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

1995; Taylor & Francis; Volume: 99; Issue: 1-4 Linguagem: Inglês

10.1080/10426509508031347

1563-5325

K. C. Kumara Swamy, Vadapalli Chandrasekhar, John J. Harland, Joan M. Holmes, Roberta O. Day, Robert R. Holmes,

Advanced NMR Techniques and Applications

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 P2t 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).