Cocrystallization of Bent Dipyridyl Type ... - ACS Publications

Cocrystallization of Bent Dipyridyl Type Compounds with Aromatic Dicarboxylic Acids: Effect of the Geometries of Building Blocks on Hydrogen-Bonding Supramolecular Patterns

CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 3 1199-1208

Miao Du,* Zhi-Hui Zhang, and Xiao-Jun Zhao College of Chemistry and Life Science, Tianjin Normal University, Tianjin 300074, People’s Republic of China Received November 30, 2004;

Revised Manuscript Received February 14, 2005

ABSTRACT: The high-yielding preparation and X-ray structures of six 1:1 molecular cocrystals consisting of bent dipyridyl bases and aromatic dicarboxylic acids, including [phthalic acid]‚[3-bpo] (1), [phthalic acid]‚[4-bpo] (2), [isophthalic acid]‚[3-bpo] (3), [isophthalic acid]‚[4-bpo] (4), [terephthalic acid]‚[3-bpo] (5), and [terephthalic acid]‚ [4-bpo] (6), are described, in which 3-bpo refers to 2,5-bis(3-pyridyl)-1,3,4-oxadiazole and 4-bpo its 4-N-donor analogue 2,5-bis(4-pyridyl)-1,3,4-oxadiazole. A supramolecular synthon I [R22(7)] containing classical O-H‚‚‚N and weak C-H‚‚‚O interactions, usually observed in organic cocrystals of carboxylic acids with other heterocyclic bases, is again shown to be involved in constructing most of these hydrogen-bonding networks. In compounds 3-5, tapes of acid/base components are formed via synthon I, which are further extended to result in two-dimensional (2-D) supramolecular sheets via additional C-H‚‚‚O interactions. Compound 2 contains similar but helical tapes with synthon I, and the neighboring twisted chains are cross-linked via weak C-H‚‚‚O hydrogen bonds to show a 2-D corrugated network. Compound 6 contains a similar 2-D layered structure to that of 3-5, although synthon I was not found due to the stereochemistry effect. Unexpectedly, adjacent layers are further combined into a threedimensional (3-D) architecture through acid-base C-H‚‚‚O hydrogen bonds. However, in compound 1, through intermolecular O-H‚‚‚N and C-H‚‚‚O bonds, a novel 3-D structure was formed, in which acid and base subunits arrange alternately to exhibit a helical motif along the crystallographic [100] direction. For all these cocrystals, the common head-to-tail hydrogen-bonded ring motif [synthon II R22(8)] between carboxylic acids is absent. Melting points of cocrystals 1-6 are mainly correlated with the nature of the acid component and crystal packing. Thermogravimetric analysis of mass loss for six compounds has been shown to correlate with the strength of hydrogen bonds in the packing fraction. Introduction Nowadays, hydrogen bonding is a master key in the area of crystal engineering, supramolecular chemistry, material science, and biological recognition.1-4 The application of intermolecular hydrogen bonds is a wellknown and efficient tool to regulate the molecular arrangement in a crystal structure.4 The retrosynthesis of crystalline solids from multifunctional molecules may be achieved through supramolecular synthons,5 which are defined as structural units within supermolecules that can be assembled by hydrogen bonds and/or other intermolecular interactions. Great efforts have been made to identify supramolecular synthons, so that the variety inherent in crystal packing is appropriately classified and simplified in structural analysis.5-7 For example, hydrogen bonds between carboxy groups or carboxy and carboxylate groups usually compose a strong supramolecular synthon.8 It has been shown that the “supramolecular synthon” strategy may be utilized in the deliberate design of organic solids, possessing controlled and ordered architectures, and further leading to desired chemical, physical, or optical properties.9-15 Stronger classical hydrogen bonds (such as O-H‚‚‚O, O-H‚‚‚N, etc.) have been * To whom correspondence should be addressed. Fax: 86-2223540315. Tel: 86-22-23538221. E-mail: [email protected].

proven to be ideal tools to rationalize and systemize the relationship between molecular and supramolecular structures; however, weaker C-H‚‚‚X (where X ) N, O, π, etc.) interactions have more problems.15 Although specific examples have been noted,14-16 the general application of C-H‚‚‚X based synthons in crystal design is still challenging. As addressed in Desiraju’s recent account, weak hydrogen bonds need to be considered carefully in crystal engineering because they can affect crystal packing in unpredictable ways.17 Aromatic carboxylic acids have been most used and favored by supramolecular chemists for their ability to form strong, directional hydrogen bonds18 and also the different placements/binding modes of the carboxylic groups in the creation of novel frameworks through metal-carboxylic coordination.19 On the other hand, recently, angular dipyridyl-donor basic compounds, such as 2,5-bis(4-pyridyl)-1,3,4-oxadiazole (4-bpo) and its 3-Ndonor analogue 2,5-bis(3-pyridyl)-1,3,4-oxadiazole (3bpo) (see Scheme 1), were successfully applied to produce a series of infinite/discrete coordination polymers/ supramolecules with interesting structures and properties.20 We chose such unusual compounds as bent building blocks mainly considering their advantages compared with the common rodlike ligands (e.g., the famous 4,4′-bipyridine): (i) They have multiple hydrogenbonded acceptors or metal-coordination sites and thus

