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 () and report the synthesis and stereochemical characterization of the series of novel ruthenium(II) polypyridyl complexes Ru(bpy)(L) ([]-), where L is ligand , its methyl ether, 1,3-bis(methylthio)-2-methoxypropane (), or its carboxymethyl ether, 1,3-bis(methylthio)-2-(carboxymethoxy)propane (). Coordination of ligands - to the bis(bipyridine)ruthenium center gives rise to 16 possible isomers, consisting of 8 possible Λ diastereoisomers and their Δ enantiomers. We found that the synthesis of []- is diastereoselective, yielding a racemic mixture of the Λ-()-eq-()-ax-OH-[] and Δ-()-ax-()-eq-OH-[] isomers. Upon irradiation with blue light in water, []- selectively substitute their bis(thioether) ligands for water molecules in a two-step photoreaction, ultimately producing [Ru(bpy)(HO)] as the photoproduct. The relatively stable photochemical intermediate was identified as -[Ru(bpy)(κ-L)(HO)] by mass spectrometry. Global fitting of the time evolution of the UV-vis absorption spectra of []- was employed to derive the photosubstitution quantum yields (Φ) 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.