Catecholamine complexes of ruthenium-edta and their redox chemistry

The electrochemical and spectroelectrochemical behavior of some neurotransmitters (dopamine and L-dopa) and their corresponding novel blue ruthenium(III)-edta complexes were investigated in aqueous solutions. At pH 7-10, the free ligand species can be electrochemically oxidized in the range of 0.1-0.6 V versus SHE, yielding primarily quinone products susceptible to pH-dependent, secondary intramolecular chemical reactions, which make the redox processes irreversible. When coordinated to the ruthenium(III)-edta complex, their electrochemical and spectroelectrochemical behavior is dramatically changed, approaching that of metal complexes with noninnocent dioxolene ligands. Reduction of the ruthenium(III) moiety proceeds reversibly above pH 9, in the region from -0.5 to -0.7 V. The oxidation process centered on the catecholate ligands becomes reversible and leads exclusively to the formation of the semiquinone species, with no evidence of complications from further reactions. These changes in the electrochemical behavior of the neurotransmitters make their cyclovoltammetric waves for reduction/oxidation more defined, favoring more precise quantitative analyses.