| Résumé | The [CuPy₄(NO₃)₂]*2Py (Py=pyridine) clathrate showed a phase transformation at 46 °C with an enthalpy change of 3.27(5) kJ/mol. The unit cell parameters were measured in the -173 to +77 °C temperature range by single-crystal XRD, showing an expansion of two unit cell parameters and a contraction of the third, with discontinuous changes at the transition point and nonlinearity over a wide temperature range. Both polymorphs are orthorhombic; above the transition point the primitive unit cell (Pnna) transforms into a C-centered cell (Ccca). The precise transition point was determined from the temperature dependence of the intensity of the characteristic reflections which became systematic absences for the high-temperature polymorph. The crystal structure of the high-temperature polymorph was determined at +52 °C and compared with available data for the low-temperature polymorph. In both forms, the host molecule has a copper atom in a distorted octahedral environment, coordinated by four pyridine nitrogens in the equatorial plane and two nitrate oxygens above and below the equatorial plane. Guest pyridine molecules are located in channels. The changes associated with the polymorphous transformation are related to the dynamics of the nitrate group in the structure, and they take place over the entire temperature range. The nitrate ligand plane is inclined to the axis of the complex and shows significant motion. Above the transition point the nitrate ligand acquires the ability to flip from one side of the axis of the complex to the other. The results of this study make it possible for previous data to be rationalized, and provide evidence of a generic relation between two structural types of the [MPy₄X₂]*2(guest) family of clathrates. |
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