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(μ-adpc)][PF6]2
(bpy = 2,2‘-bipyridine, trpy = 2,2‘:6‘,2‘ ‘-terpyridine, apc- = 4-azo(phenylcyanamido)benzene, and adpc2- =
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)}2(μ-adpc)][PF6]2·2(acetonitrile)·2(diethyl ether). Crystal structure data are as follows:  crystal
system triclinic, space group P1̄, 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) Å3, 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 Kc = 1.3 × 1013 and an intervalence
band at 1920 nm (εmax = 10 000 M-1 cm-1), in dimethylformamide solution. The results of this study strongly
suggest that the bridging ligand adpc2- can mediate metal−metal coupling through both hole-transfer and electron-transfer superexchange mechanisms.