As one of the basic member of the mechanically interlocked structures, pseudorotaxanes and rotaxanes have become one of the hottest research area in recent years because they are not only able to become the fundamental precursors for novel supramolecular species [1, 2], but also realize some functionality and show the response to external stimulus, which could be the prototypes of simple molecular machines . Among various interlocked pseudorotaxanes and rotaxanes, pseudorotaxane and rotaxanes typically contain the wheel and axle that are connected in one molecule with a fast or slow exchange process between threaded and free forms or with a stable threaded form in solution or solid state . In particular, pseudorotaxanes and rotaxanes can be extensively utilized as molecular machines to show corresponding response to external stimuli due to their reversible conversion behavior . Therefore, the efficient construction of fascinating rotaxanes attracted continual attentions in supramolecular chemistry .
In the past decade, the well-known pillararenes have been widely used as wheel components in constructing various mechanically interlocked molecules such as pseudorotaxanes, rotaxanes and catananes with diverse functions [7-14]. The mono-functionalized pillararenes with longer side chains were also employed for construction of various pseudorotaxanes and rotaxanes [15-20]. For this purpose, various longer chain axels and larger functionalized stopers have been introduced into the rim of pillararenes [21-26]. Recently, we have successfully synthesized mono-functionalized pillararene Schiff base, thiourea, pyridylimine and polyamide derivatives and found that these functionalized pillararene derivatives with longer side chains tending to form stable pesudorotaxane and rotaxane both in solution and in solid state [27-34]. In order to assembly novel mechanically interlocked structures based on the metal coordination mode, we initialized the project on construction of functionalized rotaxanes with pillararene as the wheel and versatile ligand terpyridine as the stopper. A literature survey indicated that there are very few reports about the combination of terpyridine and pillararene in supramolecular field [35-37]. Herein, we wish to report the convenient synthesis of a series of pillararene alkylenediamido-bridged terpyridine derivatives and the efficient formation of the fascinating rotaxanes.
The synthetic rout for pillararene diamido-bridged terpyridine derivatives was illustrated in Scheme 1. According to our previously established reaction conditions for the synthesis of pilararene polyamide derivatives [31-33], a mixture of mono-amide-functionalized pillararenes 1a-d (m=2, 3, 4, 6) and (4-([2, 2′:6′, 2′′-terpyridin]-4′-yl)phenoxy)acetic acid 2a in chloroform in the presence of HOBT/EDCl was stirred at room temperature for 24 h. After workup, the expected pillararene diamido-bridged terpyridine derivatives 3a-d (m=2, 3, 4, 6; n=1) were successfully prepared in 17%-35% yields. When 4-(4-([2, 2′:6′, 2′′-terpyridin]-4′-yl)phenoxy)butanoic acid was employed in the reaction under same reaction conditions, the corresponding pillararene diamido-bridged terpyridine derivatives 4a-d (m=2, 3, 4, 6; n=3) were also conveniently synthesized in lower yields. The structures of the obtained pillararene-terpyridines 3a-d and 4a-d were fully characterized by IR, HRMS, 1H and 13C NMR spectroscopy.
The 1H NMR spectra gave the useful information about the interlocked molecular structures of the prepared pillararene-terpyridnes. In the 1H NMR spectrum of pillararene-terpyridine 3a in CDCl3, the methylene unit (OCH2CO) and ethylenediamido unit (NHCH2CH2NH) displayed normal absorptions in the range of 4.37-3.58 ppm. There are no any characteristic absorptions in the high magnetic field, which clearly indicated that the ethylenediamido unit did not inserted into the cavity of the pillararene. Therefore, the scaffold of terpyridine was only connected to the pillararene by ethylenediamido chain from the outside in pillararene-terpyridine 3a (Fig. 1a). On the other hand, the pillararene-terpyridine 3b-d clearly showed several absorptions at high magnetic field. There are two peaks at δ -0.38 and δ -1.52 for the propylenediamido unit in compound 3b, two peaks at δ 0.60 and δ (-1.79)-(-2.07) for butylenediamido unit in compound 3c and four peaks at δ (-0.07)-(-0.10), δ (-0.90)-(-1.03), δ (-1.61)-(-1.68), δ (-2.25)-(-2.27) for hexylenediamido unit in compound 3d, respectively. These chemical shift signals in negative field (δ < 0) ambiguously revealed that the alkylenediamido chain inserted into the cavity of pillararene to form the rotaxane (Fig. 1b). This result is concordance with our previously reported formation of rotaxanes based on the functionalized pillararene mono-and diamides [27-34], in which the length of the ethylenediamido unit is short for the formation of rotaxane and the relative longer alkylene such as propylene, butylene, and hexylene chains are long enough for the formation of the unique rotaxane.
