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Triple-Stranded Helical and Plywood-Like Arrays: Two Uncommon Framework Isomers Based on the Common One-Dimensional Chain Structures Xing-Qiang Lu¨,† Yu-Qin Qiao,† Jian-Rong He,† Mei Pan,† Bei-Sheng Kang,† and Cheng-Yong Su*,†,‡

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 8 1910-1914

State Key Laboratory of Optoelectronic Materials and Technologies, Institute of Photoelectronic and Functional-Composite Materials, School of Chemistry and Chemical Engineering, Sun Yat-Sen UniVersity, Guangzhou 510275, P. R. China, and State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China ReceiVed April 22, 2006; ReVised Manuscript ReceiVed May 23, 2006

ABSTRACT: Two 1D polymeric chain structures, namely, {[Ag(3-impmd)]‚(SbF6)‚CH2Cl2}∞ (1) and {[Ag(3-impmd)]‚(NO3)‚ MeOH}∞ (2), were assembled from the flexible organic spacer N,N′-bis(3-imidazol-1-yl-propyl)-pyromellitic diimide (3-impmd) with AgSbF6 and AgNO3, respectively. Complex 1 crystallizes with the 1D chains displaying cross-like arrangement, while Complex 2 presents a triple-stranded helical structure, which is dictated by the anions and further assembled into 3D architectures via various weak supramolecular interactions. The luminescent properties of both complexes were investigated in solution and solid state. Introduction As one of great challenges encountered nowadays in crystal engineering of functional materials, the structural diversity of coordination polymers exhibited by similar metal-organic building blocks is of intense interest as a result of its scientific significance as well as the intrinsic aesthetic appeal.1 Much effort has been devoted to the study of framework connectivity and topological analysis;2 however, prediction or control over the molecular packing in the solid-state remains a hard nut to crack, although various weak supramolecular forces are believed to play important roles and have been vigorously investigated.3 The structural diversity derived from interpenetration, polycatenation, interweaving, or supramolecular isomerism has already been well documented;1 by contrast, the diversity with respect to the molecular packing is seldom considered by far.4 The infinite one-dimensional (1D) chain is the simplest topology of coordination polymers, whereas its packing in crystals is by no means simple.5 Scheme 1 illustrates a few known crystal packing modes observed so far for 1D chain structures, among which the parallel fashion A is overwhelmingly predominant over the nonparallel fashion B with crosslike arrangement of the chains.6 Such cross-like arrangement of the 1D chain can be described as a 3D “plywood-like array”, which is either arranged with two differently orienting neighboring layers of AB type6a-h or even with three different neighboring layers of ABC type.6i-j As for 1D chain entanglements, the infinite multiple helices (C) represent a fascinating mode of 1D association1c with merely a few examples,7 and even rarer is the “woof-and-weft” cloth-like threading mode (D).8 Using rigid or flexible bipyridyl-type ditopic ligands with Py (pyridyl) or Bim (benzimidazolyl) as terminal donor groups,9 we have previously reported the first ‘two-over/two-under’ two-dimensional (2D) cloth-like entanglement of 1D chains (D),8b and exemplified another type of interesting structural diversity related to 1D ladders.9b In this paper, we demonstrate how two other nonparallel 1D chain associations can be achieved through assembly of a flexible bipyridyl-type ditopic ligand, N,N′-bis†

Sun Yat-Sen University. ‡ Chinese Academy of Sciences.

Scheme 1.

Representation of 1D Chain Packing Fashionsa

a (A) Parallel array; (B) plywood-like array; (C) helical array; (D) clothlike array.

Scheme 2.

Molecular Structure of the Ligand 3-impmd

(3-imidazol-1-yl-propyl)-pyromellitic diimide (3-impmd, Scheme 2), with different Ag(I) salts, namely, a three-dimensional (3D) cross-linking {[Ag(3-impmd)]‚(SbF6)‚CH2Cl2}∞ (1) and a triplestranded helical {[Ag(3-impmd)]‚(NO3)‚MeOH}∞ (2). In addition, we would like to introduce the notion of “framework isomers”10 to describe the structurally similar but conformationally distinguished {[Ag(3-impmd)]+}∞ cationic frameworks in 1 and 2, which arise from different anions and solvent molecules. Experimental Section All chemicals were of reagent grade from commercial sources and used without further purification. Elemental analyses were taken on a Perkin-Elmer 240C elemental analyzer. IR spectra were recorded on a

