Highly Stable Spherical Metallo-Capsule from a Branched Hexapodal

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Highly Stable Spherical Metallo-capsule From a Branched Hexapodal Terpyridine and Its Self-Assembled Berry-type Nanostructure Mingzhao Chen, Jun Wang, Die Liu, Zhilong Jiang, Qianqian Liu, Tun Wu, Haisheng Liu, Weidong Yu, Jun Yan, and Pingshan Wang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10707 • Publication Date (Web): 27 Jan 2018 Downloaded from http://pubs.acs.org on January 27, 2018

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Highly Stable Spherical Metallo-Capsule From a Branched Hexapodal Terpyridine and Its Self-Assembled Berry-type Nanostructure Mingzhao Chen,‡ Jun Wang,‡ Die Liu, Zhilong Jiang, Qianqian Liu, Tun Wu, Haisheng Liu, Weidong Yu, Jun Yan, Pingshan Wang* Department of Organic and Polymer Chemistry, College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan-410083, China Terpyridine, Spherical structure, Hierarchical self-assembly, Host–guest chemistry ABSTRACT: Discrete spherical metallo-organic capsules at the nanometer scale, especially those constructed from unique building blocks, have received significant attention recently because of their fascinating molecular aesthetics and potential applications due to their compact cavities. Here, the synthesis and characterization of a hexapodal, branched terpyridine ligand is presented along with the nearly quantitative self-assembly of the resulting tetrameric metallo-nanosphere. This metallo-nanosphere exhibited four quasi-triangular and six rhombus-like facets, all of which were made by the same hook-like bisterpyridine. Collision-induced dissociation experiments were done to investigate overall stability. The metallo-architecture and host–guest chemistry was investigated with coronene and fully characterized by 1D and 2D NMR, ESI−MS, and transmission electron microscopy. Furthermore, this metallo-nanosphere was observed to hierarchically self-assemble into berry-type structures in an acetonitrile/methanol mixture, in virtue of counterion-mediated attractions. The functional molecular metallo-nanosphere presented here expands the reach of terpyridine coordination systems into molecular containers and other model systems.

INTRODUCTION Inspired by the aesthetically pleasing structures of various natural virus capsids and naturally beautiful polyhedrons both in the biological and mineral worlds,1-3 chemists have designed and synthesized a number of elegant three-dimensional (3D) molecular mimicks.4,5 These objects not only permeate the literature as a form of nano-art but also let us view chemistry from a different perspective. Most of recently reported 3D capsules (organic and inorganic) with size- and shape-discrimination were formed via hydrogen bonding6,7 dynamic covalent chemistry,8,9 or coordination.10-12 These 3D molecular structures have attracted significant attention as a result of the potential applications arising from their hollow cavities such as host-guest chemistry,13,14 chemical separations,15,16 catalytic chemistry,17,18 reaction control inside the cavity and more.19,20 The synthesis of various categories of discrete 3D supramolecular structures is a challenge due to many arduous limitations; one major drawback is that the undefined directionality of multiple coordinating ligands tend to form polymer networks or mixtures.21 For pure organic systems, Cooper22 and Mastalerz9 et al. presented the formation of different 3D architectures including octahedral, cubic and cuboctahedral structures by using imineconnectivity and boronate ester chemistry. Atwood 23,24 and Rebek25,26 et al. reported a series of functional nanocapsules based on noncovalent interactions. Over the past few decades, the coordination-driven self-assembly

has been used to construct simple and elaborate 3D supramolecular architectures, and it is worth mentioning that N-donor bridging accounted for most of chemistry in this area. Notable work was demonstrated by Fujita,27,28 Stang,29-31 Nitschke,32,33 Clever34,35 and others36-40 since the 1990s. Some of well-constructed 3D spherical metal complexes successfully synthetized by Fujita et al. were made from functionalized angled bispyridine ligands and palladium(II) ions having the formula M nL2n, in which n is geometrically constrained to be 6, 12, 24, 30, and even 48.41 The above reported discrete 3D architectures were mainly synthetized upon reaction of ditopic or tritopic ligands. Terpyridine (tpy) units have been highlighted for their use in the preparation of unusual coordination nanostructures, especially those which are unattainable through pyridine-based and other ligands.42-44 Though limited new 3D supramolecular structures demonstrate the utility of the coordinating self-assembly of the tpy ligand with metal ions,45-47 one-step assembly for a stable metallo-capsule is quite challenging partly due to the limitation of the proper geometry and orientation required for the tpyM(II)-tpy connectivity. More recently, Newkome et al. reported a series of concentration-dependant 3D metallospheres by utilizing rigid or flexible multitopic tpy ligands. 48-52 In another recent example, a Ru 6L4 metallonanosphere was completed in 35% yield, where L was a tripodal ligand. However, attempts to exchange the coordinating metal ions from Ru 2+ to Cd2+, Fe2+ or Zn2+ for the

