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Covalent Post-assembly Modification Triggers Structural Transfor-mations of Borromean rings Wen-Xi Gao, Hui-Jun Feng, Yue-Jian Lin, and Guo-Xin Jin J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 22, 2019

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Journal of the American Chemical Society

Covalent Post-assembly Modification Triggers Structural Transformations of Borromean rings Wen-Xi Gaoa, Hui-Jun Fenga, Yue-Jian Lina, and Guo-Xin Jina,* Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, State Key Laboratory of Molecular Engineering of Polymers, Department of Chemistry, Fudan University, Shanghai 200433, P. R. China. a

Supporting Information Placeholder ABSTRACT: A series of supramolecular transformation cascades are presented here, employing ligand exchange reactions, concentration-dependent Borromean ring linking & unlinking, and inverse electron-demand Diels-Alder (IEDDA) reactions. The new family of template-free, tetrazine-edged Borromean rings (BRs) are constructed by using ligand exchange reactions, whose concentration-dependent linking & unlinking reaction has been observed. Moreover, Borromean precursors have been demonstrated to further undergo supramolecular structural transformations induced by rapid and efficient IEDDA reactions to afford corresponding post-assembly modified architectures. Remarkably, subtle steric increases of the pyridazine fragments obtained by IEDDA reactions is regarded to induce controlled topological transformations in the cascade, unlinking the Borromean structures by using electron-rich dienophiles as triggering signals.

Coordination-driven bottom-up approaches are now wellestablished in the construction of a range of aesthetically pleasing supramolecular structures.1-6 Taking advantage of the dynamic and reversible nature of metal-ligand coordination, supramolecular transformation serves as another powerful approach to generate diverse metallosupramolecular architectures with additional specific functions.7-11 Moreover, the disassembly and reassembly processes of the building blocks open up new insights to endow such “smart” structures allosteric regulation responding to the external stimuli.12 Molecular Borromean rings (BRs), a kind of topologically fascinating interlocked architectures, have drawn increasing attention as their topological aesthetics also the potential applications.13-15 To realize these functionality and responsiveness, the selectively construction and controllable transformation of Borromean rings play a pivotal role. Meanwhile, solution-based post-assembly modification (PAM), which has been regarded as a useful tool to reveal the full potential of discrete supramolecular coordination architectures in terms of late-stage derivatization,16-18 have been rarely investigated on such intricate Borromean topologies. Compatible PAM reactions employed to modify such architectures should not interfere with the fragile metal-ligand coordination holding supramolecular structures together19-22 but ideally proceed quantitatively under mild conditions without lengthy purification processes. Taking advantage of its efficient construction of desired products under mild conditions with minimal generation of undesirable by-products, the so-called inverse electron-demand

Diels-Alder (IEDDA) reaction23-24 between electron-rich dienophiles and electron-deficient tetrazines has been regarded as a promising PAM reaction for application to discrete supramolecular coordination complexes without tedious purification steps.25-26 Herein, by utilizing ligand exchange reactions, a series of rationally designed Borromean rings (BRs) containing potentially reactive tetrazine sites are obtained from accessible chloro-bridged macrocycles. Taking advantage of the IEDDA reaction between these tetrazine moieties and electron-rich dienophiles, covalent PAM have been developed to realize topological unlinking transformations from Borromean rings to the corresponding component monomers. To verify the potential of using IEDDA reactions as a general approach for the rapid and facile modification of supramolecular complexes, this reaction was carried out on tetranuclear complex 1, prepared simply by subcomponent self-assembly of organometallic binuclear fragment [Cp*RhCl(OTf)]2 with ligand 3,6-bis(4pyridyl)-1,2,4,5-tetrazine (BPTZ) in high yield (93.1%). NMR spectra were consistent with the expected rectangular symmetry and the HR-ESI-MS data of 1 in CH3CN shows a peak at 932.0153 m/z assigned to [1 − 2OTf]2+. Moreover, a single-crystal X-ray diffraction analysis unambiguously confirmed the rectangular structure of 1 in the solid state with a separation between tetrazine moieties of 3.6 Å (Figure 1a).

