Metal−Ligand Coordination-Induced Self-Assembly of

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Metal-Ligand Coordination-Induced Self-Assembly of Bolaamphiphiles Bearing Bipyrimidine Bo Song, Guanglu Wu, Zhiqiang Wang, and Xi Zhang* Key Lab of Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China

Mario Smet* and Wim Dehaen Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F Box 2404, B-3001 Leuven, Belgium Received September 3, 2009. Revised Manuscript Received October 23, 2009 A new type of bolaamphiphile bearing bipyrimidine (bpym-8) has been designed and synthesized. The bipyrimidine moiety allows for metal-ligand coordination, thereby influencing the self-assembly of the bolaamphiphile. Before coordination, bpym-8 self-assembles in water to form spherical aggregates. An interesting finding is that the coordination of the Cu(II) ion with bipyrimidine can induce the assembly of bpym-8 to change from spheres to clustered aggregates. It should be noted that the assembly of bpym-8 can be reversibly converted back by removing the Cu(II) ion from the coordination. This study presents a new type of bolaamphiphile that is able to coordinate with metal ions, which may provide a new clue in fabricating reversibly tunable supramolecular nanomaterials.

The self-assembly of amphiphilic molecules is a simple yet efficient bottom-up way of fabricating nanostructured materials,1-7 which have potential practical applications as soft materials or in mimicking biomineralization processes.8-10 The study of amphiphilic molecules has been continuous over the past 100 years and becomes more and more interesting when supramolecular chemistry meets with nanoscience and technology.11-18 Efforts have been made not only in understanding but also in adjusting the self-assembled morphologies by rational molecular *Corresponding authors. (X.Z.) E-mail: [email protected]. Fax: þ86-010-62792406. (M.S.) E-mail: [email protected].

(1) Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem., Int. Ed. 1988, 27, 113. (2) Ahlers, M.; Muller, W.; Reichert, A.; Ringsdorf, H.; Venzmer, J. Angew. Chem., Int. Ed. 1990, 29, 1269. (3) Kunitake, T. Angew. Chem., Int. Ed. 1992, 31, 709. (4) Lehn, J. M. Supramolecular Chemstry: Concepts and Perspectives. VCH: Weinheim, Germany, 1995. (5) Candau, S. J.; Hirsch, E.; Zana, R.; Adam, M. J. Colloid Interface Sci. 1988, 122, 430. (6) Shimizu, T.; Masuda, M.; Minamikawa, H. Chem. Rev. 2005, 105, 1401. (7) Meister, A.; Blume, A. Curr. Opin. Colloid Interface Sci. 2007, 12, 138. (8) Kato, T.; Mizoshita, N.; Kishimoto, K. Angew. Chem., Int. Ed. 2006, 45, 38. (9) Colfen, H. Macromol. Rapid Commun. 2001, 22, 219. (10) Kurth, D. G.; Lehmann, P.; Volkmer, D.; Colfen, H.; Koop, M. J.; Muller, A.; Du Chesne, A. Chem.;Eur. J. 2000, 6, 385. (11) McBain, J. W.; Laing, M. E.; Titley, A. F. J. Chem. Soc. 1919, 115, 1279. (12) Gaudin, A. M.; Fuerstenau, D. W. Trans. AIMME 1955, 202, 958. (13) Zhu, B. Y.; Gu, T. R. J. Chem. Soc., Faraday Trans. I 1989, 85, 3813. (14) Zhu, B. Y.; Gu, T. R.; Zhao, X. L. J. Chem. Soc., Faraday Trans. I 1989, 85, 3819. (15) Kunitake, T.; Okahata, Y. J. Am. Chem. Soc. 1977, 99, 3860. (16) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1991, 113, 1451. (17) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409. (18) Manne, S.; Gaub, H. E. Science 1995, 270, 1480. (19) Yan, Y.; Huang, J. B.; Li, Z. C.; Ma, J. M.; Fu, H. L.; Ye, J. P. J. Phys. Chem. B 2003, 107, 1479. (20) Benvegnu, T.; Brard, M.; Plusquellec, D. Curr. Opin. Colloid Interface Sci. 2004, 8, 469. (21) Chavez, P.; Ducker, W.; Israelachvili, J.; Maxwell, K. Langmuir 1996, 12, 4111. (22) Ducker, W. A.; Wanless, E. J. Langmuir 1996, 12, 5915. (23) Grant, L. M.; Tiberg, F.; Ducker, W. A. J. Phys. Chem. B 1998, 102, 4288.

