In contrast to almost all other derivatives of silicic acid, silatranes are remarkably stable towards hydrolysis. This feature was traced back to the hypervalency of the silicon which has not only three bonds to oxygen and one to carbon, but shows, according to crystal structures, a quite low distance to nitrogen which might be interpreted as a fifth bond. The structure with a low Si-N distance (endo-structure, around 2.2Å) is expected to be more stable than the corresponding exo-species (Si-N distance around 2.6Å), where a direct interaction between nitrogen and silicon should be much weaker. The nature of this weak Si-N bond was the target of a great number of calculations [9]. Csonka and Hencsei have shown that at sufficient level of theory the pentacoordinate endo-species is the only energetic minimum on the potential surface, [10,11], although the surface seems to be very flat in the direction of the Si-N vector. Gordon performed a Bader analysis for several silatranoles [12] finding an ionic interaction between silicon and nitrogen, as well as between silicon and oxygen. These calculations were performed on AM1-optimised structures, which give only a rough approximation of solid state properties. In particular, they give considerably longer Si-N distances than observed in crystal structures (but more close to the experimental gas phase values).
We performed a number of calculations on the X-ray geometry of our chiral silatrane derived from
(S)-
-pinene
using the
AIMPAC
package [13,14] for topological analysis. To our knowledge, it is the
first topological analysis of a chiral silatrane.
For a detailed explanation of the theory of topological analysis and the terminology used, see Bader's web-site.