Figure 1. X-ray single crystal structures of cations of (a) tetranuclear complex 1 and (b) modified macrocycle 2. Bottom: Scheme of an IEDDA reaction converting the tetrazine-edged macrocycle 1 to pyridazine-edged complex 2. As a proof of this concept, the reactivity of rectangular complex 1 was demonstrated by using common Diels-Alder reagent 2,5-

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norbornadiene (NBD), which generally reacts with tetrazines, expelling one equivalent of the relatively benign cyclopentadiene and nitrogen (as shown in Scheme. S1 in Supporting Information). The reaction between 1 and NBD (2 equiv. per tetrazine) in acetonitrile solution resulted in a significant color change from

scarlet to light yellow, while the clean formation of the modified macrocyclic complex 2 was verified by NMR and ESI-MS after 6 h at 298 K. It is noteworthy that the conversion of tetranuclear complex 1 to 2 occurs cleanly and without degradation of the assembled complex.

Scheme 1. (a) Stimulus-response map depicting the major products expressed in the supramolecular transformation cascades employing ligand exchange reactions, concentration-dependent Borromean ring linking & unlinking, and IEDDA reactions.; (b) The structure of cationic Borromean rings 3-BR in a (from left to right) chemical structural representation, skeleton representation and solid-state structure of one component ring of 3-BR. Most hydrogen atoms, anions, solvent molecules and disorder are omitted for clarity (N, blue; O, red; C, gray; Rh, violet) Moreover, the versatility of the tetranuclear complex 1 was further exploited by soaking crystals of 1 with an excess of 2,5norbornadiene in diethyl ether in a sealed vessel. The extent of conversion was confirmed to be satisfactory (>90%) by NMR analysis of the modified products after rinsing the soaked crystals with ether to remove unreacted 2,5-norbornadiene guests after one week. The PAM process was also observed directly through a gradual color change from red to light yellow, corresponding to the degradation of the intensely colored tetrazine units (as shown in Figure S39 in the Supporting Information). For detailed structural comparison with 1, single-crystal X-ray diffraction analysis data were collected on modified complex 2, revealing an analogous rectangular structure, but constructed with pyridazine edge ligands (Figure 1b). In light of the encouraging PAM reaction of simple tetranuclear complex 1, IEDDA reactions were envisaged to be employed after

a ligand exchange reaction to compose a post-assembly modification cascade, giving a range of more intricate products from chloro-bridged complex 1, as shown in Scheme 1. In the ligand exchange step, the separation between the 3,6-bis(4pyridyl)-1,2,4,5-tetrazine ligand can be rationally increased by using the longer naphthazarin-based fragment (L1) to replace the chloro bridges, creating the Borromean structure complex 3-BR. The stirring of complex 1 with 2 equiv. Na2L1 in acetonitrile resulted in a clear dark green filtrate. The neat 1H NMR spectrum of the low-concentration solution (1.0 mM) clearly indicated the self-assembly of molecular rectangle 3-MONO (see Figure S22, S23 in the Supporting Information). Upon increasing the reaction concentration to 2.0 mM, new peaks were observed along with peaks from 3-MONO in the 1H NMR spectrum that indicated the formation of a new compound. This observation propelled us to carry out reactions in CD3CN in various concentrations from 0.5 to


