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tropylium cation inclusion complexes
Tetrahedron 69 (2013) 9573e9579
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Crown ether-based cryptand/tropylium cation inclusion complexes Xiujuan Wu a, Jinying Li a, Xuzhou Yan a, *, Qizhong Zhou b, * a b
Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China Department of Chemistry, Taizhou University, Taizhou 318000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 July 2013 Received in revised form 30 August 2013 Accepted 13 September 2013 Available online 20 September 2013
Hosteguest complexation between crown ether-based cryptand hosts and a carbonium ion, tropylium hexafluorophosphate was studied. 1H NMR, NOESY NMR, and electrospray ionization mass spectrometry were employed to characterize these inclusion complexes. The contrast tests of 1H NMR and association constants indicated that cryptands are much better hosts for tropylium hexafluorophosphate than the corresponding simple crown ethers. CeH/O hydrogen bonding, face-to-face p-stacking interactions, and charge-transfer interactions are thought to be the main driving forces for the formation of these hosteguest complexes. These multiple non-covalent interactions may jointly contribute to the complex formation and considerably reinforce the complex stability. Moreover, the complexation between dibenzo-24-crown-8-based cryptand 4 and tropylium hexafluorophosphate 7 can be reversibly controlled by adding KPF6 and then DB18C6 in 1:1 acetonitrile/chloroform, providing a new cation-responsive hosteguest recognition motif for supramolecular chemistry. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Supermolecular chemistry Self-assembly Pseudorotaxanes Cryptands Hosteguest chemistry
1. Introduction During the past decades, supramolecular chemistry, chemistry beyond the covalent bond, has developed rapidly with the purpose of imitating the complex self-assembled structures in the natural world.1 As an active and important part of supramolecular chemistry, hosteguest chemistry, which aims at developing sophisticated chemical systems with novel topologies and/or important functions by molecular recognition and self-organization of components based on non-covalent interactions, has played a significant role in the development of supramolecular chemistry due to its wide applications in the fabrication of artificial molecular machines,2 supramolecular polymers,3 and other advanced supramolecular materials.4 It was first demonstrated by Stoddart and co-workers in 1987 that crown ethers could bind paraquat (N,N0 -dimethyl-4,40 bipyridinium) dications in acetone.5 Later, Loeb and co-workers found that 24-crown-8 can complex 1,2-bis(pyridinium)-ethane derivatives.6 Then, these hosteguest recognition motifs have been widely used in the efficient preparation of various threaded structures including pseudorotaxanes, rotaxanes, and metaleorganic rotaxane frameworks (MORFs), which has vastly promoted the development of supramolecular chemistry.7 Therefore, designing and preparing new macrocyclic hosts and various guest molecules and establishing novel hosteguest recognition motifs, especially the
* Corresponding authors. Tel./fax: þ86 571 87953189; e-mail addresses: xzyan@ zju.edu.cn (X. Yan), [email protected] (Q. Zhou). 0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tet.2013.09.041
ones, which can respond to external stimuli have been a topic of great current interest.8 Cryptands, a kind of crown ether derivatives, which were first reported in 1968 and gained a wide range of interests during the past decade, are three-dimensional bicyclic hosts with adequate cavities that are suitable for encapsulating ions and small molecules.9 By virtue of a significant contributions made by Gibson, Huang, and co-workers, cryptands have been demonstrated to be much better hosts for organic salts compared with simple crown ether counterparts because of the introduction of additional binding sites and the preorganization of the host structures during the association process.10 Therefore, the design and investigation of new recognition motifs based on cryptands will undoubtedly push forward not only the development of cryptand-based hosteguest chemistry but also the research on threaded structures. The tropylium cation, which possesses a positive center and planar rigid structure, is a typical aromatic system. These interesting characteristics make it an effective p-acceptor and ideal guest for charge-transfer complexation with various p-electron-rich counterparts, thereby facilitating the construction of advanced supramolecular architectures. Even so, the reports on the hosteguest recognition of macrocyclic hosts to tropylium cation are still rare. A recent example is that Li and co-workers reported the chargetransfer inclusion complex of pillararenes with tropylium cation.11 It was demonstrated that tropylium cation can selectively associate with a pillar[6]arene with an association constant of 1.8103 M1. Herein, we report three novel inclusion complexes based on the recognition of crown ether-based cryptands to tropylium cation
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with high binding abilities and stimuli-responsiveness. In this paper, we select cis-dibenzo-24-crown-8 (DB24C8)-based cryptand 4, bis(m-phenylene)-26-crown-8 (BMP26C8)-based cryptand 5, and bis(m-phenylene)-32-crown-10 (BMP32C10)-based cryptand 6 as the hosts and tropylium hexafluorophosphate 7 as the guest (Fig. 1). Meanwhile, the hosteguest complexations of the corresponding simple crown ethers DB24C8 diol 1, BMP26C8 diol 2, and BMP32C10 diol 3 with tropylium hexafluorophosphate 7 were also studied as a comparison (Fig. 1). It is worth mentioning that the hosteguest inclusion complex 4I7 can be reversibly controlled by adding and removing potassium cation in 1:1 acetonitrile/ chloroform.
complexes (4I7, 5I7, and 6I7) were of 1:1 stoichiometry in solution (Fig. 2). The Job plots of 1I7, 2I7, and 3I7 showed the same results too (Fig. S1). 2.2. Electrospray ionization mass spectrometry Electrospray ionization mass spectra (ESI-MS) of the solutions of 4e6 and 7 in 1:1 acetonitrile/chloroform provided further support for the formation of 1:1 complexes between cryptands 4e6 and tropylium hexafluorophosphate 7 in solution. As shown in Fig. 3, all equimolar mixtures of 4 and 7, 5 and 7, and 6 and 7 have strong mass fragments of [HIGdPF6]þ: m/z 730.4 (100%) for
Fig. 1. Chemical structures of crown ethers 1e3, cryptands 4e6, and tropylium hexafluorophosphate 7.
