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 (CIBP) with alkyl triflates affords 8-chloro-4-alkyl-4H-bis[1,2,3]dithiazolo[4,5-b:5',4'-e]pyridin-2-ium triflates [CIBPR]-[OTf] (R = Me, Et, Pr). Reduction of these salts with decamethylferrocene affords the corresponding CIBPR 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/631 G**) 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 CIBPR (R = Me, Et, Pr) have been determined by X-ray crystallography (at 293 K). All three structures consist of slipped pi-stacks of undimerized radicals, with many close intermolecular S...S contacts. CIBPMe undergoes a phase transition at 93 K to a slightly modified slipped g-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 Huckel band calculations. The room-temperature conductivities of CIBPR systems (sigma(RT) approximate to 10(-5) to 10(-6) S cm(-1)), as well as the weak 1 D ferromagnetism exhibited by CIBPMe, are interpreted in terms of weak intermolecular overlap along the pi-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.