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Also, this research allows us to identify the predictable hydrogen-bonding synthons between 4-bpo and such acidic components containing “classicalâ€...
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CRYSTAL GROWTH & DESIGN

A Search for Predictable Hydrogen-Bonding Synthons in Cocrystallization of Unusual Organic Acids with a Bent Dipyridine

2006 VOL. 6, NO. 2 390-396

Miao Du,* Zhi-Hui Zhang, and Xiao-Jun Zhao College of Chemistry and Life Science, Tianjin Normal UniVersity, Tianjin 300074, P. R. China ReceiVed July 13, 2005; ReVised Manuscript ReceiVed October 13, 2005

ABSTRACT: A bent dipyridyl compound, 2,5-bis(4-pyridyl)-1,3,4-oxadiazole (4-bpo), was employed to crystallize with 1,4-cyclohexanedicarboxylic acid (H2chdc), hydroquinone, citric acid (H4ca‚H2O), barbituric acid, and ferrocene dicarboxylic acid (H2Fc) under general conditions, affording binary cocrystals [(H2chdc)‚(4-bpo)] (1), [(hydroquinone)‚(4-bpo)2] (2), [(H4ca)‚(4-bpo)] (3), [(barbituric acid)‚(4-bpo)] (4), and [(HFc)‚(H-4-bpo)] (5) in high yield. X-ray single-crystal structures of these compounds reveal that a supramolecular synthon [R22(7)] containing classical O-H‚‚‚N and weak C-H‚‚‚O interactions, usually observed in organic cocrystals of carboxylic acid and heterocyclic base, is again involved in constructing the hydrogen-bonding networks of 1, 3, and 5. Its ionic analogous synthon [consisting of strong N-H‚‚‚O and weak C-H‚‚‚O, R22(7)] also appears in the partly chargetransfer salt 5. For 2, only hydroxyl of the hydroquinone component can act as the hydrogen-bonding contributor, giving a strong O-H‚‚‚N bond with 4-bpo. Multiple hydrogen-bonded donating/accepting sites in compound 3 fulfill the formation of new supramolecular patterns. For 4, another heterosynthon ring [containing strong N-H‚‚‚N and weak C-H‚‚‚O, R22(7)] is formed due to the presence of the imide functional group. As a consequence, robust hydrogen-bonding interactions in these compounds afford diverse 2-D waved or planar layers (1 or 4), 1-D extended tape (2), 3-D net (3), and 1-D molecular-box type (5) supramolecular architectures. Thermal stability of these compounds has been investigated by thermogravimetric analysis (TGA) of mass loss. Introduction Hydrogen bonding has emerged to be the most powerful tool among the directional intermolecular interactions in noncovalent synthesis, which is operative in determining the molecular conformation, supramolecular aggregation, and function of a large number of chemical systems such as organic, material, and biological chemistry.1-4 The preparation of ordered functional crystalline solids, which display a variety of well-defined supramolecular networks mediated by hydrogen bonds, has attracted considerable interest.4 Invariably, evolution of an assembly depends on the nature of the functional group under consideration. To this end, assemblies involving the carboxylic group (-COOH) are very widely applied because of its ability to form robust hydrogen bonds on its own and also with the aza compounds, forming either single O-H‚‚‚N or O-H‚‚‚N/ C-H‚‚‚O pairwise hydrogen bonds, as well as coordination bonds with almost all metal ions.5-10 Recently, aromatic di- or poly(carboxylic acids) have been investigated adequately in the area of solid state and material science. For instance, terephthalic acid was commonly employed in the preparation of metal-based functional solids.10,11 Trimesic acid has a well-known chicken-wire framework12a and forms hexagonal hydrogen-bonding networks when cocrystallized with pyrene, N,N-dimethylamine, and so on.12b,c To our knowledge, cocrystallization of a dipyridyl building block with a nonaromatic organic acid is relatively rare. As far as 4,4′-dipyridine is concerned, a CSD (Cambridge Structural Database, version 5.26 of November 2004, plus three updates)13 search reveals that only 22 related cocrystals have been reported so far. Typically, 1,4-cyclohexanedicarboxylic acid (H2chdc) has not been introduced to the generation of cocrystals. It has three preponderant conformations with regard to the carboxyl groups and is usually in a mixture of cis and trans conformations. As shown in Scheme 1, it is a little more difficult when A transfers to B because the R-protons must be deprotonated to accelerate * Fax and Tel: 86-22-23540315. E-mail: [email protected].

