Strong and weak coupling in mixed-valence complexes bridged by 4,4′-azo-di(phenylcyanamido)

The complexes [{Ru(tpy∗)(bpy)}2(μ-adpc)][PF6]2 where tpy∗ is 4,4′,4″-tri-(tert-butyl)-2,2′:6′,2″-terpyridine, bpy is 2,2′-bipyridine, and adpc2− is 4,4′-azo-diphenylcyanamide dianion and trans,trans-[{Ru(tpy∗)(pc)}2(μ-adpc)] where pc− is 2-pyrazine-carboxylato were prepared and characterized by cyclic voltammetry and spectroelectrochemical methods. Intervalence band properties and IR spectroelectrochemistry of the mixed-valence complexes [{Ru(tpy∗)(bpy)}2(μ-adpc)]3+ and trans,trans-[{Ru(tpy∗)(pc)}2(μ-adpc)]+ are consistent with delocalized and valence-trapped mixed-valence properties respectively. The reduction in mixed-valence coupling upon substituting a bipyridine ligand with 2-pyrazine carboxylato strongly suggests that hole-transfer superexchange is the dominant mechanism for metal–metal coupling in these complexes.