Cyclic Pentaoxy Siliconates¹,²

Spirocyclic pentaoxy anionic silicates are synthesized and their molecular structures determined by X-ray diffraction. The K, 18-crown-6 salt of [(OCMe2CMe2O)2SiO-t-Bu] (1), formed from the reaction of the bis-pinacol spirosilicate and potassium t-butoxide, represents a new substance. Both the butylammonium salts, [(OCMe2CMe2O)2SiOR][n-BuNH3] R = OEt (2); R = OMe (3), have structures considerably displaced from a trigonal bipyramid toward a rectangular pyramid, 38.9% for 2 and 71.2% for 3, compared to 24.1% for 1. Hydrogen bonding is implicated in accounting for the greater structural displacement of 2 and 3. These pentaoxy derivatives represent the first members of this class that have been structurally characterized. Comparisons with related five-coordinated silicon and phosphorus compounds support their use as models in nucleophilic substitution reactions.