(a)
(b)
(c)
Fig. 2. Job plots showing the 1:1 stoichiometries of the complexations between 4 and 7 (a), between 5 and 7 (b), and between 6 and 7 (c) in 1:1 acetonitrile/chloroform: (a) [4]0þ[7]0¼0.500 mM; (b) [5]0þ[7]0¼0.500 mM; (c) [6]0þ[7]0¼0.500 mM. [4]0, [5]0, [6]0, and [7]0 are the initial concentrations of 4, 5, 6 and 7, respectively.
2. Results and discussion 2.1. Determination of stoichiometries of the complexations between cryptands 4e6 and tropylium hexafluorophosphate 7 Equimolar 1:1 acetonitrile/chloroform solutions of each of the cryptands 4, 5, and 6 with tropylium hexafluorophosphate 7 were yellow due to charge-transfer interactions between the electronrich aromatic rings of the cryptand hosts and the electron-poor tropylium ring of the guest 7, giving direct evidence for complexation. Job plots12 based on UVevis absorption spectroscopy of the charge-transfer band demonstrated that all these hosteguest
[4I7dPF6]þ, m/z 730.3 (100%) for [5I7dPF6]þ, and m/z 818.3 (32.5%) for [6I7dPF6]þ. No peaks with other complexation stoichiometries were found. Electrospray ionization mass spectra of equimolar 1:1 acetonitrile/chloroform solutions of 1 (or 2, 3) with 7 were studied at the same time (Figs. S7eS9). 2.3. Determination of association constants of 1I7, 2I7, 3I7, 4I7, 5I7, and 6I7 Then association constants (Ka) of complexes 1I7, 2I7, 3I7, 4I7, 5I7, and 6I7 were determined in 1:1 acetonitrile/chloroform by probing the charge-transfer band of the complexes by
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determined in the same way as that of 5I7. Ka values for all complexes were summarized in Table 1. Compared with the Ka values of simple crown ether complexes 1I7, 2I7, and 3I7, Ka values of the corresponding cryptand complexes 4I7, 5I7, and 6I7 increased about 38-, 427-, and 27fold, respectively. Similar to cryptand/paraquat complexes reported previously,10c,d the improvement of Ka values from crown ether complexes 1I7, 2I7, and 3I7 to cryptand complexes 4I7, 5I7, and 6I7 is due to the preorganization of the cryptand hosts and the introduction of the additional binding sites. The Ka value of complex 5I7 (4.70103 M1) is about 1.8 times higher than that of smaller cryptand complex 4I7 (2.61103 M1), and four times higher than that of bigger cryptand complex 6I7 (1.14103 M1). Similarly, the Ka value of crown ether/tropylium hexafluorophosphate complex 1I7 (67.8 M1) is about six times higher than that of bigger crown ether complex 2I7 (11.0 M1), and 1.6 times higher than that of the biggest crown ether complex 3I7 (42.0 M1). The results indicated that the cavity size of the hosts has an important influence on the binding ability to the tropylium hexafluorophosphate 7 and the bis(m-phenylene)-26-crown-8based cryptand 5 is the best host here for tropylium cation, while the corresponding cryptand host 4 or 6 is either too small or too big. 2.4. The 1H NMR study of the complexation between hosts 1e6 and guest 7 To further investigate the complexations between the six hosts and the tropylium hexafluorophosphate, proton NMR spectra of equimolar (2.00 mM) 1:1 CDCl3/CD3CN solutions of 7 with each of 1, 2, 3, 4, 5, and 6 were examined (Fig. 5e7). Only one set of peaks were found in the proton NMR spectra, indicating that all of the six complexation systems are fast exchange on the proton NMR time scale. Taking the 1H NMR spectra of the complexations of cryptand 5 and the corresponding crown ether 2 towards tropylium hexafluorophosphate 7, for example, comparing the partial proton NMR spectra of 5 (Fig. 6, spectrum a) and 7 (Fig. 6, spectrum c) with the partial proton NMR spectrum of inclusion complex 5I7 (Fig. 6, spectrum b), significant upfield shifts of aromatic protons H5d, H5e, and benzyl protons H5c and a-ethyleneoxy protons H5a on 5 and protons H7a of 7 and downfield shifts of pyridyl protons H5b, H5a,
Fig. 3. Electrospray ionization mass spectra of equimolar 1:1 acetonitrile/chloroform solutions of 7 with (a) 4, (b) 5, and (c) 6.
