Encapsulated alkali metal cations

In an interesting aside, many years after these synthetic capsules were first prepared, it was found that cells produce protein vaults that have similar encapsulating abilities but on a much larger scale. Whereas cryptands bind single alkali metal cations, vault proteins bind RNA composed of over 140 bases and other similarly sized guests. These biological crypts are discussed later. 

The covalent assembly of guest-encapsulating host species has long been an area of research interest. The first examples of host species binding their guests within a three-dimensional array of interactions were the class of compounds known as cryptands [26,27]. The cryptands were designed as hosts for alkali metal cations and are based on macrobicyclic-polyethers (Fig. 2). Typically, cryptands are synthesised by the addition of a diacyl-chloride to an azacrownether (Scheme 1). 

In general, discrete Zintl anions, whether they obey the simple octet rule or are electronically more complex, can often be obtained intact from the initial Zintl phase by substituting tetraalkylammonium ions for the alkali metal cations or by encapsulating the latter in a cryptand such as cryptate-222 (see 1-XXII).25... 

If empty zeolite LTA is exposed to cesium gas, and redox reaction with the exchangeable cations of the zeolite can occur, then the product Cs" " ions may seal the zeolite crystals. Everything within the zeolite may be encapsulated, including the product atoms, perhaps as neutral clusters. Well, nearly all cations, including Na" ", are reduced by Cs(g) only some of the other alkali-metal cations may resist reaction. Depending upon the mobility of the Cs" " ions at the reaction temperature, the surfaces of the crystals may be closed initially. If the reaction proceeds, the entire volume of the zeolite may be sealed. The ultimate outcome of the process will depend upon the mobilities of all the atoms and ions in the structure at the reaction temperature, and upon considerations of the stability and lability of any structural subunits which may form. 

The results described above indicate that dealuminated zeolites, which were exchanged with alkali-metal cations, can form a variety of supports suitable for the encapsulation of Mo(CO)fi. 

The size of the alkali metal cation has an impact on the basic properties of the final catalysts this is reflected in the order of basic strength found for several metal oxides encapsulated in X-zeolite Na O/NaX < K,0/KX = Rb O/RbX < CSj O/CsX. This order indicates that stronger basic sites are formed when the radius of the monovalent cation increases the sites are even stronger than those measured for the corresponding bulk alkaline oxides [67], A similar order of basicity was observed for a series of alkaline earth oxides occluded inside the cages of faujasite [70]. 

As covered in comprehensive texts [2,3], early efforts in the study of salt partitioning by pure solvents were directed toward understanding the basic equilibria involved and the chemical factors influencing partitioning behavior. Figure 3 essentially encapsulates that early work. For the high-dielectric solvents, such as nitrobenzene or nitromethane, it was readily established that solvent-phase salts of the alkali metal cations with various... 

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