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

• Published in: Journal of the American Chemical Society 2002-08, Vol.124 (32), p.9498-9509
• Main Authors: Beer, Leanne, Brusso, Jaclyn L, Cordes, A. Wallace, Haddon, Robert C, Itkis, Mikhail E, Kirschbaum, Kristin, MacGregor, Douglas S, Oakley, Richard T, Pinkerton, A. Alan, Reed, Robert W
• Format: Article
• Language: English
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Electron paramagnetic resonance and relaxation; Exact sciences and technology; Free radicals; Heterocyclic compounds; Magnetic resonances and relaxations in condensed matter, mössbauer effect; Organic compounds; Physics; Structure of solids and liquids; crystallography; Structure of specific crystalline solids
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja026118s

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.