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

Abstract

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 ( E redox), or the peak potential of the irreversible reduction ( E p c ) followed a linear correlation involving the Hammett parameters σ m and σ p related to the phenyl group substituent. Good linear correlation relationships were obtained regardless of the characteristic potential ( E p c or E redox) or the temperature: at 20 °C E p c x (V/Fc +/Fc)=0.20 σ x−1.54 ( r=0.96) and at 90 °C E redox x (V/Fc +/Fc)=0.16 σ x−1.46 ( r=0.97). In the case of strong electron-withdrawing substituents ( para-CN or para-CO 2CH 3) the compounds showed quasi-reversible behaviour even at room temperature.