EPR and voltammetric evidence for the reversible dimerization of anion radicals of aromatic meta-substituted diesters and dithioic S,S9-diesters

The electrochemical behaviour of some alkyl meta-substituted (R = Me or Pr) diesters and dithioic S,S9-diesters of pyridine and benzene (trivial names dipicolinate and isophthalate esters) have been studied by cyclic voltammetry (scan rates = 0.1–50 V s) in acetonitrile with 0.1 M tetrabutylammonium hexafluorophosphate as the supporting electrolyte. The cyclic voltammetry data can be interpreted as suggesting that the compounds undergo a one electron reduction to form the corresponding radical anions, which rapidly decompose via a chemical step, i.e. an EC mechanism, with the chemical step occurring in the order of several milliseconds. However, in situ electrochemicalEPR experiments performed during the one electron reduction of the diesters have confirmed the existence of relatively stable radical anions with a much greater lifetime (t1/2 > 2–5 s). Furthermore, the very simple nature of the EPR spectra, combined with product analysis data from bulk controlled potential electrolysis experiments, provide very good evidence that the anion radicals are the primary radicals formed by one electron reduction of the starting materials. Therefore, the apparent discrepancy in the lifetimes of the radical anions obtained by voltammetric and spectroscopic methods have been rationalized by considering a reversible dimerization mechanism. Rate constants have been fitted to the cyclic voltammetry data by digital simulation techniques and were estimated to be kf ≈ 10–10 L mol s (for a radical–radical coupling step) and kb ≈ 10–10 s (for the dianion dimer to dissociate to form two anion radicals). Only approximate rate constants could be derived from the experimental curves because a large number of variables needed to be included in the simulations, meaning that no unique combination of variables would give a reasonable data fit. The dimerization reaction is also complicated by a competing reaction where the radical anions or dianions irreversibly decay to form stable products. Thus, the competing decay reaction has also been included in the electrochemical simulations and gives values of kd ≈ 10–10 s (assuming a first-order decay).