Structural, electronic and acid/base properties of [Ru(bpy(OH)2)3]2+ (bpy(OH)2 = 4,4′-dihydroxy-2,2′-bipyridine)

Abstract

We have synthesized the complex [Ru(bpy(OH)₂)₃]²⁺ (bpy(OH)₂ = 4,4′-dihydroxy-2,2′-bipyridine) containing ligands that can be readily deprotonated. Both experimental and computational techniques were utilized to perform a thorough analysis of the structural and electronic properties of the complex in both the protonated and deprotonated state. The complex [Ru(bpy(OMe)₂)₃]²⁺ (bpy(OMe)₂ = 4,4′-dimethoxy-2,2′-bipyridine) was also synthesized and studied, because the bpy(OMe)₂ ligand has electron-donating properties like bpy(OH)₂, but does not contain deprotonatable groups. Cyclic voltammetry of [Ru(bpy(OH)₂)₃]²⁺ yields a reversible Ru^III/II wave that shifts 1.43 V to lower energy upon deprotonation of the complex. UV/Visible absorbance spectroscopy reveals several Metal-to-Ligand Charge Transfer (MLCT) transitions that shift to lower energy upon deprotonation of the complex. This observation is in contrast to mixed-ligand systems containing deprotonatable groups, such as [Ru(bpy)₂(bpy(OH)₂)]²⁺ (bpy = 2,2′-bipyridine) that demonstrate different types of electronic transitions assigned as mixed Metal-Ligand to Ligand Charge Transfer (MLLCT). The more symmetrical nature of the tris-bpy(OH)₂ complex most likely prevents the metal molecular orbitals from significantly mixing with the molecular orbitals of the deprotonated ligand. Luminescence studies were carried out on [Ru(bpy(OH)₂)₃]²⁺ and reveal a shift to lower energy and quenching of the excited state upon deprotonation in accordance with the energy gap law.