Synthesis, Characterization, and Photophysical Properties of Three Pt(II) Complexes Bearing Fluorescent Analogues of the Di-(2-pyridyl)methane Ligand

Abstract

Three new ligands of the general formula [RNHCH(py)2] (py = pyridine; R = tosyl, Ts-dpm; R = dansyl, Ds-dpm; R = 7-nitro-1,2,3-benzoxadiazole, NBD-dpm) have been synthesized and characterized. Reactions of these ligands with cis-[Pt(DMSO)2Cl2] (DMSO = dimethyl sulfoxide) in methanol affords [Pt(Ts-dpm)Cl2] (1), [Pt(Ds-dpm)Cl2] (2), and [Pt(NBD-dpm)Cl2] (3). The crystal structures of these complexes reveal bidentate coordination of the ligands to the Pt center with nonplanar chelate rings. Because of inequivalent substituents on the methine carbon atom of the ligands, distinct exo and endo isomers exist in the three complexes. X-ray analyses indicate that 1 crystallizes in the endo conformation, 2 in the exo conformation, and 3 as a mixture of the two conformers. The 1H NMR and 195Pt NMR spectra of the complexes display two sets of independent signals corresponding to the chemically inequivalent exo and endo conformers. The exo conformer was determined by 2D NMR spectroscopy to be thermodynamically favored for all three complexes. Density functional theory (DFT), time-dependent DFT, and atoms in molecules calculations were carried out for both conformers of 3 to investigate differences in their electronic structures and to explore intramolecular interactions. In the presence of dioxygen, 1 thermally decomposes at 60 °C to form several unidentified products. Compound 2 is thermally stable even in the presence of dioxygen and water but upon light exposure decomposes to form a new platinum(II) species with a 195Pt NMR shift of −2177 ppm. Compound 3 reacts both thermally and photochemically in the presence of dioxygen and trace amounts of water to form both 4-amino-7-nitro-2,1,3-benzoxadiazole and [Pt(dpk)Cl2] (dpk = di-2-pyridyl ketone). Oxidation of 1 and 3 with H2O2 in acetic acid affords a mixture of compounds, two of which contain dpm ligands bound in a tridentate manner to platinum.