Amine-Templated Aluminoborates Exhibiting Graphite and Diamond

Jan 11, 2010 - 2010, Vol. 10. 765–774. Amine-Templated Aluminoborates Exhibiting Graphite and. Diamond Nets. Avijit Kumar Paul and Srinivasan Natara...
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DOI: 10.1021/cg901161z

Amine-Templated Aluminoborates Exhibiting Graphite and Diamond Nets

2010, Vol. 10 765–774

Avijit Kumar Paul and Srinivasan Natarajan* Framework Solids Laboratory, Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560012, India Received September 22, 2009; Revised Manuscript Received December 15, 2009

ABSTRACT: A solvothermal reaction of Al2O3, H3BO3, pyridine, and H2O at 180 °C/7 days in the presence of organic amine molecules gave rise to four new aluminoborates, [(C6H18N2)(AlB6O13H3)], I; [(C5H16N2)(AlB5O10)] 3 2H2O, II; [(C5H16N2)(AlB5O10)], III; and [(C5H17N3)(AlB5O10)] 3 H2O, IV, with two- and three-dimensional structures. All the structures have been formed by the connectivity involving Al3þ ions and [B5O10] cyclic pentaborate units. In I, the 3-connected trigonal nodes form a layer that resembles a graphite structure has been observed. The compounds II, III, and IV, have 4-connected nodes that forms a diamond related three-dimensional structure. The formation of solvatomorphs in II and III is noteworthy and has been observed first time in a family of amine templated aluminoborates. A comparison of the various aluminoborate structures reveals subtle relationships between the organic amines (length of the amines) and the final framework structures. The compounds have been characterized using a variety of techniques including IR, second-order optical behavior, and MAS NMR studies.

Introduction Compounds exhibiting open structures have grown to occupy a prime position in materials chemistry.1 Continued research over the years clearly suggests that the open-framework structures accommodate many anions such as sulfates,2 sulfites,3 selenates,4 and selenites,5 in addition to the more well-known silicates and phosphates.6 The use of borate anions as a structure builder could be attractive for the chemist as boron exhibits varying coordination environments (trigonal and tetrahedral). Thus, it offers more flexibility to manipulate the coordination environment of not only the cations but also the anions. The ease of polymerization of borate anions, under suitable conditions, giving rise to polyborate species of different dimensionalities is an added incentive for investigating borate frameworks. Earlier attempts at the incorporation of boron in openframework structures resulted in the formation of borosilicate7 and borophosphate8 compounds. Metal borates have also been prepared employing borate fluxes at high temperatures (T > 800 °C).9 The borate frameworks, formed at high temperatures, are generally neutral or cationic in nature.9,10 Synthesis of anionic framework of metal borate compounds have been attempted, recently, employing hydrothermal methods in the presence of organic amine molecules.11 Though the number of available anionic borate frameworks is still limited, the observed structural features in aminetemplated open-framework borates clearly indicated that the borate frameworks can also exhibit the vast structural diversity that are more commonly found in silicate and phosphate frameworks. Furthermore, the borates also exhibit structures that can be related to classical inorganic ones such as CrB4,11a diamond,11b SrAl2,11c etc. nets. As part of the study to explore and incorporate newer anions as network builders, we have recently established the use of thiosulfate anions.12 In continuation of the theme,

we have been studying the formation of anionic borate frameworks employing solvothermal approaches. Our studies were successful and we have now isolated four new amine-templated aluminoborate structures, [(C6H18N2)(AlB6O13H3)], I; [(C5H16N2)(AlB5O10)] 3 2H2O, II; [(C5H16N2)(AlB5O10)], III; and [(C5H17N3)(AlB5O10)] 3 H2O, IV. The structures are formed by the connectivity involving Al3þ and [B5O10] units forming two-dimensional graphitic and three-dimensional diamond related network structures. In this paper, we present the synthesis, structure, and characterization of the aluminoborate phases. Experimental Section

*To whom correspondence should be addressed. E-mail: snatarajan@ sscu.iisc.ernet.in.

Synthesis and Initial Characterization. All the compounds were prepared employing the solvothermal method. In a typical synthesis, for I, Al2O3 (0.104 g, 1.0 mM) was dispersed in a solution of 0.6 mL of H2O and 4.40 mL of pyridine. To this, boric acid (0.186 g, 3 mM) and 2-methyl-1,5-diaminopentane (2-Me 1,5-DAP; 0.65 mL, 5 mM) were added under continuous stirring. The mixture was homogenized for 30 min at room temperature. The final mixture with a composition, 1:3:5:54:33 Al2O3/H3BO3/2-Me 1,5-DAP/pyridine/H2O was then sealed in a 23 mL PTFE lined autoclave and heated at 180 °C for 7 days under autogenous pressure. The initial pH of the reaction mixture was 8, and no appreciable change in pH was noted after the reaction. The final product, containing large quantities of colorless rod-type crystals, was filtered, washed with deionized water under a vacuum, and dried at ambient conditions (yield ∼ 75% based on Al). A similar synthesis procedure was employed for the preparation of other compounds, but by varying the organic amine molecules or the solvent mixtures. The synthesis conditions are summarized in Table 1. All the compounds were characterized by powder X-ray diffraction (PXRD), IR, UV-vis, thermogravimetric analysis (TGA) and MAS NMR studies. The PXRD patterns were recorded in the 2θ range 5-50° by using Cu-KR radiation (Philips, X’pert-Pro). The XRD patterns indicated that the products were new and the patterns were entirely consistent with the simulated XRD pattern generated based on the structures determined using the single crystal XRD (see Supporting Information, Figure S1). Infrared (IR) spectroscopic studies have been carried out in the mid-IR region (4000 to 400 cm-1) on KBr pellets (Perkin-Elmer, SPECTRUM

r 2010 American Chemical Society

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Table 1. Synthesis Composition and Conditions Employed for the Compounds I-IVa S. no. 1 2 3 4

composition

temp (°C)

time (days)

pH

1(Al2O3) þ 3(H3BO3) þ 5(2-methyl-1,5-diaminopentane) þ 54(pyridine) þ 33(H2O) 1(Al2O3) þ 3(H3BO3) þ 5(1,5-diaminopentane) þ 54(pyridine) þ 33(H2O) 1(Al2O3) þ 3(H3BO3) þ 5(1,5-diaminopentane) þ 58(pyridine) 1(Al2O3) þ 3(H3BO3) þ 5(N-ethylamine-1,3diaminopropane) þ 54(pyridine) þ 33(H2O)

