Supramolecular Self-Assembly of Zigzag Coordination Polymer

Supramolecular Self-Assembly of Zigzag Coordination Polymer Chains: A Fascinating Three-Dimensional Polycatenated Network Featuring an Uneven “Densi...
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Supramolecular Self-Assembly of Zigzag Coordination Polymer Chains: A Fascinating Three-Dimensional Polycatenated Network Featuring an Uneven “Density of Catenations” and a Three-Dimensional Porous Network

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 9 2061-2065

Xin-Long Wang, Chao Qin, En-Bo Wang,* and Lin Xu Institute of Polyoxometalate Chemistry, Department of Chemistry, Northeast Normal UniVersity, Changchun, Jilin 130024, P. R. China ReceiVed February 24, 2006; ReVised Manuscript ReceiVed June 11, 2006

ABSTRACT: Reaction of ZnII or CdII nitrate with mixed ligands biphenylethene-4,4′-dicarboxylate (bpea) and 1,10-phenanthroline (phen) under the same conditions affords two zigzag polymer chains [Zn(bpea)(phen)] (1) and [Cd(bpea)(phen)(H2O)]‚H2O (2). The zigzag chains in 1 propagate in two non-coplanar directions (rotated by 73°) that are assembled by supramolecular forces into an intriguing three-dimensional (3D) polycatenated network featuring an uneven “density of catenation” for an individual interlocked component. In 2, however, all the zigzag chains extend in the same direction and are linked to one another by supramolecular forces into a 3D porous network featuring rectangular channels. Thermal stability of the two compounds also was studied in this paper. Introduction The current interest in coordination polymer frameworks not only stems from their potential applications in microelectronics, nonlinear optics, porous materials, and catalysis1 but also from their intriguing variety of topologies and entanglement motifs.2,3 Interpenetration has been the most investigated type of entanglement, as shown by the two comprehensive reviews by Batten and Robson.4 More recently, a complete analysis of all the threedimensional (3D) interpenetrated structures contained in the CSD database also was carried out with a rationalization and classification of the topology of the interpenetration.5 Remarkably, the increasing number of coordination polymers reported in the literature has led to new and more complex types of entanglements being recognized4b,6 in polycatenated, polythreaded, and polyknotted species that are reminiscent of molecular catenanes, rotaxanes, and knots. Species such as these are expected to be more flexible than the usual networks based entirely on coordination bonds and thus may have potential applications ranging from drug-delivery vehicles to sensor devices.7 Therefore, the exploitation of these new types of entangled species not only increases the structural diversity of coordination polymers but also provides new insights into the relationships between structure and function of these materials. Ongoing research in our laboratory has been directed toward the design and synthesis of novel metal-organic frameworks showing new modes of entanglements.8 Generally, long ligands are good candidates for the assembly of interpenetrated structures due to their propensity to form large voids. Intriguingly, as evidenced by the reports of us and other groups, when another larger heterocyclic aromatic ligand was introduced simultaneously, the resulting nets usually assumed uncommon types of entanglements, such as a clothlike warp-and-woof sheet structure,9 a two-dimensional (2D) polyrotaxane assembled by molecular rhombi,10 and the unprecedented 9-fold interlocking homochiral helices.8a This can be attributed to the following reasons: (i) the steric hindrance at the metal center is increased when the bulky aromatic ligand binds to the metal ion; this reduces the dimension of the net formed. Lower dimensional * To whom correspondence should be addressed. Tel: +86-431-5098787. Fax: +86-431-5098787. E-mail: [email protected].

