Stellated Cuboctahedron Through Post-Stellation of an M<Sub>

ARTICLES PUBLISHED ONLINE: 11 MARCH 2012 | DOI: 10.1038/NCHEM.1285

An M18L24 stellated cuboctahedron through post-stellation of an M12L24 core Qing-Fu Sun, Sota Sato and Makoto Fujita*

Platonic and Archimedean polyhedra, well-known to mathematicians, have been recently constructed by chemists at a molecular scale by deﬁning the vertices and the edges with metal ions (M) and organic ligands (L), respectively. Here, we report the ﬁrst synthesis of a concave-surface ‘stellated polyhedron’, constructed by extending the faces of its precursor polyhedron until they intersect, forming additional nodes. Our approach involves the formation of an M12L24 cuboctahedron core, the linkers of which each bear a pendant ligand site that is subsequently able to bind an additional metal centre to form the stellated M18L24 cuboctahedron. During this post-stellation process, the square faces of the M12L24 core are closed by coordination of the pendant moieties to the additional metal centres, but they are re-opened on removing these metals ions from the vertices. This behaviour is reminiscent of the analogous metal-triggered gate opening–closing switches found in spherical virus capsids.

tellated polyhedra1 are a unique family of polyhedra with resonance (NMR) spectroscopy and mass spectrometry (MS). The concave surfaces, constructed by extending the faces of a large square windows (Fig. 1) of the M12L24 cuboctahedron are Spolyhedron until they intersect (Fig. 1). Although the closed by the stellation, but can be re-opened by removal of the molecular self-assembly of polyhedral architectures is a topic of metal ions from the vertices, thereby providing a reversible gate current interest2–18, their stellated derivatives—in which the stellated opening–closing function for a large cuboctahedral cage. This frameworks are clearly defined by metal vertices—have never been post-stellation approach also provides an easy and efficient synthesized. In the literature, an M6L8 ‘stellated octahedron’ has method for functionalizing and stabilizing the original polyhedral been described by Hardie and colleagues19, in which the stellated core framework. vertices are defined by the pronounced pyramidal shape of the 20–25 ligand. Topologically similar M6L8 complexes have been Results and discussion described using terms such as ‘cubes’20, ‘balls’21,22 or ‘spheres’24. Ligand 1 was synthesized from 3,5-dibromobenzyl alcohol in two Hardie and colleagues have also reported a unique M4L4 cube steps: Mitsunobu displacement with 4-hydroxypyridine at the whose hexameric aggregate is closely related to the stellated benzylic position, followed by Suzuki coupling with 4-pyridylborate truncated hexahedron26. Here, we report the self-assembly of a at the 3,5-positions (Supplementary Scheme S1). In ligand 1, the stellated cuboctahedron in which all the vertices are clearly coordination ability of the pyridyl groups at coordination sites A defined by metal centres, through the post-stellation of a polyhedral and B is comparable. It was therefore anticipated that, upon precursor by metal coordination. addition of metal ions, the random complexation of these two sites would generate numerous oligomeric products, which N in turn would be kinetically trapped, resulting in a gel or a B precipitate. However, when a 2:1 mixture of 1 and Pd(BF4)2 was heated in dimethyl sulfoxide-d6 (DMSO-d6)at808C for 12 h, selective complexation was observed (by 1H-NMR) at site A only. O Only site A pyridyl protons were significantly shifted downfield,

A A N N 1

On complexation of tris(4-pyridyl) ligands (comprising a benzene core bearing two A and one B coordination sites) (1) with Pd(II) ions, coordination sites A (1,3-positions of the benzene core) predo- minantly participate in self-assembly to form an M12L24 cuboctahedral core with an M:L ratio of 1:2, leaving sites B uncomplexed. On further addition of Pd(II), the dangling pyridyls at sites B coordinate Figure 1 | Cartoons of a cuboctahedron (left) and its stellated counterpart to these additional metal centres, converting the cuboctahedron into (right). Note that the mathematically deﬁned stellated cuboctahedron is an M18L24 stellated cuboctahedron. These structures were sub- stellated on every triangular face of the cuboctahedron, whereas in our sequently characterized by X-ray crystallography, nuclear magnetic molecular assembly only the square windows are stellated.

