Volatile and Thermally Stable Polymeric Tin Trifluoroacetates

Tin
trifluoroacetates are effective vapor phase single-source precursors
for F-doped SnO2, but their structures have been poorly
understood for decades. Here we undertook a comprehensive structural
analysis of these compounds in both the solid and gas phases through
a combined single-crystal X-ray crystallography, gas phase electron
diffraction, and density functional theory investigation. Tin­(II)
bis­(trifluoroacetate) (1) thermally decomposes into a
1:1 mixture of 1 and ditin­(II) μ-oxybis­(μ-trifluoroacetate)
(2) during sublimation, which then polymerize into hexatin­(II)-di-μ3-oxyoctakis­(μ-trifluoroacetate) (3) upon
solidification. Reversible depolymerization occurred readily upon
heating, making 3 a useful vapor phase precursor itself.
Tin­(IV) tetrakis­(trifluoroacetate) (5) was also found
to be polymeric in the solid state, but it evaporated as a monomer
over 130 °C lower than 3. This counterintuitive
improvement in volatility by polymerization was possibly due to the
large entropy change during sublimation, which offers a strategic
new design feature for vapor phase deposition precursors.