refluxed for 2 h. Addition of solid KCl (20 g) led to a pale yellow product, which was filtered off after 15 min and recrystallized at least twice from hot water. The light yellow crystals were obtained overnight. Yield 10.0 g (31.3%) Anal. Calc.: As, 2.57; Ce, 7.20; K, 6.02; W, 56.64; H2O, 6.90; NH4, 0.31. Found: As, 2.25; Ce, 6.50; K, 5.83; W, 55.85; H2O, 6.80%. 2.2.3. K7H5[As2W18Ce3O71(H2O)3]·14H2O (2) Five grams of 1 were dissolved in 20 ml warm water and passed through a column containing Potassium salt of strongly acidic ion exchange resin. Two-three grams of KCl was added to the effluent; the precipitated solid was refluxed in water for 1 h. The product was obtained overnight, which filtered off and re crystallized from warm water. The yellow crystals suitable for X-ray crystallography were obtained overnight. Yield 3.2 g (64%). Anal. Calc.: As, 2.62; Ce, 7.36; K, 6.84; W, 57.91; H2O, 4.41%. Found: As, 2.34; Ce, 7.12; K, 6.50; W, 57.45; H2O, 4.30%.
Fig. 1. IR spectra (metal-oxygen stretching region) of (a) Na8HAsW9O34·x H2O (b) 4 (c) 3 (d) 2 (e) 1.
2.2.4. K12[As2W18Ce3O71(H2O)3]·20H2O (3) One gram of 1 was dissolved in 10 ml water. The pH was raised to 8.14 from 4.45 with 0.5 M KOH. The yellow color of the solution faded predominately; addition of solid KCl (0.7 g) led to a cream-colored precipitation, which was re crystallized from water. Cream-colored crystalline powder was obtained overnight. Yield: 0.5 g (50%) Anal. Calc.: As, 2.54; Ce, 7.13; K, 7.95; W, 56.10; H2O, 6.10%. Found: As, 2.24; Ce, 6.78; K, 8.10; W, 55.60; H2O, 5.80%. 2.2.5. (NH4)10H2[As2W18Ce3O71(H2O)3]·27H2O (4) The synthetic procedure of 1 was followed using NH4Cl (25 g) instead of KCl. Yield: 8.9 g (28%). Anal. Calc.: As, 2.61; Ce, 7.33; W, 57.68; (cation content þ H2O), 12.54%. Found: As, 2.39; Ce, 7.00; W, 57.00; (cation content þ H2O), 12.34%. 2.3. X-ray crystallography A light yellow prism of 2 with dimensions 0.11 £ 0.15 £ 0.18 mm3 was used for data collection at 110(2) K on a Bruker SMART 1000 CCD single crystal diffractometer with graphite-monochromated Mo Ka Of the 40,871 unique reflections radiation ðl ¼ 0:71073 AÞ:
M.H. Alizadeh et al. / Journal of Molecular Structure 688 (2004) 33–39