Department of Applied Chemistry, School of Engineering, The University of Tokyo, and Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Corporation (JST), 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. *e-mail: [email protected]

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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1285 ARTICLES a characteristic of metal coordination, and no distinct chemical N α' shift change was observed for the site B pyridyl protons β ' (Fig. 2a,b). The number of signals derived from ligand 1 remained O unchanged, indicating a product with a highly symmetrical β 12 Pd(II) structure. Diffusion-ordered NMR spectroscopy (DOSY) showed 24 α Self-assembly the formation of a single product with a single band at diffusion N N coefficient D ¼ 3.80 × 10211 m2 s21 (log D ¼ 210.42); this value is comparable to that of previously reported M12L24 complexes with the same shell framework12. These observations strongly support the formation of M12L24 cuboctahedron 2, 1 2 in which 48 site A pyridyl groups are selectively used for self- b α' assembly. Finally, the formation of 2 in solution was proven Ligand 1 β by cold-spray ionization time-of-flight mass spectrometry α 27 β' (CSI-TOF-MS) , which revealed a series of prominent peaks . 2 2 mþ for [Pd12(1)24 (BF4 )24–(BF4 )m] (m ¼ 11–7) (Fig. 2c) (see Supplementary Information). 2 Complex Selective complexation at site A to form cuboctahedron 2 is explained by the kinetic trap of the stable M12L24 core in the self- assembly process. We have previously shown that, once self- assembled from bis(4-pyridyl) bent ligands and Pd(II) ions, the 1098765ppm M12L24 framework gains remarkable kinetic stability with a half- c life of 20 days for ligand exchange28. A range of metal–ligand 9+(sim.) 9+ oligomers are initially formed, but because the highly symmetric 10+ M12L24 cuboctahedral framework (2) is the only stable structure, it 9+(obs.) is the one that eventually assembles during equilibration. After counter-anion exchange with PF 2, the structure of + 6 11 + 2 8 2 (PF6 salt) was unambiguously determined by X-ray crystallogra- 1,191 1,193 phy. Slow vapour diffusion of ethyl acetate into a DMSO solution of 2 + 7 2 (PF6 salt) resulted in single crystals suitable for X-ray analysis. Structure refinement revealed the M12L24 cuboctahedral skeleton with dangling uncoordinated pyridyl arms at site B (Fig. 3). The 900 1,100 1,300 1,500 m/z dangling arms are disordered but aligned towards the centre of the square windows of the cuboctahedron, presumably to optimize Figure 2 | Synthesis and structural analyses of cuboctahedron 2. a, crystal packing. Cartoon illustration of the self-assembly of 2. b, 1H NMR spectra (500 MHz, It was expected that the dangling pyridyl arms (site B)atthe convex of 2 could further accommodate six metal ions on the 300 K, DMSO-d6) of ligand 1 and complex 2, with blue and red dashed lines indicating the change of chemical shift on pyridyl sites A (a, b)and square windows of the cuboctahedron to give an M18L24 stellated ′ ′ 2 cuboctahedron 3 (Fig.4a).Accordingly,Pd(BF4)2 was added B (a , b ). c, CSI-TOF mass spectrum of complex 2 (BF4 salt). Inset: expanded spectrum of the 9þ peaks (top, simulated; bottom, observed). portionwise to a solution of 2, and the complexation was moni- tored by 1H NMR spectroscopy (Fig. 4b). The proton signals of the pyridyl groups in site B were gradually shifted downfield (indicated by blue and red dashed lines in Fig. 4b), while those of site A on the M12L24 core remained unchanged. DOSY also demonstrated the formation of a new discrete product with a similar diffusion coefficient (Supplementary Fig. S17). Unlike complexation at site A, free and complexed pyridyl groups at sites B are averaged on the NMR timescale, indicating weaker coordination at site B than at site A. We assumed that each Pd(II) ion is cooperatively coordinated by the four site B pyridyl groups at every square window. A Job plot confirmed this hypothesis—the clear 1:1 complexation of one Pd(II)ion per square window (1/6 partial structure of 2) was revealed from the plot of x.Dd versus x (where x ¼ [Pd(II)]/([Pd(II)] þ 1/6.[2], and Dd indicates the change in chemical shift in 1H-NMR) (Fig. 4c). Nonlinear curve fitting revealed cooperative 1:4 complexation at every Pd(II) centre with an apparent associate constant Ka (between the Pd(II) ion and the square window) of 1.79 × 103 M21 (Fig. 4d). Owing to the weaker coordination ability of the site B pyridyls 2 Figure 3 | Crystal structure of 2 (PF6 salt) looking down through a at the vertices, CSI-TOF mass measurements failed to detect triangular window (left) and a square window (right). Below each structure, the expected M18L24 species directly. Nonetheless, prominent . 2 2 nþ palladium atoms (yellow balls) that define the cuboctahedral framework peaks corresponding to [Pd12þm(1)24 (BF4 )24þ2m 2 (BF4 )n] (shown in blue) have been extracted from the crystal structure. A disordered (m ¼ 0–3, n ¼ 14–8) were observed, probably generated by the model was applied to the flexible pyridyl groups (pink) on the square loss of some of the Pd(II) ions from 3 under CSI-MS conditions, 2 windows. All the counterions (PF6 ) present at the apical positions of each taking into account the weaker Pd(II)–pyridine coordination at Pd(II) centre are omitted for clarity. site B (Supplementary Figs S22 and S23).

