Synthesis, Structure, and Reactivity of the First Kinetically Stabilized Silanethione

Abstract

Diaryltetrathiasilolanes 2a,b, bearing bulky aryl groups such as 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt), mesityl (Mes), and 2,4,6-triisopropylphenyl (Tip), were obtained by thermal reaction of dihydrosilane 3a with elemental sulfur, reaction of highly hindered disilene 4 with sulfur, or reduction of dibromosilane 6 followed by addition of sulfur. The molecular structure of 2a was determined by X-ray crystallographic analysis. Desulfurization of 2a at −78 °C afforded a silicon−sulfur double bond compound (silanethione 7a). Although the formation of 7a was confirmed by various trapping reactions, 7a dimerized at room temperature to give 1,3,2,4-dithiadisiletane 8, whose molecular structure was determined by X-ray crystallographic analysis. Desulfurization of 2b afforded the first stable silanethione, Tbt(Tip)SiS (7b), as yellow crystals, which were thermally stable under inert gas up to its melting point (185−189 °C) and showed λmax (n→π*) at 396 nm and νSiS (Raman) at 724 cm-1. The chemical shift of δSi 166.56 in the 29Si NMR spectrum is greatly downfield-shifted, characteristic of a silathiocarbonyl unit. Comparison of the electronic spectra (n→π*) of a series of R1R2MS (M = C, Si, Ge, Sn) compounds revealed an interesting trend: λmax is significantly blue-shifted on going from M = C to M = Si, whereas λmax's for R1R2MS (M = Si, Ge, Sn) are red-shifted on increasing the atomic number of M. This trend is also found in calculated values for H2MS (M = C, Si, Ge, Sn). The X-ray crystallographic analysis of 7b revealed that there are two nonidentical silanethiones in the asymmetric unit and the silathiocarbonyl units have completely trigonal-planar geometry, with silicon−sulfur double bond lengths of 1.948(4) and 1.952(4) Å, which are about 0.2 Å shorter than the typical Si−S single bond length (ca. 9% shortening), indicating an unambiguous 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 cycloadduct. Theoretical calculations were carried out for the σ and π bond energies of H2MX (M = C, Si, Ge, Sn, Pb; X = O, S, Se, Te) and for the single and double bond lengths of H3MXH and H2MX, and the theoretical backgrounds of the chemistry of group 14−group 16 double bond species were discussed.