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That’s No Moon: It’s a Molecular Capsule

2016; Elsevier BV; Volume: 1; Issue: 1 Linguagem: Inglês

10.1016/j.chempr.2016.06.008

2451-9308

Ben S. Pilgrim, Jonathan R. Nitschke,

Metal complexes synthesis and properties

In this issue of Chem, Fujita et al. have produced the largest synthetic self-assembled capsule to date through subtle modification of the flexibility and geometry of its constituent ligands. In this issue of Chem, Fujita et al. have produced the largest synthetic self-assembled capsule to date through subtle modification of the flexibility and geometry of its constituent ligands. Molecules are small—even those of a non-scientific background appreciate this fact. Chemists have a precise view of the molecular scale; the smallest molecules are of the order of an Ångström in size (0.1 nm), whereas the largest discrete synthetic structures are rarely larger than a couple of nanometers. The word discrete here is crucial, because there are numerous particulate materials, including metal nanoparticles, metal-organic frameworks, and polymers, whose constituent units can range in size from tens to hundreds of nanometers. However, in the realm of materials chemistry, particles are polydisperse, such that any particular batch can contain particles of many sizes. Building large structures with absolute monodispersity requires a carefully controlled bottom-up approach. Molecular self-assembly provides such an approach. Taking advantage of the well-defined coordination geometries of transition-metal ions and carefully synthesizing organic ligands to coordinate to these metals make it possible to pre-program smaller components to come together into more complex architectures with complete control. This technique has seen extensive use in recent years in the construction of a plethora of molecular capsules, most of which are based on Platonic solids such as the tetrahedron and cube, where metal ions occupy the vertices and organic ligands occupy the edges. The guest binding properties of these capsules have enabled their employment in a host of applications, from catalysis to sensing. However, they are still limited to little more than a nanometer or two in diameter. Larger capsules require larger pieces, and apart from the obvious drawback of more laborious ligand synthesis, the greater flexibility of longer ligands can reduce the preference for assembly into a single structural type. In recent years, the Fujita group has been in the vanguard of the campaign to synthesize large well-defined capsules. Rather than focus on simple tetrahedra and cubes, they have targeted structures incorporating much larger numbers of building blocks that do not define symmetry axes directly. This approach provides access to greater sizes while allowing the individual pieces to remain small. Their efforts must be described as a tour de force in self-assembly and have enabled them to construct progressively larger M6L12 octahedra,1Suzuki K. Tominaga M. Kawano M. Fujita M. Chem. Commun. (Camb.). 2009; : 1638-1640Crossref PubMed Scopus (126) Google Scholar M12L24 cuboctahedra,2Tominaga M. Suzuki K. Kawano M. Kusukawa T. Ozeki T. Sakamoto S. Yamaguchi K. Fujita M. Angew. Chem. Int. Ed. Engl. 2004; 43: 5621-5625Crossref PubMed Scopus (352) Google Scholar and M24L48 rhombicuboctahedra.3Sun Q.-F. Iwasa J. Ogawa D. Ishido Y. Sato S. Ozeki T. Sei Y. Yamaguchi K. Fujita M. Science. 2010; 328: 1144-1147Crossref PubMed Scopus (615) Google Scholar These polyhedra all contain a Pd2+ ion at each vertex and a bent organic ligand along each edge. Their ligands incorporate a pyridine nitrogen at each end to coordinate to the metal and are carefully tailored to favor a particular structural type. Figure 1 shows the ligands that self-assemble into each of these capsules, along with spheres scaled to illustrate the relative sizes of the capsules and onto which the polyhedral structure has been projected to show how the symmetry of these capsules more closely approaches that of a sphere as they increase in size and nuclearity. The earlier structures reported by the Fujita group are dwarfed in size by the subject of their most recent study, an M30L60 icosidodecahedron, which is reported in this issue of Chem.4Fujita D. Ueda Y. Sato S. Yokoyama H. Mizuno N. Kumasaka T. Fujita M. Chem. 2016; 1: 91-101Abstract Full Text Full Text PDF Scopus (192) Google Scholar This structure is a far more challenging target than the previous three. In addition to the increased entropic penalty incurred in the assembly of capsules of ever greater nuclearity, this capsule is the first in the series with overall icosahedral (Ih) rather than octahedral (Oh) point-group symmetry. The angles around the vertex in a true icosidodecahedron are 108° and 60°, a considerable departure from the ideal 90° of square planar Pd2+. The key breakthrough was the introduction of tetramethylphenylene