Clathrates synthesis

Before developing a description of the clathrate synthesis mechanism we first note the large unit cell volume expansion for the NaSi Zintl phase recorded between 60 and 500 °C (Fig. 4.3). The thermal expansion coefficient has a remarkably high value (4.54 x that is comparable with ionic con-... 

Our results lead us to a proposed mechanistic description of the NaSi -> NagSi46 clathrate synthesis process, drawing on key observations described above and in combination with essential chemical reasoning. 

Despite the fact that clathrate hydrate science can be considered to be a mature field, numerous challenges remain. In part, this is because hydrates exist in nature and hence exhibit the complexity of mineral phases, where the structure and composition reflect not only the conditions of synthesis but also subsequent exposure to changing conditions after synthesis. Another part is that the principal component of these materials is water, likely the most important material on earth, and the clathrates offer a possibility to study the diverse ways in which water interacts with other materials. In this respect, it is interesting to note that new structures continue to be found. Structurally related families of materials such as the sUica-based zeosils and clathrasils tend to show a far greater structural diversity. In part, this is because of the covalent nature of the silica lattices that give far more robust frameworks than the water-based ones however, each structure in these classes of materials could be considered as a candidate for clathrate synthesis. 

Diederich F, Jonas U, Gramlich V, Herrmann A, Ringsdorf H and Thilgen C 1993 Synthesis of a fullerene derivative of benzo[18]crown-6 by Diels-Alder reaction complexation ability, amphiphilic properties, and x-ray crystal structure of a dimethoxy-1,9-(methano[1, 2]benzomethano)fullerene[60] benzene clathrate Helv. Chim. Acta 76 2445-53... 

Lokshin, K.A. Zhao, Y. (2006). Fast synthesis method and phase diagram of hydrogen clathrate hydrate. App. Phys. Lett., 88, 131909. 

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 - ... 

Cross-metathesis applications, 11, 200 enynes, 11, 282 in ethenolysis, 11, 198 Lewis-basic substrates, 11, 193 in one-pot reactions, 11, 197 for reagent synthesis, 11, 188 as simple metathesis reaction, 1, 171 Crotyltributyltins, with aldehydes, 9, 352 Crown ether clathrates, diorganozinc compounds, 2, 335 Crown ether-pendant polysilanes, preparation, 3, 577 Crown-ethers, as hosts, 12, 813... 

Figure 1.2 Schematic illustrating the difference between a cavitate and a clathrate (a) synthesis and conversion of a cavitand into a cavitate by inclusion of a guest into the cavity of the host molecule (b) inclusion of guest molecules in cavities formed between the host molecules in the lattice resulting in conversion of a clathrand into a clathrate (c) synthesis and self-assembly of a supramolecular aggregate that does not correspond to the classical host-guest description.

Cram [49a] elaborated further on this concept by enclosing space in his carcerands and hemicarcerands (See Scheme 1) to form a new inner phase which he has referred to as a new phase of matter . In contrast to the hollow space found inside clathrates and zeolites for instance, the cages of these molecules are independent of the form and physical state. For example, hemicarcerands and related supramolecular systems (i.e. hemicarceplexes) prevail in the solid, liquid, or gas phase. This characteristic-hollow space, the inside surface — is maintained across all phase transitions. The inner surfaces and spaces of these systems are not manifested as bulk properties. An extensive review on the synthesis of these materials has been published recently [205]. 

When Cd(CN)2 is crystallized in the presence of other molecules that can stuff cavities or tunnels, many different structures are formed depending on the size and shape of the guests that stuff the cavities. Similar behavior is, of course, found elsewhere, e.g., in gas hydrates and hydrothermal synthesis of zeolites. These cadmium cyanide structures may be considered as a new class of clathrates. 

Included among the interesting properties currently being pursued in some Zintl phases are the metal-nonmetal transition and colossal magnetoresistance. Zintl phases have also been used as precursors in the synthesis of novel solid-state materials. For example, afullerene-type silicon clathrate compound, Na2BagSi46 (clathrates are covalent crystals, whereas fullerides are molecular crystals), with a superconducting transition at... 

A simple possibility for the synthesis of esters, the reaction of an acid chloride with an alcohol, was used by Schrage and Vogtle [58] for a two-step synthesis of the macrocycle 63 from the alcohol 61 and the acid chloride 62. Compound 63, an example from the field of host/guest chemistry, forms a cavity, as studied with CPK-models, which could include planar, aromatic guests. Crystals obtained from benzene/ -heptane point to a 1 2 stoichiometry of 63 and benzene according to NMR-spectroscopic data. However, whether this is a molecular inclusion complex or just a clathrate is not yet known. 

Site III). Each pseudo unit cell contains 27 water molecules, and 20 of these form a pentagonal dodecahedron that fines the a-cage (Figure 17). This water structure is found in some water clathrates and in LTA surrounds the central Na ion with its attendant three water molecules. It is cation water assemblages like this that are hkely to be the precursors for the formation of zeolite frameworks in synthesis gels (see Section. 4.2.3). 

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