nets are usually less likely to interpenetrate because there are more possible ways to maximize the packing efficiency,11 and (ii) the large aromatic system can provide potential supramolecular recognition sites for π-π stacking interactions that can be used to govern the process of self-assembly. In this regard, as an extension of our previous work, for our synthetic strategy we make use of long biphenylethene-4,4′-dicarboxylate (bpea) and chelate 1,10-phenanthroline (phen) as mixed ligands and expect that the integration of them may offer new opportunities to construct new types of entangled frames. Fortunately, we have now isolated a new species [Zn(bpea)(phen)] (1), containing polymeric chains that are assembled via molecular recognition into an intriguing 3D polycatenated array featuring an uneven “density of catenation”. We also report on a strictly related species, [Cd(bpea)(phen)(H2O)]‚H2O (2), which, despite the similar polymeric motif, shows a completely different frame. Experimental Section Materials. All chemicals purchased were of reagent grade and used without further purification. All syntheses were carried out in 20 mL Teflon-lined autoclaves under autogenous pressure. The reaction vessels were filled to approximately 60% volume capacity. Water used in the reactions was distilled water. Synthesis of [Zn(bpea)(phen)] (1). A mixture of Zn(NO3)2‚6H2O (0.5 mmol, 0.149 g), H2bpea (0.5 mmol, 0.134 g), 1,10-phenanthroline (0.5 mmol, 0.099 g), triethylamine (0.15 mL), and water (10 mL) was stirred for 15 min in air, then transferred and sealed in a 23 mL Parr Teflon-lined stainless steel vessel, heated to 160 °C for 5 days, and then cooled to room temperature at a rate of 10 °C/h. The resulting colorless crystals were filtered, washed, and dried in air; yield 0.123 g, 48% based on Zn. Elemental analysis found: C, 65.52%; H, 3.36%; N, 5.55%. Calcd. for C, 65.70%; H, 3.55%; N, 5.47%. IR data (KBr, cm-1): 1645s, 1628s, 1580m, 1570s, 1468s, 1401s, 1369s, 1286m, 1163w, 1039w, 831m, 779m, 750s, 686m, 549m, 416w. Synthesis of [Cd(bpea)(phen)(H2O)]‚H2O (2). The same synthetic method as for 1 was used except that Zn(NO3)2‚6H2O was replaced by Cd(NO3)2‚4H2O (0.5 mmol, 0.154 g), affording colorless block crystals in yield 45% (0.133 g, based on Cd). Elemental analysis found: C, 56.38%; H, 3.58%; N, 4.63%. Calcd. for C, 56.53%; H, 3.73%; N, 4.71%. IR data (KBr, cm-1): 3220m, 1660s, 1622m, 1584w, 1572s, 1480s, 1432s, 1390s, 1364s, 1280m, 1160m, 1044m, 823s, 775s, 755w, 683m, 635w, 509m, 424w. X-ray Crystallography. Single crystals of compounds 1 and 2 were glued on a glass fiber. Data were collected on a Rigaku R-AXIS RAPID

10.1021/cg0600997 CCC: $33.50 © 2006 American Chemical Society Published on Web 07/18/2006

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Table 1. Crystal Data and Structure Refinement of 1 and 2

a

compound

1

2

empirical formula M T/K λ/Å crystal system space group a/Å b/Å c/Å R/deg β/deg γ/deg V/Å3 Z µ/mm-1 R1a [I > 2σ(I)] wR2b

C28H18N2ZnO4 511.81 293(2) 0.71073 monoclinic P21/n 11.682(2) 16.800(3) 12.929(3) 90 105.40(3) 90 2446.3(8) 4 1.040 0.0543 0.1241

C28H22N2CdO6 594.88 293(2) 0.71073 monoclinic P21/n 10.257(2) 18.988(4) 15.296(3) 90 105.51(3) 90 2870.5(10) 4 1.377 0.0520 0.1249

Figure 1. Structure of the 1D zigzag chain in 1.

Scheme 1

R1 ) ∑||F0| - |Fc||/∑|F0|. b wR2 ) ∑[w(F02 - Fc2)2]/∑[w(F02)2]1/2.

