Tuning Electron Delocalization via Protonation in a Cyanidometal-Bridged Trinuclear Ruthenium Complex

A cyanidometal-bridged trinuclear ruthenium complex trans-[Cp*(dppe)RuCN-Ru(DMPZ)₂Cl₂-NCRu(dppe)Cp*] (1) (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl, dppe = 1,2-bis(diphenylphosphino)ethane, DMPZ = 2,6-dimethylpyrazine) and its one-, two-electrons-oxidized products 1⁺ and 1²⁺ were synthesized. Through different protonation conditions, one-electron-oxidized singly protonated (1-H)²⁺, doubly protonated (1-2H)²⁺, one-electron-oxidized doubly protonated (1-2H)³⁺, and two-electron-oxidized doubly protonated (1-2H)⁴⁺ were characterized. Significantly, the single-crystal X-ray diffraction structures of (1-H)²⁺, (1-2H)²⁺, and (1-2H)³⁺ were successfully obtained and well characterized. The supramolecular structure of (1-H)²⁺ suggests a possible proton pathway between the phenyl fragment of the dppe ligand on the terminal Ru center and the DMPZ ligand on the central Ru center which belong to an adjacent unit cell along the c axis. Upon protonation, the fully delocalized 1²⁺ is converted into the fully localized (1-2H)⁴⁺, while the electronic structure of 1⁺ is regulated by a stepwise protonation first to (1-H)²⁺, then to highly delocalized (1-2H)³⁺. The proton-induced intervalence charge transfer from 1²⁺ to (1-2H)⁴⁺ is investigated by electrochemistry, spectroelectrochemistry, and (TD)DFT calculation.