Electron-transfer reactions accompanied by substantial structural changes: Oxidation of 9,10-bis(dimethylamino)anthracene and 3,6-bis(dimethylamino)durene

The electrochemical oxidation of N,N,N',N'-tetramethyl-1,4-phenylenediamine (1), 9,10-bis(dimethylamino)anthracene(2) and 3,6-bis(dimethylamino)durene (3) has been studied at glassy carbon electrodes in acetonitrile. 1 shows two one-electron oxidation steps with the normal ordering of potentials, the second electron being 0.6 V more difficult to remove than the first. Each electron-transfer reaction is inherently rapid. 2 and 3, by contrast, show a single two-electron oxidation peak by cyclic voltammetry. Potentiometric titrations suggest for 2 and demonstrate for 3 that the reversible formal potentials are inverted for these species, with the second electron being removed more easily than the first. For 3, the overall two-electron reversible potential is -0.035 V vs. ferrocene, and the difference in the one-electron formal potentials is estimated to be more than 0.20 V. Evaluation of the standard heterogeneous electron-transfer rate constants for the individual oxidation steps was carried out for 2 and 3, and it was found that the values are unusually small, much smaller than seen for 1. This observation has been interpreted to mean that substantial structural change accompanies the oxidation of 2 and 3, but not 1. AM1 calculations have been used to illuminate the causes of sluggish electron-transfer kinetics for 2 and 3, and to rationalize the observed inversion of the potentials.