Prediction in Cocrystallization of Flexible

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CRYSTAL GROWTH & DESIGN

Synthons Competition/Prediction in Cocrystallization of Flexible Dicarboxylic Acids with Bent Dipyridines

2006 VOL. 6, NO. 1 114-121

Miao Du,* Zhi-Hui Zhang, Xiao-Jun Zhao, and Hua Cai College of Chemistry and Life Science, Tianjin Normal UniVersity, Tianjin 300074, People’s Republic of China ReceiVed May 24, 2005; ReVised Manuscript ReceiVed July 17, 2005

ABSTRACT: On the basis of the primary carboxyl/pyridyl hydrogen-bonded synthon, 2,5-bis(3-pyridyl)-1,3,4-oxadiazole (3-bpo) and its 4-N-donor isomer 2,5-bis(4-pyridyl)-1,3,4-oxadiazole (4-bpo) were assembled with flexible dicarboxylic acids possessing variable chain lengths to afford a series of organic cocrystalline materials. [Fumaric acid]‚[3-bpo] (1) has a two-dimensional (2-D) layered supramolecular pattern. [Fumaric acid]‚[4-bpo] (2) has linear tapes connected via carboxyl/pyridyl heterosynthons and further creates a three-dimensional (3-D) square net with the CdSO4 topology combining with other weak interactions. [Suberic acid]‚[3bpo] (3), obtained in ambient conditions, exhibits a 3-D corrugated network; however, [suberic acid]‚[4-bph] (4), produced unexpectedly by hydrothermal synthesis, displays a 3-D framework expanding from 2-D layers, in which N,N′-bis(4-picolinoyl) hydrazine (4bph) is generated in situ from the hydrolysis of 4-bpo. [Benzene-1,4-dioxyacetic acid]‚[3-bpo] (5) takes on a nearly planar 2-D sheet, while [benzene-1,4-dioxyacetic acid]‚[4-bpo] (6) presents an unusual bilayered structure. For all binary compounds except 1, the acid-base components are connected by the carboxyl/pyridyl heterosynthon [O-H‚‚‚N and C-H‚‚‚O, R22(7)], which has been well exploited to engineer plenty of ordered crystalline architectures, to generate linear or zigzag tapes. Via subsidiary C-H‚‚‚O and/or N-H‚‚‚O (only for 4) hydrogen bonds between the neighboring tapes, these 1-D arrays are further extended to diverse networks from 2-D to 3-D. The similarities/differences between the hydrogen-bonded motifs of the cocrystals based on the flexible (alkyl) and rigid (aromatic) dicarboxylic acids with the same bent dipyridyl bases are also discussed, which may further enhance the crystal engineering of organic solids. Introduction The success of crystal engineering1 has attracted extensive interest in the area of prediction and assembly of desired structural aggregates. According to the opinions of Prof. Desiraju, a pioneer and leading scientist in this field, it is a dominating task to find recurring packing fashions adopted by certain functional groups and to rely on a variety of such motifs to achieve new solid-state crystalline materials.2 Such repetitive motifs are called supramolecular synthons.3 As an efficient instrument, “supramolecular synthons” strategy is widely utilized to generate new structures, which can be introduced to the design of new materials.4 Proper mastery of binary organic/organic relations is crucial from the view that the existence of stable and incidental molecular combinations can significantly complicate or simplify many isolation and purification strategies.5 Among the acid-base crystalline adducts, the frequently employed base building blocks are dipyridine, pyrazine, and their analogues.6 Meantime, aromatic carboxylic acids have been most favored in supramolecular and solid sciences due to their active ability to participate in hydrogen bonds7 and metalcarboxylic coordination.8 To further understand the nature of the intermolecular interactions and validate the main role of carboxyl/pyridyl heterosynthon [R22(7)] in supramolecular assembly, we have synthesized a series of binary cocrystals based on aromatic diand/or poly-carboxylic acids and the angular dipyridyl-donor compounds, 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).9 For these cocrystalline materials, the primary strong O-H‚‚‚N and the auxiliary weak C-H‚‚‚O hydrogen-bonding interactions between the carboxylic and pyridyl groups usually afford 1-D linear, zigzag, or helical infinite arrays. Combining with other * Corresponding author. Telephone: 86-22-23538221. Fax: 86-2223540315. E-mail: [email protected].

