Isolable Anion Radical of Blue Disilene ( t Bu 2 MeSi) 2 SidSi(SiMe t Bu 2 ) 2 Formed upon One-Electron Reduction: Synthesis and Characterization Akira Sekiguchi,* Shigeyoshi Inoue, Masaaki Ichinohe, and Yoriko Arai Contribution from the Department of Chemistry, UniVersity of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan Received March 9, 2004; E-mail: sekiguch@staff.chem.tsukuba.ac.jp Abstract: The highly twisted tetrakis(di-tert-butylmethylsilyl)disilene 4 was prepared and reacted with t BuLi in THF, producing disilene anion radical 5 upon one-electron reduction. The anion radical 5 was isolated in the form of its lithium salt as extremely air- and moisture-sensitive red crystals. The molecular structure of 5 was established by X-ray crystallography, which showed a nearly orthogonal structure (twisting angle of 88°) along the central Si-Si bond, with a length of 2.341(5) Å, which is 3.6% elongated relative to that of 4. The interesting feature of 5 is that one of the central Si atoms has radical character, whereas the other Si atom has silyl anion character. An electron spin resonance (ESR) study of the hyperfine coupling constants of the 29 Si nuclei indicates that rapid spin exchange occurs between these central Si atoms on the ESR time scale. Introduction The introduction of a silyl group to a π system causes remarkable steric and electronic perturbations. 1 The unusual chemical and physical properties of tetrakis(trimethylsilyl)- ethylene (1) and its derivatives arising from the distorted nonplanar structure around the CdC double bond have been reported. 2 An interesting feature of 1 is that a cation radical was readily generated upon oxidation with aluminum trichloride, and also an anion radical and, subsequently, a dianion were formed by reduction with alkali metals. 3,4 Disilenes with an Sid Si double bond are also known to undergo reduction because of their low-lying LUMOs compared with those of the corre- sponding alkene analogues. 5,6 Meanwhile, we have reported the effective steric protection by di-tert-butylmethylsilyl groups that permits us to successfully isolate various types of unsaturated compounds of heavier Group 14 elements. 7,8 Therefore, it is quite interesting to introduce four bulky t Bu 2 MeSi groups on the sp 2 Si atoms of a disilene to cause steric and electronic perturbations, as found in 1. Herein, we report our study of the synthesis of the highly twisted tetrakis(di-tert-butylmethylsilyl)- disilene (4) and its unusual reaction with t BuLi resulting in the clean formation of the isolable anion radical of 5 upon one- electron reduction. ESR studies on tetra-tert-butyldisilene and tetramesityldisilene anion radicals, which were generated during reduction of the corresponding 1,2-dichlorodisilanes, have been reported by Weidenbruch et al. 9 Later, Kira et al. demonstrated the direct reduction of the disilene derivatives to form the corresponding anion radicals. 10 However, these results report only on ESR spectra, and no structural study has been reported. The anion radical of 5 presented here is the first example of an anion radical isolable in a pure form that has been fully characterized, including X-ray crystallography. 