ð2umax ¼ 50:068Þ; 13,446 reflections ðRint ¼ 0:0797Þ were considered observed ðI . 2sðIÞÞ: The final cycle of refinement converged at R ¼ 0:055 and Rw ¼ 0:1297 ðI . 2sðIÞÞ: Crystallographic data and structure refinement parameters are listed in Table 1, atomic coordinates in Table 2 and selected atomic distances and angles in Table 3. Intensities were corrected for Lorenz-polarization effects, and the SADABS program [16] was applied for absorption. The structure was refined with SHELEX -97 [17] by full-matrix Least-squares of F2. In the final differences map the deepest hole and the highest peak were 2 2.996 and ˚ 23 respectively. The unit cell 2 is shown in [Fig. 2]. 6.144 e A
3. Result and discussion 3.1. Structure The novel arsenotungstocerate [As2W18Ce3O71(H2O)3]122 consists of two lacunary A-a-[AsW9O34]92 Keggin moieties linked via a belt containing three oxygen atoms alternating with three cerium atoms leading to a sandwich-type structure (Fig. 3). Although the synthetic procedure is similar to that phosphorous analogue [9], [P2W18Ce3O71(H2O)2]122 however, the structures have some different. The crystal structure of [P2W18Ce3O71(H2O)2]122 reveals that three cerium atoms have dissimilar coordination (two seven- and one six-) and only two cerium atoms have external water ligands. But crystal structure of [As2W18Ce3O71(H2O)3]122 shows that Table 1 Crystal data and structure refinement for 2 Empirical formula Formula weight Temperature (K) ˚) Wavelength (A Crystal system Space group Unit cell dimensions ˚) a (A ˚) b (A ˚) c (A a (8) b (8) g (8) Volume Z Density Absorption coefficient F(000) Crystal size Reflections collected Goodness-of-fit on F 2 Final R (I . 2s(I)) R indices (all data)
H39As2Ce3K7O88W18 5600.51 110(2) 0.71073 Triclinic P1 11.615(5) 17.638(7) 19.448(8) 73.643(7) 88.799(7) 88.078(7) ˚ 3821(3) A 2 4.868 mg m23 30.089 mm21 4896 0.11 £ 0.15 £ 0.18 mm3 40,871 1.057 R1 ¼ 0:0555 wR2 ¼ 0.1297 R1 ¼ 0:0808 wR2 ¼ 0.1403
R1 ¼ SlF0 l 2 lFc ll=SF0 l; wR2 ¼ ½S½wðF02 2 Fc2 Þ2 =S½wðF02 Þ2 1=2 :
35
Table 2 Atomic coordinates ( £ 104) and equivalent isotropic displacement ˚ 2 £ 103) for 2 parameter (A Atom
x
y
z
U(eq)
W(1) W(2) W(3) W(4) W(5) W(6) W(7) W(8) W(9) W(10) W(11) W(12) W(13) W(14) W(15) W(16) W(17) W(18) Ce(1) Ce(2) Ce(3) As(1) As(2) O(1) O(2) O(3) O(4) O(5) O(6) O(7) O(8) O(9) O(10) O(11) O(12) O(13) O(14) O(15) O(16)
7455(1) 7636(1) 5022(1) 6764(1) 9397(1) 9650(1) 7253(1) 4341(1) 4103(1) 8454(1) 8691(1) 6042(1) 7308(1) 9925(1) 10,177(1) 7790(1) 4862(1) 4646(1) 8085(1) 8320(1) 5116(1) 6925(2) 7460(2) 7981(13) 6075(11) 5938(12) 6736(13) 8738(13) 8934(11) 7068(12) 4702(11) 4527(11) 8417(13) 9773(11) 8874(12) 5708(11) 3435(12) 5339(12) 7079(13)