180

7

8.0

[(C6H18N2)(AlB6O13H3)], I

product

yield (%) 75

180

7

8.0

[(C5H16N2)(AlB5O10)] 3 2H2O, II

80

180

7

8.0

[(C5H16N2)(AlB5O10)], III

77

180

7

8.0

[(C5H17N3)(AlB5O10)] 3 H2O, IV

72

a Elemental analysis: for I: calcd (%) C 17.11, H 5.02, N 6.65; found C 17.19, H 5.09, N 6.79; for II: calcd (%) C 15.75, H 5.29, N 7.34; found 15.87, H 5.37, N 7.29; for III: calcd (%) C 17.39, H 4.67, N 8.11; found C 17.48, H 4.59, N 8.20; for IV: calcd (%) C 15.87, H 5.06, N 11.10; found C 15.96, H 5.16, N 11.0.

Table 2. Crystal Data and Structural Refinement Parameters of [(C6H18N2)(AlB6O13H3)], I; [(C5H16N2)(AlB5O10)] 3 2H2O, II; [(C5H16N2)(AlB5O10)], III and [(C5H17N3)(AlB5O10)] 3 H2O, IVa structural parameter

I

II

III

IV

empirical formula formula weight crystal system space group a (A˚) b (A˚) c (A˚) R (°) β (°) γ (°) V (A˚3) Z D (calc/g cm-3) μ (mm-1) λ (Mo KR/A˚) θ range (°) total data collected unique data Rint R indexes [ I > 2σ(I)] R indexes (all data)

[(C6H18N2)(AlB6O13H3)] 421.09 monoclinic Cc (No. 9) 19.713(2) 7.4085(4) 16.2048(15) 90.000 128.216(16) 90.000 1859.4(3) 4 1.504 0.175 0.71073 3.80-25.98 8490 3432 0.0545 R1 = 0.0553; wR2 = 0.1091 R1 = 0.0952; wR2 = 0.1192

[(C5H16N2)(AlB5O10)] 3 2H2O 381.23 monoclinic P21 (No. 4) 9.102(5) 10.402(5) 9.283(5) 90.000 110.941(5) 90.000 1062.0(2) 2 1.526 0.185 0.71073 2.35-26.00 7357 2926 0.0920 R1 = 0.0741; wR2 = 0.1462 R1 = 0.1227; wR2 = 0.1650

[(C5H16N2)(AlB5O10)] 345.23 monoclinic C2 (No. 5) 9.1093(3) 11.1484(3) 7.9463(3) 90.000 117.733(2) 90.000 714.28(4) 2 1.605 0.194 0.71073 2.90-25.98 6076 1300 0.0261 R1 = 0.0394; wR2 = 0.1106 R1 = 0.0408; wR2 = 0.1115

[(C5H17N3)(Al(B5O10)] 3 H2O 378.25 monoclinic P21 (No. 4) 8.9210(5) 10.0784(4) 8.9621(7) 90.000 109.686(7) 90.000 758.68(8) 2 1.647 0.197 0.71073 3.15-26.00 4852 2598 0.0221 R1 = 0.0332, wR2 = 0.0822 R1 = 0.0349, wR2 = 0.0828

)

)

a R1 = Σ F0| - |Fc /Σ|F0|; wR2 = {Σ[w(F02 - Fc2)2]/Σ[w(F02)2]}1/2. w = 1/[σ2(F0)2 þ (aP)2 þ bP], P = [max(F02,0) þ 2(Fc)2]/3, where a = 0.0574 and b = 0.0000 for I, a = 0.0628 and b = 0.0000 for II, a = 0.0601 and b = 1.4551 for III, a = 0.0565 and b = 0.0000 for IV.

1000). IR spectra for all the compounds were found to be comparable (see Supporting Information, Figure S2). The solid state UV-vis spectroscopic studies were carried out at room temperature (Perkin-Elmer model Lambda 35 UV-vis spectrometer). The observed reflectance spectra were converted into a absorption like spectra using the Kubelka-Munk function (see Supporting Information, Figure S3). Single-Crystal Structure Determination. A suitable colorless single crystal of each compound was carefully selected under a polarizing microscope and glued to a thin glass fiber with a cyanoacrylate (superglue) adhesive. The single crystal data were collected on a Bruker AXS smart Apex CCD diffractometer at 293(2) K. The X-ray generator was operated at 50 kV and 35 mA using Mo KR (λ=0.71073 A˚) radiation. Data were collected with ω scan width of 0.3°. A total of 606 frames were collected at three different settings of j (0, 90, 180°) keeping the sample-to-detector distance fixed at 6.03 cm and the detector position (2θ) fixed at -25°. The data were reduced using SAINTPLUS,13 and an empirical absorption correction was applied using the SADABS program.14 The structure was solved and refined using SHELXL9715 present in the WinGx suit of programs (Version 1.63.04a).16 The hydrogen positions for the two lattice water molecules in II and one lattice water molecule in IV were not located. The hydrogen positions for the amine molecules were, however, located in the difference Fourier maps. For the final refinement, the hydrogen atoms were placed in geometrically ideal positions and refined using the riding mode. The final refinement included atomic positions for all the atoms, anisotropic thermal parameters for all the nonhydrogen atoms, and isotropic thermal parameters for all the hydrogen atoms. Full-matrix least-squares refinement against |F2|

