Cocrystallization of Trimesic Acid and Pyromellitic Acid with Bent

Adrian E. Ion, Andreea Dogaru, Sergiu Shova, Augustin M. Madalan, Oluseun Akintola, Sorana Ionescu, Mariana Voicescu, Simona Nica, Axel Buchholz, ...
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CRYSTAL GROWTH & DESIGN

Cocrystallization of Trimesic Acid and Pyromellitic Acid with Bent Dipyridines

2005 VOL. 5, NO. 3 1247-1254

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 December 22, 2004;

Revised Manuscript Received February 22, 2005

ABSTRACT: Bent dipyridyl bases, 2,5-bis(3-pyridyl)-1,3,4-oxadiazole (3-bpo) and its 4-N-donor analogue 2,5-bis(4-pyridyl)-1,3,4-oxadiazole (4-bpo), were applied to crystallize with trimesic acid (benzene-1,3,5-tricarboxylic acid, H3TMA) and pyromellitic acid (benzene-1,2,4,5-tetracarboxylic acid, H4BTA), affording new binary molecular cocrystals [(H3TMA)‚(3-bpo)]‚DMF (1), [(H3TMA)‚(4-bpo)] (2), [(H4BTA)‚(3-bpo)]‚2H2O (3), and a partly charge-transfer salt [(H2BTA)1/2‚(H-4-bpo)‚(H4BTA)1/2]‚H2O (4) under general conditions. An unexpected compound 5 (pyromellitic dihydrazide) was obtained during the attempt to attain a cocrystal of 4-bpo and H4BTA by hydrothermal synthesis. X-ray single-crystal diffraction studies reveal that supramolecular synthon I [R22(7)] containing classical O-H‚‚‚N and weak C-H‚‚‚O interactions is again testified to be involved in constructing the binary hydrogen-bonding networks 1-4, in which the heteromeric intermolecular interactions are often utilized as effective synthetic tools to provide a significant portion of the stabilization energy of molecular crystals. In compound 1, 1-D acid/base zigzag chains formed by synthon I are extended to a 2-D layered architecture via additional C-H‚‚‚O interactions, the cavities of which are occupied by guest solvents (N,N-dimethylformamide, DMF). Compound 2 displays fascinating 1-D triplehelical motifs via synthon I (formation of the single strand helix) and C-H‚‚‚O interactions, which are further expanded to a 3-D supramolecular array by multiple hydrogen bonds. Self-recognition via hydrogen bonding has been evidently embodied in this unique supramolecular system. In compounds 3 and 4, water moieties are present in both resultant assemblies and play different roles in the formation of 3-D supramolecular networks. For 3, each water molecule acts as a 3-connected node, linking three 1-D acid/base double chains resulting from the combination of synthon I and C-H‚‚‚N bonds, to afford a 3-D sandwich network. For 4, water molecules only behave as guests to further stabilize the robust 3-D hydrogen-bonding host framework. Compound 5 has a 2-D sheet structure via a pair of strong head-to-tail N-H‚‚‚O interactions. The thermal stability of cocrystals 1-4 has been investigated by thermogravimetric analysis of mass loss. Introduction Crystal engineering has gradually been shown to have wider implications for material and solid science,1-4 though the concept was originally introduced in the context of stereochemical control of photochemical reactions.5 The ultimate goal of crystal engineering is to control the molecular assembly and orientation within a solid-state structure, which further leads to predictable and desired structural aggregates. Hydrogen bonds often play a dominant role in crystal engineering because of their selectivity and directionality to control the design of various molecular assemblies.6,7 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.8-11 To achieve this aim, “supramolecular synthon” strategy has been well applied to design stable open organic frameworks with potential applications in nonlinear optical material,12 homogeneous and heterogeneous separations, reversible guest exchange,13 and selective organic catalysis.14 Supramolecular synthons based on hydrogen bonds can be categorized into two motives.15 One is the “supramolecular homosynthons” from self-complementary half units, such as the dimer formed by carboxylic acids [synthon R22(8)];16 the other motif is “supramo* To whom correspondence should be addressed. Fax & Tel: 86-2223540315. E-mail: [email protected].

