Silatranes geometry

The principles of silatrane geometry have been discussed in a recent review article20 which considered 40 molecular structures of silatranes and silatrane-like derivatives. As these features also allow more general conclusions to be drawn for neutral pentacoordi-nate silicon complexes as a whole they will be summarized here. Derivatives... 

A single hexacoordinated silatrane 46 possessing a very weak additional Si NMe2 bond (2.952 A) was obtained recently by Corriu and coworkers44. The hexacoordination in 46 is most likely due to the rigid geometry of its 8-dimethylaminonaphthyl group. The reluctance of silicon to become hexacoordinated in the silatrane structure is its special characteristic feature which, as will be shown below (Section II.C), predetermines the low reactivity of silafunctional silatranes toward nucleophiles. 

The energetic preference of the endo form was demonstrated by the first quantum chemical calculations of silatranes performed on 1-hydro- and 1-fluorosilatranes12,181. These calculations employed the CNDO/2 semiempirical method with spd basis set and geometries of the endo and exo forms corresponding to those obtained for 1-methylsilatrane by modified molecular mechanics method. The energy of the Si-e- N coordinative bonding was found to be about 20-25 kcalmol-1 and to be the main contributor to stabilization of the endo structure of silatranes. 

AMI and PM3 computation on 1-methyl-191, 1-fluoro-180, 1-chloro-192 and 1-isothiocyanatosilatranes68 using full geometry optimization confirmed the small energy cost for a shortening of the SN— N bond in silatranes. The potential profiles of these molecules were shown to be very flat in the 2.00-3.60 A range of the Si — N distance, with two distinct minima related to the endo and exo forms. Both parametrizations found the endo structure to be the more stable in the case of 1-isothiocyanato- and 1-halosilatranes, where the equilibrium Si —N distances are shorter than in 1-methylsilatrane. Both... 

In contrast to the semiempirical results, full optimization of the molecular geometry of 1-fluorosilatrane by restricted Hartree-Fock (RHF) calculations using the 3-21G, 3-21G and 6-31G basis sets did not find evidence for the existence of an endo minimum on the energy hypersurface193. Since the exo form was never found experimentally, the ab initio results support the view of the exclusive existence of silatranes in the endo form. As for the equilibrium Sk—N distance, the ab initio calculations using polarized basis sets overestimated its value (2.556 A) even more than the AMI and PM3 methods. 

Although no four-coordinate Si compounds have been found to have planar geometry, the five-coordinate anion (53), prepared as shown in equation (57), has a geometry at silicon distorted very much away from trigonal bipyramidal towards square pyramidal geometry. The O Si angles across the spirocychc ring are about 166° and 143°. The stmctures of numerous silatranes have been determined and are discussed... 

This review is not the first to discuss the structural chemistry of organosilicon compounds. There have been a few earlier reviews discussing various aspects of the structural chemistry of organosihcon compounds. The stereochemistry of elements of Group 15 and 16 bonded to silicon was reviewed in 1973 °. In 1985 the geometry of silatranes was reviewed and simulation of the reaction pathway for Sis/2 substitution reactions at tetrahedral silicon using structural data was published. Later on, in 1986, the structural chemistry of tricoordinate silicon was reviewed and the X-ray and NMR studies on penta- and hexacoordinate silicon compounds were summarized. The most comprehensive review on the structural chemistry of organosilicon compounds was published in 1989 by W. S. Sheldrick ... 

Subsequent theoretical calculations confirmed the important role of the Si-<—N interaction in the formation of the silatrane structure. A significant Si- —N bond order was determined at the CNDO/2 level using the experimental solid state geometries of silatranes Thus, the Wiberg indexes of this bond in the crystalline a, and y... 

Investigations carried out by Verkade and coworkers show that the Si shielding of most 1-substituted triazasilatranes are sensitive to the phase and temperature - . In this respect triazasilatranes resemble silatranes. Ab initio calculations predict that the differences between the gas phase and the solution or solid state geometries of triazasilatranes should be rather less than in the case of silatranes . The lower sensitivity of... 

---