Substituent effects of 1,2-dithiole groups on the electrochemical oxidation of some ferrocenyl-1,2-dithiole compounds

New ferrocenyl compounds substituted by sulfur containing groups were synthesized leading to ferrocenyl-3H-1,2-dithiole-3-thiones and related compounds. The substituent of the ferrocene was a [3-thioxo-3H-1,2-dithiol]-4 or 5-yl, a [3-oxo-3H-1,2-dithiol]-4 or 5- yl or a [3-methylsulfanyl-3H-1,2-dithiolium]-4 or 5-yl cation group. Their anodic behavior was studied by cyclic voltammetry at a Pt electrode in aprotic solvent. All synthesized ferrocenes exhibited a one-electron reversible oxidation leading to the corresponding ferricinium cation. At low potential scan, the irreversible oxidation of 5-ferrocenyl-3H-1,2-dithiole-3-thione was observed and attributed to a dimerization involving the dithiolethione group. Redox potential of the reversible oxidation allowed the determination of the electronic effect of the 1,2-dithiole groups. The Hammett σ_p constants of the dithiole substituents were obtained from linear correlation between oxidation potentials and electronic effects. The results showed that the [3-thioxo-3H-1,2-dithiol]-5-yl and the [3-methylsulfanyl-3H-1,2-dithiolium]-5-yl cation groups were strong inductive electron withdrawing substituents characterized by σ_p values of 0.55 and 0.97, respectively.