Role of the Phosphine Ligands on the Stabilization of Monoadducts of the Model Nucleobases 1-Methylcytosine and 9-Methylguanine in Platinum(II) Complexes
journal contribution
posted on 2008-04-07, 00:00 authored by Diego Montagner, Ennio Zangrando, Bruno LongatoThe addition of 1-methylcytosine (1-MeCy) or 9-methylguanine (9-MeGu) to solutions of <i>cis</i>-(PPh<sub>3</sub>)<sub>2</sub>Pt(ONO<sub>2</sub>)<sub>2</sub> (<b>1a</b>), in a molar ratio of 1:1, affords the monoadducts <i>cis</i>-[(PPh<sub>3</sub>)<sub>2</sub>Pt(1-MeCy)(ONO<sub>2</sub>)]NO<sub>3</sub> (<b>2a</b>) and <i>cis</i>-[(PPh<sub>3</sub>)<sub>2</sub>Pt(9-MeGu)(ONO<sub>2</sub>)]NO<sub>3</sub> (<b>3a</b>) and only trace amounts of the bisadducts <i>cis</i>-[(PPh<sub>3</sub>)<sub>2</sub>Pt(1-MeCy)<sub>2</sub>](NO<sub>3</sub>)<sub>2</sub> (<b>4a</b>) and <i>cis</i>-[(PPh<sub>3</sub>)<sub>2</sub>Pt(9-MeGu)<sub>2</sub>](NO<sub>3</sub>)<sub>2</sub> (<b>5a</b>), respectively. The X-ray structural determination of <b>2a</b> and <b>3a</b> indicates a strong π−π stacking interaction between one of the PPh<sub>3</sub> phenyl groups and the pyrimydinic N3-platinated cytosine or the imidazole part of the N7-coordinated guanine base. The addition of a further equiv of nucleobase to the monoadducts forms quantitatively the bisadducts that have been isolated as pure compounds <b>4a</b> and <b>5a</b>. Under the same experimental conditions, the dinitrato analogue <i>cis</i>-[(PMePh<sub>2</sub>)<sub>2</sub>Pt(ONO<sub>2</sub>)<sub>2</sub>] (<b>1b</b>) forms the monoadducts <b>2b</b> and <b>3b</b> in equilibrium with a relatively high concentration (20–30%) of the bisadducts <i>cis</i>-[(PMePh<sub>2</sub>)<sub>2</sub>Pt(1-MeCy)<sub>2</sub>](NO<sub>3</sub>)<sub>2</sub> (<b>4b</b>) and <i>cis</i>-[(PMePh<sub>2</sub>)<sub>2</sub>Pt(9-MeGu)<sub>2</sub>](NO<sub>3</sub>)<sub>2</sub> (<b>5b</b>), which have been structurally characterized by single-crystal X-ray analysis. The characterization of the isolated complexes by multinuclear NMR spectroscopy is also described.