UVevis spectroscopy and employing a titration method. For example, progressive addition of a 1:1 acetonitrile/chloroform solution with high guest 7 concentration (15.0 mM) and low host 5 concentration (1.00 mM) to a 1:1 acetonitrile/chloroform solution with the same host 5 concentration (1.00 mM) resulted in an increase of the intensity of the charge-transfer band of the complex (Fig. 4a). Treatment of the collected absorbance data at l¼381 nm with a non-linear curve-fitting program afforded the corresponding Ka values: 4.70 (0.070)103 M1 for 5I7. The association constants for complexes 1I7, 2I7, 3I7, 4I7, and 6I7 were 2.0
(b)
1.0
0.5
0.0 300
2.0
1.6
1.5
Absorbance Intensity
Absorbance Intensity
(a)
350
400
450
500
550
1.2
0.8
0.4
0.0
0.000
Wavelength(nm)
0.001
0.002
0.003
0.004
Concentration of 7 (M)
Fig. 4. The absorption spectral changes of 5 upon addition of 7 (a) and the absorbance intensity changes upon addition of 7 (b). The red solid line was obtained from the non-linear curve-fitting.
Table 1 Association constants Ka (M1) of inclusion complexes 1I7, 2I7, 3I7, 4I7, 5I7, and 6I7 in 1:1 acetonitrile/chloroform at 295 K
Association constants (M1)
1I7
2I7
3I7
4I7
5I7
6I7
67.86.60
11.02.00
42.04.26
(2.610.250)103
(4.700.070)103
(1.140.030)103
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Fig. 5. Partial 1H NMR spectra (500 MHz, 1:1 CDCl3/CD3CN, 295 K) of (a) 2.00 mM 4; (b) 2.00 mM 4þ2.00 mM 7; (c) 2.00 mM 7; (d) 2.00 mM 1þ2.00 mM 7; (e) 2.00 mM 1.
Fig. 6. Partial 1H NMR spectra (500 MHz, 1:1 CDCl3/CD3CN, 295 K) of (a) 2.00 mM 5; (b) 2.00 mM 5þ2.00 mM 7; (c) 2.00 mM 7; (d) 2.00 mM 2þ2.00 mM 7; (e) 2.00 mM 2.
and b-ethyleneoxy protons H5b, g-ethyleneoxy protons H5g on 5 were observed. These results suggested that pep stacking occurs between electronically complementary aromatic rings and CeH/O hydrogen bonding forms between protons H on 7 and O (N) on 5. The chemical shift changes of protons on cryptand hosts 4 and 6 showed similar characteristics as cryptand host 5 after complexation with 7 (Figs. 5 and 7). While comparing the partial proton NMR
spectra of 2 (Fig. 6, spectrum e) and 7 (Fig. 6, spectrum c) with the partial proton NMR spectrum of 2I7 (Fig. 6, spectrum d), besides slight upfield shifts of protons H7a of 7 and aromatic protons H2a on 2 were observed, no obvious chemical shifts of all the other protons on 2 were observed. These results revealed that BMP26C8 diol has a very week interaction with tropylium hexafluorophosphate 7. Analogously, the complexes 1I7 and 3I7 also have very week
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Fig. 7. Partial 1H NMR spectra (500 MHz, 1:1 CDCl3/CD3CN, 295 K) of (a) 2.00 mM 6; (b) 2.00 mM 6þ2.00 mM 7; (c) 2.00 mM 7; (d) 2.00 mM 3þ2.00 mM 7; (e) 2.00 mM 3.
interactions between the host component and guest moiety (Figs. 5 and 7). 2.5. The 1H NMR study of the potassium cation-controlled hosteguest complexation of DB24C8-based cryptand 4 towards guest 7 Moreover, the formation of the inclusion complex based on cryptand 4 towards tropylium hexafluorophosphate 7 can be
controlled by adding and removing potassium cation. When KPF6 is added, cryptand 4 can form a more stable complex with Kþ, which can cause the complex between 4 and 7 to disassemble. Subsequently, the complex 4I7 can form again when enough DB18C6 is added to trap the added Kþ. This reversible process was confirmed by proton NMR experiments (Fig. 8). When KPF6 (2.00 equiv) was added to 4 and 7 (2.00 mM) in 1:1 CDCl3/CD3CN (0.500 mL), the chemical shifts corresponding to the protons on guest 7 returned to almost their uncomplexed values (Fig. 8, spectra
Fig. 8. Partial 1H NMR spectra (500 MHz, 1:1 CDCl3/CD3CN, 295 K) of (a) 2.00 mM 4; (b) 2.00 mM 4þ2.00 mM 7; (c) after addition 0.37 mg of KPF6 (2.00 equiv) to b; (d) after addition 1.44 mg of DB18C6 (2.00 equiv) to c; (e) 2.00 mM 7.
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c and e). Correspondingly, the color of the solution changed from yellow to colorless, indicating the total dissociation of the 4I7 complex. However, after addition of DB18C6 (2.00 equiv) to this solution, the complexation between 4 and 7 recovered; large chemical shift changes corresponding to the protons on 7 were observed again (Fig. 8, spectra d and e) and accordingly the yellow color of the solution recovered, indicating the reformation of the 4I7 complex. 2.6. NOESY NMR experiments The assignment and correlation of the protons on inclusion complexes 4I7, 5I7, and 6I7 were validated by NOESY NMR spectra, which were all obtained in 1:1 CDCl3/CD3CN. As shown in Fig. 9, the NOESY NMR examination of inclusion complex 5I7 exhibited unequivocal correlation peaks between the signal of protons H7a of the guest tropylium hexafluorophosphate 7 and aromatic protons H5d, H5e and ethyleneoxy protons H5a, H5b, H5g of the cryptand 5. All these correlation signals indicated that in 1:1 CDCl3/CD3CN the tropylium hexafluorophosphate was incorporated into the cavity of cryptand 5. NOESY NMR spectra of the other two inclusion complexes 4I7 and 6I7 showed similar correlation signals (Figs. S10 and S11).
