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.
ASJC Scopus subject areas
- Atomic and Molecular Physics, and Optics