CRYSTAL GROWTH & DESIGN
Multidimensional Networks of π-Conjugated Oligomers: Crystal Structures of 4,4′:2′,2′′:4′′,4′′′-Quaterimidazole in Hydrate, Protonated Salt, and Dinucleic Copper Complexes
2006 VOL. 6, NO. 4 1043-1047
Tsuyoshi Murata,† Yasushi Morita,*,†,‡ Kozo Fukui,‡ Yumi Yakiyama,† Kazunobu Sato,§ Daisuke Shiomi,§ Takeji Takui,§ and Kazuhiro Nakasuji*,† Department of Chemistry, Graduate School of Science, Osaka UniVersity, Toyonaka, Osaka 560-0043, Japan, PRESTO, Japan Science and Technology Agency (JST), Honcho Kawaguchi, Saitama 332-0012, Japan, and Departments of Chemistry and Materials Science, Graduate School of Science, Osaka City UniVersity, Sumiyoshi-ku, Osaka 558-8585, Japan ReceiVed February 3, 2006
ABSTRACT: Hydrogen-bonded networks of 4,4′:2′,2′′:4′′,4′′′-quaterimidazole, a new molecular building block for supramolecular assemblies based on π-conjugated oligomers, are investigated in tetrahydrate, hydroiodic acid salt, and dinucleic copper complexes. We emphasize that multidimensional networks are constructed by taking advantage of a two-directional nature of hydrogen-bonding interactions inherent in the imidazole-ring system. In the crystal structure of the tetrahydrate, hydrogen-bonding interactions through water molecules construct a three-dimensional network including a π-stacking columnar structure. The hydroiodic acid salt forms a one-dimensional tape-like structure by double hydrogen bondings across the iodide anions and solvent molecules. The dicopper complex of quaterimidazole exhibits intra- and intermolecular hydrogen bondings to construct a one-dimensional chain by N-H‚ ‚‚O‚‚‚H-N hydrogen bonding through water molecules. These hydrogen-bonding modes of the quaterimidazole system are discussed in view of a structural relationship with 2,2′- and 4,4′-biimidazole systems. Introduction Recent studies on π-conjugated oligomers have acquired a growing importance in many areas of material chemistry.1 For instance, oligo(thiophene)s and oligo(pyrrole)s possess high electron- and energy-transfer abilities and have been studied for conducting, luminescent, and FET properties.1 In the chemistry of oligomer-based molecular materials, the π-stacking interaction highly affects the molecular aggregation and plays a crucial role in the electronic properties in the solid state.1,2 Thus, in consideration of the construction of new moleculebased materials using these π-conjugated oligomers, aggregation of the component molecules with desirable molecular packing and orientation is one of the most important issues.3 From these points of view, oligo(thiophene)s4 and oligo(pyrrole)s,5 having hydrogen-bonding (H-bonding) functional groups as substituents on π-conjugated backbones, have been studied with the aim of controlling multidimensional structures using directional and strong H-bonding interactions.6 However, examples of structurally well-defined π-conjugated oligomers in which the backbones are directly linked to each other by intermolecular interactions such as H-bonding, π-stacking, and coordination bonds7 have never been reported so far, because most oligomers known to date are composed of aromatic-ring systems having only one or no interaction sites toward H-bonding and coordination bonds.1 The imidazole-ring system possesses two nitrogen atoms and thus can form two-direction H-bonding or coordination bonds directing opposite sites. Taking this advantage, the imidazolering system can be recognized as a suitable building block for supramolecular assemblies with multidimensional networks.8 Actually, the 2,2′-biimidazole system (2,2′-H2Bim), a dimer of * To whom correspondence should be addressed. Tel: (+81)-6-68505393. Fax: (+81)-6-6850-5395. E-mail:
[email protected]. † Osaka University. ‡ PREST, JST. § Osaka City University.
