Preparation and Characterization of a Neutral π-Radical Molecular Conductor

The synthesis and solid-state characterization of the heterocyclic π-radical 1,2,5-thiadiazolo[3,4-b]-1,2,3-dithiazolo[3,4-b]pyrazin-2-yl, 1,2,3-TDTA, is described. The ESR spectrum of 1,2,3-TDTA (in CH2Cl2, 293 K, g = 2.009) confirms a highly delocalized spin distribution, with observable hyperfine coupling to
all five nitrogen atoms of the tricyclic molecule (aN = 0.514, 0.343, 0.109, 0.051, and 0.045 mT). While
chemical and electrochemical oxidation (E1/2(ox) = 1.14 V vs SCE) of 1,2,3-TDTA requires relatively harsh
conditions, reduction is extremely facile (E1/2(red) = 0.15 V vs SCE). More importantly both the observed
cell potential Ecell and computed (MNDO) gas-phase enthalpy ΔHdisp for the disproportionation of this and
other 1,2,3-dithiazolyls are significantly lower than those observed for their 1,3,2-isomers. Crystals of 1,2,3-TDTA are monoclinic P21/n, with a = 6.6749(16) Å, b = 11.7178(14) Å, c = 8.6148(14) Å, β =
103.297(16)°, and Z = 4. The crystal structure consists of slipped stacks of head-to-tail (centrosymmetric)
π-dimers. The closest intradimer S- - -S contact (S2- - -S3) is 3.2331(15) Å. Variable-temperature magnetic
susceptibility measurements establish that 1,2,3-TDTA is essentially diamagnetic at room temperature. The
magnetic data, along with the results of variable-temperature single-crystal conductivity measurements (1,2,3-TDTA exhibits a room-temperature conductivity σ = 1 × 10-4 S cm-1), are interpreted in terms of one-dimensional hopping mechanism for charge transport.