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Journal of the American Chemical Society 10 mM. The reaction carried out at 10 mM led to almost complete disappearance of the original signal of 3-MONO. A 1H diffusion ordered spectroscopy (DOSY) NMR spectrum of a mixture of 3 (5 mM) in CD3CN also confirmed the NMR signals associated with two diffusion coefficients, namely D = 4.93 × 10–10 m2s–1 for the monomeric ring and 3.99 × 10–10 m2s–1 for the new signals, indicating the generation of more advanced structures. Single crystals X-ray diffraction analysis has confirmed the Borromean structure of 3-BR in the solid state shown in Scheme 1b. As expected, the topologically intricate Borromean rings obtained consist of three mechanically interpenetrating but chemically equivalent rings, each of them adopting a distorted rectangle-like conformation with an average length and width of 15.4 and 8.4 Å, respectively. The three distorted rectangles are held together by donor-acceptor stacking interactions (ca. 3.4 Å) between naphthazarin and electron-deficient tetrazine moieties as evidenced from single-crystal X-ray diffraction analysis (see Figure S33 in the Supporting Information).

Figure 2. (a) In situ 1H NMR monitoring (400 MHz, 298 K, CD3CN) of the reaction between Borromean ring 3-BR (10 mM) and NBD (2 equiv. per tetrazine) to give modified macrocycle 4. (b) 1H NMR spectrum of 3-BR. (c - f) 1H NMR spectrum of the reaction mixture of 3-BR and excess NBD at 0h, 3.5 h, 5 h and 12 h respectively, revealing clean formation of 4 and cyclopentadiene after 12 h. Red circles denote NBD signals; purple triangles denote cyclopentadiene signals. (g) 1H DOSY spectrum of the reaction mixture after 12 h. Unit of D is m2/s.

The success of the IEDDA reaction employed to modified Borromean ring 3-BR was verified by a shift in the 1H resonance for the pyridyl protons from 9.36, 7.94 ppm in 3-BR to 8.51, 8.17 ppm in 4 and the appearance of corresponding new proton resonances assigned to pyridazine at 8.18 ppm. As shown in figure 2, treating 3-BR with NBD in CD3CN at 10mM resulted in almost quantitative formation of pyridazine-edged monomeric complex 4 after 12 h at 298K, together with an equimolar quantity of cyclopentadiene. Furthermore, DOSY NMR spectroscopy of the modified products indicated a slightly smaller hydrodynamic radius than 3-BR, but an almost identical value to that of monomeric ring 3-MONO, confirming that the covalent PAM reaction induces transformations of Borromean ring structure 3-BR to generate corresponding monomeric macrocycle 4. To further investigate the possibility of topological unlinking of Borromean rings upon modification by IEDDA reactions, the lengthened proligand L2 (K2Pd(opba), opba = o-phenylenebis (oxamato)) was used in the place of the naphthazarin fragment to further increase the separation between the parallel tetrazine ligands. Remarkably, the new Borromean structure 5 was obtained as revealed by NMR spectroscopy, ESI-MS and single-crystal Xray diffraction analysis. The 1H NMR spectrum of 5 is much more complex than that of normal metallarectangles, while all peaks between 6.47 and 9.16 ppm displayed a single diffusion constant of 3.55 × 10–10 m2s–1, slightly smaller than that observed for 3-BR, in a DOSY NMR spectrum (Figure S15), suggesting that they belong to a single species of larger size than 3-BR. The structure of 5 in solution was also supported by ESI-MS investigation, where the prominent peaks at m/z = 1489.1831 ([5 − 5OTf−]5+), 1898.7128 ([5 − 4OTf−]4+) and 2581.2666 ([5 − 3OTf−]3+) were in good agreement with their theoretical distribution (Figure S27 - S29). The X-ray crystal structure of 5 unambiguously confirmed its Borromean ring structure (Figure 3), wherein each of the three equivalent rings adopt an inward concave rectangle with dimensions of 17.1 and 10.7 Å (Rh···Rh nonbonding distances). Intriguingly, a subsequent covalent PAM reaction of 5 engendered subtle configurational changes of the corresponding pyridazineedged Borromean structure 6, which inherits a similar geometric construction to pre-modified Borromean ring 5 as shown in Figure 3. The BRs based on the longer precursor L2 retains its configuration as an inseparable ensemble made up by three interpenetrating but non-catenated rectangles after Diels-Alder PAM reactions with the electron-rich dienophile, while the same reaction induces structural transformations from BRs bridged by the naphthazarin fragment (L1) to monomeric rings. Comparing the two different structures resulting from PAM reactions, the insufficient separation of the dipyridine ligands and increased steric hindrance of the postmodified pyridazine ligand in complex 3-BR are likely the determinants to trigger Borromean topological unlinking transformations. In the case of complex 5, a pair of dipyridine ligands pillared by L2 are far enough apart to allow both the tetrazine or modified pyridazine ligands of another ring to thread between them, thus the Borromean structures can form before and after the PAM reaction. By contrast, the distance between Rh centers linked by naphthazarin moieties is only 8.4 Å, which only just allows the smaller pre-modified tetrazine to thread inside it, but not the pyridazine ring with its two additional hydrogen atoms, thus inhibiting the Borromean ring form. Given these differences, the slight further reduction of the space between pre-modified dipyridine fragments, without increasing the steric hindrance via PAM reactions, was also predicted to block the formation of Borromean rings. Thus, 2,5-dichloro-3,6-dihydroxyp-benzoquinone (L3), a similar but slightly shorter (0.4 Å) bridge ligand than L1, was strategically selected to test this hypothesis. As expected, the combination of L3 and the tetrazine ligand afforded



