SYNTHESIS, STRUCTURE, AND REACTIVITY OF THE FIRST KINETICALLY STABILIZED SILANETHIONE

Diaryltetrathiasilolanes 2a,b, bearing bulky aryl groups such as 2,4,6 -tris[bis(trimethylsilyl)methyl]-phenyl (Tbt), mesityl (Mes), and 2,4, 6-trisopropylphenyl (Tip), were obtained by thermal reaction of dihydr osilane 3a with elemental sulfur, reaction of highly hindered disilene 4 with sulfur, or reduction of dibromosilane 6 followed by addition o f sulfur. The molecular structure of 2a was determined by X-ray crysta llographic analysis. Desulfurization of 2a at -78 degrees C afforded a silicon-sulfur double bond compound (silanethione 7a). Although the f ormation of 7a was confirmed by various trapping reactions, 7a dimeriz ed at roam temperature to give 1,3,2,4-dithiadisiletane 8, whose molec ular structure was determined by X-ray crystallographic analysis. Desu lfurization of 2b afforded the first stable silanethione, Tbt(Tip)Si=S (7b), as yellow crystals, which were thermally stable under inert gas up to its melting point (185-189 degrees C) and showed lambda(max) (n --> pi) at 396 MI and nu(Si=S) (Raman) at 724 cm(-1). The chemical sh ift of delta(Si) 166.56 in the Si-29 NMR spectrum is greatly downfield -shifted, characteristic of a silathiocarbonyl unit. Comparison of the electronic spectra (n-->pi) of a series of (RRM)-R-1-M-2=S (M = C, S i, Ge, Sn) compounds revealed an interesting trend: lambda(max) is sig nificantly blue-shifted on going from M = C to M = Si, whereas lambda( max)'s for (RRM)-R-1-M-2=S (M = Si, Ge, Sn) are red-shifted on increas ing the atomic number of M. This trend is also found in calculated val ues for H2M= S (M = C, Si, Ge, Sn). The X-ray crystallographic analysi s of 7b revealed that there are two nonidentical silanethiones in the asymmetric unit and the silathiocarbonyl units have completely trigona l-planar geometry, with silicon-sulfur double bond lengths of 1.948(4) and 1.952(4) Angstrom, which are about 0.2 Angstrom shorter than the typical Si-S single bond length (ca. 9% shortening), indicating an una mbiguous double bond character between silicon and sulfur. Despite the presence of two very bulky substituents on silicon, 7b easily reacted with water, methanol, phenyl isothiocyanate, mesitonitrile oxide, and 2,3-dimethyl-1,3-butadiene to afford the corresponding adduct or cycl oadduct. Theoretical calculations were carried out for the sigma and p i bond energies of H2M=X (M = C, Si, Ge, Sn, Pb; X = O, S, Se, Te) and for the single and double bond lengths of H3M-XH and H2M=;X, and the theoretical backgrounds of the chemistry of group 14-group 16 double b ond species were discussed.