The First Electrochemical Study of a Silene. An Unusually Low Oxidation Potential, Comparable to Those of Strong Organic π-Electron Donors Michael Bendikov, ² Victoria Kravchenko, ² Yitzhak Apeloig,* ,² and James Y. Becker* ,‡ Department of Chemistry and the Lise Meitner-Minerva Center for Computational Quantum Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel, and Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel Received September 25, 2003 Summary: The first electrochemical study of a compound containing a SidC double bond, i.e., (t-BuMe 2 Si)(Me 3 - Si)Sid2-Ad, reveals an extremely low oxidation potential of 0.28 V (vs Ag/AgCl), which is comparable to those of of the best known organic electron donors. Since the first report of firm evidence for the existence of compounds with a SidC double bond (silenes) in 1967, 1 several physical properties of transient and stable silenes have been determined. 2-4 However, one of the most basic physical properties of silenes, their redox potentials, have not been studied yet. Here we report the first cyclic voltammetry (CV) measurements of a stable silene, (t-BuMe 2 Si)(Me 3 Si)Sid2-Ad (1), 5 which provide important insight into the electronic structure of silenes. CV measurements (Figure 1) of silene 1, which were carried out in THF with Bu 4 NClO 4 as an electrolyte, show a one-electron quasi-reversible 6 reduction couple at E 1/2 red )-1.80 V (vs Ag/AgCl) and a broad, ill-defined oxidation wave, centered at ∼0.4 V. The redox couple did not disappear even after seven consecutive scans. This behavior indicates that, although the anion radical 1 •- is stable during the time scale of the voltammogram measurement in this medium, the cation radical is not. When the voltammogram was measured in CH 2 Cl 2 - Bu 4 NClO 4 , an oxidation couple corresponding to the cation radical 1 •+ was detected at E 1/2 ox )+0.28 V, but no reduction wave was found up to -2 V. However, the quasi-reversible 6 couple disappears quite rapidly upon consecutive scans (only a trace of it remained at the third cycle), indicating that 1 is relatively short-lived (seconds) and undergoes decomposition in this solvent- electrolyte system. 6,7 In any case, this is the first reported observation by CV at room temperature of both the anion radical and the cation radical of the same organosilicon compound. 8-10 * To whom correspondence should be addressed. Fax: Y.A., +972- 48233735. E-mail: Y.A., chrapel@tx.technion.ac.il; J.Y.B., becker@ bgumail.bgu.ac.il. ² Technion-Israel Institute of Technology. ‡ Ben-Gurion University of the Negev. (1) Gusel’nikov, L. E.; Flowers, M. C. Chem. Commun. 1967, 864. (2) For the most recent comprehensive review on silenes see: Mu ¨ ller, T.; Ziche, W.; Auner, N. In The Chemistry of Organic Silicon Com- pounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K., 1998; Vol. 2, Chapter 16. (3) For previous reviews see: (a) Raabe, G.; Michl, J. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: Chichester, U.K., 1989; Chapter 17. (b) Brook, A. G.; Brook, M. A. Adv. Organomet. Chem. 1996, 39, 71. (c) Raabe, G.; Michl, J. Chem. Rev. 1985, 85, 419. (4) (a) Buffy, J. J.; West, R.; Bendikov, M.; Apeloig, Y. J. Am. Chem. Soc. 2001, 123, 978. (b) Bendikov, M.; Solouki, B.; Auner, N.; Apeloig, Y. Organometallics 2002, 21, 1349. (c) Bendikov, M.; Apeloig, Y.; Bukalov, S.; Garbuzova, I.; Leites, L. J. Phys. Chem. A 2002, 106, 4880. (5) Apeloig, Y.; Bendikov, M.; Yuzefovich, M.; Nakash, M.; Bravo- Zhivotovskii, D.; Bla ¨ ser, D.; Boese, R. J. Am. Chem. Soc. 1996, 118, 12228. (6) Peak separations (ΔEp) of ∼120 mV were found in both solvents. This value is greater than the theoretical 58 mV, which is the reason for calling the electron-transfer process “quasi-reversible”. However, it is quite common to obtain peak separations larger than 58 mV in solvents with small dielectric constants, such as THF and CH 2Cl2. Since the silene is relatively stable in THF, the current function (ip/ (Cν 1/2 ) of the reduction peak could be evaluated and was found to be constant over the range of 50-500 mV/s, indicating that the limiting current is diffusion controlled. Also, a comparison with Fc + /Fc as external standard (diffusion controlled) indicates that the electron- transfer process involves one electron. (7) Control experiments show that silene 1 is indefinitely stable only in nonpolar solvents such as hexane or toluene but slowly decomposes in more polar solvents such as THF and CH 2Cl2. Since we found that on the time scale of the CV measurements the decomposition is considerably faster in CH 2Cl2, it is probable that only a fraction of silene 1 survives during the measurements. (8) (a) We disregard cases in which the organosilicon compound is oxidized or reduced not at the silicon atom but at different atoms. (b) Reversible chemical oxidation and reduction of cyclotetrasilenyl radical were reported recently; Matsuno, T.; Ichinohe, M.; Sekiguchi, A. Angew. Chem., Int. Ed. 2002, 41, 1575. (9) A two-electron redox process leading to a dication or dianion is highly unlikely (HOMO-1 and LUMO+1 separated from FMO by at least 1.4 eV, at MP2/6-31G*//B3LYP/6-31G* and B3LYP/6-31G*// B3LYP/6-31G*). (10) Fuchigami, T. In The Chemistry of Organosilicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K., 1998; Vol. 2, Chapter 23. Figure 1. Cyclic voltammograms of silene 1 (2-3 mM) at a Pt working electrode (Fc/Fc + ) 0.50 V vs Ag/AgCl), in CH 2 Cl 2 -0.1 M Bu 4 NClO 4 (positive scan) and in THF-0.1 M Bu 4 NClO 4 (negative scan). The sweep rate was 100 mV/s. 921 Organometallics 2004, 23, 921-923 10.1021/om0341909 CCC: $27.50 © 2004 American Chemical Society Publication on Web 12/20/2003