Scheme 1

the equilibrium, although B is thermodynamically more stable. Meanwhile, conformation C is the least stable and easily attains style B.14 Hydroquinone, though having weak acidity, is also a good hydrogen-bonding participant in the assembly of crystalline materials with desired crystallographic architectures.15 As a metal chelating agent, citric acid (H4ca‚H2O) is often deprotonated to different levels such as in singly, doubly, or triply ionized form.16 Even when weak or moderately strong organic bases exist, it is relatively rare for citric acid to keep the neutral form in the complex adducts.17 Barbituric acid has imide functional groups, which could be considered as the second best hydrogen-bond donor2a,7b to produce new supramolecular solids.18 Ferrocene dicarboxylic acid (H2Fc) has attracted intense interest in generating the hybrid organic-organometallic or organometallic-organometallic hydrogen-bonding networks, together with the fascinating properties of the ferrocene group.19 One of the powerful strategies is to utilize acid-base reactions to afford cocrystals based on charge-assisted hydrogen bonds.19b On the other hand, according to our recent research on cocrystallization of 2,5-bis(4-pyridyl)-1,3,4-oxadiazole (4-bpo) with aromatic dicarboxylic acids,20a the hydrogen-bonded driven assembly of 4-bpo with such organometallic components may

10.1021/cg0503292 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/11/2005

Search for Predictable Hydrogen-Bonding Synthons

Crystal Growth & Design, Vol. 6, No. 2, 2006 391 Scheme 2

also achieve a heterosynthon between carboxyl and pyridyl moieties with the absence of the carboxyl-carboxyl homodimer. As a continuation of our investigation in supramolecular assembly of binary hydrogen-bonding cocrystals20 or functional coordination solids21 on the basis of the oxadiazole-containing compounds such as 4-bpo, we anticipate that the incorporation of this dipyridyl-type building block with the salient organic acids mentioned above (see Scheme 2) within a binary complex would lead to the formation of new organic/organometallic crystalline materials. Also, this research allows us to identify the predictable hydrogen-bonding synthons between 4-bpo and such acidic components containing “classical” donor and acceptor groups (-COOH, -OH, -CONHR, etc.). Here, we will describe the high-yielding noncovalent preparation and crystal engineering of five new cocrystals, [(H2chdc)‚(4-bpo)] (1), [(hydroquinone)‚(4-bpo)2] (2), [(H4ca)‚(4-bpo)] (3), [(barbituric acid)‚(4-bpo)] (4), and [(HFc)‚(H-4-bpo)] (5), which display diverse 1-D extended tape or molecular-box, 2-D waved or planar layers, and 3-D net supramolecular architectures. Experimental Section General Materials and Methods. All reagents were analytical grade and used as received from commercial sources, with the exception of 4-bpo, which was prepared according to the literature method.22 Melting points of the new compounds were recorded on a WRS-1B digital thermal apparatus without correction, and elemental (C, H, and N) analyses were performed on a CE-440 (Leemanlabs) analyzer. Fourier transform (FT) IR spectra (KBr pellets) were carried out on an AVATAR-370 (Nicolet) spectrometer. Thermogravimetric analysis (TGA) experiments were taken on a Dupont thermal analyzer from 25 to 800 °C under nitrogen atmosphere at a heating rate of 10 °C/min. Preparation of [(H2chdc)‚(4-bpo)] (1). 4-Bpo (22.4 mg, 0.1 mmol) and H2chdc (17.2 mg, 0.1 mmol) were mixed in methanol/water (v/v ) 1:1, 10 mL) with stirring. Upon slow evaporation of the solvents, colorless prism single crystals suitable for X-ray diffraction were produced over 3 days. Yield: 90%, mp 172.9-175.3 °C. Anal. Calcd for C20H20N4O5: C, 60.60; H, 5.09; N, 14.13%. Found: C, 60.48; H, 5.06; N, 14.12%. FT-IR (KBr pellet, cm-1): 3047w, 2959m, 2539m, 1919m, 1694Vs, 1611s, 1567m, 1539m, 1482m, 1446w, 1414s, 1370w, 1307s, 1230s, 1195s, 1058m, 1004s, 968m, 915w, 840s, 744m, 718s, 637w, 531w, 494w. Preparation of [(Hydroquinone)‚(4-bpo)2] (2). 4-Bpo (22.4 mg, 0.1 mmol) was dissolved in CH3OH (5 mL), to which a CH3OH/DMF solution (v/v ) 1:1, 10 mL) of hydroquinone (11.0 mg, 0.1 mmol) was added with stirring. The resultant colorless solution was filtered and left to stand at room temperature. Orange lamellar crystals were obtained by slow evaporation of the solvents over a period of 2 weeks. Yield: 85%, mp 266.8-268.1 °C. Anal. Calcd for C30H22N8O4: C, 64.51; H, 3.97; N, 20.06%. Found: C, 64.26; H, 4.09; N, 19.80%. FT-IR (KBr pellet, cm-1): 3095b, 2857w, 2751w, 1611m, 1568m, 1539m, 1476Vs, 1418s, 1364m, 1246s, 1218s, 1120w, 1093w, 998w, 826m, 761m, 715s, 502m. Preparation of [(H4ca)‚(4-bpo)] (3). An ethanol/water solution (v/v ) 2:1, 6 mL) of citric acid (21.0 mg, 0.1 mmol) was carefully layered onto a CHCl3 (4 mL) solution of 4-bpo (22.4 mg, 0.1 mmol) in a straight

