Highly Flexible Molecule
“Chameleon”:
Reversible Thermochromism and Phase Transitions in Solid Copper(II)
Diiminate Cu[CF<sub>3</sub>C(NH)CFC(NH)CF<sub>3</sub>]<sub>2</sub>
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Abstract
Three thermochromic phases (α, green; β,
red; γ,
yellow) and six polymorphic modifications (α<sub>1</sub>, monoclinic, <i>P</i>2<sub>1</sub>/<i>n</i>, <i>Z</i> =
2; β<sub>1</sub>, monoclinic, <i>P</i>2<sub>1</sub>/<i>c</i>, <i>Z</i> = 4; β<sub>2</sub>,
triclinic, <i>P</i>1̅, <i>Z</i> = 4; β<sub>3</sub>, monoclinic, <i>P</i>2<sub>1</sub>/<i>n</i>, <i>Z</i> = 4; γ<sub>1</sub> and γ<sub>2</sub>, tetragonal, <i>P</i>4<sub>2</sub>/<i>n</i>, <i>Z</i> = 4) have been found and structurally characterized for
copper(II) diiminate Cu[CF<sub>3</sub>C(NH)CFC(NH)CF<sub>3</sub>]<sub>2</sub> (<b>1</b>). 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 <b>1</b>. 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<sub>1</sub>⇌β<sub>1</sub> 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<sub>1</sub>→γ<sub>1</sub>, β<sub>2</sub>→γ<sub>1</sub>, β<sub>3</sub>→γ<sub>1</sub>, and γ<sub>1</sub>⇌γ<sub>2</sub> phase transitions can be triggered in <b>1</b> by temperature.
It has been suggested that, under equilibrium conditions, the α<sub>1</sub>→γ<sub>1</sub> and β<sub>2</sub>→γ<sub>1</sub> phase transitions should proceed stepwise through the α<sub>1</sub>→β<sub>1</sub>→β<sub>2</sub>→β<sub>3</sub>→γ<sub>1</sub> and β<sub>2</sub>→β<sub>3</sub>→γ<sub>1</sub> stages, respectively. The mechanism
of the phase transitions is discussed on the basis of experimental
and theoretical data
