Electron-transfer reactions with significant inner reorganization energies. Two-electron oxidation of derivatives of 1,4-bis(dialkylamino)-1,3-butadiene

Oxidation of compounds 1a-1c was studied in acetonitrile by cyclic voltammetry and homogeneous redox catalysis. Each compound undergoes a two-electron oxidation with removal of the second electron occurring more easily than the first. Thus, the one-electron product, radial cation 1(+), is unstable with respect to disproportionation. Results of UV-vis absorption, ESR, and potentiometric titration experiments were consistent with this conclusion. The rate constant for removal of the first electron at glassy carbon, gold, or platinum electrodes is unusually small, consistent with a substantial structural change accompanying this step. At a glassy carbon electrode, the rate constant for the first electron transfer is only 2% that of the second, and by homogeneous redox catalysis it was found that the rate constant for electron transfer between 1a(+) and ferrocene was sufficiently small. to cause this reaction to be rate-limiting compared to reaction of ferrocene with the dication. AM1 calculations indicate that the structural changes accompanying removal of the first electron are quite substantial and are larger for the loss of the first electron compared to the second for seven structural parameters that were investigated. The calculations suggest that the inner reorganization contribution to the enthalpic barrier for the first electron transfer is about 9 kcal/mol compared to 3 kcal/mol for the second step of oxidation, a result that is qualitatively consistent with the experimental results.