was carried out using the WinGx package of programs. Details of the structure solution and final refinements for all the structures are given in Table 2. The pictures of the four single crystals have also been given (see Supporting Information, Figure S4). The crystallographic data for compounds can be found in CCDC: 748030-748033 by free of charge from The Cambridge Crystallographic Data Center (CCDC) via www.ccdc.cam.ac.uk/ data_request/cif.

Results Structure of [(C6H18N2)(AlB6O13H3)], I (Graphite net). The asymmetric unit of I contains 28 non-hydrogen atoms. There is only one crystallographically independent Al3þ ion, which is coordinated by four oxygen atoms of the borate unit forming a tetrahedral environment, (AlO4, CN = 4). The average Al-O bond length is 1.728 A˚ and the average O-Al-O bond angle is 109.5°. Of the four oxygen atoms that are connected to Al, one belongs to a terminal O-B(OH)2 unit and the other three are from the B5O10H cyclic borate units. The average B-O-Al bond angle is 146.6°. There are five triangular (B1 to B5) and one tetrahedral boron (B6) atom in the asymmetric unit. The triangular boron atoms have an average B-O distance of 1.366 A˚ and the tetrahedral boron atom has a longer B-O distance with an average of 1.473 A˚. The average O-B-O bond angle for the triangular boron atoms is 120.0° and the average

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Table 3. Selected Bond Distances Observed in [(C6H18N2)(AlB6O13H3)], I; [(C5H16N2)(AlB5O10)] 3 2H2O, II; [(C5H16N2)(AlB5O10)], III; and [(C5H17N3)(AlB5O10)] 3 H2O, IVa moiety

distance (A˚)

moiety

distance (A˚)

Al(1)-O(1) Al(1)-O(2) Al(1)-O(3) Al(1)-O(4) B(1)-O(1) B(1)-O(5) B(1)-O(6) B(2)-O(2)#3 B(2)-O(9) B(2)-O(10) B(3)-O(3) B(3)-O(9)

Compound I 1.723(3) B(3)-O(11) 1.717(3) B(4)-O(4) 1.722(3) B(4)-O(12) 1.751(3) B(4)-O(13) 1.324(6) B(5)-O(5) 1.408(6) B(5)-O(7) 1.347(6) B(5)-O(8) 1.328(6) B(6)-O(6)#4 1.387(6) B(6)-O(8)#4 1.372(5) B(6)-O(10) 1.332(6) B(6)-O(11) 1.397(6)

Al(1)-O(1) Al(1)-O(2) Al(1)-O(3) Al(1)-O(4) B(1)-O(1) B(1)-O(8) B(1)-O(9) B(2)-O(2)#5 B(2)-O(5) B(2)-O(8)

Compound II 1.716(5) B(3)-O(3) 1.723(4) B(3)-O(7) 1.721(4) B(3)-O(10)#6 1.740(4) B(4)-O(4) 1.362(8) B(4)-O(6) 1.352(7) B(4)-O(10) 1.348(8) B(5)-O(5) 1.314(8) B(5)-O(6)#7 1.374(7) B(5)-O(7)#8 1.397(7) B(5)-O(9)

1.370(8) 1.349(8) 1.413(8) 1.321(8) 1.352(8) 1.386(8) 1.487(7) 1.448(7) 1.482(7) 1.482(7)

Al(1)-O(1) Al(1)-O(1)#1 Al(1)-O(2) Al(1)-O(2)#1 B(1)-O(1) B(1)-O(4) B(1)-O(5)

Compound III 1.735(2) B(2)-O(2) 1.735(2) B(2)-O(3) 1.738(2) B(2)-O(4)#2 1.738(2) B(3)-O(3)#3 1.333(4) B(3)-O(3)#4 1.388(4) B(3)-O(5) 1.370(4) B(3)-O(5)#5

1.330(4) 1.388(4) 1.387(4) 1.454(4) 1.454(4) 1.480(4) 1.480(4)

Al(1)-O(1) Al(1)-O(2) Al(1)-O(3) Al(1)-O(4) B(1)-O(1) B(1)-O(5) B(1)-O(6) B(2)-O(2) B(2)-O(8) B(2)-O(10)

Compound IV 1.7298(18) B(3)-O(3)#4 1.7334(18) B(3)-O(9) 1.7349(17) B(3)-O(10) 1.7391(16) B(4)-O(4)#5 1.319(3) B(4)-O(6) 1.379(3) B(4)-O(7) 1.396(3) B(5)-O(5)#6 1.340(3) B(5)-O(7)#6 1.370(3) B(5)-O(8) 1.394(3) B(5)-O(9)

1.339(3) 1.361(3) 1.386(3) 1.337(3) 1.392(3) 1.365(3) 1.465(3) 1.465(3) 1.470(3) 1.467(3)

1.377(5) 1.342(6) 1.377(7) 1.366(7) 1.380(6) 1.363(6) 1.329(5) 1.472(6) 1.495(7) 1.458(6) 1.456(6)

a

Symmetry transformations used to generate equivalent atoms. I: #3 x, y þ 1, z; #4 x þ 1/2, -y þ 1/2, z þ 1/2. II: #5 -x, y - 1/2, -z þ 1; #6 -x þ 1, y þ 1/2, -z þ 2; #7 x - 1, y, z; #8 -x, y - 1/2, -z þ 2. III: #1 -x, y, -z; #2 x þ 1/2, y - 1/2, z; #3 x - 1/2, y þ 1/2, z; #4 -x þ 1/2, y þ 1/2, -z þ 1; #5 -x, y, -z þ 1. IV: #4 -x þ 1, y þ 1/2, -z þ 1; #5 -x, y þ 1/2, -z; #6 x, y, z þ 1.