focusing on using this new recognition motif to construct stimuliresponsive supramolecular systems. 4. Experimental section 4.1. General DB24C8 diol 1,10b BMP26C8 diol 2,10e BMP32C10 diol 3,10a dibenzo-24-crown-8-based cryptand 4,10b bis(m-phenylene)-26crown-8-based cryptand 5,9g bis(m-phenylene)-32-crown-10based cryptand 6,8g and tropylium hexafluorophosphate 711 were synthesized according to literature procedures. All reagents were commercially available and used as supplied without further purification. Solvents were either employed as purchased or dried according to procedures described in the literature. 1H NMR and NOESY spectra were recorded with a Bruker Avance DMX 500 spectrophotometer with use of the deuterated solvent as the lock and the residual solvent or TMS as the internal reference. Low resolution electrospray ionization mass spectra were recorded on a Bruker Esquire 3000 Plus spectrometer. Acknowledgements This work was supported by the Natural Science Foundation of China (21172166). Supplementary data These date include determination of Job plots, association constants, NOESY NMR spectra, electrospray ionization mass spectra, and other materials are available. Supplementary data associated with this article can be found in the online version at http:// dx.doi.org/10.1016/j.tet.2013.09.041. References and notes
Fig. 9. Partial 2D NOESY NMR (500 MHz, 1:1 CDCl3/CD3CN, 295 K) spectrum of an equimolar solution of 5 and 7 at 30.0 mM.
3. Conclusions In summary, we have demonstrated the hosteguest interactions of crown ether-based cryptands with a carbonium ion, tropylium hexafluorophosphate. The results obtained from NMR and ESI mass spectrometry indicated the formation of charge-transfer inclusion complexes. The contrast tests of 1H NMR and association constants indicated that cryptands are much better hosts for tropylium hexafluorophosphate than the corresponding simple crown ethers. All the hosteguest systems are fast exchange on the proton NMR time scale. CeH/O hydrogen bonding, face-to-face p-stacking interactions, and charge-transfer interactions are thought to be the main driving forces between the hosteguest complexes. These multiple non-covalent interactions may jointly contribute to the complex formation and considerably reinforce the complex stability. Moreover the complexation between 4 and 7 can be reversibly controlled by adding KPF6 and then DB18C6 in 1:1 acetonitrile/ chloroform, providing a new cation-responsive hosteguest recognition motif for supramolecular chemistry. Currently, we are
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W. Macromolecules 2009, 42, 6483e6494; (d) Trabolsi, A.; Fahrenbach, A. C.; Dey, S. K.; Share, A. I.; Friedman, D. C.; Basu, S.; Gasa, T. B.; Khashab, N. M.; Saha, S.; Aprahamian, I.; Khatib, H. A.; Flood, A. H.; Stoddart, J. F. Chem. Commun. 2010, 871e873; (e) Jiang, Y.; Cao, J.; Zhao, J.-M.; Xiang, J.-F.; Chen, C.-F. J. Org. Chem. 2010, 75, 1767e1770; (f) Xu, Z.; Jiang, L.; Feng, Y.; Zhang, S.; Liang, J.; Pan, S.; Yang, Y.; Yang, D.; Cai, Y. Org. Biomol. Chem. 2011, 9, 1237e1243; (g) Yan, X.; Chi, X.; Wei, P.; Zhang, M.; Huang, F. Eur. J. Org. Chem. 2012, 6351e6356. (a) Jiang, W.; Winkler, H. D. F.; Schalley, C. A. J. Am. Chem. Soc. 2008, 130, 13852e13853; (b) Guo, D.-S.; Chen, S.; Qian, H.; Zhang, H.-Q.; Liu, Y. Chem. Commun. 2010, 2620e2622; (c) Li, C.; Zhao, L.; Li, J.; Ding, X.; Chen, S.; Zhang, Q.; Yu, Y.; Jia, X. Chem. Commun. 2010, 9016e9018; (d) Yan, X.; Zhang, M.; Wei, P.; Zheng, B.; Chi, X.; Ji, X.; Huang, F. Chem. Commun. 2011, 9840e9842; (e) Yan, X.; Wei, P.; Zhang, M.; Chi, X.; Liu, J.; Huang, F. Org. Lett. 2011, 13, 6370e6373; (f) Shu, X.; Chen, S.; Li, J.; Chen, Z.; Weng, L.; Jia, X.; Li, C. Chem. Commun. 2012, 2967e2969; (g) Yan, X.; Wei, P.; Xia, B.; Huang, F.; Zhou, Q. Chem. Commun. 2012, 4968e4970; (h) Yan, X.; Wu, X.; Wei, P.; Zhang, M.; Huang, F. Chem. Commun. 2012, 8201e8203; (i) Duan, Q.; Xia, W.; Hu, X.; Ni, M.; Jiang, J.; Lin, C.; Pan, Y.; Wang, L. Chem. Commun. 2012, 8532e8534; (j) Li, C.; Shu, X.; Li, J.; Fan, J.; Chen, Z.; Weng, L.; Jia, X. Org. Lett. 2012, 14, 4126e4129; (k) Xia, W.; Hu, X.-Y.; Chen, Y.; Lin, C.; Wang, L. Chem. Commun. 