R EPORTS product analyses of a few chemical-trapping reactions (12, 13). The present results demon- A Stable Bicyclic Compound strate that the chemistry of Si¢Si double bonds ¢ is not as limited as had been assumed, and it with Two Si Si Double Bonds may even have comparable importance to or- ganic alkene chemistry. Takeaki Iwamoto, Makoto Tamura, Chizuko Kabuto, Mitsuo Kira* Compound 1. In contrast to carbon, silicon does not readily form double bonds, and com- pounds containing silicon-silicon double bonds can usually be stabilized only by protection with bulky substituents. We have isolated a silicon analog of spi- ropentadiene 1, a carbon double-ring compound that has not been isolated to date. In the crystal structure of tetrakis[tri(t-butyldimethylsilyl)silyl]spiro- pentasiladiene 2, a substantial deviation from the perpendicular arrangement of the two rings is observed, and the silicon-silicon double bonds are shown to be distorted. Spectroscopic data indicate pronounced interaction between two We have found that tetrakis[tri(tert-bu- remote silicon-silicon double bonds in the molecule. Silicon-silicon bonds may tyldimethylsilyl)silyl]spiropentasiladiene (2) be more accessible to synthesis than previously assumed. forms as a by-product during the preparation of cyclotrisilene 3 (Eq. 1) (7). The reaction of 4 In contrast to alkenes, Si¢Si doubly bonded isolated to date (11). Parent spiropentadiene 1 with potassium graphite at Ϫ78°C and subse- compounds (disilenes) are usually unstable and and 1,1Ј-dichlorinated spiropentadiene have quent work-up resulted in a brownish red solid, can only be isolated when the double bonds are been generated in solution but decompose with- which contained spiropentasiladiene 2 and cy- protected effectively by bulky substituents. in a few minutes even below Ϫ100°C; their clotrisilene 3 at a ratio of 1:8, as determined by Since the first isolation of tetramesityldisilene structures were identified only by nuclear mag- 1H NMR spectroscopy. Crystallization from by West et al. in 1981 (1), a number of stable netic resonance (NMR) spectroscopy and by hexane gave dark red crystals of 2 in 3.5% disilenes have been prepared, but the types of isolated disilenes remain very limited (2–4). The chemistry of disilenes cannot be compared to the extensive alkene chemistry, even though very recently a few silicon analogs of cycloalk- enes (5–9) and butadienes (10) have been syn- thesized. We report the isolation of a silicon analog of spiropentadiene 1, which is a fasci- nating type of compound because of the possi- ble through-space interaction between the two perpendicular double bonds, but it has not been Department of Chemistry, Graduate School of Sci- ence, Tohoku University, Aoba-ku, Sendai 980-8578, Japan. *To whom correspondence should be addressed. E- mail: [email protected] Equations. Fig. 1. Molecular view of tet- rakis[tri(t-butyldimethylsilyl) si- lyl]spiropentasiladiene 2 in the solid state. Selected bond lengths (Å), angles (degrees), and dihedral angles (degrees): Si1-Si6 2.186(3), Si1-Si11 2.320(2), Si6-Si11 2.323(2), Si1-Si2 2.393(3), Si6-Si7 2.396(3), Si11-Si1-Si6 61.99(9), Si11-Si6-Si1 61.82(9), Si1-Si11- Si6 56.19(7), Si11-Si1-Si2 148.1(1), Si11-Si6-Si7 144.1(1), Si2-Si1-Si6 148.8(1), Si7-Si6- Si1 152.1(1), Si2-Si1-Si6-Si7 30.0(5), and plane 1 (Si11-Si1- Si6)Ðplane 2 (Si11-Si1*-Si6*) 78.26(0). 504 20 OCTOBER 2000 VOL 290 SCIENCE www.sciencemag.org R EPORTS yield. The structure of 2 was determined by 1H, 104]: 560 (0.253), 500 (0.364), 428 (1.17), and result of the effective through-space interaction 13C, and 29Si NMR spectroscopy and x-ray 383 (1.81)} were observed for 2. The longest between the two remote ␲* orbitals, as suggest- crystallography (14). Although spiropentasila- absorption maximum was highly red-shifted ed in parent spiropentadiene 1 (22–24). In the diene 2 is sensitive to air, it is thermally very relative to those for 3 (7) and 5 (8), which show distorted spiropentasiladiene 7 (D2), the degen- ␲3␲ ␲ stable, in contrast to spiropentadiene itself; 2 * absorption maxima at 482 and 466 nm, eracy of the bonding orbitals in 7 (D2d)is melts at 216° to 218°C without decomposition respectively. removed efficiently by the through-space inter- (Eq. 1). High-lying occupied and low-lying unoccu- action between the ␲ orbitals. The pronounced X-ray single-crystal analysis (15) shows that pied molecular orbitals and their energy levels through-space interactions between ␲ orbitals ␲ the two three-membered rings in 2 are not of cyclotrisilene 12 (C2v), parent spiropentasi- as