Redox properties of ruthenium(III/II)-edta complexes with o-phenylenediamine and o-benzoquinone diimine ligands

The reaction of [Ru-III (edta)(H2O)](-) with o-phenylenediamine (opda) in water, under aerobic conditions. affords the diamagnetic [Ru-II (edta) (bqdi)](2-) product (where edta stands for the ethylenediaminetetraacetate co-ligand, and bqdi represents the non-innocent o-benzoquinone alpha,alpha'-diimine ligand). In the current communication, the redox chemistry of this system in aqueous solution is described in details on the basis of electrochemical and spectroelectrochemical studies. The electrochemical behavior of "free" opda is rather complicated with further chemical reactions following the irreversible two-proton/two-electron oxidation (opda --> bqdi + 2e(-) + 2H(+)), whereas its complex is electrochemically well-behaved with two chemically reversible redox processes: the monoelectronic couple associated with the metal ion (Ru-III/Ru-II) and another bielectronic step centered on the coordinated ligand (bqdi/opda). The set of UV-Vis electronic spectra were obtained by electrolytical generation. ill situ, of all the redox species accessible in the CV working conditions (i.e., the starting [Ru-II(edta) (bqdi)](2-) the fully oxidized [Ru-III(edta)(bqdi)](-), and the fully reduced [Rull(edta)(opda)](2-) species), which are stable and totally interconvertible. The electrochemistry and absorption spectroscopy of these complexes in water were found to be comparable with the tetraammine counterparts. A remarkable difference in redox behavior between the diimine- and the analogous dioxolene-complexes was also revealed by comparison of the system reported herein with the one derived from catechol, and rationalized in terms of the quite efficient pi-accepting electronic nature of the bqdi ligand. (C) 2002 Published by Elsevier Science B.V.