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ARTICLES NATURE CHEMISTRY DOI: 10.1038/NCHEM.1285 a applications in host–guest chemistry for giant guests on a biomo- lecular level, such as a small protein.

6 Pd(II) N TMEDA

O O

2 3

b [Pd(II)]/(1/6·[2]) DHO DMSO N N 0.00

0.33 5 0.67 1.00 Cuboctahedral complex 2 has eight triangular and six square windows. The diameters of the inscribed circles of the triangular 1.33 and square windows are 0.88 and 1.24 nm, respectively, indicating 2.00 that the square windows are much larger portals for potentially 2.67 incorporating a guest into the cuboctahedral cage. It is worth 3.33 noting that the stellation can act as a gate closing process for the 4.00 large square windows of 2. The re-opening of the windows was also examined by removal of the six Pd(II) ions from the vertices. 4.67 ′ ′ We found that the addition of N,N,N ,N -TMEDA to a solution of After removal of Pd(II) 3 selectively sequestered the metals from the vertices to result in gate-opened complex 2 (Fig. 4b, bottom trace). DOSY measure- 109876543ppm ments also conﬁrmed that the M12L24 spherical framework was c not damaged by the addition of the TMEDA (Supplementary 0.16 d 0.6 Fig. S18). We therefore achieved reversible gate opening/closing 0.14 0.5 0.12 (stellation/destellation) of the cuboctahedral structure. 0.4 0.10

Δδ 0.08 0.3 Δδ •

χ Conclusions 0.06 0.2 0.04 We have established a post-stellation method for the self-assembly 0.02 0.1 of an M18L24 stellated cuboctahedron by structural extension from 0.00 0.0 an M L cuboctahedron core. The method features the bottom- 0.00 0.25 0.50 0.75 1.00 12 24 0.0 1.0 2.0 3.0 4.0 5.0 up construction of new, complicated, three-dimensional molecular χ 2 [Pd(II)]/(1/6·[2]) = [Pd(II)]/([Pd(II)] + 1/6·[ ]) polyhedra and, equally importantly, the manipulation and func- tionalization of known polyhedral molecular objects. We further Figure 4 | Reversible conversion between an M12L24 cuboctahedron (2) and an M18L24 stellated cuboctahedron (3) through gate opening– closing. a, Cartoon illustration of the metal-triggered structural switch between 2 and 3 (the colour code of the ligand is the same as in Fig. 2a). b, 1H NMR tracing of the conversion. Complex 2 was converted to 3 by the addition of Pd(II) with an increasing [Pd(II)]/(1/6.[2]) ratio from 0.00 to 4.67. The bottom chart shows the reverse conversion from 3 to 2 on sequestration of Pd(II)ionsbythe addition of N,N,N′,N′-TMEDA. Blue and red dashed lines indicate the chemical shifts of the a′ and b′ protons on the pyridyl site B, respectively. c, Job plot conﬁrming the 1:1 binding of Pd(II) per square window (Supplementary Figs S6–S8). d, Nonlinear ﬁtting, from which × 3 21 the apparent binding constant Ka was estimated to be 1.79 10 M (Supplementary Figs S9 and S10).

