Supramolecular electrochemistry: Redox series of cyano-bridged polynuclear bipyridine ruthenium(II) complexes

Abstract

The electrochemical behaviour of the binuclear complex NC-Ru II(bpy) 2-CN-Ru II(bpy) 2-CN + and of the trinuclear complex NC-Ru II(bpy) 2-CN-Ru II(bpy) 2-NC-Ru II(bpy) 2-CN 2+, where bpy = 2,2'-bipyridine, has been studied in acetonitrile and in N, N-dimethylformamide. Because of the high number of oxidizable and reducible centres, these polynuclear complexes give rise to very extended redox series, consisting of two or three metal-localized one-electron reversible oxidation steps and seven or ten ligand-localized one-electron reversible reduction steps for the binuclear and trinuclear complex. Such behaviour, for the reduction processes, is observed only at low temperature (ca. −50°C) and at a sufficiently high sweep rate (50 V/s in the case of the binuclear complex). On the other hand, at lower sweep rates and higher temperatures, the injection of the fifth, respectively seventh electron into the molecule causes its decomposition. Comparison of the behaviour of the polynuclear complexes and Ru(bpy) 2(CN) 2 and Ru(bpy) 2+ 3 has allowed a satisfactory localization of the redox centres in the molecules. The study of the characteristics of the redox series has led to the inference that rather strong metal-metal interactions via CN bridges are operative, while remote ligand interactions (via the M-CN-M chain) are weaker. Furthermore, the behaviour of these complexes, which can be considered as consisting of identical building blocks connected by asymmetric bridges, appears to be affected remarkably by the asymmetry of the bridge. In fact, the more easily oxidizable Ru(II) centres are those containing the largest number of N-bonded cyanides, and the bpy molecules connected to such Ru(II) centres are conversely the least easily reduced.