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Crystal Growth & Design, Vol. 5, No. 3, 2005 Scheme 1

are anticipated to form novel networks combined with both driven forces via self-assemblies. (ii) The structural geometries of these building blocks could be changeful and adaptable to meet the requirements for constructing different frameworks. Especially for 3-bpo, three possible conformations (one trans and two cis) were observed under approximate conditions. (iii) Pyridyl and oxadiazole rings within each system usually tend to involve aromatic stacking interactions, further affecting the crystal packing. Organic crystalline materials composed of aromatic carboxylic acid and dipyridyl compounds such as 4,4′-bipyridine or 4,4′-bipyridylacetylene have been documented, in which, however, the base component is always linear.21 Thus, if the base species is changed into such a bent one, what will happen? To search for the similarity and difference between cocrystal materials of aromatic phenyl diacids with linear/ angular base components and further understand the role of C-H‚‚‚X synthons in crystal engineering, we will report herein the preparation, crystal structures, and properties of six related acid-base cocrystals consisting of 4- or 3-bpo and the most typical aromatic carboxylic acids, including phthalic, isophthalic, and terephthalic acids (see Scheme 1). Experimental Section Materials and Methods. All the reagents and solvents for synthesis were commercially available and used as received, except for 3-bpo and 4-bpo, prepared according to the literature procedures.22 The melting points of six new compounds were recorded on a WRS-1B digital apparatus without correction, and Fourier transform (FT) IR spectra (KBr pellets) were taken on an AVATAR-370 (Nicolet) spectrometer. Carbon, hydrogen, and nitrogen analyses were performed on a CE-440 (Leemanlabs) analyzer. Thermogravimetric analysis (TGA) experiments were carried out on a Dupont thermal analyzer from room temperature to 800 °C under nitrogen atmosphere at a heating rate of 10 °C/min. Syntheses of the Compounds. (a) [Phthalic acid]‚[3bpo] (1). 3-Bpo (22.4 mg, 0.1 mmol) and phthalic acid (16.6 mg, 0.1 mmol) were mixed and dissolved in methanol (10 mL) with stirring. Upon slow evaporation of the solvents, colorless needle-shaped single crystals suitable for X-ray analysis were obtained after 2 days in 90% yield; mp 172-173 °C. Anal. Calcd for C20H14N4O5: C, 61.48; H, 3.59; N, 14.35%. Found: C, 61.49; H, 3.49; N, 14.50%. IR (cm-1): 2464b, 1856w, 1703m, 1591m, 1274vs, 1196w, 1045s, 820w, 732m, 696m. (b) [Phthalic acid]‚[4-bpo] (2). 4-Bpo (22.4 mg, 0.1 mmol) and phthalic acid (16.6 mg, 0.1 mmol) were dissolved in a

