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

Abstract

The electrochemical behaviour of some alkyl meta-substituted (R = Me or Pr) diesters and dithioic S,S′-diesters of pyridine and benzene (trivial names dipicolinate and isophthalate esters) have been studied by cyclic voltammetry (scan rates = 0.1–50 V s^–1) 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 electrochemical-EPR experiments performed during the one electron reduction of the diesters have confirmed the existence of relatively stable radical anions with a much greater lifetime (t_1/2 > 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 k_f ≈ 10³–10⁴ L mol^–1 s^–1 (for a radical–radical coupling step) and k_b ≈ 10^–1–10⁰ s^–1 (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 k_d ≈ 10^–1–10⁰ s^–1 (assuming a first-order decay).

References

1. R. D. Webster, A. M. Bond and R. G. Compton, J. Phys. Chem., 1996, 100, 10288.
2. (a) R. D. Webster, A. M. Bond and T. Schmidt, J. Chem. Soc., Perkin Trans. 2, 1995, 1365; (b) R. D. Webster and A. M. Bond, J. Org. Chem., 1997, 62, 1779; (c) R. D. Webster and A. M. Bond, J. Chem. Soc., Perkin Trans. 2, 1997, 1079.
3. R. D. Webster, A. M. Bond and D. C. Coomber, J. Electroanal. Chem., 1998, 422, 217.
4. (a) A. Yildiz and H. Baumgärtel, Ber. Bunsenges. Phys. Chem., 1977, 81, 1177; (b) M. A. Oturan and A. Yildiz, J. Electroanal. Chem., 1984, 161, 377.
5. (a) O. Hammerich and V. D. Parker, Acta Chem. Scand., Ser. B, 1981, 35, 341; (b) V. D. Parker, Acta Chem. Scand., Ser. B, 1981, 35, 349; (c) O. Hammerich and V. D. Parker, Acta Chem. Scand., Ser. B, 1983, 37, 379; (d) O. Hammerich and V. D. Parker, Acta Chem. Scand., Ser. B, 1983, 37, 851.
6. (a) C. Amatore, J. Pinson and J. M. Savéant, J. Electroanal. Chem., 1982, 137, 143; (b) C. Amatore, J. Pinson and J. M. Savéant, J. Electroanal. Chem., 1982, 139, 193; (c) C. Amatore, D. Garreau, M. Hammi, J. Pinson and J. M. Savéant, J. Electroanal. Chem., 1984, 184, 1.
7. (a) C. Z. Smith and J. H. P. Utley, J. Chem. Res. (S), 1982, 18; (b) A. S. Mendkovich, L. V. Michalchenko and V. P. Gultyai, J. Electroanal. Chem., 1987, 224, 273.
8. R. M. Crooks and A. J. Bard, J. Electroanal. Chem., 1988, 240, 253.
9. (a) A. B. Suttie, Tetrahedron Lett., 1969, 12, 953; (b) E. T. Denisov and I. V. Khudyakov, Chem. Rev., 1987, 87, 1313; (c) J. A. Richards, P. E. Whitson and D. H. Evans, J. Electroanal. Chem., 1975, 63, 311; (d) J. A. Richards and D. H. Evans, J. Electroanal. Chem., 1977, 81, 171.
10. (a) D. H. Evans, P. J. Jimenez and M. J. Kelly, J. Electroanal. Chem., 1984, 163, 145; (b) P. Hapiot and J. Pinson, J. Electroanal. Chem., 1993, 362, 257.
11. (a) A. Smie and J. Heinze, Angew. Chem., Int. Ed. Engl., 1997, 36, 363; (b) P. Tschuncky, J. Heinze, A. Smie, G. Engelmann and G. Koβmehl, J. Electroanal. Chem., 1997, 433, 223; (c) J. Heinze, P. Tschuncky and A. Smie, J. Solid State Electrochem., 1998, 2, 102.
12. (a) P. Audebert and P. Hapiot, Synthetic Metals, 1995, 75, 95; (b) P. Hapiot, L. Gaillon, P. Audebert, J. J. E. Moreau, J. P. LerePorte and M. W. C. Man, J. Electroanal. Chem., 1997, 435, 85.
