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 (<b>terpys</b>) are promising candidates for anticancer therapy and catalysis. Their structural and optical properties can be tuned by modifying the <b>terpy</b> 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 (<b>dtpys</b>). <b>dtpy</b>-based copper(II) complexes (<b>Cu-dtpys</b>), despite their applicative potential, are barely characterized in the literature. Here, the series of <b>Cu-dtpys</b> (<b>1</b>-<b>13</b>) 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 <b>Cu-dtpys</b> (<b>14</b>-<b>24</b>) and their terpy analogues (<b>Cu-terpy-1</b> ÷ <b>Cu-terpy-24</b>). The detailed analysis revealed that five-coordinate <b>Cu-dtpys</b> complexes adopt a square pyramidal geometry comparable to that of <b>Cu-terpys</b> complexes but with markedly smaller deviations from the ideal square pyramid. Compared with <b>Cu-terpys</b>, Cu-Cl_apical bonds are shorter, while Cu-N_central bonds are elongated. The <b>Cu-dtpy</b> systems usually present the longest wavelength of the lowest energy absorption band in comparison to <b>Cuterpys</b>. 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 <b>4</b>-<b>6</b>, <b>10</b>-<b>13</b>, <b>16</b>-<b>17</b>, and <b>22</b>, for which a newly formed intraligand charge transfer band is formed.