Scheme 1

weak C-H‚‚‚O and/or C-H‚‚‚N interactions, such heterosynthons extend the supramolecular architectures to higher dimensions as compared with the cases utilizing linear dipyridyl derivatives as the building blocks,6b-d,10 which is attributed to the multiple hydrogen-bonding sites and relatively flexible conformation of bpo.11 On the other hand, flexible alkyl carboxylic acids have also been applied in similar assemblies; for example, the 1,2-disubstituted fumaric acid was used to cocrystallize with 4,4′-bipyridine derivatives.12 However, far less common has been the exploration of such flexible components in construction of cocrystalline systems,13 and the resultant solid structures must be difficult to predict and design and must not readily fit into a current pattern due to their flexibility and conformational freedom. For the sake of enriching such interesting hydrogen-bonding supramolecular systems and as a continuation of our research, we choose the 1,2-disubstituted fumaric acid, 1,6-disubstituted suberic acid, and 1,8-disubstituted flexible benzene-1,4-dioxyacetic acid to cocrystallize with changeable 3-bpo/4-bpo (see Scheme 1). In the present contribution, six related 1:1 acidbase cocrystals were prepared to search for the similarities and

10.1021/cg050229w CCC: $33.50 © 2006 American Chemical Society Published on Web 09/14/2005

Synthons Competition/Prediction in Dicarboxylic Acids

differences between cocrystalline materials of bpo with rigid and flexible diacids and to further understand the role of weak C-H‚‚‚N/C-H‚‚‚O interactions in crystal engineering. To the best of our knowledge, this is the first time for benzene-1,4dioxyacetic acid to be applied in cocrystal synthesis, although some related metal complexes have been reported recently.14 Remarkably, the in situ generation of a new organic compound N,N′-bis(4-picolinoyl) hydrazine (4-bph) from the hydrolysis of 4-bpo was detected during cocrystallization of 4-bpo with suberic acid under hydrothermal conditions. Experimental Section General Materials and Methods. 3-Bpo and 4-bpo were prepared using the literature procedures,15 and other reagents and solvents (analysis grade) were commercially available and used as received. The melting points of the new compounds were recorded on a WRS-1B digital thermal apparatus without correction. Fourier transform (FT) infrared data were collected on an AVATAR-370 (Nicolet) spectrometer by transmission through the sample deposited on a KBr pellet. Elemental analyses were performed on a CE-440 (Leemanlabs) analyzer. Thermogravimetric analysis (TGA) experiments were performed on a Dupont thermal analyzer in the temperature range of 25800 °C under nitrogen atmosphere at a heating rate of 10 °C/min. Syntheses of the Compounds. [Fumaric Acid]‚[3-bpo] (1). 3-Bpo (22.4 mg, 0.1 mmol) and fumaric acid (11.6 mg, 0.1 mmol) were mixed and dissolved in methanol (10 mL) with stirring. The clear solution was allowed to evaporate at ambient conditions, affording colorless lamellar crystals after 2 days in 85% yield; mp 185-186 °C. Anal. Calcd for C16H12N4O5: C, 56.47; H, 3.55; N, 16.46%. Found: C, 56.56; H, 3.45; N, 15.98%. IR (cm-1): 2782w, 2502w, 2361Vs, 2338s, 1701s, 1589m, 1461m, 1443m, 1294Vs, 1176Vs, 1127w, 1078w, 1045w, 1005w, 923w, 826m, 776w, 732m, 698m, 670m, 629Vs, 552w, 514w. [Fumaric Acid]‚[4-bpo] (2). 4-Bpo (22.4 mg, 0.1 mmol) and fumaric acid (11.6 mg, 0.1 mmol) were dissolved in a methanol/water mixture (v:v ) 1:1, 10 mL). Upon slow evaporation of the solvents, colorless lamellar single crystals suitable for X-ray analysis were obtained over a period of 3 days in 90% yield; mp 261 °C. Anal. Calcd for C16H12N4O5: C, 56.47; H, 3.55; N, 16.46%. Found: C, 56.69; H, 3.50; N, 16.40%. IR (cm-1): 3092m, 2421b, 1880w, 1709m, 1613m, 1571m, 1533m, 1487m, 1416s, 1298Vs, 1214m, 1161s, 1056m, 1011m, 968m, 922w, 849s, 778m, 746m, 726s, 710m, 641w, 550s, 502m. [Suberic Acid]‚[3-bpo] (3). The same synthetic procedure as for 2 was used except that the reactants were replaced by 3-bpo (22.4 mg, 0.1 mmol) and suberic acid (17.4 mg, 0.1 mmol), giving colorless block crystals by slow evaporation of the solvents after 5 days in 80% yield; mp 131-133 °C. Anal. Calcd for C20H22N4O5: C, 60.29; H, 5.57; N, 14.06%. Found: C, 59.94; H, 5.59; N, 14.03%. IR (cm-1): 3063w, 2940m, 2860m, 2504m, 1879m, 1701Vs, 1595s, 1543m, 1464m, 1436m, 1407m, 1373m, 1332s, 1240Vs, 1178Vs, 1127w, 1082m, 1025m, 932w, 903w, 824m, 732m, 702m, 668m, 635m, 523w, 498w. [Suberic Acid]‚[4-bph] (4). A water (10 mL) solution containing 4-bpo (44.8 mg, 0.2 mmol) and suberic acid (34.8 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. Orange needlelike crystalline products were collected in 80% yield; mp 187191 °C. Anal. Calcd for C20H24N4O6: C, 57.69; H, 5.81; N, 13.45%. Found: C, 57.67; H, 5.53; N, 13.85%. IR (cm-1): 2943m, 2856w, 2509m, 1904m, 1702Vs, 1610m, 1565m, 1542m, 1487m, 1467m, 1412s, 1375m, 1333s, 1244Vs, 1179Vs, 1058m, 1007s, 899w, 841s, 792w, 721s, 695m, 670m, 527w, 502m, 412w. [Benzene-1,4-dioxyacetic Acid]‚[3-bpo] (5). A CH3OH/C2H5OH (v:v ) 1:1, 6 mL) solution of benzene-1,4-dioxyacetic acid (22.6 mg, 0.1 mmol) was carefully layered onto a solution of 3-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 2 weeks in 70% yield; mp 254-256 °C. Anal. Calcd for C22H18N4O7: C, 58.67; H, 4.03; N, 12.44%. Found: C, 58.77; H, 4.04; N, 12.49%. IR (cm-1): 2914w, 2457w, 1840w, 1737s, 1603m, 1537w, 1506s, 1465w, 1441m, 1417m, 1383m, 1289m, 1248m, 1211Vs, 1118m, 1088s, 1039s, 917w, 840m, 793s, 730m, 693m, 640m, 535w, 491w.

