Cu(II) and Zn(II) Complexes of New 8-Hydroxyquinoline Schiff Bases: Investigating Their Structure, Solution Speciation, and Anticancer Potential

[Image: see text] We report the synthesis and characterization of three novel Schiff bases (L1–L3) derived from the condensation of 2-carbaldehyde-8-hydroxyquinoline with amines containing morpholine or piperidine moieties. These were reacted with CuCl(2) and ZnCl(2) yielding six new coordination compounds, with the general formula ML(2), where M = Cu(II) or Zn(II) and L = L1–L3, which were all characterized by analytical, spectroscopic (Fourier transform infrared (FTIR), UV–visible absorption, nuclear magnetic resonance (NMR), or electron paramagnetic resonance (EPR)), and mass spectrometric techniques, as well as by single-crystal X-ray diffraction. In the solid state, two Cu(II) complexes, with L1 and L2, are obtained as dinuclear compounds, with relatively short Cu–Cu distances (3.146 and 3.171 Å for Cu(2)(L1)(4) and Cu(2)(L2)(4), respectively). The free ligands show moderate lipophilicity, while their complexes are more lipophilic. The pK(a) values of L1–L3 and formation constants of the complex (for ML and ML(2)) species were determined by spectrophotometric titrations, with the Cu(II) complexes showing higher stability than the Zn(II) complexes. EPR indicated the presence of several species in solution as pH varied and binding modes were proposed. The binding of the complexes to bovine serum albumin (BSA) was evaluated by fluorescence and circular dichroism (CD) spectroscopies. All complexes bind BSA, and as demonstrated by CD, the process takes several hours to reach equilibrium. The antiproliferative activity was evaluated in malignant melanoma cells (A375) and in noncancerous keratinocytes (HaCaT). All complexes display significant cytotoxicity (IC(50) < 10 μM) but modest selectivity. The complexes show higher activity than the free ligands, the Cu(II) complexes being more active than the Zn(II) complexes, and approximately twice more cytotoxic than cisplatin. A Guava ViaCount assay corroborated the antiproliferative activity.