Achiral Benzoic Acid Derivatives as Chiral Cocrystal Building Blocks in

CRYSTAL GROWTH & DESIGN

Achiral Benzoic Acid Derivatives as Chiral Cocrystal Building Blocks in Supramolecular Chemistry: Adducts with Organic Amines

2006 VOL. 6, NO. 5 1186-1193

Ting-Feng Tan,† Jie Han,† Mei-Li Pang,† Hai-Bin Song,‡ Yu-Xin Ma,† and Ji-Ben Meng*,† Department of Chemistry, Nankai UniVersity, Tianjin 300071, People’s Republic of China, and The State Key Laboratory of Elemento-organic Chemistry, Nankai UniVersity, Tianjin 300071, People’s Republic of China ReceiVed January 7, 2006; ReVised Manuscript ReceiVed February 25, 2006

ABSTRACT: Achiral benzoic acid derivatives were cocrystallized with a range of amines, affording binary or ternary cocrystals [(p-nitrobenzoic acid)‚(2-aminopyridine)] (1), [(p-nitrobenzoic acid)‚(4,4′-bipyridine)] (2), [(p-nitrobenzoic acid)‚(p-methoxyaniline)] (3), [(3,5-dinitrobenzoic acid)‚(2,2′-bipyridine)] (4), [(3,5-dinitrobenzoic acid)‚(4, 4′-bipyridine)] (5), and [(phthalic acid)‚(2aminobenzimidazole)] (6) under specified conditions. All the prepared cocrystals have a common structural feature of the 21 screw axis. It is noteworthy that the cocrystals 1*, 4*, and 6*, achiral benzoic acid derivatives and nitrogen bases, are noncentrosymmetric and chiral cocrystals. Strong N-H‚‚‚O interactions are involved in the chiral cocrystals, undoubtedly following the best donor/ acceptor guidelines. X-ray single-crystal diffraction studies reveal that the stronger N-H‚‚‚O and O-H‚‚‚N interactions, as well as weaker C-H‚‚‚O ones, are all among the driving forces for the construction of the hydrogen-bonding networks in 1-6. Thus, a two-dimensional (2D) framework is constructed for 2, and a three-dimensional (3D) column framework is constructed for 3 by multiple hydrogen bonds. There are pairwise interwoven sheets in 4 and 6. For 5, a V-shaped arrangement of molecules exhibits a parallel orientation. Introduction The crystal engineering of self-assembled supramolecular architectures is currently of great interest, due to its intriguing topologies and its applications in materials, particularly in optoelectronics, conductivity and superconductivity, chargetransfer and magnetism, nanoporus materials, and biomimetic materials.1,2 Nowadays, hydrogen bonding is a master key in the areas of crystal engineering, supramolecular chemistry, material science, and biological recognition.3-5 In the discussion about the packing modes of the molecular crystal, it is important to understand weaker forces such as the N-H‚‚‚O, O-H‚‚‚N, and C-H‚‚‚O hydrogen bondings and π-π stacking interactions,6 which play a critical role in controlling the packing modes of molecular crystals. In particular, such interactions have been extensively used in the field of organic crystal engineering to assemble organic molecular building blocks into well-defined crystalline materials.7 Chiral crystallization for two-component molecular crystals is a powerful tool for designing absolute asymmetric syntheses8 and new functional solid materials.9 Noncentrosymmetric and chiral crystals formed from achiral organic compounds10 inherently have second-order nonlinear optical character. Studies on preparation of noncentrosymmetric and chiral crystals have revealed that the cocrystallization approach by combining two different achiral molecules provides a higher feasibility for chiral crystal architecture than a single-component approach.11 In this paper, we report the preparation, crystal structures, and properties of six cocrystals (Scheme 1). Among them, three chiral cocrystals were prepared from achiral benzoic acid derivatives and amines under routine conditions for cocrystallization. One key component has an amino group or nitrogen atom as an electron acceptor, and another has a carboxyl group * To whom correspondence should be addressed. Tel.: 86-22-23509933. Fax: 86-22-23502230. E-mail: [email protected]. † Department of Chemistry. ‡ The State Key Laboratory of Elemento-Organic Chemistry.