10.1021/cg060240b CCC: $33.50 © 2006 American Chemical Society Published on Web 06/24/2006

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Table 1. Crystallographic Data for Complexes 1 and 2 formula Mw crystal system space group a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Z Dc, g cm-3 µ, mm-1 R1 wR2

C23H22AgCl2F6N6O4Sb 860.99 monoclinic Cc 17.418(5) 10.740(3) 16.615(5) 90 111.952(5) 90 2882.7(14) 4 1.984 1.883 0.0409 0.1032

C23H24AgN7O8 634.36 monoclinic P2(1)/c 16.256(7) 11.091(5) 15.798(7) 90 113.061(7) 90 2620.6(19) 4 1.608 0.830 0.0593 0.1587

Bruker EQUINOX55 FT-IR spectrophotometer using KBr disks in the 4000-400 cm-1 regions.1H NMR measurements were carried out on INOVA 500NB spectrometer or Mercury-Plus 300 spectrometer with SiMe4 as internal standard. The XRD patterns were recorded on a D/Max-IIIA diffractometer with Cu KR radiation (λ ) 1.540 56 Å) at a scanning rate of 1° min-1 with 2θ ranging from 5° to 55°. The excitation and emission spectra were obtained on an HITACHI 850 spectrometer. N,N′-Bis(3-imidazol-1-yl-propyl)-pyromellitic Diimide, 3-impmd. The ligand 3-impmd was prepared by reaction of pyromellitic dianhydride with 2 molar equivalents of N-3-aminopropyl-imidazole in DMF solution. Typically, a solution of 20 mL of DMF containing 2.20 g (0.01 mol) pyromellitic dianhydride and 2.60 mL (0.021 mol) N-3aminopropyl-imidazole was heated to reflux for 6 h. Upon cooling, the pale yellow crude product precipitated, and the solid was recrystallized using DMF/EtOH (3/1, V/V) mixture giving 2.6 g white product. Yield: 63%. Anal. Calcd for C22H20O4N6: C, 61.11; H, 4.66; N, 19.43%. Found: C, 61.23; H, 4.56; N, 19.38%. 1H NMR (δH, 300 MHz, DMSO-d6): 8.146 (s, 2H), 7.585 (s, 2H), 7.150 (s, 2H), 6.824 (s, 2H), 4.013 (t, 4H), 3.582 (t, 4H), 2.053 (m, 4H). IR (KBr, cm-1): 3110(w), 3000(m), 2965(m), 2869(w), 2363(w), 1921(w), 1764(s), 1703(vs), 1569(w), 1503 (m), 1438(s), 1389(m), 1343(m), 1275(w), 1230(m), 1140(w), 1120(m), 1090(m), 1038(m), 990(m), 949(w), 908(w), 868(m), 837(w), 818(m), 759(w), 732(m), 662(w), 557(w). {[Ag(3-impmd)]‚(SbF6)‚CH2Cl2}∞, 1. AgSbF6 (18 mg, 0.05 mmol) dissolved in MeOH (5 mL) was layered onto a solution of 3-impmd (22 mg, 0.05 mol) in CH2Cl2 (10 mL). Crystals started to form after several days at the interface between MeOH and CH2Cl2 layers. Yield: 57%. Anal. Calcd for C23H22Cl2AgF6N6O4Sb: C, 32.09; H, 2.58; N, 9.76%. Found: C, 32.86; H, 2.48; N, 9.89%. 1H NMR (δH, 300 MHz, DMSO-d6): 7.998 (s, 2H), 7.713 (s, 2H), 7.370 (s, 2H), 6.857 (s, 2H), 5.739 (s, 2H), 4.176 (t, 4H), 3.724 (t, 4H), 2.255 (m, 4H). IR (KBr, cm-1): 3138(m), 2945(w), 2362(w), 1771(s), 1714(vs), 1519(w), 1446(w), 1397(s), 1366(m), 1349(m), 1234(w), 1130(w), 1098(w), 1038(w), 990(w), 837(w), 786(w), 731(m), 659(s), 559(w). {[Ag(3-impmd)]‚(NO3)‚MeOH}∞, 2. AgNO3 (17 mg, 0.1 mmol) dissolved in MeOH (5 mL) was layered onto a solution of 3-impmd (44 mg, 0.1 mol) in CH2Cl2 (10 mL). Crystals started to form after several days at the interface between MeOH and CH2Cl2 layers. Yield: 38%. Anal. Calcd for C23H24AgN7O8: C, 43.55; H, 3.81; N, 15.46%. Found: C, 43.46; H, 3.78; N, 15.59%. 1H NMR (δH, 300 MHz, DMSO-d6): 8.015 (s, 2H), 7.743 (s, 2H), 7.371 (s, 2H), 6.881 (s, 2H), 4.184 (t, 4H), 3.726 (t, 4H), 2.250 (m, 4H). IR (KBr, cm-1): 3460(b), 3121(w), 2946(w), 1767(m), 1710(vs), 1523(m), 1455(m), 1396(s), 1367(s), 1338(s), 1240(m), 1121(m), 1092(m), 1039(m), 876(w), 831(w), 728(m), 660(w), 594(w), 506(w). X-ray Crystallography. Experimental details of the X-ray analysis as well as the crystallographic data are listed in Table 1. All diffraction data were collected on a Bruker Smart 1000 CCD diffractometer with graphite monochromated Mo KR radiation (λ ) 0.710 73 Å) at room temperature using the program SMART11 and processed by SAINT+.12 Absorption corrections were applied using the SADABS program.13 Space groups were determined from systematic absences and further justified by the results of the refinements. In all cases, the structures were solved by direct methods and refined using the full-matrix leastsquares method against F2 using SHELXTL software.14 The coordinates of the non-hydrogen atoms were refined anisotropically. All hydrogen

