Calorimetric and vibrational spectroscopic investigations of phase transitions in crystalline [Cr(OS(CH3)2)6](BF4)3

Two phase transitions – one sharp at T1 = 247.4 K and the other very broad at T2 ∼ 135 K – were observed for [Cr(OS(CH3)2)6](BF4)3 by adiabatic calorimetry. They are of the order–disorder type, and their transition entropies are 28.4 and 8.3 J K−1 mol−1, respectively. IR absorption and Raman scattering spectroscopy measurements revealed distinct changes in the molecular dynamics of both CH3 groups and BF4− anions at T1 and reduced symmetry of the [Cr(OS(CH3)2)6]3+ complex cations and OS(CH3)2 ligands below T2. The spectral width of an IR and Raman active band associated with the δas(FBF)F2 mode yielded an activation energy (Ea) ∼ 9 kJ mol−1 for the reorientation of BF4− anions, which is comparable with that obtained for other coordination compounds with BF4− anions.

► Phase transitions are observed at 247.4 and 135 K for [Cr(DMSO)6](BF4)3.
► Both transitions are of the order–disorder type.
► The reorientational dynamics of CH3 and BF4− are changed across the transition at 247.4 K.
► Reduced symmetry is revealed for cations and DMSO ligands below 135 K.