Organometallic rotaxane dendrimers with fourth-generation mechanically interlocked branches Wei Wanga, Li-Jun Chena, Xu-Qing Wanga, Bin Suna,b, Xiaopeng Lib, Yanyan Zhangc, Jiameng Shic, Yihua Yuc, Li Zhangd, Minghua Liud, and Hai-Bo Yanga,1 aShanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, Shanghai 200062, People’s Republic of China; bDepartment of Chemistry and Biochemistry, Texas State University, San Marcos, TX 78666; cShanghai Key Laboratory of Magnetic Resonance, Department of Physics, East China Normal University, Shanghai 200062, People’s Republic of China; and dKey Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, People’s Republic of China Edited by Vivian Wing-Wah Yam, The University of Hong Kong, Hong Kong, China, and approved March 31, 2015 (received for review January 11, 2015) Mechanically interlocked molecules, such as catenanes, rotaxanes, Herein, we describe the synthesis, characterization, and func- and knots, have applications in information storage, switching tionalization of higher-generation (up to fourth-generation) organ- devices, and chemical catalysis. Rotaxanes are dumbbell-shaped ometallic rotaxane branched dendrimers. A divergent strategy was molecules that are threaded through a large ring, and the relative employed for the dendrimer synthesis in which the host–guest motion of the two components along each other can respond to complex of a pillar[5]arene and a neutral alkyl chain were used as external stimuli. Multiple rotaxane units can amplify responsiveness, the rotaxane subunits. The formation of platinum–acetylide bonds — — and repetitively branched molecules dendrimers can serve as vehi- wasthegrowthstepinthesynthesis;it produced satisfactory yields cles for assembly of many rotaxanes on single, monodisperse com- and allowed construction of the targeted structures. The introduction pounds. Here, we report the synthesis of higher-generation rotaxane of macrocyclic wheels enhanced the rigidity of the resultant rotaxane dendrimers by a divergent approach. Linkages were introduced as dendrimers and reduced self-folding. Electrochemically active spacer elements to reduce crowding and to facilitate rotaxane motion, rotaxane dendrimers substituted with different numbered ferrocenes even at the congested periphery of the compounds up to the fourth generation. The structures were characterized by 1D multinuclear (1H, were also prepared by direct surface modification. 13 31 C, and P) and 2D NMR spectroscopy, MALDI-TOF-MS, gel perme- Results and Discussion ation chromatography (GPC), and microscopy-based methods in- Synthesis. cluding atomic force microscopy (AFM) and transmission electron To synthesize rotaxane branched dendrimers, the microscopy (TEM). AFM and TEM studies of rotaxane dendrimers mechanically interlocked functions must be repeating subunits of vs. model dendrimers show that the rotaxane units enhance the the targeted structures. The rotaxane building blocks must be rigidity and reduce the tendency of these assemblies to collapse stable enough to handle and incorporate repeatedly during the by self-folding. Surface functionalization of the dendrimers with growth processes. We used organometallic [2] rotaxane 1 (Fig. 2) ferrocenes as termini produced electrochemically active assemblies. as the basic precursor for the divergent dendrimer growth for CHEMISTRY The preparation of dendrimers with a well-defined topological the following reasons: (i) 1 can be quickly synthesized by using structure, enhanced rigidity, and diverse functional groups opens Ogoshi’s available pillar[5]arene and its neutral alkyl chain guest previously unidentified avenues for the application of these mate- (20–22); (ii) 1 contains a platinum–acetylide unit that prevents rials in molecular electronics and materials science. the macrocycle from escaping the thread; (iii) 1 can react with a free alkyne to generate a stable organometallic bond in good rotaxane dendrimer | controllable divergent approach | platinum yield under mild conditions (23–25); (iv) 1 contains protected acetylide | surface modification | dynamic supramolecular systems alkynes that can be gently exposed for dendrimer growth; and endritic molecules containing rotaxane components are a Significance Drecently developed subset of mechanically bonded super- – molecules (1 3). The combination of the characteristics of both In this study, the preparation of organometallic rotaxane den- rotaxanes (sliding and rotary motion) and dendrimers (repetitive drimers with a well-defined topological structure and enhanced branching with each generation) provides the resultant rotaxane rigidity was developed. Starting from a simple rotaxane building