Table 2. Selected Bond Lengths [Å] and Angles [°] for 1 and 2a 1 Zn(1)-O(1) Zn(1)-O(2) O(3)-Zn(1)-O(1) O(1)-Zn(1)-N(2) O(1)-Zn(1)-N(1) O(3)-Zn(1)-O(2) N(2)-Zn(1)-O(2)

2.003(3) 2.327(3) 125.26(13) 127.57(12) 100.43(13) 97.54(14) 94.56(13)

Cd(1)-O(2A) Cd(1)-O(3) Cd(1)-N(1) O(2A)-Cd(1)-O(5) O(2A)-Cd(1)-O(3) O(5)-Cd(1)-O(3) O(2A)-Cd(1)-N(2) O(3)-Cd(1)-N(2) O(2A)-Cd(1)-O(4) O(3)-Cd(1)-O(4) N(2)-Cd(1)-O(4)

2.218(4) 2.331(4) 2.338(5) 88.16(18) 103.82(17) 143.38(16) 153.34(17) 95.32(16) 122.44(16) 54.65(15) 83.79(15)

Zn(1)-N(2) Zn(1)-N(1) O(3)-Zn(1)-N(2) O(3)-Zn(1)-N(1) N(2)-Zn(1)-N(1) O(1)-Zn(1)-O(2) N(1)-Zn(1)-O(2)

2.072(3) 2.090(4) 101.42(13) 111.96(14) 79.93(14) 60.11(12) 150.50(12)

Cd(1)-O(5) Cd(1)-O(4) Cd(1)-N(2) O(2A)-Cd(1)-N(1) O(5)-Cd(1)-N(1) O(3)-Cd(1)-N(1) O(5)-Cd(1)-N(2) N(1)-Cd(1)-N(2) O(5)-Cd(1)-O(4) N(1)-Cd(1)-O(4)

2.279(4) 2.430(4) 2.375(5) 85.14(17) 104.07(15) 111.26(17) 87.33(17) 70.56(16) 89.58(14) 150.00(15)

2

a

Symmetry codes: (A) x + 3/2, -y + 1/2, z + 1/2 for 2.

IP diffractometer with Mo KR monochromated radiation (λ ) 0.71073 Å). Empirical absorption correction was applied. The structures were solved by the direct method and refined by the full-matrix least-squares method on F2 using the SHELXTL crystallographic software package.12 Anisotropic displacement parameters were applied to all non-hydrogen atoms. The organic hydrogen atoms were generated geometrically; the aqua hydrogen atoms were located from difference maps. The crystal data and structure refinement of compounds 1 and 2 are summarized in Table 1. Physical Measurements. Elemental analyses (C, H, and N) were performed on a Perkin-Elmer 2400 CHN elemental analyzer. FTIR spectra were recorded in the range 400-4000 cm-1 on an Alpha Centaurt FTIR spectrophotometer using a KBr pellet. TG analyses were performed on a Perkin-Elmer TGA7 instrument in flowing N2 with a heating rate of 10 °C min-1.

Results and Discussion Crystal Structures. Singe-crystal X-ray diffraction reveals that the structure of 1 consists of one-dimensional (1D) polymeric chains spanning two different directions, which, however, display a quite fascinating supramolecular organization. As shown in Figure 1, [Zn(phen)]2+ molecular corners are joined by crystallographically independent bpea spacers to generate a 1D zigzag chain with a period of 28.78 Å. The bpea ligands assume two distinct coordination modes (see Scheme 1a,b), namely, chelating bis(bidentate) and bridging bis(monodentate). The zinc ions show a distorted trigonal bipyramidal

