Reactivity of radical-anions and dianions derived from electrochemical reduction of tetraphenylbutatriene and 1,1,4,4-tetraphenyl-1,3-butadiene

Abstract

Cyclic voltammograms recorded at a hanging mercury drop in dimethylformamide containing tetra-n-butylammonium perchlorate show that tetraphenylbutatriene and 1,1,4,4-tetraphenyl-1,3-butadiene are each reduced in stepwise fashion to a radical-anion and to a dianion. For the first stage of reduction, the radical-anion derived from each compound is chemically stable for sweep rates ranging from 50 mV s −1 to 100 V s −1 and the heterogeneous electron-transfer process is reversible. Dianions generated from the starting materials react with proton donors in the medium, but the electron-transfer process involving the radical-anion and dianion can be rendered reversible at sweep rates larger than 1 V s −1 for tetraphenylbutatriene and 20 V s −1 for 1,1,4,4-tetraphenyl-1,3-butadiene. Experimentally acquired cyclic voltammograms agree well with those calculated by means of digital simulation if a diffusion-controlled comproportionation reaction is included in the mechanism for reduction of each compound. Pseudo-first-order rate constants for protonation by the medium of the dianions derived from tetraphenylbutatriene and 1,1,4,4-tetraphenyl-1,3-butadiene are 22 and 200 s −1, respectively. Addition of excess phenol causes the first voltammetric wave to increase in magnitude at the expense of the second wave, and the behavior of each compound changes from that of an EE mechanism to that of a DISP1 scheme where the second electron is transferred homogeneously in solution by the radical-anion to the neutral radical at a diffusion-controlled rate. Pseudo-first-order rate constants for protonation of the radical-anions formed by reduction of tetraphenylbutatriene and 1,1,4,4-tetraphenyl-1,3-butadiene in the presence of a ten-fold excess of phenol are 0.8 and 11.3 s −1, respectively.