Figure 1. (a) 2D sheet assembled via weak Ag‚‚‚O interactions and C-H‚‚‚O hydrogen bonds (dash lines) in 1 and (b) representation of cross-like arrangement of 1D chains in ABAB fashion to generate 3D plywood-like array in 1. atoms were introduced in calculated positions and refined with fixed geometry with respect to their carrier atoms. CCDC reference numbers are 281897 and 281898.

Results and Discussion Synthesis and Characterization. The new ligand 3-impmd was obtained from the reaction of pyromellitic dianhydride with 3-imdazol-1-yl-propylamine and features a long, flexible exobidentate character in comparison to the familiar rigid rod-like 4,4′-bipyridyl ligands. The functional pyromellitic diimide (pmd) fragment provides potential H-bond donors (C-H) and acceptors (CdO), while the propyl linkers between the central pmd and the terminal imidazolyl (Im) rings endow the ligand with flexibility to adopt different conformations. Reaction of 3-impmd with AgSbF6 or AgNO3 in CH2Cl2-MeOH mixtures led to formation of complexes 1 or 2, respectively. Both were identified by means of element analysis and IR spectra, and their bulk phase purity was confirmed by X-ray powder diffractions. The 1H NMR spectra of the complexes are similar, closely resembling that of the ligand, indicating that the solid structures may not be retained in the polar DMSO solution. Crystal Structure. Single-crystal X-ray diffraction studies reveal that the asymmetric units of both 1 and 2 are composed of one [Ag(3-impmd)]+ cation, one counteranion, and one solvent molecule. Each ligand connects two symmetry-related Ag(I) ions, and each Ag(I) ion is bound to two different ligands, thus generating a 1D chain in which the Ag(I) ions display linear coordination geometry with the Ag-N distances in the range 2.093(5)-2.125(8) Å and N-Ag-N angles close to linearity (177.6(3)° for 1 and 172.6(2)° for 2). Both the anions and the solvated molecules are “free” without participating in coordination with Ag(I) ions. The unusual feature of 1 is that the 3-impmd ligand adopts a “Z” conformation to connect Ag(I) ions into zigzag chains, which span two directions to give rise to a plywood-like stacking fashion, parallel on each layer but nonparallel on alternate layers. As shown in Figure 1a, the chains that run parallel are linked with each other through multiple supramolecular forces involving weak Ag‚‚‚O interactions (3.07-3.09 Å) and C-H‚‚‚O

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Figure 2. Channels hosting anions and solvent molecules in c direction in 1.