coordination environment containing two nitrogen atoms of a chelating phen ligand and three oxygen atoms from one chelating and one bridging carboxylate ends of two bpea ligands (Figure S1, Supporting Information). The peculiar structural feature of this compound consists of the unique supramolecular organization of these polymeric chains that extend in two noncoplanar directions. As schematized by the rod-packing of Figure 2a, these polymeric chains are arranged on parallel levels in different propagating directions, rotated by 73° on passing from one level to the successive one, thus resulting in an ABAB sequence. As far as we know, the packing of 1D polymers usually occurs with a parallel orientation of all chains; less commonly they can span two different directions on alternate layers.13 At first glance, these chains seem to be independent of one another. However, a more careful examination leads to the exceptional finding that as far as a specific chain (labeled by the star in Figure 2a) of the A or B level is concerned, the bpea phenyl rings are almost perpendicular to the phen groups of the chains lying in displaced positions from the two second nearest neighboring layers of the same type (Figure 2b), exhibiting strong edge-to-face C-H‚‚‚π interactions between phen protons and a phenyl ring (H‚‚‚π 3.08 Å and C-H‚‚‚π 140°).14 Taking into account these supramolecular interactions, two identical sets of 2D layers are formed originating, respectively, from polymeric chains of A type levels only or B only (highlighted in black in Figure 2a, and Figure S2, Supporting Information) that span two different stacking directions. Considering the Zn atoms and the C atoms of the µ2-carboxylate groups as nodes (keep the bpea ligand as a spacer and so the ligand will give two three-connected nodes), we can see that the layers are comprised of octagonal and rhombic meshes with a 3-connected (4.82) topology (Figure 2c). The diagonals of the rhombic windows are about 12.2 and 9.5 Å, while the dimensions of the distorted octagonal windows, estimated from the maximum distances between opposite vertices, are about 23.8 × 28.2 Å. Interestingly, these layers are interlocked in such a way that the inclined catenation not only occurs on the larger octagonal windows but also on the smaller ones due to the large voids, thus giving an overall unique 3D polycatenated array with interlocked four- and eight-membered rings, as illustrated in Figure 3a. The remarkable topological feature of this interlock-

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Figure 3. Schematic view of the polycatenation in 1 (a) and the schematic and space-filling presentations of the two kinds of windows in one layer unevenly catenated by those from other independent layers (b and c). Two sets of layers are shown in red and green.

Figure 2. (a) Rod-packing picture of the 1D chains spanning two different directions (cyan and purple) in 1. The specific chains chosen from one type of level are marked with a star. The 2D sheet obtained by C-H‚‚‚π interactions is highlighted in black. (b) A space-filling model of the 2D sheet, showing the C-H‚‚‚π interactions between chains. (c) A schematic view of the 2D sheet of (4‚82) topology. Black (C atoms) and blue (Zn atoms) are indicated the 3-connected nodes.

ing mode is that the octagonal meshes of a single layer are catenated by three inclined layers (Figure 3c), while the tetragonal ones are catenated only by one (Figure 3b); that is to say, the density of catenation (Doc for short15) of the two types of meshes in an individual layer is uneven; therefore, no index of Doc can be given for this case. To the best of our knowledge, the sole known example with uneven catenation is [Cd(SO4)(bpp)3]16 (bpp ) 1,3-bis(4-pyridyl)propane) sustained by covalent bonds. The catenation mode, however, is different from that of 1 in which hexagonal windows are catenated by

four inclined layers with a topology of (42‚62)(4‚62), while the square ones are catenated only by two. Moreover, entanglement of layers with (4.82) topology is not very common if compared to layers that show the more common square (44) or hexagonal (63) topologies, and only few examples are known that include 2-fold17 and 3-fold18 parallel interpenetrated sheets. Despite the large dimensions of the windows in a single layer of 1, the entanglement with other layers fills almost all the free voids, with only 10.9% of the cell volume left. Infinite catenation of polymeric motifs represents a new type of supramolecular entanglement that has the peculiarity that the whole catenated array displays an interesting “dimensional expansion” phenomenon compared with the component motifs and that each individual motif is catenated only with the surrounding ones but not with all the others.6 Known examples include a polycatenated 1D ladder that give a 2D or 3D array,19 2D simple3,20 or multilayers21 that result in 3D architectures, and the more complex cases involving motifs of different dimensionality.22 We very recently also have reported on a catenated 2D network based on a 1D nanotube.8b Obviously, compound 1 belongs to the subclass of “polycatenation of 2D motifs”; however, in contrast to the most previous examples, it has an uneven “density of catenation” for an individual interlocked component. The structure of compound 2, obtained under the same reaction condition as that of 1 but using CdII ions, also consists

2064 Crystal Growth & Design, Vol. 6, No. 9, 2006

Figure 4. (a) Structure of the 1D zigzag chain in 2. (b) A view of a 2D layer in 2 formed via π-π stacking interactions between phen and bpea groups. (c) Interlayer C-H‚‚‚π (top) and hydrogen bonding (bottom) interactions.