13. S. Picart and E. Genies, J. Electroanal. Chem., 1996, 408, 53.
14. L. J. Kepley and A. J. Bard, J. Electrochem. Soc., 1995, 142, 4129.
15. V. D. Parker, Acta Chem. Scand., 1998, 52, 154.
16. (a) E. M. Kosower and J. L. Cotter, J. Am. Chem. Soc., 1964, 86, 5524; (b) M. J. Blandamer, M. F. Fox, M. C. R. Symons and G. S. P. Verma, J. Chem. Soc., Chem. Commun., 1966, 844; (c) M. J. Blandamer, J. A. Brivati, M. F. Fox, M. C. R. Symons and G. S. P. Verma, Trans. Faraday Soc., 1967, 63, 1850; (d) J. F. Stargardt and F. M. Hawkridge, Anal. Chim. Acta, 1983, 146, 1; (e) M. Lapkowski and G. Bidan, J. Electroanal. Chem., 1993, 362, 249; (f) J. Mizuguchi and H. Karfunkel, Ber. Bunsenges. Phys. Chem., 1993, 97, 1466; (g) K. Komeers, J. Chem. Res. (S), 1994, 293; (h) R. D. Webster, R. A. W. Dryfe, J. C. Eklund, C.-W. Lee and R. G. Compton, J. Electroanal. Chem., 1996, 402, 167; (i) J. A. Alden, J. A. Cooper, F. Hutchinson, F. Prieto and R. G. Compton, J. Electroanal. Chem., 1997, 432, 63.
17. (a) B. M. Bezilla and J. T. Maloy, J. Electrochem. Soc., 1979, 126, 579; (b) R. D. Grypa and J. T. Maloy, J. Electrochem. Soc., 1975, 122, 509.
18. M. Svaan and V. D. Parker, Acta Chem. Scand., 1985, 39, 445.
19. M. Sertel, A. Yildiz, R. Gambert and H. Baumgärtel, Electrochimica Acta, 1986, 31, 1287.
20. M. L. Andersen, M. F. Nielsen and O. Hammerich, Acta Chem. Scand., 1997, 51, 94.
21. D. H. Evans, S. E. Treimer and K. Hu, J. Electroanal. Chem., 1997, 438, 173.
22. S. A. Lerke, D. H. Evans and P. J. Fagan, J. Electrochem. Soc., 1997, 144, 4223.
23. M. M. Bates, T. J. Cardwell, R. W. Cattrall, L. W. Deady and C. G. Gregorio, Talanta, 1995, 42, 999.
24. (a) T. Osa and T. Kuwana, J. Electroanal. Chem., 1969, 22, 389; (b) A. J. Fry and W. E. Britton, in Laboratory Techniques in Electroanalytical Chemistry, ed. P. T. Kissinger and W. R. Heineman, Marcel Dekker, New York, 1984, ch. 13..
25. (a) Digisim by Bioanalytical Systems Inc. (BAS), 2701 Kent Avenue, West Lafayette IN 47906, USA; (b) M. Rudolph, D. P. Reddy and S. W. Felberg, Anal. Chem., 1994, 66, 589A.
26. C. Nervi, Electrochemical Simulations Package, Version 2.4, http://chpc06.ch.unito.it/esp_manual.html.
27. (a) B. A. Coles and R. G. Compton, J. Electroanal. Chem., 1983, 144, 87; (b) R. G. Compton and A. M. Waller, in Spectroelectrochemistry: Theory and Practice, ed. R. J. Gale, Plenum Press, New York, 1988, ch. 7.; (c) R. D. Webster, A. M. Bond, B. A. Coles and R. G. Compton, J. Electroanal. Chem., 1996, 404, 303.
28. (a) R. S. Nicholson and I. Shain, Anal. Chem., 1964, 36, 706; (b) R. S. Nicholson, Anal. Chem., 1966, 38, 1406.
29. J. H. Wagenknecht, R. D. Goodin, P. J. Kinlen and F. E. Woodard, J. Electrochem. Soc., 1984, 131, 1559.
30. C. R. Wilke and P. Chang, AIChE J., 1955, 1, 264.
31. R. L. Wang, K. Y. Tam and R. G. Compton, J. Electroanal. Chem., 1997, 434, 105.
32. R. J. Gale ed., in Spectroelectrochemistry: Theory and Practice, Plenum Press, New York, 1988.
33. M. Hirayama, Bull. Chem. Soc. Jpn., 1967, 40, 2234.
34. J. Voss, W. Schmüser and K. Schlapkohl, J. Chem. Res. (S), 1977, 144.