Synthesis and structures of highly conducting charge-transfer salts of selenium containing TTM-TTP derivatives

Selenium-substituted TTM-TTP (2,5-bis[4,5-bis(methylthio)-1,3-dithiol-2- ylidene]-1,3,4,6-tetrathiapentalene) derivatives (1a-1d), in which several 1,3-dithiole rings of the bis-fused TTF framework are replaced by 1,3-diselenole rings, have been prepared. In particular, we have accomplished the first selenium substitution of the inner TTP part (1a). The 1:1 composition iodine salt, (1a)I₃, is an insulator constructed of the donor trimers. The 1:1 GaCl₄ salt, (1a)GaCl₄, has a uniform column isostructural to the sulfur analog, (TTM-TTP)FeBr_1.8Cl_2.2; it exhibits metallic conduction down to about 60 K, the lowest metal-insulator (M-I) transition temperature in 1:1 salts. The donors 1c and 1d give the iodine salts isostructural to the sulfur analog, (TTM-TTP)(I₃) _5/3. By the selenium substitution, the M-I transition of (1d)(I ₃)_5/3 is entirely suppressed.