O-B-O bond angle for the tetrahedral boron atoms is 109.4°. The B atoms are bonded together, through the B-O-B linkages, forming a cyclic polyborate cluster, [B5O10H]. The various geometric parameters observed in I are comparable to other similar aluminoborate structures reported in the literature.11 Selected bond distances are listed in Table 3 and the bond angles are given in Table S1 (Supporting Information). The compound I has a two-dimensional layered structure formed by the linkages involving the cyclic pentaborate clusters and the Al3þ ions (Figure 1a). The cyclic pentaborate cluster, [B5O10H], contains four triangular borate (BO3) units and one tetrahedral borate (BO4) unit. The connectivity between the borate units is such that they form two sixmembered rings, B3O3, fused with the common tetrahedral

Figure 1. (a) View of the two-dimensional layer in [(C6H18N2)(AlB6O13H3)], I. Note that the terminal -OB(OH)2 units hang from Al centers (see text). (b) View of the arrangement of layers in I along the b axis. The layers are held together by nearly linear B-O-H 3 3 3 O-B hydrogen bonds (see text).

boron at the center. The six-membered rings are connected in a way that they are almost perpendicular to each other with the dihedral angle between the two rings of ∼80.0°. Out of four triangular borate units, three are connected with the aluminum centers through the oxygen atom and remaining one is a terminal hydroxyl unit. Thus, one pentaborate unit is bonded with three aluminum centers. The aluminum atom, in turn, is connected to three [B5O10H] units and the fourth vertex is bonded to a terminal -H2BO3 unit (see Supporting Information, Figure S5). The connectivity between the [B5O10H] units and Al3þ ions gives rise to a two-dimensional layer structure (Figure 1a). The layers are arranged one-over the other in a AAAA... fashion. The arrangement of layers is such that there is a supermesh of aperture of ∼8.5 A˚ (shortest atom-atom contact distances, not including the van der Waals radii), which penetrate the entire structure in a direction perpendicular to the layers (Figure 1b). The organic amine molecules occupy these pseudo one-dimensional channels and participate extensively in hydrogen bond interactions. The presence of terminal -H2BO3 group hanging from the Al-center is an important feature in this structure and has been observed for the first time in the amine templated borates. The -H2BO3 groups participate in

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intra- as well as interlayer hydrogen bond interactions through the O-H 3 3 3 O interactions (Figure 1b). The formation of classical, near linear hydrogen bonds17 in this structure suggests that the interactions between the layers are strong. Interactions of this type are generally observed in purely hydrogen bonded organic systems rather than inorganic layered compounds with extended structures.18 The complete list of hydrogen bond interactions is given in Table 4. Structures of [(C5H16N2)(AlB5O10)] 3 2H2O, II, [(C5H16N2)(AlB5O10)], III, and [(C5H17N3)(AlB5O10)] 3 H2O, IV (Diamond net). The asymmetric units of II, III and IV contain 25, 13, and 25 non-hydrogen atoms, respectively. The Al3þ ions in all the compounds are tetrahedrally coordinated by four oxygen atoms of the borate unit (AlO4, CN = 4). The average Al-O bond lengths are 1.725 A˚ (II), 1.736 A˚ (III), and 1.734 A˚ (IV) and the average O-Al-O bond angles are 109.4° (II), 109.5° (III), and 109.5° (IV). In II, out of the five independent boron atoms, four (B1 to B4) have triangular and one (B5) has a tetrahedral environment with respect to the oxygens. The average B-O distances are 1.361 A˚ (triangular) and 1.474 A˚ (tetrahedral). In III, out of the three independent B atoms, B1 and B2 have triangular connectivity and B3 has a tetrahedral connectivity with respect to the oxygens. The average B-O distances are 1.366 A˚ (triangular) and 1.467 A˚ (tetrahedral). In IV, out of the five independent boron atoms, four have triangular (B1 to B4) and one has a tetrahedral (B5) environment with respect to the oxygens, similar to II. The average B-O distances are 1.365 A˚ (triangular) and 1.466 A˚ (tetrahedral). The average O-B-O bond angles are 120.0° (triangular) and 109.4° (tetrahedral), respectively. Selected bond distances are listed in Table 3 and the bond angles are given in Table S1 (Supporting Information). Similar to I, the triangular and tetrahedral boron atoms are linked together forming the cyclic pentaborate unit, [B5O10], which is the common unit present in II, III, and IV. The two six-membered borate rings lie more or less perpendicular to each other with the dihedral angles between the rings of ∼84.0° (II), ∼85.0° (III), and ∼83.0° (IV). The three-dimensional structures of the aluminoborates are formed by the linkages involving the [B5O10] cluster units and the Al3þ ions. The Al centers are connected to four [B5O10] units and each [B5O10] unit is connected to four Al centers (see Supporting Information, Figures S6-S8). In II, the connectivity between [B5O10] and Al3þ ions are such that they form both the right-handed as well as the left-handed helices (Figure 2a). This suggests that the Al-borate chains lie on the 21 axis. The left- and the right-handed helices couple with each other to form the three-dimensional structure with helical channels of size 9.6 A˚  7.7 A˚ (shortest atom-atom contact distances, not including the van der Waals radii) (Figure 2b). The channels are occupied by the fully protonated amine molecule (1,5-DAP) along with the free water molecules, which participate in hydrogen bond interactions (Table 4). The three-dimensional structure of III also has one-dimensional channels of size 9.5 A˚  7.4 A˚ (shortest atom-atom contact distances, not including the van der Waals radii), in which the protonated amine molecules (1,5-DAP) are located (Figure 3). In this structure, the unusual positioning of the amine molecules did not favor any hydrogen-bond interactions. In the structure IV, the connectivity between [B5O10] units and Al3þ ions are exactly similar to that

