Multistate redox processes of structurally asymmetric diamines with a cyclometalated ruthenium bridge

Two structurally asymmetric cyclometalated ruthenium complexes 3(PF₆) and 4(PF₆), with an amine substituent on the cyclometalating and noncyclometalating ligand of each complex respectively, have been prepared and characterized. Three consecutive anodic redox couples are observed for these complexes in the potential window of 0 ~ +1.5 V vs Ag/AgCl, assigned to the stepwise oxidations of the ruthenium component and the two amine substituents. Upon stepwise oxidations with SbCl₅, these complexes exhibit three-step absorption spectral changes in the near-infrared (NIR) region. At the singly-oxidized state, single-line or axial-like EPR signals are detected for 3²⁺ and 4²⁺, respectively. The NIR absorptions of 3ⁿ⁺ and 4ⁿ⁺ at different charge states (n = 2 – 4) are rationalized by time-dependent density functional theory calculations.