Fulvalenes having large

Equation (171) illustrates a route by which a double bond can be formed between two dithiole rings. By this type of reaction, a large variety of unsymmetrical tetrathia- and dithiadiselena-fulvalenes have been prepared in exploration of new molecular-based organic metals. 

Whereas the fulvalenes 1-6 are relatively unstable hydrocarbons and therefore largely of theoretical interest, their heteroatom analogs demand considerable attention in synthetic chemistry and material sciences. Tlie general principle of heterocyclic chemistry to relate heterocyclic compounds to carbocyclic ones was the driving force for the synthesis and their application to heteroful-valenes. Numerous heterocyclic derivatives iso-rr-electronic with, for example, heptafulvalene 3 were accessible in which pairs of carbon atoms linked by double bonds were replaced by heteroatoms capable of contributing two tt-electrons. By this principle, the well-known tetrathiafulvalene and its derivatives have been synthesized successfully (Scheme 2). 

Nitrogen-containing fulvalenes, particularly their di- and tetraazaderiva-tives, have to be classified in the large group of organic compounds which comprise two-step redox systems (78AG927) (Scheme 46). 

The trithionium salts also condense with the weak C—H acids cyclopentadiene, indene, and fluorene, but again only when the latter are present as their sodium salts. As we have already pointed out, however, the reactions with such strong bases are rather unpredictable, and the desired 1,2-dithiafulvalenes (146) may be obtained (if at all) only in very low yields, together with a large number of by-products. The formation of some of these by-products is attributed to the fact that the 1,2-dithiafulvalenes, like the sesqui-fulvalenes, also react as fulvenes and add on strong bases.112... 

Brown and Ikeda [25, 26] have reported thermal and photo-assisted decomposition of Cp2Ti(N)3)2 to deposit TiN. The fact that these films are generally contaminated with a large amount of carbon indicates that the cyclopentadiene ligands do not form volatile leaving groups such as HCp or fulvalenes. On the other hand, cyclopentadi-enyl compounds of titanium have been successfully applied to the preparation of TiC films by MOCVD [27]. 

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