A comprehensive study on electrochemical oxidation of phenothiazine in water-acetonitrile mixture: Electrosynthesis of phenothiazine dimers

Abstract

Electrochemical oxidation of phenothiazine (PTZ) was investigated in an acetonitrile/water (50/50, v/v) mixture using differential pulse voltammetry, cyclic voltammetry, choronopotentiometry, constant current electrolysis and controlled-potential coulometry. Our findings show that the oxidation pathway of PTZ at different pH values does not follow a simple pattern and varies with respect to pH. Our results show that the oxidation process at pH ≤ 1, occurs via two one-electron steps, while this process at 1 < pH ≤ 7 is a two-electron process. Surprisingly, this oxidation at pH values > 7 again turns into two one-electron steps. We have performed this study to achieve the following goals. (a) To gain in-depth insights into the oxidation mechanism of PTZ and (b) the development of a new strategy for PTZ dimerization. To achieve the second goal, we performed electrooxidation of PTZ at pH 3. Using the obtained electrochemical data, we succeeded in synthesizing two new types of PTZ dimer (D1 and D2). The structures of D1 and D2 are fully characterized by 1H NMR, 13C NMR, mass spectrometry, and FTIR techniques. The synthesis of D1 and D2 is performed in a mixture of acetonitrile and water (50/50, v/v), without the use of the oxidant or catalyst, using an undivided cell and carbon electrode, under mild conditions, using an air-assisted electrochemical method.