STERICALLY OVERCROWDED OR CHARGE PERTURBED MOLECULES .23. HEXAKIS(TRIMETHYLSILYL)DISILANE - STRUCTURE AND PHOTOELECTRON-SPECTRUM OF A STERICALLY OVERCROWDED MOLECULE

Hexakis(trimethylsilyl)disilane [(H3C)3Si]3Si-Si[Si(CH3)3]3 crystallizes in the rhombohedral space group R3cBAR with six molecules of D3 Symmetry per unit cell. The steric overcrowding by the 18 peripheral methyl groups shows up especially in the elongation of the central SiSi bond to 240 pm, in the differing dihedral angles omega(SiSi-SiSi) of 43-degrees and 77-degrees and in the rather short non-bonded C...C distances of only 352 pm, which reveals an inter-penetration of the methyl groups within their van der Waals radii of about 200 pm. The H-1 nuclear magnetic resonance (NMR) spectrum in DCCl3 at 190 K, however, does not exhibit any signal splitting of the 42 equivalent methyl hydrogens and so the rotational barriers must be below 20 kJ mol-1. The photoelectron spectrum shows in its low-energy region between 7.2 eV and 9.3 eV a resolved alpha(SiSi) ionization pattern, which can be satisfactorily reproduced by a LCBO-MO model parametrized in terms of the different SiSi/SiSi bond interactions, and the higher ionization energies are assigned on the basis of a radical cation state comparison with [(H3C)Si]3SiCl, (H3C)3SiCl and (H3C)3CCl. Although the first vertical ionization energy is only 7.70 eV, which is the lowest observed so far for a disilane derivative, and in accord with the cyclovoltammetrically determined irreversible first oxidation potential, no persistent radical cation of hexakis(trimethylsilyl)disilane can be generated in solution.