All attempts to obtain single crystals of 3 were unsuccessful, possibly due to the equilibrium in the stellation step. To circumvent this problem, crystals of an analogous M18L24 complex (4) were obtained. This was prepared from ligand 5 (see Supplementary Information) to crystallographically support the structure of 3. A single crystal of high enough quality for X-ray analysis was obtained by the slow vapour diffusion of 1,4- Figure 5 | Crystal structure of 4 looking down through a triangular window 2 dioxane into a DMSO solution of 4 (NO3 salt). The X-ray (left) and a stellated vertex (right). Below each structure, palladium atoms structure revealed the expected M18L24 stellated polyhedron with (yellow balls) that deﬁne the stellated cuboctahedral framework (shown in six clipping Pd(II) ions on the square windows (Fig. 5), further blue) have been extracted from the crystal structure (the stellated parts are 2 supporting the suggested structure of complex 2. This M18L48 shown in green in the lower panel). All the counterions (NO3 )present framework has a huge void space with a separation of 3.64 nm at the apical positions of each Pd(II) centre and the solvents are omitted between the most distant Pd(II) ions, suggesting potential for clarity.

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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1285 ARTICLES

envisage that the reversible gate opening–closing process at the 13. Sun, Q.-F. et al. Self-assembled M24L48 polyhedra and their sharp structural square windows will enable us to incorporate giant guests such as switch upon subtle ligand variation. Science 328, 1144–1147 (2010). 14. Leininger, S., Olenyuk, B. & Stang, P. J. Self-assembly of discrete cyclic proteins and DNA through the opened gate and to subsequently nanostructures mediated by transition metals. Chem. Rev. 100, 853–908 (2000). incarcerate them in the stellated polyhedron by gate closing. We 15. Fujita, M. et al. Molecular paneling via coordination. Chem. Commun. also note that such a controllable structural transformation by the 509–518 (2001). crosslinking of component subunits resembles the metal-triggered 16. Caulder, D. L. & Raymond, K. N. Supermolecules by design. Acc. Chem. Res. 32, gate opening–closing switches found in spherical virus capsids29–32. 975–982 (1999). 17. Mu¨ller, A., Ko¨gerler, P. & Dress, A. W. M. Giant metal-oxide-based spheres and Methods their topology: from pentagonal blocks to keplerates and unusual spin systems. Coord. Chem. Rev. 222, 193–218 (2001). X-ray structure determination. The diffraction data were collected at synchrotron 18. Cotton, F. A., Lin, C. & Murillo, C. A. Supramolecular arrays based on dimetal facilities (SPring-8, beamline BL38B1 for 2; KEK, beamline NE3A for 4). Despite building units. Acc. Chem. Res. 34, 759–771 (2001). many attempts to optimize the measurement conditions based on synchrotron 19. Ronson, T. K., Fisher, J., Harding, L. P. & Hardie M. J. Star-burst prisms with radiation, the crystals diffracted very weakly because of the large amount of cyclotriveratrylene-type ligands: a [Pd L ]12þ stella octangular structure. disordered solvents and anions inside the huge void in the crystal. Most of the 6 8 Angew. Chem. Int. Ed. 46, 9086–9088 (2007). disordered molecules and residues could not be modelled, and the SQUEEZE33 20. Hong, M. et al. A nanometer-sized metallosupramolecular cube with O