Substituent effect on the electrochemical behaviour of some ortho-substituted (aryl)(2-nitrobenzo[b]thiophen-3-yl)amines. A combined experimental and computational study

Abstract

The reduction potential values of the title compounds (1Ar,o) have been evaluated by cyclic voltammetry at a platinum electrode in the presence of 0.1 M N(Et)₄BF₄ in DMSO. Compounds 1Ar,o give reversible reduction peaks. The ortho-substituent affects their values basically by electronic effects (good LFER have been observed). The peculiar behaviour of (2′-hydroxyphenyl)(2-nitrobenzo[b]thiophen-3-yl)amine 1Ar,o(i) (showing two quasi reversible systems of peaks) has been explained on the basis of the special effect of the 2′-hydroxy substituent, which is able to assist a proton transfer through a network of hydrogen bonds involving the amine nitrogen atom (proton shuttle). This behaviour is unexpected in anhydrous DMSO and has similarities with the reaction features observed in water. The above hypothesis has been confirmed by quantum-mechanical DFT calculations carried out on 1Ar,o(i). In this theoretical investigation all the possible species, that can form as a consequence of two mono-electron reduction processes and the relevant various chemical transformations involving the intermediate species, have been carefully investigated.