Trimethylsilyl derivatives for the study of silicate structures. Part 7. Calcium silicate structures

Abstract

A detailed study was made of the yield and products of trimethylsilylation of calcium orthosilicate (Ca₂[SiO₄]), rankinite (Ca₃[Si₂O₇]), and pseudowollastonite (Ca₃[Si₃O₉]) under various experimental conditions. With hexamethyldisiloxane, chlorotrimethylsilane, and isopropyl alcohol as reagents, in the absence of added water Ca₂[SiO₄] yielded predominantly SiO₄(SiMe₃)_x(Prⁱ)_4–x(x= 1–4) and a lesser amount of Si₂O₇(SiMe₃)_x(Prⁱ)_6–x(x= 2–6), Ca₃[Si₂O₇] gave predominantly Si₂O₇(SiMe₃)_x(Prⁱ)_6–x(x= 2–6), and Ca₃[Si₃O₉] gave predominantly Si₃O₉(SiMe₃)_x(Prⁱ)_6–x(x= 2–6) and Si₃O₁₀(SiMe₃)_x(Prⁱ)_8–x(x= 3–8). Mixed trimethylsilyl–isopropyl derivatives were converted to the fully trimethylsilylated derivatives SiO₄(SiMe₃)₄, Si₂O₇(SiMe₃)₆, and Si₃O₁₀(SiMe₃)₈ on treatment with Amberlyst 15 ion-exchange resin in the presence of hexamethyldisiloxane. In the trimethylsilylation of Ca₂[SiO₄] reaction of the mineral itself is complete within ca. 6 min at room temperature, under the conditions employed, and the majority of unwanted side reactions which yield Si₂O₇ and Si₃O₁₀ derivatives occur during the first 3–4 min. Side reactions may be suppressed by increasing the proportion of isopropyl alcohol and decreasing that of Ca₂[SiO₄] in the reagents. The presence of both isopropyl alcohol and chlorotrimethylsilane is necessary for reaction to occur. Isopropyl alcohol may be replaced by either ethyl alcohol or water, although the yield is reduced. When isopropyl alcohol is replaced by water, only the fully trimethylsilylated derivatives are formed. The mechanism of the reaction is discussed and experimental conditions are described which give highest yields of the desired products.