Extensive redox series in dinuclear and dendritic Ru(II) complexes

The electrochemical behavior of a dinuclear and a hexanuclear dendritic ruthenium(II) bipyridine complexes with 2,3-bis(2-pyridyl)pyrazine (2,3-dpp) as bridging ligands has been investigated in highly purified liquid SO2 or N,N-dimethylformamide (DMF) solutions. The compounds have general formula [Ru-n(bpy)(n+2)(2,3-dpp)(n-1)](2n+), where n = 2 or 6; bpy is 2,2 ' -bipyridine. The wide anodic and cathodic potential windows explored in liquid SO2 and DMF, at low temperature, (up to ca. 4.3 and - 3.1 V vs. SCE, respectively) have allowed the observation of several metal-centered oxidations and ligand-centered oxidations and reductions, leading to the most extensive redox series so far reported, comprising up to 26 reversible ligand-centered reductions and 14 reversible metal- or ligand-centered oxidations, for a total of 40 redox processes in the hexanuclear complex. The redox standard potentials for overlapping processes in multielectron waves have been obtained from the analysis of the voltammetric curves and their digital simulation, and the localization of all the redox processes and an evaluation of the mutual interactions between the redox centers have been obtained. These results allowed to clarify the important role played by bridging ligands in mediating the interactions between the equivalent redox sites. Furthermore, the localization of the redox sites and the determination of relative standard potentials allowed the mapping of MLCT excited states within the dendrimeric structure, through the correlation between the absorption spectroscopic data and the relevant voltammetric data. (C) 2001 Elsevier Science Ltd. All rights reserved.