Du et al. methanol/water mixture (v/v ) 1: 1, 10 mL). The solution was allowed to evaporate at ambient conditions, resulting in clear, colorless block crystals after 2 days in 85% yield; mp 187 °C. Anal. Calcd for C20H14N4O5: C, 61.48; H, 3.59; N, 14.35%. Found: C, 61.30; H, 3.52; N, 14.18%. IR (cm-1): 2450b, 1705s, 1612w, 1569w, 1537w, 1413s, 1276vs, 1206s, 1011s, 836w, 741m, 723m. (c) [Isophthalic acid]‚[3-bpo] (3). 3-Bpo (22.4 mg, 0.1 mmol) was dissolved in methanol (5 mL), to which a methanol solution (5 mL) of isophthalic acid (16.6 mg, 0.1 mmol) was added. A large amount of white precipitate was immediately formed, which was dissolved after adding excessive CH3CN (10 mL). The resultant colorless solution was filtered and left to stand at room temperature. Lamellar crystals were obtained by slow evaporation of the solvents after 1 week. Yield: 80% (mp 251-252 °C). Anal. Calcd for C20H14N4O5: C, 61.48; H, 3.59; N, 14.35%. Found: C, 61.51; H, 3.50; N, 14.19%. IR (cm-1): 2361m, 2340w, 1707vs, 1432w, 1287vs, 1257s, 1189w, 1074w, 1036m, 822w, 730m, 697m, 641m. (d) [Isophthalic acid]‚[4-bpo] (4). A CH3OH (5 mL) solution of isophthalic acid (16.6 mg, 0.1 mmol) was carefully layered onto a solution of 4-bpo (22.4 mg, 0.1 mmol) in CHCl3 (3 mL) in a test tube. Colorless block crystals appeared on the tube wall over a period of 3 days in 90% yield; mp > 300 °C. Anal. Calcd for C20H14N4O5: C, 61.48; H, 3.59; N, 14.35%. Found: C, 61.44; H, 3.47; N, 14.44%. IR (cm-1): 2377w, 2317w, 1695m, 1608m, 1481w, 1415m, 1318s, 1286m, 1006m, 839m, 725s. (e) [Terephthalic acid]‚[3-bpo] (5). A water (10 mL) and DMF (2 mL) solution containing 3-bpo (44.8 mg, 0.2 mmol) and terephthalic acid (33.2 mg, 0.2 mmol) was placed in a Parr Teflon-lined stainless steel vessel (20 mL) under autogenous pressure, which was heated to 140 °C for 72 h and subsequently cooled to room temperature at a rate of 5 °C/h. Colorless needlelike crystalline products were collected in 80% yield; mp > 300 °C. Anal. Calcd for C20H14N4O5: C, 61.48; H, 3.59; N, 14.35%. Found: C, 61.29; H, 3.55; N, 14.74%. IR (cm-1): 2450b, 1700s, 1606m, 1423w, 1271vs, 1082m, 1039m, 964w, 819m, 731s, 698m, 637m. (f) [Terephthalic acid]‚[4-bpo] (6). A hot C2H5OH/DMF solution (10 mL) of terephthalic acid (16.6 mg, 0.1 mmol) was carefully layered onto a solution of 4-bpo (22.4 mg, 0.1 mmol) in CHCl3 (4 mL) in a straight glass tube. Colorless block crystals were observed on the tube wall over a period of 3 weeks in 85% yield; mp > 300 °C. Anal. Calcd for C20H14N4O5: C, 61.48; H, 3.59; N, 14.35%. Found: C, 61.21; H, 3.52; N, 14.37%. IR (cm-1): 2467b, 1702m, 1612w, 1571w, 1494w, 1419m, 1287s, 1209m, 1112m, 1054m, 1009m, 839m, 733s, 708m, 674w. Single-Crystal X-ray Diffraction Studies. X-ray singlecrystal diffraction data for cocrystals 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 the structural analysis, and semiempirical absorption corrections were applied using the SADABS program. The program SAINT was used for integration of the diffraction profiles.23 All structures were solved by direct methods using the SHELXS program of the SHELXTL package and refined with SHELXL.24 The final refinement was performed by fullmatrix least-squares methods with anisotropic thermal parameters for all the non-hydrogen atoms on F2. 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 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 Preparation of Compounds 1-6. 3-Bpo and 4-bpo have good solubility in water and all common organic

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Table 1. Crystal Data and Structure Refinement Summary for Compounds 1-6 empirical formula M crystal size/mm 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/independent reflections parameters Rint R,a Rwb GOFc residuals/e Å-3 a

1

2

3

4

5

6

C20H14N4O5 390.35 0.24 × 0.22 × 0.18 orthorhombic Pnna 13.718(5) 10.550(4) 12.339(4) 90 90 90 1785.8(11) 4 1.452 0.107 808 -15/16,-12/12,-9/14 8317/1544

C20H14N4O5 390.35 0.20 × 0.14 × 0.10 monoclinic C2/c 17.569(6) 10.547(4) 20.831(7) 90 111.660(5) 90 3587(2) 8 1.446 0.107 1616 -20/20,-12/10,-24/24 9033/3167

C20H14N4O5 390.35 0.24 × 0.20 × 0.16 triclinic P1 h 6.880(4) 7.543(6) 18.983(11) 84.750(16) 87.374(12) 63.034(10) 874.4(10) 2 1.483 0.110 404 -8/5,-8/8,-22/16 4526/3053

C20H14N4O5 390.35 0.32 × 0.14 × 0.12 monoclinic P21/c 13.160(4) 6.773(2) 23.049(8) 90 120.395(6) 90 1772.0(10) 4 1.463 0.108 808 -15/9,-7/8,-21/27 8871/3130

C20H14N4O5 390.35 0.20 × 0.16 × 0.12 triclinic P1h 6.969(4) 9.947(6) 13.530(8) 97.531(11) 103.608(11) 100.683(9) 880.6(9) 2 1.472 0.109 404 -8/8,-11/9,-13/16 4483/3045

