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)](PF 6 ) 2 ([ 1 ]-[ 3 ](PF 6 ) 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 ](PF 6 ) 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 ](PF 6 ) 2 selectively substitute their bis(thioether) ligands for water molecules in a two-step photoreaction, ultimately producing [Ru(bpy) 2 (H 2 O) 2 ] 2+ as the photoproduct. The relatively stable photochemical intermediate was identified as cis -[Ru(bpy) 2 (κ 1 -L)(H 2 O)] 2+ by mass spectrometry. Global fitting of the time evolution of the UV-vis absorption spectra of [ 1 ]-[ 3 ](PF 6 ) 2 was employed to derive the photosubstitution quantum yields (Φ 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.