Chart 1
imidazole, constructs a variety of H-bonded supramolecular structures by two-direction H-bonding in the neutral state, protonated salts, and assembled metal complexes.9,10 Our recent synthesis of 4,4′-biimidazole (4,4′-H2Bim), which is an isoelectronic isomer of 2,2′-H2Bim with a centrosymmetric nature, revealed its high potential in crystal engineering and supramolecular chemistry based on the directionality of H-bonding interactions intrinsically different from that of the 2,2′-H2Bim system.11 This H-bonding feature resulted in the construction of diverse and characteristic H-bonded structures depending on the protonated and deprotonated states in its protonated salt,11a charge-transfer complexes,11b and metal complexes.12 4,4′:2′,2′′:4′′,4′′′-Quaterimidazole (H4Qim), a tetramer of the imidazole-ring system, possesses one 2,2′-H2Bim unit and two 4,4′-H2Bim units in its skeleton (Chart 1).11a Thus, the H4Qim system can be expected to exhibit H-bonding and coordination modes seen in the 2,2′- and 4,4′-H2Bim systems. Furthermore, H4Qim may behave as a bridging ligand of dinucleic metal complexes. These structural features point out that H4Qim is one of the promising π-conjugate oligomers to construct a multidimensional network using H-bonding and π-stacking interactions, as well as coordination bonds. Herein we have disclosed crystal structures of the H4Qim system in the hydrate, protonated salt, and dinucleic copper complexes and demonstrated the first realization of imidazole-based π-conjugated oligmers exhibiting multidimensional network structures based
10.1021/cg0600602 CCC: $33.50 © 2006 American Chemical Society Published on Web 03/03/2006
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Table 1. Crystallographic Data for (H4Qim)‚(H2O)4, (H6Qim2+)I-2‚(dioxane)2, and [Cu2(H4Qim)Cl-4‚(DMSO)2]H2O empirical formula formula weight temp (K) crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (g cm-1) µ (Mo KR) (cm-1) unique reflns reflns used refined params R1, wR2 [I > 2σ(I)] goodness-of-fit
(H4Qim)‚(H2O)4
(H6Qim2+)I-2‚(dioxane)2
[Cu2(H4Qim)Cl-4‚(DMSO)2]H2O
C12H18N8O4 338.33 200 monoclinic P21/n 7.2964(4) 20.930(1) 10.7900(6) 90 103.888(2) 90 1599.6(1) 4 1.405 1.09 3668 2318 257 0.088, 0.228 1.04
C20H28N8O4I2 698.30 100 monoclinic P21/n 9.3170(1) 11.6417(1) 25.1539(2) 90 93.5469(6) 90 2723.11(4) 4 1.703 23.50 6207 2479 307 0.067, 0.153 0.811
C16H18N8O3S2Cl4Cu2 697.35 296 triclinic P1h 7.6970(2) 13.2760(3) 14.5091(6) 68.154(1) 84.772(1) 82.781(2) 1363.75(7) 2 1.698 21.38 6050 4407 408 0.038, 0.108 1.04
on H-bonding, π-stacking interactions, and coordination bonds. Furthermore, H-bonding modes of the H4Qim system depending on the protonated and coordination states are discussed for the first time, showing a resemblance to those of the 2,2′- and 4,4′H2Bim systems. These studies are of particular interest in designing oligomer-based molecular materials with an intriguing molecular functionalitiy controlled structurally and electronically by intermolecular interactions, especially by H-bondings.13
X-ray Analysis. X-ray crystallographic measurements were made on a Rigaku Raxis-Rapid Imaging Plate with graphite monochromated Mo KR radiation (λ ) 0.710 75 Å). All structures were determined by direct method using SHELXS-8614 or SIR-92 and refined by full-matrix least squares on F2 with SHELXL-97.15 All non-hydrogen atoms were refined anisotropically, and all hydrogen atoms were included but not refined. Selected crystal data and data collection parameters are given in Table 1.