of similar zigzag chains but all running along the [3 0 1] direction with a period of 30.48 Å (Figure 4a). The CdII coordination is of distorted octahedral geometry surrounded by a chelating phen group, two carboxylate groups, and one aqua ligand (Figure S3, Supporting Information). The bpea ligands adopt a chelate-monodentate coordination mode (Scheme 1c). These same directional chains are first extended into 2D sheets in the bc plane via strong π-π stacking interactions between phen and bpea groups (face-to-face distance of ca. 3.48 Å, Figure 4b). Then the layers superimpose in the third direction under the directions of significant hydrogen bonds involving coordinated aqua molecules and carboxylate groups (O5‚‚‚O1 2.654(6) Å and O5‚‚‚O4 2.738(6) Å), as well as C-H‚‚‚π interactions between phen protons and a phenyl ring (H‚‚‚π 2.62 Å and C-H‚‚‚π 160°, Figure 4c) to give a 3D porous network with an effective rectangular window size of 6.7 × 5.6 Å (the size determined by considering van der Waals radii for C atom 1.7 Å),23 as shown in Figure 5. Free water molecules occupy the interlayer regions and form hydrogen-bonding interactions with a carbolxylate oxygen atom (O1W‚‚‚O1 2.883(9) Å). The effective free voids calculated by PLATON correspond to ca. 23.0% of the cell volume. Such different structures for 1 and 2, having the same stoichiometry of metal-to-ligand, are difficult to rationalize. However, it can be reasoned that the nature of metal ions and the presence of coordinated and solvated water molecules in 2 may play some role, especially in templating the porous structure of 2. Thermal Stability Analyses. To examine the thermal stability of the two compounds, thermal gravimetric (TG) analyses were carried out for 1 and 2 between 30 and 800 °C (Figure S4,

Wang et al.

Figure 5. (a) A perspective view of the packing down the a-axis in 2, showing the free water molecules lying in the interlayer regions. (b) A space-filling diagram of the packing down the a-axis in 2, showing the rectangular channels.

Supporting Information). Probably due to the existence of strong C-H‚‚‚π and hydrogen-bonding interactions in the two compounds that further stabilize the whole structures, they exhibit good thermal stability. For 1, no weight losses were observed up to 280 °C; after this, significant weight losses occurred and ended at ca. 720 °C, indicating the complete deposition of the complex to form ZnO as a final product. This conclusion is supported by the percentages of the residues (15.50%), which is in accordance with the expected value (15.91%). The TG curve of 2 indicates the release of a guest water molecule up to 130 °C (observed 3.03%, calcd. 3.66%) to give the product of guest-free [Cd(bpea)(phen)(H2O)], which is stable up to 320 °C. Then, the framework collapsed in the temperature range of 320-800 °C before formation of the final product CdO (observed 20.67%, calcd. 21.59%), which implied that the coordinated water molecule could not be released before the framework collapsed due to the strong hydrogen bonds participated by coordinated water molecules. To further investigate the porosity of 2, a freshly ground sample was heated inside a vacuum oven at 200 °C for 8 h to release the guest water molecule. An X-ray powder diffraction (XRPD) pattern after desolvation indicated that the porous framework structure constructed by packing of these chains was maintained, although the main sharp diffraction peaks show an evident shift and broadening of the lines (Figure S5, Supporting Information). The stability of the porous structure after desolvation might be attributable to the existence of π-π stacking, hydrogen bonds, and C-H‚‚‚π interactions between chains.

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Crystal Growth & Design, Vol. 6, No. 9, 2006 2065

Conclusions In summary, two interesting supramolecular networks constructed by packing of the polymer chains, [Zn(bpea)(phen)] (1) and [Cd(bpea)(phen)(H2O)]‚H2O (2), have been synthesized and characterized that show intriguing 3D polycatenation and porous networks, respectively. The successful isolation of the two species not only provides intriguing examples of chemical topology but also demonstrates that the contemporary use of long ligands and large aromatic ligands opens a promising route for the construction of novel supramolecular networks, a goal we are actively pursuing. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (No. 20371011). Supporting Information Available: Additional plots of the structures, TG curves of 1 and 2, and X-ray crystallographic information files (CIF) for 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org.

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