PENTACOORDINATED SILICON-COMPOUNDS - INTRAMOLECULAR RING-CLOSURE, SITE PREFERENCES OF SUBSTITUENTS AND THE STABILITY OF THE RESULTING CHELATES

Dynamic NMR studies have been made of chelation in 2-(dimethylaminomethyl)phenylsilanes and 2-[1-(dimethylamino)ethyl]phenylsilanes with a wide range of substituents on the silicon atom. Temperature-dependent 19F spectra of compounds of the type Me2NCH2C6H4SiMeFX in which the geometry about the silicon atom is trigonal bipyramidal with the donor nitrogen atom axial, have established the preference of the substituent X for the axial site trans to the donor nitrogen atom, relative to that of the fluorine atom. Combined with other structural data this leads to te apicophilicity series: H < alkyl < aryl < OR,NR2 < F ≈ SR < Cl,OCOR. It is concluded that the apicophilicity is closely correlated with the ability of the bond trans to the donor atom to be stretched by electron donation to the central atom. The stability of the chelates has been investigated by dynamic 1H NMR spectroscopy of the potentially diastereotopic NMe2 groups. When the ligand that occupies the site trans to the donor nitrogen atom is electronegative, the stability is largely determined by the nature of that ligand, and increases with the apicophilicity. Intramolecular coordination is barely detectable in the case of an alkoxy group. Compounds containing only alkyl and aryl substituents in addition to the bidentate ligand are also not chelated, but dihydrogeno-alkyl (or -aryl) derivatives, in which the hydrogen atoms occupy the equatorial sites, and trihydrogeno derivatives, are relatively strongly coordinated. © 1990.