2013, 5085e5087. (a) Simmons, H. E.; Park, C. H. J. Am. Chem. Soc. 1968, 90, 2428e2429; (b) Lehn, J. M. Science 1985, 227, 849e856; (c) Yamaguchi, N.; Gibson, H. W. Angew. Chem. , Int. Ed. 1999, 38, 143e147; (d) Huang, F.; Gibson, H. W.; Bryant, W. S.; Nagvekar, D. S.; Fronczek, F. R. J. Am. Chem. Soc. 2003, 125, 9367e9371; (e) Li, S.; Liu, M.; Zhang, J.; Zheng, B.; Zhang, C.; Wen, X.; Li, N.; Huang, F. Org. Biomol. Chem. 2008, 6, 2103e2107; (f) Li, S.; Liu, M.; Zhang, J.; Zheng, B.; Zhang, C.; Wen, X.; Li, N.; Huang, F. Eur. J. Org. Chem. 2008, 6128e6133; (g) Wang, F.; Zhou, Q.; Zhu, K.; Li, S.; Wang, C.; Liu, M.; Li, N.; Fronczek, F. R.; Huang, F. Tetrahedron 2009, 65, 1488e1494; (h) Zhu, K.; Wu, L.; Yan, X.; Zheng, B.; Zhang, M.; Huang, F. Chem. dEur. J. 2010, 16, 6088e6098. (a) Gibson, H. W.; Nagvekar, D. S. Can. J. Chem. 1997, 75, 1375e1384; (b) Gibson, H. W.; Wang, H.; Slebodnick, C.; Merola, J.; Kassel, W. S.; Rheingold, A. L. J. Org. Chem. 2007, 72, 3381e3393; (c) Pederson, A. M.-P.; Ward, E. M.; Schoonover, D. V.; Slebodnick, C.; Gibson, H. W. J. Org. Chem. 2008, 73, 9094e9101; (d) Li, S.; Liu, M.; Zheng, B.; Zhu, K.; Wang, F.; Li, N.; Zhao, X.-L.; Huang, F. Org. Lett. 2009, 11, 3350e3353; (e) Xu, Z.; Huang, X.; Liang, J.; Zhang, S.; Zhou, S.; Chen, M.; Tang, M.; Jiang, L. Eur. J. Org. Chem. 2010, 1904e1911; (f) Liu, M.; Yan, X.; Hu, M.; Chen, X.; Zhang, M.; Zheng, B.; Hu, X.; Shao, S.; Huang, F. Org. Lett. 2010, 12, 2558e2561; (g) Niu, Z.; Slebodnick, C.; Bonrad, K.; Huang, F.; Gibson, H. W. Org. Lett. 2011, 13, 2872e2875. Fan, J.; Deng, H.; Li, J.; Jia, X.; Li, C. Chem. Commun. 2013, 6343e6345. Job, P. Ann. Chim. 1928, 9, 113e203.
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Tetrahedron journal homepage: www.elsevier.com/locate/tet
Crown ether-based cryptand/tropylium cation inclusion complexes Xiujuan Wu a, Jinying Li a, Xuzhou Yan a, *, Qizhong Zhou b, * a b
Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China Department of Chemistry, Taizhou University, Taizhou 318000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 July 2013 Received in revised form 30 August 2013 Accepted 13 September 2013 Available online 20 September 2013
Hosteguest complexation between crown ether-based cryptand hosts and a carbonium ion, tropylium hexafluorophosphate was studied. 1H NMR, NOESY NMR, and electrospray ionization mass spectrometry were employed to characterize these inclusion complexes. The contrast tests of 1H NMR and association constants indicated that cryptands are much better hosts for tropylium hexafluorophosphate than the corresponding simple crown ethers. CeH/O hydrogen bonding, face-to-face p-stacking interactions, and charge-transfer interactions are thought to be the main driving forces for the formation of these hosteguest complexes. These multiple non-covalent interactions may jointly contribute to the complex formation and considerably reinforce the complex stability. Moreover, the complexation between dibenzo-24-crown-8-based cryptand 4 and tropylium hexafluorophosphate 7 can be reversibly controlled by adding KPF6 and then DB18C6 in 1:1 acetonitrile/chloroform, providing a new cation-responsive hosteguest recognition motif for supramolecular chemistry. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Supermolecular chemistry Self-assembly Pseudorotaxanes Cryptands Hosteguest chemistry
1. Introduction During the past decades, supramolecular chemistry, chemistry beyond the covalent bond, has developed rapidly with the purpose of imitating the complex self-assembled structures in the natural world.1 As an active and important part of supramolecular chemistry, hosteguest chemistry, which aims at developing sophisticated chemical systems with novel topologies and/or important functions by molecular recognition and self-organization of components based on non-covalent interactions, has played a significant role in the development of supramolecular chemistry due to its wide applications in the fabrication of artificial molecular machines,2 supramolecular polymers,3 and other advanced supramolecular materials.4 It was first demonstrated by Stoddart and co-workers in 1987 that crown ethers could bind paraquat (N,N0 -dimethyl-4,40 bipyridinium) dications in acetone.5 Later, Loeb and co-workers found that 24-crown-8 can complex 1,2-bis(pyridinium)-ethane derivatives.6 Then, these hosteguest recognition motifs have been widely used in the efficient preparation of various threaded structures including pseudorotaxanes, rotaxanes, and metaleorganic rotaxane frameworks (MORFs), which has vastly promoted the development of supramolecular chemistry.7 Therefore, designing and preparing new macrocyclic hosts and various guest molecules and establishing novel hosteguest recognition motifs, especially the
