Polynuclear heteroleptic ruthenium(II) photoredox catalysts: Evaluation in blue-light-mediated, regioselective thiol-ene reactions

Polynuclear heteroleptic ruthenium(II) photosensitisers combining either imine or amine-functionalised bipyridyl ligands were synthesised (via Schiff-base condensation/reductive amination reactions), characterised and investigated for their photoreactivity in the hydrothiolation reaction. Furthermore, electrochemical, electronic absorption and emission studies of the complexes were conducted. All the ligand-modified, heteroleptic complexes show red-shifted emission spectra (614–633 nm) relative to the canonical [Ru(bpy)3](PF6)2 complex (609 nm), attributed to the transition from the triplet MLCT excited state (3MLCT) to the ground state. The complexes were evaluated as visible-light photoredox catalysts in the radical hydrothiolation reaction of olefins (thiol-ene coupling) to afford thioethers. Control reactions performed in the absence of the photocatalyst resulted in either significantly lower yields (6%) or no product formation, demonstrating the role of the complexes as photoredox catalysts. The reactions carried out using trinuclear complexes (Ered (Ru2+*/+) = +0.300 V vs Ag/Ag+) resulted in increased yields in comparison with their respective mononuclear congeners and greater than those reported for [Ru(bpy)3](PF6)2, demonstrating the benefits of using polynuclear photocatalysts for photoinitiated redox catalysis reactions.