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Figure 1. Cartoon representation of [Zn6L4]12+ and stable 24+ [Zn12LH4] metallo-nanosphere based on tripodal and hexapodal ligands.

sake of quantitative self-assembly failed to gain the expected M6L4 (M = Cd, Fe or Zn). In addition, a 3D metallo-capsule derived from the rigid pentaterpyridine or hexaterpyridine has not been reported so far. Herein, we present a C6 symmetrical homologous hexapodal ligand LH that could assemble with Zn2+ to generate a metallonanosphere [Zn12LH4]24+ in nearly quantitative yield. The successful construction of the metallo-nanosphere may be due to the more compact characteristics achieved upon complexation with the zinc ions, thus providing more benefits for encapsulating guest molecules (Figure 1). Based on intermolecular interactions (e.g., π−π stacking, counterion-mediated attraction), we utilized [Zn12LH4]24+ for encapsulating a coronene guest, and investigated the self-assembling into berry-type nanostructures.

RESULTS AND DISCUSSION Ligand synthesis and metallo-nanosphere assembly. The original design involved a nonplanar hook-like ditopic bisterpyridyl ligand LB (Scheme 1). A twofold Suzukicoupling reaction of 4,4''-dibromo-1,1':2',1''-terphenyl (with 60°angle) and 3-terpyridinyl-B(OH)2 was conducted by employing Pd(PPh3)4 as the catalyst (Supporting Information, Scheme S1). After purification by column chromatography (Al 2O3) eluting with the mixed solvent of petroleum ether and CH 2Cl2, LB was obtained as pure white solid (75%). When treating LB with Zn(NO3)2·6H2O in a stoichiometric ratio of 1:1 in CHCl3/MeOH (1:1.5, v/v) at 75 °C for 6 h (Scheme 1), the coordination dimer [Zn2LB2]4+ was obtained (95%) as a pale yellow solid after counterion exchange with excess NH4PF6 in MeOH. Chemical shifts ( 1H NMR) of [Zn2LB2]4+ showed a single peak for tpyH3',5', confirming that the product was a pure complex (Figure S1). Further evidence was provided by ESI-MS (Figure S1); the intense signals in charge states 4+ (m/z 455.09), 3+ (m/z 655.11) and 2+ (m/z 1055.16), with its distinguishable isotope patterns for each charge state, verified the 3D metallo-macrocyclic, rhombus-like dimer [Zn2LB2]4+ with 4 PF6- counterions. Scheme 1. (A) Synthetic route to ligand LB and LH. (B) Self-Assembly of rhombus-like dimer [Zn2LB2]4+ and metallo-nanosphere [Zn12LH4]24+.

Following the initial results, a more complicated hexapodal organic ligand LH was designed and synthesized. A benzene core was functionalized with six 120°angular terpyridines, where all terpyridyl arms could rotate freely. Treating LH with Zn2+ yielded metallo-nanosphere [Zn12LH4]24+ quantitatively via self-assembly (Scheme 1). The sphere-like structure was supported by 1D and 2D NMR and ESI-MS. The nano-capsule, made of 16 components, has a high degree of symmetry. It possessed four quasi-triangular and six rhombus-like faces; notably, all faces/edges were of free-form curved surfaces. Compared with the rhombus-like dimer [Zn2LB2]4+, the quasitriangular face was made from the same hook-like bisterpyridine LB; that is, the ligand with the same angles and degree of rotational freedom gave different products. The key hexapodal terpyridine LH was generated through a six-fold Suzuki-coupling reaction of hexa(4bromophenyl)benzene and 3-terpyridinyl-B(OH)2, with Pd(PPh3)4 as the catalyst (Scheme S2). LH was purified through recrystallization from MeOH/CHCl 3. The 1H NMR spectrum of LH showed two singlets at 8.65 ppm and 8.00 ppm having a 2:1 radio assigned to tpyH3',5' and Ph-Ha, respectively. Other protons assignments were confirmed by the 2D COSY and 2D NOESY. Direct evidence for the the ligand was confirmed by the high-resolution ESI-MS with m/z peak at 2400.85 (M+Na+) (ESI, Figure S25). Mixing LH and Zn(NO3)2·6H2O in a 1:3 molar ratio in CHCl3/MeOH (2:3, v/v) at 75 °C for 12 h, a clear solution was achieved. After cooling at ambient temperature, excess NH 4 PF 6 in MeOH was added to produce a paleyellow precipitate, which was filtered and washed thoroughly with MeOH. Complex [Zn12LH4]24+ with 24 PF6counterions was obtained (96%) as a faint yellow solid. Initial analysis on the metallo-sphere was conducted by 1 H NMR (Figure 2A), which showed two single peaks at 8.87 ppm and 8.27 ppm with a 2:1 ratio attributed to tpyH3',5' and Ph-Ha, respectively. The results were similar