C20H14N4O5 390.35 0.20 × 0.16 × 0.12 orthorhombic Cmc21 22.252(7) 10.912(4) 7.290(2) 90 90 90 1770.1(10) 4 1.465 0.108 808 -20/26,-12/11,-7/8 4488/1543

143 0.0518 0.0651, 0.1528 1.121 0.292, -0.257

265 0.0452 0.0436, 0.1001 1.037 0.204, -0.154

263 0.0866 0.0676, 0.1150 0.989 0.164, -0.182

264 0.0397 0.0519, 0.1102 1.014 0.204, -0.165

264 0.0453 0.0641, 0.1220 0.980 0.396, -0.238

134 0.0279 0.0332, 0.0764 0.994 0.106, -0.159

R ) ∑||Fo| - |Fc||/∑|Fo|. b Rw ) [∑[w(Fo2 - Fc2)2]/∑w(Fo2)2]1/2. c GOF ) {∑[w(Fo2 - Fc2)2]/(n - p)}1/2.

Scheme 2

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

3

4

solvents, such as CH3OH, C2H5OH, CH3CN, CHCl3, and CH2Cl2. All crystallizations of bpo and different aromatic dicarboxylic acids were carried out in a 1:1 ratio, considering the number of hydrogen-bonding donor/ acceptor groups in each component. For the preparation of 1 and 2, phthalic acid was mixed directly with equivalent base in CH3OH and/or H2O solution, which was allowed to evaporate at ambient conditions to give the final crystalline products. In the case of 5, due to the poor solubility of terephthalic acid in the majority of organic solvents, the cocrystals suitable for X-ray determination were achieved via hydro/solvothermal synthesis, usually applied in the preparation of inorganic or hybrid crystalline materials. For compounds 3, 4, and 6, the combination of acid and base led to a mass of white precipitation in CH3OH/H2O or CH3OH/ DMF solution. Single crystals for X-ray diffraction of 3 were obtained by recrystallizing the powder solid in CH3CN, and cocrystallization of 4 or 6 was tried using a test tube to facilitate the slow diffusion of the two reagents. Molecular and Supramolecular Structures of Compounds 1-6. The schematic representations of the different types of hydrogen-bonding synthons related to

5

6

D-H‚‚‚A (Å) O2-H2‚‚‚N2a C3-H3‚‚‚O3′b O2-H2‚‚‚N4c O4-H4‚‚‚N1d C11-H11‚‚‚O3e C12-H12‚‚‚O3f C1-H1‚‚‚O5g O3-H3‚‚‚N4h O4-H4‚‚‚N1c C1-H1‚‚‚O5 C5-H5‚‚‚O5e C11-H11‚‚‚O2i C16-H16‚‚‚O2e C10-H10‚‚‚O3j O2-H2A‚‚‚N4k O4-H4A‚‚‚N1l C1-H1‚‚‚O5m C5-H5‚‚‚O5n C8-H8‚‚‚O3o C12-H12‚‚‚O3p O2-H2‚‚‚N1q O4-H4‚‚‚N4r C1-H1‚‚‚O3s C11-H11‚‚‚O5t C12-H12‚‚‚O5r C16-H16‚‚‚O3u O3-H3‚‚‚N1v C4-H4‚‚‚O2w C5-H5‚‚‚O2x

D‚‚‚A (Å) H ‚‚‚A (Å) D-H‚‚‚A (deg) 2.688(6) 3.205(6) 2.682(3) 2.662(3) 3.266(3) 3.140(3) 3.485(4) 2.680(6) 2.609(6) 3.255(8) 3.490(6) 3.188(8) 3.196(8) 3.578(8) 2.662(2) 2.681(3) 3.136(2) 3.194(3) 3.182(2) 3.221(2) 2.696(5) 2.671(5) 3.106(6) 3.228(6) 3.322(6) 3.237(6) 2.645(2) 3.626(2) 3.269(3)

1.87 2.35 1.86 1.85 2.58 2.49 2.90 1.86 1.80 2.41 2.94 2.50 2.51 2.80 1.85 1.86 2.34 2.59 2.44 2.60 1.90 1.85 2.40 2.47 2.71 2.48 1.83 2.76 2.46