hydrogen bonds (H‚‚‚O 2.48-2.56 Å), resulting in a 2D sheet extending in the ab plane. The crystal is stacked up by these sheets in an ABAB way as seen from Figure S1, Supporting Information. It is noteworthy that such ABAB stacking is caused by alternate cross-like arrangement of 1D chains on every two neighboring sheets as depicted in Figure 1b. Therefore, the crystal packing exhibits a plywood-like fashion of AB type corresponding to mode B in Scheme 1. In contrast to the parallel mode A (Scheme 1), little attention has been paid to this plywood-like packing fashion in the literature,6 though such a nonparallel array may render unique characteristics to the crystal such as anisotropic properties relying on orientation of the chains. The porosity known for 2D and 3D networks can also be achieved through nonparallel arrangements of 1D chains,6d and in 1, channels are formed in the c direction hosting SbF6anions and CH2Cl2 solvated molecules (Figure 2). By contrast, in 2 the 3-impmd ligand takes on a “U” conformation to join Ag(I) ions to form helical chains extending along the b axis. It is interesting that three such helical chains intertwine one another to result in a rare triple-stranded helix with a long helical pitch of 33.3 Å, thrice that of the b axis, as shown in Figure 3. A peculiar feature of the triple helix is that a rectangular-shaped tubular channel is formed by the three strands, in which the NO3- anions and the solvated methanol molecules are located (Figure 3). To the best of our knowledge, such unique inclusion phenomenon of mixed guest molecules via formation of tubular channels inside the triple helices has not been oberved in a few known examples of triple helical coordination polymers.7 Crystal structural analysis indicates no significant intermolecular interactions among three strands except van der Waals interactions;7b however, there exist supramolecular interactions between the adjacent helices. As depicted in Figure 4, every single strand of the triple helix forms weak π‚‚‚π interactions (centroid-to-centroid distance 3.80 Å) with a neighboring strand belonging to the adjacent triple helix, although the Ag‚‚‚Ag separation (3.48 Å) is just comparable with the van der Waals sum of two Ag atoms (3.44 Å). This causes closely interdigitating packing and hence directs the handedness of the neighboring triple helices. The right-handed and left-handed triple helices are aligned alternately by such π‚‚‚π interactions to result in 2D racemic layers extending on the bc plane. The crystal is stacked up by these 2D layers with rather weak C-H‚‚‚O hydrogen bonds (H‚‚‚O 2.66 Å) formed between the layers. Topological Analysis. A noticeable point behind the simple and common 1D chain structures themselves of 1 and 2 is that they offer a good example demonstrating how structural diversity could be achieved even from the simplest polymeric topology. The cationic frameworks for both 1 and 2 have exactly

Figure 3. (a) Triple-stranded helix formed in 2 hosting NO3- anions and CH3OH solvent molecules (top) and space-filling mode representation of three strands (bottom) and (b) a perspective view of the tubular channels formed inside the triple helix hosting guest molecules (spacefilling mode) and C-H‚‚‚O hydrogen bonds (dash lines) connecting triple helices in the a direction.

Figure 4. The π‚‚‚π and Ag‚‚‚Ag interactions (dash lines) formed between the single strands belonging to the adjacent triple-stranded helices (top, only one strand of each triple-stranded helix is shown for clarity), and two adjacent triple-stranded helices with opposite handedness (bottom, right-handed above and left-handed below) in 2.

the same composition, [Ag(3-impmd)]+, but different conformations, zigzag in 1 and helical in 2. This is apparently caused by the different “Z-like” and “U-like” conformations of 3-impmd in 1 and 2, respectively, finally leading to different crystal

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Figure 5. Supramolecular interactions formed between anions and chain frameworks in 1 (left) and 2 (right).

