Reactivity of bi- and monometallic trifluoroacetates towards amorphous SiO2

Abstract

The reactivity of alkali–manganese(II) and alkali trifluoroacetates towards amorphous SiO₂ (a-SiO₂) was studied in the solid-state. K₄Mn₂(tfa)₈, Cs₃Mn₂(tfa)₇(tfaH), KH(tfa)₂, and CsH(tfa)₂ (tfa = CF₃COO^–) were thermally decomposed under vacuum in fused quartz tubes. Three new bimetallic fluorotrifluoroacetates of formulas K₄Mn₃(tfa)₉F, Cs₄Mn₃(tfa)₉F, and K₂Mn(tfa)₃F were discovered upon thermolysis at 175 °C. K₄Mn₃(tfa)₉F and Cs₄Mn₃(tfa)₉F feature a triangular-bridged metal cluster of formula [Mn₃(μ₃-F)(μ₂-tfa)₆(tfa)₃]⁴⁻. In the case of K₂Mn(tfa)₃F, fluoride serves as an inverse coordination center for the tetrahedral metal cluster K₂Mn₂(μ₄-F). Fluorotrifluoroacetates may be regarded as intermediates in the transformation of bimetallic trifluoroacetates to fluoroperovskites KMnF₃, CsMnF₃, and Cs₂MnF₄, which crystallized between 250 and 600 °C. Decomposition of these trifluoroacetates also yielded alkali hexafluorosilicates K₂SiF₆ and Cs₂SiF₆ as a result of the fluorination of fused quartz. The ability to fluorinate fused quartz was observed for monometallic alkali trifluoroacetates as well. Hexafluorosilicates and heptafluorosilicates K₃SiF₇ and Cs₃SiF₇ were obtained upon thermolysis of KH(tfa)₂ and CsH(tfa)₂ between 200 and 400 °C. This ability was exploited to synthesize fluorosilicates under air by simply reacting alkali trifluoroacetates with a-SiO₂ powder.