tetranuclear monomeric macrocycle 7, which was characterized by NMR and single-crystal X-ray diffraction analysis. As shown in Figure S40 in Supporting Information, compared with Borromean ring structure 3 wherein the narrow separation of dipyridine ligands is strained to its limit (8.6 Å) to allow another ligand to thread

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through it, the further decreased distance between the tetrazine ligands linked by the chloranilicacid no longer allows other tetrazine moieties through the macrocycle to form Borromean rings.

Figure 3. (a) Self-assembly of Borromean ring 5 and its modification giving pyridazine-edged Borromean ring structure 6 by treatment with NBD; (b) Chemical structural representation of 5 and 6; (c) solid-state structure of one component monomer of 6 for clarity (C, gray; N, blue; O, red; F, green; S, yellow; Rh, violet; Pd, orange); (d) solid-state structure of modified BRs 6 hosting a triflate anion inside its central cavity. In conclusion, we present a series of supramolecular transformation cascades employing ligand exchange reactions, concentrationdependent Borromean ring linking & unlinking, and IEDDA reactions. As a proof-of-concept, the starting chloro-bridged tetranuclear macrocycle 1 has been demonstrated to undergo IEDDA reactions to form the corresponding pyridazine-edged complex 2 both in solution and crystalline form. Accordingly, the verified PAM reaction, with NBD serving as external stimulus, was then further employed to trigger supramolecular transformation of Borromean ring structure 3-BR to obtain monomeric ring 4. By varying the separation of parallel tetrazine ligands, the increased steric hindrance of the modified pyridazine ligand is shown to be crucial to induce unlinking of the Borromean ring. We believe that a deeper understanding of supramolecular transformation cascades induced by PAM reactions will be helpful to construct bespoke architectures and responsive supramolecular systems, opening up new insights into the field of smart materials.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.XXXXXXX.

ASSOCIATED CONTENT

ACKNOWLEDGMENT

Experimental procedures, rystallography

spectroscopic

data

and

X-ray (PDF).

X-ray data for 1 (CCDC 1901497), 2 (CCDC 1901498), 3-BR (CCDC 1901499), 5 (CCDC 1901500), 6 (CCDC 1901501), 7 (CCDC 1901502) (CIF).

AUTHOR INFORMATION Corresponding Author *[email protected]

Notes The authors declare no competing financial interests.

Supporting Information


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Journal of the American Chemical Society We are indebted to the National Science Foundation of China (21531002, 21720102004) and the Shanghai Science Technology Committee (13JC1400600) for continued financial support over the years of our studies in supramolecular chemistry. G.-X.J. thanks the Alexander von Humboldt Foundation for a Humboldt Research Award.

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