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Figure 2. (A) Comparison of the 1H NMR spectra (500 MHz) of star-shaped ligand LH in CDCl3 and assembled metallo24+ 24+ nanosphere [Zn12LH4] in CD3CN; (B) DOSY of [Zn12LH4] (500 MHz, 25 °C, CD3CN).

of [Zn12LH4]24+ was shown through the peaks of tpyH6,6'' and tpyH5,5'' shifting upfield characteristically upon complexation, owing to the electron shielding effects. Other proton peak assignments were confirmed based on the 2D COSY and 2D NOESY NMR experiments (Figures S15 and S16). Remarkably, in comparison with the 1H NMR spectrum of ligand LH, the original two signals of PhHe,f,g,h were split into four signals in spheric [Zn12LH4]24+, indicating Ph-He,f and Ph-Hg,h were no longer the same. The Ph-He,f at δ = 7.40 ppm was split into two doublets at δ = 7.48 and 7.43 ppm, while the Ph-Hg,h at δ = 7.15 ppm was split into two doublets at δ = 7.73 and 7.65 ppm (Figure 2A). The two different chemical environments was compelling evidence for the endo–exo conformation of the tpy arms, where all six arms were limited in what might be an unfavourable conformation (all are in one direction) after forming the favoured metallonanosphere.53,54 In addition, the diffusion-ordered NMR spectroscopy (DOSY) spectrum was done to roughly estimate radii for the metallo-nanosphere structure (Figure 2B), with a single band near a diffusion coefficient D= 2.59 × 10−10 m2 s−1. According to the Stokes-Einstein Equation, the diameter of the spherical complex was ca. 4.6 nm. The result was consistent with the molecular modelling data. The HR ESI-MS spectrum of [Zn12LH4]24+ revealed a series of peaks at m/z 839.25, 914.51, 1003.23, 1107.79, 1232.86, 1386.04, 1577.31 and 1823.34 corresponding to

to LH, indicating that a single and highly symmetrical complex was generated. More evidence for the formation

Figure 3. (A) ESI-MS of [Zn12LH4]24+; (B) Three charge states of [Zn12LH4]24+, Theoretical (top, blue) and experimental (bottom, red).



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the charge states from 14+ to 7+, respectively (Figure 3A). The experimental m/z values and isotope patterns for all charge states were consistent with the corresponding theoretically calculated values of [Zn12LH4]24+ (Figure 3B). Upon changing the counterion from PF 6− to BPh4−, the structure of [Zn12LH4]24+ remained stable (Figure S21). The clear-cut size of the metallo-nanosphere facilitated further characterization by TEM (Figure 4).The sample was dissolved in MeCN at the concentrations of ~10−7 M, and then the solution was drop casted onto a carboncoated copper grid (Cu, 400 mesh); extra solution was absorbed by filter paper to avoid aggregation. TEM images provided visualization of [Zn12LH4]24+, displaying both the size and shape of dispersed molecules directly. The outline of a single molecule located on the film where dots with uniform size could be observed. The measured mean diameter of 4.2 nm correlates well with the size shown in optimized molecular models.

24+

Figure 5. ESI-MS of [Zn12LH4] at m/z 1577.31 with different 24+ collision energies and ESI-MS of [Zn12LH4] (PF6)21(NO3)3 at m/z 1546.17 as a representative dissociation peak and the fragmentation behavior.

