Preparation of Anionic Metal-Seamed Pyrogallol[4]arene

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Preparation of Anionic Metal-Seamed Pyrogallol[4]arene Nanocapsules via Surface Functionalization Chen Zhang,† Rahul S. Patil,† Charles L. Barnes,† and Jerry L. Atwood*,† †

Department of Chemistry, University of Missouri-Columbia, 601 South College Avenue, Columbia, Missouri 65211, United States S Supporting Information *

ABSTRACT: We present here the preparation of anionic dimeric and hexameric C-alkylpyrogallol[4]arene nanocapsules by attaching multiple nitrate ions to the nanocapsule surface via metal coordination. These anionic nanocapsules can be utilized for encapsulation of cationic guest molecules and have potential applications in catalysis, sensing, and biomedicine. This study demonstrates that the surface functionalization may act as an effective method to tailor and introduce new properties to metal− organic nanocapsules. Scheme 1. C-Alkylpyrogallol[4]arenea

R

ational design and preparation of novel supramolecular nanocapsules have aroused a surge of interest among nanoscientists.1−11 These supramolecular species not only show architectural beauty by mimicking biological structures found in nature, but also have potential applications in many areas such as biomedicine,12,13 catalysis,14−17 selective gas adsorption,18,19 and sensing. 20,21 The self-assembly of multiple small components via noncovalent interactions, typically hydrogen bonding and metal coordination, results in the formation of a large group of capsules/cages with nanometer-sized cavities. For example, Rebek and co-workers reported the first tennisball-shaped nanocapsule by utilizing hydrogen bonding between two hemispheres. Fujita,10,22 Stang,2,11 and co-workers have conducted extensive research on self-assembly of cationic metal−organic nanocapsules (MONCs) via coordination between palladium ions and various pyridine-like bridging ligands. Raymond and co-workers have developed a watersoluble anionic capsule by utilizing coordination between Ga3+ or Fe3+ and diaminonaphthalene biscatechol amide ligands.4 This capsule contains a large hydrophobic cavity (300−500 Å3) which has been utilized to encapsulate cationic guests.17,20 In our previous studies, we have shown various dimeric and hexameric MONCs via self-assembly of C-alkylpyrogallol[4]arene (PgCn, Scheme 1) and metal ions such as Zn2+, Cu2+, Ni2+, Co2+, Mg2+, and Ga3+.23−31 The dimeric nanocapsules are typically constructed from eight metal ions and two pyrogallol[4]arene molecules, while the hexameric nanocapsules usually involve 24 metal ions and six pyrogallol[4]arene molecules. As opposed to the work cited above on MONCs, all dimeric/hexameric pyrogallol[4]arene MONCs are neutral. As the emphasis switches from structure to function, functionalization of MONCs has gathered increasing attention as an alternative way to introduce new function and properties. One strategy to functionalize MONCs is using organic ligands with various pendant groups, which has been widely applied in the synthesis of functional metal−organic species.12,32 Another © XXXX American Chemical Society

a

R is the functional group at the bridging carbon of pyrogallol[4]arene.

strategy is to functionalize MONCs via metal coordination. For instance, we have reported that surface functionalization of MONCs with multiple pyridine (py) molecules prevents them from further assembling into hierarchical structures and leads to the formation of discrete MONCs.29 In this communication, we present the preparation of anionic dimeric (I, [Co8(PgC3OH)2(py)8(NO3)]−[Hpy]+ · DMF) and hexameric (II, [Mg24(PgC6)6(H2O)44(NO3)4]4−[Hpy]4+ · 2py) pyrogallol[4]arene MONCs by surface functionalization with multiple nitrate groups via metal coordination. The molecular structures of these anionic nanocapsules have been determined by single crystal X-ray diffraction (SCXRD), and structural differences are revealed by comparison with their neutral counterparts (III, [Co 8 (PgC 2 ) 2 (py) 11 · 7py], and IV, [Mg24(PgC3)6(DMF)4(H2O)44]). The synthesis of anionic dimeric capsule, I, is accomplished by thoroughly mixing 0.1 mmol of C-propan-3-olpyrogallol[4]arene (PgC3OH) and 0.4 mmol of cobalt nitrate in N,Ndimethylformamide (DMF)/acetonitrile (MeCN) (1:1), followed by the addition of 100 μL of pyridine (see Supporting Received: July 12, 2017 Published: July 24, 2017 A