procedure of PLATON34 was used in the refinement. Results for complex 2: cubic, h symmetry. J. Am. Chem. Soc. 122, 4819–4820 (2000). Im–3m, a ¼ 39.346(5) Å, V ¼ 60,912(12) Å3, Z ¼ 2, T ¼ 300 K. Anisotropic 21. Duriska, M. B. et al. A nanoscale molecular switch triggered by thermal, light, least-squares refinement for the atoms on the polyhedral backbone and and guest perturbation. Angew. Chem. Int. Ed. 48, 2549–2552 (2009). isotropic refinement for the other atoms on 2,437 independent merged reflections 22. Duriska, M. B. et al. Systematic metal variation and solvent and hydrogen-gas (R ¼ 0.0800) converged at residual wR ¼ 0.3715 for all data, residual R ¼ 0.1194 int 2 1 storage in supramolecular nanoballs. Angew. Chem. Int. Ed. 48, for 2,374 observed data (I . 2s(I)) and goodness of fit (GOF) ¼ 1.822. Results for 8919–8922 (2009). complex 4: cubic, F432, a ¼ 53.334(6) Å, V ¼ 151,709(31) Å3, Z ¼ 4, T ¼ 293 K. 23. Chand, D. K., Biradha, K., Fujita, M., Sakamoto, S. & Yamaguchi, K. A molecular Anisotropic least-squares refinement for the atoms on the polyhedral backbone and sphere of octahedral symmetry. Chem. Commun. 2486–2487 (2002). isotropic refinement for the other atoms on 6,350 independent merged reflections 24. Moon, D. et al. Face-driven corner-linked octahedral nanocages: M L cages (R ¼ 0.0675) converged at residual wR ¼ 0.2887 for all data, residual R ¼ 0.0882 6 8 int 2 1 formed by C -symmetric triangular facial ligands linked via C -symmetric for 4,948 observed data (I . 2s(I)) and goodness of fit (GOF) ¼ 1.192. 3 4 square tetratopic PdII ions at truncated octahedron corners. J. Am. Chem. Soc. The X-ray crystallographic coordinates for structures reported in this Article 128, 3530–3531 (2006). have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under 25. Hiraoka, S. et al. Isostructural coordination capsules for a series of 10 different deposition nos CCDC 840807 (2) and CCDC 840814 (4). These data can be d5–d10 transition-metal ions. Angew. Chem. Int. Ed. 45, 6488–6491 (2006). obtained free of charge (http://www.ccdc.cam.ac.uk/data_request/cif). 26. Ronson, T. K. et al. Stellated polyhedral assembly of a topologically complicated Full experimental details and crystallographic analysis are given in the Pd L ‘Solomon cube’. Nature Chem. 1, 212–216 (2009). Supplementary Information. 4 4 27. Yamaguchi, K. Cold-spray ionization mass spectrometry: principle and Received 12 October 2011; accepted 31 January 2012; applications. J. Mass Spectrom. 38, 473–490 (2003). 28. Sato, S., Ishido, Y. & Fujita, M. Remarkable stabilization of M L spherical published online 11 March 2012 12 24 frameworks through the cooperation of 48 Pd(II)-pyridine interactions. J. Am. Chem. Soc. 131, 6064–6065 (2009). References 29. Hsu, C. H., Sehgal, O. P. & Pickett, E. E. Stabilizing effect of divalent metal ions 1. Wenninger, M. J. Dual Models (Cambridge Univ. Press, 2003). on virions of southern bean mosaic virus. Virology 69, 587–595 (1976). 2. Fujita, M. et al. Self-assembly of ten molecules into nanometre-sized organic 30. Rayment, I., Johnson J. E. & Rossmann, M. G. Metal-free southern bean mosaic host frameworks. Nature 378, 469–471 (1995). virus crystals. J. Biol. Chem. 254, 5243–5245 (1979). 3. MacGillivray, L. R. & Atwood, J. L. A chiral spherical molecular assembly held 31. Robinson, I. K. & Harrison, S. C. Structure of the expanded state of tomato bushy together by 60 hydrogen bonds. Nature 389, 469–472 (1997). stunt virus. Nature 297, 563–568 (1982). 