Chart 1

lecular heterosynthons” consisting of two or more components, for example, the dimer formed by pyridine with carboxylic acid [synthon R22(7)].15,17-21 The carboxylic acid/pyridine heterosynthon has been observed in the field of liquid crystalline materials, 2-D beta networks,17 2-D corrugated sheets,18 and “ternary supermolecules”.19 Similar heterosynthons have also been used as templates for controlling solid-state reactions20 or predictably generating binary pharmaceutical crystalline phases.15 Recently, 2,5-bis(3-pyridyl)-1,3,4-oxadiazole (3-bpo) and its 4-N-donor analogue 2,5-bis(4-pyridyl)-1,3,4oxadiazole (4-bpo) (see Chart 1) have been initially applied by us and others to achieve a series of coordination polymers or supramolecules with interesting ex-

10.1021/cg0495680 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/29/2005

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tended frameworks and properties.22 The specific bent geometry of such compounds may result in either discrete or divergent supramolecular networks under appropriate conditions. Moreover, they have abundant robust heteroatoms with free electron pairs that could be considered as the potential active hydrogen-bonding acceptors.23 Especially for 3-bpo, the possibility of three potential conformations24 allows it to self-adjust according to the requirements in the process of self-assembly, which is unusual compared with common building blocks.21,25 Meanwhile, trimesic acid (benzene-1,3,5tricarboxylic acid, H3TMA) and pyromellitic acid (benzene-1,2,4,5-tetracarboxylic acid, H4BTA, see Chart 1) were frequently chosen as building blocks for crystal engineering due to their predictable and interesting supramolecular properties (e.g., catenation or interpenetration, polymorphism and inclusion),26,27 which are the consequences of their higher molecular symmetry and robust hydrogen-bonding abilities.28 On the other hand, both carboxylic acids can form heterodimers with a variety of basic components such as pyridine, pyrazine, and pyrimidine, in which18,27f the most classical synthon R22(7) (involving both O-H‚‚‚N and C-H‚‚‚O interactions) is always present by the best-donor (carboxylic group) and the best-acceptor (heterocyclic nitrogen atom) of hydrogen bonds.29,30 Thus, we anticipate that the incorporation of both distinctive polycarboxylic- and dipyridyl-type building blocks within a binary molecular crystal would lead to the formation of new organic crystalline materials with fascinating supramolecular structures. In this paper, we will describe a systemic investigation on such cocrystals, including molecular crystals [(H3TMA)‚(3-bpo)]‚DMF (1), [(H3TMA)‚(4-bpo)] (2), [(H4BTA)‚(3-bpo)]‚2H2O (3), and a proton-transfer salt [(H2BTA)1/2‚(H-4-bpo)‚(H4BTA)1/2]‚H2O (4), displaying diverse 2-D layer, 3-D sandwich, cross-link, and unique triple-helix networks. The solvents play a key role in producing such ordered networks. Additionally, pyromellitic dihydrazide (5) was occasionally obtained during our attempt to achieve a binary molecular crystal of 4-bpo and H4BTA under hydrothermal conditions, the molecular/supramolecular structure of which was also analyzed. Experimental Section Materials and Methods. All materials for synthesis were from commercial sources and used as received, with the exception of 3-bpo and 4-bpo, prepared according to the literature procedures.31 The melting points of the new compounds were recorded on a WRS-1B digital thermal apparatus without correction, and Fourier transform (FT) IR spectra (KBr pellets) were taken on an AVATAR-370 (Nicolet) spectrometer. Elemental (C, H, and N) analyses were performed on a CE440 (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. Preparation of [(H3TMA)‚(3-bpo)]‚DMF (1). 3-Bpo (22.4 mg, 0.1 mmol) and H3TMA (21.0 mg, 0.1 mmol) were mixed in a methanol/water mixture (v/v ) 1:1, 10 mL). A white precipitate appeared immediately, which was dissolved after adding excessive CH3CN and DMF (v/v ) 1:1, 10 mL) with vigorous stirring at ambient temperature for ca. 30 min. The resultant colorless solution was filtered and left to stand at room temperature. Colorless rhombic crystals were obtained by slow evaporation of the solvents over a period of 1 week. Yield, 80%; mp > 300 °C. Anal. Calcd for C24H21N5O8: C, 56.75;

