Multiple Metal Binding to the 9‑Methyladenine Model Nucleobase Involving N1, N6, and N7: Discrete Di- and Trinuclear Species with Different Combinations of Monofunctional PdII and PtII Entities

Several di- and trinuclear metal complexes consisting of the model nucleobase 9-methyladenine (9-MeA) or its mono-deprotonated form (9-MeA–) and monofunctional (dien)PdII, (dien)PtII, (NH3)3PtII, or (trpy)PdII in different combinations have been prepared and/or studied in solution by NMR spectroscopy: [{Pd(dien)}3(9-MeA–-N1,N6,N7)]Cl3.5(PF6)1.5·3H2O (1), [(dien)Pd(N1-9-MeA-N7)Pt(NH3)3](ClO4)4·9.33H2O (2), [(dien)Pt(N1-9-MeA-N7)Pt(NH3)3](ClO4)4·H2O (3), and [{(trpy)Pd}2(N1,N6-9-MeA–-N7)Pt(NH3)3](ClO4)5·3H2O (4). A migration product of 3, [(dien)Pt(N6-9-MeA–-N7)Pt(NH3)3]3+ (3a), has been identified in solution. Unlike Pt-adenine bonds, Pd-adenine bonds are substantially labile, and consequently all Pd-containing complexes discussed here (1, 2, 4) exist in aqueous solution in equilibria of slowly interconverting species, which give rise to individual resonances in the 1H NMR spectra. For example, 1 exists in an equilibrium of five adenine-containing species when dissolved in D2O, 2 undergoes dissociation to [Pt(NH3)3(9-MeA-N7)]2+ or forms the migration product [(dien)Pd(N6-9-MeA–-N7)Pt(NH3)3]3+ (2a), depending on pD, and 4 loses both (trpy)PdII entities as the pD is increased. In no case is Pd binding to N3 of the adenine ring observed. A comparison of the solid-state structures of the two trinuclear complexes 1 and 4 reveals distinct differences between the Pd atoms bonded to N1 and N6 in that these are substantially out of the nucleobase plane in 1, by ca. 0.6 Å and −1.0 Å, respectively, whereas they are coplanar with the 9-MeA– plane in 4. These out-of-plane movements of the two (dien)PdII units in 1 are not accompanied by changes in hybridization states of the N1 and N6 atoms.