dendrimers with unusual topological features and potentially use- block, high-generation rotaxane branched dendrimers were syn- ful properties. For example, the introduction of stimuli-responsive thesized and characterized. The fourth-generation structure de- rotaxanes (4) such as muscle-like bistable rotaxanes or daisy chains scribed is among the highest-generation organometallic rotaxane can impart switchable features to the resultant dendrimers that are dendrimers reported to date. The introduction of pillar[5]arene “smart” to external inputs. The applications of dendrimers in rotaxane units activates dynamic features in the dendrimer and materials science (5, 6) suggest that rotaxane dendrimers could enhances the rigidity of each branch of the supermolecules. This serve as supramolecular dynamic materials. research offers a facile approach to the construction of high-gen- A variety of rotaxane dendrimers have been designed and con- eration rotaxane branched dendrimer, which not only enriches the structed over the past few years. For examples, mechanically library of rotaxne dendrimer but also provides the further insight interlocked units were used either as cores or end groups, by Vögtle into their applications as supramolecular dynamic materials. and coworkers (7), Stoddart and coworkers (8–13), Gibson et al. (14), Kim and coworkers (15, 16), and Kaifer and coworkers (17, Author contributions: W.W. and H.-B.Y. designed research; W.W., L.-J.C., X.-Q.W., B.S., X.L., Y.Z., J.S., Y.Y., L.Z., and M.L. performed research; W.W. contributed new reagents/ 18). Compared with these simpler systems, rotaxane dendrimers analytic tools; W.W., X.L., and H.-B.Y. analyzed data; and W.W., X.L., and H.-B.Y. wrote with interlocking ring components on the branches or at the branch the paper. points are rare. Specifically, Kim et al. (16) and Leung et al. (19) The authors declare no conflict of interest. have reported the only two cases of rotaxane branched dendrimers This article is a PNAS Direct Submission. up to the second generation. Third- or higher-generation rotaxane 1To whom correspondence should be addressed. Email: [email protected]. dendrimers equipped with mechanically interlocked functions on This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. the branches (Fig. 1) are unknown to us. 1073/pnas.1500489112/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1500489112 PNAS | May 5, 2015 | vol. 112 | no. 18 | 5597–5601 Downloaded by guest on October 6, 2021 1,each31P NMR spectrum of the rotaxane dendrimers displayed a downfield shift (Δδ ’ 2.4 ppm), which also supports the forma- tion of platinum–acetylide bonds during dendrimer growth. As in the 1H NMR spectra, different chemical shifts were observed for Rotaxane the phosphine ligands in each generation in the growth of the rotaxane dendrimers, indicating the nonequivalent chemical en- vironment of the phosphorous ligands (SI Appendix, Fig. S80). MALDI-TOF-MS studies were performed on all of the rotaxane dendrimers. The spectra provided direct support for the formation of mechanically interlocked compounds (Fig. 4). For the first-gen- eration rotaxane dendrimer G1, the MALDI-TOF-MS spectrum in reflectron mode exhibited a single peak at m/z = 6,661.5, which was + Rotaxane Dendrimer attributed to [G1 + H] with a theoretical monoisotopic mass at 6,661.9 Da. This peak was isotopically resolved and agreed well with Dendrimer the theoretical distribution. The corresponding peaks were also ob- served in the MS spectra of the higher-generation rotaxane den- Fig. 1. Schematic representation of a rotaxane dendrimer with mechan- drimers G2 and G3, confirming the synthesis of the targeted ically interlocked moieties incorporated on the branches. compounds. [With increasing molecular weight (for G2, theoretical average Mr = 18,760 Da; for G3, theoretical average Mr = 42,948 v 1 Da), the peaks became broader, with a rational deviation from the ( ) has active alkyne units that can be functionalized to impart theoretical mass in linear acquisition mode. This broadening effect further structural diversity and function. was attributed to the binding of sodium and potassium ions to large We synthesized organometallic [2]rotaxane 1 in a few steps, SI Appendix rotaxane dendrimers, along with the proton signals.] For these high- as indicated in ,SchemeS1. The rotaxane formation step generation architectures, high charge states, i.e., 2+ and/or 3+, were proceeded in good yield (86%) from three components and allowed also observed in MALDI-TOF-MS in addition to singly charged the preparation of 1 on gram scales. The building block 1 proved ions, as shown in Fig. 4 B and C. In the MS spectrum