glass tube, which was allowed to stand under ambient environment. Colorless prism crystals were observed on the tube wall over a period of 2 weeks. Yield: 90%, mp 177 °C. Anal. Calcd for C18H16N4O8: C, 51.93; H, 3.87; N, 13.46%. Found: C, 51.82; H, 4.02; N, 13.27%. FT-IR (KBr pellet, cm-1): 3459m, 3071b, 2362m, 1897w, 1724Vs, 1617m, 1548m, 1493w, 1429m, 1370m, 1291w, 1210s, 1121s, 1058m, 1012m, 843m, 801w, 714m, 586m, 537w, 503m. Preparation of [(Barbituric acid)‚(4-bpo)] (4). The same synthetic procedure as for 1 was used except that H2chdc was replaced by barbituric acid (16.4 mg, 0.1 mmol), resulting in block brown crystals in 90% yield after 2 days. Mp > 300 °C. Anal. Calcd for C16H12N6O4: C, 54.55; H, 3.43; N, 23.85%. Found: C, 54.20; H, 3.19; N, 24.27%. FT-IR (KBr pellet, cm-1): 2968m, 2729m, 1744m, 1704Vs, 1609w, 1565w, 1538w, 1477m, 1415m, 1387m, 1350s, 1244m, 1205w, 1044w, 1002w, 936w, 836m, 743m, 717s, 692w, 639w, 489s. Preparation of [(HFc)‚(H-4-bpo)] (5). An orange solution of H2Fc (27.6 mg, 0.1 mmol) in CH3OH (5 mL) was carefully layered onto a CHCl3 (5 mL) solution of 4-bpo (22.4 mg, 0.1 mmol) in a straight glass tube, which was allowed to stand at room temperature in the dark. Red block crystals were collected on the tube wall over 3 weeks. Yield: 85%, mp > 300 °C. Anal. Calcd for C24H10FeN4O5: C, 58.80; H, 2.06; N, 11.43%. Found: C, 58.67; H, 2.53; N, 11.62%. FT-IR (KBr pellet, cm-1): 3407b, 1699w, 1606m, 1563s, 1534m, 1483s, 1416s, 1329m, 1272m, 1219m, 1114w, 1064w, 988w, 833s, 742m, 713Vs, 686w, 499m. Single-Crystal X-ray Diffraction Determination and Refinement. Single-crystal X-ray diffraction measurements of 1 (0.28 × 0.27 × 0.22 mm3), 2 (0.60 × 0.40 × 0.08 mm3), 3 (0.36 × 0.18 × 0.14 mm3), 4 (0.32 × 0.30 × 0.18 mm3), and 5 (0.22 × 0.20 × 0.18 mm3) were collected on a Bruker Apex II CCD diffractometer at 293(2) K with Mo KR radiation (λ ) 0.710 73 Å). There was no evidence of crystal decay during data collection. Semiempirical absorption corrections were applied using SADABS program, and the program SAINT was used for integration of the diffraction profiles.23 The structures were solved by direct methods using the SHELXS program of the SHELXTL package and refined with SHELXL.24 Fe(II) ion in 5 was located from the E-maps and other non-H atoms were generated in successive difference Fourier syntheses. The final refinement was performed by full-matrix least-squares methods on F2 with anisotropic thermal parameters for all non-H atoms. H atoms bonded to C were placed geometrically with fixed thermal parameters. H atoms binding to O or N were first located in difference Fourier maps and then placed in the calculated sites and included in the final refinement. Further details of the crystallographic parameters are listed in Table 1.