Crystal Growth & Design, Vol. 6, No. 1, 2006 115 [Benzene-1,4-dioxyacetic Acid]‚[4-bpo] (6). A CH3OH (4 mL) and CH3CN (2 mL) solution containing benzene-1,4-dioxyacetic acid (22.6 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 prism crystals were observed on the tube wall after 4 days in 85% yield; mp 234 °C. Anal. Calcd for C22H18N4O7: C, 58.67; H, 4.03; N, 12.44%. Found: C, 58.44; H, 4.06; N, 12.38%. IR (cm-1): 3099w, 2912w, 2453m, 1848w, 1745m, 1610m, 1567m, 1509s, 1418s, 1386m, 1324w, 1286s, 1223Vs, 1199s, 1115m, 1082s, 1056s, 1011s, 968w, 880w, 843m, 819m, 798s, 745m, 725s, 661m, 503m, 416m. X-ray Crystallography. X-ray single-crystal diffraction data for 1-6 were collected on a Bruker Apex II CCD diffractometer at room temperature using a fine-focus molybdenum KR tube (λ ) 0.71073 Å). There was no evidence of crystal decay during data collection. A semiempirical absorption correction was applied (SADABS), and the program SAINT was used for integration of the diffraction profiles.16 All structures were solved by direct methods with SHELXS and refined by full-matrix least-squares on F2 with the SHELXL program of the SHELXTL package.17 All H atoms were first found in difference electron density maps, and then placed in the calculated sites and included in the final refinement with fixed thermal factors. Crystallographic data and structural refinement parameters are summarized in Table 1.

