The Ring Size Influence on ²⁹Si N.M.R. Chemical Shifts of Some Spirocyclic Tetra- and Penta-coordinate Diolato Silicates

Abstract

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. ²⁹Si n.m.r. studies have demonstrated that the chemical shifts are sensitive to both coordination number and ring size. Ring contributions to the ²⁹Si 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 ²⁹Si n.m.r. spectroscopy, which demonstrates that no ring contribution to the chemical shift is apparent for this ring size.