173 153 176 170 131 128 122 177 170 151 119 131 131 119 171 178 143 123 137 125 165 176 133 139 124 139 171 155 146

a x - 1/2, -y + 1/2, z - 1/2. b x, -y + 1/2, 3/2 - z. c x + 1, y, z. -x + 3/2, y - 1/2, -z + 1/2. e x - 1, y, z. f -x + 1, y, -z + 1/2. g -x + 3/2, y + 1/2, -z + 1/2. h x + 1, y + 1, z - 1. i x - 1, y - 1, z + 1. j x, y - 1, z + 1. k x, -y + 3/2, z + 1/2. l x - 1, -y + 3/2, z - 1/2. m x + 1, -y + 1/2, z + 1/2. n x + 1, -y + 3/2, z + 1/2. o x, -y + 1/2, z - 1/2. p x, -y + 3/2, z - 1/2. q x, y - 1, z. r -x, -y, -z + 1. s x, y + 1, z. t -x + 1, -y, -z + 1. u 1 - x, 1 - y, 2 - z.v x, y, z - 1. w x, -y, z + 1/2. x -x + 1/2, y + 1/2, z + 1. d

this work are summarized in Scheme 2. Hydrogen bond geometries of compounds 1-6 are listed in Table 2. The primary supramolecular synthons observed so far indicate that there are numerous examples of hydrogenbonded motifs consisting of two strong or two weak hydrogen bonds, but motifs with one strong and one

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Figure 1. (a) ORTEP view of 1 with atom labeling of the asymmetric unit; (b) acid (left, half of 3-bpo molecules were omitted for clarity) and base (right) unit in 1 showing the hydrogen bonds (indicated as dashed lines in this and subsequent figures); (c) 3-D hydrogen-bonding architecture of 1, viewing along the [100] direction.

weak bond are relatively rare.25-28 In this study, for compounds 2-5, the carboxylic acid and bpo furnish a hydrogen-bonded synthon I with an orbicular R22(7) pattern, which is one of the most frequently observed synthons in organic acid-base cocrystals. Although the other two cocrystals 1 and 6 also contain strong O-H‚‚‚ N and weak C-H‚‚‚O hydrogen bonds, synthon I was not formed due to the relative twist orientation of neighboring acid and base subunits. (a) [Phthalic acid]‚[3-bpo] (1). As depicted in Figure 1a, the molecular structure of 1 has a C2 symmetry with the O3 atom of phthalic acid applied with a disorder model. The dihedral angle between the 3-bpo molecule and the benzene ring of phthalic acid is 23.6(4)°. Within each 3-bpo, which adopts the unusual cisoid-II configuration (always cisoid-I in its metal complexes),20c,g two terminal pyridyl rings make a dihedral angle of 2.9(3)° with the central oxadiazole system, and the dihedral angle of them is 5.0(2)°. Further analysis of the crystal packing suggests that the intramolecular S(7) hydrogen bond (synthon III, Scheme 2), which is a common occurrence in 1,2disubstituted dicarboxylic acids in either neutral29 or deprotonated30 form, is not found. Instead of this, phthalic acid in 1 forms intermolecular interactions with 3-bpo, including strong O-H‚‚‚N and weak C-H‚‚‚O hydrogen bonds. As shown in Figure 1b, each acid molecule connected four 3-bpo subunits through two O2-H2‚‚‚N2 and two C3-H3‚‚‚O3′ hydrogen bonds, and due to this stereochemistry effect, synthon I (Scheme 2) could not be formed properly. Meanwhile, around

each base molecule, there are also four acid molecules through such interactions. Thus, each acid or base component acts as a four-connected node to build up a 3-D supramolecular network, as illustrated in Figure 1c, in which acid and base units are alternately arranged to show helix arrays along the crystallographic a axis. (b) [Phthalic acid]‚[4-bpo] (2). In the local structure of cocrystal 2 as shown in Figure 2a, the mean plane of 4-bpo makes a dihedral angle of 12.3(2)° with the acid molecule. Within each 4-bpo subunit, the dihedral angle between two pyridyl rings is 8.2(1)°, and they make the dihedral angles of 9.1(2) and 2.0(1)°, respectively, with the central oxadiazole ring. Unlike compound 1, two carboxylic groups in each acid molecule take part in different hydrogen-bonding modes in compound 2. First, each -COOH group of a phthalic acid connects the adjacent pyridyl ring of the bent base 4-bpo via synthon I [O2-H2‚‚‚N4 and C11-H11‚‚‚O3; O4H4‚‚‚N1 and C1-H1‚‚‚O5], and these hydrogen bonds extend the acid-base subunits to form a unique helical supramolecular tape along the crystallographic [010] direction, as shown in Figure 2b left. For related cocrystals of phthalic acid and rodlike 4,4′-bipyridine or 4,4′-bipyridylacetylene,21b 1-D zigzag chains through O-H‚‚‚N bonds were observed. Second, each carboxylic O3 atom acts as the acceptor of a C12-H12‚‚‚O3 bond, and as a consequence, the 1-D helical chains were further connected by such weak C-H‚‚‚O interactions to form a two-dimensional corrugated network viewed from the ab plane, as illustrated in Figure 2c. Another



