Electronic Coupling in a Bis-Cyclometalated Ruthenium Complex Bridged by 3,3′,5,5′-Tetrakis(1<i>H</i>-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-1<i>H</i>-1,2,3-triazol-4-yl)­biphenyl, and dtab is 1,3-bis­(1-butyl-1<i>H</i>-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 <i>K</i>_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 (ν_max = 5550 cm^–1, ε_max = 5100 M^–1 cm^–1, Δν_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 <i>H</i>_ab = 530 cm^–1. It displayed a metal-centered rhombic EPR signal at 77 K with an isotropic <i>g</i> factor of 2.109 and total <i>g</i> anisotropy of 0.256, which points to an appreciable spin distribution on the bridging ligand in addition to the metal centers