Results and Discussion Molecular and Supramolecular Structures of 1-5. The schematic representations of the hydrogen-bonding synthons related to this work are summarized in Scheme 3. Hydrogen bond geometries for compounds 1-5 are listed in Table 2, and other bond parameters are in the archived CIF files (Supporting Information). Supramolecular synthons based on hydrogen bonds can be categorized into two motifs.2d One is “supramolecular homosynthons” from the self-complementary half units, such as the R22(8) ring of two carboxyl groups [synthon I];12a the other is “supramolecular heterosynthons” consisting

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Du et al.

Table 1. Crystal Data and Structure Refinement Summary for Compounds 1-5 1 chemical formula formula weight (M) crystal system space group a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Z µ, mm-1 total/independent reflns params Rint Ra, Rwb GOFc residuals, e Å-3

2

3

C20H20N4O5 396.40

C30H22N8O4 558.56

C18H16N4O8 416.35

orthorhombic Pca21 10.3372(12) 6.4643(8) 28.328(3) 90 90 90 1893.0(4) 4 0.102 10123/1712

triclinic P1h 6.7853(11) 10.0404(17) 10.3991(17) 77.810(2) 74.370(2) 78.391(2) 658.99(19) 1 0.098 3793/2286

orthorhombic P212121 5.3972(8) 17.449(3) 19.711(3) 90 90 90 1856.3(5) 4 0.120 10095/3294

264 0.0263 0.0316, 0.0754 1.159 0.115, -0.166

191 0.0112 0.0353, 0.0980 1.044 0.109, -0.185

276 0.0395 0.0396, 0.0914 1.131 0.149, -0.137

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

D-H‚‚‚A

D‚‚‚A (Å)

H‚‚‚A (Å)

∠D-H‚‚‚A (deg)

1

O3-H3‚‚‚N4a O4-H4‚‚‚N1b C2-H2‚‚‚O5c C3-H3A‚‚‚O5d C10-H10‚‚‚O2e C11-H11‚‚‚O2f O2-H2‚‚‚N4g C11-H11‚‚‚N3h O2-H2‚‚‚3i O4-H4‚‚‚N4g O7-H7‚‚‚N1j O8-H8‚‚‚O7h C2-H2A‚‚‚O6k C3-H3‚‚‚O6l C9-H9‚‚‚O5m C12-H12‚‚‚O3n C17-H17A‚‚‚O8g N3-H3A‚‚‚N2 C4-H4‚‚‚O3g C5-H5‚‚‚O3 C9-H9B‚‚‚O2h O3-H3‚‚‚N1k N4-H4B‚‚‚O4o C2-H2‚‚‚O2j C3-H3A‚‚‚O2p C10-H10‚‚‚O5o C11-H11‚‚‚O5q

2.722(3) 2.715(3) 3.215(4) 3.465(4) 3.444(4) 3.136(4) 2.784(3) 3.253(2) 2.832(3) 2.621(3) 2.650(3) 3.104(3) 3.001(3) 3.468(2) 3.320(4) 3.300(4) 3.278(3) 2.851(2) 3.217(2) 3.355(2) 3.296(3) 2.710(5) 2.820(7) 3.443(6) 3.154(6) 3.554(7) 3.129(9)

