Substituent Effect of the Bridging Ligand in the Trinuclear Ru Complexes on Photocatalytic Oxygenation of a Sulfide and Alkenes

To examine the effect of tuning the electron transfer ability between photosensitizers and reaction centers on the activity of visible-light-driven catalysts, a series of trinuclear Ru photocatalysts containing two Ru­(II) photosensitizers and a Ru­(II) reaction center bridged by a substituted bipyrimidine was prepared. The introduction of electron-donating or electron-withdrawing groups on the ligands was found to affect the photophysical and electrochemical properties of these catalysts. Density functional theory calculations of the mononuclear Ru complexes with nonsubstituted and substituted bipyrimidine ligands, which correspond to the reaction center of the trinuclear complexes, revealed how the electronic effect of the substituents affect the frontier orbital energies. For photocatalytic oxygenation of a sulfide and terminal alkenes, the catalyst with a 5,5′-dibromo-2,2′-bipyrimidine ligand showed higher activity than the catalyst with 5,5′-dimethyl-2,2′-bipyrimidine and nonsubstituted bipyrimidine ligands, as expected according to the electronic effects of these ligands. In contrast, the catalyst with the nonsubstituted bipyrimidine ligand showed the highest activity toward inner alkenes, which may be due to steric effects.