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 (α(1), monoclinic, P2(1)/n, Z = 2; β(1), monoclinic, P2(1)/c, Z = 4; β(2), triclinic, P1[overline], Z = 4; β(3), monoclinic, P2(1)/n, Z = 4; γ(1) and γ(2), tetragonal, P4(2)/n, Z = 4) have been found and structurally characterized for copper(II) diiminate CuCF(3)-C(NH)-CF═C(NH)-CF(3) (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 α(1)<--> β(1) 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 α(1)→γ(1), β(2)→γ(1), β(3)→γ(1), and γ(1)<-->γ(2) phase transitions can be triggered in 1 by temperature. It has been suggested that, under equilibrium conditions, the α(1)→γ(1) and β(2)→γ(1) phase transitions should proceed stepwise through the α(1)→β(1)→β(2)→β(3)→γ(1) and β(2)→β(3)→γ(1) stages, respectively. The mechanism of the phase transitions is discussed on the basis of experimental and theoretical data.