Results and Discussion Preparation of Compounds 1-6. All crystallizations of 3-bpo/4-bpo and flexible dicarboxylic acids were performed in a 1:1 ratio, considering the number of hydrogen-bonding acceptor/donor groups in each building block. For the preparation of 1-3, diacid was mixed directly with bpo, which was allowed to evaporate at ambient conditions to give the crystalline products. For 5, cocrystallization of benzene-1,4-dioxyacetic acid and 3-bpo was experimented using a test tube to facilitate the slow growth of larger crystals suitable for X-ray diffraction. In the cases of 4 and 6, the immediate combination of acid and base components led to a mass of white precipitation. Wellshaped crystals of 6 were achieved using the slow diffusion method similar to 5; however, only the crystals of suberic acid were obtained when we attempted cocrystallization of it with 4-bpo utilizing the same procedure. Our previous research results9 have testified that hydro/solvothermal synthesis is also a useful strategy to prepare such cocrystals. However, an unexpected product 4, that is, [suberic acid]‚[4-bph], came into being during this process, in which a new organic compound N,N′-bis(4-picolinoyl) hydrazine (4-bph) was generated in situ. According to our recent work,9b,18 when 4-bpo was handled under hydrothermal circumstance, it may hydrolyze into isonicotinic acid and hydrazine, which could be rationally deduced from the synthetic process of 4-bpo.15 The formation mechanism of 4-bph may be interpreted as a ring-open course (part hydrolyzation) of the oxadiazole ring of 4-bpo, which, at the same time, cocrystallizes with suberic acid to form the final binary compound 4. Molecular and Supramolecular Structures of 1-6. The schematic representations of related hydrogen-bonding synthons are summarized in Scheme 2. Hydrogen-bond parameters of compounds 1-6 are listed in Table 2. [Fumaric Acid]‚[3-bpo] (1). In the molecular structure of 1 (Figure 1a), the bpo component has a C2 symmetry and fumaric acid is centrosymmetric, which should be related to their molecular backbones. The 3-bpo molecule adopts a cisoid-II configuration, which is unusual in the metal complexes11 but, however, is common in its binary cocrystals with aromatic acids.9 Within each 3-bpo, two terminal pyridyl rings make a dihedral angle of 13.4(2)° with the central oxadiazole system, and the dihedral angle is 26.3(3)°. Each acid molecule connected four 3-bpo components (Figure 1b) via two O3-H3‚‚‚N1 bonds

116 Crystal Growth & Design, Vol. 6, No. 1, 2006

Du et al.

Table 1. Crystal Data and Structure Refinement Summary for Compounds 1-6

a

1

2

3

empirical formula M crystal size/mm crystal system space group a/Å b/Å c/Å β/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

C16H12N4O5 340.30 0.20 × 0.18 × 0.16 monoclinic C2/c 22.271(8) 6.611(2) 10.553(4) 100.225(6) 1529.1(9) 4 1.478 0.113 704 -27/21, -6/8, -11/13 4278/1578

C16H12N4O5 340.30 0.20 × 0.18 × 0.12 monoclinic P2/c 10.585(6) 11.337(5) 13.697(6) 104.733(6) 1589.6(13) 4 1.422 0.109 704 -6/13, -14/13, -17/16 8936/3250

C20H22N4O5 398.42 0.30 × 0.13 × 0.11 monoclinic C2/c 5.1367(11) 13.129(3) 29.552(6) 91.771(4) 1992.1(7) 4 1.328 0.097 840 -6/6, -16/16, -38/30 6899/2315

116 0.0333 0.0429, 0.0998 1.048 0.194, -0.151

229 0.0552 0.0565, 0.0806 0.984 0.154, -0.170

134 0.0373 0.0506, 0.1162 0.983 0.205, -0.145

4

5

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

C20H24N4O6 416.43 0.22 × 0.16 × 0.12 triclinic P1h 4.8911(14) 8.073(2) 13.274(4) 94.337(5) 93.437(5) 104.298(5) 504.8(3) 1 1.370 0.103 220 -6/5, -10/8, -16/16 2930/2041

C22H18N4O7 450.40 0.53 × 0.30 × 0.24 monoclinic C2/c 19.521(2) 6.6365(8) 17.255(2) 90 115.3010(10) 90 2021.1(4) 4 1.480 0.113 936 -18/23, -7/7, -20/20 5522/1779

C22H18N4O7 450.40 0.23 × 0.14 × 0.09 triclinic P1h 8.395(7) 11.051(9) 11.518(9) 107.226(10) 93.103(11) 93.958(11) 1015.0(14) 2 1.474 0.112 468 -9/9, -13/12, -13/13 5502/3535

137 0.0223 0.0473, 0.1007 1.035 0.169, -0.207

152 0.0167 0.0327, 0.0970 1.083 0.171, -0.172

301 0.0197 0.0374, 0.0898 1.034 0.161, -0.174

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

and two pairs of C4-H4 ‚‚‚O2/C5-H5‚‚‚O2 weak interactions [synthon I, R12(5)].19 Due to the stereochemistry effect within this structure (the pyridyl plane is inclined to the adjacent fumaric acid with a rather large angle of 35.7(1)°), the most frequent carboxyl/pyridyl synthon II is absent. Meanwhile,

