Prismanes Comprising Silicon and Germanium

The six, 2,6-diisopropylphenyl groups attached to the each silicon in 25 are equivalent on the NMR time scale. Si NMR spectroscopy in solution demonstrated that the six silicons of the skeleton are equivalent, with a single resonance appearing at -22.3 ppm. However, CPMAS Si NMR of solid 25 showed signals at -22.2 and -30.8 ppm with a relative intensity of 2 1. The Si resonance is shifted considerably upheld from that of the octasilacubane 16. 

Structural Parameters of Prismanes Comprising Group 14 Elements 

Hexasilaprismane 25 is photosensitive (7). On irradiation of 25 with light having a wavelength of 360-380 nm in solution (3-MP or 2-MeTHF) at -50°C or in a glass matrix at 77 K, new absorption bands appeared at 335, 455, and 500 nm assignable to the absorption bands of hexasila- 

Dewar benzene 29. Excitation of the bands with wavelengths longer than 460 nm resulted in the immediate regeneration of 25. 

The hexasila-Dewar benzene 29 is thermally stable at -150°C, however, it gradually reverted to the hexasilaprismane 25. The half-life is tj = 0.52 min at 0°C in 3-MP (first-order rate constant, k = 2.21 x 10 s )-The activation parameters for hexasila-Dewar benzene 29 to the hexasilaprismane 25 are Ea = 13.7 kcal/mol, AH = 13.2 kcal/mol, AS - -17.8 eu. The small Ea value is consistent with the high reactivity of Si=Si 

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