Anal. Chem. 1996, 68, 3879-3881
Chromatography of Mechanically Interlocked Molecular Compounds Masumi Asakawa, Dario Pasini, Franc¸ isco M. Raymo, and J. Fraser Stoddart*
School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
Catenanes and rotaxanes are molecules composed of mechanically interlocked components which are not linked to each other by covalent bonds. These molecular assemblies behave as discrete molecules with defined properties significantly different from those of the parent “free” components. High-performance liquid chromatography has been employed successfully to characterize some tetracationic catenanes and rotaxanes incorporating either cyclobis(paraquat-p-phenylene) or cyclobis(paraquat-4,4′-biphenylene) as the charged components and either hydroquinone-containing macrocycles or dumbbellshaped entities as the neutral components. In each case, significant differences in the retention times of the mechanically interlocked molecular compounds, in comparison with those of their components as their “free” forms, were observed. [n]Catenanes1 are molecules composed of n macrocyclic mechanically interlocked components. Similarly, [n]rotaxanes1 are molecules composed of n - 1 macrocyclic components encircling a thread-like species bearing, at both its ends, bulky groups in order to prevent mechanically the unthreading of the macrocycles. In both cases, no intercomponent covalent bonds exist. Rather, “mechanical bonds”, together with superimposed noncovalent bonds, are responsible for holding the components together. As a result, dismembering of either a catenane or a rotaxane can occur only after cleaving an intracomponent covalent bond. Thus, these mechanically interlocked compounds must be considered as discrete molecules, rather than as assemblies of two or more molecules. Efficient template-directed synthetic approaches for the construction of catenanes and rotaxanes, such as the [2]catenane 1‚4PF6,2 the [3]catenane 4‚4PF6,3 and the [2]rotaxane 6‚4PF64 (Figure 1), have been developed5 recently by us. This methodology relies upon a range of cooperative noncovalent-bonding (1) (a) Schill, G. Catenanes, Rotaxanes and Knots; Academic Press: New York, 1971. (b) Walba, D. M. Tetrahedron 1985, 41, 3161-3212. (c) DietrichBuchecker, C. O.; Sauvage, J. P. Chem. Rev. 1987, 87, 795-810. (d) Lipatov, Y. S.; Lipatova, T. E.; Kosyanchuk, L. F. Adv. Polym. Sci. 1989, 88, 4976. (e) Dietrich-Buchecker, C. O.; Sauvage J. P. Bioorg. Chem. Front. 1991, 2, 195-248. (f) Gibson, H. W.; Marand, H. Adv. Mater. 1993, 5, 11-21. (g) Chambron, J. C.; Dietrich-Buchecker, C. O.; Sauvage, J. P. Top. Curr. Chem. 1993, 165, 131-162. (h) Gibson, H. W.; Bheda, M. C.; Engen., P. T. Prog. Polym. Sci. 1994, 19, 843-945. (i) Amabilino, D. B.; Parson, I. W.; Stoddart, J. F. Trends Polym. Sci. 1994, 2, 146-152. (j) Amabilino, D. B.; Stoddart, J. F. Chem. Rev. 1995, 95, 2725-2828. (k) BeˇlohradskÄ y, M.; Raymo, F. M.; Stoddart, J. F. Collect. Czech. Chem. Commun. 1996, 61, 1-43. (2) Anelli, P. L.; Ashton, P. R.; Ballardini, R.; Balzani, V.; Delgado, M.; Gandolfi, M. T.; Goodnow, T. T.; Kaifer, A. E.; Philp, D.; Pietraszkiewicz, M.; Prodi, L.; Reddington, M. V.; Slawin, A. M. Z.; Spencer, N.; Stoddart, J. F.; Vicent, C.; Williams, D. J. J. Am. Chem. Soc. 1992, 114, 193-218. S0003-2700(96)00460-X CCC: $12.00
© 1996 American Chemical Society
Figure 1. [2]Catenane 1‚4PF6, [3]catenane 4‚4PF6, [2]rotaxane 6‚4PF6, and their “free” component compounds.
