TY - JOUR
T1 - Polysilyl radicals
T2 - EPR study of the formation and decomposition of star polysilanes
AU - Apeloig, Y.
AU - Bravo-Zhivotovskii, D.
AU - Yuzefovich, M.
AU - Bendikov, M.
AU - Shames, A. I.
N1 - Funding Information:
This paper is dedicated to Professor Paul von Rague Schleyer, a great chemist, on the occasion of his 70th birthday. This research was supported by the Israel Science Foundation administrated by the Israel Academy of Sciences and Humanities, by the Fund for the Promotion of Research at the Technion and by the Minerva Foundation in Munich. A. S. and D. B.-Z. are grateful to the Israel Ministry of Immigrant Absorption for Kamea and Gileadi scholarships.
PY - 2000/1/1
Y1 - 2000/1/1
N2 - Electron paramagnetic resonance (EPR) spectroscopy was fruitfully used for studying the formation and the reactions of the star polysilane radical (Me3SiMe2Si)3Si (1). 1, which was successfully generated both thermally and photochemically from a variety of precursors, was found to be significantly more stable kinetically than the (Me3Si)3Si radical. Thus, (Me3SiMe2Si)3Si̇ has a half-life time of ca. 6 min at 20°C, while (Me3Si)3Si̇ can be observed only at -25°C. Density-functional quantum-mechanical calculations show that 1 and (Me3Si)3Si̇ have the same thermodynamic stability. The high kinetic stability of 1 is attributed to its backfold "umbrella"-type conformation where the β-silyl groups point "inwards" towards the radical center. This conformation protects the radical center of 1 from dimerization and other reactions. The EPR spectrum of 1 and in particular the Si α-hyperfine coupling constant of 5.99 mT shows that 1 is less pyramidal than (Me3Si)3Si̇ but is more pyramidal than (i-Pr3Si)3Si̇, with an estimated SiSiSi bond angle around the radical center of 118°. Photolysis and thermolysis of [(Me3SiMe2Si)3Si]2 also involves the intermediacy of 1. Photolysis of [(Me3SiMe2Si)3Si]2 leads to (Me3SiMe2Si)4Si, while thermolysis produced the less strained isomer of 1, (Me3SiMe2Si)3SiSi-Me2Si(Me 3SiMe2Si)2SiMe3. In this study we provide the first direct evidence that silyl radicals are involved as intermediates in the reactions of silanes with di(tert-butyl)mercury.
AB - Electron paramagnetic resonance (EPR) spectroscopy was fruitfully used for studying the formation and the reactions of the star polysilane radical (Me3SiMe2Si)3Si (1). 1, which was successfully generated both thermally and photochemically from a variety of precursors, was found to be significantly more stable kinetically than the (Me3Si)3Si radical. Thus, (Me3SiMe2Si)3Si̇ has a half-life time of ca. 6 min at 20°C, while (Me3Si)3Si̇ can be observed only at -25°C. Density-functional quantum-mechanical calculations show that 1 and (Me3Si)3Si̇ have the same thermodynamic stability. The high kinetic stability of 1 is attributed to its backfold "umbrella"-type conformation where the β-silyl groups point "inwards" towards the radical center. This conformation protects the radical center of 1 from dimerization and other reactions. The EPR spectrum of 1 and in particular the Si α-hyperfine coupling constant of 5.99 mT shows that 1 is less pyramidal than (Me3Si)3Si̇ but is more pyramidal than (i-Pr3Si)3Si̇, with an estimated SiSiSi bond angle around the radical center of 118°. Photolysis and thermolysis of [(Me3SiMe2Si)3Si]2 also involves the intermediacy of 1. Photolysis of [(Me3SiMe2Si)3Si]2 leads to (Me3SiMe2Si)4Si, while thermolysis produced the less strained isomer of 1, (Me3SiMe2Si)3SiSi-Me2Si(Me 3SiMe2Si)2SiMe3. In this study we provide the first direct evidence that silyl radicals are involved as intermediates in the reactions of silanes with di(tert-butyl)mercury.
UR - http://www.scopus.com/inward/record.url?scp=0034418032&partnerID=8YFLogxK
U2 - 10.1007/BF03162156
DO - 10.1007/BF03162156
M3 - Article
AN - SCOPUS:0034418032
SN - 0937-9347
VL - 18
SP - 425
EP - 434
JO - Applied Magnetic Resonance
JF - Applied Magnetic Resonance
IS - 3
ER -