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 - Fe_nF_n(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.