Other Channel Clathrates

Urea has the remarkable property of forming crystalline complexes or adducts with straight-chain organic compounds. These crystalline complexes consist of a hoUow channel, formed by the crystallized urea molecules, in which the hydrocarbon is completely occluded. Such compounds are known as clathrates. The type of hydrocarbon occluded, on the basis of its chain length, is determined by the temperature at which the clathrate is formed. This property of urea clathrates is widely used in the petroleum-refining industry for the production of jet aviation fuels (see Aviation and other gas-TURBINE fuels) and for dewaxing of lubricant oils (see also Petroleum, refinery processes). The clathrates are broken down by simply dissolving urea in water or in alcohol. 

Aromatic derivatives of cyclotriphosphazenes, rigid six-membered ring systems built on a framework of alternating P and N atoms, provide one of the more beautiful early examples of hosts that form channel-type clathrates and can be useful for molecular separations 32,42>, Although these clathrate systems were discovered by accident, the conclusions that emerged from their investigation have been extremely helpful for the molecular design of other potential host molecules. 

One of the first reactions to be studied systematically in a channel-type clathrate was the polymerization of olefins or diolefins. The ordering of the monomers within the clathrate lattice leads to stereoregular products that are not available by other techniques (232-234). Such radiation-induced stereospecific polymerization has been reported for a number of clathrate hosts (235). 

The zeolites are aluminosilicate framework minerals of general formula M", [AI4Sil0lr+>JJ -zH20.y They are characterized by open structures that permit exchange of catioas and water molecules (Fig. 16.2). In the synthetic zeolites the aperture and channel sizes may sometimes be controlled by a sort of template synthesis—the zeolite is synthesized around a particular organoammonium canon. This yields channels of the desired size. The zeolite framework thus behaves in some ways like a clathrate cage about a guest molecule (Chapter 8). The synthesis of zeolites also involves several other factors such as the Al/Si ratio, the pH. the temperature and pressure, and the presence or absence of seed crystals - ... 

Cl-4H2Op [790], on the other hand, are not clathrates but are channel-type inclusion compounds formed from cross-linking nets and ribbons into framework structures, with anions integrated in the water structure. The lower hydrates of... 

More commonly, clathrates are formed by linking both octahedrally and tetrahe-drally coordinated cadmium ions. For example, simply by crystallization of Cd(CN)2 from aqueous alcohols21 (or in similar ways22) one obtains clathrates having infinite channels, as shown in Fig. 15-3 for the products obtained with Pr"OH and Pr OH. Similar structures with similar (or even wider) channels are obtained with DMF and DMSO,23 and in the compound Cd(CN)2-%H20-Bu 0H.24 There are other clathrates that include a combination of water and ether molecules,25 and still... 

By direct action of NH3 on HgO one obtains Hg2N0H-2H20 (Millon s base), which is a clathrate. The framework is made of (Hg2N)m and the cavities and channels contain the OH ions and water molecules. In addition to the chloride mentioned above, many other salts of Millon s base, Hg2NX-nH20, with X = N03, CIO4, Br , I-, and n = 0-2, are known. These compounds are a type of ion exchanger. 

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