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 12+, 32+, and 52+ 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 (Me3tctpy), respectively]. Three other complexes with mixed terminal ligands (Mebip-Me3tctpy for 62+; Mebip-ttpy for 72+; ttpy-Me3tctpy for 82+) 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 52+) to 280 mV (for 62+). 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 ( Eop ≈ 5750 cm-1), with the electronic coupling parameter Vab of 1240, 1220, and 970 cm-1 for 13+, 33+, and 53+, respectively. The nonsymmetric series show significantly different Eop (7600, 5850, and 6650 cm-1 for 63+, 73+, and 83+, respectively) but Vab values that were comparable to that of 13+. DFT and TDDFT calculations have been performed to complement these experimental results. [ABSTRACT FROM AUTHOR]