Paul and Natarajan

Figure 2. (a) The helical arrangement of the aluminoborate chains in II, [(C5H16N2)(AlB5O10)] 3 2H2O. The central rod-like unit is a guide to the eye. (b) The three-dimensional structure of II in the ab plane. Note that the amine molecules protrude out of the channels and are arranged in a helical way. The lattice water molecules are avoided for clarity.

observed in the structure II. The three-dimensional structure contains one-dimensional channels of pore size of 9.7 A˚  7.0 A˚ (shortest atom-atom contact distances, not including the van der Waals radii) (Figure 4). The channels are occupied by one lattice water molecule and a biprotonated amine molecule (EADAP) (see Supporting Information, Figure S8). The structures of II and IV are comparable as the connectivities are similar, but the compound [(C5H16N2)(AlB5O10)] 3 2H2O, II has one additional water molecule compared to [(C5H17N3)(AlB5O10)] 3 H2O, IV. The amine molecule (EADAP) in IV has one additional nitrogen atom compared to the 1,5-DAP molecule present in II, which probably occupies the space of the lattice water in the framework in IV. In the structure of III, the different orientation and positioning of the amine molecules (1,5DAP) did not have sufficient space to have additional solvent molecules. The protonated amine molecules are stabilized through the extensive hydrogen boding interactions with the framework borate units in all the borate compounds, except III (Table 4). Thermal Studies. Thermogravimetric analysis (MettlerToledo TG850) for all the compounds was carried out in

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Table 4. Important Hydrogen Bond Interactions Observed in [(C6H18N2)(AlB6O13H3)], I; [(C5H16N2)(AlB5O10)] 3 2H2O, II; and [(C5H17N3)(AlB5O10)] 3 H2O, IVa D-H (A˚)

D-H 3 3 3 A

H3 3 3A (A˚)

D3 3 3A (A˚)

D-H 3 3 3 A (°)

N(1)-H(1) 3 3 3 O(12)#1 N(1)-H(2) 3 3 3 O(10)#2 N(1)-H(3) 3 3 3 O(11)#3 N(2)-H(4) 3 3 3 O(9)#4 N(2)-H(5) 3 3 3 O(8)#5 N(2)-H(6) 3 3 3 O(4) O(7)-H(7) 3 3 3 O(13)#6 O(12)-H(12) 3 3 3 O(5)#2 O(13)-H(13) 3 3 3 O(6)

Compound I 0.89 2.00 0.89 2.06 0.89 2.13 0.89 1.94 0.89 2.08 0.89 2.06 0.82 1.91 0.82 1.95 0.82 1.91

2.846(8) 2.918(5) 2.924(5) 2.811(5) 2.933(8) 2.891(5) 2.728(4) 2.731(4) 2.710(6)

158 161 149 168 161 156 177 159 164

N(1)-H(11A) 3 3 3 O(100)#7 N(1)-H(11B) 3 3 3 O(9)#8 N(1)-H(11C) 3 3 3 O(7)#9 N(2)-H(12A) 3 3 3 O(200)#10 N(2)-H(12B) 3 3 3 O(5) N(2)-H(12C) 3 3 3 O(6)#10

Compound II 0.89 1.95 0.89 1.94 0.89 2.00 0.89 1.96 0.89 2.01 0.89 1.95

2.812(9) 2.827(6) 2.879(7) 2.807(8) 2.900(7) 2.814(6)

164 172 168 159 177 164

Compound IV 0.90 1.97 0.89 2.15 0.88 2.07 0.83 2.15 0.89 1.97

2.846(4) 2.923(3) 2.937(3) 2.924(3) 2.846(4)

164 145 167 155 169

N(1)-H(1) 3 N(1)-H(2) 3 N(1)-H(3) 3 N(2)-H(4) 3 N(3)-H(7) 3

3 3 3 3 3

#11

3 N(3)#12 3 O(7) 3 O(8)#13 3 O(9) #14 3 N(1)

Figure 4. (a) The three-dimensional structure of [(C5H17N3)(AlB5O10)] 3 H2O, IV in the bc plane. Note the amine molecules occupy the center of the channels. The lattice water molecules are omitted for clarity.

a Symmetry transformations used to generate equivalent atoms: #1 -1/2 þ x, 1/2 - y, -1/2 þ z; #2 x, 1 - y, -1/2 þ z; #3 x, -y, -1/2 þ z; #4 1/2 þ x, 1/2 - y, 1/2 þ z; #5 1/2 þ x, -1/2 þ y, z; #6 x, 1 - y, 1/2 þ z; #7 2 - x, 1/2 þ y, 1 - z; #8 1 þ x, y, 1 þ z; #9 1 - x, 1/2 þ y, 1 - z; #10 1 - x, -1/2 þ y, 1 - z; #11 -1 þ x, y, z; #12 -x, -1/2 þ y, 1 - z; #13 1 - x, -1/2 þ y, 2 þ z; #14 1 - x, -1/2 þ y, 1 - z.

