PENTACOORDINATE ACYCLIC AND CYCLIC ANIONIC OXYSILICATES. A 29Si NMR AND X-RAY STRUCTURAL STUDY

Abstract

Abstract Variable temperature 29Si NMR spectral measurements revealed the formation of acyclic anionic silicates, [R'nSi(OR)5-n] [K, 18-crown-6], n = 0-3, from reactions of alkoxy and aryloxysilanes with the respective potassium alkoxide or potassium aryloxide in the presence of 18-crown-6. This elusive class of substances has been proposed as model intermediates in the sol-gel process. In the case of [PhSi(OCH2CF3)4][K, 18-crown-6], isolation as a crystalline solid was achieved. The29Si chemical shifts move progressively downfield from Si(OR)5 to R'Si(OR)4 − to R'2Si(OR)3 −. Pentaalkoxysilicate anions react rapidly with pinacol to form the corresponding five-coordinated bicyclic anionic silicate. Independent synthesis and an X-ray structural study verified the formation of the bis(pinacolate), [(Me4C2O2)2SiO-i-Pr][K, 18-c-6] (1). For comparison of structural distortions, the synthetic and X-ray characterization of the related anionic oxysilicates, [(C6H4(CF3)2CO)2SiC6H11][Et4N] (2) and [(CH4O2)2SiC6H11][...