The remarkable behavior of crystalline [Fe(η5-C5H4CHO)2]:: Two solid-to-solid phase transitions and a solid-state reaction

The crystal architecture, stability, and behavior with temperature of bis(formyl)ferrocene, [Fe(eta(5)-C5H4CHO)(2)], have been investigated by variable-temperature X-ray diffraction experiments, differential scanning calorimetry, and thermogravimetry experiments. [Fe(eta(5)C(5)H(4)CHO)(2)] is present with two independent molecules with cisoid and transoid relative orientations of the two formyl groups in the crystals obtained from the reaction sequence (phase RT-1). The role of C-H- - -O interactions involving the formyl groups has been examined. When RT-1 is heated, the first irreversible phase transition to a plastic phase (phase HT) is observed at ca. 38 degrees C (311 K). When it is cooled, phase HT transforms into a new room-temperature phase (RT-2). Once RT-2 has been formed, the system switches reversibly between HT and RT-2 (transition temperature in the heating cycles ca. 35 degrees C), while RT-1 can no longer be obtained. Further heating of phase HT shows the occurrence of an exothermic reaction at ca. 150 degrees C (423 K) leading to the formation of a ferrocene-based polymer.