Tetraselenafulvalene complexes

The synthesis of the mixed sulfur-selenium donor BEDS-TTF (BEST) via the tetralithiation of TTF was mentioned above [53]. Bis(ethylenedithio) tetraselenafulvalene (BEDT-TSF, BETS) is obtained in good yield following the sequence outlined in Scheme 14. The synthesis makes use of a titanium-cyclopentadiene complex to stabilize a problematic, unstable synthetic intermediate (35) [7]. 

The synthesis of the zinc complex 115 according to Scheme 3 presents an efficient alternative to the previous method based on reduction of carbon diselenide by sodium metal <1990JCD115>. Compounds 22, 98-101, and 116 containing either (2-methoxycarbonyl)ethyl or 2-cyanoethyl as thiolate protecting groups are versatile compounds for the synthesis of a large selection of functionalized tetraselenafulvalenes via the (1) base-promoted deprotection and (2) thiolate alkylation protocol. 

Cyclic voltammetry on the cation 192 was not reversible attempted reoxidation of the reduction product of 192 produced the cation-radical of the tetraselenafulvalene 193. Apparently, the initially formed radicals dimerize rapidly with elimination of the SeEt groups at ambient temperatures. The cation-radical of 193 and various substituted derivatives have been reported,As with the sulfur analogs (see Section III,B,7,b), no comprehensive survey of the solid-state properties of these materials is attempted. In general, it is found that 193 and its derivatives, although rather more difficult to oxidize than tetrathiafulvalene counterparts, nevertheless do form electrically conducting salts and complexes. 

Like their sulfur analogues, tetraselenafulvalenes and the mixed sulfur/selenium systems are excellent electron donors which are able to form radical cation salts and charge transfer complexes with appropriate electron acceptors. 

A plethora of selenium-containing heterocycles are known to act as the donor component for donor-acceptor complexes that have properties as electrical conductors. Two examples that have provoked widespread interest are tetraselenafulvalene (54) and tetraselenotetracene (55). A variety of selenium-containing polymers such as (7) are also of interest in this regard. 

T. Jigami, K. Takimiya, T. Otsubo and Y. Aso, Novel selenocycle-fused TTF-type of electron donors forming conducting molecular complexes bis(ethyleneseleno)tetrathiafulvalene (BES-TTF), diselenolotetrathiafulvalene (DS-TTF) and bis(ethyleneseleno)tetraselenafulvalene (BES-TSF), J. Org. Chem., 63, 8865-8872 (1998). 

Replacement of the four sulphur atoms of TTF by four selenium atoms leads, in the TCNQ complex, to enhanced metallic behaviour. The required tetraselenafulvalene (56 R = H, X = Se) was prepared by deselenization of l,3-diselenole-2-selone a convenient alternative route has been described. Similar methods have been used to prepare tetramethyltetraselenafulvalene (56 R = Me, X = Se) (which forms two salts with TCNQ, one a red insulator and the other a black conductor), a series of similar derivatives, and a tetrakis-methylselenotetraselenafulvalene (56 R = MeSe, X = Se). ... 

Tetraselenafulvalene and diselenadithiafulvalene have been compared (by cyclic voltammetry and measurement of gas-phase ionization potentials) with TTF and are found to be weaker electron donors. The TCNQ complex of... 

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