Copper(II) Complexes with 4-Substituted 2,6-Bis(thiazol-2-yl)pyridines-An Overview of Structural-Optical Relationships.

Copper(II) complexes with 2,2':6',2″-terpyridines (terpys) are promising candidates for anticancer therapy and catalysis. Their structural and optical properties can be tuned by modifying the terpy backbone, including a substitution at the 4' position or the replacement of peripheral pyridines with thiazole rings, forming 2,6-bis(thiazol-2-yl)pyridines (dtpys). dtpy-based copper(II) complexes (Cu-dtpys), despite their applicative potential, are barely characterized in the literature. Here, the series of Cu-dtpys (1-13) was synthesised and characterized by FT-IR, HRMS, X-ray diffraction, and UV-Vis spectroscopy. Their structural and optical features were compared to previously studied Cu-dtpys (14-24) and their terpy analogues (Cu-terpy-1 ÷ Cu-terpy-24). The detailed analysis revealed that five-coordinate Cu-dtpys complexes adopt a square pyramidal geometry comparable to that of Cu-terpys complexes but with markedly smaller deviations from the ideal square pyramid. Compared with Cu-terpys, Cu-Clapical bonds are shorter, while Cu-Ncentral bonds are elongated. The Cu-dtpy systems usually present the longest wavelength of the lowest energy absorption band in comparison to Cuterpys. The analysis of the relationship between Hammett's constant and wavelength of absorption indicates that the most promising from the photophysical point of view are compounds 4-6, 10-13, 16-17, and 22, for which a newly formed intraligand charge transfer band is formed.