High-Temperature, High-Pressure Hydrothermal Synthesis, Crystal

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High-Temperature, High-Pressure Hydrothermal Synthesis, Crystal Structure, Thermal Stability, and Solid State NMR Spectroscopy of an Aluminum Borate, Ba[AlB4O8(OH)] Chun-Chi Peng,† Hsin-Kuan Liu,† and Kwang-Hwa Lii*,†,‡ †

Department of Chemistry, National Central University, Zhongli, Taiwan 320, Republic of China Institute of Chemistry, Academia Sinica, Taipei, Taiwan 115, Republic of China

‡

S Supporting Information *

ABSTRACT: A new aluminum borate, Ba[AlB4O8(OH)], has been synthesized under high-temperature, high-pressure hydrothermal conditions at 550 °C and 1400 bar and its structure characterized by single-crystal X-ray diffraction, IR, and MAS 11 B, and 27Al NMR spectroscopy. It crystallizes in the monoclinic space group P21/n with a = 7.0695(5) Å, b = 15.108(1) Å, c = 7.0746(5) Å, β = 93.593(2)°, and Z = 4. Its 2D layer structure is formed of corner-sharing B4O8(OH) clusters and AlO4 tetrahedra with the charge-compensating Ba2+ cations between the layers. While the same in the framework composition, the title compound and the hydrate, Ba[AlB4O8(OH)]·H2O, differ greatly in structure. Although the title compound contains an OH group, it is thermally stable up to 740 °C and then decomposes into Ba2Al2B8O17, as indicated by high-temperature DSC/TG analysis and powder X-ray diffraction.

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INTRODUCTION The synthesis of nonlinear optical materials with a wide range of ultraviolet transparency has been the subject of intensive studies because of the materials’ applications in optical technology.1 Some beryllium borates such as the well-known KBe2BO3F2 and Sr2Be2B2O7 possess large energy band gaps such that their transmission cutoff wavelengths are very short ( 99.5%), 20.5 mg of H3BO3 (Merck, 99.8%), and 0.238 mL of H2O (molar ratio Ba/Al/B = 1/3/ 10) in a 3.4-cm-long gold ampule (inside diameter = 0.48 cm), welded completely closed, was heated in a Tem-Pres high pressure vessel at 550 °C for 2 days. The degree of filling of the pressure vessel by water at r.t. was 55%, and the pressure was estimated to be 1400 bar at 550 °C according to the P−T diagram for H2O.16 The vessel was then cooled to 350 °C at 5 °C/h and quenched in air at r.t. by removing it from the tube furnace. The reaction product was separated by suction filtration, washed with water, rinsed with ethyl alcohol, and dried in a desiccator at r.t. Colorless columnar crystals of BaAlB4O8(OH) were obtained in a yield of 75%. In a separate reaction, a mixture of 18.0 mg of Ba(OH)2·8H2O, 8.9 mg of Al(OH)3, 70.5 mg of H3BO3, 0.028 mL of 10 M NaOH (aq), and 0.308 mL of H2O (molar ratio Ba/Al/B = 1/ Received: November 11, 2017

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DOI: 10.1021/acs.inorgchem.7b02855 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

valence sum calculations were performed.20 The values for Ba, Al, and B were 2.18, 2.89, and 3.04−3.05, respectively, which are consistent with their valences. O(9) had valence sum of 1.15, and all other O atoms had values close to 2. The value for O(9) is indicative of a hydroxyl group. However, the H atom could not be located from difference Fourier maps. The final cycles of least-squares refinement included the atomic coordinates and anisotropic thermal parameters for all atoms and converged at R1 = 0.0253 and wR2 = 0.0492 for 1897 reflections with I > 2σ(I), GoF = 1.066, ρmax,min = 0.85, and −1.02 e· Å−3. The SHELXTL version 6.14 software package was used for all calculations.21 The crystallographic data and selected bond distances are given in Tables 1 and 2, respectively.