Figure 3. (a) The three-dimensional structure of [(C5H16N2)(AlB5O10)], III in the bc plane. Note the position of the amine molecules is such that they occupy the channels. Note also the position with respect to the amine in II (see text).

an atmosphere of flowing oxygen (flow rate=50 mL min-1) in the temperature range 30-850 °C (heating rate = 5 °C min-1). The results (see Supporting Information, Figure S9) indicated that the compound I shows a continuous weight loss up to ∼750 °C in multiple steps. The total observed weight loss of 31% corresponds to the loss of the amine molecules (calcd 29%). The compound II also behaves

Figure 5. The MAS NMR spectra for the aluminoborates, I-IV. (a) 27Al spectra. (b) 11B spectra.

similarly with a continuous weight loss up to ∼700 °C. The total observed weight loss of 36% for II corresponds to the loss of the lattice water and the amine molecules (calcd 37%). The compound III shows an initial weight loss in the

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Figure 6. (a) Node connectivity between Al and [B5O10H] units in I. Note the formation of graphite-like layers (honeycomb net). Inset shows the representation of [B5O10H] and Al as single point nodes. (b) The arrangement of the layers and the positioning of the amine in I. (c) The graphite-like layers in [Zn(dien)2][{Al(OH)}{B5O9F}]. (d) The arrangement of layers and the positioning of the cation in [Zn(dien)2][{Al(OH)}{B5O9F}]. Note the close resemblance between the two structures.

range 100-350 °C followed by a continuous weight loss up to 750 °C. The total weight loss of 33.5% corresponds to the loss of the amine molecules (calcd 30.5%). For compound IV, again we observed a continuous weight loss up to ∼700 °C. The observed weight loss of 39.5% for IV corresponds to the loss of the lattice water and the amine molecules (calcd 36.5%). The final calcined product in all the cases was found to be crystalline as identified by powder XRD and corresponds to a mixture of aluminoborate (Al4B2O9; ICDD No: 09-0158) and boron trioxide (B2O3; ICDD No: 06-0634).

Optical Studies. The aluminoborates, I-IV, have space groups Cc (No. 9), P21 (No. 4), C2 (No. 5), and P21 (No. 4), which suggests that they could exhibit second-order nonlinear optical properties.19 We have carried out a study of the second harmonic generation (SHG) property on the powdered samples of I-IV using a laser beam of energy 2.5 mJ/ pulse. The observed responses for the compounds were 6 mV, 8 mV, 12.5 mV, and 9 mV, respectively, for I-IV. These values are much lower than the values observed for KDP (KH2PO4, 75 mV). Since, the optical properties were studied on well ground and powdered samples, the random

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771

Table 5. Summary of the Aluminoborate Framework Structures Formed with [B5O10] Units and Aluminum Centers initial amine (atoms no.) 1,4-DAB (6) 1,5-DAP (7) 1,5-DAP (7) EADAP (8) N-cyclohexl-1,3-DAPr(11) TETA (10)

empirical formula [NH3(CH2)4NH3][AlB5O10] 3 2H2O [NH3(CH2)5NH3][AlB5O10] 3 2H2O [NH3(CH2)5NH3][AlB5O10] [NH3(CH2)2NH(CH2)3NH3][AlB5O10] 3 H2O [CH3NH2(CH2)3NH3][AlB5O10] [{NH3(CH2)2NH(CH2)2NH(CH2)2NH3}2][{AlB5O10}2] 3 0.25H2O

orientation of the crystallites could have resulted in the lower values for the SHG behavior. The fact that we did observe some response for SHG behavior suggests that compounds indeed crystallized in noncentrosymmetric space groups. MAS NMR Studies. The present compounds contain both Al and B, which are amenable for probing using MAS NMR investigations. The 27Al and 11B MAS NMR spectroscopic studies have been carried out on a Bruker-AVIII 500 spectrometer. Al(NO3)3 and H3BO3 were used as the external standard for the Al and B, respectively, and the chemical shifts of the compounds are relative to the standard samples. All the four compounds exhibit more or less similar MAS NMR signals with respect to both the nucleus. One singlet at 0.3 ppm was observed for 27Al MAS NMR spectra (Figure 5a). The compounds have only one chemically distinct Al center and the observation of a single peak would be expected. Similar observations have also been made before.20 In the case of 11B NMR, one doublet in the range -5 to -12 ppm and a singlet at -18 ppm have been observed (Figure 5b). The doublet signals observed around -5 to -12 ppm would correspond to the three-coordinated boron species and the singlet observed around -18 ppm correspond to the four-coordinated boron species. It has been established that the second-order nuclear electric quadrupolar interactions in trigonal boron species would affect the NMR signals and leads to shift toward the upfield region, and hence, we observed weak up-shifted signals for trigonal boron species.21 In addition, we have also evaluated the bond strength values of the boron species in all the compounds using the method of Brown22 and correlated that with the 11B NMR signals. It has been shown that the isotropic chemical shifts of the NMR spectra shift up/downfield depending on the bond-strength of the probe nucleus.23 We find that the bond strength values of the trigonal boron (5.7 to 6.2) are reasonably less compared to the tetrahedral boron (∼8.0) (see Supporting Information, Table S2). Thus, in the present compounds the lower bond strength values suggest that the trigonal boron would exhibit higher chemical shift compared to the tetrahedral boron; which is the observed behavior. Discussion By varying the organic amine molecule in the synthesis mixture or modifying the solvent mixture, four new aluminoborate phases have been isolated. The conditions for the reactions in all the cases being the same, the observed differences between the structures could only result from the subtle role played by the organic amine molecule. In compounds I-III, the amine molecule employed is similar, with the exception of the presence of a terminal -CH3 group in [(C6H18N2)(AlB6O13H3)], I. The hanging -CH3 group would facilitate weak C-H 3 3 3 O hydrogen bond interactions along with the N-H 3 3 3 O interactions. Such extended interactions

templated amine (no. of non-hydrogen atoms)

type of net

short symbol

ref

1,4-DAB (6) 1,5-DAP (7) 1,5-DAP (7) EADAP (8)

