Platinum−Nitrogen Bond Rearrangements in Isomeric cis‐PtII(NH3)2‐bis(9‐methyladenine) Complexes under Alkaline Conditions

Abstract

AbstractThe migration of coordinated PtII in the adenine ring was studied in basic aqueous solutions for the isomeric bis(9‐methyladenine) complexes: cis‐[Pt(NH3)2(9‐made‐N7)2]2+ (1) and cis‐[Pt(NH3)2(9‐made‐N1)(9‐made‐N7)]2+ (2). In both compounds, coordinated PtII migrates from the endocyclic nitrogen N(7) or N(1), respectively, to the exocyclic amino group, upon displacement of an NH2 proton, to give the same first‐generation product, viz. cis‐[Pt(NH3)2(9‐made‐N6)(9‐made‐N7)]2+ (3). Although both reactions undergo an intramolecular reaction, the N(1)⇄N(6) step is faster and far more efficient than the N(7)⇄N(6) step. This may be attributed to the more favorable electronic effects and to the spatial positioning of the N(1)‐bound PtII in comparison to the platinum coordinated to the N(7) site. However, the contribution of a Dimroth‐type rearrangement to the overall N(1)⇄N(6) conversion cannot be completely ruled out. Attempts to obtain second migration in the N(6),N(7)‐bound species to give the N(6),N(6) species were unsuccessful. Rather, prolonged treatment of the N(6),N(7)‐bound complex in strongly basic solution resulted in the deamination of the N(7)‐bound 9‐methyladenine yielding a mixed ligand complex consisting of N(6)‐bound 9‐methyladenine and N(7)‐bound 9‐methylhypoxanthine, i.e. cis‐[Pt(NH3)2(9‐made‐N6)(9‐mhyp‐N7)]2+ (4). To the best of our knowledge, this is the first example of such a deamination reaction in PtII complexes. Both reaction products, 3 and 4, were characterized by 1H and 195Pt NMR spectroscopy and by X‐ray crystallography of the diperchlorate. In dicationic 3 and 4, the N(6)‐bound 9‐made exists in the rare imino tautomer with the N(1) site bearing a proton. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)