Non-rigid molecule of copper(II) diiminate Cu[CF3C(NH)C(F)C(NH)CF3]2, its conformational polymorphism in crystal and structure in solutions (Raman, UV–vis and quantum chemistry study)

• Copper(II) diiminate Cu[CF3C(NH)C(F)C(NH)CF3]2 shows a double-well potential curve with a low barrier.
• The difference between various polymorphs of the copper(II) diiminate was studied by optical spectroscopy methods.
• The new brown polymorph of the copper(II) diiminate was investigated by X-ray diffraction analysis.
• Evident distinctions between the polymorph spectra reflect the differences in copper(II) coordination geometry.
• Spectral features of the copper(II) diiminate polymorphs are determined by intermolecular interactions and packing favors.

Calculation of potential energy surface (PES) of isolated molecule of copper(II) diiminate Cu[CF3С(NH)C(F)C(NH)CF3]2 (1) resulted a double-well curve with the minima corresponding to equivalent screwed conformations. The low barrier leads to molecular non-rigidity which seems to be the reason of conformational polymorphism in crystals, reported in [1]. For one of newly found polymorphs, the X-ray structure was determined. The differences in the Raman and UV–vis spectra between differently colored species and their solutions were revealed, they are determined by different geometries of Cu(II) coordination polyhedron and different systems of intermolecular interactions in crystals. Transformations of the polymorphs under thermal, mechanical and photo exposures were studied.