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.