Copper(ii) complexes based on 2-ferrocenyl-1,10-phenanthroline: structure, redox properties, cytotoxicity and apoptosis

Abstract

Two new copper(II) coordination compounds, [CuL(NO₃)₂] (1) and [CuLCl₂]·0.7H₂O (2), were synthesized by the reaction of CuX₂ salts (X = NO₃⁻ or Cl⁻) with 2-ferrocenyl-1,10-phenanthroline (L). The complexes were characterized by elemental, thermogravimetric and high-resolution electrospray mass spectrometric analysis; IR and EPR spectroscopy; cyclic voltammetry; and powder and single-crystal X-ray diffraction analysis. The coordination environment of copper(II) in complex 1 is 4 + 2 according to single-crystal X-ray diffraction analysis. The stability of the ligand and complexes in different solvent mixtures (water, phosphate-buffered saline, and cell medium DMEM/F12) was studied using UV-vis spectroscopy and conductometry. Complex 1 was shown to dissociate into [CuL(solvent)_n]²⁺ in DMSO and ethanol solutions, while complex 2 was partially dissociated. The observed time-dependent changes in UV-vis spectra likely reflected the oxidation of Fe²⁺ to Fe³⁺. The cyclic voltammogram (CV) of L and 1 in DMSO revealed reversible oxidation peaks at 0.61 V and 0.64 V, respectively (vs. Ag/AgCl), assigned to the Fe(II)/Fe(III) couple whereas the CV of the solution of 2 showed two oxidation peaks. In vitro cytotoxic activity was evaluated against three tumor cell lines (A549, Hep2, and HepG2) and non-tumor lung fibroblasts (MRC5) using 2D and 3D cell models. Compared to the ligand L, both complexes exhibited more pronounced dose-dependent cytotoxicity against all tested cell lines, with LC₅₀ values in the low micromolar concentration range. Despite containing the redox-active ligand L, neither complex induced significant reactive oxygen species (ROS) production after 1-h incubation with Hep2 cells at concentrations of 1.2–10 μM. However, apoptosis was detected in this cell line, suggesting a possible mechanism of action for the complexes. The effect of complexes on the mitochondrial potential of A549 cells was studied using TMRM.