Thermal, structural, and spectroscopic characterization of an ionic [Cu(H ₂ O) ₄ ](ReO ₄ ) ₂ compound in a wide temperature range

We present complementary investigations of water and anion dynamics in an ionic [Cu(H 2 O) 4 ](ReO 4 ) 2 . This compound has one reversible phase transition in the solid state upon heating and cooling at 247 and 224 K, respectively. The value of entropy change indicates a moderate degree of molecular dynamical disorder. At room temperature, tetraaquacopper(II) rhenate(VII) crystallizes in a triclinic crystal system, within the space group No. 2 = P‐1, with one molecule in the unit cell. X‐ray single crystal diffraction measurements indicate that the crystal structure does not change significantly at the phase transition. However, neutron diffraction experiments performed on the powdered sample revealed some subtle changes in the registered patterns. Vibrational–reorientational dynamics of H 2 O ligands in the high (I) and low‐temperature (II) phases of [Cu(H 2 O) 4 ](ReO 4 ) 2 was probed by a set of complementary spectroscopy techniques in a wide temperature range. The dynamics of the H 2 O molecules in both phases was investigated by means of band shape analysis performed for infrared bands. The H 2 O ligands perform fast (τ R ≈ 10 −11 to 10 −12 s) stochastic reorientational motions in the phases I and II with a mean value of activation energy: 6.13 kJ⋅mol −1 . However, a continuous change of bands connected with hydrogen bonds is observed. The Raman measurements revealed that in the phase transition region the intensity of bands connected with perrhenate anion significantly increases. Using vibrational spectroscopy methods all characteristic wavenumbers of the H 2 O and ReO 4 − vibrations were detected.