Unusual Locking of Silicon Chains into all-transoid Conformation by Pentacoordinate Silicon Atoms Ibrahim El-Sayed, Yasuo Hatanaka,* Shun-ya Onozawa, and Masato Tanaka* National Institute of Materials and Chemical Research Tsukuba, Ibaraki 305-8565, Japan ReceiVed NoVember 27, 2000 Conformation of flexible linear chains are of fundamental importance in determining the physical properties of organic and inorganic polymers. 1 Over the past decade, a great deal of effort has been made for conformational control of silicon chains 2 because electronic and optical properties of σ-conjugated silicon polymers such as polysilanes and polycarbosilanes are highly sensitive to subtle changes in the backbone conformation. 3 In this regard, conformational locking of the silicon chains into an all- transoid form is essential for realizing the useful physical properties of silicon compounds, 4 since all-transoid conformation allows effective σ-conjugation along the silicon chain. 2g,3b How- ever, conformational control of a silicon chain is extremely difficult, since the rotational barriers about Si-Si single bonds are so small that essentially free rotation occurs at room temperature. For example, the energy barrier to the rotation about the Si2-Si3 bond of Si 4 Me 10 is predicted to be only 3.5 kcal/ mol, which is significantly smaller than that of the corresponding C3-C4 bond of octamethylhexane (>20 kcal/mol), revealing the greater flexibility of silicon chains compared with carbon chains. 5 We report herein unusual locking of silicon chains into all- transoid conformation by pentacoordinate silicon atoms. Thus, the internal rotation about the Si-Si single bonds of pentacoor- dinate pentasilane 1 was found to be nearly completely inhibited even in a room-temperature solution, although there are no significant steric interactions between the substituents (Figure 1). Pentacoordinate pentasilane 1 was prepared by the reaction of N-methyl-N-trimethylsilylacetamide with {Me 3 SiSi(CH 2 Cl)- Cl} 2 SiMe 2 (1:1 mixture of diastereomers) in hexane at room temperature. 6 The reaction stereoselectively gave the dl-isomer of 1, which was recrystallized from hexane/benzene to give analytically pure 1 in 81% yield as colorless, benzene-containing crystals. X-ray analysis of 1 revealed that strong intramolecular O f Si coordination in an N-[(chlorosilyl)methyl]amide system led to the almost undistorted trigonal bipyramidal (TBP) structure of 1 as indicated by the high %TBP a and %TBP e values for the pentacoordinate silicon atoms (89 and 99%, respectively). 7 The most remarkable feature of the crystal structure of 1 is the all- transoid conformation of the silicon backbone. Thus, pentaco- ordinate pentasilane 1 has a stretched silicon chain; the Si-Si bond lengths (2.3353(8), 2.3480(9) Å), Si-Si-Si bond angles (113.50(3), 111.66(4)°), and Si-Si-Si-Si dihedral angle (163.61- (3)°) meet expectations for the all-transoid conformation. The solid-state UV spectrum of the thin film of 1 exhibited an intense absorption at 257 nm, which is attributed to the σ SiSi f σ* SiSi excitation of the silicon backbone with the all-transoid conformation (Figure 2a). 6b A weak absorption around 220 nm is assignable to the amide chromophore. To our surprise, the UV spectrum of 1 in ether solution was essentially similar to the solid- state spectrum, showing an intense absorption at 257 nm (ǫ ) 22 000 M -1 cm -1 ) (Figure 2b). The obvious similarity between the solid state and the solution spectra reveals that the most stable conformation of 1 is all-transoid even in a room-temperature solution, because UV spectra of oligosilanes are highly sensitive to the conformational change of the silicon backbones. 3a In sharp contrast, the conformational properties of tetracoordinate oligosi- (1) (a) Dehong, H.; Ji, Y.; Kim, W.; Bagchi, B.; Rossky, P. J.; Barbara, P. F. Nature 2000, 405, 1030-1033. (b) Mark, J. E.; Allcock, H. R.; West, R. Inorganic Polymers; Prentice Hall: New Jersey, 1992. (2) (a) Obata, K.; Kabuto, C.; Kira, M. J. Am. Chem. Soc. 1997, 119, 11345-11346. (b) Yuan, C.-H.; West, R. Macromolecules 1994, 27, 629- 630. (c) KariKari, E. K.; Greso, A. J.; Farmer, B. L.; Miller, R. D.; Rabolt, J. F. Macromolecules 1993, 26, 3937-3945. (d) Schilling, F. C.; Lovinger, A. J.; Davis, D. D.; Bovey, F. A.; Zeigler, J. M. Macromolecules 1993, 26, 2716-2723. (e) Harrah, L. A.; Zeigler, J. M. Macromolecules 1987, 20, 601- 608. (f) Miller, R. D.; Hofer, D.; Rabolt, J. J. Am. Chem. Soc. 1985, 107, 2172-2174. (g) Miller, R. D.; Michl, J. Chem. ReV. 1989, 89, 1359-1410 and references therein. (3) a) Imhof, R.; Teramae, H.; Michl, J. Chem. Phys. Lett. 1997, 270, 500- 505. (b) Klingensmith, K. A.; Downing, J. W.; Miller, R. D.; Michl, J. J. Am. Chem. Soc. 1986, 108, 7438-7439. (4) The term “all-transoid” denotes a backbone conformation whose dihedral angle is close to 165°. In the recent past, this conformation has been confused with “all-trans conformation”; however, “all-trans” should refer to a dihedral angle of 180°. (5) Neumann, F.; Teramae, H.; Downing, J. W.; Michl, J. J. Am. Chem. Soc. 1998, 120, 573-582. (6) Recently, we have reported that introduction of pentacoordinate silicon atoms into oligosilanes leads to a drastic change in the electron transition energies of the Si-Si bonds: (a) El-Sayed, I.; Hatanaka, Y.; Muguruma, C.; Shimada, S.; Tanaka, M.; Koga, N.; Mikami, M. J. Am. Chem. Soc. 1999, 121, 5095-5096. (b) Muguruma, C.; Koga, N.; Hatanaka, Y.; El-Sayed, I.; Mikami, M.; Tanaka, M. J. Phys. Chem. A 2000, 104, 4928-4935. (7) %TBP values indicate the degree of pentacoordinate character of a silicon atom: Tamao, K.; Hayashi, T.; Ito, Y.; Shiro, M. Organometallics 1992, 11, 2099-2114. Figure 1. X-ray structure of 1. All hydrogen atoms and crystalline solvent are omitted for clarity. Selected bond distances (Å), bond angles (deg), and dihedral angle (deg): Si1-Si2 2.3480(9), Si2-Si3 2.3353(8), Si2- O1 1.990(2), Si2-Cl1 2.3499(7); C4-Si2-Cl1 88.22(7), Si1-Si2-Cl1 97.24(3), Si3-Si2-Cl1 90.96(3), Si1-Si2-C4 122.06(7), Si1-Si2-Si3 113.50(3), Si3-Si2-C4 124.08(7), O1-Si2-Cl1 170.70(5), Si2-Si3- Si2* 111.66(4); Si1-Si2-Si3-Si2* 163.61(3). Figure 2. UV spectra of (a) a thin film of 1, (b) 1 in ether solution, (c) a thin film of 2, and (d) 1 in acetonitrile solution. 3597 J. Am. Chem. Soc. 2001, 123, 3597-3598 10.1021/ja0040621 CCC: $20.00 © 2001 American Chemical Society Published on Web 03/21/2001