21289(1) 2552(1) 2685(1) 33(1) 157(1) 997(1) 1596(1) 1445(1) 746(1) 5637(1) 6359(1) 6259(1) 3642(1) 3739(1) 4543(1) 5181(1) 5061(1) 4331(1) 2020(1) 3001(1) 2836(1) 833(1) 4419(1) 21345(9) 2891(10) 21486(8) 2794(8) 2698(8) 221(9) 439(8) 305(8) 2255(8) 2243(9) 624(8) 1309(9) 1631(9) 1036(10) 407(8) 962(9)
3087(1) 1270(1) 2126(1) 4226(1) 3428(1) 1404(1) 392(1) 1321(1) 3122(1) 3727(1) 1914(1) 2715(1) 4561(1) 3730(1) 1708(1) 707(1) 1622(1) 3438(1) 3716(1) 1336(1) 2394(1) 2313(1) 2634(1) 2156(8) 1404(9) 2798(8) 3704(8) 3089(9) 1451(8) 701(8) 1455(7) 2851(8) 4244(8) 2417(7) 482(8) 739(8) 2180(8) 3779(8) 4404(9)
30(1) 30(1) 28(1) 24(1) 25(1) 24(1) 24(1) 23(1) 24(1) 23(1) 22(1) 26(1) 20(1) 21(1) 21(1) 21(1) 22(1) 20(1) 24(1) 25(1) 25(1) 15(1) 14(1) 40(4) 37(4) 32(4) 30(4) 36(4) 28(3) 28(3) 23(3) 26(3) 35(4) 22(3) 33(4) 31(4) 36(4) 29(3) 35(4)
O(17) O(18) O(19) O(20) O(21) O(22) O(23) O(24) O(25) O(26) O(27) O(28) O(29) O(30) O(31) O(32) O(33) O(34) O(35) O(36)
9470(12) 9752(11) 7492(12) 4296(11) 4077(11) 7883(13) 8111(13) 3875(13) 6345(13) 10,659(12) 11,016(12) 7052(12) 3305(12) 2923(13) 6784(11) 7583(13) 7715(12) 5582(10) 9080(11) 7222(13)
1124(8) 2041(9) 2588(9) 2427(8) 1769(9) 22216(9) 21003(9) 21223(9) 2568(8) 2295(9) 794(9) 1746(9) 1546(9) 375(10) 2101(8) 770(8) 1366(9) 1265(8) 6365(8) 6852(8)
3604(8) 29(3) 1343(7) 27(3) 411(7) 29(3) 1459(7) 23(3) 3121(7) 29(3) 3665(9) 39(4) 635(9) 37(4) 1999(8) 35(4) 5046(8) 33(4) 3763(8) 32(4) 1118(9) 34(4) 2500(8) 33(4) 660(8) 34(4) 3646(8) 35(4) 2211(8) 24(3) 3101(7) 29(3) 1628(7) 28(3) 2294(7) 19(3) 2890(8) 29(3) 2069(7) 32(4) (continued on next page)
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M.H. Alizadeh et al. / Journal of Molecular Structure 688 (2004) 33–39 Table 3 ˚ ), angles (8) and valence bond values for 2 Selected bond distances (A
Table 2 (continued) Atom
x
y
z
U(eq)
O(37) O(38) O(39) O(40) O(41) O(42) O(43) O(44) O(45) O(46) O(47) O(48) O(49) O(50) O(51) O(52) O(53) O(54) O(55) O(56) O(57) O(58) O(59) O(60) O(61) O(62) O(63) O(64) O(65) O(66) O(67) O(68) O(69) O(70) O(71) O(1W) O(2W) O(3W)
7027(12) 7549(11) 9576(11) 9800(11) 7923(11) 5524(11) 5371(12) 8981(12) 10,247(11) 9455(12) 6210(10) 3994(11) 5850(11) 7304(12 9719(11) 10,022(11) 7799(12) 4576(12) 4412(12) 9054(13) 9453(12) 5041(13) 7015(13) 11,255(12) 11,589(11 7662(12) 3886(11) 3543(11) 7612(12) 7983(11) 8162(11) 6058(11) 8703(13) 6584(12) 6404(12) 9196(13) 9653(13) 2816(12)