* Corresponding authors. Tel./fax: þ86 571 87953189; e-mail addresses: xzyan@ zju.edu.cn (X. Yan), [email protected] (Q. Zhou). 0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tet.2013.09.041
ones, which can respond to external stimuli have been a topic of great current interest.8 Cryptands, a kind of crown ether derivatives, which were first reported in 1968 and gained a wide range of interests during the past decade, are three-dimensional bicyclic hosts with adequate cavities that are suitable for encapsulating ions and small molecules.9 By virtue of a significant contributions made by Gibson, Huang, and co-workers, cryptands have been demonstrated to be much better hosts for organic salts compared with simple crown ether counterparts because of the introduction of additional binding sites and the preorganization of the host structures during the association process.10 Therefore, the design and investigation of new recognition motifs based on cryptands will undoubtedly push forward not only the development of cryptand-based hosteguest chemistry but also the research on threaded structures. The tropylium cation, which possesses a positive center and planar rigid structure, is a typical aromatic system. These interesting characteristics make it an effective p-acceptor and ideal guest for charge-transfer complexation with various p-electron-rich counterparts, thereby facilitating the construction of advanced supramolecular architectures. Even so, the reports on the hosteguest recognition of macrocyclic hosts to tropylium cation are still rare. A recent example is that Li and co-workers reported the chargetransfer inclusion complex of pillararenes with tropylium cation.11 It was demonstrated that tropylium cation can selectively associate with a pillar[6]arene with an association constant of 1.8103 M1. Herein, we report three novel inclusion complexes based on the recognition of crown ether-based cryptands to tropylium cation
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with high binding abilities and stimuli-responsiveness. In this paper, we select cis-dibenzo-24-crown-8 (DB24C8)-based cryptand 4, bis(m-phenylene)-26-crown-8 (BMP26C8)-based cryptand 5, and bis(m-phenylene)-32-crown-10 (BMP32C10)-based cryptand 6 as the hosts and tropylium hexafluorophosphate 7 as the guest (Fig. 1). Meanwhile, the hosteguest complexations of the corresponding simple crown ethers DB24C8 diol 1, BMP26C8 diol 2, and BMP32C10 diol 3 with tropylium hexafluorophosphate 7 were also studied as a comparison (Fig. 1). It is worth mentioning that the hosteguest inclusion complex 4I7 can be reversibly controlled by adding and removing potassium cation in 1:1 acetonitrile/ chloroform.
complexes (4I7, 5I7, and 6I7) were of 1:1 stoichiometry in solution (Fig. 2). The Job plots of 1I7, 2I7, and 3I7 showed the same results too (Fig. S1). 2.2. Electrospray ionization mass spectrometry Electrospray ionization mass spectra (ESI-MS) of the solutions of 4e6 and 7 in 1:1 acetonitrile/chloroform provided further support for the formation of 1:1 complexes between cryptands 4e6 and tropylium hexafluorophosphate 7 in solution. As shown in Fig. 3, all equimolar mixtures of 4 and 7, 5 and 7, and 6 and 7 have strong mass fragments of [HIGdPF6]þ: m/z 730.4 (100%) for
Fig. 1. Chemical structures of crown ethers 1e3, cryptands 4e6, and tropylium hexafluorophosphate 7.
(a)
(b)
(c)
Fig. 2. Job plots showing the 1:1 stoichiometries of the complexations between 4 and 7 (a), between 5 and 7 (b), and between 6 and 7 (c) in 1:1 acetonitrile/chloroform: (a) [4]0þ[7]0¼0.500 mM; (b) [5]0þ[7]0¼0.500 mM; (c) [6]0þ[7]0¼0.500 mM. [4]0, [5]0, [6]0, and [7]0 are the initial concentrations of 4, 5, 6 and 7, respectively.
