Synthesis, structure, photophysics, and biological activity of platinum (II) complexes

Abstract

The synthesis, structure, and photophysical properties of luminescent platinum (II) complexes with different coordination patterns, (C^C*N^N), (N^C*N), (N^N*C) and (N^N^C) are reported, where "C^N or N^N" denotes a bidentate coordination to the platinum to form a five-membered metallacycle and "C*N" denotes a bidentate coordination to the platinum to form a six-membered metallacycle. Sixteen cyclometalated platinum complexes have been synthesized with different coordination patterns, which include six complexes with tridentate N^C*N cyclometalating ligands (13, 14, 15, 16, 17, and 18), five complexes with N^N*C cyclometalating ligands (19a, 19b, 20a, 20b, and 24), three complexes with N^N^C cyclometalating ligands (21a, 21b, and 29) and two complexes with tetradentate C^C*N^N cyclometalating ligands (33 and 34). The structures of the platinum complexes 13, 15, 16, 18, 19a, 19b, 20a, 21a, 21b, 24, 29, 33, and 39 were determined by single crystal X-ray diffraction. In platinum complexes with five-six membered metallacycle (15, 16, 18, 19a, 19b, 20a, 24, and 39), the square geometry of the complexes is improved when compared to that of platinum complexes with five-five membered metallacycle as the biting angle is increased. The tetradentate coordination (C^C*N^N), square planar geometry of complex 33 are revealed from its X-ray crystal structure. The DFT calculations have been carried out on complexes 13, 14, 15, 16, 17, 18, 33, and 34 to examine the molecular orbital character of the complexes, which are used in interpreting the electronic spectra of the complexes. The photophysical properties of the platinum complexes were studied and a majority of the complexes were highly emissive in solution at room temperature. Complex 13 exhibited the highest quantum yield (65%) among all of the complexes. Self quenching was not observed in a majority of the platinum complexes at lower concentrations. The cytotoxicity (IC₅₀) of the complexes in three lung cancerous cell lines and one prostate cancer cell line were determined by MTT assay. The toxic platinum complexes induced the cell death by triggering apoptosis. The interactions of the platinum complexes with plasmid and calf thymus DNA were studied. Activation of caspase -7, PARP, and p53 were also observed in RV1 and HCC827 cell lines when treated with platinum complexes. Complexes 35, 37 and 38 were more potent than the clinically approved drug, cis-platin.