Scheme 1

as an electron donor to produce a chiral space group. The nitrogen atom of the pyridine moiety can also act as a cationic binding site, or the amino group can act as a hydrogen donor. Simultaneously, the carboxyl group was used as a strong anionic connector. Experimental Section Materials and Methods. All materials were commercially available and used as received. Fourier transform infrared spectra were recorded from KBr pellets on a Bio-Rad FTS 135 spectrometer. Elemental analyses were carried out with a Yanaco CHN CORDER MT-3 apparatus. The melting points of six new compounds were recorded on a X-4 digital apparatus without correction. Thermogravimetric analyses (TGA) were performed under nitrogen atmosphere using a Netzsch TG 209 analyzer with a heating rate of 10 °C min-1. Preparation of [(p-Nitrobenzoic acid)‚(2-aminopyridine)] (1). The crystallization of p-nitrobenzoic acid (0.042 g, 0.25 mmol) with 2-aminopyridine (0.024 g, 0.25 mmol) in 30 mL of acetone afforded colorless block crystals of 1, mp ) 198-200 °C, within 5 days. Anal. Calcd. for C12H11N3O4: C, 55.17; H, 4.24; N, 16.08%. Found: C, 55.31; H, 4.18; N, 16.16%. IR (cm-1): 3265 (br), 2963 (br), 2447 (br), 1730, 1687, 1521, 1383, 1340, 1286, 1124, 1073, 1009, 824, 720. Preparation of [(p-Nitrobenzoic acid)‚(4,4′-bipyridyl)] (2). The crystallization of p-nitrobenzoic acid (0.084 g, 0.50 mmol) with 4,4′bipyridyl (0.048 g, 0.25 mmol) in 30 mL of methanol afforded colorless crystals of 2, mp ) 192-194 °C. Anal. Calcd. for C12H9N2O4: C, 58.78; H, 3.70; N, 11.42%. Found: C, 58.86; H, 3.45; N, 11.31%. IR (cm-1): 3112, 3056, 2392, 1680, 1603, 1530, 1491, 1408, 1341, 1313, 1063, 1005, 877, 810, 797.

10.1021/cg060009y CCC: $33.50 © 2006 American Chemical Society Published on Web 04/19/2006

Adducts with Organic Amines

Crystal Growth & Design, Vol. 6, No. 5, 2006 1187

Table 1. Crystal Data and Structure Refinement Summary for Compounds 1-6a 1* empirical formula M crystal size/mm3 crystal system space group a/Å b/Å c/Å R/deg β/deg γ/deg V/Å3 Z Fcalcd/g cm-3 µ/mm-1 F (000) range of h.k.l total/ind. reflns parameters Rint R1 [I > 2σ (I)] wR2 [all] GOF residuals e Å-3 a

2

3

4*

5

6*

C12H11N3O4

C24H18N4O8

C14H14N2O5

C24H16N6O12

C24H16N6O12

C22H20N6O4

261.24 0.32 × 0.22 × 0.10

490.42 0.32 × 0.24 × 0.20

290.27 0.25 × 0.25 × 0.25

580.43 0.40 × 0.32 × 0.22

580.43 0.22 × 0.12 × 0.10

432.44 0.32 × 0.20 × 0.14

monoclinic

monoclinic

monoclinic

monoclinic

monoclinic

orthorhombic

P21 8.175(4) 6.719(3) 11.189(5) 90.00 98.991(6) 90.00 607.0(5) 2 1.429 0.110 272 -8/9, -7/7, -13/10 3319/2091

P21/n 7.8399(19) 6.8494(17) 20.788(5) 90.00 92.555(4) 90.00 1115.2(5) 4 1.461 0.112 508 -9/9, -7/8, -25/20 6011/2270

P21/n 4.2446(9) 23.732(5) 13.884(3) 90.00 97.441(4) 90.00 1386.8(5) 4 1.390 0.107 608 -5/5, -27/29, -17/14 7815/2865

P21 5.6782(17) 21.221(6) 10.394(3) 90.00 91.946(4) 90.00 1251.7(6) 2 1.540 0.127 596 -6/6, -25/24, -6/12 6789/4268

P21/c 6.169(2) 22.252(9) 10.396(3) 90.00 116.387(17) 90.00 1278.4(8) 4 1.508 0.124 596 -6/7, -22/26, -11/12 6787/2264

