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

摘要

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-OHeq-[>Ru]²⁺ and Δ-(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(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 quantumyields (Φ₄₄₃) for each of the two photochemical reactionsteps separately, revealing very high quantum yields of 0.16–0.25for the first step and lower values (0.0055–0.0093) for thesecond step of the photoreaction. The selective and efficient photochemicalreaction makes the photocleavable bis(thioether) ligand scaffold reportedhere a promising candidate for use in e.g. ruthenium-based photo-activatedchemotherapy.