Cyclometalated Diruthenium Complexes Bridged by 3,3′,5,5′-Tetra(pyrid-2-yl)biphenyl: Tuning of Electronic Properties and Intervalence Charge Transfer by Terminal Ligand Effects

Six cyclometalated diruthenium complexes bridged by 3,3,5,5-tetra(pyrid-2-yl)biphenyl have been prepared, in which 1(2+), 3(2+), and 5(2+) are symmetric complexes with different terminal ligands [2,6-bis(N-methylbenzimidazolyl)pyridine (Mebip), 4-tolyl-2,2:6,2-terpyridine (ttpy), and trimethyl-4,4,4-tricarboxylate-2,2:6,2-terpyridine (Me(3)tctpy), respectively]. Three other complexes with mixed terminal ligands (Mebip-Me(3)tctpy for 6(2+); Mebip-ttpy for 7(2+); ttpy-Me(3)tctpy for 8(2+)) are structurally nonsymmetric. These complexes display two consecutive Ru(III/II) redox couples between 0.20 and 0.90 V vs. Ag/AgCl, with the potential splitting ranging from 150 (for 5(2+)) to 280 mV (for 6(2+)). The mixed-valent states of these complexes have been generated by stepwise electrolysis. The symmetric complexes show intervalence charge transfer (IVCT) transitions at similar energy (E-op approximate to 5750 cm(-1)), with the electronic coupling parameter V-ab of 1240, 1220, and 970 cm(-1) for 1(3+), 3(3+), and 5(3+), respectively. The nonsymmetric series show significantly different E-op (7600, 5850, and 6650 cm(-1) for 6(3+), 7(3+), and 8(3+), respectively) but V-ab values that were comparable to that of 1(3+). DFT and TDDFT calculations have been performed to complement these experimental results.