Figure 2. (a) ORTEP view of 2 with atom labeling of the asymmetric unit; (b) (left) 1-D helical hydrogen-bonding array with synthon I and (right) dimer-of-dimer hydrogen-bonding subunit of 2; (c) 2-D corrugated supramolecular network viewing along the c axis in the unit cell of 2.

alternative description, perhaps more revealing from the viewpoint of supramolecular network topology, is as follows: as depicted in Figure 2b right, a pair of carboxylic group and pyridine loop (synthon I, patterns A and A′) could be considered as a dimer and two such adjacent dimers, related by a crystallographic C2 symmetry, are further linked through a pair of weak C12H12‚‚‚O3 hydrogen bonds to form the “dimer-of-dimer” structural unit. In this unit, another hydrogen-bonded motif B, that is, synthon IV [R24(10)], is found. If such a dimer-of-dimer unit is treated as a single node, then the final 2-D network topology could be considered as a simple (4,4) notation,31 which is most observed so far, especially in metal-organic coordination frameworks. Further analysis of the crystal packing of 2 indicates that these 2-D waved layers are interdigitated and take on an antiparallel stacking mode in the unit cell. (c) [Isophthalic acid]‚[3-bpo] (3) and [Isophthalic acid]‚[4-bpo] (4). The molecular structures of 3 and 4 are depicted in Figures 3a and 4a, respectively. The dihedral angles between acid and base components are 11.4(4)° for 3 and 11.5(3)° for 4. For 3-bpo in 3, also exhibiting the cisoid-II configuration, two pyridyl rings within each bpo molecule deviate by 14.2(5)° from

coplanarity and form the dihedral angles of 7.9(4) and 8.6(3)°, respectively, with the central oxadiazole plane. Corresponding values for 4-bpo in 4 are 3.6(3), 1.8(3), and 1.9(2)°. It is notable that the structural geometries of isophthalic acid are different in 3 and 4: two carboxylic groups are in trans- or cis-arrangement, respectively, to fulfill the hydrogen-bonded patterns as described below. Both 3 and 4 exhibit similar 2-D supramolecular structures, as illustrated in Figures 3b and 4b, respectively. In 3, an extended zigzag tape is generated via synthon I [O3-H3‚‚‚N4 and C11-H11‚‚‚ O2; O4-H4‚‚‚N1 and C5-H5‚‚‚O5]. Significantly, there is a C16-H16‚‚‚O2 hydrogen bond between the acid molecules in adjacent tapes. Furthermore, a pair of weak interactions [C1-H1‚‚‚O5 and C10-H10‚‚‚O3] helps to maintain the coplanarity of the diacid and the angular dipyridyl units, and thus, these tapes align parallel to each other to form a nearly-planar supramolecular sheet. Four types of hydrogen-bonded patterns A-D, notated as R22(7), R33(17), R34(24), and R23(11), are observed in this 2-D array. No significant interaction such as hydrogen bonds or aromatic stacking exists between these sheets. Adjacent layers stack in the antiparallel alignment with some offset along the ac

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Figure 3. (a) ORTEP view of 3 with atom labeling of the asymmetric unit; (b) 2-D hydrogen-bonding sheet along the crystallographic ac plane.

plane. The arrangements of molecules in the unit cell and 2-D hydrogen-bonding supramolecular sheet in 4 are similar to that of 3, except the absence of C-H‚‚‚O interaction between the acid molecules in the latter case. Additionally, the structure of 4 is almost the same as that of the cocrystal based on isophthalic acid and 4,4′bipyridine.21a (d) [Terephthalic acid]‚[3-bpo] (5). While synthesizing the binary crystals in this work, we encountered many difficulties in obtaining a particular cocrystal suitable for X-ray diffraction because of the mismatched solubility between acid and base components, especially for terephthalic acid with very poor solubility. Similar cases were also reported previously while preparing the molecular cocrystals.32 As stated above, well-shaped single crystals of 5 were obtained by the hydro/solvothermal approach, in which each cisoid-II 3-bpo unit crystallizes with two-half molecules of terephthalic acid, as depicted in Figure 5a. Both diacid subunits are centrosymmetric, affording the dihedral angles of 8.8(3) and 6.7(4)° with 3-bpo, in which two pyridine rings form the dihedral angles of 8.0(4) and 8.3(3)°, respectively, with the oxadiazole plane and the dihedral angle of 2.1(4)° with each other. Analysis of the crystal packing of 5 suggests that terephthalic acid and 3-bpo molecules locate alternately, also generating a zigzag tape with the formation of a pair of synthon I [O4-H4‚‚‚N4 and C12-H12‚‚‚O5; O2-H2‚‚‚N1 and C1-H1‚‚‚O3, motifs A and A′ in Figure 5b]. Adjacent tapes are further connected through weak C16-H16‚‚‚O3 interactions,

