Redox, Magnetic, and Structural Properties of 1,3,2-Dithiazolyl Radicals. A Case Study on the Ternary Heterocycle S 3 N 5 C 4

The characterization of the heterocyclic radical 1,2,5-thiadiazolo[3,4- b ]-1,3,2-dithiazolo[3,4- b ]pyrazin-2-yl (TDP-DTA) is described. The compound is prepared by treatment of 5,6-dithiolo-1,2,5-thiadiazolo[3,4- b ]pyrazine with S 3 N 3 Cl 3 and purified by fractional sublimation in vacuo. The results of cyclic voltammetry and ESR analysis of TDP-DTA and related heterocyclic dithiazolyls indicate that the spin distributions and donor/acceptor properties of these radicals are extremely sensitive to the nature of the 4,5-substituents. The crystal structure of TDP-DTA has been determined at two temperatures. At 293 K, the crystals are triclinic, space group P 1̄, with a = 4.4456(8), b = 8.407(2), c = 9.671(3) Å, α = 71.34(2), β = 89.28(2), γ = 87.80(2)°, Z = 2 (for C 4 N 5 S 3 ); at 150 K the crystals are triclinic, space group P 1̄, with a = 7.489(7), b = 9.593(4), c = 10.759(6) Å, α = 65.77(4), β = 74.10(6), γ = 74.64(6)°, Z = 2 (for C 8 N 10 S 6 ). At 293 K the structure consists of ribbons of TDP-DTA radicals packed in a slipped π-stack arrangement. In the low temperature phase alternate layers of ribbons are shifted laterally to produce arrays of dimers. Within these dimers three long (3.401(5), 3.460(5) and 3.511(5) Å) interannular S---S contacts link the two molecules. A multiple “tectonic plate” slippage mechanism is proposed to account for the interconversion of the two phases. The structural results are discussed in the light of variable temperature magnetic susceptibility measurements.