DOI: 10.1021/acs.cgd.7b00968 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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prepared from a solvothermal reaction between PgC3 and Mg(NO3)2 in DMF/MeCN (1:1) with the aid of imidazole as a modulator.28 Herein, we applied the same strategy and successfully synthesized an anionic hexameric pyrogallol[4] arene nanocapsule (II) by attaching multiple nitrate ions to the nanocapsule surface. In a typical synthesis, C-hexylpyrogallol[4]arene (PgC 6 ), magnesium nitrate, and pyridine (PgC6:Mg2+:py = 1:4:12) are thoroughly mixed in MeCN, and then heated in an oven at 100 °C overnight to yield a dark green solution (see Supporting Information for details). After cooling to room temperature for a few hours, large green crystals are collected and taken for SCXRD analysis. Nanocapsule II crystallizes in the triclinic space group P1,̅ while the crystal structure of IV reveals the monoclinic space group C2/m. The core structure of both II and IV can be viewed as a truncated octahedron with a [Mg3O3] sixmembered unit on each face (Figures 2 and S2). A single

Information for details). Thereafter, the reaction mixture is heated in an oven at 100 °C overnight to yield a dark purple solution. After cooling and being held at room temperature for 1 week, brown plate-like crystals are produced. Its neutral counterpart, III, has already been reported in our previous study (CCDC 742756),24 which can be synthesized by mixing MeCN solutions of Co(NO3)2 · 4py and PgC2 followed by recrystallization of the resultant precipitate in pyridine. SCXRD analysis reveals that compound I crystallizes in the monoclinic space group P21/n while III has an orthorhombic unit cell in the space group Fddd. The core structure of both I and III contains eight cobalt ions and two C-alkylpyrogallol[4]arene ligands (Figures 1 and S1). In each pyrogallol moiety,

Figure 1. Top view of cobalt-seamed dimeric pyrogallol[4]arene nanocapsules I (A) and III (B), showing the coordination environment for cobalt centers around the nanocapsule periphery. Color codes: cobalt (light blue), carbon (gray), oxygen (red), and nitrogen (blue). Hydrogen bonds are represented by red dashed lines. Hydrogen atoms and alkyl tails of pyrogallol[4]arene have been omitted for clarity.

Figure 2. Side view of magnesium-seamed hexameric pyrogallol[4]arene nanocapsules II (A) and IV (B). Color codes: magnesium (green), carbon (gray), oxygen (red), and nitrogen (blue). Hydrogen bonds are represented by red dashed lines. Hydrogen atoms, axial water ligands around magnesium centers that are not hydrogenbonded to pyridine molecules, and alkyl tails of pyrogallol[4]arene have been omitted for clarity.

two of the three phenolic groups are deprotonated to counterbalance the charges of eight Co2+ cations, which makes III a neutral capsule. The nondeprotonated phenolic groups afford the formation of strong O−H···O hydrogen bonds (O···O = 2.39−2.41 Å). Capsule I has one nitrate ion η2coordinated with a cobalt ion, which endows the capsule with one negative charge (Figure 1A). This extra charge is counterbalanced by a pyridinium cation located within the capsule (Figures 1A and S1A). The endo-capsule pyridinium ion is hydrogen-bonded to the nanocapsule interior, as evidenced by the close distance between the nitrogen atom of the pyridine molecule and oxygen atom of a phenolic group (N···O = 3.00 Å). The exterior eight pyridine ligands and one nitrate ion result in an irregular 6−5−5−6−5−5−5−5 coordination pattern around the capsule periphery (Figure 1A). The pyridinium ion within capsule I provides a general model for encapsulating cationic guest molecules using this anionic nanocapsule as a host. The interior of capsule III is occupied by a disordered pyridine molecule which also shows a weak interaction with a Co center (N···Co = 2.82 Å) (Figures 1B and S1B).24 Together with ten exterior pyridine ligands, capsule III employs an irregular 6−5−6−5−6−5−5−5 coordination pattern around the capsule periphery (Figure 1B). We recently reported a magnesium-based hexameric pyrogallol[4] arene nanocapsule, IV (CCDC 1543737),

capsule contains 24 Mg2+ and six C-alkylpyrogallol[4]arene molecules. Two of the three phenolic groups in the pyrogallol moieties are deprotonated to counterbalance the charges of the 24 Mg2+ cations, which makes the net charge of IV zero. The crystal structure of II shows that the exterior axial positions of the Mg2+ are occupied by four nitrate ions and 20 water molecules, while 24 water molecules are axially cooordinated with the Mg2+ on the interior of the nanocapsule (Figures 2A and S2A). The exterior four nitrate ions endow the nanocapsule with four negative charges that are counterbalanced by four pyridinium cations adjacent to the nitrate ions. Instead of acting as a bidentate ligand in I, these nitrate ions are monodentate, which result in six-coordination for all magnesium ions. In addition, two pyridine molecules located at the orthogonal direction are hydrogen-bonded to the exterior axial water ligands evidenced by their short distance (N···O = 2.72 Å) (Figures 2A and S2A). By contrast, the four exterior nitrate ions are replaced by four DMF molecules in the crystal structure of IV (Figures 2B and S2B). The anionic hexameric nanocapsule retains the large enclosed cavity as found in its neutral counterpart (1400 Å3)28 and offers the chance for encapsulation of cationic guest molecules within the cavity. B

