Electrode reactions of 8-mercaptoquinoline and manganese(II)-8-mercaptoquinolinato complexes in non-aqueous solvent

Abstract

The electrode reactions of 8-mercaptoquinoline and manganese (II)-8-mercaptoquinolinato complexes at a dropping mercury electrode (DME) and a glassy, carbon electrode (GCE) have been studied in N,N-dimethylformamide (DMF). The sodium salt of 8-mercaptoquinoline (8-MQNa) undergoes a reversible one-electron oxidation wave ( E 1/2=−0.878 V vs. SCE) and an irreversible two-electron reduction wave ( E 1/2=−2.188 V vs. SCE) at the DME. The existence of bis- and tris-complexes of manganese (II), i.e. [Mn(8-MQ) 2] and [Mn(8-MQ) 3] −, has been established by analysing the polarograms and absorption spectra. These redox processes have been examined by polarography, cyclic voltammetry, controlled potential electrolysis, and spectrophotometry. These complexes undergo a one-electron oxidation wave at the GCE and a two-electron reduction wave at the DME. Manganese (II) compounds show complex behaviour for the oxidation waves at the DME. The proposed oxidation mechanisms at the DME are: [Mn(8-MQ) 2]+Hg→Mn 2++[Hg(8-MQ) 2]+2 e − or 2[Mn(8-MQ) 3] −+Hg ag2[Mn(8-MQ) 2]+[Hg(8-MQ) 2]+2 e − [Mn(8-MQ) 2]+Hg→Mn 2++[Hg(8-MQ) 2]+2 e − and the reduction steps can be expressed by the following schemes: [Mn(8-MQ) 2]+2 e −→[Mn(8-MQ) 2] 2− or [Mn(8-MQ) 3] −+2 e −→[Mn(8-MQ) 2] 2−+(8-MQ) − (8-MQ) −+2 e −→(8-MQ) 3− [Mn(8-MQ) 3] − undergoes a single one electron oxidation step at the GCE: [Mn(8-MQ) 3] − ag[Mn(8-MQ) 3]+ e − The oxidation mechanism of [Mn(8-MQ) 2] at the GCE is also proposed.