Highly Flexible Molecule “Chameleon”: Reversible Thermochromism and Phase Transitions in Solid Copper(II) Diiminate Cu[CF3C(NH)CFC(NH)CF3]2

Three thermochromic phases (α, green; β, red; γ, yellow) and six polymorphic modifications (α1, monoclinic, P21/n, Z = 2; β1, monoclinic, P21/c, Z = 4; β2, triclinic, P1̅, Z = 4; β3, monoclinic, P21/n, Z = 4; γ1 and γ2, tetragonal, P42/n, Z = 4) have been found and structurally characterized for copper(II) diiminate Cu[CF3C(NH)CFC(NH)CF3]2 (1). The α phase is stable under normal conditions, whereas the high-temperature β and γ phases are metastable at room temperature and transform slowly into the more stable α phase over several days or even weeks. X-ray diffraction study revealed that the title molecules adopt different conformations in the α, β, and γ phases, namely, staircase-like, twisted, and planar, respectively. The investigation of the α, β, and γ phases by differential scanning calorimetry showed that the three endothermic peaks in the range 283, 360, and 438 K are present on their thermograms upon heating/cooling. The two peaks at 283 and 360 K correspond to the solid–solid phase transitions, and the high-temperature peak at 438 K belongs to the melting process of 1. The temperature and thermal effect of all the observed transitions depend on the prehistory of the crystalline sample obtained. A reversible thermochromic single-crystal-to-single-crystal α1⇌β1 phase transition occurring within a temperature interval of 353–358 K can be directly observed using a CCD video camera of the X-ray diffractometer. A series of other solid–solid α1→γ1, β2→γ1, β3→γ1, and γ1⇌γ2 phase transitions can be triggered in 1 by temperature. It has been suggested that, under equilibrium conditions, the α1→γ1 and β2→γ1 phase transitions should proceed stepwise through the α1→β1→β2→β3→γ1 and β2→β3→γ1 stages, respectively. The mechanism of the phase transitions is discussed on the basis of experimental and theoretical data.