Photolabile RuIIHalf-Sandwich Complexes Suitable for Developing “Caged” Compounds: Chemical Investigation and Unexpected Dinuclear Species with Bridging Diamine Ligands

Six half-sandwich RuII coordination compounds of the general formula [Ru([9]aneS3)(L–L)(L′)][PF6]n {2–7; [9]aneS3 = 1,4,7-trithiacyclononane, L–L = 2,2′-bipyridine (bpy), 1,2-diaminoethane (en), (±)-trans-1,2-diaminocyclohexane (dach), picolinate (pic); L′ = pyridine (py), 3-acetylpyridine (3-acpy), imidazole (im); n = 1 or 2, depending on the nature of L–L} were prepared and characterized. If irradiated with blue light (λ = 400–490 nm) in aqueous solution, they readily dissociate the monodentate L′ ligand to generate selectively the corresponding aqua species. The extent and rate of photoinduced ligand release depend primarily on the nature of the chelating ligand (bpy >> en ≈ dach > pic) and, to a minor extent, on that of the leaving ligand (py > im > 3-acpy). Photolabile compounds 2–5 showed no significant antiproliferative activity against the MDA-MB-231 human mammary carcinoma cell line, both in the dark and upon irradiation with blue light. The clean photodissociation process that characterizes this class of half-sandwich RuII compounds, and the substantial lack of toxicity of the photogenerated Ru aqua species, suggest that they might be suitable for the preparation of “caged” RuII compounds. In the frame of this work, two unexpected dinuclear compounds, namely, [{Ru([9]aneS3)(en)}2(μ-en)][CF3SO3]4 (9) and [{Ru([9]aneS3)}2(μ-dach)(μ-CH3O)2][CF3SO3]2·2CH3OH (10)―both containing rare examples of bridging en and dach ligands―were also isolated and structurally characterized.