Results and Discussion Experimental Section H4Qim was prepared according to our previous paper,11a and single crystals of neutral H4Qim containing water molecules were obtained as colorless blocks by recrystallization from a 10:1 mixture of water and DMSO. Reagents and solvents were used as purchased without further purification. 1H NMR spectra were recorded at 270 MHz with DMSO-d6 as solvent and Me4Si as an internal standard. Elemental analyses were performed at the Graduate School of Science, Osaka University. Infrared and electronic spectra were measured using KBr plates on JASCO FT/IR-660M and Shimadzu UV/Vis-NIR scanning spectrophotometer UV-3100 PC, respectively. The magnetic susceptibility measurement for [Cu2(H4Qim)Cl-4‚(DMSO)2]H2O was performed for the polycrystalline solid on a Quantum Design SQUID magnetometer MPMS XL with an applied field of 0.1 T in the temperature range of 1.9-298 K. Hydroiodic Acid Salt of 4,4′:2′,2′′:4′′,4′′′-Quaterimidazole [(H6Qim2+)I-2‚(dioxane)2]. H4Qim (138 mg, 0.52 mmol) was placed in a 30-mL round-bottomed flask and dissolved with EtOH (8 mL). HI (55% aqueous solution, 0.46 mL, 3.17 mmol) was added to this mixture at room temperature. The reaction mixture was refluxed for 3 h and then cooled to 0 °C. Et2O (10 mL) was added to this mixture; then the resulting powder was collected by filtration and washed with Et2O (5 mL) to give the salt (196 mg) as a soft yellow powder. Yellow platelet crystals suitable for X-ray crystal structure analysis were obtained by the vapor diffusion method using 1,4-dioxane-MeOH. Due to the high efflorescence and hygroscopic nature of this salt, the elemental analysis did not give the appropriate value. Mp 235-240 °C (dec); 1H NMR (270 MHz, DMSO-d6) δ 7.75 (s, 2), 7.78 (s, 2), 9.18 (s, 2); IR (KBr) 3300-2300, 1635, 1559, 1540 cm-1. Anal. Calcd for (C12H12N8I2)(C4H8O2)(H2O)3: C, 28.93; H, 3.95; N, 16.87. Found: C, 28.91; H, 3.48; N, 16.62. Dinucleic Copper Complex of 4,4′:2′,2′′:4′′,4′′′-Quaterimidazole {[Cu2(H4Qim)Cl-4‚(DMSO)2]H2O}. H4Qim (26.6 mg, 0.10 mmol) and CuCl2 (26.9 mg, 0.20 mmol) was placed in a 30-mL round-bottomed flask and dissolved with the mixture of MeOH (10 mL) and DMSO (1 mL). This mixture was subjected to vapor diffusion using acetone. The resulting crystals were collected by filtration and washed with acetone (10 mL) to give the copper complex (42.0 mg, 76%) as a yellowish brown crystals. Mp > 300 °C; IR (KBr) 3412, 3300-2600, 1650, 1550 cm-1; UV (KBr) 220, 326, 838 nm. Anal. Calcd for C16H24Cu2N8O3S2Cl4: C, 27.09; H, 3.41; N, 15.79. Found: C, 27.45; H, 3.24; N, 16.03.
Crystal Structure of (H4Qim)‚(H2O)4. The tetrahydrate of H4Qim crystallizes in the space group P21/n with four molecules in a unit cell. Figure 1a shows the ORTEP drawing of the molecular structure. The H4Qim molecule possesses a nearly planar structure, and twist angles of neighboring imidazole rings are 4°-12°. The two 4,4′-H2Bim units adopt trans conformations. As expected in the molecular design, neutral H4Qim molecule shows the two-direction H-bonding interactions inherent in the imidazole-ring system to construct a three-dimensional Hbonding network including water molecules. The nitrogen atoms at the NNH position of the