P212121 8.2519(18) 11.814(3) 22.267(5) 90.00 90.00 90.00 2170.7(8) 4 1.323 0.095 904 -9/9, -14/11, -26/26 11815/3826

172 0.0184 0.0341 0.0849 1.022 0.102, -0.168

164 0.0406 0.0442 0.1306 0.987 0.156, -0.169

193 0.0491 0.0441 0.1269 1.015 0.218, -0.148

381 0.0170 0.0332 0.0832 1.099 0.117, -0.108

192 0.0634 0.0817 0.2497 1.011 0.366, -0.217

290 0.0489 0.0457 0.1160 1.051 0.239, -0.188

Compounds with askerisks represent chiral compounds.

Preparation of [(p-Nitrobenzoic acid)‚(p-methoxyaniline) (3). A solution of p-nitrobenzoic acid (0.084 g, 0.50 mmol) and p-methoxyaniline (0.062 g, 0.50 mmol) in 30 mL of methanol was allowed to evaporate slowly at room temperature. Brown crystals of 3, mp ) 164166 °C, were obtained. Anal. Calcd. for C14H14N2O5: C, 57.93; H, 4.86; N, 9.65%. Found: C, 57.86; H, 4.73; N, 9.87%. IR (cm-1): 2841, 2606, 2143, 1885, 1632, 1591, 1518, 1468, 1383, 1342, 1260, 1192, 1171, 1105, 1025, 879, 825, 800, 725. Preparation of [(3,5-Dinitrobenzoic acid)‚(2,2′-bipyridyl)] (4). A solution of 3,5-dinitrobenzoic acid (0.106 g, 0.5 mmol) and 2,2′bipyriddyl (0.039 g, 0.25 mmol) in 30 mL of acetonitrile was left undisturbed to evaporate slowly under ambient conditions. Colorless crystals of 4, mp ) 148-149 °C, were obtained. Anal. Calcd. for C24H16N6O12: C, 49.66; H, 27.78; N, 14.48%. Found: C, 49.56; H, 17.89; N, 14.64%. IR (cm-1): 3091, 1653, 1538, 1458, 1347, 1163, 1075, 905, 766. Preparation of [(3,5-Dinitrobenzoic acid)‚(4,4′-Bipyridyl)] (5). Compound 5 was obtained via reaction of 3,5-dinitrobenzoic acid (0.106 g, 0.5 mmol) and 4,4′-bipyridyl (0.048 g, 0.25 mmol) in 30 mL of acetone. Colorless crystals, mp ) 158-160 °C, appeared within 14 days. Anal. Calcd. for C12H8N3O6: C, 49.66; H, 2.78; N, 14.48%. Found: C, 49.76; H, 2.54; N, 14.34%. IR (cm-1): 3100, 2876, 2404 (br), 1607, 1540, 1457, 1409, 1347, 1070, 814, 724. Preparation of [(Phthalic Acid)‚(2-Aminobenzimidazole)] (6). A solution of phthalic acid (0.042 g, 0.25 mmol) and 2-aminobenzimidazole (0.067 g, 0.5 mmol) in 30 mL of acetonitrile evaporated slowly at room temperature. Colorless crystals of 6, mp ) 255-256 °C, suitable for X-ray crystallography appeared within 2 days. Anal. Calcd. for C22H20N6O4: C, 61.10; H, 4.66; N, 19.43%. Found: C, 61.25; H, 4.38; N, 19.24%. IR (cm-1): 3288, 3068, 2780, 1699, 1610, 1475, 1364, 1270, 1160, 1043, 801. Single-Crystal X-ray Diffraction Determination and Refinement. X-ray single-crystal diffraction data for 1-6 were collected on a Bruker Smart 1000 CCD diffractometer at 293(2) K with Mo KR radiation (λ ) 0.71073 Å) by ω scan mode. There was no evidence of crystal decay during data collection for all compounds. All the measured independent reflections were used in structural analysis, and semiempirical absorption corrections were applied using the SADABS program. The program SAINT was used for integration of the diffraction profiles.12 The structures were solved by direct methods using the SHELXS program of the SHELXTL package and refined with SHELXL.13 The final refinement was performed by full-matrix least-squares methods on F2 with anisotropic thermal parameters for all non-H atoms. Most of the hydrogen atoms were first observed in difference electron density maps and then placed in the calculated sites and included in the final