4. Stang, P. J., Olenyuk, B., Muddiman, D. C. & Smith, R. D. Transition-metal- 32. Speir, J. A., Munshi, S., Wang, G., Baker, T. S. & Johnson, J. E. Structures of the mediated rational design and self-assembly of chiral, nanoscale supramolecular native and swollen forms of cowpea chlorotic mottle virus determined by X-ray polyhedra with unique T symmetry. Organometallics 16, 3094–3096 (1997). crystallography and cryo-electron microscopy. Structure 3, 63–78 (1995). 5. Caulder, D. L., Powers, R. E., Parac, T. N. & Raymond, K. N. The self-assembly of 33. Spek, A. L. & Van Der Sluis, P. BYPASS: an effective method for the refinement a predesigned tetrahedral M4L6 supramolecular cluster. Angew. Chem. Int. Ed. of crystal structures containing disordered solvent regions. Acta Crystallogr. A 37, 1840–1843 (1998). 46, 194–201 (1990). 6. Takeda, N., Umemoto, K., Yamaguchi, K. & Fujita, M. A nanometre-sized 34. Spek A. L. Structure validation in chemical crystallography. Acta Crystallogr. D hexahedral coordination capsule assembled from 24 components. Nature 398, 65, 148–155 (2009). 794–796 (1999). 7. Olenyuk, B., Whiteford, J. A., Fechtenko¨tter, A. & Stang, P. J. Self-assembly Acknowledgements of nanoscale cuboctahedra by coordination chemistry. Nature 398, This research was supported by the CREST project of the Japan Science and Technology 796–799 (1999). Agency (JST), for which M.F. is the principal investigator, and also in part by KAKENHI, 8. Olenyuk, B., Levin, M. D., Whiteford, J. A., Shield, J. E. & Stang, P. J. Self- MEXT. Synchrotron X-ray diffraction studies were performed at KEK and SPring-8. The assembly of nanoscopic dodecahedra from 50 predesigned components. J. Am. authors thank T. Ozeki for valuable suggestions on the refinements of the X-ray structures. Chem. Soc. 121, 10434–10435 (1999). 9. Abrahams, B. F., Egan, S. J. & Robson, R. A very large metallosupramolecular 3 capsule with cube-like 4 topology assembled from twelve Cu(II) centers and Author contributions eight tri-bidentate tri-anionic ligands derived from 2,4,6-triphenylazo-1,3,5- M.F. and Q.-F.S. conceived and designed the experiments and wrote the manuscript. Q.-F.S. trihydroxybenzene. J. Am. Chem. Soc. 121, 3535–3536 (1999). performed the experiments and analysed the data. S.S. contributed the diffraction and 10. Moulton, B., Lu, J., Mondal, A. & Zaworotko, M. J. Nanoballs: nanoscale faceted NMR studies and participated in discussions throughout. polyhedra with large windows and cavities. Chem. Commun. 863–864 (2001). 11. Eddaoudi, M. et al. Porous metal2organic polyhedra: 25 Å cuboctahedron Additional information constructed from 12 Cu2(CO2)4 paddle-wheel building blocks. J. Am. Chem. Soc. The authors declare no competing financial interests. Supplementary information 123, 4368–4369 (2001). accompanies this paper at www.nature.com/naturechemistry. Reprints and permission 12. Tominaga, M. et al. Finite, spherical coordination networks that self-organize information is available online at http://www.nature.com/reprints. Correspondence and from 36 small components. Angew. Chem. Int. Ed. 43, 5621–5625 (2004). requests for materials should be addressed to M.F.

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Stellated Cuboctahedron Through Post-Stellation of an M&lt;Sub&gt;

Stellated Cuboctahedron Through Post-Stellation of an M<Sub>