Tuning the Electronic Coupling in Cyclometalated Diruthenium Complexes through Substituent Effects: A Correlation between the Experimental and Calculated Results

A common bridging ligand, 3,3′,5,5′‐tetrakis(N‐methylbenzimidazol‐2‐yl)biphenyl, and four terpyridine terminal ligands with various substituents (amine, tolyl, nitro, and ester groups) have been used to synthesize ten cyclometalated diruthenium complexes 1 ²⁺– 10 ²⁺. Among them, compounds 1 ²⁺– 6 ²⁺ are redox nonsymmetric, and others are symmetric. These complexes show two Ru^III/II processes and an intervalence charge transfer (IVCT) transition in the one‐electron oxidized state. The potential separation (ΔE) of 1 ²⁺– 10 ²⁺ has been correlated to the energy difference ΔG⁰, the energy of the IVCT band E_op, and the ground‐state delocalization coefficient α². Time‐dependent (TD)DFT calculations suggest that the absorptions in the visible region of 1 ²⁺– 6 ²⁺ are mainly associated with the metal‐to‐ligand charge‐transfer transitions from both ruthenium ions and to both terminal ligands and the bridging ligand. However, the energies of these transitions vary significantly. DFT calculations have been performed on 1 ²⁺– 6 ²⁺ and 1 ³⁺– 6 ³⁺ to give information on the electronic structures and spin populations of the mixed‐valent compounds. The TDDFT‐predicted IVCT excitations reproduce well the experimental trends in transition energies. In addition, three monoruthenium complexes have been synthesized for a comparison study.