Du et al. H, 4.14; N, 13.79%. Found: C, 56.88; H, 4.11; N, 13.75%. IR (cm-1): 2473b, 1712vs, 1633s, 1606m, 1441s, 1237vs, 1199s, 1129w, 1101m, 1033m, 738m, 677m, 636m, 503w. Preparation of [(H3TMA)‚(4-bpo)] (2). 4-Bpo (44.8 mg, 0.2 mmol) and H3TMA (42.0 mg, 0.2 mmol) were mixed and dissolved in methanol (10 mL) with stirring. Upon slow evaporation of the solvents, colorless rhombic single crystals suitable for X-ray analysis were produced after 3 days. Yield, 90%; mp > 300 °C. Anal. Calcd for C21H14N4O7: C, 58.01; H, 3.22; N, 12.89%. Found: C, 57.99; H, 3.17; N, 12.83%. IR (cm-1): 2438b, 1726vs, 1614m, 1571m, 1284s, 1214s, 1161s, 1097w, 1014m, 881w, 840m, 743m, 665m, 505m. Preparation of [(H4BTA)‚(3-bpo)]‚2H2O (3). 3-Bpo (22.4 mg, 0.1 mmol) was dissolved in methanol (5 mL), to which a methanol/water solution (v/v ) 1:1, 5 mL) of H4BTA (25.4 mg, 0.1 mmol) was added with stirring and heating for 1 h at 50 °C. Then the colorless solution was filtered and allowed to evaporate at ambient conditions, resulting in lamellar crystals after 5 days. Yield, 85%; mp 236.2-237.7 °C. Anal. Calcd for C22H18N4O11: C, 51.32; H, 3.49; N, 10.88%. Found: C, 51.30; H, 3.40; N, 10.87%. IR (cm-1): 3527m, 2500b, 1713vs, 1611w, 1467w, 1266s, 1244m, 1126w, 1092w, 1051m, 821m, 791w, 760m, 730m, 692m, 522w. Preparation of [(H2BTA)1/2‚(H-4-bpo)‚(H4BTA)1/2]‚H2O (4). A CH3OH/H2O solution (v/v ) 1:1, 5 mL) of H4BTA (25.4 mg, 0.1 mmol) was carefully layered onto a solution of 4-bpo (22.4 mg, 0.1 mmol) in CHCl3 (5 mL) in a straight glass tube. Colorless block crystals were observed on the tube wall over 2 weeks in 80% yield, mp 248.4-248.8 °C. Anal. Calcd for C22H16N4O10: C, 53.18; H, 3.22; N, 11.28%. Found: C, 53.49; H, 2.95; N, 11.38%. IR (cm-1): 3089w, 2461b, 1703m, 1638w, 1565m, 1481m, 1357s, 1286m, 1163m, 1086w, 979w, 946m, 852s, 749m, 709w, 600m. Synthesis of Pyromellitic Dihydrazide (5). A water (10 mL) solution containing 4-bpo (44.8 mg, 0.2 mmol) and H4BTA (50.8 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. Dark brown block crystalline products were collected in 70% yield. Single-Crystal X-ray Diffraction Determination and Refinement. X-ray single-crystal diffraction data for 1-5 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. 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.32 The structures were solved by direct methods using the SHELXS program of the SHELXTL package and refined with SHELXL.33 The final refinement was performed by full-matrix least-squares methods on F 2 with anisotropic thermal parameters for all non-H atoms. H atoms attached to C were placed geometrically and allowed to ride during subsequent refinement with an isotropic displacement parameter fixed at 1.2 times Ueq of the parent atoms. H atoms bonded to O or N were first located in difference Fourier maps and then placed in calculated sites and included in the refinement. Further details for unit-cell parameters and refinement conditions are listed in Table 1.

