COMPROPORTIONATION IN THE REDUCTION OF PYRIDINIUM DERIVATIVES - A COMBINED ESR AND ELECTROCHEMICAL STUDY

Combination of voltammetry at various types of solid electrodes (mostl y Pt) with electrochemical generation of radical intermediates and the ir ESR investigation enabled a detailed elucidation of the reduction m echanism of 1,2,4,6-tetrasubstituted pyridinium cations and of analogo us 1,1'-X-hexasubstituted bipyridinium dications. The experiments were carried out in aprotic solvents such as eg dimethylformamide. The mea surements confirmed the reversible stepwise one-electron reduction of the quaternary pyridinium site leading to the formation of the ESR-vis ible radical (or radical cation) and followed by the uptake of a furth er electron. These reactions result in the formation of the correspond ing anion and its protonation (possibly by traces of water present in the solution). The measurement of the change in the ESR intensity with time of the radical without applied voltage revealed that the redox m echanisms of the compounds are complicated by synproportionation betwe en the starting substance and theanion formed by the uptake of two ele ctrons per a single pyridinium group. (C) 1997 Elsevier Science Ltd.