Table 2. Possible Intermolecular Hydrogen Bonds for Compounds 1-6a compound

D-H‚‚‚A (Å)

D‚‚‚A (Å)

H‚‚‚A (Å)

D-H‚‚‚A (deg)

1*

N2-H2‚‚‚O4a N3-H3A‚‚‚O3b N3-H3B‚‚‚O3c O1-H1‚‚‚N2d C3-H3‚‚‚O2e C8-H8‚‚‚O4f C12-H12‚‚‚O2g N2-H2A‚‚‚O2h N2-H2B‚‚‚O1i N2-H2C‚‚‚O2j C9-H9‚‚‚O4k O7-H7‚‚‚O5l N1-H1‚‚‚O6m C7-H7‚‚‚O8 N3‚‚‚O12n O5-H5‚‚‚N3o N1-H1A‚‚‚O3p N1-H1B‚‚‚O1q N2-H2‚‚‚O2r N3-H3A‚‚‚O1s N4-H4A‚‚‚O4t N4-H4B‚‚‚O2 N5-H5A‚‚‚O4 N6-H6A‚‚‚O3u

2.684(2) 2.812(3) 2.830(5) 2.606(3) 3.257(3) 3.165(5) 3.203(3) 2.715(3) 2.718(2) 2.733(3) 3.453(6) 2.473(2) 2.691(4) 3.258(7) 2.879(5) 2.554(23) 2.915(3) 2.776(3) 2.836(3) 2.691(3) 2.775(3) 2.967(3) 2.711(3) 2.752(3)

1.825 1.953 1.981 1.789 2.441 2.473 2.554 1.830 1.835 1.852 2.559 1.667 1.910 2.352

175.77 177.05 168.97 174.25 146.4 131.4 127.3 172.17 171.53 170.18 161.3 166.94 150.36 164.3

1.747 2.128 1.928 1.987 1.844 1.941 2.171 1.857 1.897

167.54 152.00 168.37 169.08 168.03 163.00 153.92 171.68 172.50

2

3

4*

5 6*

a Symmetry codes: ax, y - 1, z; bx, y - 1, z; c-x + 1, y - 1/2, -z; dx, y - 1, z; e1 - x, 1 - y, -z; f1/2 - x, 1/2 + y, 1/2 - z; gx, y - 1, z; hx, y - 1, z; i-x + 2, -y + 1, -z; jx - 1, y - 1, z; k1 - x, 1 - y, 1 - z; lx 1, y, z; mx - 1, y, z + 1; n1 - x, y - 1/2, -z; ox - 2, -y + 1/2, z - 1/2; px - 1/2, -y + 1/2, -z; qx - 1, y, z; rx - 1, y, z; sx - 1/2, -y + 1/2, -z; t-x + 2, y - 1/2, -z + 1/2; u-x + 2, y - 1/2, -z + 1/2.

refinement in the riding model approximation with fixed thermal factors. Further details for crystallographic data and structural analysis are listed in Table 1.

Results and Discussion Molecular and Supramolecular Structures of Compounds 1-6. The schematic representations of the different types of hydrogen-bonding synthons in this work are summarized in Scheme 2. Hydrogen-bond geometries of compounds 1-6 are listed in Table 2. Chiral Crystallization for Two-Component Molecular Crystals. Molecular assembly in crystals is controlled by the molecular structure and intermolecular interactions. For example,

1188 Crystal Growth & Design, Vol. 6, No. 5, 2006

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Scheme 2

homochirality of the molecular building blocks ensures noncentrosymmetricity in the bulk material, and conformational flexibility can have a profound influence on the molecular organization in the solid state. In the chiral crystallization, the key component often has an amino group or nitrogen atom as an electron acceptor.14 The nitrogen atom of pyridine moiety or bipyridyl also acts as a cationic binding site, and the achiral benzoic acid derivative is used as a strong anionic connector. In this work, the chiral column crystal 1* provides a typical example shown in Figure 1, since building blocks in 1* all have a considerably similar structure with the 21 screw axis along the b-axis. The column structure of 1* is formed by the aggregation of such building blocks via the strong N-H‚‚‚O hydrogen bonding (Scheme 3).15 The 2D structures of cocrystals 1*, 4*, and 6* are shown in Figure 1 (1*), Figure 4 (4*) and Figure 6 (6*), respectively. Accordingly, we now assume that building blocks with 21-column structure would aggregate in the opposite orientation if there are no effective interactions between them. Hence, some certain driving forces such as