CrB4 (crb) diamond (dia) diamond (dia) diamond (dia)

4.65 66 66 66

11a compound II compound III compound IV

N-Me-1,3-DAPr (6) TETA (10)

diamond (dia) SrAl2 (sra)

66 42.63.8

11b 11c

would possibly favor the formation and stabilization of a lower dimensional structure. In addition, we observed the presence of one terminal borate units (H2BO3) in [(C6H18N2)(AlB6O13H3)] (I), which also actively participates in hydrogen bond interactions (Figure 1b). In fact, the O-H 3 3 3 O interactions between the borate units resemble the classical acetate-dimers observed in the literature before.24 The extended hydrogen bonded interactions stabilize the two-dimensional structure in I. Though compound [(C5H16N2)(AlB5O10)] 3 2H2O (II) and [(C5H16N2)(AlB5O10)] (III) are formed by the use of 1,5-diaminopentane by varying the solvent mixture, the observed framework structures are the same. The amine molecules are oriented differently in II and III. The use of pyridine exclusively in the reaction mixture gave rise to a dehydrated phase in [(C5H16N2)(AlB5O10)] (III), whereas the water molecule was also incorporated as the guest in [(C5H16N2)(AlB5O10)] 3 2H2O (II). The two structures can thus, be considered as solvatomorphs in which the final structures differ in the amount of the solvent molecules. To the best our knowledge, compound II and III represent the first examples of solvatomorphs in amine templated metal borate family of compounds. Compound [(C5H17N3)(AlB5O10)] 3 H2O (IV) also has the same framework structure as that of compounds II and III, but the organic amine employed was different. The present study, thus, clearly illustrates the subtle effects of the organic amine molecules in stabilizing a particular framework. From a topological point of view, since all the compounds are formed by the same building units, it is worth comparing the structures. Compound [(C6H18N2)(AlB6O13H3)] (I) has B5O10H units with one terminal B-OH group rendering the cyclic borate unit to essentially form only three bonds (3connected). Similarly, the Al3þ ion also has a terminal H2BO3 unit and forms three bonds (3-connected). Thus, compound I has 3-connected nodes, which is also observed for the graphite structure (honeycomb topology) (Figure 6a). The corresponding Schl€ afli symbol for the net is 63. The Similar honeycomb topology has also been observed in the aluminoborate structure, [Zn(dien)2][{Al(OH)}{B5O9F}] (Figure 6c).25 Comparing the reported structure with the present structure, one can easily see the close relationship. In the [Zn(dien)2][{Al(OH)}{B5O9F}] structure, the honeycomb layers are separated by [Zn(dien)2] complex and the layers are anionic (Figure 6d). The present structure also has anionic layers separated by the organic amine molecules (Figure 6b). Extensive hydrogen bond interactions have also been observed in I compared to [Zn(dien)2][{Al(OH)}{B5O9F}]. The compounds [(C5H16N2)(AlB5O10)] 3 2H2O, II, [(C5H16N2)(AlB5O10)], III, and [(C5H17N3)(AlB5O10)] 3 H2O, IV, form with the same framework structure. The [B5O10] unit is 4-connected and the Al3þ ion is also 4-connected resulting in a 4-connected tetrahedral node structure. The topological analysis of the three-dimensional structures reveal that the

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Figure 7. (a) The CrB4 (crb) net observed in [NH3(CH2)4NH3][AlB5O10] 3 2H2O. Note that the amine molecules occupy the channels fully. The diamond (dia) net observed in the compounds (b) II, (c) III, (d) IV, and (e) [CH3NH2(CH2)3NH3][AlB5O10]. (f ) The SrAl2 (sra) net observed in [{NH3(CH2)2NH(CH2)2NH(CH2)2NH3}2][{AlB5O10}2] 3 0.25H2O. Note that the amine molecules extend over many channels.

