Construction of Cylindrical Nanotubular Materials by Self-Assembly of

Seung Uk Son, Kang Hyun Park, Bo Yun Kim, and Young Keun Chung*. School of Chemistry and Center for Molecular Catalysis, Seoul National University,...
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Construction of Cylindrical Nanotubular Materials by Self-Assembly of Co(NCS)2 with Bent-Building Blocks Having Diimidazole Rings Seung Uk Son, Kang Hyun Park, Bo Yun Kim, and Young Keun Chung*

CRYSTAL GROWTH & DESIGN 2003 VOL. 3, NO. 4 507-512

School of Chemistry and Center for Molecular Catalysis, Seoul National University, Seoul 151-747, Korea Received January 27, 2003;

Revised Manuscript Received April 18, 2003

ABSTRACT: Three cylindrical nanotubular supramolecular structures (S1, S2, and S3) were synthesized by selfassembly of Co(NCS)2 with 1,3-diimidazolyl benzene (L1), 1,2-diimidazolyl benzene (L2), or 2,6-diimidazolyl pyridine (L3), respectively. They all showed similar top views. Columnar packing or assembly of helices was used to explain the construction of tubular materials with bent ligands. The supramolecular material, S1, derived from L1 was a tubular structure based on columnar packing, and others formed from L2 or L3 were similar structures based on the assembly of helices and were a rare example of Meso-helical architectures. These three tubular materials showed interesting 3-D-packing features because of various π-π interactions between 2-D supramolecules. The free heteroatom, nitrogen, in L3 plays an important role in determining the supramolecular structures. The structures show very high thermal stability up to 291 °C for Co(NCS)2(L1)2, 300 °C for Co(NCS)2(L2)2, and 256 °C for Co(NCS)2(L3)2, respectively. Crystal data for the three structures: Co(NCS)2(L1)2, monoclinic, C2/m, a ) 7.403(1) Å, b ) 17.561(1) Å, c ) 10.275(1) Å, β ) 107.201(4)°, V ) 1276.0(2) Å3, Z ) 4; Co(NCS)2(L2)2, orthorhombic, Cmca, a ) 7.868(1) Å, b ) 18.614(1) Å, c ) 7.794(1) A, V ) 2592.2(4) Å3, Z ) 8; Co(NCS)2(L3)2, monoclinic, P21/n, a ) 11.1230(1) Å, b ) 9.2170(1) Å, c ) 12.8680(1) Å, β ) 95.967(2)°, V ) 1312.1(2) Å3, Z ) 4. Introduction Cylindrical nanotubular materials have attracted a great deal of interest among many scientists because of their potential applications, such as in photoelectric devices, ion exchange, molecular sieves, sensors, biomimetics, catalysts, artificial strorages, and templates for the synthesis of nanomaterials such as nanowires in nanotechnology.1,2 Among the synthetic methods of the nanotubular materials, self-assembly is a widely used synthetic tool by designing diverse building blocks. Especially, coordination polymers formed by self-assembly between various ligands and metals occupy a special position because of easy introduction of functional groups to the structure. In this case, columnar packings and assembly of helices or helix-like channels3 are considered as two rationales4 for the construction of channels. For example, a novel nanotubular coordination polymer was recently reported by Liu’s group using a porous cyclodextrin as a building block.5 The structure was formed by columnar packing between porous building blocks. Examples of the construction of cylindrical materials based on helical architectures were numerous.3 In this case, the chirality related to the rotational direction of the helix was paid much attention by crystal engineers.6 Square-grid tubular coordination polymers synthesized by self-assembly between linear building blocks and metals were most extensively studied and wellestablished.7 Thus, introduction of one or more functional groups to the tubular structure is relatively easy and predictable. Most square-grid polymers having cylindrical channels were formed by columnar packing between porous layers. As compared to linear ligands, * Corresponding author. Fax: 82-2-889-0310. Tel: 82-2-880-6662. E-mail: [email protected].

the use of bent ligands for the synthesis of tubular coordination polymers is relatively rare because it is not so easy to predict the supramolecular structure.8 A few interesting examples of tubular materials were recently prepared by Jung’s group9 or our group10 using a bent or flexible ligand as a building block. Although these supramolecular structures show beautiful and interesting structures, the structures can be hardly predicted because they are formed by very complicated selfassembly. Thus, modification of the stucture seems to be rather difficult. It is necessary to develop a simple rationale for the construction of channels with a bent ligand that will assist a facile modification of the stucture. With this in mind, we prepared three simple bent building blocks, 1,3-diimidazolyl benzene (L1), 1,2diimidazolyl benzene (L2), and 2,6-diimidazolyl pyridine (L3) and used these in the synthesis of supramolecular compounds (S1, S2, and S3) by assembling with Co(NCS)2. Studies on self-assembly using 1,4-diimidazolyl benzene have been previously performed, but to the best of our knowledge, supramolecular structures using L1-3 as building blocks have not yet been reported.