Results and Discussion Preparation of Compounds 1-5. Both base-type and acid-type reagents selected in this research have good solubility in common organic solvents, such as CH3OH, C2H5OH, CH3CN, CHCl3, and CH2Cl2. All crystalline materials 1-4 were obtained as 1:1 binary compounds, either carried out in 1:1 or 2:1 base/acid molar ratio of the two components during the reactions. For the preparation of 1 and 4, direct combination of the acid and base species led to the formation of a large

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Table 1. Crystal Data and Structure Refinement Details for Compounds 1-5 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

C24H21N5O8 507.46 0.28 × 0.18 × 0.12 monoclinic C2/c 14.812(3) 14.615(3) 22.143(4) 90 99.523(3) 90 4727.4(15) 8 1.426 0.110 2112 -18/19, -18/18, -24/28 16279/5377

C21H14N4O7 434.36 0.22 × 0.20 × 0.14 monoclinic P21/n 10.306(3) 12.894(4) 15.321(4) 90 100.207(5) 90 2003.6(10) 4 1.440 0.111 896 -12/9, -16/14, -19/18 11385/4112

C22H18N4O11 514.40 0.40 × 0.20 × 0.18 monoclinic C2/c 8.730(3) 16.337(5) 16.476(6) 90 104.030(6) 90 2279.9(13) 4 1.499 0.123 1064 -9/10, -20/12, -20/20 6461/2333

C22H16N4O10 496.39 0.24 × 0.16 × 0.12 triclinic P1 h 7.779(2) 11.569(3) 12.540(4) 101.478(5) 102.953(5) 90.168(5) 1076.5(5) 2 1.531 0.124 512 -7/9, -13/13, -12/14 5605/3769

C10H6N4O4 246.19 0.20 × 0.18 × 0.12 orthorhombic Fmmm 17.237(6) 8.983(2) 6.1602(15) 90 90 90 953.8(5) 4 1.714 0.137 504 -21/13, -11/11, -6/7 1384/297

340 0.0507 0.0480, 0.1049 0.996 0.196, -0.167

289 0.0459 0.0510, 0.0997 1.008 0.169, -0.245

172 0.0298 0.0423, 0.0997 1.022 0.231, -0.165

326 0.0204 0.0442, 0.0974 1.040 0.197, -0.229

29 0.0183 0.0486, 0.1477 1.101 0.368, -0.256

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.

Chart 2

Table 2. Hydrogen-Bond Geometries in the Crystal Structures of 1-5 compound

D-H‚‚‚A (Å)

D‚‚‚A (Å)

H‚‚‚A (Å)

D-H‚‚‚A (deg)

1

O3-H3‚‚‚N4a O5-H5‚‚‚N1b O7-H7‚‚‚O8c C1-H1‚‚‚O4d C2-H2‚‚‚O2 C9-H9‚‚‚O2e C11-H11‚‚‚O6f O3-H3A‚‚‚O7g O4-H4A‚‚‚N4g O6-H6A‚‚‚N1h C1-H1‚‚‚O5g C5-H5‚‚‚O7i C5-H5‚‚‚O2j C9-H9‚‚‚N3k C10-H10‚‚‚O5l O2-H2‚‚‚O6m O4-H4‚‚‚N1m O6-H6A‚‚‚O5n O6-H6B‚‚‚O3c C5-H5‚‚‚O5° C7-H7‚‚‚N2c N1-H1A‚‚‚O2a O3-H3A‚‚‚O1 O5-H5A‚‚‚N4m O7-H7‚‚‚O10p O10-H10A‚‚‚O4 O10-H10B‚‚‚O6c C11-H11‚‚‚O1a C15-H15‚‚‚N2m C19-H19‚‚‚O3q C20-H20‚‚‚O6° C22-H22‚‚‚O8a N1-H1‚‚‚Or

