**4. Conclusions**

A set of four thiosemicarbazone ligands derived from naphthaldehyde and anthraldehyde have been synthesized and characterized also X-ray diffraction. The reactivity of the ligands toward Cu(II) ions revealed unexpected behavior. Cu2+ was able to oxidize a part of the naphthaldehyde thiosemicarbazones to a 1,3,4-thiadiazole derivative; this was unusual, given that this reactivity is usually associated with Fe3+ ions and not with other redox metal ions. The remaining naphthaldehyde thiosemicarbazone molecules present in solution yielded a Cu(I) complex. The anthraldehyde thiosemicarbazones were even more reactive towards Cu(II) ions and were oxidized to multiple species (not clearly identifiable) that probably contained not only 1,3,4-thiadiazole derivatives, but also 1,2,4-triazolines derivatives. Between the two Cu(I) complexes isolated, only [CuI(L1)2](HSO4) (**1**) was stable under the tested conditions, and moreover, did not undergo reoxidation, at least for the duration of the biological tests. Therefore, it was selected, together with its parent ligand, for further biological investigations. Both the ligand and its complex revealed an interaction with DNA. For both compounds, the observed circular dichroism behavior was consistent with a possible conformational change of DNA from B to C, and with the simultaneous presence of more than one mode of interaction. The UV experiments confirmed multiple ways for both compounds to interact with DNA, excluding complex intercalation as the prevalent interaction, and suggesting an external mode of action toward DNA, as determined by electrostatic interactions and by the formation of a grea<sup>t</sup> network of hydrogen bonds. Both the ligand and the complex had an affinity for BSA; this was probably due to the thiosemicarbazone moiety of the compounds that interacted with the protein, since there was no significant difference between the ligand and the complex behavior. The most interesting outcome of this research is that complex [CuI(L1)2](HSO4) (**1**) showed an important inhibition effect on U937 leukemic cell line viability, reaching a very low IC50 value of 5.46 μM, a property that is absent in the ligand alone.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/compounds2020011/s1, Crystal data and Cell viability curve after complex treatment. Figure S1: U937 cell viability curve after treatment with [CuI(L1)2](HSO4) (**1**) at different concentrations; Figure S2: Representation of complex [CuI2(SO4)(L2)5] (**2**).

**Author Contributions:** Conceptualization, G.P. and F.B.; Data curation, F.B.; Formal analysis, G.P., S.P. and F.B.; Methodology, F.B.; Supervision, G.P. and F.B.; Writing—original draft, F.B.; Writing—review & editing, G.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by UNIPR Fil.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable. **Data Availability Statement:** CCDC 2164721-2164726 contain the supplementary crystallographic data. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving. html (accessed on 6 April 2022), or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: (+44)-1223-336-033, or E-mail: deposit@ccdc.cam.ac.uk.

**Conflicts of Interest:** The authors declare no conflict of interest.