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design or controlling the outer conditions.19-31 Introducing strong supramolecular interactions, such as π-π stacking or hydrogen bonding interactions, between the construction modules has been proven to be a feasible way to enhance the stability of the self-assemblies.32-34 Bolaamphiphiles, which contain a hydrophobic skeleton and two water-soluble groups on both ends, have shown a unique property in enhancing the stability of the self-assemblies.35-37 In this letter, we want to design and synthesize a new type of bolaamphiphile with the ability to coordinate. It is expected that the introduction of a metal-ligand into the bolaamphiphile allows for the study of how the coordination influences the self-assembly behavior and whether the assembly process can be reversibly tuned. It is anticipated that this line of research may provide new insight in fabricating reversibly tunable supramolecular nanomaterials. (24) Lamont, R. E.; Ducker, W. A. J. Am. Chem. Soc. 1998, 120, 7602. (25) K€ohler, K.; F€orster, G.; Hauser, A.; Dobner, B.; Heiser, U. F.; Ziethe, F.; Richter, W.; Steiniger, F.; Drechsler, M.; Stettin, H.; Blume, A. J. Am. Chem. Soc. 2004, 126, 16804. (26) Meister, A.; Bastrop, M.; Koschoreck, S.; Garamus, V. M.; Sinemus, T.; Hempel, G.; Drescher, S.; Dobner, B.; Richtering, W.; Huber, K.; Blume, A. Langmuir 2007, 23, 7715. (27) Gao, S.; Zou, B.; Chi, L. F.; Fuchs, H.; Sun, J. Q.; Zhang, X.; Shen, J. C. Chem. Commun. 2000, 1273. (28) Qiu, D. L.; Song, B.; Lin, A. L.; Wang, C. Y.; Zhang, X. Langmuir 2003, 19, 8122. (29) Song, B.; Liu, G. Q.; Xu, R.; Yin, S. C.; Wang, Z. Q.; Zhang, X. Langmuir 2008, 24, 3734. (30) Zou, B.; Wang, L. Y.; Wu, T.; Zhao, X. Y.; Wu, L. X.; Zhang, X.; Gao, S.; Gleiche, M.; Chi, L. F.; Fuchs, H. Langmuir 2001, 17, 3682. (31) Zou, B.; Wang, M. F.; Qiu, D. L.; Zhang, X.; Chi, L. F.; Fuchs, H. Chem. Commun. 2002, 1008. (32) Song, B.; Wang, Z. Q.; Chen, S. L.; Zhang, X.; Fu, Y.; Smet, M.; Dehaen, W. Angew. Chem., Int. Ed. 2005, 44, 4731. (33) Song, B.; Wang, Z. Q.; Zhang, X. Pure Appl. Chem. 2006, 78, 1015. (34) Chen, S. L.; Song, B.; Wang, Z. Q.; Zhang, X. J. Phys. Chem. C 2008, 112, 3308. (35) Fuhrhop, J. H.; Mathieu, J. J. Chem. Soc., Chem. Commun. 1983, 144. (36) Fuhrhop, J. H.; Liman, U. J. Am. Chem. Soc. 1984, 106, 4643. (37) Fuhrhop, J. H.; Wang, T. Y. Chem. Rev. 2004, 104, 2901.