2. Results and discussion 2.1. Determination of stoichiometries of the complexations between cryptands 4e6 and tropylium hexafluorophosphate 7 Equimolar 1:1 acetonitrile/chloroform solutions of each of the cryptands 4, 5, and 6 with tropylium hexafluorophosphate 7 were yellow due to charge-transfer interactions between the electronrich aromatic rings of the cryptand hosts and the electron-poor tropylium ring of the guest 7, giving direct evidence for complexation. Job plots12 based on UVevis absorption spectroscopy of the charge-transfer band demonstrated that all these hosteguest
[4I7dPF6]þ, m/z 730.3 (100%) for [5I7dPF6]þ, and m/z 818.3 (32.5%) for [6I7dPF6]þ. No peaks with other complexation stoichiometries were found. Electrospray ionization mass spectra of equimolar 1:1 acetonitrile/chloroform solutions of 1 (or 2, 3) with 7 were studied at the same time (Figs. S7eS9). 2.3. Determination of association constants of 1I7, 2I7, 3I7, 4I7, 5I7, and 6I7 Then association constants (Ka) of complexes 1I7, 2I7, 3I7, 4I7, 5I7, and 6I7 were determined in 1:1 acetonitrile/chloroform by probing the charge-transfer band of the complexes by
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determined in the same way as that of 5I7. Ka values for all complexes were summarized in Table 1. Compared with the Ka values of simple crown ether complexes 1I7, 2I7, and 3I7, Ka values of the corresponding cryptand complexes 4I7, 5I7, and 6I7 increased about 38-, 427-, and 27fold, respectively. Similar to cryptand/paraquat complexes reported previously,10c,d the improvement of Ka values from crown ether complexes 1I7, 2I7, and 3I7 to cryptand complexes 4I7, 5I7, and 6I7 is due to the preorganization of the cryptand hosts and the introduction of the additional binding sites. The Ka value of complex 5I7 (4.70103 M1) is about 1.8 times higher than that of smaller cryptand complex 4I7 (2.61103 M1), and four times higher than that of bigger cryptand complex 6I7 (1.14103 M1). Similarly, the Ka value of crown ether/tropylium hexafluorophosphate complex 1I7 (67.8 M1) is about six times higher than that of bigger crown ether complex 2I7 (11.0 M1), and 1.6 times higher than that of the biggest crown ether complex 3I7 (42.0 M1). The results indicated that the cavity size of the hosts has an important influence on the binding ability to the tropylium hexafluorophosphate 7 and the bis(m-phenylene)-26-crown-8based cryptand 5 is the best host here for tropylium cation, while the corresponding cryptand host 4 or 6 is either too small or too big. 2.4. The 1H NMR study of the complexation between hosts 1e6 and guest 7 To further investigate the complexations between the six hosts and the tropylium hexafluorophosphate, proton NMR spectra of equimolar (2.00 mM) 1:1 CDCl3/CD3CN solutions of 7 with each of 1, 2, 3, 4, 5, and 6 were examined (Fig. 5e7). Only one set of peaks were found in the proton NMR spectra, indicating that all of the six complexation systems are fast exchange on the proton NMR time scale. Taking the 1H NMR spectra of the complexations of cryptand 5 and the corresponding crown ether 2 towards tropylium hexafluorophosphate 7, for example, comparing the partial proton NMR spectra of 5 (Fig. 6, spectrum a) and 7 (Fig. 6, spectrum c) with the partial proton NMR spectrum of inclusion complex 5I7 (Fig. 6, spectrum b), significant upfield shifts of aromatic protons H5d, H5e, and benzyl protons H5c and a-ethyleneoxy protons H5a on 5 and protons H7a of 7 and downfield shifts of pyridyl protons H5b, H5a,
Fig. 3. Electrospray ionization mass spectra of equimolar 1:1 acetonitrile/chloroform solutions of 7 with (a) 4, (b) 5, and (c) 6.
UVevis spectroscopy and employing a titration method. For example, progressive addition of a 1:1 acetonitrile/chloroform solution with high guest 7 concentration (15.0 mM) and low host 5 concentration (1.00 mM) to a 1:1 acetonitrile/chloroform solution with the same host 5 concentration (1.00 mM) resulted in an increase of the intensity of the charge-transfer band of the complex (Fig. 4a). Treatment of the collected absorbance data at l¼381 nm with a non-linear curve-fitting program afforded the corresponding Ka values: 4.70 (0.070)103 M1 for 5I7. The association constants for complexes 1I7, 2I7, 3I7, 4I7, and 6I7 were 2.0
(b)
1.0
0.5
0.0 300
2.0
1.6
1.5
Absorbance Intensity
Absorbance Intensity
(a)
350
400
450
500
550
1.2
0.8
0.4
0.0
0.000
Wavelength(nm)
0.001
0.002
0.003
0.004
Concentration of 7 (M)
Fig. 4. The absorption spectral changes of 5 upon addition of 7 (a) and the absorbance intensity changes upon addition of 7 (b). The red solid line was obtained from the non-linear curve-fitting.
Table 1 Association constants Ka (M1) of inclusion complexes 1I7, 2I7, 3I7, 4I7, 5I7, and 6I7 in 1:1 acetonitrile/chloroform at 295 K
Association constants (M1)
1I7
2I7
3I7
4I7
5I7
6I7
67.86.60
11.02.00
42.04.26
(2.610.250)103
(4.700.070)103
(1.140.030)103
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Fig. 5. Partial 1H NMR spectra (500 MHz, 1:1 CDCl3/CD3CN, 295 K) of (a) 2.00 mM 4; (b) 2.00 mM 4þ2.00 mM 7; (c) 2.00 mM 7; (d) 2.00 mM 1þ2.00 mM 7; (e) 2.00 mM 1.
Fig. 6. Partial 1H NMR spectra (500 MHz, 1:1 CDCl3/CD3CN, 295 K) of (a) 2.00 mM 5; (b) 2.00 mM 5þ2.00 mM 7; (c) 2.00 mM 7; (d) 2.00 mM 2þ2.00 mM 7; (e) 2.00 mM 2.
and b-ethyleneoxy protons H5b, g-ethyleneoxy protons H5g on 5 were observed. These results suggested that pep stacking occurs between electronically complementary aromatic rings and CeH/O hydrogen bonding forms between protons H on 7 and O (N) on 5. The chemical shift changes of protons on cryptand hosts 4 and 6 showed similar characteristics as cryptand host 5 after complexation with 7 (Figs. 5 and 7). While comparing the partial proton NMR
spectra of 2 (Fig. 6, spectrum e) and 7 (Fig. 6, spectrum c) with the partial proton NMR spectrum of 2I7 (Fig. 6, spectrum d), besides slight upfield shifts of protons H7a of 7 and aromatic protons H2a on 2 were observed, no obvious chemical shifts of all the other protons on 2 were observed. These results revealed that BMP26C8 diol has a very week interaction with tropylium hexafluorophosphate 7. Analogously, the complexes 1I7 and 3I7 also have very week
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Fig. 7. Partial 1H NMR spectra (500 MHz, 1:1 CDCl3/CD3CN, 295 K) of (a) 2.00 mM 6; (b) 2.00 mM 6þ2.00 mM 7; (c) 2.00 mM 7; (d) 2.00 mM 3þ2.00 mM 7; (e) 2.00 mM 3.
