The coordination of the DNA model base 9‐methyladenine (9‐MeAde) to both complexes cis‐[Ru(bpy)2Cl2] and α‐[Ru(azpy)2(NO3)2] is reported. Structural characterisation using 2D NMR techniques and variable‐temperature NMR studies between 25 and −55 °C show that in the compounds α‐[Ru(azpy)2(9‐MeAde)](PF6)2 (1) and cis‐[Ru(bpy)2(9‐MeAde)](PF6)2 (2), 9‐MeAde is coordinated to the ruthenium ion in a chelating mode via its N7 and exocyclic N6 atoms. The NMR spectroscopic data unambiguously prove the occurrence of the rare imine tautomer of 9‐MeAde in these complexes, which is clearly stabilised by the bidentate coordination of 9‐MeAde in both complexes. Variable‐temperature NMR spectra and 2D COSY data show that the H2 resonance of 9‐MeAde in 1 and 2 appears as a doublet at low temperatures and shows a COSY cross peak to the NH1 resonance of 9‐MeAde, which confirms the protonation at the N1 site. This protonated N1 site, together with an observed pKa of ca. 6.5 of compound 1 is in agreement with the presence of the imine tautomeric form of 9‐MeAde. As the binding mode of 9‐MeAde to both cis‐[Ru(bpy)2] and α‐[Ru(azpy)2] moieties is the same, this binding mode does not explain the earlier observed difference in cytotoxicity between cis‐[Ru(bpy)2Cl2] and α‐[Ru(azpy)2Cl2].

Adenine bases have recently been found to coordinate to the cytotoxic alpha isomer of dichlorobis(2-phenylazopyridine)ruthenium(II) complexes in the rare imine form, stabilised by N6,N7-didentate coordination, which is now proven by X-ray structure determination of the compound -[Ru(azpy) 2 (3-MeAde ؊H )]PF 6 (azpy ‫؍‬ 2-phenyl-azopyridine, 3-MeAde ؊H ‫؍‬ deprotonated 3-methyladenine).Ruthenium complexes have attracted much attention in the search for new anticancer agents.1 The so-called α-isomer of the dichlorobis(2-phenylazopyridine)ruthenium() complexes, 2 α-[Ru(azpy) 2 Cl 2 ] (α corresponds to the isomer in which the coordination pairs Cl, N(py), and N(azo) are cis, trans and cis, respectively) shows remarkably high cytotoxicity against a series of human tumour cell lines.3 It is generally accepted that DNA might be the ultimate target for antitumour-active ruthenium complexes, 1 as in the case of antitumour-active platinum complexes.4 Therefore, the interaction of the cytotoxic α-[Ru(azpy) 2 Cl 2 ] with DNA-model bases has been studied. With 9-ethylguanine (9-EtGua) the monofunctional adduct α-[Ru(azpy) 2 (9-EtGua)(H 2 O)](PF 6 ) 2 is formed, fully characterised by NMR spectroscopy.5 The coordination of adenine bases to the α-[Ru(azpy) 2 ] moiety results in didentate coordination via the N6 and N7 atoms as concluded from NMR data 6 of α-[Ru(azpy) 2 (9-MeAde)](PF 6 ) 2 , and shown in the X-ray structure of α-[Ru(azpy) 2 (3-MeAde ϪH )](PF 6 ) 2 reported here. The single-crystal structure determination of α-[Ru(azpy) 2 -(3-MeAde ϪH )]PF 6 , 1, ‡ is the first crystallographic evidence of a mononuclear ruthenium() complex in which an adenine model base is present in the imine form and stabilised by chelating coordination via both its N7 and N6 atoms. There are some differences between 9-MeAde and 3-MeAde like the increased ligating power of 3-alkylated 6-aminopurines over the 9-alkylated 6-aminopurines 7 and tautomeric structures 8 (Fig. 1). Nevertheless, the coordination mode of 9-MeAde and Another example is a rhodium complex with a bridging 3-MeAde ligand (via the N7 and N6 sites). 10The molecular structure of 1 (Fig. 2) shows the 3-MeAde base coordinated in a didentate fashion via its N6 and N7 atoms. In fact the 3-MeAde ligand is found to be disordered over two positions, related by a two-fold rotation around the axis bisecting the angle N8-Ru-N28 (see Fig. S1, ESI). Since bond distances and valence angles of both components are linked, only geometric parameters for the major component are listed. The Ru-N6 distance of 2.215(10) Å and Ru-N7 distance of 2.033(11) Å are comparable to those in the polynuclear ruthenium structures [{Ru(ade)(η3ϩ (9-EtAde ϪH = deprotonated 9-ethyladenine) and [{Ru(5-AMP)(η 6 -p-MeC 6 -H 4 Pr)} 3 ]ؒ7.5H 2 O (5Ј-AMP = adenosine 5Ј-monophosphate) in which the adenine derivatives also coordinate in a N6,N7-didentate fashion.11 In contrast to 1, these complexes are polynuclear: the adenine derivatives function as bridging ligands via the N1 or N9 sites.The 3-MeAde ligand in 1 sho...