around each 3-bpo, there are also four acid molecules binding via the above-mentioned hydrogen bonds. As a consequence, each acid or base component acts as a 4-connected node to build up a 2-D hydrogen-bonded network, in which the acid and base arrange alternately along the [001] direction. The adjacent sheets are parallel and align with some offset. [Fumaric Acid]‚[4-bpo] (2). The asymmetry unit of 2 (Figure 2a) contains a pair of half 4-bpo molecules (base1/base2, C2 symmetry) and the corresponding fumaric acid (acid1/acid2, centrosymmetry). Each 4-bpo crystallizes with a nearly parallel fumaric acid with the incline angles of only 2.2(1)° for acid1/ base1 and 3.8(2)° for acid2/base2, and a pair of such acid/base “combos” form a crossed arrangement. The dihedral angle between two pyridyl rings is 1.8(2)° for base1 and 4.3(2)° for base2, and the pyridyls are inclined to oxadiazole with the dihedral angle of 1.4(2)° for base1 and 3.1(2)° for base2. The dihedral angle of acid1/acid2 is 123.6(1)°, and that of base1/ base2 is 124.1(1)°. The adjacent acid and base components are linked to form a linear tape via synthon II (O4-H4‚‚‚N4 and C11-H11‚‚‚O3 for acid1/base1; O5-H5‚‚‚N2 and C5H5A‚‚‚O6 for acid2/base2). Both nitrogens of each oxadiazole

Synthons Competition/Prediction in Dicarboxylic Acids

Crystal Growth & Design, Vol. 6, No. 1, 2006 117

Table 2. Hydrogen Bond Parameters in the Crystal Structures of 1-6 D-H‚‚‚A

D‚‚‚A (Å)

H‚‚‚A (Å)

D-H‚‚‚A (deg)

O3-H3‚‚‚N1a C4-H4‚‚‚O2b C5-H5‚‚‚O2b O4-H4‚‚‚N4c O5-H5‚‚‚N2d C3-H3‚‚‚N3e C4-H4A‚‚‚O4f C5-H5A‚‚‚O6f C6-H6‚‚‚O3g C9-H9‚‚‚N1 C10-H10‚‚‚O5 C11-H11‚‚‚O3c C12-H12‚‚‚O6h O3-H3‚‚‚N1i C1-H1‚‚‚O2j C2-H2‚‚‚O2k N1-H1A‚‚‚O1k O2-H2A‚‚‚N2 C1-H1‚‚‚O3 C2-H2‚‚‚O3l C5-H5‚‚‚O2m O3-H3‚‚‚N2n C4-H4‚‚‚O3g C5-H5‚‚‚O4o C10-H10‚‚‚O4e O2-H2‚‚‚N4p O7-H7‚‚‚N1q C2-H2A‚‚‚O3 C3-H3‚‚‚O6r C10-H10‚‚‚O3s C16-H16‚‚‚O6t

2.710(3) 3.164(3) 3.191(3) 2.609(3) 2.664(3) 3.454(4) 3.456(3) 3.231(3) 3.240(4) 3.485(4) 3.431(4) 3.214(4) 3.209(3) 2.686(3) 3.418(2) 3.220(3) 2.947(2) 2.658(2) 3.684(6) 3.207(3) 3.430(3) 2.649(4) 3.337(2) 3.677(2) 3.155(2) 2.661(3) 2.728(3) 3.271(4) 3.292(2) 3.162(3) 3.305(4)

1.90 2.56 2.58 1.79 1.84 2.54 2.53 2.57 2.42 2.57 2.57 2.55 2.36 1.87 2.83 2.39 2.17 1.85 3.15 2.53 2.53 1.84 2.59 3.14 2.40 1.84 1.91 2.49 2.62 2.46 2.57

169 123 124 177 178 168 171 129 147 167 155 129 152 173 122 149 150 169 118 130 162 169 138 118 138 179 178 142 130 132 136

