Multiple metal binding to the 9-methyladenine model nucleobase involving N1, N6, and N7: Discrete di- and trinuclear species with different combinations of monofunctional Pd II and Pt II entities

Abstract

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)Pd II, (dien)Pt II, (NH 3) 3Pt II, or (trpy)Pd II in different combinations have been prepared and/or studied in solution by NMR spectroscopy: [{Pd(dien)} 3(9-MeA-N1,N6,N7)]- Cl 3.5(PF 6) 1.53H 2O (1), [(dien)Pd(N1-9-MeA-N7)Pt(NH 3) 3](ClO 4) 49.33H 2O (2), [(dien)Pt-(N1-9-MeA-N7)Pt(NH 3) 3](ClO 4) 4H 2O (3), and [{(trpy)Pd} 2(N1,N6-9-MeA --N7)Pt(NH 3) 3]- (ClO 4) 53H 2O (4). A migration product of 3, [(dien)Pt(N6-9-MeA- N7)Pt(NH 3) 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 D 2O, 2 undergoes dissociation to [Pt(NH 3) 3(9-MeA-N7)] 2+ or forms the migration product [(dien)Pd(N6-9-MeA --N7)Pt(NH 3) 3] 3+ (2a), depending on pD, and 4 loses both (trpy)Pd II 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)Pd II units in 1 are not accompanied by changes in hybridization states of the N1 and N6 atoms. © 2012 American Chemical Society.