Published on Web 11/02/2009

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Scheme 1. Molecular Structure of bpym-8 and Schematic Illustration of the Binding and Unbinding of bpym-8 and the Cu(II) Ion

2,20 -Bipyrimidine is known as a unique donor ligand in preparing 3D complexes by coordinating with different metal ions such as Cu(I), Cu(II), Co(II), and Ni(II).38-41 Herein, we attempt to introduce a coordination interaction between amphiphilic molecules, thus 2,20 -bipyrimidine is chosen as the ligand and Cu(II) is chosen as the metal ion. On the basis of this idea, we have designed and synthesized a new type of bolaamphiphile, 5,50 bis-(8-(1-pyridiniumchloro)-octyloxy)-2,20 -bipyrimidine, noted as bpym-8, as shown in Scheme 1a. A model complex of 2,20 bipyrimidine bound with the Cu(II) ion has been well resolved by Demuno et al.41 through X-ray analysis, indicating that a planar structure should be formed when bpym-8 coordinates with the Cu(II) ion. Thus, it is expected that the variation of the arrangement of bpym-8 induced by the Cu(II) ion may lead to a change in the self-assembled structure of bpym-8 aggregates. Moreover, if the removal of the Cu(II) ion can be realized, then the change in the structure above should be reversible. A schematic illustration of this process is shown in Scheme 1b. bpym-8 itself is an amphiphilic molecule, so it is not surprising that it can self-assemble in aqueous solution and form aggregates, which can be judged by the Tyndall effect. To observe the morphology of the aggregates formed by bpym-8, freshly cleaved mica is incubated in a bpym-8 solution at a concentration of 1  10-3 mol/L for 10 min, removed, and air dried. The sample is then visualized by atomic force microscopy (AFM). As shown in Figure 1a, spherical structures are observed with a diameter of about 23 nm, which agrees well with the size measured by dynamic light scattering (DLS) of 25.4 nm diameter. To confirm this point, the same solution is dropped onto a copper grid and air dried, and then the sample is investigated by transmission electron microscopy (TEM). The morphologies (Figure 1b) obtained from TEM have similar shapes and sizes to those shown in the AFM image. These results indicate that bpym-8 itself can self-assemble into spherical aggregates in aqueous solution. We wondered if the coordination can influence the self-assembly of bpym-8. To answer this question, copper(II) triflate, noted as Cu(OTf)2, is employed as the Cu(II) ion source. Because we expected 1 mol of bpym-8 to bind 1 mol of Cu(II) ions, we first used this stoichiometry and kept the concentration of bpym-8 at 1.0  10-3 mol/L. The color of the solution turns from light (38) Martin, S.; Barandika, M. G.; de Larramendi, J. I. R.; Cortes, R.; FontBardia, M.; Lezama, L.; Serna, Z. E.; Solans, X.; Rojo, T. Inorg. Chem. 2001, 40, 3687. (39) Bekiari, V.; Thiakou, K. A.; Raptopoulou, C. P.; Perlepes, S. P.; Lianos, P. J. Lumin. 2008, 128, 481. (40) Colacio, E.; Lloret, F.; Navarrete, M.; Romerosa, A.; Stoeckli-Evans, H.; Suarez-Varela, J. New J. Chem. 2005, 29, 1189. (41) De munno, G.; Julve, M.; Lloret, F.; Cano, J.; Caneschi, A. Inorg. Chem. 1995, 34, 2048.

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Figure 1. Topology images of the self-assembled structure before (AFM (a), TEM (b)) and after (AFM (c), TEM (d)) the addition of the Cu(II) ion.

