Proton-induced tuning of metal-metal communication in rack-type dinuclear Ru complexes containing benzimidazolyl moieties

Ru complexes bearing a bis-tridentate benzimidazolyl ligand have been synthesized. The dinuclear ones act as a bibasic acid with pK_a1=4.36 and pK_a2=5.90. The protonated form of the dinuclear complex exhibited two one-electron oxidations at +0.91 and +1.02V versus the ferrocenium/ ferrocene (Fc/Fc⁺) couple (the potential difference (ΔE)=0.11V), but the di-deprotonated form showed two waves at +0.50 and +0.58V versus Fc/Fc⁺ (ΔE=0.08V). Since the potential difference between two waves reflects the strength of the metal-metal interaction, the deprotonation of the benzimidazole moieties in the complexes weakened the Ru-Ru communication. The degree of electronic coupling between two metal centers, estimated from the intervalence charge transfer (IVCT) band, was greater for the protonated form. DFT calculations for the protonated and deprotonated forms of the dinuclear complex suggest that the Ru^II-L(H₂) π* interaction plays a key role in the Ru-Ru interaction.