Cyclic tetranuclear half-sandwich ruthenium(II) complexes with 4,7-phenanthroline and hydroxo bridges: Crystal structure, solution behaviour and binding to nucleosides

The reaction between [(η6-p-cymene)Ru(H2O)3]X2 and 4,7-phenanthroline (phen) leads to the formation of the rectangular tetranuclear complexes [(η6-p-cymene)4Ru4(μ-4,7-phen-N4,N7)2(μ-OH)4]X4 (X = NO3, 1a; SO3CF3, 1b) which have been structurally characterised by X-ray crystallography. 1H NMR spectroscopic studies suggest the presence of a partially dissociated dinuclear species of type [(η6-p-cymene)2Ru2(μ-4,7-phen-N4,N7)(solv)4]4+ in equilibrium with the tetranuclear cyclic species found in the solid state. The temperature effect for this equilibrium was studied by variable temperature 1H NMR experiments in D2O and MeOD. The results reveal that the proportion of the tetranuclear species increases with the polarity of the solvent which favour stacking interactions between the phenanthroline moieties. In addition, the reactivity of the tetranuclear species towards the nucleosides guanosine (Guo), cytidine (Cyt), 2′-deoxythymidine (Thy) and 2′-deoxyadenosine (dAdo) has been monitored by 1H NMR as a potential model for the interaction of the 1 species with the probable DNA target. The results reveal that the 1 systems are able to bind the nucleobases endocyclic nitrogen atoms of Guo Cyt, and dAdo.