| Research Abstract |
Hypervalent compounds such as sigma-sulfuranes and thiathiophthenes are of great interest in structural chemistry and organic synthesis. Electron-density distributions of two tetraazathiapentalene derivatives, I and II,with typical hypervalent S-N bonds of 1.90 - 1.96 A were investigated in order to shed light on the character of hypervalent bonds and to compare electronic features of these sulfur atoms. Electron-density distributions of dibenzothiophene derivative, III,with a weak intramolecular S・・・S contact of 3.012 A were also investigated. Intensities of X-ray diffraction were measured at 143K up to 2rhetamax=100ﾟ (88ﾟ for III) and the structure refinements were performed using multipole expansion atomic scattering factors up to the hexadecapole expansion for S atom. In model-deformation maps of the tetraazathiapentalene planes, positive electron densities along S-N bonds are observed near the nitrogen atoms, not in the center of the bonds. The net atomic charges derived from the multipole refinement suggest a polarized character of S-N bond. For each compound, two lone-pair electron densities are observed in the section perpendicular to the pentalene plane through the S-C bonds. The angles of C-S-L,C-S-L' and L-S-L' (L and L' = peaks of lone-pair densities) are about 120ﾟ.Structure around the hypervalent S atom is a trigonal bipyramid with equatorial sp^2 hybrid of S-C bond and lone-pair electrons and apical polarized S-N bonds. In III lone-pair electron densities are observed perpendicular to the thiophene ring and the C-S-C sulfide planes. Lobes of lone-pair electrons of two sulfide groups are orthonormal with each other to avoid a steric hindrance between lone pairs of two S atoms.
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