2.680(2) 2.751(2) 2.547(2) 3.064(3) 3.325(3) 3.116(3) 3.311(3) 2.674(2) 2.644(3) 2.616(2) 3.272(3) 3.118(3) 3.182(3) 3.417(3) 3.382(4) 2.608(2) 2.569(2) 2.790(2) 2.908(2) 3.213(3) 3.447(3) 2.665(3) 2.398(3) 2.550(3) 2.586(3) 2.707(3) 2.717(3) 3.224(8) 3.325(4) 3.255(4) 3.496(5) 3.422(4) 2.743(3)

1.86 1.94 1.75 2.35 2.44 2.41 2.44 1.76 1.71 1.68 2.38 2.47 2.44 2.59 2.80 1.79 1.75 1.94 2.08 2.55 2.54 1.72 1.44 1.59 1.78 1.85 1.84 2.64 2.47 2.55 2.97 2.50 1.89

179 172 164 133 160 133 157 174 160 173 160 126 136 149 121 172 174 175 165 129 164 176 172 172 167 171 168 123 154 132 118 169 173

2

3

amount of white precipitation in CH3OH/H2O. To obtain the single crystals suitable for X-ray determination, recrystallizing of the powder solid 1 in CH3CN/DMF was tried, and cocrystallization of 4 was put in practice using a test tube to facilitate the slow diffusion of the two reagents. In the cases of 2 and 3, 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 products. Different from molecular crystals 1-3, a proton transfer from acid to base occurs in 4. A hydro/solvothermal approach is usually applied in preparation of inorganic or hybrid crystalline materials. Attempts to cocrystallize 4-bpo and H4BTA from water at high temperature appear to have caused hydrolysis of 4-bpo and formation of hydrazine,31,34 leading ultimately to formation of the known dihydrazide of pyromellitic acid 5.35 Molecular and Supramolecular Structural Descriptions of 1-5. The schematic representations of hydrogen-bonding synthons related to this work are summarized in Chart 2. Hydrogen-bond geometries of 1-5 are listed in Table 2. The supramolecular structures of 1-4 display complicated hydrogen-bonded networks, resulting from the supramolecular synthons

4

5

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

and other weak C-H‚‚‚O and/or C-H‚‚‚N interactions, and a classical heterosynthon I with an orbicular R22(7) pattern is involved in all structures. This bimolecular synthon affords a large number of crystal structures for analysis, which occurred the most frequently among

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Figure 1. (a) Molecular structure of 1 with atom labeling. (b) 2-D supramolecular sheet along the crystallographic bc plane (the hydrogen bonds are indicated as broken lines in this and the subsequent figures); the DMF molecules in the cavities are marked in cyan.

hydrogen bonding between carboxylic acid and complementary functional groups.36 [(H3TMA)‚(3-bpo)]‚DMF (1). In the local structure [Figure 1a] of 1, the 3-bpo molecule exhibits the unusual transoid configuration34 in the interest of forming steady synthon I with H3TMA, and the dihedral angle between them is 2.6°. Within each 3-bpo component, the dihedral angle between two pyridyl rings is 3.8°, and they make dihedral angles of 9.4 and 8.5°, respectively, with the central oxadiazole ring. Analysis of the intermolecular interactions in 1 reveals that trimesic acid does not introduce typical extended honeycomb topology,37 which assembles with 3-bpo and DMF solvent via both strong and weak hydrogen bonds, as shown in Figure 1b. Three carboxylic groups in each H3TMA molecule are involved in different hydrogen-bonding modes. First, two -COOH groups in a acid unit connect two adjacent pyridyl rings of the distinct bent bases 3-bpo via synthon I [O3H3‚‚‚N4 and C9-H9‚‚‚O2; O5-H5‚‚‚N1 and C1-H1‚‚‚ O4], and thus these hydrogen bonds extend the acidbase subunits to form quite undulating supramolecular tapes along the crystallographic [001] direction. Second, each carboxylic O2/O6 atom acts as an acceptor of C2-H2‚‚‚O2/C11-H11‚‚‚O6 bonds, and as a conse-