spacer units between the terminal pyridine moieties and the central thiophene unit (which enables the requisite bend angles). This spacer unit engenders sufficient flexibility (described as a "slight sag" in the ligand) to allow the angles between ligands around the Pd2+ to approach 90° while still maintaining a ligand bend angle close to the ideal 150°. Remarkably, this additional flexibility did not lead to the formation of lower-nuclearity assemblies, as is common with flexible ligands. The effect of the tetramethylphenylene spacer must thus be more nuanced than just allowing a sag in the ligand; it will affect factors such as the dihedral angles between adjacent rings, and the amplification of subtle differences such as these can have significant ramifications in the structures of large assemblies. The capsule has been well characterized by mass spectrometry, 1H nuclear magnetic resonance (NMR), and 1H diffusion-ordered spectroscopy NMR, even though employing these techniques is challenging on an object of this size. However, the pièce de résistance of characterization is clearly the authors' single-crystal X-ray structure. Obtaining data on this enormous structure with such a large void volume is truly remarkable, and their special operational procedures, such as the refrigerated handling of samples and the collection of data at room temperature, are key points of note. Growing single crystals of sufficient quality for X-ray analysis has been many chemists' bane. The authors' comment that this structure was obtained only after years of perseverance should be a source of inspiration to all in the field: hard work pays off. As for the potential applications of this structure, the interior volume of 157,000 Å3 provides opportunities for encapsulation of larger guests than has previously been possible, and the authors comment that this is sufficiently large to encapsulate proteins. The realization of protein encapsulation nonetheless represents a still-significant challenge and a worthy goal. Self-assembly is the key construction technique for making the higher-order structures of life; virus capsids and the iron-storage protein capsule ferritin provide examples of how hierarchical complexity can be generated within living systems. Synthetic chemists still have a long way to go to compete with nature's benchmark in this field. The poliovirus, part of which is pictured in Figure 1, is viewed as one of the simplest significant viruses. It contains 60 protein subunits organized into 12 pentamers that self-assemble to form the 30-nm-diameter icosahedral procapsid of the virus.5Hogle J.M. Chow M. Filman D.J. Science. 1985; 229: 1358-1365Crossref PubMed Scopus (884) Google Scholar When placed alongside these largest spherical synthetic capsules to date, it still towers over them. The Fujita group's efforts are clearly making rapid progress—this latest 8.2-nm-diameter capsule represents an important milestone on the road to accessing self-assembled structures of the same size and complexity as their natural counterparts. Achieving the synthesis of such large, closed capsules represents one of the salient challenges for this discipline over the next quarter century. It is truly an exciting time for the field of supramolecular cages and capsules—new structures and functions are just beyond the horizon. Perhaps even higher-order polyhedra can be assembled, and there might still be more exotic, non-polyhedral structures that could be accessed by this synthetic methodology. It is now clear that discrete, well-defined molecular objects can be prepared on a length scale of 10 nm, extending the previous state of the art by an order of magnitude. Just as the crew of the Millennium Falcon initially couldn't believe that the large spherical object in the distance could have been assembled by man and that it had to be a moon,6Lucas G. Star Wars: Episode IV – A New Hope. 20th Century Fox, 1977Google Scholar we must readjust our preconceptions of what is possible in the world of molecular self-assembly. The possibilities out there are endless, but if size is what you are after, then clearly these are the capsules you're looking for. B.S.P. wrote the original draft, and editing was done by both authors. Self-Assembly of M30L60 IcosidodecahedronFujita et al.ChemJuly 07, 2016In BriefBottom-up construction of giant structures by the self-assembly of a large number of components (n = ∼100) has been a daunting challenge. Here, Fujita and colleagues report the self-assembly of a spherical metal polyhedron, possessing a hitherto unreported icosidodecahedron geometry with 30 vertices and 60 edges. The authors succeeded in controlling the self-assembly by intensive tuning of the ligand flexibility. X-ray crystallographic analysis confirmed that the complex is the largest well-defined spherical molecular capsule, comparable with the size of a typical protein. Full-Text PDF Open Archive