compound 1 2

3 4

5

6

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

take part in the intermolecular C3-H3‚‚‚N3/ C9-H9‚‚‚N1 interactions between base1 and base2, affording a new synthon III (denoted as R22(11))20 and further linking the adjacent acid/ base tapes. Meanwhile, acid1/acid2 also create abundant weak interactions with base2/base1 (C4-H4A‚‚‚O4 and C6H6‚‚‚O3; C10-H10‚‚‚O5 and C12-H12‚‚‚O6) from the neighboring acid/ base tapes (Figure 2b), further consolidating this 3-D structure. Another alternative description from the viewpoint of network topology is as follows: two crystallographically independent hydrogen-bonded chains of acid1/base1 and acid2/ base2 are oriented at an angle of ca. 60° and connected via hydrogen bonds stated above. If the centers of synthons II and III are considered as square nodes, this orthogonal stacking is topologically identical to a tetragonal CdSO4 framework (Figure 2c).21 Recently, Nangia and co-workers22 reported an unusual silver(I) complex of isonicotinamide with 5-fold interpenetrated CdSO4 network and suggested that such 3-D hydrogen-bonded nets be named by their inorganic structure equivalents to avoid ambiguity. It is also notable that the inversion-related base2 molecules are π-stacked along the [100] direction with the center-to-center distance of 3.81 Å between the neighboring pyridyl groups, which are weaker noncovalent interactions in nature but further stabilize the network. [Suberic Acid]‚[3-bpo] (3). In the molecular structure of 3 (Figure 3a), the local asymmetric unit contains half suberic acid and half 3-bpo. For 3-bpo subunit with a C2 symmetry and also a cisoid-II configuration, two terminal pyridyl rings deviate by 12.9(2)° from coplanarity and form the dihedral angle of 6.7(3)° with oxadiazole. In the centrosymmetric suberic acid

Figure 1. (a) ORTEP view of 1 with atom labeling of the asymmetric unit; (b) 2-D hydrogen-bonding architecture of 1, viewing along the [100] direction. Hydrogen bonds are shown as dashed lines.

component, two carboxylic groups are trans-arrangement to selfadjust the generation of the favorite hydrogen-bonded patterns as described below. As expected, the suberic acid and 3-bpo locate alternately; thus an extended zigzag tape is generated via synthons II (O3-H3‚‚‚N1 and C1-H1‚‚‚O2, Figure 3b, top). Obviously, in accordance with the anti-backbone of the diacid component, two base molecules around it are also in antifashion arrangement to fulfill the formation of this 1-D tape via heterosynthon II (Figure 3b, bottom). Significantly, these zigzag tapes are highly interdigitated and further connected through weak C2-H2‚‚‚O2 hydrogen bonds between acid and base subunits, resulting in a 3-D integrated network (Figure 3c). [Suberic Acid]‚[4-bph] (4). In the molecular structure of 4, each 4-bph crystallizes with a suberic acid, and both components are centrosymmetric. Clearly, two related terminal pyridyls in 4-bph are parallel, and the appropriate distance between pyridyl and carboxyl facilitates the formation of “intramolecular” hydrogen bonds (Figure 4a). Thus, 4-bph and suberic acid are connected via strong O2-H2A‚‚‚N2 and weak C1-H1‚‚‚O3 (synthon II) interactions, affording 1-D linear tapes. As viewed from the ac plane, such 1-D tapes are combined to generate a 2-D layer, through weaker C2-H2‚‚‚O3 and C5-H5‚‚‚O2 bonds between acid and base components from adjacent tapes (Figure 4b), in which two additional synthons IV [A ) R44(10)]13e and V [B ) R24(10)]9a are included. Interestingly, suberic acid and 4-bph arrange alternately along each direction

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Figure 3. (a) ORTEP view of 3 with atom labeling of the asymmetric unit; (b) (top) a side view of the 1-D zigzag hydrogen-bonding array via synthon II, (bottom) a perspective view of the 1-D zigzag chain along the [100] direction; (c) 3-D supramolecular architecture of 3. Figure 2. (a) ORTEP view of 2 with atom labeling of the asymmetric unit; (b) 3-D hydrogen-bonding array with synthon II and other weak interactions; (c) 3-D square network with CdSO4 topology.

of this plane, which is unusual for such 2-D cocrystalline net.9 Furthermore, there exists an interlayered N1-H1A‚‚‚O1 hydrogen-bonding motif, that is, amide-amide dimer [synthon VI, R22(10)],23 which extends the adjoining parallel sheets to afford a 3-D array (Figure 4c). This cocrystalline architecture based on amide-amide homometric interactions follows the bestdonor/best-acceptor (second best-donor)/(second best-acceptor) hierarchical fashion of hydrogen bonds to maximize the electrostatic attractions.24 [Benzene-1,4-dioxyacetic Acid]‚[3-bpo] (5). The molecular structure of 5 (Figure 5a) is similar to those of 1 and 3, presenting half base and half acid subunits. The 3-bpo molecule also exhibits the cisoid-II configuration in the interest of forming steady synthon II with centrosymmetric diacid. The terminal pyridyl rings in 3-bpo form the dihedral angle of 1.6(2)° with the oxadiazole plane, and that of 3.0(1)° with each other. Analysis of the crystal packing of 5 implies that the acid and base subunits locate alternately, generating a zigzag tape via