6298(9) 4836(8) 4886(7) 5552(7) 6006(8) 5923(8) 5366(9) 3620(7) 4048(8) 5130(8) 5145(8) 4886(8) 3908(8) 2624(8) 2706(8) 3609(8) 4206(8) 4089(8) 3380(8) 5983(8) 7157(9) 6987(8) 3597(8) 3728(9) 4730(8) 5807(7) 5638(8) 4438(8) 5344(8) 3756(7) 4355(7) 4256(7) 2536(8) 2979(9) 2348(8) 1473(10) 3387(9) 3175(9)
3499(8) 4258(7) 3623(7) 1967(7) 1223(7) 1941(7) 3320(8) 4590(7) 2739(7) 815(7) 1035(7) 2507(7) 4123(7 4566(7) 3792(7) 1528(8) 568(7) 1582(7) 3327(7) 4384(8) 1432(9) 2734(9) 5466(8) 4087(8) 1532(7) 2147(8) 1019(7) 4017(7) 2724(7) 3382(7) 1896(7) 2559(7) 2540(7) 1695(9) 3267(8) 4924(8) 136(8) 2316(8)
32(4) 22(3) 21(3) 21(3) 22(3) 25(3) 31(4) 24(3) 24(3) 25(3) 21(3) 23(3) 21(3) 27(3) 22(3) 25(3) 25(3) 29(4) 26(3) 31(4) 38(4) 35(4) 31(4) 31(4) 23(3) 32(4) 25(3) 22(3) 24(3) 19(3) 18(3) 20(3) 30(4) 36(4) 29(4) 39(4) 36(4) 31(4)
Valence bond values
the cerium atoms have similar coordination and each cerium atom also has an external water ligand. Each cerium atom adopts tri-capped trigonal-prismatic coordination, the local 3-fold axis of the CeO6 group lying into the Ce3O3 plane. Trigonal-prismatic achieved by two the terminal oxygen of an edge shared paired of WO6 octahedra to each A-a-[AsW9O34]92 moiety and two oxygen from the belt. The cap achieved by one m3-O (As, W2) to each A-a-[AsW9O34]92 moiety and one external water ligand. The algorithm derived by Brown and Altermatt [18] from bond lengths stored in the Inorganic Crystal Database indicates that O(As, W2) atoms, oxygen atoms linked to one As and two W atoms, have valance sums in the range (1.773 – 1.877) average 1.818(4), and presumably therefore have residual bonding capacity. By contrast the valance sums for O(As, W3), which are linked to one As and three W atoms, are 2.002(2). The valance bond sums average for As, Ce and W are 4.868 (1), 3.932(1), and 6.078(1) respectively. The Ce –O
W 2 Od W– Ob W– Oc W– Od(Ce) W– O(As,W3) W– O(As,W2) As–O Ce –Od Ce –Obelt Ce –O(As,W2) Ce –O(nW) Ce1· · ·Ce2 Ce1· · ·Ce3 Ce2· · ·Ce3 Ce1 – Ce2 – Ce3 Ce2 – Ce1 – Ce3 Ce1 – Ce3 – Ce2 O32 –Ce1 –O66 O71 –Ce1 –O(1W) O71 –Ce1 –O17 O71 –Ce1 –O16 O71 –Ce1 –O32 O17 –Ce1 –O51 O17 –Ce1 –O50 O17 –Ce1 –O66 Ce1 – O71 –Ce3 Ce2 – O70 –Ce3 Ce2 – O69 –Ce1
1.724(2) 1.924(6) 1.938(2) 1.806(2) 2.386(2) 2.377(2) 1.695(2) 2.345(4) 2.212(9) 2.887(3) 2.682(5) 4.468(3) 4.303(2) 4.193(2) 59.49(3) 57.08(3) 63.44(4) 141.1(4) 143.7(5) 134.2(6) 81.0(5) 76.4(5) 79.8(5) 140.8(5) 134.0(4) 156.3(8) 160.8(9) 150.6(7)
1.687(9) 0.983(1) 0.941(4) 1.353(7) 0.286(3) 0.290(2) 1.217(7) 0.505(6) 0.727(19) 0.116(1) 0.203(3)
Od, Ob and Oc show terminal, corner shared and edge shared oxygen atoms, respectively.
