THE RING SIZE INFLUENCE ON SI-29 NMR - CHEMICAL-SHIFTS OF SOME SPIROCYCLIC TETRACOORDINATE AND PENTACOORDINATE DIOLATO SILICATES

A series of tetracoordinate spirocyclic silicates has been prepared from the reaction of a range of diols with tetraethoxysilane, Si(OEt)(4). The silicates can be converted into anionic pentacoordinate silicates by reaction with KOBu(t)/18-crown-6 in toluene. Alternatively, the pentacoordinate spirocycles can be prepared directly without prior preparation of a tetracoordinate spiro silicate. Si-29 n.m.r. studies have demonstrated that the chemical shifts are sensitive to both coordination number and ring size. Ring contributions to the Si-29 chemical shifts are apparent for the five-membered ring spiro silicates, those for the pentcoordinate species being less than those for the tetracoordinate species. Acyclic and six-membered ring spiro silicates are virtually indistinguishable by Si-29 n.m.r. spectroscopy, which demonstrates that no ring contribution to the chemical shift is apparent for this ring size.