Interaction in molecular crystals, 155. Crystallization and structure determination of the radicalcation salt [Tetrahydrotetrathiafulvalenium(center dot circle plus)] [AlCl4 circle minus]

The dark red tetrahydrstetrathiafulvalenium radical cation has been generat ed 20 years ago by iodine oxidation of hexathia-dispiro[4.0.4.4]decane and characterized in solution by its temperature-dependent ESR spectrum. Here w e report the single crystal growth of its tetrachloroaluminate salt as well as its low temperature structure determination, which proves the C=C doubl e bond to be elongated to 140 pm length and one molecular half to be twiste d by 7 degrees. The assumed slight cyanine type distortion is supported by structural comparison with tetrakis(dimethylamino)ethene dication salts and wi th the 1:3 complex of tetrahydrotetrathiafulvalene mercury dichloride. Unrestricted Density Functional Theory calculations predict both the spin a s well as the positive charge to be distributed predominantly within the pi molecular center {S2C=CS2}.