A New Hydrated Cesium Heptaborate Cs2[B7

ABSTRACT: The new hydrated cesium borate Cs2B7O9(OH)5 containing the first example of an isolated heptaborate anion was synthesized by the reaction of...
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CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 6 1247-1249

Communications A New Hydrated Cesium Heptaborate Cs2[B7O9(OH)5]: Synthesis and Crystal Structure Zhi-Hong Liu* and Lian-Qing Li School of Chemistry and Materials Science, Shaanxi Normal UniVersity, Xi’an 710062, P. R. China ReceiVed July 4, 2005; ReVised Manuscript ReceiVed February 21, 2006

ABSTRACT: The new hydrated cesium borate Cs2B7O9(OH)5 containing the first example of an isolated heptaborate anion was synthesized by the reaction of H3BO3 with Cs2CO3 at 170 °C. Its crystal structure was determined from single-crystal X-ray diffraction data and further characterized by FT-IR and DTA-TG. It crystallizes in the monoclinic space group P21/n with unit cell of dimensions a ) 8.365(2) Å, b ) 12.033(3) Å, c ) 14.164(4) Å, R ) 90°, β ) 103.52(2)°, γ ) 90°, V ) 1386.13(61) Å3, Z ) 4. It consists of an isolated [B7O9(OH)5]2polyborate anion and two cesium ions. Dehydration of this compound occurs in one step and leads to an amorphous phase which undergoes crystallization. Introduction In the past several decades, much interest has been focused on studies of alkali borate compounds because some of these compounds show interesting physical properties, such as nonlinear optical behavior for CsLiB6O10 (CLBO),1 CsB3O5 (CBO),2 and KB5O8‚4H2O (KB5). Boron atoms coordinate with oxygen not only in 4-fold coordination (tetrahedral, BO4) but also in 3-fold coordination (triangular, BO3). These BO3 and BO4 groups may further link via common oxygen atoms to form isolated rings and cages or polymerize into infinite chains, sheets, and networks. Many examples of isolated boron oxoanions containing one to six borons exist in mineral and synthetic borates. However, isolated boron oxoanions having more than six borons are rare.3 Schubert et al. reported two borates, [C(NH2)3]3[B9O12(OH)6] and [C3H5N2]3[B9O12(OH)6], which contain the first examples of the isolated nonaborate anion [B9O12(OH)6]3-.4 The present paper deals with a new hydrated cesium heptaborate Cs2B7O9(OH)5, which contains the first example of an isolated [B7O9(OH)5]2- heptaborate anion. Experimental Section Synthesis and Characterization. All reagents used in the synthesis were analytic grade. The title compound was obtained from H3BO3 and Cs2CO3 with a B/Cs molar ratio of 3. A mixture of Cs2CO3 and H3BO3 was sealed in a Teflon-lined bomb and heated at 170 °C for 7 days and then cooled to room temperature. The resulting colorless and transparent crystals of Cs2B7O9(OH)5 were recovered by filtration, washed by deionized water, and dried in a vacuum dryer to a constant mass at room temperature. The yield was about 45.4% based on H3BO3. The compound was characterized by FT-IR spectroscopy (recorded over the 400 to 4000 cm-1 region on a Nicolet NEXUS 670 spectrometer with KBr pellets at room temperature) and by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) (performed on a TA-SDT Q600 thermal analyzer under N2 atmosphere with a heating rate of 10 °C/min). * Corresponding author. E-mail address: [email protected]

