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 ΔG(0) , the energy of the IVCT band Eop , and the ground-state delocalization coefficient α(2) . 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.