Recent Synthetic Advances of Tetrathiafulvalene-Based Organic Conductors

The synthetic chemistry of tetrathiafulvalene (TTF)-type electron donors has been markedly promoted by introducing two key reactions, the one-pot preparation of 1,3-dichalcogenole-2-chalcogenones from terminal alkynes and the formation of heterocyclic rings via transalkylation on a chalcogen atom. In combination with a conventional trialkyl phosphite-promoted coupling reaction of 1,3-dichalcogenole-2-chalcogenones, the first reaction constitutes a ready access to the tetrachalcogenafulvalene skeleton. As a result, a number of previously inaccessible TTF-type electron donors were readily obtained. These included tetraselenafulvalene (TSF) derivatives, the selenium/sulfur or selenium/tellurium hybrid systems, dimeric TSFs linked with a single spacer group, and sophisticated stacked TSF phanes. The second reaction is very useful in constructing additional heterocyclic ring(s) on the tetrachalcogenafulvalene skeleton. A wide variety of five-, six-, and seven-membered heterocycle-fused TTF-type compounds were thus synthesized. These were systematically examined for superior electron donors of conductive molecular complexes. This study resulted in the discovery of two novel electron donors, methylenedithio-tetraselenafulvalene and methylenedithio-diselenadithiafulvalene, that form unique superconductors with exceptionally high conductivities at room temperature. The scopes and limitations of these synthetic reactions are described in connection with the recent research trends of TTF-based organic (super)conductors.