Linear correlation of electrochemical reduction potential with substituent effect in a 5-phenyl-1,2-dithiole-3-thione series

A range of 19 different 5-phenyl-1,2-dithiole-3-thiones, meta- or para-substituted, were investigated in a DMF electrolyte at a platinum electrode. In all cases, excepted for the para-nitrophenyl derivative, the reduction appeared as a quasi-reversible process at 90 °C while it was generally irreversible at 20 °C. The redox potential of the quasi-reversible reduction (Eredox), or the peak potential of the irreversible reduction (Epc) followed a linear correlation involving the Hammett parameters σm and σp related to the phenyl group substituent. Good linear correlation relationships were obtained regardless of the characteristic potential (Epc or Eredox) or the temperature: at 20 °C Epcx (V/Fc+/Fc)=0.20σx−1.54 (r=0.96) and at 90 °C Eredoxx (V/Fc+/Fc)=0.16σx−1.46 (r=0.97). In the case of strong electron-withdrawing substituents (para-CN or para-CO2CH3) the compounds showed quasi-reversible behaviour even at room temperature.