interactions which drive the self-assembly in solution of the highly complementary π-electron-rich hydroquinone-based and π-electrondeficient bipyridinium-based components into the catenanes or rotaxane. As a result of its relative simplicity and efficiency, this self-assembly procedure has been employed6 successfully by many other investigators to construct a wide range of catenanes and rotaxanes, structurally analogous to those shown in Figure 1. The [2]catenane 1‚4PF6 incorporates the hydroquinone-based macrocyclic polyether 2 and the bipyridinium-based cyclophane 3‚4PF6. Similarly, the [3]catenane 4‚4PF6 is composed of two of the macrocyclic polyethers 2 and the larger tetracationic cyclophane 5‚4PF6, while the [2]rotaxane 6‚4PF6 is made up of the dumbbell-shaped compound 7 and the tetracationic cyclophane 3‚4PF6. In previous work,2-5 we showed that the mechanically (3) Amabilino, D. B.; Ashton, P. R.; Brown, C. L.; Co´rdova, E.; Godı´nez, L. A.; Goodnow, T. T.; Kaifer, A. E.; Newton, S. P.; Pietraszkiewicz, M.; Philp, D.; Raymo, F.M.; Reder, A. S.; Rutland, M. T.; Slawin, A. M. Z.; Spencer, N.; Stoddart, J. F.; Williams, D. J. J. Am. Chem. Soc. 1995, 117, 12711293. (4) Asakawa, M.; Ashton, P. R.; Iqbal, S.; Stoddart, J. F.; Tinker, N. D.; White, A.J. P.; Williams, D. J. Chem. Commun. 1996, 483-486. (5) (a) Philp, D.; Stoddart, J. F. Synlett 1991, 445-458. (b) Amabilino, D. B.; Stoddart, J. F. Pure Appl. Chem. 1993, 65, 2351-2359. (c) Pasini, D.; Raymo, F. M.; Stoddart, J. F. Gazz. Chim. It. 1995, 125, 431-443. (d) BeˇlohradskÄ y, M.; Philp, D.; Raymo, F. M.; Stoddart J. F. In Organic Reactivity: Physical and Biological Aspects; Golding, B. T., Griffin, R. J., Maskill, H., Eds.; RSC Special Publication 148; Royal Society of Chemistry: Cambridge, UK, 1995; pp 387-398. (e) Philp, D.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1996, 35, 1154-1196.
Analytical Chemistry, Vol. 68, No. 21, November 1, 1996 3879
interlocked molecular compounds 1‚4PF6, 4‚4PF6, and 6‚4PF6 have different properties compared with those of their parent “free” components. Fast atom bombardment spectrometric analysis of 1‚4PF6, 4‚4PF6, and 6‚4PF6 reveals peaks at m/z values corresponding to [M - nPF6]+ ions, arising from the loss of one or more hexafluorophosphate counterions, where M is the sum of the molecular weights of the components, including the counterions, of the mechanically interlocked compound. On the contrary, when equimolar mixtures of their corresponding “free” components were analyzed by fast atom bombardment mass spectrometry, only peaks at lower m/z values corresponding to the molecular ions of the separate components were observed. A similar marked difference was observed in the absorption UVvisible, 1H NMR, and 13C NMR spectra of the mechanically interlocked compounds in comparison with those of their parent components. Furthermore, variable-temperature 1H NMR spectroscopic analyses showed that, within these catenanes and rotaxanes, the interlocked components circumrotate and traverse rapidly, one with respect to each other. Here, we report the investigation of the chromatographic behavior of 1‚4PF6, 4‚4PF6, and 6‚4PF6 and relate each of them to the chromatographic behavior of their components. EXPERIMENTAL SECTION Reagents. The compounds 1‚4PF6,2 2,2 3‚4PF6,2 4‚4PF6,3 5‚4PF6,3 6‚4PF6,4 and 74 were synthesized as described in the