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

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 Λ diastereoisomers and their Δ enantiomers. We found that the synthesis of [1]-[3](PF₆)₂ is diastereoselective, yielding a racemic mixture of the Λ-(S)-eq-(S)-ax-OH_eq-[Ru]²⁺ and Δ-(R)-ax-(R)-eq-OH_eq-[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(κ¹-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 (Φ₄₄₃) 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.