Diastereoselective Synthesis and Two-Step Photocleavage of Ruthenium Polypyridyl Complexes Bearing a Bis(thioether) Ligand

Thioethers are good ligands for photoactivatable ruthenium(II) polypyridyl complexes, as they form thermally stable complexes that are prone to ligand photosubstitution. Here, we introduce a novel symmetric chelating bis(thioether) ligand scaffold, based on 1,3-bis(methylthio)-2-propanol (4) and report the synthesis and stereochemical characterization of the series of novel ruthenium(II) polypyridyl complexes [Ru(bpy)(2)(L)](PF6)(2 )([1]-[3]-(PF6)(2)), where L is ligand 4, its methyl ether, 1,3-bis(methylthio)-2-methoxypropane (5), or its carboxymethyl ether, 1,3-bis(methylthio)-2-(carboxymethoxy)propane (6). Coordination of ligands 4-6 to the bis(bipyridine)ruthenium center gives rise to 16 possible isomers, consisting of 8 possible Lambda diastereoisomers and their Delta enantiomers. We found that the synthesis of [1]-[3](PF6)(2) is diastereoselective, yielding a racemic mixture of the Lambda-(S)-eq-(S)-ax-OHeq-[Ru](2+) and Delta-(R)-ax-(R)-eq-OHeq-[Ru](2+) isomers. Upon irradiation with blue light in water, [1]-[3](PF6)(2) selectively substitute their bis(thioether) ligands for water molecules in a two-step photoreaction, ultimately producing [Ru(bpy)(2)(H2O)(2)](2+) as the photoproduct. The relatively stable photochemical intermediate was identified as cis-[Ru(bpy)(2) (kappa(1)-L)(H2O)](2+) by mass spectrometry. Global fitting of the time evolution of the UV-vis absorption spectra of [1]-[3](PF6)(2) was employed to derive the photosubstitution quantum yields (Phi(443)) for each of the two photochemical reaction steps separately, revealing very high quantum yields of 0.16-0.25 for the first step and lower values (0.0055-0.0093) for the second step of the photoreaction. The selective and efficient photochemical reaction makes the photocleavable bis(thioether) ligand scaffold reported here a promising candidate for use in e.g. ruthenium-based photo-activated chemotherapy.