Structural Evolution of Iron(III) Trifluoroacetate upon Thermal Decomposition: Chains, Layers, and Rings

Metal trifluoroacetates
(TFAs) are commonplace precursors for bulk
and nanostructured metal fluorides and oxides obtained usually through
their thermal decomposition. Very little, however, is known about
the atomistic mechanism of such a thermal conversion, or even the
crystal structure of the precursors. In this study, we detail the
structural evolution of Fe­(TFA)3 upon its thermal decomposition
into rhombohedral iron­(III) fluoride. Several distinct structural
motifs have been identified in the temperature range of 250–350
°C. In particular, Fe­(TFA)3, Fe2F­(TFA)5, and FeF­(TFA)2 are composed of infinite chains.
In addition, several volatile molecular species with ring structureFenFn(TFA)2n (n = 6–10)have been
characterized both by single-crystal X-ray diffraction and by mass
spectrometry. Further conversion of TFA groups into fluoride ions
leads to a layered FeF2(TFA). All these compounds feature
bridging TFA ligands and octahedral fluoro- or oxo-coordination of
Fe. The retention of the +3 oxidation state is confirmed by 57Fe Mössbauer spectroscopy. Magnetization measurements point
to the ferrimagnetic ordering in FeF2(TFA) at T ≤ 150 K, unlike Fe­(TFA)3, which remains paramagnetic
at as low as 10 K. This study highlights the potential of metal TFAs
for the rational synthesis of molecular and solid-state hybrid compounds
by their versatile thermal decomposition into fully inorganic materials.