Two New Potassium Borates, K4B10O15(OH)4 with Stepped Chain and KB5O7(OH)2‚H2O with Double Helical Chain Hong-Xia Zhang,† Jie Zhang,† Shou-Tian Zheng,† and Guo-Yu Yang*,†,‡
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 1 157-161
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China, and State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing, Jiangsu 210093, People’s Republic of China Received March 25, 2004;
Revised Manuscript Received July 9, 2004
ABSTRACT: Two unusual chainlike potassium borates of K4B10O15(OH)4 (1) and KB5O7(OH)2‚H2O (2) have been synthesized under mild solvothermal conditions at 170 °C. The structures were determined by single-crystal X-ray diffraction and further characterized by FTIR, powder X-ray diffraction, and thermogravimetric analysis. Compound 1 crystallized in the monoclinic space group C2/c, a ) 18.077(6) Å, b ) 6.857(2) Å, c ) 13.266(4) Å, β ) 95.271(4)°, V ) 1637.4(9) Å3, Z ) 4, R ) 0.0234; and compound 2 crystallized in the monoclinic space group P2(1)/c, a ) 9.4824(8) Å, b ) 7.5180(6) Å, c ) 11.4154(6) Å, β ) 97.277(3)°, V ) 807.2(1) Å3, Z ) 4, R ) 0.0502. The stepped chains of 1 are built up from corner sharing of B3O5(OH)23- and B4O96- groups to form new fundamental building blocks (FBBs) of B5O10(OH)27- constructed by corner sharing of one B3O5(OH)23- cluster with a three-membered ring, one BO33- unit, and BO45- tetrahedron, while the double helical chain of 2 is built up by FBBs of B5O7(OH)2-. Adjacent chains in 1 and 2 are further connected into a 3-D structure by K+ cations and H-bonding interactions. Introduction Borates have provided a rich area of research for over 50 years due to their rich structural chemistry and potential applications in mineralogy1 and nonlinear optical materials.2 As a result, many borate systems including alkali metal, alkaline earth metal, rare-earth, and transition metals have been widely studied.1c,3 In general, the boron atom possesses two kinds of coordination modes of triangularly coordinated boron atoms and tetrahedrally coordinated boron atoms (∆, trigonal BO3 and T, tetrahedral BO4). The BO3 and BO4 groups favor polymerization via common corners into large polynuclear anion units including isolated or finite clusters,1c,4 chains,1c,5 sheets,1c,6 and frameworks.1c,7 Recently, Burns et al.1 have developed a comprehensive description based on FBBs (fundamental building blocks) to have a clearer nomenclature for the borates with more complicated structures. To date, FBBs containing one to 63 boron atoms have been reported.1,3,8,9 Despite the richness of the crystal chemistry of borate materials, the rational design of new borates is still not possible, predominantly due to our poor understanding of the mechanism for the formation of such materials. So far, hydrated borates, except the borate minerals, are generally synthesized under atmospheric pressure, which possess rich terminal hydroxyl groups and exist in isolated structure.3 These hydrated borates can be used as precursors for the syntheses of anhydrous borates via the dehydration process including the condensation/polymerization of terminal hydroxyl groups of FBBs under high-temperature conditions.3 Lately, a hydrothermal approach has been used for the synthesis * Corresponding author. E-mail:
[email protected]. † Chinese Academy of Sciences. ‡ Nanjing University.
