Synthesis, Characterization, and Crystal Structure of α-[Ru(azpy)₂(NO₃)₂] (azpy = 2-(Phenylazo)pyridine) and the Products of Its Reactions with Guanine Derivatives

The synthesis and characterization of α-[Ru(azpy)₂(NO₃)₂], 1, are reported (azpy is 2-(phenylazo)pyridine; α indicates the isomer in which the coordinating pairs ONO₂, N(py), and N(azo) are cis, trans, and cis, respectively). The solid-state structure of 1 has been determined by X-ray crystallography. Crystal data:  orthorhombic a = 15.423(5) Å, b = 14.034(5) Å, c = 10.970(5) Å, V = 2374(2) ų, space group P2₁2₁2₁ (No. 19), Z = 4, D_calc = 1.655 g cm^-3. The structure refinement converged at R1 = 0.042 and wR2 = 0.118 for 3615 unique reflections and 337 parameters. The octahedral complex shows monodentate coordination of the two nitrate ligands. The Ru−N(azo) bond distances (2.014(4) and 1.960(4) Å), slightly shorter than the Ru−N(py) bonds (2.031(4) and 2.059(4) Å), agree well with the π-back-bonding ability of the azo groups. The binding of the DNA-model bases 9-ethylguanine (9egua) and guanosine (guo) to 1 has been studied and compared with previously obtained results for the binding of model bases to the bis(bipyridyl)ruthenium(II) complex. The ligands 9egua and guo appear to form monofunctional adducts, which have been isolated as α-[Ru(azpy)₂(9egua)Cl]PF₆, 2, α-[Ru(azpy)₂(9egua)(H₂O)](PF₆)₂, 3, α-[Ru(azpy)₂(guo)(H₂O)](PF₆)₂, 4, and α-[Ru(azpy)₂(guo)Cl]Cl, 5. The orientations of 9egua and guo in these complexes have been determined in detail with the use of 2D NOESY NMR spectroscopy. In 2 and 5, H8 is directly pointed toward the coordinated Cl, whereas, in 3 and 4, H8 is wedged between the pyridine and phenyl rings. The guanine derivatives in the azpy complexes can have more orientations than found for related cis-[Ru(bpy)₂Cl₂] species. This fluxionality is considered to be important in the binding of the α-bis(2-(phenylazo)pyridine)ruthenium(II) complex to DNA. In complex 1, ruthenium is the chiral center and in the binding to guanosine, two diastereoisomers each of adducts 4 and 5 have been clearly identified by NMR spectroscopy.