brown to yellow after adding the Cu(II) ion. To observe the morphology of the self-assembled structure after coordination, AFM and TEM are also employed for parallel comparison. As shown in Figure 1c by AFM and in Figure 1d by TEM, after the addition of the Cu(II) ion to the solution of bpym-8, the spherical aggregates turn into clustered aggregates with ramified topology. To exclude the substrate effect, a control experiment is carried out using hydrophilic modified silicon wafers as substrates (treated with 7:3 v/v concentrated sulfuric acid/30% H2O2), and there is no clear difference observed. This suggests that the formation of the clustered aggregates is independent of the substrates, which should reflect the morphology of the assemblies in the aqueous solution. These results indicate that the coordination of the Cu(II) ion and bpym-8 does change the self-assembled structure. To investigate the interaction between the Cu(II) ion and bpym-8, UV-vis spectroscopy is employed. First, the absorption spectrum of a pure solution of bpym-8 is captured; then Cu(II) ion is added stepwise, and the spectra are also recorded following each step. Figure 2a shows all of the UV-vis spectra of the titration experiments. The main absorption band of bpym-8 is around 265 nm. After the addition of Cu(II) ions, the absorption peak of the 265 nm band starts to decrease and at the same time a new absorption band around 325 nm appears. This new peak is ascribed to the metal-ligand charge transfer (MLCT) between the Cu(II) ion and bpym-8, which reflects the formation of a coordination interaction. With further addition of Cu(II), the MLCT band around 325 nm increases accordingly. An isosbestic point is observed at 283 nm, which suggests that bpym-8 aggregates transform from one form to another without any other extra aggregates formed in this process. When the molar ratio of bpym8 to Cu(II) varies from 1:0 to 1:1, the absorption intensities of the related peaks change gradually. To see this change more clearly, the intensity is plotted with the corresponding concentration of Cu(II) (the concentration of bpym-8 is kept constant). As shown in Figure 2b, when the molar ratio is smaller than 1:1, the intensity increases almost linearly with the increase in the Cu(II) concentration; at around 1:1, an inflection appears, and after the inflection, the intensity changes little with the increase in the Cu(II) ion concentration. From the UV-vis titration experiments, the binding constant Ka of bpym-8 with Cu(II) in water is calculated to be 1.92((0.21)  105 L/mol through the nonlinear DOI: 10.1021/la903321b

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Figure 2. (a) UV-vis absorption spectra of bpym-8 mixed with different numbers of Cu(II) ions. (b) Absorption intensity of bpym-8 at 325 nm vs the concentration of Cu(II) ion in the mixture; the concentration of bpym-8 is 1.0  10-4 mol/L.

Figure 3. Job’s plot of bpym-8 and the Cu(II) ion complex.

curve fitting in Figure 2b.42 This result also suggests that 1 mol of bpym-8 combines with 1 mol of Cu(II) ions. A Job’s plot was employed to confirm this binding stoichiometry further43 and is popularly used to determine the stoichiometry of two components (e.g., A and B). First, a series of samples with different molar ratios of A to B are prepared by keeping the total concentration of these two components constant. Second, the UV-vis absorption for each sample is recorded. Finally, from the Job’s plot, the molar ratio that refers to the maximum adsorption is the binding stoichiometry of these two components. Thus, from the analysis of the Job’s plot, one can feasibly obtain the stoichiometry of the two binding components. In doing so, the total concentration of bpym-8 and Cu(II) ion is fixed at 1.0  10-4 mol/L, and then the molar ratio of the Cu(II) ion in the total concentration is varied from 0 to 1. The intensity of the UV-vis absorption peak at 325 nm is recorded every time when changing the molar ratio of the Cu(II) ion. Then the dependence of the intensity on the corresponding molar ratio of the Cu(II) ion is determined via the Job’s plot of bpym-8 bound with Cu(II), as shown in Figure 3. It is obvious that the absorption reaches its maximum when the molar ratio is 0.5 and the plot is symmetric about the dividing line x = 0.5, which indicates that the combining stoichiometry between bpym-8 and Cu(II) ions is 1:1. The 1H NMR titration experiments provide solid evidence for the above conclusion. Similar to the UV-vis experiment, herein the concentration of bpym-8 is also kept constant at 1.0  10-3 mol/L and the molar ratio of bpym-8 to Cu(II) is varied from (42) Conners, K. A. Binding Constants: The Measurement of Molecular Complex Stability; Wiley: New York, 1987. (43) Job, P. Ann. Chim. Ser. 1928, 9, 113.