Cylindrical structures can be retrosynthesized as shown in Scheme 1. Channels in cylindrical structures can be formed by columnar packing as shown Scheme 1b or by helical assembly as shown in Scheme 1c,d. In

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Scheme 1.

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Retrosynthesis of Cylindrical Tubular Structuresa

a (a) Cylindrical material, (b) cylindrical material formed by columnar packing, (c) cylindrical material consisting of P-helices, and (d) cylindrical material consisting of M-helices.

the cases of helical assembly, many structural isomers are possible depending on the rotational direction of the helix. Experimental Procedures Synthesis of Ligands L1, L2, and L3. Ligands L1, L2, and L3 were synthesized by the literature method11 from 1,3dibromobenzene, 1,2-dibromobenzene, and 2,6-dibromopyridine, which were purchased form Aldrich Chemical Co. Compound Co(NCS)2 was also purchased from Aldrich Chemical Co. and used as received. Elemental analyses were done at National Center for Inter-University Research Facilities, Seoul National University. Thermogravimetric analyses were done at the Korea Basic Science Institute. Synthesis of Coordination Polymers. A methanol solution (5 mL) of L1 (30 mg, 0.14 mmol) was slowly diffused into a water solution (3 mL) of Co(NCS)2 (20 mg, 0.11 mmol) for a week. Pink cystals (31 mg, 71%) were grown at the interface. Anal. Calcd. of S1 (C26H20N8S2Co): C, 52.44; H, 3.38; N, 23.52; S, 10.77. Found: C, 52.47; H, 3.43; N, 2.41; S, 10.80. Crystals of S2: yield, 65%. Anal. Calcd. of S2 (C26H20N8S2Co): C, 52.44; H, 3.38; N, 23.52; S, 10.77. Found: C, 52.57; H, 3.42; N, 23.55; S, 10.80. Crystals of S3: yield, 75%. Anal. Calcd. of S3 (C24H18N10S2Co): C, 48.24; H, 3.04; N, 28.13; S, 10.73. Found: C, 47.87; H, 3.13; N, 27.62; S, 10.82. Single-Crystal X-ray Analysis. X-ray data for single crystals were collected on an Enraf-Nonius CCD single-crystal X-ray diffractometer at room temperature using graphitemonochromated MoKR radiation (λ ) 0.71073 Å). The structures were solved by direct methods (SHELXS-97) and refined against all F2 data (SHELXS-97). All non-hydrogen atoms were refined with anisotropic thermal parameters, and the hydrogen atoms were treated as idealized contributions.

Results and Discussion Self-assembly between Co(NCS)2 and L1 in MeOH/ H2O was carried out. We anticipated that a 1-D coordination polymer having pores would be formed. After one week, pink crystals stable in air were obtained. The X-ray structural analysis of the crystal revealed the

Figure 1. (a) An ORTEP drawing of asymmetric unit with 30% ellipsoid probability and (b) the columnar packing structure of a 1-D polymer formed by L1.

formation of supramolecular S1 (Co(NCS)2(L1)2) in the monoclinic space group C2/m (Tables 1 and 2; for detailed data, see Supporting Information). Figure 1a shows an ORTEP drawing of an asymmetric unit of Co(NCS)2(L1)2. Each cobalt atom is in an octahedral environment. Figure 1b shows that 1-D coordination polymers are packed in a columnar manner to form a 2-D cylindrical tubular structure as shown in Scheme 1b. The size of channel in a 2-D-cylindrical structure is 8.78 × 9.89 Å, and the effective size calculated from the model with van der Waals radii is 3.45 × 8.24 Å. The distance between layers of the 1-D-coordination polymer is 7.40 Å, and NCSs are located in channels with an inclined manner. Thus, the tubular structure S1 can be a prototype for the construction of cylindrical tubular material using a bent ligand by columnar packing. Next, we investigated the self-assembly of Co(NCS)2 with L2. Owing to the steric effect or improper angle, it was expected that coordination of two imidazole rings of L2 to a cobalt center might be more difficult than L1 because the cobalt atom and two imidazole rings should be in the same plane. Under the same conditions as above, pink crystals stable in air were obtained. The

