Ruthenium complexes with asymmetric hydroxy- and methoxy-substituted bipyridine ligands

Abstract The development of metal complexes with ligands capable of proton transfer can result in significant advances in Proton Coupled Electron Transfer (PCET) chemistry, specifically understanding how these reactions occur mechanistically in the excited state. The synthesis of three ruthenium complexes containing asymmetric bipyridine ligands: [Ru(bpy)2(4bpyOH)]2+, [Ru(bpy)2(4bpyOMe)]2+, and [Ru(bpy)2(44′bpy(OH)(OMe))]2+ (bpy = 2,2′-bipyridine; 4bpyOH = 4-hydroxy-2,2′-bipyridine; 4bpyOMe = 4-methoxy-2,2′-bipyridine; 44′bpy(OH)(OMe) = 4-hydroxy-4′-methoxy-2,2′-bipyridine) are reported. These complexes were studied using both experimental and computational methods. These studies indicate that the methoxy-substitution gives similar electron-donating properties to the hydroxy-substitution, meaning that the methoxy-substitution is a good control when examining electron transfer in the absence of proton transfer. Potential versus pH diagrams show that [Ru(bpy)2(4bpyOH)]2+ has a RuIII/II reduction potential of 0.97 V vs. Ag/AgCl in aqueous acidic solution that decreases to 0.76 V vs. Ag/AgCl when deprotonated. The [Ru(bpy)2(44′bpy(OH)(OMe))]2+, with an additional methoxy-substituent, has a RuIII/II reduction potential that is slightly lower with a potential of 0.92 V vs. Ag/AgCl in acidic aqueous solution that decreases to 0.74 V vs. Ag/AgCl when deprotonated. These complexes are comparable to other hydroxy- and methoxy-substituted polypyridyl ruthenium complexes reported previously, showing the additive effects of hydroxy- and methoxy-substitutions on the reduction potential of the RuIII/II wave and spectroscopic shifts that occur as the ligand scaffold is altered.