9-Methyladenine complexes of platinum(II) stabilized by trimethylphosphine: Use of N-15 nuclear magnetic resonance spectroscopy to assign the coordination site

The new 9-methyladenine (made) complexes cis-[Pt(PMe3)2(made)2][NO3] 2 and cis-[{Pt(PMe3)2(made - H)}2][NO3]2 have been prepared and characterized. They are formed by treating stoichiometric amounts of the nucleobase with an aqueous solution of cis-[Pt(PMe3)2(NO3)2] at autogenous and neutral pH, respectively. A detailed multinuclear (1H, 31P, 195Pt, 13C and 15N at natural abundance) NMR investigation in (CD3)2SO indicated that in the mononuclear complex the adenine ligands are N(1)-co-ordinated and that rotation around the metal-nitrogen bonds is slow on the NMR time-scale leading to the presence of two conformers with a relative abundance of ca. 2:1. In the dinuclear complex the NH2-deprotonated adenine acts as a bridging ligand through the N(1) and N(6) atoms. The sites of metal binding are clearly evidenced in the 15N NMR spectra by the remarkable shifts shown by the co-ordinated nitrogens (60 ppm upfield for the endocyclic atom and 25 ppm downfield for the exocyclic one, with respect to the unco-ordinated nucleobase) and their coupling with the 31P nucleus of the ligand in mutual trans position (2JNP ca. 60 Hz).