Determination of the Crystal Structure. A colorless, transparent crystal of size 0.38 × 0.28 × 0.26 mm3 was selected for the crystal structure measurements. The crystal structure determination by X-ray diffraction was performed on a Siemens P4 diffractometer with graphite-monochromated MoKR (λ ) 0.71073 Å) in the ω scaning mode at room temperature. The structure was solved by direct methods. The Cs atoms were first located, and the boron and oxygen atoms were found in the final difference Fourier map. Hydrogen atoms in the compound 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. All non-hydrogen atoms were refined anisotropically. Of the 3004 reflections measured (2.25 e θ e 25.50°), 2577 unique reflections in the compound were used to solve the structure. On the basis of all these data, R1 ) 0.0259, [I > 2σ (I)], wR2 ) 0.0579, and the goodness-of-fit on F2 is 0.986. Crystal data and conditions of intensity measurements are given in Table 1. Results and Discussion FT-IR Spectroscopy. The FT-IR spectrum of this compound (Figure 1) exhibited the following absorption bands, and they were assigned referring to literature.5 The band at 3426 cm-1 is the stretching modes of O-H. The bands at 1413, 1354, and 931 cm-1 might be the asymmetric and symmetric stretching modes of B-O in BO3. The bands at 1097, 810 cm-1 are assigned as the asymmetric and symmetric stretching of B-O in BO4. The band at 742 cm-1 is the out-of-plane bending mode of B-O in BO3. The strong band at 666 cm-1 might be the characteristic peak of [B7O9(OH)5]2-. The band at 460 cm-1 is the bending mode of B-O in BO4. Description of Crystal Structure. The crystal was found to be monoclinic, with space group P21/n, a ) 8.365(2) Å, b ) 12.033(3) Å, c ) 14.164(4) Å, β ) 103.52(2)°. The main bond lengths and angles are listed in Table 2. The crystal structure of Cs2B7O9(OH)5 consists of an isolated [B7O9(OH)5]2- polyborate anion and two cesium ions (Figure 2). The [B7O9(OH)5]2- heptaborate group

10.1021/cg0503200 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/19/2006

1248 Crystal Growth & Design, Vol. 6, No. 6, 2006

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Table 1. Crystal Data and Structure Refinement for Cs2B7O9(OH)5 empirical formula formula weight crystal system space group a b c γ volume Z calculated density crystal size temperature wavelength absorption coefficient F(000) theta range for data collection limiting indices reflections collected absorption correction max and min transmission refinement method goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff peak and hole

H5B7Cs2O14 570.53 monoclinic P21/n 8.365(2) Å 12.033(3) Å 14.164(4) Å 90° 1386.13(61) Å3 4 2.734 g/cm3 0.38 × 0.28 × 0.26 mm 298(2) K 0.71073 Å 5.328 mm-1 1048 2.25-25.50° 0 e h e 10, 0 e k e 14, -17 e l e 16 3004 empirical 1.0000 and 0.6880 full-matrix least-squares on F2 0.986 R1 ) 0.0259, wR2 ) 0.0579 R1 ) 0.0352, wR2 ) 0.0598 0.873, -0.612 e Å-3

consists of three six-membered rings in which five trigonal BO2(OH) units and two tetrahedral BO4 units are linked by vertical oxygen atoms. [B1, B2, B4, B6, and B7] are surrounded by two oxygen atoms and one hydroxyl and the other two boron atoms [B3 and B5] are surrounded by four oxygen atoms. In other words, BO2(OH) triangles and BO4 tetrahedra are linked by oxygen atoms, and the three six-membered rings are linked through their vertices [B3 and B5] to form the heptaborate anion [B7O9(OH)5]2-. In the triangular borons, the B-O bonds vary from 1.324 to 1.389 Å, and the mean B-O distance is 1.359 Å. In the tetrahedral borons, the B-O bonds vary from 1.423 to 1.504 Å, and the mean B-O distance is 1.472 Å. This compares very well with the corresponding

Figure 1. The FT-IR spectrum of Cs2B7O9(OH)5.

