The novel mononuclear and dinuclear complexes [Ru(trpy)(bpy)(apc)][PF 6] and [{Ru(trpy)(bpy)} 2(μ -adpc)][PF 6] 2 (bpy = 2,2′-bipyridine, trpy = 2,2′:6′,2″-terpyridine, apc - = 4-azo(phenylcyanamido)benzene, and adpc 2- = 4,4′ -azodi(phenylcyanamido)) were synthesized and characterized by 1H NMR, UV-vis, and cyclic voltammetry. Crystallography showed that the dinuclear Ru(II) complex crystallizes from diethyl ether/acetonitrile solution as [{Ru(trpy)(bpy)} 2gt-adpc)][PF 6] 2·2(acetonitrile)·2(diethyl ether). Crystal structure data are as follows: crystal system triclinic, space group P1, with a, b, and c = 12.480(2), 13.090(3) and 14.147(3) Å, respectively, α, β, and γ = 79.792(3), 68.027(3), and 64.447(3)°, respectively, V = 1933.3(6) Å, and Z = 1. The structure was refined to a final R factor of 0.0421. The mixed-valence complex with metal ions, separated by a through-space distance of 19.5 Å, is a class III system, having the comproportionation constant K c. = 1.3 × 10 13 and an intervalence band at 1920 nm (ε max = 10 000 M -1 cm -1), in dimethylformamide solution. The results of this study strongly suggest that the bridging ligand adpc 2- can mediate metal-metal coupling through both hole-transfer and electron-transfer superexchange mechanisms.

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Journal Inorganic Chemistry
Mosher, P.J., Yap, G.P.A., & Crutchley, R. (2001). A donor - Acceptor - Donor bridging ligand in a class III mixed-valence complex. Inorganic Chemistry, 40(6), 1189–1195. doi:10.1021/ic001268u