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M. Fujita and Doraemon's Panacea

Science 1 September 2006: Vol. 313. no. 5791, pp. 1273 - 1276
DOI: 10.1126/science.1129830

Fluorous Nanodroplets Structurally Confined in an Organopalladium Sphere

Sota Sato, Junya Iida, Kosuke Suzuki, Masaki Kawano, Tomoji Ozeki, Makoto Fujita

Astract: The distinct properties of fluorous phases are practically useful for separation, purification, and reaction control in organic synthesis. Here, we report the formation of a liquid-like fluorous droplet, composed of 24 perfluoroalkyl chains confined in the interior of a 5-nanometer-sized, roughly spherical shell that spontaneously assembled in solution from 12 palladium ions and 24 bridging ligands. Crystallographic analysis confirmed the rigid shell framework and amorphous interior. Perfluoroalkanes can dissolve in this well-defined fluorous phase, whereas they can hardly dissolve in a surrounding polar organic solution, and their solubility (up to ~eight perfluoroalkane molecules per spherical complex) can be finely controlled by tuning the length of perfluoroalkyl chains tethered to the shell.

Self-assembly and guest incorporation. Cages containing 12 positively charged palladium atoms (yellow) are generated using bananashaped linkers (dark blue) with fluorous or polyether substituents (red spheres). Void spaces in the core of the fluorinated cages allow the selective incorporation of fluorous molecules. Nonfluorous molecules such as hexafluorobenzene do not function as guests, but smaller gas molecules such as O2 and CH4 might have appreciable solubilities in fluorocarbons. The polyether cages reversibly bind metal ions.

Figure 4

Molecular structure of 2a. (A) The x-ray crystal structure of the shell framework of 2a. C6F13CH2-side chains at the curvature point of the ligands are disordered and could not be located. (B) The C6F13(CH2)2-side chains (orange) are modeled, and only the chains are optimized, by force-field calculations. (C and D) Six molecules of perfluorooctane (3, red) were placed at the central void of 2a, and structural annealing was conducted from 2000 to 300 K by molecular dynamics (MD) simulation. The images show one of the energy minimum structures obtained after MD simulation followed by force-field optimization. In (D), host 2a is represented by wire frames, whereas the accommodated guest molecules are represented by space-filling models.

see also John A. Gladysz's Perspectives:

Fluorous to the Core

Fluorous molecules--which are fluorine-rich and based on saturated carbon --commonly separate from aqueous and organic phases. This "molecular xenophobia," which is often reversible at elevated temperatures, is caused not by repulsive forces but rather by the much greater attractions between water molecules or organic molecules. Fluorous molecules have been widely used to separate products and catalysts or products and reagents, purify mixtures, and control reactions. Many fluorous microenvironments have been created, including micelles, vesicles, microbubbles, tubules, and hollow fibers, dendrimers, nanoparticles, and modified solid phases. They have been exploited as contrast agents for ultrasound imaging, drug delivery, and oxygen transport.

However, none of these assemblies feature the elegant design elements and the degree of molecular control that characterize the complexes reported by Sato et al. on page 1273 of this issue. The authors use coordination-driven self-assembly to generate a polycationic cage consisting of 12 palladium atoms and 24 bent pyridine-based linkers (see the figure). Attached to the concave side of the linkers are ponytails of the formula -OCH2CH2X, where X is a completely fluorinated chain of six to nine saturated carbon atoms. These chains reside in the cage, creating fluorous environments. Nuclear magnetic resonance (NMR) data support the proposed structures, which might be viewed as prototypes for "inverse dendrimers," with branches in a converging region of space.

Models suggest that if X is a six-carbon fluorinated chain, the chains do not completely fill the cage, leaving a void space. The fluorous guest molecule perfluoro-n-octane can then be incorporated in the cage from a surrounding dimethyl sulfoxide suspension. NMR data indicate that an average of 5.8 molecules of perfluoro-n-octane can be accommodated. Single crystals of the inclusion complex can be grown. Synchroton x-ray studies of these crystals indicate a slightly oval geometry (4.9 nm by 4.2 nm); the fluorinated segments are disordered, suggesting a fluidlike or "nano-droplet" environment.

Other experiments support this host/guest model. For example, complexes with longer fluorinated carbon chains should have less interior void space. When these are similarly treated with perfluoro-n-octane, fewer molecules are incorporated. For the nonfluorous hexafluorobenzene molecule, no host/guest complex can be detected.

Thus, in contrast to the inaccessible interiors of large fullerenes, the cages reported by Sato et al. can readily take up suitable guests. It remains unclear, however, how the guests gain access. Dissociation of linkers from the cages would generate transient channels or pores, but this process is known to be slow. Hence, transport probably occurs through one of the existing large portals.

The same research team has previously reported closely related complexes with other types of interior functionality. For example, when X is a polyether segment, the core of the cage complex features a dense array of oxygen donor atoms that should be able to bind metal ions. Indeed, when acetonitrile solutions of this complex were treated with sources of La3 ions, about 20 ions were incorporated. When dimethyl sulfoxide (which strongly solvates many metal cations) was added, the La3 ions were extracted, demonstrating that the formation of host/guest complexes is reversible.

How might such assemblies be exploited in future work? One major impetus for the industrial development of fluorous chemistry during the 1990s was the hope that fluorous media might be used in the selective oxidation of methane to methanol. Small gaseous molecules such as methane and oxygen are usually highly soluble in fluorous phases. Methanol, because of its much greater polarity, might be rapidly scavenged by a nonfluorous phase before further oxidation could occur. Reactions of such guest molecules with the fluorous cages reported by Sato et al. are therefore of particular interest. The next logical step would be to immobilize a fluorous oxidation catalyst in the cage interior and treat the system with a mixture of methane and oxygen.

The highly positively charged cages might also be attractive for anionic fluorous guests. Because of toxicity concerns and environmental persistence, several commercial fluorous carboxylates and sulfonates have been removed from the market in recent years. It is possible that they could be scavenged by the fluorous cages or by second-generation derivatives.

The results reported by Sato et al. are likely to inspire many more ideas for applications. Given that such assemblies can be prepared from a variety of linkers, that guests can easily be incorporated, and that more voluminous analogs are likely to be available soon, this initial study is certain to be followed by many exciting discoveries.

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平凡的水果世界,平凡中的不平凡。 今朝看水果是水果 ,看水果还是水果 ,看水果已不是水果。这境界,谁人可比?在不平凡的水果世界里,仁者见仁,智者见智。




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