Fourier transform infrared spectroscopy and differential scanning calorimetry studies of the phase transitions and molecular motions in polycrystalline [Al(DMSO)6](ClO4)3 and [Al(DMSO)6](BF4)3

•[Al(DMSO)6](ClO4)3 and [Al(DMSO)6](BF4)3 have been synthesised and characterized.•Thermal behavior of both complexes has been studied between 100 and 1485 K.•One solid phase transition has been discovered in both complexes.•The transition has been investigated using infrared absorption spectroscopy.•Reorientational dynamics of ligands and anions is changed across the transition.

Vibrational spectra, reorientational motions and phase transitions of polycrystalline [Al(DMSO)6](ClO4)3 and [Al(DMSO)6](BF4)3, where DMSO = (CH3)2SO, were studied by Raman Scattering (RS), Fourier transform infrared absorption spectroscopies (FT-IR) and by differential scanning calorimetry (DSC). One solid–solid phase transition was detected in [Al(DMSO)6](ClO4)3 at TCh = 185.6 K and in [Al(DMSO)6](BF4)3 at TCh = 116.8 K (on heating). It was evidenced by FT-MIR measurements performed in a function of temperature that these phase transitions are associated with a sudden change of reorientational dynamics of CH3 groups (from (CH3)2SO ligands) and BF4− anions. It is a result of weak (not conventional) hydrogen bond like interactions occurring between these groups. Calculated for the high temperature phases of [Al(DMSO)6](ClO4)3 and [Al(DMSO)6](BF4)3 activation energy values for the CH3 reorientations (Ea(CH3)) are comparable for both compounds, and equal to ca. 11 and 9 kJ mol−1, respectively. The values of activation energy for the anions’ reorientations Ea(ClO4−) and Ea(BF4−) are also comparable for both compounds, and equal to ca. 8 and 9 kJ mol−1, respectively. Observed splitting of some bands in infrared absorption spectra indicated that a change of the crystal structure symmetry at the phase transition can take place. By thermal analysis methods (TG/QMS) it was established that in argon atmosphere [Al(DMSO)6](BF4)3 is thermally stable up to 380 K and decomposes in two main steps between 380 and 1485 K, with creation of Al2O3 as a final product of decomposition.