imidazole rings (N1, N4, N5, and N8) are linked with water molecules (O3, O2, O1, and O4, respectively) (Figure 1b). The nitrogen atoms at the NN positions of the imidazole rings (N2, N3, N6, and N7) interact with two water molecules, which are linked with the NNH nitrogen atoms belonging to neighboring H4Qim molecules (Figure 1b). These H-bonding interactions across water molecules including O1‚ ‚‚O1 and O1‚‚‚O2 H-bondings construct a two-dimensional sheet structure parallel to the ab-plane (Figure 1b). These H-bonding distances are shorter than the sums of the van der Waals radius of nitrogen and oxygen atoms (3.07 Å) and two oxygen atoms (3.04 Å).16 In addition, the H4Qim molecule stacks in the nearlyeclipsed and slipped-stacking manner with the face-to-face distances of ca. 3.9 and ca. 3.6 Å, respectively, to form a columnar structure (Figure 1c). Furthermore, a H-bonding interaction between water molecules, O2‚‚‚O4, connects the sheet structure by the H-bondings and π-stackings to build up a three-dimensional structure. Crystal Structure of (H6Qim2+)I-2‚(dioxane)2. The dihydroiodic acid salt crystallizes in the space group P21/n with four H6Qim2+ molecules in a unit cell and contains 1,4-dioxane as crystal solvent. Figure 2a shows the ORTEP drawing of the molecular structure of H6Qim2+. Similarly to the molecular structure of the neutral H4Qim, the H6Qim2+ skeleton in this
Multidimensional Networks of π-Conjugated Oligomers
Crystal Growth & Design, Vol. 6, No. 4, 2006 1045
Figure 2. Crystal structure of (H6Qim2+)I-2‚(dioxane)2: (a) molecular structure and atomic numbering scheme of H6Qim2+; (b) onedimensional structure by double H-bonding interactions across iodide anions and 1,4-dioxane molecules; (c) two-dimensional structure constructed by H-bondings and π-stackings. H-bonding distances (D‚ ‚‚A in Å): N1‚‚‚I2, 3.45; N2‚‚‚O3, 2.72; N4‚‚‚O3, 2.79; N5‚‚‚I2, 3.60; N7‚‚‚I1, 3.53; N8‚‚‚O2, 2.77.
Figure 1. Crystal structure of (H4Qim)‚(H2O)4: (a) molecular structure and atomic numbering scheme of H4Qim; (b) partial packing diagram showing H-bonding pattern; (c) crystal packing viewed along the a-axis showing H-bonded network and the columnar structures. Dotted lines represent H-bondings, and the light gray molecules are the next ones in the π-stackings. H-bonding distances (D‚‚‚A in Å): N1‚‚‚O3, 2.77; N2‚‚‚O2, 2.81; N2‚‚‚O3, 3.05; N3‚‚‚O1, 2.92; N3‚‚‚O2, 2.99; N4‚‚‚ O2, 2.85; N5‚‚‚O1, 2.76; N6‚‚‚O1, 2.79; N7‚‚‚O3, 2.82; N7‚‚‚O4, 3.00; N8‚‚‚O4, 2.83; O1‚‚‚O1, 2.88; O1‚‚‚O2, 2.92; O2‚‚‚O4, 2.82.
salt is nearly planar with the twist angle of 5°-9°, and the two 4,4′-H2Bim units adopt trans conformations. In this salt, two terminal imidazole rings of H6Qim2+ are protonated, probably due to the Coulomb repulsion between two positive charges. In the intramolecular C-C and C-N bond lengths, noticeable difference is not found between neutral H4Qim and protonated H6Qim2+. In this crystal structure, the H6Qim2+ molecule possesses a similar two-direction H-bonding interaction to that seen in the crystal structure of (H4Qim)‚(H2O)4. The nitrogen atoms at the NNH positions of the imidazole rings (N1 and N4; N5 and N8) linearly link the iodide anions and the 1,4-dioxane molecules, and these H-bondings connect neighboring H6Qim2+ molecules to form a one-dimensional tape-like structure (Figure 2b). In addition, N2 and N7 atoms interact with the iodide anion and the 1,4-dioxane molecule. N5‚‚‚I2 H-bonding is slightly longer than the sum of the van der Waals radius of nitrogen and iodine atoms (3.53 Å), and the other H-bonding distances are close to
