Triphenylphosphine Thiolate Gold(I) Complexes with Redox-Active Schiff Bases: Synthesis, Electrochemical Properties, and Biological Activity

New gold(I) phosphine thiolate complexes [(Ph3P)Au(SLn)] I–V with Schiff bases LnSH containing redox-active catechol, phenol, or quinone methide moieties were synthesized and characterized. The molecular structure of compound I in the crystalline state was established by X-ray diffraction (CCDC no. 2237815). The electrochemical behavior of compounds I–V was studieв by cyclic voltammetry. The proposed electrooxidation mechanism of the complexes involves the Au–S bond cleavage, the disulfide formation, as well as the oxidation of the redox active group of the ligand. In the cathode region, complexes I–III tend to form relatively stable monoanionic species. The radical scavenging activity of complexes decreases in comparison to free ligands in the reactions with synthetic radicals and the CUPRAC test. Compounds I, II, IV, and V have no clear-cut effect on the promoted DNA damage; however, they show antioxidant action in the non-enzymatic lipid peroxidation of rat liver homogenate. Compounds I–V demonstrate a weak antibacterial activity against Staphylococcus aureus strains. The gold(I) complexes cytotoxicity was studied against A-549, MCF-7, and HTC-116 cancer cell lines using MTT assay. The test compounds are characterized by higher selectivity to certain types of cells than the sulfur-containing Schiff bases. The presence of quinone methide moiety in the ligand in case of V significantly increases the cytotoxicity against all of the cell lines.