Figure 2. (a) Molecular structure of 2 with atom labeling. (b) (Left) The essential hydrogen-bonded patterns occurring in 2; (right) the unique trihelix motif recognized by hydrogen bonds. (c) Space-filling model of the 2-D supramolecular architecture consisting of trihelix units.

quence, these 1-D curving tapes are further connected to form a 2-D network viewing from the bc plane. Via strong O7-H7‚‚‚O8 interactions with trimesic acids, the solvents (DMF) are captured to locate in the cavities of the host framework. Further analysis of the crystal packing indicates that these 2-D layers take on an A(-A)A′(-A)′ antiparallel stacking mode in the unit cell. The distance between two adjacent layers is ca. 4.1 Å, and no significant interaction such as hydrogen-bonded or aromatic stacking is found between them. [(H3TMA)‚(4-bpo)] (2). The molecular structure of 2 is depicted in Figure 2a. The dihedral angle between acid and base components is 17.1°. Within each 4-bpo subunit, two pyridyl rings deviate by 12.2° from coplanarity and form dihedral angles of 12.3 and 13.9°,

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Figure 3. (a) (left) Molecular structure of 3 with atom labeling of the asymmetric unit; (right) perspective (top) and side (bottom) view of the 1-D double chain hydrogen-bonding array via synthons I and VIII. (b) (left) Hydrate hydrogen-bonding ring showing the interactions between water and pyromellitic acid molecules; (right) sketch map of the 3-D supramolecular arrangement of 3; the bridging water molecules are indicated as gray balls and 1-D double-chain units as two adjacent bold lines, and the broken lines represent the hydrogen bonds between them.

respectively, with the oxadiazole plane. It is notable that the arrangement of three carboxylic groups of H3TMA in 2 is different from that in 1 (with C3 molecular symmetry), probably in order to fulfill the formation of suitable hydrogen-bonded patterns as described below. Similar to 1, two carboxylic groups of each trimesic acid in 2 also attend the creation of synthon I with two pyridyl rings. However, the third carboxylic group joins a new pattern of supramolecular synthon V [R22(6)], via strong O3-H3A‚‚‚O7 and weak C5-H5‚‚‚O2 and C5H5‚‚‚O7 interactions [see Figure 2b left]. An extended helical tape is generated via synthon I [O6-H6A‚‚‚N1 and C5-H5‚‚‚O7; O4-H4A‚‚‚N4 and C10-H10‚‚‚O5] along the b axis. Significantly, there is an additional C1-H1‚‚‚O5 hydrogen bond between each acid molecule and a base molecule in the adjacent tape, and as a consequence, three 1-D helixes are entangled to build up a unique trihelix motif, as shown in Figure 2b right. That is, three adjacent helical tapes recognize each other in sequence via such interchain hydrogen bonds. Furthermore, the O2-C13-O3 groups in H3TMA, being not involved in synthon I, locate at two sides of each 1-D trihelix pattern [see Figure 2b right] and extend these parallel trihelix motifs in “hand in hand” fashion via synthons V along the [001] direction, resulting in a 2-D layer [see Figure 2c]. Additionally, dimeric synthon VII [R22(10)] (see Chart 2 and Figure 2b left) between a pair of 4-bpo molecules (C9-H9‚‚‚N3) cannot be neglected, which connects the neighboring layers, stacking in the parallel alignment with some offset, along the a axis to form a 3-D network. [(H4BTA)‚(3-bpo)]‚2H2O (3). As depicted in Figure 3a left, the molecular structure of 3 has C2 symmetry,