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Figure 4. 1H NMR titration experiments. The concentration of bpym-8 is kept constant at 1.0  10-3 mol/L, and the molar ratio of bpym-8 to the Cu(II) ion is varied as 1:0, 1:0.2, 1:0.5, 1:1, and 1:2.

1:0 to 1:2. The corresponding 1H NMR spectra are recorded, as shown in Figure 4. After the addition of Cu(II), the signal change in the spectra mainly happens to Hp, HR, Hβ, and Hγ. Upon addition of Cu(II) (molar ratio of bpym-8 to Cu(II) is 1:0.2), the Hp signal disappears and at the same time the HR, Hβ, and Hγ signals become broad, weak, and shift to higher field. As we know, Cu(II) is paramagnetic. When the bipyrimidine coordinates with Cu(II), the upfield shift and weakening of the proton signals are commonly observed because of the paramagnetic shielding effect. The closer the proton is to Cu(II), the weaker and the more upfield shifted the signal (e.g., HR, Hβ, and Hγ). Thus, the closest proton Hp to the ligand site exhibits the disappearance of the signal. Further addition of Cu(II) to the system up to a molar ratio of 1:1 makes the HR, Hβ, and Hγ signals broader, weaker, and shifted more to high field, which indicates that more and more bpym-8 entities are combined in the process. It is worth noting that the NMR signal of Hγ shows a very interesting position shift. It moves gradually from the left side to the right side of the peak next to the Hγ peak. As the titration proceeds, no further signal change is observed with a molar ratio of up to 1:2, which means that all the bpym-8 species have been coordinated with Cu(II). All of these results indicate that the coordination stoichiometry between bpym-8 and Cu(II) is 1:1, suggesting that the two species alternately combine in the supramolecular complex. Knowing that the addition of Cu(II) ions induces the reassembly of bpym-8 from spherical structures to clustered aggregates, we still want to know whether this process is reversible. For this purpose, ethylenediaminetetraacetic acid (EDTA) is employed to Langmuir 2009, 25(23), 13306–13310

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Figure 5. (a) UV-vis absorption spectra of the pure bpym-8 (curve a), bpym-8 þ equimolar Cu(II) ion (curve b), and bpym-8 þ equimolar Cu(II) ion þ equimolar EDTA (curve c). (b) TEM image of the self-assembled structure of bpym-8 after the addition of EDTA to remove the Cu(II) ion from the association.

remove the Cu(II) ion from coordination with bpym-8 because of the stronger coordination between EDTA and Cu(II). As shown in Figure 5a, curve a is the UV-vis spectrum of pure bpym-8, which changes to curve b by adding an equimolar amount of Cu(II). After further addition of an equimolar amount of EDTA to the complexed system of bpym-8 bound with Cu(II), the absorption spectrum, curve c, can be recovered and almost totally overlaps the spectrum corresponding to the solution of pure bpym-8, curve a, which means that the MLCT interaction between the 2,20 -bipyrimidine residue and Cu(II) is broken. Thus, one can draw the conclusion that the Cu(II) bound to bpym-8 can be completely removed by adding an equivalent amount EDTA. Besides the UV-vis spectra, DLS also gives further evidence. After the addition of Cu(II), the peak around 25 nm for aggregates of pure bpym-8 disappears. Considering that the clustered aggregates are not spherical, DLS is not suitable for characterizing the aggregates of bpym-8 bound with Cu(II). However, when EDTA is added to the aggregate of bpym-8 bound with Cu(II), it shows that the size of the aggregates become 22.8 nm in diameter, corresponding to the previous size of the aggregates of pure bpym-8 solution, which supports the assumption that the morphology transformation can be reversibly controlled by adding and removing Cu(II). As shown in Figure 5b, the TEM images also indicate that the clustered aggregates are completely gone after the replacement and the self-assemblies return to the spherical structures. However, with careful observation, one will find that the size of the aggregates after EDTA treatment is larger than that formed by pure bpym-8. We suggest that it is caused by the residual Cu-EDTA complex in the solution during TEM experiments. In addition, this difference between DLS and TEM is understandable because DLS is measured in situ and TEM is measured ex situ. Therefore, by removing the Cu(II) ion from the complex, bpym-8 can be reversibly returned to the original self-assembled structure. In summary, metal-ligand coordination is introduced into the amphiphilic assembly system. The addition of Cu(II) induces the self-assembly of bpym-8 from spheres to clustered aggregates. It is proven that the two coordination species require an alternate method to combine in the supramolecular complex. Moreover, the morphology transformation can be reversibly controlled by removing the Cu(II) ion from the complex, thus it may provide a new avenue to prepare reversible self-assembled nanomaterials.