bond lengths in CeO 6 group, Ce – O(As, W 2 ) and ˚ average Ce – O(nW) are in the range 2.117 –2.400(14) A ˚ ˚ ˚ and 2.300(9) A, 2.865(14) –2.944(13) A average 2.887(3) A ˚ ˚ 2.624(15) – 2.716(14) A average 2.682(5) A respectively (Table 3). Structural different of [As2W18Ce3O71(H2O)3]122 and phosphorous analogue can be attributed to different of bond lengths of As – O and P –O in AsO4 and PO4 in the compounds, respectively. The bond lengths average of ˚ ) is about 11% longer than that of the As – O (1.688(2) A ˚ P – O (1.499(3) A) [9]. Bond length longer in AsO4 makes available condition for coordination of m3-O (As, W2) to cerium atoms. 3.2. IR spectra IR spectra of [As2W18Ce3O71(H2O)3]122 with that of A-a-[AsW9O34]92 are shown in Fig. 4. As expected, IR spectra of Keggin-like sandwich-type of [X2W18M4O68(H2O)2]n2 (X ¼ As, P, Si; M ¼ Co, Cu, Mn, Ni, Fe, Zn, Cd) and those of [P2W18M3O68(H2O)3]122 (M ¼ Co, Cu, Fe, Ni, Mn, Pd, Zn) are similar to those of Keggin and the trivacant lacunary structures, respectively [8,10 –11,19]. IR spectra of [As2W18Ce3O71(H2O)3]122 anion are very
M.H. Alizadeh et al. / Journal of Molecular Structure 688 (2004) 33–39
37
Fig. 2. Unit cell of 2.
similar to that of A-a-[AsW9O34]92. The following points can be drawn from of IR spectra: (i)
All of the characteristic vibrational frequencies increased compared with those of A-a-AsW9O92 34 ,
which is attributed to the decrease of negative charges per the A-a-AsW9O92 34 moiety in the anion [20]. (ii) The asymmetry stretching vibration frequency of W –Oc –W in the anion is more up shifted (40 cm21) and also its intensity amplified, which are attributed to
Fig. 3. Structure of 2.
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M.H. Alizadeh et al. / Journal of Molecular Structure 688 (2004) 33–39
Fig. 4. UV-visible spectrum of 2 (1 £ 1025 M). Inset: absorption spectrum of 2 (1.3 £ 1023 M).
the present of cerium cation (directly bound to the terminal Oxygen of an edge shared pair of WO6) and duplication of A-a-[AsW9O34]92 in the anion. (iii) The vibrational frequency of As – O bonds overlaps those W –Ob – W bonds [21]. 3.3. Electronic spectra The UV-visible spectra of 2 have three absorption bonds (Fig. 5). Two the higher energy bonds at ca. 208 and 260 nm are attributed to charge transfer ðOc =Ob Þ ! W and Od ! W respectively [1]. The bond at 359 nm with 1 ¼ 1750 M 21 cm12 is attributed to charge transfer O ! Ce. 3.4. Acid/base titration The acid/base titration curves of 1 and 2 show two break points at 1.0 and 2.0 equivalent of added base. It shows that the anion has two different titrable protons. Whereas the lacunary A-a-[AsW9O34]92anion, [MAs2W18U2O72]132
(M ¼ K, NH4) (pH 7) and [As2W18U3O74]122 (pH 5) [14] anions are unstable in solution, 1 and 2 have an unprecedented range of pH stability. The anions appear to be stable between pH 2.5 and 9. When 2 is titrated with NaOH up to pH 11.2, large amount of the anion can be isolated as 3 by precipitating with KCl and then re crystallization.
4. Conclusion A new example of polyoxometalate complexes incorporating nine-coordinate (tri-capped trigonal-prismatic) hetero atoms has been prepared from the reaction of A-a-Na8H[AsW9O34] with (NH4)4Ce(SO4)4. X-Ray single crystal analysis of K7(H3O)5[As2W18Ce3O71(H2O)3]·9H2O confirms the structure and shows that it has some different with that phosphorous analogue, K9(H3O)3[P2W18Ce3O71(H2O)2], in which is attributed to longer As –O bond in compared to P– O bond.
Acknowledgements We thank Prof. I. David Brown, Department of Physics and Astronomy McMaster University Hamilton, for his advice on valance bond calculation.
References
Fig. 5. Potentiometric titration of a 3.57 £ 1023 M solution of 2.
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