Cyclometalated Platinum(II) Complexes Comprising 2‐(Diphenylphosphino)pyridine and Various Thiolate Ligands: Synthesis, Spectroscopic Characterization, and Biological Activity

The known cyclometalated platinum(II) complex [Pt(ppy)(DMSO)(Cl)] (A) in which ppy = 2‐phenylpyridinate, was treated with 2‐(diphenylphosphino)pyridine (PPh₂py, PN) (1 equiv.) and readily afforded complex [Pt(ppy)(PPh₂py‐κ1P)(Cl)] (1). The transphobia effect (T) and single‐crystal X‐ray diffraction crystallography confirmed that the phosphine ligand in 1 is located trans to the nitrogen atom of the cyclometalating fragment (ppy). Compound 1 was treated with various thiolate reagents (1 equiv.) at room temperature, leading to the displacement of the chloride ligand in 1 by thiolate ligands through a salt metathesis reaction. The reaction led to the formation of a series of monomeric complexes with the general formula [Pt(ppy)(PPh₂py‐κ1P)(SR)] (2a–2d), where SR = deprotonated form of pyridine‐2‐thiol (HSpy, 2a), pyrimidine‐2‐thiol (HSpyN, 2b), thiophenol (HSPh, 2c), and 2‐thiazoline‐2‐thiol (HSt, 2d). All complexes have been characterized by NMR spectroscopy. The biological activities of the complexes were evaluated against three human cancer cell lines, including A549 (human lung cancer), SKOV3 (human ovarian cancer), and MCF7 (human breast cancer), by means of the MTT assay [MTT = 3‐(4,5‐dimethylthiazol‐yl)‐2,5‐diphenyltetrazolium bromide]. Compounds 2a and 2b presented effective, potent cytotoxic activity regarding the cell lines. Electrophoretic mobility shift assays on plasmid DNA and molecular modeling investigations were also performed to determine the specific binding mode or the binding orientation of the complexes to DNA.