Metal Chelation-Assisted Amine-Amine Electronic Coupling through the 4,4′-Positions of 2,2′-Bipyridine

A redox-active diamine ligand, 4,4'-bis(di-p-anisylamino)-2,2'-bipyridine (NNbpy), has been prepared. Electrochemical and spectroscopic studies suggest that little electronic coupling is present between two amine groups in NNbpy. After chelation with Ru(bpy)(2) (bpy is 2,2'-bipyridine), the resulting complex displays two N center dot+/0 processes at +1.02 and +1.16 V versus Ag/AgCl. In the mixed-valent state, rich near-infrared absorptions have been observed, which are believed to consist of multiple metal-to-ligand charge transfer and intervalence charge transfer transitions in the low-energy region. These results suggest that the amineamine electronic coupling has been enhanced by chelation with Ru(bpy)2. In contrast, no efficient electronic coupling can be realized by chelation with Ir(ppy)(2) (ppy is 2'-phenylpyridine) or Re(CO)(3)Cl. A ruthenium ion-mediated electron transfer mechanism, instead of through-space coupling, has been proposed to explain this phenomenon. For the purpose of comparison, a monoamine-substituted bpy ligand and corresponding Ru(bpy)(2) complex have been synthesized and studied. In addition, EPR, DFT, and TDDFT studies have been performed to complement the experimental results.