"Front Matter". In: Organosilanes in Radical Chemistry - Index of
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"Front Matter". In: Organosilanes in Radical Chemistry - Index of

"Front Matter". In: Organosilanes in Radical Chemistry - Index of "Front Matter". In: Organosilanes in Radical Chemistry - Index of

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Structural Properties of Silyl Radicals 11 photolysis of poly(di-n-alkylsilanes) in solution via a complex reaction mechanism [8]. Radical 14 (Hx ¼ n-hexyl) with g ¼ 2:0047, a(a- 29 Si) ¼ 75 G and a(b- 29 Si) ¼ 5:8 G, showed line-broadening effects as the temperature was lowered. This observation has been correlated to the restricted rotational motion about the C w Si: bond and, in particular, to a rocking interchange of the two a-hydrogens. Isolation of ‘allylic-type’ silyl radical 15 has also been achieved [51]. The EPR spectrum consists of a broad singlet (g ¼ 2:0058) with three doublet satellite signals due to coupling with 29 Si of 40.7, 37.4 and 15.5 G. The two doublets with 40.7 and 37.4 G broaden upon raising the temperature and coalesce at 97 8C due to the rotation of the t-BuMe2Si group. The magnitude of 29 Si hfs constants is consistent with the delocalization of the unpaired electron over the three silicon atoms in the ring, but it is noteworthy that the coupling constants of the outer Si atoms are not equal. This is explained below. Hx Si Si Hx Hx Hx 14 Hx 1.2.3 CRYSTAL STRUCTURES Si t Bu2 MeSi t Bu Si Si Si t Bu Si SiMe tBu 2 15 SiMe tBu 2 The crystal structures of two isolable silyl radicals have recently been reported. The bulky substituted (t-Bu2MeSi) 3Si: radical was isolated as air-sensitive yellow needles [13], whereas the conjugated and bulky substituted cyclotetrasilenyl radical 15 was obtained as red–purple crystals [51]. Figure 1.2 shows a completely planar geometry around the Si1 atom of (t-Bu2MeSi) 3Si: radical. Indeed, the bond angles Si2 w Si1 w Si3, Si2 w Si1 w Si4 and Si3 w Si1 w Si4 are 119.498, 120.088 and 120.438, respectively, their sum being exactly 3608. The Si w Si bonds are larger (2:42 0:01 A ˚ ) than normal. Interestingly, all the methyl substituents at the a-Si atoms (i.e., C1, C4 and C7) are located in the plane of the polysilane skeleton in order to minimize steric hindrance. As reported in the previous section, the planarity of this radical is retained in solution. Figure 1.3 shows the ORTEP drawing of the conjugated radical 15. The fourmembered ring is nearly planar with the dihedral angle between the radical part Si1 w Si2 w Si3 and Si1 w Si4 w Si3 being 4.78. The Si1 and Si2 atoms have planar geometry (the sums of the bond angles around them are 360.08 and 359.18, respectively) whereas the Si3 atom is slightly bent (356.28). This small asymmetry of the moiety where the radical is delocalized is also observed in the

12 Formation and Structures of Silyl Radicals C1 Si2 C2 C3 C6 C5 Si3 Si1 Figure 1.2 Molecular structure of (t-Bu2MeSi) 3Si: radical with thermal ellipsoids drawn at the 30 % level (hydrogen atoms are omitted for clarity). Reprinted with permission from Reference [13]. Copyright 2002 American Chemical Society. Si7 Si3 C32 Si2 Si4 Figure 1.3 ORTEP Drawing of cyclotetrasilenyl radical 15. Hydrogen atoms are omitted for clarity. Reprinted with permission from Reference [51]. Copyright 2001 American Chemical Society. Si w Si bond length, the Si1 w Si2 being slightly shorter than Si2 w Si3 (2.226 vs 2.263 A ˚ ), and explains the magnetic inequivalence of Si1 and Si3 noted above. The reaction of (t-Bu2MeSi) 3Si: radical with lithium in hexane at room temperature afforded the silyllithium 16 for which the crystal structure shows Si6 C4 C28 C7 Si1 C8 Si4 C9 Si5

12 Formation and Structures <strong>of</strong> Silyl <strong>Radical</strong>s<br />

C1<br />

Si2<br />

C2<br />

C3<br />

C6<br />

C5<br />

Si3<br />

Si1<br />

Figure 1.2 Molecular structure <strong>of</strong> (t-Bu2MeSi) 3Si: radical with thermal ellipsoids drawn at<br />

the 30 % level (hydrogen atoms are omitted for clarity). Repr<strong>in</strong>ted with permission from<br />

Reference [13]. Copyright 2002 American Chemical Society.<br />

Si7<br />

Si3<br />

C32<br />

Si2<br />

Si4<br />

Figure 1.3 ORTEP Draw<strong>in</strong>g <strong>of</strong> cyclotetrasilenyl radical 15. Hydrogen atoms are omitted for<br />

clarity. Repr<strong>in</strong>ted with permission from Reference [51]. Copyright 2001 American Chemical<br />

Society.<br />

Si w Si bond length, the Si1 w Si2 be<strong>in</strong>g slightly shorter than Si2 w Si3 (2.226 vs<br />

2.263 A ˚ ), and expla<strong>in</strong>s the magnetic <strong>in</strong>equivalence <strong>of</strong> Si1 and Si3 noted above.<br />

The reaction <strong>of</strong> (t-Bu2MeSi) 3Si: radical with lithium <strong>in</strong> hexane at room<br />

temperature afforded the silyllithium 16 for which the crystal structure shows<br />

Si6<br />

C4<br />

C28<br />

C7<br />

Si1<br />

C8<br />

Si4<br />

C9<br />

Si5

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