Changing the electrochemical behavior of catechols by means of their conversion into the corresponding ruthenium(III)–dioxolene complexes

The rather complicated electrochemical behavior, in aqueous solution, exhibited by catechol (1,2-dihydroxybenzene) and its derivatives containing carboxylate and sulphonate groups, can be conveniently controlled by means of their conversion into the corresponding blue ruthenium(III)–edta complexes (edta=ethylenedinitrilotetraacetate ligand). In the free form, the catechol species undergo a primary electrochemical oxidation process in the range of 0.3–0.6 V versus SHE, yielding quinone products susceptible to a number of pH dependent, secondary chemical reactions. When coordinated to the ruthenium(III)–edta unit, their electrochemical and spectroelectrochemical behavior is dramatically changed, approaching that of metal complexes with non-innocent dioxolene ligands. In this case, the reduction of the ruthenium(III) metal ion proceeds reversibly above pH 9, in the −(0.5–0.6) V range and the oxidation process centered on the catecholate ligands becomes reversible, leading exclusively to the formation of the semiquinone species, with no evidence of complications from the secondary reactions.