literature. High-performance liquid chromatography (HPLC) grade MeCN and H2O were purchased from BDH Laboratory Supplies (Poole, UK) and degassed by bubbling He. Trifluoroacetic acid was purchased from Acros Organics (Geel, Belgium) and used as received. Experimental Method. Known amounts of the 1‚4PF6, 2, 3‚4PF6, 4‚4PF6, 5‚4PF6, 6‚4PF6, and 7 were dissolved individually in MeCN (HPLC grade) to afford stock solutions of known concentration (0.001 M). The resulting solutions (injected volume 20 µL) were analyzed at ambient temperature by HPLC [flow rate 1.0 mL/min; mobile phase, pump A 0.1% CF3CO2H in H2O, pump B MeCN/0.1% CF3CO2H in H2O 95:5; time (min)/pump A (%) ) 0/100, 8/100, 28/0, 42/0, 45/100] by employing a Hypersil BDS C18 column (length 25 cm, inside diameter 5.6 mm) operated by a Gilson 714 system, fitted with a UV-visible detector (detector wavelength 260 nm). In order to assess the reproducibility of (6) (a) Gunter, M. J.; Johnston, M. Tetrahedron Lett. 1990, 31, 4801-4804. (b) Gunter, M. J.; Johnston, M. R. J. Chem. Soc., Chem. Commun. 1992, 1163-1165. (c) Gunter, M. J.; Hockless, D. C. R.; Johnston, M. R.; Skelton, B. W.; White, A. H. J. Am. Chem. Soc. 1994, 116, 4810-4823. (d) Gunter, M. J.; Johnston, M. R. J. Chem. Soc., Chem. Commun. 1994, 829-830. (e) Gunter, M. J.; Johnston, M. R. J. Chem. Soc., Perkin Trans. 1 1994, 9951008. (f) Vo ¨gtle, F.; Muller, W. M.; Muller, U.; Bauer, M.; Rissanen, K. Angew. Chem., Int. Ed. Engl. 1993, 32, 1295-1297. (g) Bauer, M.; Mu ¨ ller, W. M.; Mu ¨ ller, U.; Rissanen, K.; Vo ¨gtle, F. Liebigs Ann. Chem. 1995, 649656. (h) Benniston, A. C.; Harriman, A. Angew. Chem., Int. Ed. Engl. 1993, 32, 1459-1461. (i) Benniston, A. C; Harriman, A.; Lynch, V. M. Tetrahedron Lett. 1994, 35, 1473-1476. (j) Benniston, A. C.; Harriman, A.; Lynch, V. M. J. Am. Chem. Soc. 1995, 117, 5275-5291. (k) Marsella, M. J.; Carrol, P. J.; Swager, T. M. J. Am. Chem. Soc., 1994, 116, 9347-9348. (l) Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1994, 117, 7017-7018. (m) Marsella, M. J.; Carroll, P. J.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 98329841. (n) Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 1259312602. (o) Li, Z. T.; Stein, P. C.; Svenstrup, N.; Lund, K. H.; Becher, J. Angew. Chem., Int. Ed. Engl. 1995, 34, 2524-2528. (p) Li, Z. T.; Stein, P. C.; Becher, J.; Jensen, D.; Mørk, P.; Svenstrup, N. Chem. Eur. J. 1996, 2, 624-633. (q) Li, Z. T.; Becher, J. Chem. Commun. 1996, 639-640. (r) Mirzoian, A.; Kaifer, A. E. J. Org. Chem. 1995, 60, 8093-8095. (s) Staley, S. A.; Smith, B. D. Tetrahedron Lett. 1996, 36, 283-286. (t) Lipton, M. A. Tetrahedron Lett. 1996, 37, 287-290.
3880 Analytical Chemistry, Vol. 68, No. 21, November 1, 1996
Table 1. HPLC Retention Times of the [2]Catenane 1‚4PF6, [3]Catenane 4‚4PF6, [2]Rotaxane 6‚4PF6, and Parent Component Compounds retention time b (min)
compound 1‚4PF6 2 3‚4PF6 1‚4PF6 + 2 + 3‚4PF6a 4‚4PF6 5‚4PF6 2 + 4‚4PF6 + 5‚4PF6a 6‚4PF6 7 3‚4PF6 + 6‚4PF6 + 7a
19 25 16 16 18 18 16
19 22
25
22 19
25
19
21 21
a Solutions containing the mechanically interlocked compound and equimolar amounts of its two parent component compounds. b The retention times are considered accurate to (0.5 min.
Figure 2. HPLC traces of (a) the “free” macrocyclic polyether 2, (b) the “free” tetracationic cyclophane 3‚4PF6, (c) the [2]catenane 1‚4PF6, and (d) an equimolar mixture of 1‚4PF6, 2, and 3‚4PF6.