of borate materials.5c,7d,7e,10 The investigations on the M-B-O system (M ) metal) invariably lead to the formation of unusual clusters under hydrothermal conditions.7e Ozin et al. have proposed a new mode for the formation of high-dimensional aluminophosphate structures from the transformation of linear chains, which described that hydrolysis and condensation reactions of the linear chain can produce a large number of other more complex structures.11 If this hydrolysis and condensation reaction mode allows for the rational design of other inorganic structures, such as borates with rich terminal hydroxyl groups, there may be an enormous growth in the structural diversity of inorganic materials. During our investigation of borate materials, we applied a self-assembly process under solvothermal conditions. Herein, we first describe the syntheses and structural characterization of two new potassium borates of K4B10O15(OH)4 (1) and KB5O7(OH)2‚H2O (2). Compound 1 possesses a stepped chain containing the first example of the FBBs of B5O10(OH)27- constructed from corner sharing of one B3O5(OH)23- cluster, one BO33- unit, and BO45- tetrahedron. Compound 2 has a double helical chain, composed of FBBs of B5O7(OH)2-. Experimental Procedures Syntheses and Characterization of Compounds. Compound K4B10O15(OH)4 (1) was initially obtained from a solvothermal reaction designed to synthesize a templated transitionmetal borogermanate; therefore, the synthetic mixture contained GeO2, organic amine, and transition-metal salt as shown in runs 1 and 2 in Table 1. Later, GeO2, organic amine, and transition-metal salt were removed from the reaction system, and it was found that the syntheses could be carried out at the same temperature (runs 3 and 4 in Table 1). Compound 1 is synthesized solvothermally under autogenous pressure. In a typical synthesis of 1, 1.830 g of K2[B4O5(OH)4]‚2H2O was dissolved into the mixed solvent of 5.3 mL of pyridine (Py) and
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run
Pya (mL)
GeO2 (g)
boride (g)
1 2 3 4 5 6 7 8 9 10
3.2 3.2 5.3 4.2 3.2 3.2 3.2 3.2 4.2 3.2
0.104 0.104
0.916 A 0.610 A 1.830 A 1.525 A 0.186 B 0.186 B 0.544 C 0.272 C 0.232 B 0.232 B
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Table 1. Variation in the Combinations of Reagents and Obtained Product
a
KOH (g)
NMe4OH (mL)
amine (mL)
H2O (mL)
salt (g)
product
1.1
1 dien 2 dien
0.2 1.0 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2
0.115 MnCO3 0.238 MnCO3
1, this paper 1, this paper 1, this paper 1, this paper K2[B5O8(OH)](H2 O)212 ref 12 ref 12 ref 12 2, this paper 2, this paper
0.050 0.030
0.138 K2CO3 0.276 K2CO3 0.138 K2CO3 0.138 K2CO3
Py ) pyridine; A ) K2[B4O5(OH)4]‚2H2O; B ) H3BO3; C ) NH4B5O8‚4H2O; NMe4OH ) N(CH3)4OH; and dien ) H2N(CH2)2NH(CH2)2NH2.
0.2 mL of H2O. The mixture with the molar ratio of 60 K2[B4O5(OH)4]‚2H2O/657 Py/111 H2O was stirred under ambient conditions. The resulting mixture, with a pH of about 6.0, was sealed in a Teflon-lined bomb and heated at 170 °C for 7 days and then cooled to room temperature. The resulting colorless prismlike crystals of 1 were recovered by filtration, washed with distilled water, and dried in air (49.0% yield based on boron). The pH of solution remained at 6 during crystallization. Compound KB5O7(OH)2‚H2O (2) is also synthesized solvothermally under autogenous pressure. Typically, 0.232 g of H3BO3 and 0.050 g of KOH were dissolved into the mixed solvent of 4.2 mL of Py and 1.1 mL of H2O. The mixture with the molar ratio of 37 H3BO3/9 KOH/520 Py/611 H2O was stirred under ambient conditions. The resulting mixture, with a pH of about 6.0, was sealed in a Teflon-lined bomb and heated at 170 °C for 7 days and then cooled to room temperature. The colorless prismlike crystals of 2 were obtained that were recovered by filtration, washed with distilled water, and dried in air (26.7% yield based on boron). The pH of solution remained at 6 during crystallization. It seems that the formation of compounds 1 and 2 is greatly influenced by the boron source, as shown in Table 1. Using H3BO3 and NH4B5O8‚4H2O as a boron source, and K2CO3 as potassium source, a known compound K2[B5O8(OH)](H2O)212 was always obtained in our case. By optimizing the reaction conditions, such as changing the amount of pyridine as solvent and