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

Abstract

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 (²J_NPca. 60 Hz).