Thermal analysis, phase transitions and molecular reorientations in [Fe(OS(CH3)2)6](ClO4)2

Abstract

Thermogravimetric analysis connected with quadruple mass spectroscopy (TG/MS) for an identification of [Fe(OS(CH3)2)6](ClO4)2 decomposition products, carried out to determine its thermal stability, has indicated that the title compound does not change its mass till ca. 385 K. Above this temperature, it starts slowly to lose a part of (CH3)2SO ligands, which begins to form a liquid phase, in which the title compound partially dissolves. Finally, at ca. 476 K, when two from six coordinated (CH3)2SO were detached from central atom, the [Fe(OS(CH3)2)4](ClO4)2 is formed. At ca. 514 K this sample explodes. Differential scanning calorimetry (DSC) measurements performed in the temperature range of 100–443 K revealed existence of two anomalies on DSC curves. The first, a big one, at Tc ≈ 338 K is associated with the phase transition: crystal phase Cr. 1 ↔ rotational phase Rot. 1, and the second, a small one, at Tm1 ≈ 414 K is associated with two parallel processes, which are: decomposition of [Fe(OS(CH3)2)6](ClO4)2 with the DMSO release and dissolution of [Fe(OS(CH3)2)6−x](ClO4)2 in DMSO. The large value of solid–solid phase transition entropy change (∆Sc ≈ 79.3 J mol−1 K−1) and small value of the melting process (∆Sm ≈ 5.8 J mol−1 K−1) indicate on such large configurational disorder in the high-temperature phase that this phase can be considered as a rotational phase (so called also as “plastic crystals”). The results of the vibrational and reorientational dynamics of (CH3)2SO ligands and ClO4− anions in the high- and low-temperature phases of [Fe(OS(CH3)2)6](ClO4)2, investigated by Fourier transform infrared absorption spectroscopy, show that even in the low-temperature phase the CH3 groups in (CH3)2SO ligands and also the ClO4− anions perform fast (correlation time τR ≈ 10−12 s) reorientational motions. These reorientational motions above Tc temperature became so fast that in the rotational phase they turn into nearly free rotational motions.