Electrochemistry of multicomponent systems, redox series comprising up to 26 reversible reduction processes in polynuclear ruthenium(II) bipyridine-type complexes

The electrochemical behavior of a family of polynuclear ruthenium(II) bipyridine-type complexes with 2,3-bis(2-pyridyl)pyrazine (2,3-dpp) and 2,5-bis(2-pyridyl)pyrazine (2,5-dpp) as bridging ligands has been investigated in highly purified N,N-dimethylformamide solution. The compounds studied contain two, three, four, and six metal centers and have general formula [Ru-n(bpy)(n+2)(2,X-dpp)(n-l)](2n+), where n = 2, 3, 4, or 6, X = 3 or 5, and bpy is 2,2'-bipyridine, The wide cathodic potential window explored (up to ca. -3.1 V vs SCE) has allowed us to observe the most extensive ligand-centered odor series so far reported, comprising up to 26 reversible reduction processes for 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. The localization of each redox process and the mutual interactions of the redox centers have been elucidated through the analysis and comparison of the redox series of the various compounds. For the dinuclear species, the assignment of the redox sites has been confirmed by semiempirical molecular orbital calculations (ZINDO) and spectroelectrochemical experiments. Calculations also substantiate the important role played by bridging ligands in mediating the interactions between equivalent redox sites. Finally, it has been shown that the size of the supporting electrolyte cation has an influence on the processes occurring at the extreme cathodic region.