Materials and Methods Methods. AFM images were taken with a commercial multimode Nanoscope IV AFM with tapping-mode scanning. RTESP silicon cantilevers were purchased from the same company. TEM was performed on a JEMO 2010 electron microscope, operating at an acceleration voltage of 120 kV. The samples were prepared by drop coating the aqueous solution on the carbon-coated Langmuir 2009, 25(23), 13306–13310

copper grids. DLS was detected with an ALV/CGS-5022F (He-Ne Laser, 632.8 nm, 22 mW). UV-vis spectra were measured with a Hitachi U-3010 spectrophotometer. NMR spectroscopy (600 MHz) was employed in studying the aggregation state of amphiphiles, and the sample was dissolved in deuterium oxide. The substrates involved were commercial mica and silicon slides. Before use, the mica was freshly cleaved and the silicon slide was treated with piranha solution (7:3 v/v concentrated sulfuric acid/ 30% H2O2 ) to obtain a hydrophilic surface. Materials. The target molecule was synthesized by the following procedure. Synthesis of 5,50 -Dibromo-2,20 -dipyrimidine (1).44 2,20 -Bipyrimidine (1.0 g, 6.3 mmol) and Br2 (2.2 g, 14 mmol) were added to a sealed tube, which was immersed in oil and heated to 155 °C and kept at this temperature for 24 h. After cooling to room temperature, the gray solid was grounded and washed with a 1 M aqueous solution of Na2SO3 and NaOH and extracted with CH2Cl2 (50 mL) 10 times, yielding a yellow solution. The crude product was purified by column chromatography using 1:10 v/v MeOH/CH2Cl2 as the eluent. Product 1 (0.8 g) was collected in a yield of 40%. 1H NMR (300 MHz, CDCl3, 25 °C, TMS): δ 9.042 (d, 3J(H-H) = 4.8 Hz, 4H, 4-H 6-H pyrimidine), 7.459 (t, 3J(H-H) = 4.8 Hz, 2H, 5-H pyrimidine).

Synthesis of 8-(Tetrahydro-pyran-2-yloxy)-octan-ol (2). 45 A 100 mL flask was charged with 1,8-octanediol (1.5 g, 10 mmol), 30 mL of a mixture of 2,3-dihydropyran(DHP) and toluene (5:95 v/v, 0.67 mmol DHP), and 5 mL of a 5 M aqueous solution of NaHSO4. The mixture was stirred overnight at 60 °C and cooled to room temperature, and then diethyl ether (100 mL) was added and the solution was washed with brine (100 mL). The organic solution was dried with MgSO4, filtered, and evaporated under reduced pressure. The product was purified by column chromatography using 1:25 v/v MeOH/CH2Cl2 as the eluent. Product 2 (0.9 g) was collected in a yield of 38%. 1H NMR (300 MHz, CDCl3, 25 °C, TMS): δ 4.576 (1H; O-CH(CH2)-O), 3.873-3.367 (m, 6H; CH2-O), 1.827-1.337 (m, 18H; the other CH2).