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Table 1. X-ray Differaction Informations of Coordination Polymers by L1, L2, and L3 chem. formula a (Å) b (Å) c (Å) β (deg) Fw cryst. syst. space group V (Å3) Z density θ range h k l R(int) goodness -of-fit R1 wR2

S1

S2

S3

C13H10Co0.50N5S

C13H10Co0.50N5S

C12H9Co0.50N6S

7.403(1) 17.561(1) 10.275(1) 107.201(4) 297.79 monoclinic C2/m 1276.0(2) 4 1.550 2.07-27.50 -9 e h e 9 -20 e k e 22 -13 e l e 13 0.0414 1.041

17.868(1) 18.614(1) 7.794(1) 90 297.79 orthorhombic Cmca 2592.2(4) 8 1.526 2.19-27.49 -23 e h e 23 -23 e k e 19 -9 e l e 9 0.0729 1.005

11.123(1) 9.217(1) 12.868(1) 95.967(2) 298.78 monoclinic P21/n 1312.1(2) 4 1.512 2.30-27.54 -14 e h e 14 -11 e k e 11 -16 e l e 16 0.0304 1.029

0.0153 0.1415

0.0430 0.1143

0.0368 0.1016

Figure 3. 3. The π-π interactions between 2-D cylindrical layers by L1 (a) and L2 (b) act like walls in channels.

Figure 2. (a) An ORTEP drawing of asymmetric unit with 30% ellipsoid probability and (b) the helical structure of 2-D polymers formed by L2.

X-ray structural analysis of the crystal revealed the formation of supramolecular S2 (Co(NCS)2(L2)2) in the orthorhombic space group, Cmca (Tables 1 and 2; for detailed data, see Supporting Information). Figure 2a shows an ORTEP drawing of an asymmetric unit of Co(NCS)2(L2)2. Surprisingly, it turned out that the 2-D polymers having helical channels were synthesized (Figure 2). Preparation of the 1-D helical coordination polymers and their transformation to other supramolecular structures by intermolecular forces such as π-π interaction have been extensively studied.3 However, as compared to 1-D helical coordination polymers, 2-D helical coor-

Figure 4. Top views of 3-D packing structures of coordination polymers formed by L1 (a) and L2 (b), respectively. NCSs were omitted for clarity.

dination polymers were rarely studied. When an achiral building block was used, 1-D helices right- or lefthanded were usually found in the crystal. So, the 1-D meso-helices were relatively rare.6 Similary, in cases of 2-D helical coordination polymers, more than two structures can be possible depending on the rotational direction of helices (Scheme 1c,d). In our case, the neighboring helix in each channel of 2-D tubular polymers rotates in the different direction as described in Scheme 1d. This structure can be called a 2-D meso-

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Table 2. Selected Bond Lengths (Å) and Angles (deg) for S1, S2, and S3 S1 Co-N(3) Co-N(1) N(3)-Co-N(3)#1 N(3)#1-Co-N(1) N(1)#2-Co-N(1)

S2 2.153(5) 2.157(3) 180.000(1) 91.52(14) 91.61(18)

Co-N(2) Co-N(3) N(3)-Co-N(3)#1 N(2)-Co-N(3) N(2)-Co-N(2)#2

Figure 5. An ORTEP drawing (a) of an asymmetric unit (ellipsoids are drawn at the 30% probability level), top view (b) of cylindrical structures (red, clockwise rotation; green, counterclockwise rotation), and the cylindrical tubular structure (c) formed by L3. NCSs were omitted for clarity in panels b and c.

helix. To the best our knowledge, this is a rare example of a 2-D meso-helix.6d,e The size of a channel is 8.94 × 9.31 Å, and the effective size calculated from the model with van der Waals radii is 5.39 × 6.44 Å. NCS ligands are located parallel to the channels and play important roles as walls among the channels.

S3 2.153(3) 2.177(5) 180.00(19) 90.26(14) 90.24(17)

Co-N(6) Co-N(3) Co-N(5) N(6)#1-Co-N(6) N(6)-Co-N(3) N(6)-Co-N(5) N(3)-Co-N(5)

2.116(2) 2.150(2) 2.177(2) 180.0 91.55(8) 89.56(9) 87.61(8)