or shorter than the sum of the van der Waals radius.16 N3 and N6 atoms at the NN position of the imidazole ring do not form H-bonds. In addition to this H-bonded structure, the H6Qim2+ molecule stacks to form a π-stacking column with the face-toface distance of ca. 3.4 Å. These intermolecular interactions construct a two-dimensional structure of this crystal (Figure 2c). Crystal Structure of [Cu2(H4Qim)Cl-4‚(DMSO)2]H2O. The copper complex crystallizes in the space group P1h with two molecules in a unit cell. The structure of the H4Qim molecule in this dinucleic copper complex shows remarkable differences from those of the neutral and protonated species (Figure 3a); the H4Qim skeleton is not planar and twists by 31° at the central C6-C7 bond. In addition, the two 4,4′-H2Bim units of H4Qim possess cis conformation unlike the trans conformation seen in the neutral and protonated species of H4Qim. The two 4,4′-H2Bim units with cis conformation independently coordinate to copper atoms as bidentate chelating ligands to form the dinucleic complex. The copper atoms are coordinated also by two chloride anions and a DMSO molecule to construct trigonal bipyramidal coordination spheres. The Cu‚‚‚N coordination lengths of 1.972.07 Å are close to those of copper complexes of 4,4′-H2Bim (1.98-2.02 Å), and N‚‚‚Cu‚‚‚N chelating angles of ∼80.7° are slightly smaller than those of copper complexes of 4,4′-H2Bim (81.9°-82.6°).12 In addition to the coordination to copper atoms, the H4Qim molecule interacts with counteranions and water molecules to form the two-directional interactions inherent in the imidazolering system. The inner N-H groups (N4 and N5) form intramolecular H-bonds with chloride anions (Cl4 and Cl2, respectively), which coordinate to the copper atoms (Figure 3b). The terminal N-H groups (N1 and N8) are linked with a water molecule, and this N-H‚‚‚O‚‚‚H-N H-bonding interaction forms a one-dimensional chain (Figure 3b). In addition, the N1 atom interacts with the Cl3 atom. The water molecule interacts
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Figure 4. Hydrogen-bonding modes of the 4,4′-H2Bim system in (a) trans and (b) cis conformation and of the H4Qim system seen in the crystal structures of (c) (H4Qim)‚(H2O)4, (d) (H6Qim2+)I-2‚(dioxane)2, and (e) [Cu2(H4Qim)Cl-4‚(DMSO)2]H2O. Figure 3. Crystal structure of [Cu2(H4Qim)Cl-4‚(DMSO)2]H2O: (a) molecular structure and atomic numbering scheme of [Cu2(H4Qim)Cl-4‚ (DMSO)2] unit, (b; one-dimensional chain structure formed by intermolecular N-H‚‚‚O‚‚‚H-N H-bonding across water molecule including the intramolecular N-H‚‚‚Cl H-bonding interactions; (c) twodimensional structure constructed by intermolecular H-bonding interactions and π-stackings. H-bonding distances (D‚‚‚A in Å): N1‚‚‚O3, 2.84; N1‚‚‚Cl2, 3.29; N4‚‚‚Cl4, 3.10; N5‚‚‚Cl2, 3.10; N8‚‚‚O3, 2.95; O1‚‚‚O3, 2.83; O3‚‚‚Cl2, 3.21.