K2[B4O5(OH)4]‚2H2O, compound 1 was successfully isolated as the sole product of reaction. If the compound of K2[B4O5(OH)4]‚2H2O as starting materials was replaced by H3BO3 and KOH, compound 2 was produced. The infrared (IR) spectrum (KBr pellets) was recorded on an ABB Bomen MB 102 spectrometer. Powder X-ray diffraction (XRD) data were obtained using a Philips X′Pert-MPD diffraction with CuKR radiation (λ ) 1.5406 Å). The thermogravimetric analysis (TGA) was performed on a Netzsch STA 449c analyzer in N2 atmosphere with a heating rate of 10 °C. Single-Crystal Structure Determination. Suitable colorless crystals of the as-synthesized compounds with dimensions of 0.65 × 0.65 × 0.30 mm3 for 1 and 0.20 × 0.18 × 0.18 mm3 for 2 were carefully selected under an optical microscope and glued to a thin glass fiber with epoxy resin. Crystal structure determination by X-ray diffraction was performed on a Siemens SMART CCD diffractometer with graphite-monochromated MoKR (λ ) 0.71073 Å) in the ω and φ scanning mode at room temperature. An empirical absorption correction was applied using the SADABS program.13 The structures were both solved by direct methods. The K atoms were first located, and the boron and oxygen atoms were found in the final difference Fourier map. Hydrogen atoms in 1 and 2 were found in the difference Fourier maps. The structures were refined on F2 by a full-matrix least-squares methods using the SHELXL-97 program package.14 All non-hydrogen atoms were refined anisotropically. Of the 11 305 reflections measured (2.26 e θ e 34.97°), 3496 unique reflections in 1 were used to solve the structure. On the basis of all these data, R1 ) 0.0234, [I > 2σ(I)], wR2 ) 0.0633, and the goodness-of-fit on F2 is 1.123. In 2, 1045 unique reflections of 2244 reflections measured (2.17 e θ e 25.04°) were used to solve the structure, with R1 ) 0.0502, [I >
Table 2. Crystallographic Data and Structure Refinement Parameters for 1 and 2 Compound Formula FW space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (g cm-3) µ (mm-1) θ range (deg) final R1 wR2 [I > 2σ(I)]
1 H4B10K4O19 572.52 C2/c 18.077(6) 6.857(2) 13.266(4) 90 95.271(4) 90 1637.4(9) 4 2.322 1.194 2.26 e θ e 34.97 0.0234 0.0633
2 H4B5KO10 257.18 P2(1)/c 9.4824(8) 7.5180(6) 11.4154(6) 90 97.277(3) 90 807.2 (1) 4 2.116 0.699 2.17 e θ e 25.04 0.0502 0.1271
2σ(I)], wR2 ) 0.1271, and the goodness-of-fit on F2 is 1.118. Experimental details for the structural determinations of 1 and 2 are presented in Table 2.
Results and Discussion Characterization. Figure 1a,b shows the powder XRD pattern of as-synthesized compound and the simulated pattern on the basis of single-crystal structure of 1 and 2, respectively. The diffraction peaks on both patterns corresponded well in position, indicating the phase purity of the two as-synthesized samples. The difference in reflection intensities between the simulated and the experimental patterns was due to the variation in crystal orientation for the powder samples. The infrared (IR) spectra of 1 contained two strong bands at about 1353 and 1011 cm-1, consistent with the existence of BO3 and BO4 groups, respectively, while the strong bands at 1285, 1216 cm-1 and 1096, 1055 cm-1 of 2 are in agreement with the existence of BO3 and BO4, respectively. The band due to OH stretching vibrations is observed at around 3314 cm-1 for 1 and 3441 cm-1 for 2, respectively.5c,7c Thermogravimetric analysis (TGA) was performed in dry N2 atmosphere from 40 to 800 °C. TGA shows 1 has one step weight loss at about 400 to 650 °C, corresponding to the dehydration process of hydroxyls (found: 6.09%; calcd: 6.29%). To 2, there is also one step weight loss between 140 and 410 °C, corresponding to the removing of H2O molecule and dehydration process of hydroxyls (found: 13.79%; calcd: 14.00%). Description of the Structure. Compound 1 crystallized in the monoclinic space group C2/c, a ) 18.077(6) Å, b ) 6.857(2) Å, c ) 13.266(4) Å, β ) 95.271(4)°. Selected bond distances and angles for 1 are listed in Table 3. Compund 1 has a 1-D stepped chainlike
Two New Potassium Borates
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Figure 2. ORTEP of the asymmetric unit of 1 (a) and 2 (b), showing the atom-labeling scheme and 50% thermal ellipsoids, respectively. Atom labels having A refer to symmetry-generated atoms.