1.90 1.90 2.31 2.87 2.87 2.24 1.97 2.57 2.02 1.81 1.83 2.42 2.56 2.88 2.50 2.38 2.38 1.91 2.60 2.67 2.36 1.89 2.02 2.52 2.48 3.14 2.50

175 174 163 123 121 163 172 130 167 168 174 142 110 123 148 171 154 170 124 131 161 177 155 170 130 109 125

2 3

4

5 4 chemical formula formula weight (M) crystal system space group a, Å b, Å c, Å V, Å3 Z µ, mm-1 total/independent reflns params Rint Ra, Rwb GOFc residuals, e Å-3

5

C16H12N6O4 352.32

C24H10FeN4O5 490.21

orthorhombic Pnma 6.7809(9) 17.439(2) 13.1867(17) 1559.4(4) 4 0.113 8328/1421

orthorhombic P212121 6.5505(8) 10.1928(13) 31.830(4) 2125.2(5) 4 0.755 11635/3760

128 0.0248 0.0319, 0.0880 1.079 0.174, -0.131

308 0.0284 0.0441, 0.1246 1.092 0.388, -0.223

a R ) ∑||F | - |F ||/∑|F |. b R ) [∑[w(F 2 - F 2)2]/∑w(F 2)2]1/2. c GOF o c o w o c o ) {∑[w(Fo2 - Fc2)2]/(n - p)}1/2.

Scheme 3

of two or more different components, for example, the dimer consisting of pyridyl and carboxyl groups [synthon II, R22(7)].25 In this study, for compounds 1, 3, and 5, the classical heterosynthon II is furnished as expected. Notably, synthon III [R22(7)], an ionic analogue of II containing strong N-H‚‚‚O and weak C-H‚‚‚O interactions, is involved in 5 due to the monoprotonation of pyridyl-N donor of 4-bpo, which is common in the charge-assisted hydrogen-bonding networks based on H2Fc and other N-containing bases, such as amines, pyridines, and aminidines.19b In the molecular cocrystal of 4, 4-bpo combines barbituric acid via synthon IV [R22(7)], in which the strong hydrogen bond is N-H‚‚‚N instead of O-H‚‚‚N in synthon II. In the structure of 2, such a heterosynthon is absent with

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

the formation of only a strong O-H‚‚‚N bond between the -OH group of hydroquinone and the pydiryl ring of 4-bpo. Structural Description of [(H2chdc)‚(4-bpo)] (1). The structural determination of 1 reveals an acentric two-component molecular crystal (space group Pca21), in which each 4-bpo crystallizes with one H2chdc molecule (Figure 1a). Within 4-bpo, two pyridyl rings form the dihedral angles of 9.3° and 8.3° with the central oxadiazole plane and the dihedral angle of 17.5° with each other. As stated above, H2chdc possesses a very flexible backbone and three possible conformations, which makes it difficult to pre-organize the resultant supramolecular architecture. Only a few coordination polymers with this unusual building block have been documented so far,26 and 1 represents the first cocrystalline compound of H2chdc. It is worth noting that only the e,a-cis conformation of H2chdc (Scheme 1) is observed in this structure, although the starting material used in the preparation is a mixture of both cis and trans conformations. Such selective hydrogen-bonding binding may develop possible application in molecular recognition and separation. Analysis of the crystal packing of 1 suggests that H2chdc and 4-bpo molecules locate alternately, generating a 1-D zigzag tape with the aid of synthon II [O3-H3‚‚‚N4 and C10-H10‚ ‚‚O2; O4-H4‚‚‚N1 and C3-H3‚‚‚O5; see Figure 1b]. In this tape, 4-bpo subunits take on trans arrangement around each H2chdc molecule to fulfill the formation of synthon II. Adjacent tapes are connected through weak C2-H2‚‚‚O5 and C11-H11‚ ‚‚O2 interactions between acid and base subunits from the neighboring tapes, resulting in a 2-D waving layered network (see Figure 1c for the side view). These 2-D layers stabilized by a combination of synthon II and further C-H‚‚‚O bonds adopt parallel alignment along the [100] direction (see Figure 1c). Structural Description of [(Hydroquinone)‚(4-bpo)2] (2). In the molecular structure of 2, each half hydroquinone unit

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Crystal Growth & Design, Vol. 6, No. 2, 2006 393

Figure 2. (a) Molecular structure of 2 with atom labeling of the asymmetric unit; (b) 1-D hydrogen-bonded supramolecular ladder along the crystallographic a axis.