mean bond lengths observed in many hydrated or anhydrous borate compounds. According to the classification of polyborate anions proposed by Heller 6 and Christ and Clark,7 the shorthand notation for [B7O9(OH)5]2- is 7:5∆ + 2T. The polyborate anion with seven boron atoms is also found in [Cu(en)2][B7O13H3],8 in which the heptaborate units are linked together through exocyclic oxygens to neighboring units to form a covalently bonded two-dimensional (2D) corrugated sheet consisting of rings with 12 boron atoms. Namely, the heptaborate unit [B7O13H3]2- in this compound is not the isolated polyborate anion. In Cs2B7O9(OH)5, the shortest Cs-B distance is 3.568 Å, and the longest Cs-O(9) distance is 3.795 Å. Only the oxygen atoms with shorter Cs-O bonds (i.e., Cs-O < Cs-B) will be considered for the oxygenated environment of Cs ions.9 Therefore, there are nine and eight oxygen atoms around each cesium ion, respectively (Figure 2). Cs1 is surrounded by four oxygen atoms [O4, O5A, O13A, and O13B] from BO2(OH), five oxygen atoms [O1, O2A, O7A, O11A, and O12A] from the B-O-B bridge. Cs2 is

Table 2. Selected Bond Lengths (Å) and Angles (°) for Cs2[B7O9(OH)5]a O(3)-B(3) O(6)-B(3) O(8)-B(5) mean: 1.472

1.501(6) 1.465(6) 1.423(6)

Tetrahedral Borons O(8)-B(3) O(7)-B(5) O(10)-B(5)

1.431(6) 1.476(6) 1.504(6)

O(2)-B(3) O(12)-B(5)

1.492(6) 1.488(6)

O(1)-B(1) O(2)-B(2) O(7)-B(4) O(13)-B(6) O(11)-B(7) mean: 1.359

1.378(6) 1.324(6) 1.340(6) 1.361(6) 1.388(6)

Triangular Borons O(3)-B(1) O(5)-B(2) O(6)-B(4) O(10)-B(6) O(12)-B(7)

1.346(6) 1.374(6) 1.368(6) 1.342(6) 1.342(6)

O(4)-B(1) O(1)-B(2) O(9)-B(4) O(11)-B(6) O(14)-B(7)

1.346(6) 1.372(6) 1.360(6) 1.389(6) 1.355(6)

Cs(1)-O(4) Cs(1)-O(1) Cs(1)-O(5)#1 mean: 3.265

2.983(4) 3.163(3) 3.387(4)

Cs(1)-O(2)#1 Cs(1)-O(7)#3 Cs(1)-O(12)#3

3.115(3) 3.278(3) 3.414(3)

Cs(1)-O(13)#2 Cs(1)-O(11)#4 Cs(1)-O(13)#4

3.137(4) 3.357(3) 3.552(4)

Cs(2)-O(14)#5 Cs(2)-O(2)#4 Cs(2)-O(5)#6 mean: 3.223

3.089(4) 3.217(3) 3.351(4)

Cs(2)-O(8)#4 Cs(2)-O(12) Cs(2)-O(10)#4

3.094(3) 3.255(3) 3.373(3)

Cs(2)-O(8) Cs(2)-O(3)

3.101(3) 3.304(3)

O(3)-B(1)-O(4) O(2)-B(2)-O(1) O(8)-B(3)-O(6) O(8)-B(3)-O(3) O(7)-B(4)-O(9) O(8)-B(5)-O(7) O(8)-B(5)-O(10) O(10)-B(6)-O(13) O(12)-B(7)-O(14)

122.5(4) 123.1(4) 113.0(4) 110.0(4) 117.1(4) 113.9(4) 110.3(4) 123.8(5) 119.3(4)

O(3)-B(1)-O(1) O(2)-B(2)-O(5) O(8)-B(3)-O(2) O(6)-B(3)-O(3) O(7)-B(4)-O(6) O(8)-B(5)-O(12) O(7)-B(5)-O(10) O(10)-B(6)-O(11) O(12)-B(7)-O(11)

121.4(4) 118.9(4) 110.4(4) 105.5(4) 123.0(4) 109.9(4) 106.1(4) 120.6(5) 122.2(4)

