Redox potentials vanadium complexes

This chapter primarily covers studies of the electrochemistry of vanadium, which have appeared in the literature during the period 1985-2005. The material is organized on the basis of the oxidation state of the starting complex and is not meant to be an exhaustive review. Cyclic voltammetry (CV) studies outnumber other methods of electrochemistry for the study of vanadium complexes, and redox potentials will be reported as referenced to the Cp2Pe /+ couple in the appropriate solvent except as noted [1]. 

Tab. 1 Redox potentials of sonne bis(arene)vanadium complexes in DME versus SCE...

From comparison of antitumor activity and toxicity of hetero-ligand vanadium(V) complexes, Djordjevic and Wampler (79) arrived at the conclusion that the hetero-ligand is able to affect the redox potential of the V(V)/V(IV) couple in such a way that intramolecular electron transfer can occur within the V(V)-peroxoadduct. As a consequence, vanadium(V) is reduced to the IV state, and the peroxo group is oxidized to a superoxide radical. It is conceivable that such a species is present also during the reaction of vanadium bromoperoxidase with H2O2. However, there is no evidence for a radical type of reaction with bro-moperoxidases. 

Figure 3.166 shows the reactivity and potential of these early transition metal pincer carbene complexes taking vanadium as an example. The broad range of compounds becomes possible owing to the rich redox chemistry of vanadium that makes different oxidation states easily accessible. Here, examples for vanadium(ll), vanadium(III) and vanadium(IV) are shown. Abstraction of halogen is facile making the introduction of different TT-donor ligands possible, even weak ones like acetonitrile. 

Catechols (and pyrogallols) readily reduce vanadium(V) to vanadium(IV) and, in some instances, further to vanadium(III). In the context of tunichromes as the presumed reducing agents in ascidians, the redox chemistry of catecholatovanadium complexes has been investigated to some extent. Results on reduction potentials for the and... 

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