Figure 1. (a) Molecular structure of compound 1* with atom labeling. C, black; N, blue; O, red; H, purple. (b) Polar assembly of molecules. Hydrogen atoms not involved in the hydrogen bond are omitted for clarity. (c) Hydrogen-bonded 21 chain along the b-axis in crystals of compound 1*. Only the carboxyl group of p-nitrobenzoic acid and the amino group of 2-aminopyridine are shown and hydrogen atoms not involved in hydrogen bonding are omitted for clarity (right).

hydrogen bonding must be required for the aggregation of 21column synthons in the same orientation. In this section, we will discuss how each column structure is aggregated and how the intermolecular interactions such as the



Figure 2. (a) Molecular structure of compound 2 with atom labeling. (b) The essential hydrogen-bonded patterns observed in compound 2. (c) Packing arrangement of the supramolecular structures obtained with compound 2 along the crystallographic b-axis.

Figure 3. (a) 2D supramolecular structure of compound 3 along the crystallographic bc-plane; the broken lines indicate the hydrogen bonds. (b) 2D supramolecular column structure of compound 3 along the crystallographic ac-plane of the small channel via N2-H2B‚‚‚O1, N2H2C‚‚‚O2, and N2-H2A‚‚‚O2 and the big channel via C9-H9‚‚‚O4, N2-H2C‚‚‚O2, and N2-H2A‚‚‚O2. (c) Projection of 3D packing of compound 3 showing the channels along the ac-plane.

Scheme 3


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Figure 4. (a) Crystal structure of ternary compound 4* (DNB-BP+DNB) with atom labeling of the asymmetric unit from crystal structure analysis. (b) The essential hydrogen-bonded patterns observed in compound 4*. The DNB-‚‚‚BP+, BP+‚‚‚DNB and DNB-‚‚‚DNB heterosynthons aggregating in the shape of a void of 3.5 × 17 Å. Only hydrogen atoms involved in hydrogen bonding and carboxyl alone are shown. (c) Structure of the hydrogen-bonded network in a molecular sheet in the crystal of compound 4*, where DNB-, BP+, and DNB are shown in red, cyan, and blue, respectively. Hydrogen atoms not involved in hydrogen bonding are omitted for clarity. (d) Spacing-filling model of network showing DNB -‚‚‚DNB spirals along the b-axis; only hydrogen atoms involved in hydrogen bonding and carboxyl alone are shown.

weaker N-H‚‚‚O hydrogen bonding affect the generation of chirality in their molecular crystals. Schematic representation of the aggregation process of column structures is shown in Scheme 3. [(p-Nitrobenzoic acid)‚(2-aminopyridine)] (1*). Compound 1* is orthorhombic, in the P21 chiral space group. The molecular structure of compound 1* is depicted in Figure 1a. The dihedral angle between acid and base components is 5.6°. The packing mode of compound 1* is depicted in Figure 1b (synthon I, Scheme 2). All of the hydrogen bonds in this crystal are asymmetrically arranged. The crystal 1* shows chiral crystallization through A-A type, which is shown in Scheme 3. Proton transfer between the p-nitrobenzoic acid and the pyridine is clearly revealed by the reduced C-O bond length of the resulting benzoate and the widened N-C-N bond angle in the pyridinium ring. The presence of the amino, pyridinium, and benzoate groups in the molecules leads to the formation of extensive hydrogen bonds in this salt.16 Extensive hydrogenbonding interactions are observed between both the nitrogen atom and the amino group of 2-aminopyridine and the carboxyl