Figure 1. Powder X-ray diffraction patterns of compound 1 (a) and 2 (b). Table 3. Selected Bond Lengths (Å) and Angles (°) for 1a B(1)-O(3) B(1)-O(1)#6 B(1)-O(2)#2 B(2)-O(4) B(2)-O(5) B(2)-O(3) B(2)-O(1) B(3)-O(10) B(3)-O(5) O(3)-B(1)-O(1)#6 O(3)-B(1)-O(2)#2 O(4)-B(2)-O(5) O(4)-B(2)-O(3) O(5)-B(2)-O(3) O(4)-B(2)-O(1) O(5)-B(2)-O(1) O(3)-B(2)-O(1) O(10)-B(3)-O(5) O(10)-B(3)-O(6)
1.3709(10) 1.3720(10) 1.3809(10) 1.4513(9) 1.4589(10) 1.4836(10) 1.5131(11) 1.3508(10) 1.3576(10) 121.44(7) 119.02(7) 110.69(6) 110.35(6) 111.22(6) 108.85(6) 106.90(6) 108.72(6) 125.89(7) 120.03(7)
B(3)-O(6) B(4)-O(8) B(4)-O(7) B(4)-O(6) B(5)-O(9) B(5)-O(2) B(5)-O(10) B(5)-O(8) O(5)-B(3)-O(6) O(8)-B(4)-O(7) O(8)-B(4)-O(6) O(7)-B(4)-O(6) O(9)-B(5)-O(2) O(9)-B(5)-O(10) O(2)-B(5)-O(10) O(9)-B(5)-O(8) O(2)-B(5)-O(8) O(10)-B(5)-O(8)
1.413(1) 1.350(1) 1.369(1) 1.383(1) 1.449(1) 1.457(1) 1.489(1) 1.506(1) 114.05(7) 122.12(7) 121.67(7) 116.20(7) 109.00(6) 111.32(7) 110.16(6) 110.69(6) 106.36(6) 109.19(6)
a Symmetry transformations used to generate equivalent atoms: #1 x,y + 1,z; #2 -x,-y, -z; #3 x,-y,z - 1/2; #4 -x+1/2, -y - 1/2,-z; #5 -x + 1/2,y + 1/2,-z + 1/2; #6 -x,y,-z - 1/2; #7 x,y 1,z; #8 -x + 1/2,y - 1/2,-z + 1/2; and #9 x,-y,z + 1/2.
structure built up of trigonal BO3 or BO2(OH) units and tetrahedral BO4 or BO3(OH) units. The asymmetric unit of 1 contains three independent triangularly coordinated boron atoms (B-O (av.) 1.372 Å), two unique tetrahedrally coordinated boron atoms (B-O (av.) 1.476Å), two unique potassium atoms, and 9.5 oxygen atoms as shown in Figure 2a. Two trigonal BO3 and BO2(OH) units and one tetrahedral BO3(OH) unit are linked through their vertexes to form a B3O5(OH)23- cluster.
Figure 3. Representation of the stepped chain and the FBBs of B5O10(OH)27- for 1.
