Resonance-Stabilized 1,2,3-Dithiazolo-1,2,3-dithiazolyls as Neutral π-Radical Conductors

Alkylation of the zwitterionic heterocycle 8-chloro-bis[1,2,3]dithiazolo[4,5-b:5‘,4‘-e]pyridine (ClBP)
with alkyl triflates affords 8-chloro-4-alkyl-4H-bis[1,2,3]dithiazolo[4,5-b:5‘,4‘-e]pyridin-2-ium triflates [ClBPR][OTf] (R = Me, Et, Pr). Reduction of these salts with decamethylferrocene affords the corresponding ClBPR
radicals as thermally stable crystalline solids. The radicals have been characterized in solution by cyclic
voltammetry and EPR spectroscopy. Measured electrochemical cell potentials and computed (B3LYP/6-31G**) gas-phase disproportionation enthalpies are consistent with a low on-site Coulombic barrier U to
charge transfer in the solid state. The crystal structures of ClBPR (R = Me, Et, Pr) have been determined
by X-ray crystallography (at 293 K). All three structures consist of slipped π-stacks of undimerized radicals,
with many close intermolecular S···S contacts. ClBPMe undergoes a phase transition at 93 K to a slightly
modified slipped π-stack arrangement, the structure of which has also been established crystallographically
(at 25 K). Variable-temperature magnetic and conductivity measurements have been performed, and the
results interpreted in light of extended Hückel band calculations. The room-temperature conductivities of
ClBPR systems (σRT ≈ 10-5 to 10-6 S cm-1), as well as the weak 1D ferromagnetism exhibited by ClBPMe,
are interpreted in terms of weak intermolecular overlap along the π-stacks. The latter is caused by slippage
of the molecular plates, a feature necessitated by the steric size of the R and Cl groups on the pyridine
ring.