group of the p-nitrobenzoic acid. These interactions lead to the 21 screw axis (Figure 1c). The short intermolecular N-H‚‚‚O bonds (rN2‚‚‚O4 ) 2.684 Å, θN2-H2‚‚‚O4 ) 175.77°; rN3‚‚‚O3 ) 2.812 Å, θ N3-H3A‚‚‚O3 ) 177.05°; rN3‚‚‚O3 ) 2.830 Å, θ N3-H3A‚‚‚O3 ) 168.97°) lead to the extended spiral chain along the b-axis. With the screw rotation (about b) being the only symmetry element present, all the chains in the crystal are oriented in the same direction (Figure 1c). Only the carboxyl group of p-nitrobenzoic acid and amine group of 2- aminopyridine are shown, and hydrogen atoms not involved in hydrogen bonds are omitted for clarity. [(p-Nitrobenzoic acid)‚(4,4′-bipyridine)] (2). The molecular structure of compound 2 is depicted in Figure 2a. The desired O-H‚‚‚N hydrogen bond is formed by the -OH group of the carboxylic acid and the bipyridine nitrogen atom (H‚‚‚N, 1.789 Å), and no proton transfer has occurred between the acid and the base. This hydrogen bond is accompanied by a C-H‚‚‚O contact involving H3, O2 atoms of the p-nitrobenzoic acid and H8, H12 atoms of 4,4′-bipyridine. The dihedral angle between the benzene ring of p-nitrobenzoic acid and pyridyl rings of


Figure 5. (a) Molecular structure of compound 5 with atom labeling; the broken lines indicate the hydrogen bonds within the unit. (b) Projection of offset parallel in compound 5 2D supramolecular sheet along the crystallographic b-axis via π‚‚‚π interactions. (c) Packing arrangement of the supramolecular structures obtained with compound 5 along the crystallographic a-axis.

bipyridine is 5.7°. As illustrated in Figure 2b, each 4,4′bipyridine connects four p-nitrobenzoic acid via the O1-H1‚‚‚N2, C8-H8‚‚‚O4, and C12-H12‚‚‚O2 hydrogen bonds (synthon II and III, Scheme 2). These interactions lead to the 21 chain along the b-axis. The supramolecular packing arrangement of compound 2 was illustrated in Figure 2c along the b-axis. [(p-Nitrobenzoic Acid)‚(p-methoxyaniline)] (3). Similar to that for compound 1*, proton transfer between p-nitrobenzoic acid and p-methoxyaniline is corroborated by the reduced C-O bond length of the resulting benzoate and the widened N2C11-C10 bond angle in the anilinium ion. The presence of the amino, aromatic hydrogen atom of anilinium ion, and benzoate group in the molecules leads to the formation of extensive hydrogen bonds in this salt.16 The dihedral angle between the benzene ring of p-nitrobenzoic acid and p-methoxyaniline is 30.6°. As shown in Figure 3a, two kinds of cavities are composed of four acids and four bases via the N2-H2B‚‚‚O1, N2-H2C‚‚‚O2, and C9-H9‚‚‚O4 (synthon IV, Scheme 2), and the aperture of the cavities are 3.4 × 3.6 Å and 8 × 12 Å, respectively. As a result, 2D supramolecular column structure with two channels in compound 3 is formed via N2-H2A‚‚‚ O2 along the crystallographic ac plane. These interactions lead to the 21 chain along the b-axis. As shown in Figure 3b (left), the 2D supramolecular column structure of the small channel in compound 3 is formed via N2-H2B‚‚‚O1, N2-H2C‚‚‚O2, and N2-H2A‚‚‚O2 interactions along the crystallographic acplane. Similarly, one of the big channels in compound 3 along


