Pentacoordinate acyclic and cyclic anionic oxysilicates. A ²⁹Si NMR and X-ray structural study

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][Me₂NH₂] (3), are reported. The structures of 1 and 2 are trigonal-bipyramidal and that of 3 is rectangular-pyramidal. The silicate 1 crystallizes in the monoclinic space group P21/c with a = 10.314 (3) Å, b = 20.401 (5) Å c = 16.608 (4) Å β= 91.04 (2)° and Z = 4. Silicate 2 crystallizes in the monoclinic space group P2_t with a = 10.148 (2) Å, b = 16.356 (6) Å, c = 11.266 (3) Å, β = 114.02 (2)°, and Z = 2. The bis catecholate 3 crystallizes in the monoclinic space group P2₁/c with a = 13.566 (2) Å, b = 14.962 (3) Å, c = 20.561 (4) Å, β = 103.56 (1)° and Z = 8. The final conventional unweighted residuals are 0.047 (1), 0.060 (2), and 0.059 (3).