The two 2-D cylindrical tubular layers formed by L1 and L2 are packed by π-π interaction (the distance between centers of the rings: 3.70 Å for S1; 3.92 Å for S2) between two central phenyl rings in the building blocks of the neighboring 2-D cylindrical tubular layers. (Figure 3). Recently, the aromatic π-π interaction between supramolecular structures was used to induce novel structures such as double-stranded helices or molecular zippers from single-stranded coordination polymers.12 In our case, the π-π interaction between 2-D cylindrical layers is induced to construct a 3-D cylindrical structure and acts like walls of channels (Figure 3). Figure 4 shows the top views of 3-D-tubular structures by L1 and L2. Although the two stuctures have channels that were formed by different rationales, interestingly, top views of two 3-D-tubular structures by L1 and L2 are very similar. Next, we investigated the use of a building block L3 to introduce a functional group (i.e., pyridyl) to the inner walls of channels of tubular materials. We anticipated that a supramolecular structure (S3) from the reaction between Co(NCS)2 and L3 would resemble that derived from L1. Slow diffusion of a solution of L3 into a solution of Co(NCS)2 gave pink crystals. The structure of the crystals was solved by an X-ray diffraction analysis (Figure 5). Monoclinic symmetry was observed (Tables 1 and 2; for detailed data, see Supporting Information).

Figure 6. (a) Top view of cylindrical, tubular structure formed by L3, (b) top view, and (c) lengths between rings participating in π-π interactions. NCSs were omitted for clarity. (d) Top view of the channel with NCS ligands.

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coordination polymers formed by self-assembly using bent building blocks and octahedral metal. The free heteroatom, nitrogen, which can form a hydrogenbonding interaction with reaction media such as MeOH or H2O played an important role in determining the supramolecular structure. The supramolecular structures showed very high thermal stability up to 291 °C for Co(NCS)2(L1)2, 300 °C for Co(NCS)2(L2)2, and 256 °C for Co(NCS)2(L3)2, respectively. Figure 7. Thermogravimetric analysis of S1, S2, and S3.

Figure 5a shows an asymmetric unit of coordination polymers formed by L3. Surprisingly, the structure resembled that derived from L2. Figure 5b,c shows M-helical channels in the cylindical 2-D polymers. It seems that the nitrogen atom in L3, capable of forming hydrogen bonds with a reaction medium, MeOH or H2O, played an important role in determining the structure. As compared to the case of the 2-D coordination polymers formed by L2, there exists another π-π interaction (distance: 3.71 Å) between the two imidazole rings of L3 in the 2-D polymers (Figure 5c). Figure 6 shows the 3-D packing-structure of the tubular 2-D coordination polymers. The channel size is 8.06 × 11.30 Å, and the effective size calculated from the model with van der Waals radii is 5.66 × 6.78 Å. Interestingly, there were some differences in 3-Dpacking structures of S2 and S3: no π-π interactions between central pyridine rings of L3 in the two 2-D coordination polymers existed, but two different π-π interactions between a pyridyl and an imidazolyl ring existed, and Figure 6c shows the distances between two rings with participating π-π interactions. The NCS ligands do not fill the channels in S3, implying that the channels are vacant. Figure 6d shows the top view of the channel with NCS ligands. Crystals derived from L1, L2, and L3 are highly stable in air. After drying in a vacuum for 1 day, the crystals formed by L1-L3 and Co(NCS)2 were analyzed by a single-crystal X-ray diffractor. The same structures were obtained with good qualities. Thermogravimetric data (Figure 7) show a high stability up to 291 °C for S1 and 300 °C for S2, respectively. As compared to S1 or S2, S3 was relatively unstable and showed stability up to 256 °C. However, this value is still quite high as compared to that of supramolecules based on linear ligands.13 The three tubular structures do not have sufficiently sized channels that can accommodate guest molecules.14 Therefore, our future work is now directed toward the preparation of tubular coordination polymers such as S1, S2, and S3 that have larger channels by using bent ligands having longer arms. In conclusion, we synthesized three cylindrical, nanotubular, supramolecular structures (S1, S2, and S3) by self-assembly of Co(NCS)2 with 1,3-diimidazolyl benzene (L1), 1,2-diimidazolyl benzene (L2), and 2,6-diimidazolyl pyridine (L3). They showed similar tubular structures, but their construction of channels with bent ligands was explained by columnar packing or helical assembly. Supermolecules reported herein represent prototypes of a new class of compounds: cylindrical

Acknowledgment. This work was supported by Korea Science Engineering Foundation (KOSEF R011999-000-0041-012002) and the KOSEF through the Center for Molecular Catalysis. S.U.S., K.H.P., and B.Y.K. thank the BK21 fellowship. Supporting Information Available: X-ray crystallographic information files (CIF) for coordination polymers S1, S2, and S3. This material is available free of charge via the Internet at http://pubs.acs.org.

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