Figure 1. (a) Molecular structure of 1 with atom labeling; (b) 2-D supramolecular layer along the crystallographic bc plane (the hydrogen bonds are indicated as broken lines in this and the subsequent figures); (c) a side view of the 3-D crystal stacking of 1 along the [010] direction.

crystallizes with one molecule of 4-bpo (Figure 2a). This composition is not consistent with the hydrogen bond donating/ accepting ability of the two subunits (equivalent -OH and -pyridyl groups) because one of the pyridyl-N donors of 4-bpo is “free” and not involved in hydrogen-bond formation as described below. The centrosymmetric hydroquinone molecule is inclined to 4-bpo with an angle of 7.5°. Two terminal pyridyl rings in 4-bpo form the dihedral angles of 5.9° and 3.6° with the oxadiazole plane and 5.3° with each other. From Figure 2b, we can clearly see that two trans base components surrounding one acid molecule are connected into a discrete trimeric motif via two strong O2-H2‚‚‚N4 bonds, and these trimers are further extended through weak C11-H11‚‚‚N3 interactions to produce a 1-D molecular ladder. This trimeric pattern is quite different from that of the cocrystallization of hydroquinone with 1,2bis(4-pyridyl)ethane, in which both terminal pyridyl nitrogens form a pair of strong O-H‚‚‚N bonds with hydroxy to afford a 1-D infinite chain.15a The crystal packing of 2 indicates that

these 1-D molecular ladder arrays take on an offset parallel stacking mode. Structural Description of [(H4ca)‚(4-bpo)] (3). X-ray diffraction of 3 suggests a spontaneously resolved enantiomorphic crystal (space group P212121). As shown in Figure 3a, although H4ca (in unusual neutral form) has abundant hydrogen-bonding participants including three carboxyls and a hydroxy, it crystallizes with only one molecule of 4-bpo. Within each 4-bpo subunit, two pyridyl rings deviate by 9.2° from coplanarity and form the dihedral angles of 3.7° and 10.1°, respectively, with the oxadiazole plane. The central carboxyl group of H4ca is nearly vertical (85.1°) to the mean plane consisting of the other C atoms. Three carboxyl groups in each acid subunit are involved in different hydrogen-bonding modes of strong and weak interactions. Only the central carboxyl connects the adjacent 4-bpo via synthon II [O7-H7‚‚‚N1 and C3-H3‚‚‚O6 in Figure 3b]. Additionally, O6 joins another 4-bpo by a rather weak C2-H2A‚‚‚O6 interaction. For two terminal -COOH groups, one connects two 4-bpo units via a strong O4-H4‚‚‚ N4 bond and a weak C9-H9‚‚‚O5 bond and the other forms a new synthon V denoted as R22(9) (O2-H2‚‚‚N3 and C12H12‚‚‚O3) by linking the oxadiazole and pyridyl rings from the same 4-bpo molecule. Furthermore, each hydroxy group joins two adjacent acid molecules via O8-H8‚‚‚O7 and C17H17‚‚‚O8 [synthon VI, R22(7)] to afford a chain of acid components along [100]. Thus, each acid molecule connects five 4-bpo and two other acid subunits by different hydrogenbonding patterns as illustrated above. Meanwhile, around each base molecule, there are also five acid molecules fixed through such interactions. The combination of these strong and weak noncovalent bonds results in a steady 3-D packing, in which the acid and base units arrange alternately in pairs along the (011) plane, as depicted in Figure 3c, and such paired acidbase units stack in the parallel alignment along [100]. Structural Description of [(Barbituric acid)‚(4-bpo)] (4). In the molecular structure of 4 (Figure 4a), a half 4-bpo

394 Crystal Growth & Design, Vol. 6, No. 2, 2006

Figure 3. (a) Molecular structure of 3 with atom labeling; (b) acid (left) and base (right) units (indicated as different color) showing the surrounding hydrogen bonds; (c) a packing diagram of acid/base subunits viewed along the a axis.