interactions between the host component and guest moiety (Figs. 5 and 7). 2.5. The 1H NMR study of the potassium cation-controlled hosteguest complexation of DB24C8-based cryptand 4 towards guest 7 Moreover, the formation of the inclusion complex based on cryptand 4 towards tropylium hexafluorophosphate 7 can be
controlled by adding and removing potassium cation. When KPF6 is added, cryptand 4 can form a more stable complex with Kþ, which can cause the complex between 4 and 7 to disassemble. Subsequently, the complex 4I7 can form again when enough DB18C6 is added to trap the added Kþ. This reversible process was confirmed by proton NMR experiments (Fig. 8). When KPF6 (2.00 equiv) was added to 4 and 7 (2.00 mM) in 1:1 CDCl3/CD3CN (0.500 mL), the chemical shifts corresponding to the protons on guest 7 returned to almost their uncomplexed values (Fig. 8, spectra
Fig. 8. Partial 1H NMR spectra (500 MHz, 1:1 CDCl3/CD3CN, 295 K) of (a) 2.00 mM 4; (b) 2.00 mM 4þ2.00 mM 7; (c) after addition 0.37 mg of KPF6 (2.00 equiv) to b; (d) after addition 1.44 mg of DB18C6 (2.00 equiv) to c; (e) 2.00 mM 7.
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c and e). Correspondingly, the color of the solution changed from yellow to colorless, indicating the total dissociation of the 4I7 complex. However, after addition of DB18C6 (2.00 equiv) to this solution, the complexation between 4 and 7 recovered; large chemical shift changes corresponding to the protons on 7 were observed again (Fig. 8, spectra d and e) and accordingly the yellow color of the solution recovered, indicating the reformation of the 4I7 complex. 2.6. NOESY NMR experiments The assignment and correlation of the protons on inclusion complexes 4I7, 5I7, and 6I7 were validated by NOESY NMR spectra, which were all obtained in 1:1 CDCl3/CD3CN. As shown in Fig. 9, the NOESY NMR examination of inclusion complex 5I7 exhibited unequivocal correlation peaks between the signal of protons H7a of the guest tropylium hexafluorophosphate 7 and aromatic protons H5d, H5e and ethyleneoxy protons H5a, H5b, H5g of the cryptand 5. All these correlation signals indicated that in 1:1 CDCl3/CD3CN the tropylium hexafluorophosphate was incorporated into the cavity of cryptand 5. NOESY NMR spectra of the other two inclusion complexes 4I7 and 6I7 showed similar correlation signals (Figs. S10 and S11).
focusing on using this new recognition motif to construct stimuliresponsive supramolecular systems. 4. Experimental section 4.1. General DB24C8 diol 1,10b BMP26C8 diol 2,10e BMP32C10 diol 3,10a dibenzo-24-crown-8-based cryptand 4,10b bis(m-phenylene)-26crown-8-based cryptand 5,9g bis(m-phenylene)-32-crown-10based cryptand 6,8g and tropylium hexafluorophosphate 711 were synthesized according to literature procedures. All reagents were commercially available and used as supplied without further purification. Solvents were either employed as purchased or dried according to procedures described in the literature. 1H NMR and NOESY spectra were recorded with a Bruker Avance DMX 500 spectrophotometer with use of the deuterated solvent as the lock and the residual solvent or TMS as the internal reference. Low resolution electrospray ionization mass spectra were recorded on a Bruker Esquire 3000 Plus spectrometer. Acknowledgements This work was supported by the Natural Science Foundation of China (21172166). Supplementary data These date include determination of Job plots, association constants, NOESY NMR spectra, electrospray ionization mass spectra, and other materials are available. Supplementary data associated with this article can be found in the online version at http:// dx.doi.org/10.1016/j.tet.2013.09.041. References and notes
Fig. 9. Partial 2D NOESY NMR (500 MHz, 1:1 CDCl3/CD3CN, 295 K) spectrum of an equimolar solution of 5 and 7 at 30.0 mM.
3. Conclusions In summary, we have demonstrated the hosteguest interactions of crown ether-based cryptands with a carbonium ion, tropylium hexafluorophosphate. The results obtained from NMR and ESI mass spectrometry indicated the formation of charge-transfer inclusion complexes. The contrast tests of 1H NMR and association constants indicated that cryptands are much better hosts for tropylium hexafluorophosphate than the corresponding simple crown ethers. All the hosteguest systems are fast exchange on the proton NMR time scale. CeH/O hydrogen bonding, face-to-face p-stacking interactions, and charge-transfer interactions are thought to be the main driving forces between the hosteguest complexes. These multiple non-covalent interactions may jointly contribute to the complex formation and considerably reinforce the complex stability. Moreover the complexation between 4 and 7 can be reversibly controlled by adding KPF6 and then DB18C6 in 1:1 acetonitrile/ chloroform, providing a new cation-responsive hosteguest recognition motif for supramolecular chemistry. Currently, we are
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