the measurements, each solution was analyzed five times under exactly the same conditions to afford the retention times reported in the Table 1. The method was scaled-up (injected volume 1.0 mL, flow rate 10.0 mL/min) on a preparative scale by employing a Hypersil BDS C18 column (length 25 cm, inside diameter 20 mm). The injected compounds were recovered unalteredsas proved by 1H NMR spectroscopy and fast atom bombardment mass spectrometrysand quantitatively after each run by evaporating the solvent under reduced pressure. In the case of the bipyridinium-based compounds, the counterions were exchanged back to hexafluorophosphate by (i) dissolving the samples in a saturated solution of NH4PF6 in H2O/Me2CO (1:1, v/v), (ii) evaporating off the Me2CO under reduced pressure, and (iii) filtering off the resulting precipitate. RESULTS AND DISCUSSION The [2]catenane 1‚4PF6 and its component compounds 2 and 3‚4PF6 were analyzed by HPLC (Figure 2), employing the conditions described in the experimental section. The HPLC traces of 2 and 3‚4PF6 (Figure 2a and b, respectively) show peaks
Figure 3. HPLC traces of (a) the “free” macrocyclic polyether 2, (b) the “free” tetracationic cyclophane 5‚4PF6, (c) the [3]catenane 4‚4PF6, and (d) an equimolar mixture of 2, 4‚4PF6, and 5‚4PF6.
Figure 4. HPLC traces of (a) the “free” dumbbell-shaped compound 7, (b) the “free” tetracationic cyclophane 3‚4PF6, (c) the [2]rotaxane 6‚4PF6, and (d) an equimolar mixture of 3‚4PF6, 6‚4PF6, and 7.
at retention times of 25 and 16 min, respectively. In contrast, the HPLC trace of 1‚4PF6 (Figure 2c) shows only one peaksconfirming that the [2]catenane behaves as a pure compoundscentered on a retention time of 19 min. Thus, the value of the retention time for the [2]catenane 1‚4PF6 is intermediate with respect to the values obtained for its two component compounds 2 and 3‚4PF6. Furthermore, when a solution containing equimolar amounts of 1‚4PF6, 2, and 3‚4PF6 was analyzed under the same conditions, the corresponding HPLC trace (Figure 2d) showed three distinct peaks at 16, 19, and 25 min, confirming that the three compounds are eluted with three different retention times. Similarly, the [3]catenane 4‚4PF6 and its parent cyclophane 5‚4PF6 were analyzed by HPLC (Figure 3) under otherwise identical conditions. The HPLC trace of the [3]catenane 4‚4PF6 (Figure 3c) reveals a single peak centered on a retention time of 22 minsi.e., some 3 min slower than the homologous [2]catenane 1‚4PF6sdemonstrating that, even although the [3]catenane is composed of three distinct components, it behaves as a single molecular compound. In addition, the value of the retention time for the mechanically interlocked compound 4‚4PF6 is once again intermediate between those of its separate components 2 and 5‚4PF6 (Figure 3a and b, respectively), as confirmed by the HPLC trace (Figure 3d) of an equimolar solution of 2, 4‚4PF6, and 5‚4PF6. The HPLC trace (Figure 4c) of the [2]rotaxane 6‚4PF6 shows only one peak at a retention time of 19 min demonstrating that, although it incorporates two mechanically interlocked components, the [2]rotaxane behaves as a pure molecular compound. Once more, the value for the retention time of 6‚4PF6 is intermediate between those of its two separate component compounds 7 and 3‚4PF6; see Figure 4, parts a and b, respectively. The HPLC trace (Figure 4d) of an equimolar mixture of 3‚4PF6, 6‚4PF6, and 7 shows three distinct peaks, demonstrating
that the three different compounds are eluted with distinct retention times. CONCLUDING REMARKS Catenanes and rotaxanes incorporate interlocked components, which are linked to each other by mechanical and noncovalent bonds rather than by covalent bonds. Nonetheless, these compounds behave as single molecular compounds. The HPLC traces of model catenanes and rotaxanes, incorporating complementary hydroquinone-based and bipyridinium-based components, reveal one peak only in each case, demonstrating the purity of these compounds. In addition, these catenanes and rotaxanes are eluted with retention times, that differ significantly from those of their two separate component compounds, and indeed, they lie in between the two. Furthermore, the synthetic methodology developed by us to self-assemble such molecular compounds is now being used by many other investigators and a wide range of catenanes and rotaxanes, structurally analogous to those employed in these investigations, are being constructed. Thus, the experimental conditions described here will be of use potentially to synthetic as well as analytical chemists. ACKNOWLEDGMENT This research was supported by the European Community Human Capital and Mobility Programme, the Ciba-Geigy Foundation (Japan) for the Promotion of Science, and the Engineering and Physical Sciences Research Council in the UK. Received for review May 9, 1996. Accepted August 5, 1996.X AC9604604 X
Abstract published in Advance ACS Abstracts, September 1, 1996.
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