O(4)-B(1)-O(1) O(1)-B(2)-O(5) O(6)-B(3)-O(2) O(2)-B(3)-O(3) O(9)-B(4)-O(6) O(7)-B(5)-O(12) O(12)-B(5)-O(10) O(13)-B(6)-O(11) O(14)-B(7)-O(11)

116.0(4) 118.1(4) 109.0(4) 108.8(3) 119.9(4) 108.0(4) 108.4(3) 115.6(4) 118.6(4)

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

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Crystal Growth & Design, Vol. 6, No. 6, 2006 1249 one-step weight loss of 8.236% from 160 to 650 °C (the minor weight loss of 0.43% from 40 to 160 °C is due to surface moisture of the sample), which corresponds to the loss of water due to the condensation of hydroxyl groups and can be compared with calculated value of 7.89%. The dehydration process can be expressed as follows: -5H2O

2Cs2B7O9(OH)5 98 Cs4B14O23

Figure 2. The asymmetric unit and coordination environments of Cs atoms in Cs2B7O9(OH)5.

This explanation is confirmed by the DTA curve (Figure 3). There are three peaks in the DTA curve. The first endothermic peak appearing at 323.20 °C is related to the dehydration and formation of the amorphous phase Cs4B14O23. This amorphous phase recrystallizes as proven by the exothermic peak at 542.19 °C. The endothermic peak appearing at 611.13 °C is related to the melting of the solid phase. In summary, we have successfully synthesized a new hydrated cesium heptaborate Cs2B7O9(OH)5 by the solid reaction of H3BO3 with Cs2CO3 at 170 °C. This compound contains the first isolated heptaborate anion [B7O9(OH)5]2-. Acknowledgment. The project is supported by Natural Science Basic Research Plan in Shaanxi Province of China. Supporting Information Available: Crystallographic files in CIF format. This material is available free of charge via the Internet at http:// pubs.acs.org.

References

Figure 3. Simultaneous TG-DTA curves of Cs2B7O9(OH)5.

surrounded by six oxygen atoms [O2B, O3, O8, O8A, O10, and O12] from the B-O-B bridge and two oxygen [O5B and O14A] from BO2(OH). The Cs1-O distances range from 2.983 to 3.552 Å with an average of 3.265 Å, and Cs2-O distances range from 3.089 to 3.373 Å with an average of 3.223 Å. The mean value is in good agreement with those found in other hydrated cesium borates such as Cs2[B4O5(OH)4]‚3H2O.10 Thermal Behavior. The thermal behavior of this compound is shown in Figure 3. The TG curve shows that Cs2B7O9(OH)5 has a

(1) Mori, Y.; Kuroda, I.; Nakajima S.; Sasaki, T.; Nakai, S. Appl. Phys. 1995, 34, 296. (2) Wu, Y. C.; Sasaki, T.; Yokotani, A.; Tang, H. G.; Chen, C. T. Appl. Phys. Lett. 1993, 62, 2614. (3) Grice, J. D.; Burns, P. C.; Hawthorne, F. C. Can. Mineral. 1999, 37, 731. (4) Schubert, D. M.; Visi, M. Z.; Knobler, C. B. Inorg. Chem. 2000, 39, 2250. (5) Li, J.; Xia, S. P.; Gao. S. Y. Spectrochim. Acta 1995, 51A, 519. (6) Heller, G. Top. Curr. Chem. 1986, 131, 39. (7) Christ, C. L.; Clark. J. R. Phys. Chem. Miner. 1977, 2, 59. (8) Sung, H. H-Y; Wu, M. M.; Williams I. D. Inorg. Chem. Commun. 2000, 3, 401. (9) Penin, N.; Seguin, L.; Gerand, B.; Touboul, M.; Nowogrocki, G. J. Alloy Compd. 2002, 334, 97. (10) Touboul, M.; Penin, N.; Nowogrocki, G. J. Solid State Chem. 1999, 143, 260.

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