6,6″-Azobis(2,2′-bipyridine) and Its Dinuclear Ruthenium Complex: A Comparative Study with Positional Isomers

Abstract

Abstract The azobis(bipyridine) ligand 6,6″-azobis(2,2′-bipyridine) (6-azobpy) was prepared as a new member of the family of azopolypyridine derivatives. This compound undergoes reversible trans/cis photoisomerization. Reaction of 6-azobpy and [Ru(bpy)2Cl2] afforded a dinuclear complex [{Ru(bpy)2}2(μ-6-azobpy)]4+. An analogue with deuterated bpy fragments was also prepared to help clarify the 1H NMR signals in the part of the 6-azobpy ligand. The analysis of 1H NMR spectra clearly identified the structure of the complex, in which the Ru2+ ions are chelated by the bpy parts of the bridging 6-azobpy ligand. Electronic absorption and electrochemical properties are characterized by the presence of a low-lying π* level of the bridging ligand, manifested by a metal-to-ligand charge-transfer band extending down to 800 nm and by two consecutive one-electron reduction waves at relatively less negative potentials in cyclic voltammetry. These properties were compared with those of previously reported positional isomers with the help of density functional theory (DFT), time-dependent DFT, and ZINDO calculations. The analysis of the redox couple of Ru3+/2+ showed that the redox potentials differ slightly between the diastereomers and that there is an intermetallic electronic interaction.