Synthesis of 5,50 -[8-(Tetrahydro-pyran-2-yloxy)-octyloxy]2,20 -bipyrimidine (3). To a solution of 1 (157 mg, 0.500 mmol)

and 2 (345 mg, 1.63 mmol) in dry DMF (20 mL) was added potassium tert-butoxide (165 mg, 1.47 mmol). The reaction mixture was stirred at 110 °C for 24 h under an argon atmosphere. After the mixture was cooled to room temperature, a saturated aqueous solution of K2CO3 (20 mL) was added and the reaction mixture was extracted with CH2Cl2 (2  50 mL). The organic layer was evaporated, and the crude product was purified by column chromatography: 1:2 v/v AcOEt/CH2Cl2, yield 18%. 1H NMR (300 MHz, CDCl3, 25 °C, TMS): δ 8.584 (s, 4H; 4-H 6-H pyrimidine), 4.576 (1H; O-CH(CH2)-O), 3.873-3.367 (m, 6H; CH2-O), 1.827-1.337 (m, 36H; the other CH2).

Synthesis of 5,50 -Bis-(8-chlorooctyloxy)-2,20 -bipyrimidine (4). The solution of 3 (50 mg, 0.087 mmol) in THF (5 mL) was

treated with 1 M HCl. The reaction mixture was stirred at room temperature for 4 h, followed by neutralization with an aqueous NaOH solution (5 mL, 1 M). Subsequently, solvents were evaporated and the residue was treated with water (10 mL) and extracted with five portions of CH2Cl2 (5  10 mL). The organic layer was dried over anhydrous MgSO4, filtered, and evaporated under reduced pressure. To a solution of the above product (without any further purification) in CH2Cl2 (10 mL) was added SOCl2 (0.1 mL) and a catalytic amount of DMF (1 drop). The reaction mixture was refluxed overnight under an argon atmosphere. Subsequently, the reaction mixture was washed with a 1 M aqueous solution of K2CO3 (20 mL), and the organic layer was dried over anhydrous MgSO4, filtered, and evaporated under reduced pressure. The crude product was purified with column chromatography using 1:10 v/v MeOH/CH2Cl2 as the eluent, (44) Ziessel, R.; Stroh, C. Tetrahedron Lett. 2004, 45, 4051. (45) Nishiguchi, T.; Haykawa, S.; Hirasaka, Y.; Saitoh, M. Tetrahedron Lett. 2000, 41, 9843.

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yield 95%. 1H NMR (300 MHz, CDCl3, 25 °C, TMS): δ 8.584 (s, 4H; 4-H 6-H pyrimidine), 4.149 (t, 3J(H-H) = 7.2 Hz, 4H; Cl-CH2-CH2), 3.547 (t, 3J(H-H) = 7.2 Hz, 4H; CH2O-pyrimidine), 1.881-1.254 (m, 24H; the other CH2).

8.605 (t, 3J(H-H) = 7.2 Hz, 2H; 4-H pyridine), 8.164 (t, 3J(H-H) = 7.2 Hz, 4H; 3-H 5-H pyridine), 4.599 (t, 3J(H-H) = 7.2 Hz, 4H; CH2-pyridine), 4.209 (t, 3J(H-H) = 7.2 Hz, 4H; CH2-O-pyrimidine), 1.881-1.254 (m, 24H; the other CH2).

pyridine (5 mL) for 24 h, yielding the target molecule by removing excess pyridine under reduced pressure and drying in vacuum, yield 98%. 1H NMR (300 MHz, DMSO, 25 °C, TMS): δ 9.100 (d, 4H; 2-H 6-H pyridine), 8.651 (s, 4H; 4-H 6-H pyrimidine),

Acknowledgment. We thank the major state basic research development program (2007CB808000) and the National Natural Science Foundation of China (50973051 and 50703022) for financial support. This work was supported by a bilateral grant (BIL07/04). B.S. and G.W. contributed equally.

Synthesis of 5,50 -Bis-(8-(1-pyridiniumchloro)-octyloxy)2,20 -bipyrimidine (bpym-8). Product 4 was refluxed in dry

13310 DOI: 10.1021/la903321b

Langmuir 2009, 25(23), 13306–13310