well as between * orbitals found in a perpendicular to each other but are slightly ladiene 7 (D2d), and a distorted spiropentasila- distorted D2 spiropentasiladiene may be the ␪ twisted with a dihedral angle ( ) of 78.26(0)° diene 7 (D2), where the ring structure is fixed to origin of the splitting of the low-energy transi- (standard deviation is in parentheses) (Fig. 1). a geometry similar to that of 2, calculated at the tion bands and the red-shift of the longest wave- The geometry around the Si¢Si bonds in 2 is HF/6-311ϩϩG(3df,2p) level are schematically length band as observed experimentally in 2 not planar but substantially twisted; the dihedral shown in Fig. 3. In 7 (D2d), there are two highly (25) (Fig. 3). ␾ ␲* 29 angle Si2-Si1-Si6-Si7 ( ) is 30.0(5)°, and the split orbitals (b1 and a2) and two degenerate The Si NMR resonance due to the unsat- ␲ ¢ sums of the bond angles are 358.9° at Si1 and orbitals (e) of the Si Si double bonds in urated silicon nuclei (Siu)in2 appeared at ¢ ␴ ϩ 358.0° at Si6, respectively. The Si Si double addition to a low-lying * orbital (a1). The 154.0 parts per million, shifted to a lower bonds in 2 are 0.05 Å longer, and the Si™Si large splitting of the ␲* orbitals is probably a field by ϳ60 ppm compared with the corre- single bonds in the three-membered ring are 0.03 to 0.04 Å shorter, than the corresponding bonds in tetrakis(di-tert-butylmethylsilyl)cyclo- trisilene 5 (8). The geometry of a model spiro- pentasiladiene 6 was optimized by ab initio molecular orbital (MO) calculations at the B3LYP/6-31G(d) level with the Gaussian 98 program (16, 17). Spiropentasiladiene 6 was found to be a minimum in D2d symmetry, sug- gesting that the substantial deformation from D2d to D2 in 2 is due to the severe steric hindrance between bulky tris(trialkylsilyl)silyl Fig. 2. Compounds 6 through 12. groups. The lengths of the Si¢Si bonds and the ™ ring Si Si bonds in 6 (D2d) were found to be longer and shorter, respectively, than the corre- sponding bond lengths in cyclotrisilene 11 (C2v). The reason for the modification of the bond distances in 2 [and 6 (D2d)] may be elec- tronic and attributed to the effective interaction between the bonding ␲-orbital of a Si¢Si bond and the low-lying Walsh-type ␴* orbitals of Si™Si bonds in the other three-membered ring (␴*–aromaticity) (18–20) (Figs. 1 and 2). Spiropentasiladiene 2 is kinetically stabi- lized as a result of steric protection by four bulky tri(t-butyldimethylsilyl)silyl groups, but isolation of 2 as the first spiropentadiene in the chemistry of group-14 elements is mainly due the relatively small strain energy (SE) of the ring systems. The SE value of parent spiropentasiladiene Si5H4 7 with D2d symmetry was calculated to be 61.1 kcal/mol as the heat of reaction of the following ho- modesmotic reaction (Eq. 2; E ϭ Si) at the B3LYP/6-311ϩϩG(3df,2p)//B3LYP/6-31G(d) level (17, 21). This SE value is much smaller than that of the corresponding spiropenta- diene 1 (D2d, 114.2 kcal/mol) calculated with the same equation (Eq. 2; E ϭ C) at the same level (22). The ultraviolet (UV)-visible spectral fea- tures of 2 in 3-methylpentane at room temper- ature were quite different from those of the monocyclic cyclotrisilenes 3 and 5 (16). Four Fig. 3. Schematic molecular orbitals and energy levels for 3,3-bis(trihydrosilyl)cyclotrisilene 12 (C2v), spiropentasiladiene 7 (D2d), and distorted spiropentasiladiene 7 (D2) at the HF/6- major absorption bands in the longer wave- 311ϩϩG(3df,2p) level. Structures of 12 (C ) and 7 (D ) were optimized at the B3LYP/6-31G(d) ␭ 2v 2d length region {absorption maxima ( max,in level. The structure of 7 (D ) was fixed to a similar ring structure to 2. The highest ␴ orbitals (b ) ε 2 2 nanometers) [molar absorption coefficient ( )/ in 7 (D2d) and 7 (D2), which are located at 8.73 and 8.66 eV, respectively, are omitted. www.sciencemag.org SCIENCE VOL 290 20 OCTOBER 2000 505 R EPORTS sponding resonances for 3 (ϩ81.9 and ϩ99.8 vice diffractometer with a graphite monochromated parent cyclotrisilene (34.5 kcal/mol) is similar to that of ppm) (7) and 5 (ϩ97.7 ppm) (8). The Si Mo-K␣ radiation (␭ϭ0.71069 Å) at 150 K. Of the cyclotrisilane (35.5 kcal/mol), whereas the SE value of u cyclopropene (55.5 kcal/mol) is 20 kcal/mol larger than resonances were well reproduced in the perti- 79,057 reflections that were collected, 14,416 were unique. The structure was solved by direct methods and that of cyclopropane (25.5 kcal/mol). Thus, the SE values nent model compounds by ab initio MO calcu- refined anisotropically for nonhydrogen atoms.