Role of the phosphine ligands on the stabilization of monoadducts of the model nucleobases 1-methylcytosine and 9-methylguanine in platinum(II) complexes

The addition of 1-methylpytosine (1-MeCy) or 9-methylguanine (9-MeGu) to solutions of cis-(Pph(3))(2)Pt(ONO2)(2) (1a), in a molar ratio of 1:1, affords the monoadducts cis-[(PPh3)(2)Pt(1-MeCy)(ONO2)]NO3 (2a) and cis-[(PPh3)(2)Pt(9-MeGu)(ONO2)]NO3 (3a) and only trace amounts of the bisadducts cis-[(PPh3)(2)Pt(1-MeCY)(2)](NO3)(2) (4a) and cis-[(PPh3)(2)Pt(9-MeGU)(2)](NO3)(2) (5a), respectively. The X-ray structural determination of 2a and 3a indicates a strong pi-pi stacking interaction between one of the PPh3 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 4a and 5a. Under the same experimental conditions, the dinitrato analogue cis-((PMePh2)(2)Pt(ONO2)(2)] (1b) forms the monoadducts 2b and 3b in equilibrium with a relatively high concentration (20-30%) of the bisadducts cis-[(PMePh2)(2)Pt(1-MeCY)(2)](NO3)(2) (4b) and cis-[(PMePh2)(2)Pt(9-MeGU)(2)](NO3)(2) (5b), which have been structurally characterized by single-crystal X-ray analysis. The characterization of the isolated complexes by multinuclear NMR spectroscopy is also described.