generating a hydrogen-bonded pattern B with R22(10) notation (diacid dimer synthon VI, Scheme 2). Through synthon VI and another weak interaction (C11-H11‚‚‚ O5) between acid and base components, neighboring tapes were combined to result in a 2-D layered net containing a new R35(31) motif C. Similar to compounds 2-4, it is likely that the layered structure of 5 is stabilized by a combination of synthon I and further C-H‚‚‚O hydrogen bonds and these sheets also adopt the antiparallel alignment along the [001] direction in the unit cell. (e) [Terephthalic acid]‚[4-bpo] (6). The structural determination of 6 reveals it is a spontaneously resolved acentric two-component molecular crystal (space group Cmc21). In the local molecular structure of 6 having a symmetric mirror for both acid and base units, as shown in Figure 6a, terephthalic acid is inclined to 4-bpo with an angle of 61.2(1)°, significantly larger than that in 1-5. The terminal pyridyl rings in 4-bpo form the dihedral angles of 7.6(2)° with the oxadiazole plane and the dihedral angle of 7.1(2)° with each other. From Figure 6b, we can clearly observe that the base and acid components are connected through strong O3-H3‚‚‚N1 and weak C5-H5‚‚‚O2 interactions, producing a 2-D wavelike structure. The 2-D structure of 6 has some noticeable differences from the 2-D networks of compounds 2-5: (i) Viewing from each direction of this plane (a or b axis), the terephthalic acid and 4-bpo arrange alternately (in 2-5, this is only the fact for one direction). (ii) There are no classical supramolecular



Figure 4. (a) ORTEP view of 4 with atom labeling of the asymmetric unit; (b) 2-D hydrogen-bonding sheet along the crystallographic bc plane.

synthons, especially synthon I in compound 6, due to the high slope between acid and base molecules. As a result, there exist interlayer C4-H4‚‚‚O2 hydrogenbonding interactions between 4-bpo and O2 donors of terephthalic acids, which connect the adjoining parallel sheets to general a 3-D polar network. A recent database study33 reveals that recognition of the -COOH group with pyridine through an O-H‚‚‚N hydrogen bond is favored 10 times more compared to dimer II and catemer V synthons (Scheme 2) with itself, indicating that the acid-acid interaction is not preferred when the pyridyl subunit is present. This is also the case for compounds 1-6. Furthermore, the involvement of a weak C-H‚‚‚O interaction between pyridine and acid units will generate a hetero-dimer ring pattern I, being the only example of a mixed strong-weak hydrogen-bonded synthon that has been systematically investigated so far.34 In this study, synthon I was found in the supramolecular structures of cocrystals 2-5, resulting in a 1-D helical or zigzag tape, which is further expanded to a 2-D network via additional weak C-H‚‚‚O bonds. Melting Points of Cocrystals 1-6. The melting points of 3-bpo and 4-bpo are 191 and 193 °C, and those of the aromatic dicarboxylic acids (phthalic, isophthalic,

and terephthalic acid) are 205, >300, and >300 °C, respectively. For the six cocrystals in this study, the melting points are as follows: 173 °C for 1, 187 °C for 2, 252 °C for 3, and >300 °C for 4-6. From the above data, we may conclude that a cocrystal in this study usually has a lower melting point than the corresponding phenyl diacid, at least for 1-3, probably due to the strong O-H‚‚‚O synthons in the diacid being destroyed in the new bimolecular crystals. On the other hand, the melting point of isophthalic or terephthalic acid is significantly higher than that of phthalic acid, and this is also the case for the corresponding cocrystals (the melting points of 1 and 2 are similar, but significantly lower than those of 3-6). This phenomenon may be relational to the nature of acid molecules, which is also the key role in generating the different crystal packing. Thermogravimetric Analysis. Compounds 1-6 are air stable and can retain their structural integrity at room temperature for a considerable length of time; thus, TGA was conducted to determine the thermal stability of these crystalline materials. For 1, the TGA curve shows two consecutive weight losses of all materials in the temperature range of 180-293 °C, peaking at 209 and 275 °C, respectively. For compound 2, the first weight loss of 57.3% from 200 to 240 °C, peaking

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Figure 5. (a) ORTEP view of 5 with atom labeling of the asymmetric unit; (b) 2-D hydrogen-bonding sheet along the crystallographic ab plane.