Two tetrahrdral BO4 units and two trigonal BO3 link through their vertexes to form a B4O96- cluster. The strictly alternating B3O5(OH)23- and B4O96- clusters are linked together to form a 1-D stepped chain structure as shown in Figure 3. It is worth noting that two threemembered rings of B4O96- unit in 1 are almost perpendicular with each other, resulting in the formation of the stepped chain. In addition, new FBBs of B5O10(OH)27built up by corner sharing of one B3O5(OH)23- cluster with three-membered ring, one BO33- unit, and BO45tetrahedron (Figure 3) were found in structure 1. All the chains in the crystal align in parallel with each other along the b axis as shown in Figure 4. Adjacent chains are further linked via H-bonding interactions into layers (O‚‚‚O (av.) 2.738 Å, Table 5). The extraframework cations of the K reside between chains compensate the negative charges of borate chains and hold the layers together into the three-dimensional structure through bonding with oxygen atoms of chains. K1 and K2 are 9and 7-fold coordinated (Figure 5a) with the K-O distances varying from 2.813 (1) to 3.194(1) Å and 2.691(2) to 2.983(2) Å, respectively. Compound 2 crystallized in the monoclinic space group P2(1)/c, a ) 9.4824(8) Å, b ) 7.5180(6) Å, c ) 11.4154(6) Å. Selected bond distances and angles for 2 are listed in Table 4. Compound
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Figure 6. View of the two adjacent chain structures in compound 2.
Figure 4. View of the chains in the crystal, which align parallel with each other along the b axis in 1. (---) H-bonds. The K-O bonds are not connected for clarity.
Figure 7. View of the helical chains in 2. The left-handed (a) and the right-handed (b), respectively.
Figure 5. Coordination environments of K atoms in 1 (a) and 2 (b). Table 4. Selected Bond Lengths (Å) and Angles (°) for 2a B(1)-O(2) B(1)-O(1) B(1)-O(3) B(2)-O(4) B(2)-O(3) B(2)-O(6)#6 B(3)-O(5) B(3)-O(1) O(2)-B(1)-O(1) O(2)-B(1)-O(3) O(1)-B(1)-O(3) O(4)-B(2)-O(3) O(4)-B(2)-O(6)#6 O(3)-B(2)-O(6)#6 O(5)-B(3)-O(1) O(5)-B(3)-O(4) O(1)-B(3)-O(4)
1.346(6) 1.348(5) 1.399(5) 1.342(5) 1.383(5) 1.384(5) 1.457(5) 1.467(5) 119.5(4) 119.9(4) 120.5(4) 121.9(4) 122.8(4) 115.4(4) 110.6(3) 108.5(3) 109.8(3)
B(3)-O(4) B(3)-O(9) B(4)-O(5) B(4)-O(7) B(4)-O(6) B(5)-O(8) B(5)-O(9) B(5)-O(7) O(5)-B(3)-O(9) O(1)-B(3)-O(9) O(4)-B(3)-O(9) O(5)-B(4)-O(7) O(5)-B(4)-O(6) O(7)-B(4)-O(6) O(8)-B(5)-O(9) O(8)-B(5)-O(7) O(9)-B(5)-O(7)
1.477(5) 1.479(5) 1.342(5) 1.384(5) 1.391(5) 1.359(5) 1.363(5) 1.386(5) 110.7(3) 108.1(3) 109.1(3) 121.5(4) 121.9(4) 116.6(3) 123.3(4) 115.1(4) 121.6(4)
a Symmetry transformations used to generate equivalent atoms: #1 -x + 2,y + 1/2,-z + 1/2; #2 x,y,z - 1; #3 x,-y + 3/2,z 1/2; #4 -x + 2,-y + 1,-z + 1; #5 x,-y + 1/2,z - 1/2; #6 -x + 2,y + 1/2,-z + 3/2; #7 x,-y + 3/2,z + 1/2; #8 x,y,z + 1; #9 x,-y + 1/2,z + 1/2; #10 -x + 2,y - 1/2,-z + 3/2; and #11 -x + 2,y - 1/ 2, -z + 1/2.