synthon II (Figure 5b, O3-H3‚‚‚N2 and C5-H5‚‚‚O4). In this tape along the [001] direction, 3-bpo subunits adopt transarrangement around the acid molecules to fulfill the formation of synthon II. Meanwhile, the carboxyl O3/O4 atoms act as the acceptors of C4-H4‚‚‚O3 and C10-H10‚‚‚O4 interactions, linking the acid/base or acid/acid components from adjacent tapes. As a consequence, these 1-D tapes are extended to a 2-D network along the bc plane, and three new hydrogen-bonded patterns marked as A [R23(12)], B [R22(16)], and C [R44(24)] come into being. In addition, the sheets adopt parallel alignment with some offset along the [100] direction. [Benzene-1,4-dioxyacetic Acid]‚[4-bpo] (6). Different from cocrystals 1-5, the molecular structure of 6 (Figure 6a) contains no crystallographically imposed symmetry. Notably, the backbone geometry of benzene-1,4-dioxyacetic acid is different from that in 5: two carboxylic groups are in cis-arrangement and possess a molecular mirror symmetry, which also represents the first structural example for such a diacid in either organic materials or metal complexes. The terminal pyridyl rings in 4-bpo make the dihedral angles of 7.1(1)° and 2.5(1)° with oxadiazole, and a dihedral angle of 7.9(1)° with each other. The base and acid subunits also align alternately (Figure 6b) to afford

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Crystal Growth & Design, Vol. 6, No. 1, 2006 119

Figure 4. (a) ORTEP view of 4 with atom labeling of the asymmetric unit; (b) 2-D hydrogen-bonding sheet along the crystallographic ac plane; (c) 3-D supramolecular arrangement of 4, where different layers are highlighted in yellow, green, and cyan, respectively.

a linear tape via synthon II (O2-H2‚‚‚N4/C10-H10‚‚‚O3; O7H7‚‚‚N1/C3-H3‚‚‚O6). However, 4-bpo components adopt the cis-arrangement around the acid molecules to meet the requirement of the generation of synthon II. Significantly, there is a C2-H2A‚‚‚O3 weak hydrogen bond between acid and base coming from two adjacent tapes, and, thus, these tapes arrange parallel to afford a 2-D nearly-planar sheet, in which the acid and base subunits are staggered in both directions (similar to 4). Further analysis of the crystal packing indicated that the additional interlayered C16-H16‚‚‚O6 interactions between the diacid molecules link two adjoining antiparallel sheets to give a double-layered motif (Figure 6c). The access of supramolecular synthon provides a considerable simplification in the design of desired architectures by using previously identified hydrogen bonds.25 In a cocrystalline solid of carboxylic acid and bpo, heterosynthon II always appears reliable to form an infinite 1-D tape pattern, except that the acid component significantly inclines to bpo. For synthons I and V, one carboxylic oxygen atom (CdO) acts as the donor of two C-H‚‚‚O hydrogen bonds in like manner, and the hydrogen atoms on sp2 carbons either from the same or from two distinct pyridyl ring(s) readily form these weak interactions. Presumably, a competition between the choices of the synthons may happen during the assembling process. On the other hand, the new building block (4-bph) generated in situ during synthesis arouses the competition of synthon VI [R22(10)] and homodimer VII [R22(8)]9b based on a pair of N-H‚‚‚O interactions. Prediction can be drawn that 4-bph might give rise to a preferred pattern VI to fulfill the steady low energy configuration. The same case has been observed in the structure of N,N′bis(2-picolinoyl) hydrazine.23 So far, a systematic research has not been performed on the choice of supramolecular patterns among two or more possible synthons with the same functional

