INTRAMOLECULAR COORDINATION AT SILICON - THE SMALL EFFECT OF EQUATORIAL LIGANDS UPON THE STABILITY OF PENTACOORDINATED ORGANOSILANES

The molecular structure of 2-[8-(dimethylamino)-1-naphthyl]-4,4,5,5-tetramethyl-1,3,2-dioxasilacyclo-pentane (1a, X = H) determined by X-ray diffraction, shows this fused system to adopt a trigonal-bipyramidal geometry with the pinacol ring spanning equatorial-apical positions, the equatorial substituent (X = H) being orthogonal to the apical dimethylamino coordinated ligand. Functional pentacoordinated organosilanes derived from 1 (X = H, F, Cl, OCOR, OPh, Ph) model the trigonal-bipyramidal (tbp) intermediates which have been postulated in nucleophilic substitution with retention at silicon, with X, the leaving group, in the equatorial position of the tbp, and the coordinated nitrogen as the apical incoming nucleophile. H-1 NMR dynamic spectroscopy shows the formation of diastereomeric pentacoordinated organosilanes, and the energy barrier for their thermal interconversion is a measure of the intramolecular coordination. Despite the large electronic changes in the equatorial X substituents, the activation energy is only slightly dependent on the nature of this ligand (DELTA-G double-ended-dagger = 15-20 kcal mol-1). The results confirm kinetic studies and ab initio calculations. The nature of the equatorial ligand has in general little effect on the energy level of the tbp species.