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(13) Jung, M.; Kim, H.; Baek, K.; Kim, K. Angew. Chem., Int. Ed. 2008, 47, 5755. (14) Yoshizawa, M.; Klosterman, J. K.; Fujita, M. Angew. Chem., Int. Ed. 2009, 48, 3418. (15) Kang, J.; Rebek, J. Nature 1997, 385, 50. (16) Brown, C. J.; Toste, F. D.; Bergman, R. G.; Raymond, K. N. Chem. Rev. 2015, 115, 3012. (17) Kaphan, D. M.; Levin, M. D.; Bergman, R. G.; Raymond, K. N.; Toste, F. D. Science 2015, 350, 1235. (18) Mastalerz, M.; Schneider, M. W.; Oppel, I. M.; Presly, O. Angew. Chem., Int. Ed. 2011, 50, 1046. (19) Patil, R. S.; Banerjee, D.; Zhang, C.; Thallapally, P. K.; Atwood, J. L. Angew. Chem., Int. Ed. 2016, 55, 4523. (20) Dalton, D. M.; Ellis, S. R.; Nichols, E. M.; Mathies, R. A.; Toste, F. D.; Bergman, R. G.; Raymond, K. N. J. Am. Chem. Soc. 2015, 137, 10128. (21) Wang, J.; He, C.; Wu, P.; Wang, J.; Duan, C. J. Am. Chem. Soc. 2011, 133, 12402. (22) Sun, Q.-F.; Iwasa, J.; Ogawa, D.; Ishido, Y.; Sato, S.; Ozeki, T.; Sei, Y.; Yamaguchi, K.; Fujita, M. Science 2010, 328, 1144. (23) Power, N. P.; Dalgarno, S. J.; Atwood, J. L. Angew. Chem. 2007, 119, 8755. (24) Atwood, J. L.; Brechin, E. K.; Dalgarno, S. J.; Inglis, R.; Jones, L. F.; Mossine, A.; Paterson, M. J.; Power, N. P.; Teat, S. J. Chem. Commun. 2010, 46, 3484. (25) Kumari, H.; Dennis, C. L.; Mossine, A. V.; Deakyne, C. A.; Atwood, J. L. ACS Nano 2012, 6, 272. (26) McKinlay, R. M.; Cave, G. W. V.; Atwood, J. L. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 5944. (27) Rathnayake, A. S.; Feaster, K. A.; White, J.; Barnes, C. L.; Teat, S. J.; Atwood, J. L. Cryst. Growth Des. 2016, 16, 3562. (28) Zhang, C.; Patil, R. S.; Li, T.; Barnes, C. L.; Atwood, J. L. Chem. Commun. 2017, 53, 4312. (29) Zhang, C.; Patil, R. S.; Liu, C.; Barnes, C. L.; Atwood, J. L. J. Am. Chem. Soc. 2017, 139, 2920. (30) McKinlay, R. M.; Thallapally, P. K.; Cave, G. W. V.; Atwood, J. L. Angew. Chem., Int. Ed. 2005, 44, 5733. (31) Zhang, C.; Patil, R. S.; Li, T.; Barnes, C. L.; Teat, S. J.; Atwood, J. L. Chem. - Eur. J. 2017, 23, 8520. (32) Liu, C.; Luo, T.-Y.; Feura, E. S.; Zhang, C.; Rosi, N. L. J. Am. Chem. Soc. 2015, 137, 10508.

In summary, we have shown the preparation of anionic dimeric and hexameric C-alkylpyrogallol[4]arene nanocapsules by attaching multiple nitrate ions to the nanocapsule surface. The negative net charge of these nanocapsules not only changes their surface properties but also affords opportunities for facile encapsulation of cationic guests. This study demonstrates that surface functionalization may act as an effective method to tailor or introduce new properties into MONCs and will shed light with regard to functionallization of metal−organic species via metal coordination.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b00968. Full details for sample preparation and characterization (PDF) Accession Codes

CCDC 1545776−1545777 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Chen Zhang: 0000-0001-5552-1960 Jerry L. Atwood: 0000-0002-3350-9618 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the University of Missouri-Columbia for financial and research facility support of this work.

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REFERENCES

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DOI: 10.1021/acs.cgd.7b00968 Cryst. Growth Des. XXXX, XXX, XXX−XXX