Hydrogen bonding in diruthenium(II,III) tetraacetate complexes with biologically relevant axial ligands

Three diadduct complexes of the mixed-valent form of diruthenium tetraacetate, [Ru2(μ-O2CCH3)4L2](PF6), where L are the biologically relevant ligands imidazole, 1, 7-azaindole, 2, and caffeine, 3, were synthesized and characterized by elemental analysis, IR and UV–Vis spectroscopy and X-ray crystallography. In order to further elucidate the potential interactions of these dimers with DNA, the nature of the ligand coordination and the secondary inter- and intramolecular hydrogen-bonding interactions in all three complexes were assessed. Complex 1 · CH2Cl2 shows, exclusively, intermolecular interactions with the PF6- counterion whereas complexes 2 · ClCH2CH2Cl and 3 · OC(CH3)2 · H2O, in addition to extensive intermolecular interactions, show intramolecular hydrogen bonding from the axial ligand to the bridging acetate oxygens, locking the ligand mean planes in place between the bridging acetate mean planes. In addition, all three complexes display π–π stacking of axial ligand rings on adjacent diadduct units.

Graphical abstractThree diruthenium(II,III) tetraacetate complexes of the form [Ru2(μ-O2CCH3)4L2](PF6), with the biologically relevant axial ligands (L) = imidazole, 7-azaindole (cation shown) and caffeine, were synthesized and structurally characterized. Each complex shows ligand binding through an unprotonated ring nitrogen as well as unique hydrogen bonding features that may have implications in how the core structure binds to DNA and elicits cytotoxicity.Figure optionsDownload full-size imageDownload as PowerPoint slide