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

Abstract

Thermal analysis (TG/DTG/QMS), performed for [Sr(OS(CH₃)₂)₆](ClO₄)₂ in a flow of argon and in temperature range of 295–585 K, indicated that the compound is completely stable up to ca. 363 K, and next starts to decompose slowly, and in the temperature at ca. 492 K looses four (CH₃)₂SO molecules per one formula unit. During further heating [Sr(DMSO)₂](ClO₄)₂ melts and simultaneously decomposes with explosion. Differential scanning calorimetry (DSC) measurements performed in the temperature range of 93–370 K for [Sr(DMSO)₆](ClO₄)₂ revealed existence of the following phase transitions: glass ↔ crystal phase Cr5 at T _g  ≈ 164 K (235 K), phase Cr5 → phase Cr4 at $T_{\text{c6}}^{\text{h}}$ ≈ 241 K, phase Cr4 → phase Cr3 at $T_{\text{c5}}^{\text{h}}$ ≈ 255 K, phase Cr3 → phase Cr2 at $T_{\text{c4}}^{\text{h}}$ ≈ 277 K, phase Cr2 ↔ phase Cr1 at $T_{\text{c3}}^{\text{h}}$ ≈ 322 K and $T_{\text{c3}}^{\text{c}}$ ≈ 314 K, phase Cr1 ↔ phase Rot2 at $T_{\text{c2}}^{\text{h}}$ ≈ 327 K and $T_{\text{c2}}^{\text{c}}$ ≈ 321 K and phase Rot2 ↔ phase Rot1 at $T_{\text{c1}}^{\text{h}}$ ≈ 358 K and $T_{\text{c1}}^{\text{c}}$ ≈ 347 K. Entropy changes values of the phase transitions at $T_{\text{c1}}^{\text{h}}$ and $T_{\text{c2}}^{\text{h}}$ (∆S ≈ 79 and 24 J mol⁻¹ K⁻¹, respectively) indicated that phases Rot1 and Rot2 are substantially orientationally disordered. The solid phases (Cr1–Cr5) are more or less ordered phases (∆S ≈ 7, 10, 4 and 3 J mol⁻¹ K⁻¹, respectively). Phase transitions in [Sr(DMSO)₆](ClO₄)₂ were also examined by Fourier transform middle infrared spectroscopy (FT-MIR). The characteristic changes in the FT-MIR absorption spectra of the low- and high-temperature phases observed at the phase transition temperatures discovered by DSC allowed us to relate these phase transitions to the changes of the reorientational motions of DMSO ligands and/or to the crystal structure changes.