packing modes as discussed above. For an easier description of their structural relationship, we would like to term the cationic skeletons of 1 and 2 as “framework isomers”, a notion already used for the mixed-metal clusters and inclusion frameworks ignoring solvents and guest molecules,10 because 1 and 2 obviously fall outside the strict definition of “supramolecular isomers”, which requires the identical molecular formulas for the whole compounds. This notion may be useful for comparisons between the closely related structures containing the same building blocks and framework backbones but different geometrical arrangements and topologies, which represents a quite common phenomenon encountered in various 1D to 3D coordination polymers. One of the advantages to use of “framework isomers” to describe the main part of such structurally associated but conformationally or topologically differentiated structures is to avoid the dilemma by using “supramolecular isomers” regardless of different solvents, simply focusing on the framework diversity derived from the same skeleton connectivity, offering a convenient way to consider the influencing factors such as counteranions and solvents. In the present cases, since the ligand, metal ions, and reaction medium for preparation of 1 and 2 are the same, their framework isomerization is evidently due to the difference of the counteranions. As shown in Figure 5, the anions SbF6- and NO3- form distinct supramolecular interactions with the chain frameworks. In 1, the SbF6- interacts with two chains (in red and purple) belonging to one layer and two chains (in blue and dark blue) belonging to the adjacent layers via formation of C-H‚‚‚F hydrogen bonds (H‚‚‚F 2.47-2.64 Å), while in 2, the NO3interacts with two chains (in red) belonging to one triple helix and two chains (in purple and cyan) belonging to two neighboring helices via formation of C-H‚‚‚O hydrogen bonds (H‚‚‚O 2.30-2.59 Å). These interactions based on the specific characters of the anions together with other weak interactions formed between chains synergistically direct the chain conformation and orientation, resulting in distinguished packing modes in 1 and 2. On the other hand, the shape and size of the anions may template the formation of unique channels in 1 and 2, which prefer different solvent molecule adsorption, namely, CH2Cl2 in 1 and CH3OH in 2. Properties. Thermogravimetric analysis (TGA) of the polycrystalline samples of 1 and 2 indicates that the solvent molecules escape at elevated temperature in the range 100-200 °C (9.3% for 1 and 4.5% for 2), and the frameworks are stable up to 310 °C for 1 and 320 °C for 2. The luminescent properties of the free ligand and complexes 1 and 2 were investigated in both solution and solid state at room temperature. In DMF solution, the luminescent spectra of 1 and 2 closely resemble that of the free ligand (Figure S2, Supporting Information), indicating that the emission bands of complexes 1 and 2 are

Figure 6. Photoinduced emission spectra of L (black line, λexcitation ) 390 nm), complex 1 (red line, λexcitation ) 386 nm), and complex 2 (green line, λexcitation ) 391 nm)) in the solid state at room temperature.

ligand centered (π f π*).15 Since the Ag-N bond is believed to be labile in polar solvent and metal-ligand exhange occurs at ambient temperature,16 the polymeric 1D chain structures existing in the single crystal are plausibly not maintained in solution17 but fall apart and result in the formation of oligomers or discrete species in exchange with the “free” ligands. In contrast, in solid state, the luminescent spectra of complexes are significantly different from that of the ligand as depicted in Figure 6. Obvious enhancement of the fluorescence intensities is observed even at room temperature, which is significantly affected by incorporation of Ag ions and in contrast to other Ag(I) complexes just emitting weak photoluminescence at low temperature.18 Complex 1 displays a wide band red-shifted by 11 nm, and complex 2 presents a more intensity enhanced blueshifted band (by 30 nm), indicating that the different chain arrangement and crystal packing in 1 and 2 exhibit significant influence on their luminescent properties. In 1, the shortest Ag‚‚‚Ag separation is as far as 8.30 Å, while in 2 it is comparable with the van der Waals sum of two Ag atoms (3.44 Å). Moreover, between the neighboring strands of the adjacent triple helices in 2, there are π‚‚‚π interactions, which are absent in 1; therefore, the electronic transition of 1 is probably mainly ligand centered, while that of 2 may include metal-to-ligand charge transfer (MLCT) character mixed in part with the ligand centered transitions (n f π* or π f π*).15,17 The weak Ag‚‚‚Ag and π‚‚‚π interactions may contribute to such luminescent difference. Overall, complexes 1 and 2 herein represent the unusual examples of room-temperature luminescent Agcontaining polymeric compounds,19 and there might be potentials for application as light-emitting diodes (LEDs).20 Conclusions We have demonstrated that two types of packing arrays of 1D polymeric chains have been constructed from the same ligand and metal ion with different counteranions, indicating that diversified higher dimensional supramolecular architectures are able to be generated even from the simplest chain topology on the basis of its different framework isomers. Such structural tuning is of interest to render important functions to the simple 1D chain coordination polymers, such as unique channels that are normally obtained in 2D or 3D structures. Photoluminescent investigation indicated that the structural variation imparted distinguished optical features to the materials in the solid state. Acknowledgment. This work was supported by the National Science Funds for Distinguished Young Scholars of China

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(Grant No. 20525310), the NSFC (Grant No. 20303027) and NSF of Guangdong Province (Grant No. 04205405). Supporting Information Available: Crystal packing for 1, solution luminescence, and CIF files for 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org.

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CG060240B