Structural, electronic and acid/base properties of [Ru(bpy)(bpy(OH)₂)₂]²⁺ (bpy = 2,2′-bipyridine, bpy(OH)₂ = 4,4′-dihydroxy-2,2′-bipyridine)

The development of metal complexes with pH dependent ligands could lead to useful design principles in altering catalysis. We have synthesized the complex [Ru(bpy)(bpy(OH)₂)₂]²⁺ (bpy = 2,2′-bipyridine, bpy(OH)₂ = 4,4′-dihydroxy-2,2′-bipyridine) to better understand how the hydroxyl groups influence the electronic and structural properties of the complex as a function of protonation state. Both experimental and computational methods were utilized to study the complex in the protonated and deprotonated state. The most notable difference observed by X-ray diffraction studies, as well as by computational structural analysis, is the shortening of the modified ligand's C-O bond length upon deprotonation due to increasing double bond character by resonance. Cyclic voltammetry studies of the complex revealed a 0.96 V decrease in Ru^III/II potential upon deprotonation. Only one ligand redox wave is observed when deprotonated, assigned to the unmodified bpy ligand. The absorption spectrum of protonated [Ru(bpy)(bpy(OH)₂)₂]²⁺ is similar to that of [Ru(bpy)₃]²⁺ with typical metal to ligand charge transfer bands at approximately 460 nm. Upon deprotonation, the absorption spectrum shifts dramatically, exhibiting a 4504 cm^-1 red shift from λ_max = 468 nm to λ_max = 593 nm in acetonitrile. Computational studies indicate that the bpy(O^-)₂ ligand's orbitals heavily mix with the Ru d-orbitals, leading to mixed metal-ligand to ligand charge transfer transitions. Luminescence studies reveal absolute quenching of the excited state upon deprotonation in accordance with the energy gap law. These studies alongside results from the previously studied [Ru(bpy)₂(bpy(OH)₂)]²⁺ and [Ru(bpy(OH)₂)₃]²⁺ complexes, provide useful insight into the impact of increasing electron donation to the metal center.