|M.Sc Student||Pinchuk Daniel|
|Subject||Chemistry of Persilyl Silenyl Anions and Radicals|
|Department||Department of Chemistry||Supervisors||? 18? Yitzhak Apeloig|
|Dr. Dmitry Bravo-Zhivotovs|
The main goal of this research was to synthesize persilyl silenyl lithium. Reaction of Br(Me2tBuSi)2SiC(O)SiMetBu2 with 3.5 eq. of tBu2MeSiLi in a 8:2 hexane:THF mixture of solvents at -78°C leads to the desired (tBuMe2Si)(tBu2MeSi)C=Si(SiMetBu2)Li•2THF (8) in 80% yield (red crystals). 8 was characterized by NMR spectroscopy and by X-ray crystallography. The chemical shifts of the central Si1=C1 atoms of 8 are at 347 ppm (29Si) and 143 ppm (13C). The X-ray structure reveals that 8 has a Si-Li bond length of 2.714 Ǻ (similar to other THF solvated silyl lithiums) and that the Li is coordinated to two THF molecules. Thus, 8 can be characterized as a silenyl lithium with a covalent Si-Li bond. Upon addition of THF vapors to the red crystals of 8, the crystals turn blue. The blue crystals were characterized by NMR spectroscopy and X-ray crystallography as [(tBuMe2Si)(tBu2MeSi)C=Si(SiMetBu2)]- [Li•4THF] (9). The chemical shift of the central Si1=C1 atom of dissolved 9 in THF appear at 408 ppm (29Si) and 134 ppm (13C). The X-ray spectra reveals that 9 has an extremely long Si-Li distance of 9.83(5) Ǻ, thus there is no interaction between these atoms. The lithium atom is coordinated to four THF molecules. 9 can therefore be characterized as a free ion-pair of the persilyl silenyl lithium. Remarkably, the solvation-dissociative process is reversible. Thus, removal of THF from 9 yields 8, and vice versa.
Several preliminary reactions of 8 with several electrophiles and oxidizing reagents, were carried out. Reaction of 8 with methyl chloride and PhBr yield coupling products, with the Si=C bond remaining intact. When the steric hindrance around the C-Br bond in the aryl bromide increases the reaction takes an entirely different course. When 8 is reacted with 2-bromo-1,3,5-tritert-butylbenzene (10) the silenyl radical (tBuMe2Si)(tBu2MeSi)C=Si•(SiMetBu2)Li (11) is observed by EPR spectroscopy. This reaction proceeds by electron transfer from 8 to 10. Addition of other SET oxidants: HgF2, Re(CO)5Br or C60 to a toluene solution of 8 also produces silenyl radical 11. Radical 11 is quite persistent, t1/2(297K) = 30min, and it does not dimerize probably due to the steric reasons. The high hfc [(a(29Si?a)= 137.5G] in the EPR spectrum of 11 points to a substantial s-character of the unpaired electron orbital and it is in agreement with the calculated strongly bent structure.