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

crystallizes with a half molecule of barbituric acid, both of which have mirror symmetry. The small dihedral angle of 3.0° between

Du et al.

the acid and base components (almost coplanarity) and the appropriate distance between them facilitate the formation of “intramolecular” hydrogen bonds. Two pyridyl rings within each 4-bpo deviate by 10.9° from coplanarity and form a dihedral angle of 5.5° with the central oxadiazole. An extended zigzag tape is generated via synthon IV [N3-H3A‚‚‚N2 and C5-H5‚ ‚‚O3], in which the base and acid subunits arrange alternately along the [010] direction (Figure 4b). Significantly, there is a weak C9-H9B‚‚‚O2 hydrogen bond between the acid molecules, locating linearly along [110]. Furthermore, a weak C4H4‚‚‚O3 interaction helps to maintain the coplanarity of the acid and dipyridyl components; as a consequence, these parallel tapes are connected to afford a 2-D supramolecular sheet. The combination of N3-H3A‚‚‚N2, C9-H9B‚‚‚O2, and C4-H4‚ ‚‚O3 bonds gives a new orbicular R33(11) pattern. No significant interaction such as hydrogen bonding or aromatic stacking exists between these sheets, which stack in an antiparallel alignment somewhat offset along [100]. Structural Description of [(HFc)‚(H-4-bpo)] (5). The crystallization of ferrocene dicarboxylic acid with 4-bpo yields an enantiomorphic charge-transfer organometallic salt 5 [space group P212121 with flack parameter of zero]. The asymmetric unit contains one deprotonated HFc anion and a monoprotonated cationic [H-4-bpo]+ (see Figure 5a). Within each [H-4-bpo]+, the terminal pyridyl and pyridinium rings make a dihedral angle of 5.6° and 172.9°, respectively, with the central oxadiazole, and the dihedral angle between them is 167.6°. The dihedral angle between the carboxyl and carboxylate groups within an HFc anion is 173.2°, indicating a nearly parallel arrangement. Each HFc anion connects two [H-4-bpo]+ subunits via synthon II (R22(7), O3-H3‚‚‚N1 and C3-H3A‚‚‚O2) and its ionic analogue III (R22(7), N4-H4B‚‚‚O4 and C10-H10‚‚‚O5). As a consequence, the [H-4-bpo]+ and HFc anionic components are connected to generate a circuitous tape (Figure 5b). Meanwhile, there is a weak C2-H2‚‚‚O2 hydrogen bond between the pyridyl ring of [H-4-bpo]+ and carboxyl O2 from the adjacent HFc anion, and a C11-H11‚‚‚O5 interaction between the pyridinium ring of [H-4-bpo]+ and carboxylate O5 from the same HFc anion, which further stabilize such a tape of anion-cation pairs in the solid state. Another alternative description, perhaps more revealing from the viewpoint of supramolecular assembly, is that two parallel alignments of HFc anions are furnished into a 1-D molecular-box array by [H-4bpo]+ cationic building blocks via hydrogen-bonding interactions. In each box, the distance of two terminal Fe atoms from distinct HFc anions linked by a [H-4-bpo]+ cation is ca. 20.14 Å, and the distance between the adjacent Fe centers along each side (a direction) is ca. 6.55 Å. Three polymorphs of ferrocene dicarboxylic acid have been reported so far, in which the hydrogen-bonded dimeric box subunits are always observed but with different lattice arrangements.27 Comparatively, the hydrogenbonding motif in 5 is considered as an elongated 1-D box with the aid of 4-bpo connectors. Two-faced pyridinium and pyridyl rings from distinct [H-4-bpo]+ cations in the box, which are hydrogen-bonded to the same HFc anion and inclined to each other with the dihedral angle of 14.2°, are involved in the π-π stacking with the center-to-plane and center-to-center separations of 3.38/3.67 and 3.74 Å, respectively. The adjacent molecular boxes are parallel packing with a separation of ca. 3.5 Å and align somewhat offset along [010], as depicted in Figure 5c. Cocrystallization of organic acid molecules with pyridine derivatives has been extensively studied with many notable examples, however, always with respect to aromatic carboxylic acids.12b,12c,28 The organic acids applied to crystallize with the