at 214 °C, corresponds to the loss of the base component (calculated: 57.4%). The residuary acid section loses weight of 42.9% (calculated: 42.6%) subsequently from 240 to 310 °C (peak: 263 °C). The decomposition temperature for 1 is slightly lower than that of 2, and this may result from the low intensity of the O-H‚‚‚N bond and less C-H‚‚‚O interactions (less thermal energy would be required to break these interactions, allowing the more facile loss). Compound 3 has abundant strong O-H‚‚‚N and weak C-H‚‚‚O hydrogen bonds, also including additional interactions between diacids. TGA measurement shows that 3 retains intact until 225 °C, and there is a sharp weight loss peaking and ending at 293 and 305 °C, respectively, corresponding to the expulsion of all base and acid components. For 4, the TGA curve shows two consecutive weight losses of all materials in the temperature range of 180-352 °C, peaking at 193 and 306 °C, respectively. For compounds 5 and 6, the TGA curves are more complicated. Three consecutive weight losses of 5 occur in the 110-344 °C range (peak positions: 139, 293, and 331 °C), and four weight losses of 6 occur in the 176-345 °C range (peak positions: 180, 191, 213, and 319 °C). Anyway, the thermal stability for each pair of cocrystals with the same diacid components clearly correlates with the strength of hydrogen bonds (primary strong O-H‚‚‚N interactions) in the packing fraction.

Conclusions and Perspectives Synthon I was usually applied in the design of supramolecular tapes of dicarboxylic acids with dipyridyl compounds. In the present contribution for molecular tectonics, an angular dipyridyl component was first introduced to such bimolecular crystalline materials, and most of the resulting supramolecular architectures also contain such a classical synthon. Obviously, the geometry of the strong O-H‚‚‚N hydrogen bond that plays the main role in controlling the 1-D supramolecular chain of the cocrystals is generally predictable in all six compounds, but the formation of a weak C-H‚‚‚O interaction that is more inclined to distort could be easily affected by the backbone of the acid and/or base components. Further analysis of the structures of 1-6 reveals that the relative location for acid and base subunits within a cocrystal makes a significant effect on forming diverse networks from two dimensions to three dimensions. The dihedral angles between acid and base components are relatively smaller (in the range of 6.7(4)-12.3(2)°) for compounds 2-5, which all form 2-D supramolecular networks via synthon I and additional C-H‚‚‚O hydrogen bonds. In the other two cases with 3-D hydrogen-bonded structures, the acid unit makes a dihedral angle with the base molecule of 23.6(4)° for 1 and 61.2(1)° for 6, which is too large to meet the



Figure 6. (a) ORTEP view of 6 with atom labeling of the asymmetric unit; (b) 2-D hydrogen-bonding sheet along the crystallographic ab plane.

requirement for formation of synthon I. This also suggests that the weak C-H‚‚‚O interactions are more readily distorted and uncertain. For the base subunit, the dihedral angle between two pyridyl rings exhibits flexible changes in the range of 2.1(4)-14.2(5)°, and the dihedral angles of each pyridyl ring with the oxadiazole plane are also diverse, making a range of 1.8(3)-9.1(2)°. Significantly, 3-bpo molecules exhibit the cisoid-II configuration in all cases, which is unusual, and spontaneously convert to afford the formation of hydrogen bonds. It can be indicated that this kind of angular building block has a significant influence on the supramolecular structures of organic solids. Their changeful and adaptable nature, observed previously in the metal complexes, can be generally used to fulfill abundant and diverse hydrogen bond patterns. For the acid component, the dihedral angle of a pair of carboxylic groups within a diacid is 132.9(5), 56.2(2), 5.5(5), 5.8(4), 0, and 1.4(3)° for 1-6, respectively. Due to the stereochemistry effect of the adjacent carboxylic groups within a phthalic acid, the dihedral angles in compounds 1 and 2 are significantly larger than those of the other four compounds, which makes it difficult to form a common planar structure, as in compounds 3-6. As a matter of fact, hydrogen bonds extend the acid-base subunits to form helical motifs in these two compounds. In conclusion, this study shows that bpo is an excellent supramolecular reagent that can produce bimolecular cocrystals with phenyl diacids in high yield. The intermolecular hydrogen-bonded motifs appear reliable and will be useful in the design of analogues for specific

applications. We are currently extending this strategy by using flexible alkyl diacids or polycarboxylic acids as the hydrogen-bonding participators to prepare new crystalline materials. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (No. 20401012), the Key Project of the Tianjin Natural Science Foundation, and Tianjin Normal University (to M. Du). Supporting Information Available: Crystallographic information files (CIF) of compounds 1-6. This material is available free of charge via the Internet at http://pubs.acs.org.

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