also with the Cl2 atom and the O1 atom of the DMSO molecule. These H-bonding interactions construct a two-dimensional sheet structure (Figure 3c). The H-bonding distances in this complex are shorter than the sums of the van der Waals radius of each atom (N and O, 3.07; N and Cl, 3.30, O and O, 3.04 and O and Cl, 3.27 Å).16 Within this sheet structure, π-π interactions between H4Qim ligands with face-to-face distances of ca. 3.3 Å are observed. The Cu1‚‚‚Cu2 distance in the dinucleic complex is 6.72 Å, while the shortest intermolecular Cu‚‚‚Cu distance is 5.73 Å. The magnetic susceptibility with a range of 1.9-298 K was measured on a polycrystalline sample of the dinucleic copper complex. The magnetic interaction of this complex is small (θ ) -0.39 K by Curie-Weiss model) because of the long distance between the copper atoms. The Curie constant and the g value are 0.868 emu K mol-1 and 2.152, respectively, falling in the ordinary range for dinucleic copper complexes. H-Bonding Modes in Quaterimidazole. Figure 4 summarizes the H-bonding modes of the 4,4′-H2Bim system in the trans and cis conformations and of the H4Qim system seen in the tetrahydrate, dihydroiodic acid salt, and dinucleic copper complexes. In the trans conformation, the NNH and NN nitrogen atoms of 4,4′-H2Bim independently connect proton acceptors to form a double linear chain mode of H-bonding (Figure 4a). The 4,4′H2Bim system in the neutral and diprotonated states shows this H-bonding mode.11a-c,e In contrast, the 4,4′-H2Bim system
having cis conformation connects two proton acceptors by the angular bridging mode of H-bonding (Figure 4b). This Hbonding mode is observed in the crystal structures of monoprotonated 4,4′-H2Bim11a,b and the copper complexes.12 The H-bonding modes featured in (H4Qim)‚(H2O)4, (H6Qim2+)I-2‚ (dioxane)2, and [Cu2(H4Qim)Cl-4‚(DMSO)2]H2O show a resemblance to those of the 2,2′- and 4,4′-H2Bim systems. In the crystal structure of neutral (H4Qim)‚(H2O)4, the H4Qim skeleton is composed of two 4,4′-H2Bim units with trans conformation. Two NNH and NN nitrogen atoms in each 4,4′H2Bim unit linearly connect water molecules to form the double linear chain mode of H-bonding (Figures 1b and 4c). In addition, the central 2,2′-H2Bim units are connected to each other by the double N-H‚‚‚O-H‚‚‚N H-bonding interactions across water molecules (Figure 1b). This H-bonding mode is a complementary mode forming a one-dimensional chain structure seen in the neutral 2,2′-H2Bim system.9a,11b The two 4,4′-H2Bim units of the diprotonated H6Qim2+ skeleton in (H6Qim2+)I-2‚(dioxane)2 possess the trans conformation similar to that in the neutral state. Two NNH nitrogen atoms in each 4,4′-H2Bim unit linearly connect proton acceptors (Figures 2b and 4d). However, the NN atoms of the inner imidazole rings possess no H atoms and do not participate in forming H-bonds. In the crystal structure of the dinucleic copper complex, [Cu2(H4Qim)Cl-4‚(DMSO)2]H2O, two 4,4′-H2Bim units possess the cis conformation. Each 4,4′-H2Bim unit coordinates to a copper atom and is linked with a solvent molecule or counteranion by the angular bridging mode of H-bondings (Figures 3b and 4e). This classification of the H-bonding modes of the H4Qim system related to the 2,2′- and 4,4′-H2Bim systems gives important information in the design for supramolecular assemblies based on oligo(imidazole) molecular materials.
Multidimensional Networks of π-Conjugated Oligomers
Conclusion The structural analyses of the H4Qim system in the neutral, protonated salt, and dinucleic copper complexes were carried out for the first time. The H4Qim system exhibited the twodirection H-bonding and coordination bonds intrinsic to the imidazole-ring system, resulting in the construction of one- or two-dimensional π-stacking structures linked to each other by H-bondings. The H-bonding natures of the H4Qim system showed a resemblance to those of the 2,2′- and 4,4′-H2Bim systems, having a complementary mode, a double linear chain mode, and an angular bridging mode.11a,b,12 These observations gave the first realization of π-conjugated oligomers exhibiting multidimensional network structures based on H-bonding, coordination, and π-stacking interactions. Importantly, these results are achieved by the two-directional nature of H-bonding and coordination bonds of the imidazole-ring system and reveal that H4Qim and oligo(imidazole)s are useful and promising molecular systems as new building blocks based on π-conjugated oligomers having intriguing electronic properties with well-defined structures. Further studies for the construction of multidimensional networks and investigation of physical properties by diverse H-bonding modes using H4Qim and sexiimidazole11a in their complexes with various metal atoms are in progress. Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research (No. 16350074) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan, by PRESTO-JST, by a grant of The Asahi Glass Foundation, and by 21COE program “Creation of Integrated EcoChemistry of Osaka University”. Supporting Information Available: The detailed crystal structures and overlap patterns of three compounds, temperature dependence of paramagnetic susceptibility of [Cu2(H4Qim)Cl-4‚(DMSO)2]H2O, and X-ray crystallographic data for each structure in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.
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