Decay of electrogenerated dianions of acidic indenes: an ‘indirect’ self-protonation mechanism

Abstract

The electrode reduction of 1,2,3-triphenylindene, giving rise to the corresponding radical anion and dianion in two successive one-electron steps, was investigated in dimethylformamide with the aim of clarifying the role of the acidic substrate in the decay of the electrogenerated bases. At –50 °C the direct protonation of the radical anion by indene (self-protonation) was too slow to be effective, and the proton transfer from indene to the dianion did appear kinetically unfavourable with respect to solution electron transfer between the same species (comproportionation). However, dianion protonation by an excess of water present in the solution can induce, through the action of OH^– ions, the substrate to transform into its conjugate base. The effect resembles that predicted for a self-protonation mechanism and can be termed an ‘indirect’ self-protonation process. The proposed reaction scheme was confirmed by comparison of cyclic voltammetric responses with those derived by mathematical computation. The rate-constant values for the relevant steps, obtained by the fitting of experimental and computed data, are consistent with the corresponding values determined for similar compounds.