Search for Predictable Hydrogen-Bonding Synthons

Crystal Growth & Design, Vol. 6, No. 2, 2006 395

attributed to their diverse acidic nature, which has also been testified in the assembly of H2Fc with bis-amidines.19a Thermogravimetric Analysis. All compounds are air stable at ambient conditions and TGA experiments were carried out to study their thermal stability. For compound 1, the TGA curve shows a sharp weight loss of all samples peaking at 259 °C in the range of 200-280 °C. The TGA curve of 2 (or 3) shows two consecutive weight losses of all substance in the temperature range of 120-285 °C (130-260 °C for 3), peaking at 149 and 272 °C (201 and 246 °C for 3). The steady 2-D network of 4 remains intact until three consecutive weight losses occur in the region of 160-285 °C (the critical temperatures are 212, 224, and 274 °C), corresponding to the stepwise removal of the acid and base subunits continuously. For 5, the TGA curve is more complicated due to the involvement of iron(II). A sharp mass loss of 50.68% occurs in the range of 200-280 °C with the peaking position of 239 °C. The subsequent two weight losses of 40.04% (peaks at 357 and 399 °C) are continuous even heated beyond 800 °C. Conclusions and Perspectives

Figure 5. (a) Molecular structure of 5 with atom labeling; (b) a perspective view of the 1-D hydrogen-bonded box (highlight in cyan and purple for top and bottom covers); (c) a space-filling view of the 3-D packing model of 5 (HFc anion is indicated as blue and [H-4bpo]+ as green).

4-bpo component in this work are uncommon, and the supramolecular structures of the resultant crystalline products are relatively difficult to predict. However, the well-known carboxylpyridyl synthon II R22(7), which is stable and reliable as the best combination of hydrogen-bonding donor/acceptor, is still an effective tool in generating the predictable hydrogenbonded motif. Furthermore, a recent work of energy calculations suggests that the formation of the robust synthon II is more energetically favorable (6.3-8.4 kJ/mol) than that of the acidacid dimeric synthon I.28b Here, as expected, synthon II is also observed in the structures of 1 and 3, and both synthons II and III (see Scheme 3) coexist in 5. This difference must be

One of the ultimate aims of supramolecular chemistry is developing reliable synthetic strategies of periodic and predictable superstructures. In the present work, a series of organic acids was introduced to construct binary crystalline materials with the angular dipyridyl 4-bpo, which has been proven to be a good building block in cocrystal formation.20 On the basis of bpo’s stable extramolecular bonding capacity with diverse hydrogen bond participants, cocrystallization strategy focuses on the function-oriented design of the superstructure and the evaluation of the crystal packing of the whole supramolecular solids. 4-Bpo exhibits favorable reproducibility in the distinct assemblies of different molecular components, and synthon II retains the same assembly-directing capacity in the design of engineering supramolecular tapes. Generally, the geometry of strong O-H‚‚‚N is more predictable, which plays the dominant role in controlling the 1-D supramolecular array. It can be indicated that 4-bpo plays a significant role in construction of the supramolecular structures of these organic solids. Its changeable nature makes general use to fulfill the abundant and diverse hydrogen-bonded patterns. As expected, its pyridyl N always acts as the best hydrogen-bonding acceptor by forming synthon II with carboxyl. The generation of a completely new synthon V in 3 further testifies that the active nitrogen atom on the oxadiazole ring may also be a favorable hydrogen-bonding acceptor. It is notable that the acentric nature of the crystalline material 5 with the ferrocene functional moiety may be applied as new nonlinear optical (NLO) material.29 It can be optimistically predicted that more and more applications of such building blocks will be developed in the crystal engineering of cocrystalline materials. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Grant No. 20401012), the Key Project of Tianjin Natural Science Foundation (Grant No. 043804111), and 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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