|Fig. 1. The illustration of free form (a) and rotaxanes (b) in pillarareneterpyridines.|
For providing reliable evidence for the above conclusion, a series of pillararene-terpyridines with longer diamido-bridge 4a-d (m=2, 3, 4, 6; n=3) were also prepared (Scheme 1). There is still no any chemical shift signals in negative field in 1H NMR spectra of the compound 4a. This means that the longer OCH2CONHCH2CH2NHCOCH2CH2CH2O chain still did not thread into the cavity of the pillararene, which is a little beyond of our expectation. A possible reason is that the ethyleneamido unit mainly exist outside of the cavity in starting material mono-amide-functionalized pillararene 1a in solution . Then, the catalytic amidation reaction between the amino group and the carboxylic group took place in the outside of the cavity of the pillararene to give the free form structure (Fig. 1a). The 1H NMR spectra of the pillararene-terpyridines 4b-d in CDCl3 indicated that some characteristic absorptions of alkylene unit were clearly observed in the high magnetic field (δ < 0). 1H NMR spectrum of 4b showed two broad singlets at δ 0.20, and δ -1.61 for the bridging propylene unit. 1H NMR spectrum of 4c gave a mixed peaks at δ (-2.07)-(-2.30) for the bridging butylene unit. 1H NMR spectrum of 4d showed four absorptions at δ -0.38, δ (-0.83)-(-1.07), δ (-1.58)-(-1.67), and δ -2.15 for the bridging hexylene unit. Therefore, it can be concluded that the similar rotaxanes was actually formed in the pillararene-terpyridines 4b-d as that of the compounds 3b-d (Fig. 1b).
The stronger evidence for formation of the rotaxanes came from 2D NOESY spectra. For examples, the 2D NOESY spectra of the compounds 3d and 4d were listed in Fig. 2, Fig. 3. From Fig. 2, it was clearly seen that the NOE correlations between proton Hm, Hn at the core of pillararene and protons Hb, Hc, Hd, He of the hexylene chain (Fig. 2, A, B, C, D), between proton Hi at the core of pillararene and protons Hb, Hc, Hd, He of the hexylene chain (Fig. 2, E, F, G, H). From the Fig. 3, the correlations between protons Ha, Hb, Hc, Hd of the hexylene unit and protons Hm, Hn at the core of pillararene (Fig. 3, A, B, C, D), between protons Ha, Hb, Hc, Hd of the hexylene unit and proton Hl at the core of pillararene (Fig. 3, E, F, G, H) were also observed. Combined the 1H NMR results where the protons of the hexylene unit obviously shifted to high magnetic field, it was confirmed the bridged hexylene chain threading into the cavity of the pillararene to form rotaxane.
In summary, we have successfully synthesized a series of novel pillararene diamido-bridged terpyridine derivatives by HOBT/EDCl catalyzed amidation reaction of aminoalkyleneamido-functionalized pillararenes with terpyridine-substituted acetic acid and butanoic acid. The analysis of 1H NMR and 2D NOESY spectra clearly indicated that the interesting rotaxanes were formed by longer propylene, butylene and hexylenediamido chains threading into the cavity of the pillararene and with larger terpyridine acting as the stopper, while pillararenes with the shorter ethylenediamido chain only exists outer of the cavity of pillararene. The convenient formation of rotaxanes with versatile complexing terpyridine can be easily employed to construct smart mechanically interlocked molecules.Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (No. 21871227) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.Appendix A. Supplementary data
Supplementary material related to this article canbefound, in the online version, at doi: https://doi.org/10.1016/j.cclet.2019.04.024.
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