A donor-acceptor-donor bridging ligand in a class III mixed-valence complex

The novel mononuclear and dinuclear complexes [Ru(trpy)(bpy)(apc)][PF6] and [{Ru(trpy)(bpy)}(2)(mu -adpc)][PF6](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 H-1 NMR, UV-vis, and cyclic voltammetry. Crystallography showed that the dinuclear Ru(II) complex crystallizes from diethyl ether/acetonitrile solution as [{Ru(trpy)(bpy)}(2)(mu -adpc)][PF6](2)(.)2(acetonitrile)(.)2(diethyl ether). Crystal structure data are as follows: crystal system triclinic, space group P (1) over bar, with a, b, and c = 12.480(2), 13.090(3) and 14.147(3) Angstrom, respectively, alpha, beta, and gamma = 79.792(3), 68.027(3), and 64.447(3)degrees, respectively, V = 1933.3(6) Angstrom (3), 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 Angstrom, is a class III system, having the comproportionation constant K-c = 1.3 x 10(13) and an intervalence band at 1920 nm (epsilon (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.