11 (1) (a) Sakurai, H. Pure Appl. Chem. 1996, 68, 327. (b) Matsuo, T.; Sekiguchi, A. Bull. Chem. Soc. Jpn. 2004, 74, 211. (2) Sakurai, H.; Nakadaira, Y.; Tobita, H.; Toriumi, K.; Itoh, H. J. Am. Chem. Soc. 1982, 104, 300. (3) (a) Sakurai, H.; Nakadaira, Y.; Kira, M.; Tobita, H. Tetrahedron Lett. 1980, 21, 3077. (b) Sakurai, H.; Tobita, H.; Kira, M.; Nakadaira, Y. Angew. Chem., Int. Ed. Engl. 1980, 19, 620. (c) Sekiguchi, A.; Nakanishi, T.; Sakurai, H. J. Am. Chem. Soc. 1989, 111, 3748. (4) (a) Sekiguchi, A.; Ichinohe, M.; Nakanishi, T.; Sakurai, H. Chem. Lett. 1993, 267. (b) Sekiguchi, A.; Ichinohe, M.; Kabuto, C.; Sakurai, H. Organometallics 1995, 14, 1092. (c) Sekiguchi, A.; Ichinohe, M.; Kabuto, C.; Sakurai, H. Bull. Chem. Soc. Jpn. 1995, 68, 2981. (d) Sekiguchi, A.; Ichinohe, M.; Kabuto, C.; Sakurai, H. Angew. Chem., Int. Ed. Engl. 1997, 36, 1533. (5) (a) West, R. Angew. Chem., Int. Ed. Engl. 1987, 26, 1201. (b) Tsumuraya, T.; Batcheller, S. A.; Masamune, S. Angew. Chem., Int. Ed. Engl. 1991, 30, 902. (c) Escudie ´, J.; Couret, C.; Ranaivonjatovo, H.; Satge ´, J. Coord. Chem. ReV. 1994, 130, 427. (d) Okazaki, R.; West, R. AdV. Organomet. Chem. 1996, 39, 231. (e) Baines, K. M.; Stibbs, W. G. AdV. Organomet. Chem. 1996, 39, 275. (f) Escudie ´, J.; Couret, C.; Ranaivonjatovo, H. Coord. Chem. ReV. 1998, 178-180, 565. (g) Power, P. P. J. Chem. Soc., Dalton Trans. 1998, 2939. (h) Power, P. P. Chem. ReV. 1999, 99, 3463. (i) Weidenbruch, M. Eur. J. Inorg. Chem. 1999, 373. (j) Escudie ´, J.; Ranaivonjatovo, H. AdV. Organomet. Chem. 1999, 44, 113. (k) Weiden- bruch, M. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; John Wiley & Sons Ltd.: New York, 2001; Vol. 3, Chapter 5. (l) Weidenbruch, M. J. Organomet. Chem. 2002, 646, 39. (m) West, R. Polyhedron 2002, 21, 467. (n) Weidenbruch, M. Organometallics 2003, 22, 4348. (6) Kira, M.; Iwamoto, T.; Yin, D.; Maruyama, T.; Sakurai, H. Chem. Lett. 2001, 910. (7) (a) Sekiguchi, A.; Lee, V. Ya. Chem. ReV. 2003, 103, 1429. (b) Lee, V. Ya; Sekiguchi, A. Organometallics 2004, 23, 2822. (8) (a) Ichinohe, M.; Arai, Y.; Sekiguchi, A.; Takagi, N.; Nagase, S. Organometallics 2001, 20, 4141. (b) Sekiguchi, A.; Izumi, R.; Lee, V. Ya.; Ichinohe, M. J. Am. Chem. Soc. 2002, 124, 14822. (c) Sekiguchi, A.; Izumi, R.; Lee, V. Ya.; Ichinohe, M. Organometallics 2003, 22, 1483. (9) (a) Weidenbruch, M.; Kramer, K.; Scha ¨fer, A.; Blum, J. K. Chem. Ber. 1985, 118, 107. (b) Weidenbruch, M.; Thom, K.-L. J. Organomet. Chem. 1986, 308, 177. (10) Kira, M.; Iwamoto, T. J. Organomet. Chem. 2000, 611, 236. (11) For reviews on Group 14 radicals, see: (a) Sakurai, H. In Free Radicals; Kochi, J. K., Ed.; John Wiley & Sons Ltd.: New York, 1973; Vol. II. (b) Chatgilialoglu, C. Chem. ReV. 1995, 95, 1229. (c) Iley, J. In The Chemistry of Organic Germanium, Tin and Lead Compounds; Patai, S., Rappoport, Z., Eds.; John Wiley & Sons Ltd.: New York, 1995; Chapter 5. (d) Power, P. P. Chem. ReV. 2003, 103, 789. Published on Web 07/17/2004 9626 9 J. AM. CHEM. SOC. 2004, 126, 9626-9629 10.1021/ja040079y CCC: $27.50 © 2004 American Chemical Society