2/20) was heated in a 5.1-cm-long gold ampule under the same reaction conditions. The reaction produced larger and well-formed crystals of the title compound in a yield of 85%. The sodium hydroxide mineralizer aids the growth of larger and better quality crystals. Both reactions produced a pure product of BaAlB4O8(OH) as indicated by powder X-ray diffraction using a Bruker D2 PHASER diffractometer with Cu Kα radiation (Figure S1). A qualitative X-ray fluorescence analysis of several colorless crystals confirmed the presence of Ba and Al. Attempts to synthesize the Sr and Ca analogues under similar reaction conditions were unsuccessful. In the case of Sr, a pure product of a known strontium borate, Sr2B5O9(OH), was obtained.12 The IR spectrum of 1 was measured within the 4000−500 cm−1 region on a JASCO FTIR-4200 spectrometer using the KBr pellet technique. The spectrum shows a medium, sharp peak at 3585 cm−1, which confirms the presence of an OH group in the structure. Strong bands corresponding to the BO3, BO4, and AlO4 groups were also observed. The IR spectrum can be seen in Figure S2. DSC/TG analysis was carried out on a powdered sample of 1 in an alumina crucible using a NETZSCHSTA 449 F3 thermal analyzer. The sample was heated from 50 to 900 °C at 5 °C/min under flowing Ar. As shown in Figure 1, there is a weight loss of 2.5% between 700 and 800

Table 1. Crystallographic Data for Ba[AlB4O8(OH)] chemical formula fw cryst syst space group a/Å b/Å c/Å β/° V/Å3 Z T, °C λ(Mo Kα), Å Dcalcd, g·cm−3 μ(Mo Kα), mm−1 R1a wR2b

HAlB4BaO9 352.57 monoclinic P21/n (No. 14) 7.0695(5) 15.108(1) 7.0746(5) 93.593(2) 754.11(9) 4 23 0.71073 3.105 5.41 0.0253 0.0492

a R1 = Σ||Fo| − |Fc||/Σ|Fo|. bwR2 = [Σ w(Fo2 − Fc2)2/Σ w(Fo2)2]1/2, w = 1/[σ2(Fo2) + (aP)2 + bP], P = [Max(Fo2,0) + 2(Fc)2]/3, where a = 0.0236 and b = 1.23.

Table 2. Selected Bond Lengths (Å) for Ba[AlB4O8(OH)]a B(1)−O(1) B(1)−O(3) B(2)−O(4) B(3)−O(2) B(3)−O(6) B(4)−O(7) B(4)−O(9) Al(1)−O(4) Al(1)−O(8)

Figure 1. DSC and TGA data for 1. The sample was heated from 50 to 900 °C at 5 °C/min under flowing argon. °C, which is close to the theoretical value of 2.56% for a half water molecule per formula unit. The endothermic peak at 740 °C in the DSC curve corresponds to the decomposition of the structure and loss of a water molecule. Meanwhile a powder sample of 1 was heated in a Pt crucible at 600, 650, 700, 750, 800, and 850 °C and cooled to r.t. for powder X-ray diffraction study (Figure S3). Compound 1 is stable up to 700 °C and decomposes into crystalline Ba2Al2B8O17 (denoted as 2) at 750 °C (Figure S4).17 Compound 2 is stable at 800 °C and decomposes into a poorly crystalline material at 850 °C. Therefore, the endothermic peak at about 810 °C in the DSC curve corresponds to the incongruent melting point of 2. Single-Crystal X-ray Diffraction. Data collection was performed on a colorless crystal of 1 with dimensions of 0.22 × 0.08 × 0.01 mm at 296 K over 16 112 frames with φ and ω scans (width 0.5°/frame) and an exposure time of 30 s/frame using a Bruker Kappa Apex II CCD diffractometer equipped with a normal focus, 3 kW sealed tube X-ray source. The SAINT program was used to determine the integrated intensities and refine the unit cell parameters.18 Absorption correction was performed using the SADABS program (Tmin/Tmax = 0.631/0.746).19 On the basis of the statistical analysis of intensity distribution and successful solution and refinement of the structure, the space group was determined to be P21/n (No. 14). The nonstandard space group was chosen instead of P21/c so that the β angle is close to 90°. The Ba, Al, and several O atoms were identified in geometrically acceptable positions by direct methods and remaining atoms located from successive difference Fourier maps. For charge balance, it needs one H atom per formula unit. At this point, bond-

a

1.343(4) 1.410(4) 1.348(4) 1.491(4) 1.443(4) 1.379(4) 1.397(4) 1.757(2) 1.706(2)

B(1)−O(2) B(2)−O(3) B(2)−O(5) B(3)−O(5) B(3)−O(7) B(4)−O(8) Al(1)−O(1) Al(1)−O(6)

1.348(4) 1.398(4) 1.355(4) 1.470(4) 1.481(4) 1.327(4) 1.756(2) 1.743(2)

Note: Ba−O distances are available from the Supporting Information.