the crystallographic ac-plane forms through C9-H9‚‚‚O4, N2H2C‚‚‚O2, and N2-H2A‚‚‚O2 interactions (Figure 3b, right). This ensemble forms channels due to the stacking of sheets along the b-axis (Figure 3c). [(3,5-Dinitrobenzoic Acid)‚(2,2′-bipyridine)] (4*). 3,5-Dinitrobenzoic acid (DNB) and 2,2′-bipyridine (BP) formed ternary cocrystalline salts leading to the stoichiometry DNB-BP+DNB (compound 4*), as shown in Figure 4a. Compound 4* is found to belong to the chiral space group P21. Proton transfer between one of the DNB molecules and the pyridine is identified by the reduced C-O bond length of the resulting benzoate and the widened C1-N1-C5 in ′BP. The presence of the pyridinium, nitro, and benzoic acid/benzoate groups in the molecules leads to the formation of extensive hydrogen bonds in these salts. The interactions of DNB-‚‚‚BP+, BP+‚‚‚DNB-, and DNB-‚‚‚DNB result in a network of 3.5 × 17 Å voids (Figure 4b), in which the protonated N1 acts as hydrogen-bond donor to carboxylate O6. There are thus four kinds of hydrogen bonding, O7-H7‚‚‚O5, N1-H1‚‚‚O6, C7-H7‚‚‚O8, and N3‚‚‚O12, which link these three-component units: the resulting supramolecular structure adopts the form of pairwise interwoven sheets built from rings. The dihedral angle between DNB- and pyridyl ring (N1-C1-C2-C3-C4-C5) is 113.6°, and the dihedral angle between DNB and pyridyl ring (N2-C6-C7C8-C9-C10) is 46.3°. The neighboring sheets are filled via interpenetration of BP+ moieties, as shown in the top view of the four neighboring sheets (Figure 4c) (synthon V and VII, Scheme 2).17-19 The distance between the helical sheet is 10.394 Å, and the screw-pitch gauge is 21.221 Å. Figure 4d shows the spacing-filling model of protonated ′BP connecting spirals of DNB-‚‚‚DNB to form a 2D polar column structure, which is A-A type (Scheme 3). These interactions lead to the 21 chain along the b-axis, and thus form a chiral column structure having a 21 screw axis along the b-axis. [(3,5-Dinitrobenzoic acid)‚(4,4′-bipyridine)] (5). The molecular structure of compound 5 is depicted in Figure 5a. Similar to that for compound 4, the desired O-H‚‚‚N hydrogen bond is formed by the -OH group of the carboxylic acid and the nitrogen atom of the 4,4′-bipyridine, but no proton transfer has occurred between the acid and the base. Crystals of compound 5, grown by slow evaporation of an acetone solution, are found to belong to the space group P21/c. The dihedral angle between the benzene ring of 3,5-dinitrobenzoic acid and pyridyl rings of bipyridine is 54.2°. Packing motif of compound 5 forms a V-shaped arrangement of molecules exhibiting a parallel orientation (Figure 5b,c) (synthon VI, Scheme 2). The net parallel molecular orientation results in the centrosymmetric space group P21/c, so the 2D supramolecular sheet of compound 5 forms via π‚‚‚π interactions along the crystallographic b-axis. The distance between the parallel molecules was determined as 3.631 Å. [(Phthalic Acid)‚(Aminobenzimidazole)] (6*). The crystals are orthorhombic, in chiral space group P212121, with Z ) 4. The molecular structure of compound 6 is depicted in Figure 6a. Proton transfer between phthalic acid and 2-aminobenzimidazole is clearly proven by the reduced C-O bond length of the resulting benzoate and the widened N-C-N in the 2-aminobenzimidazole. The dihedral angles between acid and base components are 53.7° and 68.8°, respectively. As depicted in Figure 6b, one channel is composed of six acids and six bases via synthon VIII (Scheme 2) [N1-H1A‚‚‚O3, N1-H1B‚‚‚O1, N2-H2‚‚‚O2, N3-H3A‚‚‚O1, N4-H4A‚‚‚O4, N4-H4B‚‚‚O2, N5-H5A‚‚‚O4, and N6-H6A‚‚‚O3], and the aperture of the channel is up to 11 × 11 Å voids. Two carboxylic groups in


Tan et al.

Figure 6. (a) Molecular structure of compound 6* with atom labeling. (b) The essential hydrogen-bonded patterns occurring in compound 6*; the hydrogen bonds are indicated as broken lines in this and the subsequent figures. (c) Structure of the hydrogen-bonded network in a molecular sheet in the crystal of compound 6*; hydrogen atoms not involved in hydrogen bonding are omitted for clarity. (d) Spacing-filling model of the 2D supramolecular architecture consisting of two kinds of helixes in a molecular sheet, where 2-aminobenzimidazole between the two kinds of helixes is shown in cyan. Additionally, the hydrogen bond between the carboxyl O of phthalic acid and N of 2-aminobenzimidazole are shown for the sake of clarity.

