Ni(II) Complexes of 2-Imine-8-Hydroxyquinolines: Characterization, Albumin Binding and Cytotoxicity

Nickel(II) offers an appealing base for metallodrug design as it is abundant, inexpensive and forms well-defined complexes with N,O-donor ligands. Therefore, five Ni(II) complexes containing two tridentate 2-imine-8-hydroxyquinoline Schiff base ligands, derived from morpholine, piperidine, imidazole or 2-methyl-1H-limidazole, were synthesized and characterized in solution and solid state via analytical and spectroscopic techniques. Single crystals were solved using X-ray diffraction for the complex derived from 4-(2-aminopropyl)morpholine (1). This Ni(II) complex exhibits a distorted octahedrally coordinated Ni(II) center bound to two monodeprotonated NNO-tridentate ligands, adopting a meridional coordination mode, with the two oxygen atoms arranged cis to each other. The stability of the complexes in aqueous media buffered at pH 7.4 was evaluated with UV–Vis spectroscopy and their binding to bovine serum albumin (BSA) was assessed with fluorescence titrations. All complexes show strong reversible binding to BSA (KSV ~105), which stabilizes the lipophilic complexes in aqueous media. The antiproliferative activity of the complexes was screened against colon cancer cell lines (Colo205 and Colo320). They exhibit moderate anticancer activity (IC50 = 15.1–92.5 μM), with structure–activity relationships favoring imidazole derivatives and maintaining activity against the doxorubicin-resistant cell line (Colo320) with resistance indices (1.3–3.1) comparable to or lower than doxorubicin. Overall, the complexes represent a platform to develop new anticancer Ni-based drugs.