CHARGE-TRANSFER SALTS OF BENZENE-BRIDGED 1,2,3,5-DITHIADIAZOLYL DIRADICALS - PREPARATION, STRUCTURES, AND TRANSPORT-PROPERTIES OF 1,3-[(S2N2C)C6H4(CN2S2)][X] AND 1,4-[(S2N2C)C6H4(CN2S2)][X] (X=I,BR)

Cosublimation of 1,3- and 1,4-benzene-bis(1,2,3,5-dithiadiazolyl) and iodine/bromine affords crystals of the mixed valence salts 1,3- and 1, 4-[(S2N2C)C6H4(CN2S2)][X] (X = I, Br). The crystal structures of the t wo iodide salts consist of perfectly superimposed stacks of molecular units with interannular spacing along the stacks of 3.487(3) and 3.415 (2) Angstrom for the 1,3- and 1,4-derivatives. In both compounds the i odines are disordered along the stacking direction. The 1,3-derivative has a highly one-dimensional structure; there are no short intercolum nar S-S interactions. In the 1,4-derivative, however, lateral S-S cont acts of 3.911 Angstrom afford some measure of three-dimensionality. Th e bromide salt of the 1,4-derivative consists of ribbons of alternatin g 1,4-[(S2N2C)C6H4(CN2S2)](+) units and bromide ions. Within each mole cule one heterocyclic ring is closed shell, i.e., a [CN2S2](+) cation, while the other is a discrete radical. The ribbons are layered in zig zag fashion that maximizes ion pairing and isolates the radical center s. The bromide salt of the 1,3-derivative also forms ribbon-like array s, but the unit cell repeat consists of four layers of ribbons. Within these layers the [CN2S2] rings are approximately stacked. The four ri ngs within the repeat unit along each stack consists of three rings cl ustered into a trimeric [CN2S2](+)(3) cation, while the remaining ring is a discrete [CN2S2](+) cation. Magnetic susceptibility and conducti vity measurements on the two iodide salts indicate weakly metallic beh avior at room temperature, with a charge density wave (CDW) driven met al-insulator phase transition occurring near 270 and 190 K for the 1,3 - and 1,4-derivatives, respectively. For the 1,4-derivative, analysis of the CDW wavevector associated with the transition affords a degree of charge transfer of 1/4 of electron per radical, i.e., an overall fo rmulation of [(S2N2C)C6H4(CN2S2)](0.5+)[I](0.5-). The bromide salt of the 1,3-derivative is a closed shell insulator, while in the 1.4-bromi de the isolated radical centers are antiferromagnetically coupled.