Synthesis, Crystal Structure, and Electrical Properties of Novel Radical Cation Salts of Iodine-Substituted TTF-Derived Donors with the Nitroprusside Anion Exhibiting Strong -I···NC- Interactions

Radical-cation salts of iodine-substituted TTF derivatives [diiodoethylenedithiotetrathiafulvalene (1) diiodoethylenedithiodiselenadithiafulvalene (2), or iodoethylenedithiotetrathiafulvalene (3)] with the [Fe(CN)5NO]2− nitroprusside anion, namely, 14·[Fe(CN)5NO], 24·[Fe(CN)5NO] and 33·[Fe(CN)5NO], were synthesized by electrocrystallization from solutions of the appropriate TTF derivative in CH2Cl2 with bis(tetraphenylphosphonium)nitroprusside [(PPh4)2{Fe(CN)5NO}] as electrolyte. An X-ray structure analysis indicates that the crystal structures of 14·[Fe(CN)5NO] and 24·[Fe(CN)5NO] are very similar. In both cases, molecules 1 and 2 are stacked along the c direction, with their long molecular axis parallel to each other. By contrast, in 33·[Fe(CN)5NO], molecules 3 are stacked in triad units in which two of the molecules are arranged in a face-to-face fashion and the remaining molecule is arranged in a head-to-tail fashion with respect to its neighboring molecules. In all cases, several effective iodine-nitrogen contacts between donor molecules and [Fe(CN)5NO]2− anions are observed. The room temperature electrical conductivities of 14·[Fe(CN)5NO], 24·[Fe(CN)5NO] and 33·[Fe(CN)5NO] are <10−6, 2.2 × 10−2 and 5 S cm−1, respectively. The temperature dependence of the electrical conductivity of 33·[Fe(CN)5NO] reveals semiconducting behavior with a small activation energy of 30 meV. The temperature dependence of the electrical conductivity of 24·[Fe(CN)5NO] also reveals a semiconducting behavior, but with two regimes: above 200 K the activation energy is 60 meV and becomes much smaller — 0.2 meV — below this temperature. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)