Silanethiones structure

Silanethione 38 was characterized by H, 13C, and 29Si NMR, Raman, and UV-vis spectroscopic methods. The 29Si NMR chemical shift of 38 (8Si 166.56/C6D6) for the silathiocarbonyl unit is much downfield shifted from those of the thermodynamically stabilized silanethiones, 31, 34, 35,28 and 36,29 mentioned in the previous sections, clearly indicating a genuine Si=S double bond in 38 without any intra- or intermolecular coordination. The molecular structure of 38 was successfully established by X-ray crystallographic analysis, and the detailed structural parameters are discussed in the following section. 

First, crystallographic structural analysis of the thermodynamically stabilized silanethione was established for the bulky silanethione 31b (see Scheme 8). 

Therefore, the elucidation of the intrinsic structural parameters of heavy ketones has to be done with kinetically stabilized systems. Most of the heavy ketones synthesized by steric protection with the Tbt group have provided single crystals suitable for X-ray structural analysis. The results for silanethione 38, germanethione 71b, germaneselones 75, 84, germanetellones 77,85, and stannaneselone 127, are summarized in Table III. 

FIGURE 3. Molecular structure of silanethione 74 with selected bond lengths (A)9... 

The silanethiones generated by these reactions were not isolated but their characteristic 29Si NMR chemical shifts were reported to be +22.3, +34.2 and +41.1 ppm (CDCI3) for 67, 77 and 78, respectively. The 29 Si chemical shift values of 77 and 78 are similar to those observed for 67 and 74 and therefore are indicative of their intramolecularly coordinated betaine structure. 

In contrast, the reaction of silicocene 41 with carbon disulfide did not give the reaction products derived from the expected silanethione 80 instead a surprising multistep reaction product 89 was isolated (Scheme 31)41b. In the preliminary communication413, the authors misinterpreted the structure of this reaction product as being a simple dimer 90 of the initial intermediate, thiasiliranethione 85, but the X-ray crystallographic analysis later revealed that 89 is the actual product, as shown in Scheme 3141b. 

FIGURE 4. Molecular structure of silanethione 4 with selected bond lengths (A) and angles (deg)10. Reprinted with permission from Ref. 10. Copyright (1994) American Chemical Society... 

The reaction of 59 with sulfur is somewhat more complicated (Scheme 19). At low temperatures, an intermediate is observed with a 29Si NMR chemical shift of +122.6 ppm this deshielded value is consistent with the silanethione, 69. Wanning of the solution produces the 4-ring dimer, 70. But when the reaction is carried out at room temperature this intermediate is not observed, and the major product is the trisilicon compound 71 in which a molecule of diimine has been lost from the central silicon374. The structure of 71 is somewhat similar to that of solid silicon disulfide. 

With isothiocyanates, the silanethione (84) is obtained, without oligomerization. The 29Si NMR of 84, at —21.0 ppm, shows that the silicon atom is highly shielded, consistent with a pentacoordinate structure as shown in equation 131. The chemical shift can be compared with that for singly N-coordinated silanethiones, +22 to +41 ppm, and with that for a true tricoordinate silanethione, +166.6 ppm. 

---