crystallizing with two water solvents. The dihedral angle between 3-bpo and the benzene ring of H4BTA is 4.1°, revealing the favorable coplanar character. Within each 3-bpo, adopting the cisoid-II configuration,34 two terminal pyridyl rings make a dihedral angle of 1.8° with the oxadiazole system, and the dihedral angle of them is 2.7°. As far as the pyromellitic acid component, a pair of crystallographically independent carboxylic groups is almost vertical to each other, avoiding the formation of the intramolecular S(7) hydrogen-bonded pattern (synthon IV, Chart 2) that is usually observed in H4BTA-containing compounds.27f,38 Instead of this, pyromellitic acid here forms intermolecular interactions with 3-bpo, including strong O-H‚‚‚N and weak CH‚‚‚O/C-H‚‚‚N hydrogen bonds. As shown in the top of Figure 3a right, acid and base subunits arrange alternately to exhibit a 1-D tape via synthon I [O4H4‚‚‚N1 and C5-H5‚‚‚O5]. Through further C7H7‚‚‚N2 interactions [synthon VIII, R21(3)], two such 1-D tapes are combined into a 1-D double chain. From the side view [bottom of Figure 3a right], we can clearly see the “free” O2-C11-O3 carboxylic groups, which form O-H‚‚‚O hydrogen bonds with the lattice water located between the space of the adjacent 1-D double chains. As shown in Figure 3b left, each water moiety behaves as a hydrogen-bonding 3-connector [O6H6A‚‚‚O5, O6-H6B‚‚‚O3, and O2-H2‚‚‚O6] with three carboxylic groups from distinct H4BTA molecules, and thus, two water molecules link four H4BTA to result in a closed pattern with an orbicular synthon VI [R44(12)]. Due to the attendance of the lattice water molecules, these 1-D acid/base double chains are connected to

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Figure 5. (a) Hydrogen-bonding 2-D sheet along the crystallographic ab plane. (b) Space-filling model showing the stacking fashion of 5 (indicated as different colors).

Figure 4. (a) Molecular structure of 4 with atom labeling of the asymmetric unit. (b) The essential hydrogen-bonded patterns observed in 4. (c) A portion view of 4 showing the hydrogen bonds between each water molecule and the adjacent H4BTA and H2BTA components.

afford a 3-D sandwichlike architecture, as illustrated in Figure 3b right. [(H2BTA)1/2‚(H-4-bpo)‚(H4BTA)1/2]‚H2O (4). Crystallization of pyromellitic acid with 4-bpo yields a proton-transfer organic salt 4. The asymmetric unit contains half H4BTA molecule, half deprotonated H2BTA dianion, a corresponding monoprotonated [H-4bpo]+ cation, and a water solvent. As shown in Figure 4a, both H2BTA and H4BTA units have C2 symmetry; the former is coplanar and the latter is not. The mean plane of the cationic [H-4-bpo]+ is inclined to H4BTA and H2BTA with the dihedral angles of 6.3 and 14.7°, respectively. Within each [H-4-bpo]+, two terminal pyridinio/pyridyl rings make a dihedral angle of 5.1° and 16.1° with oxadiazole, and the dihedral angle of them is 19.5°. Due to the deprotonation effect, there exists an intramolecular O3-H3A‚‚‚O1 bond between two ortho substituents in the planar H2BTA dianion. As illustrated in Figure 4b, each H2BTA dianion connects

four [H-4-bpo]+ subunits (highlighted in cyan) via two synthons III R22(7) (N1-H1A‚‚‚O2 and C11-H11‚‚‚O1) and two C19-H19‚‚‚O3 hydrogen bonds. Meanwhile, each H4BTA molecule is also surrounded by four 4-bpo subunits (highlighted in green) linked through two synthons I (O5-H5A‚‚‚N4 and C20-H20‚‚‚O6) and two weak C22-H22‚‚‚O8 interactions. Additionally, there is a C15-H15‚‚‚N2 hydrogen bond between each pyridinio group and oxadiazole ring from distinct [H-4-bpo]+ cations (indicated by a black broken line). As a consequence, via synthons III and I, each [H-4-bpo]+ component connects the neighboring H2BTA and H4BTA to form a zigzag tape, and these tapes are further extended into a 3-D cross-link network. Further analysis of the crystal packing suggests that water molecules are located in the solvent-accessible voids of this 3-D host framework, interacted with H2BTA and H4BTA via O7H7‚‚‚O10, O10-H10A‚‚‚O4, and O10-H10B‚‚‚O6 hydrogen bonds, which stabilize the total supramolecular structure. Pyromellitic Dihydrazide (5). The molecular structure of compound 5 is planar and symmetric, leading predictably to formation of a 2-D network [Figure 5a] held together by a standard pattern of hydrogen bonds (synthon IX). Layers stack along [001] with an offset that places each molecule of dihydrazide 5 over a space in the adjacent layers [Figure 5b], so π-stacking and the formation of continuous channels are avoided. Thermogravimetric Analysis. Binary compounds 1-4 are air stable and can retain their structural integrity at room temperature for a considerable length of time. TGA was conducted to determine the contents

