A variety of host compounds have been used as molecular-scale reaction vessels, protecting guests from their environment or restricting the space available around them, thus favouring particular reactions. Such molecular 'flasks' can endow guest molecules with reactivities that differ from those in bulk solvents. Here, we extend this concept to crystalline molecular flasks, solid-state crystalline networks with pores within which pseudo-solution-state reactions can take place. As the guest molecules can spontaneously align along the walls and channels of the hosts, structural changes in the substrates can be directly observed by in situ X-ray crystallography during reaction. Recently, this has enabled observation of the molecular structures of transient intermediates and other labile species, in the form of sequential structural snapshots of the chemical transformation. Here, we describe the principles, development and applications of crystalline molecular flasks.

Highly symmetric structures often appear in nature as revealed by, for example, the capsids of spherical viruses that have icosahedral symmetry consisting of 60n identical protein subunits.[1] The reason for the high symmetry lies behind the principle that increasing the number of elements with the same symmetry reduces the amount of independent structural information, which is directly related to the length of DNA. Thus, the self-organization of tiny subunits into a giant biological molecule can be regarded as the process of not only structural growth but of the amplification of molecular information. We show herein that, through metal-ligand interactions, [2,3] simple banana-shaped organic molecules self-organize into finite, spherical coordination networks with a diameter of up to 7 nm, which is in contrast to the formation of two-dimensional (2D) infinite networks that occurs with linear organic ligands. The spherical coordination networks consist of 36 components, 12 equivalent metal centers (M) and 24 equivalent ligands (L), and have cuboctahedron symmetry. By attaching a functional group (e.g., C 60 or porphyrin) to each ligand, 24 functional groups are aligned equivalently at the periphery of the sphere.Over the last decade, extensive studies have been made on infinite coordination networks that are formed by the complexation of exo-multidentate ligands with transitionmetal ions. A typical and simple example is given by a 2D grid complex that forms from a rodlike ligand and a metal (Figure 1 a). [4] We expect that, if the ligand framework is slightly bent, the coordination network will develop with a[*] Dr.

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 approximately eight perfluoroalkane molecules per spherical complex) can be finely controlled by tuning the length of perfluoroalkyl chains tethered to the shell.

An adamantanoid (H2O)10 cluster is formed within the hydrophobic cavity of a self-assembled coordination cage. This cluster is termed "molecular ice" because it is the smallest unit of naturally occurring Ic-type ice. X-ray structural analysis, coupled with neutron diffraction study, reveals that the molecular ice is formed not by a simple space-filling effect but by efficient molecular recognition within the cage via H2O:...pi interaction.

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Good solvent, poor solvent: A simple precipitation method enabled the spontaneous formation of homogeneous C70 cube crystals by self‐crystallization in cavities of a good solvent (mesitylene) surrounded by a poor solvent (isopropyl alcohol, IPA; see picture). The enormously increased photoluminescence (PL) intensity of the C70 cube crystals relative to that of C70 powder was mainly attributed to the high crystallinity of the cubes.

The photoexcitation of a self-assembled M6L4-type coordination cage accommodating photochemically inert alkane guests (e.g., adamantane and cyclooctane) led to the regioselective oxidation of the guest within the cage. Under anaerobic conditions, the guest oxidation was accompanied by the stable radical formation as indicated by ESR spectrometry (g = 2.002) and change in solution color (from colorless to blue). These phenomena were shown to be characteristic of the self-assembled molecular systems: i.e., from the M6L4 supersetGn assembly, none of the components (M, L, or G) can be eliminated for the unusual oxidation and/or radical formation.

Remote chirality at the periphery of a self-assembled cage brings about the slight twist of aromatic rings, which is yet sufficient to induce considerable asymmetric induction (up to 50% ee) in the hitherto unknown [2 + 2] cross photoadditions of fluoranthenes.