24+

Figure 4. TEM image of [Zn12LH4] . The high-magnification TEM image showed the round-shaped dots and the size is consistent with energy-minimized structures from molecular model24+ ing of [Zn12LH4] . 24+

The stability study of [ Zn12LH 4] According to the concept of “density of coordination sites” (DOCS),55 more coordination sites show more constraints, and the new pattern of metallo-nanosphere [Zn12LH4]24+ may possess a high degree of enhanced stability owing to its multiple coordination sites. Currently, the collision-induced dissociation technique has been employed to study the stability of small molecules5657 and terpyridine complexes.58-59 Therefore, we conducted the ESI-MS collision-induced dissociation experiments to investigate the stability of [Zn12LH4]24+, which revealed the exceptional stability of [Zn12LH4]24+ at collision energies ranging from 5 to 40 eV with initial dissociation at 20 eV. 8+ ions at m/z 1577.31 and 9+ ions at m/z 1380.10 were chosen as representative peaks (Figure 5 and S31). Interestingly, at m/z 1567.06, an NO3− ion remained, likely the result of an incomplete counterion exchange from NO3− to PF6− (ion exchange was performed by adding excess amount of NH 4PF6); as a result, remaining NO3− anions may be held at the center of the hollow cavity. As the collision energies increase, the signal of incomplete exchange disappeared owing to the weak intercalation. Nevertheless, a new signal at m/z 1546.17 (+8 ions) was rising, probably from the fracture of one of the tpyZn(II)-tpy moieties. The ESI-MS spectrum of [Zn12LH4]24+ (PF6)21(NO3)3 at m/z 1546.17 and 1358.28 was considered as the fragmentation behavior at 8+ and 9+, respectively (Figures

S30-S32), which showed similar ligand-binding strengths of bis-cyclometalated Ir(III) coordination complexes.58

Host–guest chemistry with Coronene. The internal volume of [Zn12LH4]24+ is estimated to be ca. ~9800 Å3 (the average distance across the interior is ∼30 Å, Figure S36);48,51 thus, the large cavity should be an ideal place for conducting host-guest chemistry and molecular recognition. Previous to this, metal-linked capsules have been used in encapsulating different guest molecules, the great work was reported by Fujita,28,60 Nitschke61,62 and others63-65. Notably, the functional molecular capsules use large aromatic panels rather than wirelike frameworks to reinforce the structure of the capsule, ensuring the guest molecules can be fully enclosed. 66-67 However, the tpy-based 3D supramolecular capsule has only been documented with the anion encapsulation.50,52 From a structural standpoint, the closed molecular nanosphere [Zn12LH4]24+ as well as the hexaphenylbenzene core from ligand LH may be employed as an ideal candidate for host-guest interactions. The initial study of aromatic encapsulating interactions was conducted utilizing coronene. Ligand LH and Zn(NO3)2•6H2O were mixed in a molar ratio of 1:3 and then coronene was added in excess to the reaction in a solvent of CHCl3/CH3OH. The prepared host–guest complexes could be confirmed by 1H NMR spectrum and ESI-MS. As shown in Figure 6A, the one-pot synthesis produced two 1H NMR signals for coronene, one was free coronene (exo), the other was encapsulated (endo), noted by the significant chemical shift from 9.05 to 7.60 ppm. This suggests the presence of coronene⊂[Zn12LH4]24+ complexation. Compared to the 1H NMR spectrum of host complex [Zn12LH4]24+, the 1H NMR of host–guest complex coronene⊂[Zn12LH4]24+ became a


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Journal of the American Chemical Society showed the upfield (endo) coronene signal was positioned within the [Zn12LH4]24+ band, suggested that coronene was within the cavity. Further evidence for the encapsulating was obtained by the ESI-MS spectrum (Figures 6B and S23), the HR ESI-MS of host– guest complex exhibited peaks for {coronene⊂[Zn12LH4]24+10PF6-}10+ and {coronene⊂[Zn12LH4]24+-9PF6-}9+ at m/z 1263.01 and 1419.23, respectively. All ESI-MS signals agreed well with their theoretical calculated isotopic values.

Figure 6. (A) Comparison of the 1H NMR spectra (500 MHz, in 24+ CD3CN) of the mixture containing coronene⊂[Zn12LH4] and 24+ 24+ empty [Zn12LH4] , coronene (middle) and [Zn12LH4] (bot24+ tom); (B) Two charge states of coronene⊂[Zn12LH4] , Theoretical (top, blue) and experimental (bottom, red).