each acid molecule take part in different hydrogen-bonding modes. The hydrogen-bonded network of compound 6* is shown in Figure 6c, which takes the form of pairwise interwoven sheets. The presence of the amino, nitrogen atom of imidazolium ion, and benzoate groups in the molecules leads to the formation of extensive hydrogen bonds in these salts.16 As shown in Figure 6d, the spacing-filling model of the 2D supramolecular architecture consists of two kinds of helixes in a molecular sheet, which shows chiral crystallization through A-A type (Scheme 3). Significantly, two kinds of helical tape are connected through N1-H1A‚‚‚O3 and N1-H1B‚‚‚O1 interactions. The screw-pitch gauges of the two spirals are both 11.814 Å, and the distance of the spiral chain is 23.747 Å. Additionally, hydrogen bondings between phthalic acid and 2-aminobenzimidazole (N1-H1A‚‚‚O3, N1-H1B‚‚‚O1, N2-H2‚‚‚O2, and N3-H3A‚‚‚O1) cannot be neglected because they connect the neighboring layers and lead to the formation of a 2D network along the 21-axis. Thermogravimetric Analysis. Compounds 1-6 are stable in air and can retain their structural integrity at room temperature

for a considerable length of time. TGA was conducted to determine the thermal stability of these crystalline materials. For 1*, the TGA curve shows that compound 1* remains intact until 150 °C, and there is a sharp weight loss peaking and ending at 243 and 254 °C, respectively, corresponding to the expulsion of all base and acid components. This may result from the high intensity of the N-H‚‚‚O bond. For 2, the first weight loss of 31.7% in the temperature range of 125-154 °C, peaking at 133 °C, corresponds to the loss of the base component (calculated: 31.8%). The residuary acid section accounting for the loss weight of 68.4% (calculated: 68.2%) is subsequently lost in the region of 154-280 °C (peak: 268 °C). For 3, the first weight loss of 42.7% in the 75-200 °C range, peaking at 182 °C, corresponds to the loss of the base component (calculated: 42.8%). The residuary acid section loses weight of 57.4% (calculated: 57.2%) subsequently from 200 to 243 °C (peak: 231 °C). TGA measurement shows that 4* remains intact below 150 °C, and there is a sharp weight loss peaking and ending at 288 and 300 °C, respectively, corresponding to the expulsion of all base and acid components. For 5, the first weight loss of


26.8% in the range of 75-210 °C, peaking at 184 °C, corresponds to the loss of the base component (calculated: 26.9%). The residuary acid section loses weight of 73.3% (calculated: 73.1%) subsequently from 210 to 294 °C (peak: 281 °C). For 6*, four consecutive weight losses occur in the 125-330 °C range (peak positions: 136, 199, 247, 290 °C), which may be attributed to the gradual loss of protonated bases. The remaining substance loses 38% weight at the 330-430 °C range (peak: 399 °C). Conclusions This study provides an approach for achiral benzoic derivatives as chiral building blocks using routine cocrystallization. Achiral aromatic acids are successful reagents to produce chiral cocrystals 1*, 4*, and 6* with various organic bases. X-ray single-crystal diffraction studies reveal that strong N-H‚‚‚O interactions are involved in all chiral cocrystals, which play a significant role in construction of the supramolecular structures of these organic solids. Chiral cocrystallization for two-component molecular crystals is a powerful tool for designing absolute asymmetric synthesis and new solid materials. With this in mind, this strategy will be helpful in the design of chiral cocrystallization. We are currently extending this strategy by using chloronitrobenzoic acids as hydrogen-bonding participators to prepare new crystalline materials. Acknowledgment. We thank the National Natural Science Foundation of China (20490210, 20372039) and N & T Joint Academy of China for financial support. We also thank Prof. Zheng-Jie He for help with the manuscript preparation (Department of Chemistry, Nankai University, People’s Republic of China).


(2)

(3) (4) (5) (6) (7) (8)

(9) (10) (11) (12) (13) (14) (15) (16) (17)

Supporting Information Available: Crystallographic information files (CIF) of compounds 1-6. This material is available free of charge via the Internet at http://pubs.asc.org.

References (1) (a) Blake, A. J.; Champness, N. R.; Hubberstey, P.; Withersby, M. A.; Schro¨der, M. Coord. Chem. ReV. 1999, 183, 117-138. (b) Lin,

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CG060009Y

Achiral Benzoic Acid Derivatives as Chiral Cocrystal Building Blocks in

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