Cocrystallization with Bent Dipyridines

of solvents and thermal stability of these crystalline materials. For 1, the TGA curve shows the first weight loss of 14.26% from 120 to 165 °C (peaking at 146 °C), corresponding to the release of a guest DMF molecule (calculated, 14.19%). The remaining 2-D host framework keeps intact until two consecutive weight losses occur in the 220-440 °C region (the critical temperatures are 322 and 414 °C), corresponding to the stepwise removal of acid and base subunits continuously. For 2, five consecutive weight losses occur in the 135-200 °C range (peak positions: 139, 169, 181, 185, and 195 °C), which may be attributed to the loss of an isonicotinic acid molecule from the decomposing of 4-bpo (observed, 28.08%; calculated, 28.34%). The remaining substance does not lose weight upon further heating, until a total weight loss occurs from 240 to 460 °C (peaking at 315 °C). For compound 3, a weight loss of 6.99% in the temperature range 120-160 °C is close to the theoretical value of 7.00% for the loss of lattice water. The TGA curve also implies that the residues remain stable below 195 °C, upon removing the water molecules. The second weight loss of 93.18% from 195 to 315 °C (peaking at 297 °C) corresponds to the loss of host component (calculated, 93.00%). The TGA curve of 4 displays a blurry weight loss from 30 to 160 °C (ca. 4%), which is attributed to the loss of the water guests (calculated, 3.62%). The residual host framework stays intact until 200 °C, and there is a sharp weight loss peaking and ending at 251 and 270 °C, corresponding to the expulsion of all base and acid components. Conclusions and Perspectives In the present contribution for molecular tectonics, we describe the supramolecular synthesis and rational analysis of four binary cocrystals based on trimesic/ pyromellitic acid and bent dipyridyl base components. It is evident that the polycarboxylic acids have robust hydrogen-bonding donors/acceptors to fulfill the diversiform motives. This work also demonstrates that bpo is an excellent supramolecular reagent that can produce bimolecular cocrystals via various synthons, and especially, the oxadiazole ring has potential hydrogen-bond acceptors to enrich the formation of homo- or heterosynthons. In cocrystals 1-4, synthon I (including synthon III in 4 due to charge transfer) is successfully applied in the design of supramolecular tapes between polycarboxylic acids and dipyridyl compounds, that is, the geometries of strong O-H‚‚‚N hydrogen bonds that play the main role in controlling the 1-D tapes is generally predictable. Meanwhile, the formation of weak C-H‚‚ ‚O interactions could be easily affected by the backbone of the acid and/or base components. A unique 1-D triplehelical structural unit in 2 is observed, in which three 1-D helical tapes via synthon I are combined by weak C-H‚‚‚O hydrogen bonds. Additionally, the relative orientation of acid and base subunits within a cocrystal also makes a significant effect on forming the final diverse networks from 2-D to 3-D. We are currently extending this strategy by using other flexible alkyl carboxylic acids as the hydrogen-bonding participators to prepare new crystalline materials. Acknowledgment. This work was financially supported by the National Natural Science Foundation of

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China (No. 20401012), the Key Project of Tianjin Natural Science Foundation, and the Starting Funding of Tianjin Normal University (to M. Du). Supporting Information Available: Crystallographic information files (CIF) of compounds 1-5. This material is available free of charge via the Internet at http://pubs.acs.org.

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