Self-assembly berry-type nanostructure. More interestingly, the metallo-nanosphere [Zn12LH4]24+ was found to self-assemble into berry-type structures in a mixed polar solvent. Liu et al. initially presented the self-assembly of viral-capsid-type vesicle-like structures from cationic metal–organic nanocages, and demonstrated that macrocations could act as counterion-mediated attraction similarly to POM macroanions self-assembly behavior.68-70 So far, those vesicle-like structures were obtained mostly from pyridine-based metallo-supramolecular structures. Nevertheless, such type of terpyridine-functionalized nanosphere has rarely been reported. Because the hydrophilic centers (charges at the Zn (II) metal centers) and the hydrophobic parts (hexapodal aromatic ligands), electrostatic and hydrophobic interactions could cause metallo-nanosphere [Zn12LH4]24+ (the counterions were nitrates) to exist as macrocations in polar solutions. Therefore, we chose a mixed solution of methanol and acetonitrile to study the solution aggregation behaviours. The sample was dissolved in CH3CN/MeOH (1:1, v/v) at the concentrations of ~10−6 M, TEM experiments provided evidence of the presence and morphology of the berry-type structures. As shown in Figure 8A-C, the berry-like balls had a relatively uniform size (∼33 nm), and there were a few berry-type ball aggregations (Figures 8A-B and S34), likely due to the multiple intermolecular interactions (e.g., π−π stacking). Moreover, some of the berry-type structures were collapsed and hence showed a biomembrane-like nature, which was similar to the reported result.68 The sizes were also consistent with the dynamic light scattering (DLS) results, which gave an experimental average hydrodynamic size of the berry-type structure of d = 30±1.0 nm (Figure 8D). Scanning transmission electron microscope (STEM) and STEM mapping (zinc and nitrogen) analyses supplied the further evidences of elemental compositions for berry-like nanospheres (Figure S35). Scheme 2. Schematic illustration of self-assembly of nanosphere [Zn12LH4]24+ into spherical berry structures.

Figure 7. (A) 2D DOSY spectrum (500 MHz) for the reaction 24+ mixture containing Coronene ⊂ [Zn12LH4] and empty 24+ [Zn12LH4] in CD3CN (peaks of coronene were labeled with blue cycles). little broad, likely due to free [Zn12LH4]24+ and coronene⊂[Zn12LH4]24+ coexisting. By controlling the proportion of coronene, the encapsulated coronene 1H NMR signal showed different intensity (Figure S19); the larger proportion of coronene in the reaction mixture resulted in a more intense signal of the encapsulated coronene. The 2D DOSY spectrum (Figure 7)



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ACKNOWLEDGMENT This research was supported by the National Natural Science Foundation of China (21274165 for PW, 51634009 and 51374247 for Wei Sun); the Distinguished Professor Research Fund from Central South University of China; Authors thank the collaboration with College of Mineral Processing and Bioengineering, Central South University of China; The authors grateful acknowledge Prof. Dr. Carol D. Shreiner for her professional consultation. Authors acknowledge the TEM and NMR measurements from the Modern Analysis and Testing Center of Central South University of China.

REFERENCES

Figure 8. TEM images of berry-type structures generated from − [Zn12LH4]24+ (24 NO3 as counterions, in CH3CN/MeOH). (A) Aggregated berry-typed ball network; (B) Two connected ball aggregations; (C) Single ball dispersed. (D) Dynamic light scattering (DLS) of berry-type structures.

CONCLUSIONS In summary, a well-defined spherical molecular architecture was constructed in nearly quantitative yield by using a hexapodal, branched terpyridine ligand via one-step assembly. The strategy of increasing coordination numbers stabilized the large, hollow 3D metallo-organic complex. High resolution ESI-MS, NMR, 2D-NOESY, and DOSY spectroscopies as well as TEM measurements unambiguously supported the formation of spherical metallo-architecture. More importantly, the fascinating metallo-nanosphere retained a relatively big and compact internal cavity, and thus became a reasonable encapsulating model for conducting research on host-guest chemistry and molecular recognition. An aromatic interaction was achieved by utilizing coronene to generate host-guest complex coronene⊂[Zn12LH4]24+. Further investigations on molecular reactor applications is currently ongoing.63,64,71 In addition, the nanosphere [Zn12LH4]24+ was shown to sequentially aggregate into large berry-like ball.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org at DOI. Experimental procedures and characterization data, including 1H, 13C, COSY, NOESY, and DOSY spectra of the new compounds and ESI-MS spectra of related compounds (PDF)

AUTHOR INFORMATION Corresponding Author * P. Wang: [email protected]

ORCID Pingshan Wang: 0000-0002-1988-7604

Author Contributions ‡ M.C. and J.W. contributed equally to this work.

Notes The authors declare no competing financial interest.

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Highly Stable Spherical Metallo-Capsule From a Branched Hexapodal Terpyridine and Its Self-Assembled Berrytype Nanostructure Mingzhao Chen, Jun Wang, Die Liu, Zhilong Jiang, Qianqian Liu, Tun Wu, Haisheng Liu, Weidong Yu, Jun Yan, Pingshan Wang*

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Highly Stable Spherical Metallo-Capsule from a Branched Hexapodal

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