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-methylcytosine (1-MeCy) or 9-methylguanine (9-MeGu) to solutions of cis-(PPh₃)₂Pt(ONO₂)₂ (1a), in a molar ratio of 1:1, affords the monoadducts cis-[(PPh₃)₂Pt(1-MeCy)(ONO₂)]NO₃ (2a) and cis-[(PPh₃)₂Pt(9-MeGu)(ONO₂)]NO₃ (3a) and only trace amounts of the bisadducts cis-[(PPh₃)₂Pt(1-MeCy)₂](NO₃)₂ (4a) and cis-[(PPh₃)₂Pt(9-MeGu)₂](NO₃)₂ (5a), respectively. The X-ray structural determination of 2a and 3a indicates a strong π−π 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)₂Pt(ONO₂)₂] (1b) forms the monoadducts 2b and 3b in equilibrium with a relatively high concentration (20–30%) of the bisadducts cis-[(PMePh₂)₂Pt(1-MeCy)₂](NO₃)₂ (4b) and cis-[(PMePh₂)₂Pt(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.