Paul and Natarajan

structures II, III, and IV exhibit the diamond (dia) net (Figure 7b-d). The corresponding Schl€ afli symbol and the vertex symbol is 66 and 62.62.62.62.62.62, respectively. It is also well-known that the structures based on dia net are often acentric and exhibit NLO property.26 Since, we have three compounds formed with the diamond structure (II-IV), we examined the spread of the Al-O-B angles in these structures. As can be noted (see Supporting Information, Table S1), the Al-O-B angle exhibits a wide range (132.7-157.1°) in the three structures. The topology of the structure, generally, depends on the connectivity between the participating entities (nodes) and we examined the dihedral angle that is present within the [B5O10] units in the threedimensional aluminoborate structures, II-IV. The dihedral angle does not show much deviation (range of 83.085.0°) suggesting that the local nodes stabilize the particular net (dia) in these structures. The diamond nets (II-IV) can be compared by the dihedral angles of the opposite nodes in the six-membered rings which are the building units of diamond structure (see Supporting Information, Figure S10). The dihedral angles in all the structures are comparable within the diamond structures for all the aluminoborates. The three-dimensional structures may also be compared with the reported aluminoborate structure, Li2[AlB5O10].27 The lithium aluminoborate structure also contains the [AlB5O10]2- building units, which were observed in the structures II, III, and IV. In the Li2[AlB5O10] structure, the threedimensional anionic [AlB5O10]2- moiety is formed by the linkages between the tetrahedral Al centers and the [B5O10] units, but the resulting diamond net exhibits 2-fold interpenetration (see Supporting Information, Figure S11a). The charge compensation in this structure is achieved by the presence of two Liþ ions. Each Liþ ion is also tertrahedrally connected with the three [B5O10] units and each [B5O10] unit is connected with six Liþ ions. The topological analysis of the overall structure reveals that the structure consists of three distinct nodes with (3,4,10) connected net (see Supporting Information, Figure S11b). The Schl€ afli symbol of the overall net is (43)2(42.64)(48.633.84). It is likely that the presence of larger and bulkier organic amine molecules could have been responsible for the absence of interpenetration in the present structures. Such observations have also been made before.28 In order to appreciate the topology of the present structures (II, III, and IV), we compared all the amine-templated aluminoborate structures reported in the literature based on the [B5O10] building unit (Table 5). From Table 5, it appears that there is some correlation between the size of the amine molecule and the resulting framework topology of the aluminoborates. Thus, when 1,4-diaminobutane was employed, the compound, [NH3(CH2)4NH3][AlB5O10] 3 2H2O,11a forms a three-dimensional structure consisting of 4-connected tetrahedral nodes, but the connectivity between the Al3þ ions and [B5O10] units forms a structure with a four- and six-membered ring. The corresponding Schl€ afli symbol and the vertex symbol is 4.65 and 4.62.6.6.6.6, respectively. This structure is similar to the connectivity of the boron atoms observed in CrB4 compound, and the net structure is known as a crb net (Figure 7a). The present structures have the diamond net and are prepared in the presence of 1,5-diaminopentane (II and III) and N-ethylamine-1,3-diaminopropane (IV). Diamond net has also been observed in [CH3NH2(CH2)3NH3][AlB5O10],11b with the N-methyl-1,3-diaminopropane (Figure 7e). When a

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much longer amine molecule, triethylenetetramine (TETA) was employed, a new compound [{NH3(CH2)2NH (CH2)2NH(CH2)2NH3}2][{AlB5O10}2] 3 0.25H2O,11c has been isolated. This compound also forms a three-dimensional structure with 4-connected tetrahedral nodes, but crystallizes with SrAl2 (sra) net and has 4-, 6-, and 8-membered rings (Figure 7f ). The Schl€afli symbol and the vertex symbol of this net are 42.63.8 and 4.6.4.6.6.82, respectively. Thus, when the amine molecule was shorter, as in 1,4-diaminobutane (six non-hydrogen atoms), the CrB4 structure was stabilized and when the amine molecule was larger, as in TETA (10 non-hydrogen atoms), SrAl2 net was stabilized. Both CrB4 and SrAl2 structures may be considered to represent the two extreme ends of the three-dimensional net based on tetrahedral nodes and the use of any other amine molecules appears to result in a diamond net. We wanted to examine this hypothesis by carrying out a reaction in the presence of 3,30 -diamino dipropylamine [HN(CH2CH2CH2NH2)2] (nine non-hydrogen atoms). The reaction yielded a known aluminum borate, [CH3NH2(CH2)3NH3][AlB5O10], formed in the presence of N-methyl-1,3-diaminopropane with a diamond net.11b It is clear that the amine decomposed during the synthesis. Similar decomposition of the amine molecule was also observed during the preparation of the aluminoborate, [CH3NH2(CH2)3NH3][AlB5O10].11b Here the N-cyclohexyl1,3-diaminopropane (11 non-hydrogen atoms) decomposed to give N-methyl-1,3-diaminopropane as in our present attempts. When we employed a long linear amine, tetraethylenepentamine ([NH3(CH2)2NH(CH2)2NH(CH2)2NH(CH2)2 NH3], 13 non-hydrogen atoms), we observed the formation of the SrAl2 structure formed during the preparation of the aluminoborate, [NH3(CH2)2NH(CH2)2NH(CH2)2NH3}2][{AlB5O10}2] 3 0.25H2O, using TETA.11c Thus, in the family of three-dimensional aluminoborate structures, only three network structures have been stabilized (crb, dia, and sra). Any modifications in the use of the amine molecules appear to result in any one of these structures only. In addition to the diamond net, during the present study we have been able to isolate a graphite related structure which may be considered as a precursor to the diamond net. Conclusion The synthesis of aluminoborate compounds with two- and three-dimensional structures has been accomplished. All the compounds have [B5O10] cyclic borate units, formed from four trigonal boron and one tetrahedral boron species, connected to Al3þ ions. The stabilization of a graphite related structure and a diamond related structure based on 3- and 4-connected nodes is noteworthy. The formation of the first solvatomorphs in II and III with diamond net structures from the same amine, 1,5-diaminopentane, by varying the solvent during the synthesis is noteworthy. To the best of our knowledge, this is the first such observation in the family of amine templated aluminoborates. It appears that [B5O10] cyclic pentaborate unit is one of the common building units in aluminoborate family of compounds. Since the borates are known to give rise to a variety of polyborate species, it is likely that this family of compounds would also exhibit diverse structures similar to the silicate structures. Acknowledgment. We thank Prof. P. K. Das of the Department of Inorganic and Physical Chemistry, Indian Institute of Science, for help with the SHG measurements. We thank Mamata Biswal, NMR center, IISc, for help with the

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collection of NMR data. The authors thank the Department of Science and Technology (DST), Government of India, for the award of a research grant. The Council of Scientific and Industrial Research (CSIR), Government of India, is thanked for the award of a research grant (S.N.) and a fellowship (A.K.P.). S.N. thanks DST for the award of a RAMANNA Fellowship. Supporting Information Available: All the bond angles, bond strength calculation, PXRD, IR-spectra, UV-vis absorbance spectra, TGA plot, crystal morphology, and the basic building rings of the successive complexes. This information is available free of charge via the Internet at http://pubs.acs.org/.

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