Structural evidence for monodentate binding of guanine to the dirhodium(II,II) core in a manner akin to that of cisplatin

The reaction of Rh₂(OAc)₄(bpy) (bpy = 2,2′- bipyridine) with 9-ethylguanine (9-EtGH) proceeds with substitution of two acetate ligands to produce [Rh₂(OAc)₂(bpy) (9-EtGH)(H₂O)₂(CH₃SO₄)] [CH₃SO₄]·(H₂O) (1), which has been structurally characterized. In compound 1, the equatorial sites of one rhodium center are occupied by a chelating bpy molecule and 9-EtGH is equatorially coordinated via N(7) to the other rhodium center. The interaction of the 9-EtGH base with the dirhodium core is further stabilized by an intramolecular hydrogen bond between the purine O(6) and the equatorial water molecule. The eight non-equivalent bpy proton resonances, as well as that of the purine H(8) (which shifts upfield upon coordination as compared to free 9-EtGH, due to bpy ring current effects), were assigned by means of 2D NMR spectroscopy. The pH titration curve of the H(8) proton reveals pH-independent behavior and a pK_a value of 8.0 for N1-H deprotonation; both observations corroborate N(7) binding of the purine base to the dirhodium unit in solution. These findings indicate that, in the presence of a chelating agent that blocks one rhodium center, 9-EtGH binds to a single rhodium center in a monodentate fashion via N(7), instead of in a bridging fashion through the N(7) and O(6) sites as previously noted.