Electronic Coupling in a Bis-Cyclometalated Ruthenium Complex Bridged by 3,3′,5,5′-Tetrakis(1H-1,2,3-triazol-4-yl)biphenyl

The linear bis-cyclometalated Ru(II) complex [(ttpy)Ru(ttabp)Ru(ttpy)](2+) was prepared from the oxidative coupling of the monometallic complex [(ttpy)Ru(dtab)](+) in good yield, where ttpy is 4'-tolyl-2,2':6',2 ''-terpyridine, ttabp is 3,3',5,5'-tetrakis(1-butyl-1H-1,2,3-triazol-4-yl)biphenyl, and dtab is 1,3-bis(1-butyl-1H-1,2,3-triazol-4-yl)benzene. The electronic properties of these two complexes were studied and compared by electrochemical and spectroscopic methods and DFT/TDDFT calculations. The complex [(ttpy)Ru(ttabp)Ru(ttpy)](2+) showed two consecutive Ru-II/III redox waves at +0.34 and +0.51 V vs Ag/AgCl with a comproportionation constant K-c of 760. One-electron oxidation of [(ttpy)Ru(ttabp)Ru(ttpy)](2+) by electrochemical methods produced the mixed-valent complex [(ttpy)Ru(ttabp)Ru(ttpy)](3+), which exhibited a symmetric intervalence charge transfer band at 1800 nm (v(max) = 5550 cm(-1), epsilon(max) = 5100 M-1 cm(-1), Delta v(1/2) = 3580 cm(-1)). The energy and shape of this band are very similar in the different solvents studied (CH3CN, DMF, CH2Cl2). [(ttpy)Ru(ttabp)Ru(ttpy)](3+) was determined to be a class II/III borderline system with the electronic coupling parameter H-ab = 530 cm(-1). It displayed a metal-centered rhombic EPR signal at 77 K with an isotropic g factor of 2.109 and total g anisotropy of 0.256, which points to an appreciable spin distribution on the bridging ligand in addition to the metal centers.