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

groups. Also, the prediction of competing synthons based on the specific geometries of different building blocks might determine the supramolecular assemblies and further the crystal packing.6e Thermogravimetric Analysis. Cocrystalline materials 1-6 are air stable at ambient conditions, and thermogravimetric experiments were implemented to investigate their thermal stabilities. The TG curve of 1 shows two consecutive weight losses of all crystalline samples from 150 to 310 °C (peaking at 204 and 287 °C, respectively). For 2, the first weight loss of 65.9% from 210 to 280 °C, peaking at 270 °C, corresponds to the loss of the base component (calculated: 65.9%). The residuary acid section subsequently loses weight of 33.9% (calculated: 34.1%) from 280 to 320 °C (peak: 290 °C). As for 3 and 4, the TGA results indicate they remain intact until 140 and 160 °C, respectively, and then there is a sharp weight loss ending at 275 °C for 3 and 300 °C for 4 (peaks: 252 °C for 3 and 258 °C for 4), corresponding to the expulsion of all base and acid components. The TGA curves of 5 and 6 also have many common grounds. The starting decomposition occurs at 200 °C, and there is a sharp weight loss of all samples, peaking and ending at 297 and 340 °C for 5, and 299 and 340 °C for 6. It can be summarized that relative to the same diacid, the decomposition temperature peak for the cocrystal with 4-bpo is usually higher than that with 3-bpo. Conclusions and Perspectives This study allows that bpo derivatives are successful reagents to produce bimolecular cocrystals with various flexible diacids, as well as with di- or poly-aromatic carboxylic acids, in good

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thermal condition, could be a good building block for constructing new organic or metal-organic crystalline materials with interesting supramolecular frameworks and properties.26 Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (No. 20401012), the Key Project of Tianjin Natural Science Foundation (043804111), and Tianjin Normal University (to M.D.). Supporting Information Available: Crystallographic information files (CIF) for compounds 1-6. This material is available free of charge via the Internet at http://pubs.acs.org.

References

Figure 6. (a) ORTEP view of 6 with atom labeling; (b) 2-D hydrogenbonding layer along the crystallographic bc plane; (c) 2-D bilayer structure viewing along the [010] direction.

output. Strong O-H‚‚‚N interactions are involved in all cocrystals 1-6, out of question following the best donor/acceptor guideline. The intermolecular hydrogen-bonded heterosynthon II appears reliable to form infinite 1-D tapes in almost all supramolecular patterns except in the case of 1, in which the acid component significantly inclines to the adjacent base molecule. With this in mind, this strategy will be helpful in the design of analogues for specific applications with related topological features. Also, in this work, some unusual (I, IV, and VI) or new (III) synthons have been created during the assemblies of cocrystals 1, 2, and 4. On the other hand, weak C-H‚‚‚O/N interactions are less robust but readily contribute to crystalline stabilization. Significantly, due to the hydrolysis of 4-bpo, the secondary N-H‚‚‚O interaction including the second-best donor (amide H atom) and second-best acceptor (carbonyl O atom) is introduced to the supramolecular structure of 4, affording a homometric dimer. Also for 4, the introduction of amide group makes the basic building block more powerful and flexible to adjust the formation of abundant synthons. In summary, we expand the mimicry from rigid to limber acids based on heterosynthon II in cocrystals within bent dipyridyl blocks. Hydrogen-bonding interactions between the pyridyl and carboxylic moieties also supply the dominating steering force for these supramolecular assemblies. However, the variations in the acid or base building blocks may modulate the hydrogen-bonded motifs and further crystal packing of these binary cocrystals. Studies are ongoing with different building blocks and crystalline conditions to flourish hierarchical rules of hydrogen-bond synthons. Finally, we anticipate that the unusual dipyridyl compound 4-bph, obtained from in situ synthesis of cocrystal of 4-bpo and suberic acid under hydro-

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Crystal Growth & Design, Vol. 6, No. 1, 2006 121 (19) For carboxyl/pyridyl cocrystals containing synthon I, see: Dale, S. H.; Elsegood, M. R. J.; Hemmings, M.; Wilkinson, A. L. CrystEngComm 2004, 6, 207. (20) This “new synthon” was identified by a CSD search (version 5.26, May 2005 update). (21) (a) Wells, A. F. Three-Dimensional Nets and Polyhedra; WileyInterscience: New York, 1977. (b) Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Oxford University Press: Oxford, 1984. (c) O’Keeffe, M.; Eddaoudi, M.; Li, H.; Reineke, T.; Yaghi, O. M. J. Solid State Chem. 2000, 152, 3. (22) Bhogala, B. R.; Thallapally, P. K.; Nangia, A. Cryst. Growth Des. 2004, 4, 215. (23) Shao, S.-C.; Zhu, D.-R.; Song, Y.; You, X.-Z.; Raj, S. S. S.; Fun, H.-K. Acta Crystallogr. 1999, C55, 1841. (24) Etter, M. C. Acc. Chem. Res. 1990, 23, 120. (25) Nangia, A.; Desiraju, G. R. Acta Crystallogr. 1998, A54, 934. (26) During the preparation of this manuscript, we have successfully synthesized and isolated the compound 4-bph and its 3-N-donor analogue. Related investigations are in progress and to be published in due course.

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