Solid State NMR Measurements. The 11B and 27Al MAS NMR measurements were performed at r.t. on a Varian Infinity-plus 500 spectrometer with a 11.74 T magnet. For magic angle spinning (MAS) NMR experiments, a 4 mm CP-MAS probe was used with a zirconia rotor at 159.95 MHz for 11B and at 129.91 MHz for 27Al resonance frequency, respectively. The sample spinning speed of 12 kHz was employed. Spectra were referenced to external NaBH4 powder and (NH4)3AlF6 powder for 11B and 27Al NMR, respectively. The 15° pulse length of 1.04 μs and repetition delay of 10 and 1 s were used for 11 B and 27Al NMR, respectively.

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RESULTS AND DISCUSSION Structure. The structure of 1 is formed of the following crystallographically distinct structural units: two BO3 triangles, one BO3H group, one BO4 tetrahedron, one AlO4 tetrahedron, and one Ba atom. All atoms are located at general positions. Figure 2 gives the fundamental building block of the structure, B

DOI: 10.1021/acs.inorgchem.7b02855 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. A fundamental building block of the structure of 1 containing three BO3 triangles and one BO4 tetrahedron, and one AlO4 tetrahedron. O(9) is a hydroxyl oxygen. The H atom is not shown.

which contains a three-membered ring of two BO3 triangles and one BO4 tetrahedron, and an additional BO3H group attached to a tetrahedral vertex. According to Burns et al., the descriptor for this decorated three-membered ring is 3△1□:△, where △ and □ represent BO3 triangle and BO4 tetrahedron, respectively.22 The borate cluster with the formula B4O8(OH) links by sharing three triangular vertices and one tetrahedral vertex with four AlO4 tetrahedra to form an aluminum borate single layer in the ac plane with the OH groups being directed perpendicular to the layer and the charge-compensating Ba2+ cations between the layers (Figures 3 and 4). Each AlO4 group links by vertex sharing to three BO3

Figure 4. Structure of 1 viewed in a direction approximately parallel to the c axis. Key: bright green triangles, BO3; green tetrahedra, BO4; yellow tetrahedra, AlO4; blue circles, Ba atoms.

Al2B8O20(OH)2, which are linked to each other via four O atoms to form bilayers. The mean B−O bond lengths of the BO3 and BO4 groups in 1 are 1.367−1.368 and 1.471 Å, respectively. The mean bond lengths of BO3 and BO4 groups are close to the grand mean [3] B−O distance of 1.370 Å and [4]B−O distance of 1.476 Å, respectively, based on the data from well refined crystal structures of 80 borate minerals.24 All oxygen atoms except O(9) are bonded to two B atoms or one B and one Al atom. The oxygen atom of the hydroxo group O(9)−H is bonded to B(4) only. The Ba2+ cations occupy eight-coordinated sites with Ba−O distances of 2.667−2.894 Å in the lamellar region between the aluminum borate layers. Hydrogen bonding was not detected because the observed O(9)···O distances (>3.2 Å) were considerably longer than the van der Waals contact distance of 2.8 Å and a sharp absorption at 3585 cm−1 in the IR spectrum. A sharp band around 3600 cm−1 was also observed in the IR spectra of hydrated alkaline earth borates.12 Thermal Stability. As indicated by powder X-ray diffraction and DSC/TG analysis, compound 1 is thermally stable up to about 700 °C. The dehydration of HBO32− occurs at a much higher temperature than that for HPO42−. This difference can be explained by Pauling’s electrostatic valence rule. The electrostatic bond strength (e.b.s.) of the B−O bond is 3/3 as compared with the e.b.s. value of 5/4 for the P−O bond, and therefore, the O−H bond in the HBO32− group is stronger such that dehydration of 1 occurs at a higher temperature than that for hydrogen phosphate. The TGA curve for 1 can be compared with that for Ba[AlB4O8(OH)]·H2O.23 The latter decomposes in two overlapping steps. The first step is because of the loss of a water molecule of crystallization, and the second step at about 700 °C corresponds to the dehydration of the hydrogen borate group and loss of a half water molecule. Compound 1 decomposes into Ba2Al2B8O17 (denoted as 2) at 740 °C according to the following equation:

Figure 3. Section of a layer in the structure of 1 viewed in a direction approximately parallel to the b axis. Key: bright green triangles, BO3; green tetrahedra, BO4; yellow tetrahedra, AlO4.

groups and one BO4 group. The topology of the aluminum borate layer is (44)-sql net with the B4O8(OH) cluster and AlO4 tetrahedron at different nodes. Recently, two isostructural metal borates, Ba[MB4O8(OH)]· H2O (M = Al, Ga),23 which contain the same B4O8(OH) cluster as that in 1, were synthesized under mild hydro(solvo)thermal conditions. However, each cluster is bidentate and bonds to a five-coordinate Al atom through two O atoms, forming a AlB4O11(OH) cluster, which can be derived from the B5O9(OH) cluster by substituting an AlO33− group for a BO+ group. The AlO5 group has a geometry of trigonal bipyramid, which is in contrast to the tetrahedral AlO4 in 1. Two AlB4O11(OH) clusters are connected by two Al−O−Al bonds to form the fundamental building unit with the formula

2BaAlB4O8(OH) → Ba 2Al 2B8O17 + H 2O C

DOI: 10.1021/acs.inorgchem.7b02855 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Compound 2 melts incongruently at 810 °C, which is consistent with the report by Kimura et al.17 11 B and 27Al MAS NMR Spectroscopy. 11B has a high natural abundance (80.4%) and high sensitivity and is therefore usually the nucleus of choice for B-NMR. It is a spin-3/2 nuclide resulting in significant peak broadening of the resonances of compounds with an unsymmetrical bonding environment. Tetrahedral BO4 has a symmetric and narrow absorption curve due to the small quadrupole interaction, whereas the trigonal BO3 has a larger coupling constant which gives rise to a broad and asymmetric line shape.25,26 It is difficult to distinguish between different types of 3-coordinate

11.74 T. The lack of intensity in the range of 0 and 30 ppm indicates a lack of 6- and 5-coordinate Al, and the sharp peak at 56.7 ppm indicates a 4-coordinate Al. These NMR experiment results are in agreement with those from crystal structure analysis. In summary, a new barium aluminum borate with a 2D layer structure was synthesized under high-temperature and highpressure hydrothermal conditions and structurally characterized by single-crystal X-ray diffraction and IR and MAS 11B and 27Al NMR spectroscopy. The borate clusters with the formula B4O8(OH) are linked by sharing three triangular vertices and one tetrahedral vertex with four AlO4 tetrahedra to form an aluminum borate single layer. This compound has the same framework composition as the barium aluminum borate hydrate, Ba[AlB4O8(OH)]·H2O, but they adopt very different crystal structures. Compound 1 decomposes into Ba2Al2B8O17 at high temperatures as indicated by DSC/TG analysis combined with powder X-ray diffraction. It crystallizes in a centrosymmetric space group, precluding it from use in nonlinear optical applications. Although it contains OH groups in the structure, the compound does not dehydrate upon heating until 740 °C. Further exploratory synthesis to discover new nonlinear optical materials is in progress.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02855. X-ray powder diffraction patterns and an IR spectrum (PDF)

Figure 5. 11B MAS NMR of 1 collected at 11.74 T.

Accession Codes

CCDC 1584629 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

boron and obtain their relative populations. Figure 5 shows the 11 B MAS NMR of 1 collected at 11.74 T. The sharp peak at δiso = 1.27 ppm corresponds to B(3)O4, and the broad signal represents a superposition of resonances of 2-connected B(4)O3 and 3-connected B(1)O3 and B(2)O3. Their chemical shifts agree well with those in the literature. 27Al MAS NMR is a powerful probe of the Al environments and easily resolves tetrahedral Al (0 ppm), 5-coordinate Al (30 ppm), and octahedral Al (50−60 ppm) if the field strength is greater than about 8.5 T.27 Figure 6 is the 27Al MAS NMR of 1 collected at

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kwang-Hwa Lii: 0000-0003-3150-1361 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Ministry of Science and Technology of Taiwan for financial support. REFERENCES

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Figure 6. 27Al MAS NMR of 1 collected at 11.74 T. D

DOI: 10.1021/acs.inorgchem.7b02855 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b02855 Inorg. Chem. XXXX, XXX, XXX−XXX