Table 5. Hydrogen-Bonding System for 1 D-H‚‚‚A
d(D-H)
d(H‚‚‚A)
d(D‚‚‚A)
∠(DHA)
O(9)-H(1)‚‚‚O(4) O(7)-H(2)‚‚‚O(1)
0.88(2) 0.87 (2)
1.91(2) 1.85 (2)
2.759(1) 2.717(1)
160(2) 172(2)
2 possesses a double helical chainlike structure built up by trigonal BO3 or BO2(OH) units and tetrahedral BO4 units. The asymmetric unit of 2 consists of four independent triangularly coordinated boron atoms (B-O (av.) 1.369Å), one unique tetrahedrally coordinated
boron atom (B-O (av.) 1.470Å), 10 oxygen atoms, and one potassium atom (Figure 2b). Four trigonal BO3 or BO2(OH) units and one tetrahedral BO4 unit are linked by vertex sharing to form the FBBs of B5O7(OH)2-, which further connects through µ2-O(6) into a 1-D chainlike structure (Figure 6). The unique structural feature of 2 is that it possesses double helical chains with 1-D helical channels in the [010] direction as shown in Figure 7. Along the [010] direction, the channels seem to have a six-membered ring aperture. In fact, there is no closed 6-ring present in the structure of 2. The unclosed -B3-O4-B2-O6B4-O5-B3-O4-B2-O6-B4-O5- linkage in the structure of 2 gives rise to two types of helices with opposite chirality along b axis. The left- and right-handed helices arrange alternately along the c axis as shown in Figure 8. There are weak hydrogen bonds not only between adjacent chains but also between chains and H2O molecule, with O‚‚‚O from 2.622(5) to 2.823(4) Å (Table 6). K atoms located between the chains link them by bonds to the O atoms of the chains. The connectivity of H-bonds and K-O bonds gives rise to 3-D structure (Figure 8). In addition, each K+ cation is 10-fold coordinated as shown in Figure 6b, with the K-O distances varying from 2.776(3) to 3.267(3) Å. Compared with previously reported chainlike borates, such as Ca[B3O4(OH)3](H2O),1c,15 KMg2H[B6O8(OH)5]2(H2O)4,1c,16 Tl2[B4O6(OH)2]‚ 2H2O,17 Pb6B11O18(OH)9,5c LiB3O5,18 Zn[B3O4(OH)3],5a and M2[B4O6(OH)2]‚nH2O (M ) Li, Na),19 the stepped chains of 1 and the double helical chains of 2 are very unusual in borates. Especially, a new FFB of B5O10(OH)27- unit was found in the structure of 1. Compared between the starting materials containing boron and the product of borate, the structures of products 1 and 2 differ greatly from their starting
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References
Figure 8. View of the structure in 2 along the b axis, showing two types of helical chains with six-membered ring channels. (---) H-bonds. The K-O bonds are not connected for clarity. Table 6. Hydrogen-Bonding System for 2 D-H‚‚‚A
d(D-H)
d(H‚‚‚A)
d(D‚‚‚A)
∠(DHA)
O(2)-H(1)‚‚‚O(10) O(8)-H(2)‚‚‚O(2) O(10)-H(3)‚‚‚O(1) O(10)-H4‚‚‚O(9)
0.78(6) 0.87 (7) 0.95(6) 0.77(6)
1.85(6) 1.97 (7) 1.76(6) 2.06(7)
2.622(5) 2.807(4) 2.686(4) 2.823(4)
174(6) 161(6) 167(5) 171(7)
materials. K2[B4O5(OH)4]‚2H2O,20 the starting material for compound 1, only consists of isolated B4O5(OH)42clusters, while product 1 contains not only B4O96- clusters but also B3O5(OH)23- units. H3BO3 as the starting material of compound 2 only exists in simple B(OH)3 units, but the product contains complex B5O10(OH)27units. These differences indicate that the solvothermal/ hydrothermal technique is a useful tool to synthesize new complex borate materials by the self-assembly process. In addition, the hydrated borates usually in isolated format can be used as precursors for the syntheses of borates with higher dimensional structure via the dehydration process including the condensation/ polymerization of terminal hydroxyl groups of FBBs under solvothermal/hydrothermal conditions. Conclusions The solvothermal synthesis of two new hydrated potassium borates with the formula K4B10O15(OH)4 and KB5O7(OH)2‚H2O, respectively, and their crystal structures have been described. Compound 1 consists of corner linking B3O5(OH)23- and B4O96- cluster units forming a stepped chain structure in which new FBBs of B5O10(OH)27- were observed, while compound 2 shows an interesting structure of a double helical chain composed of vertex-linking FBBs of B5O7(OH)2-. Supporting Information Available: Crystallographic information files (cif) and IR and TGA data for 1 and 2. This material is available free of charge via the Internet at http:// pubs.acs.org.
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