Substituent Effects on the Heteroatom. Oxidation-Reduction Potentials

SUBSTITUENT EFFECTS ON THE HETEROATOM. OXIDATION-REDUCTION POTENTIALS 

It is well known that ferrocene and related metallocene compounds undergo reversible oxidation by a variety of chemical oxidants. The particular ease with which ferrocene undergoes oxidation to its ferricinium cation allows one to determine the effect of a series of substituents on the oxidation potential of the ferrocene system. Thus a sensitive probe is available for determining the electronic effect a substituent has on the iron heteroatom and a device is made available by which one might elucidate the mechanism by which such an effect is transmitted from the substituent to the iron atom in the ferrocene system. It is unfortunate that lack of suitable derivatiyes has precluded similar studies in systems containing other transition metal atoms. 

The most widely employed methods of following oxidation of ferrocene to the ferricinium cation involve use of potentiometric, chronopotentio-metric, and polarographic techniques 35, 59, 66). However, experimental difficulties within the polarographic method limit the usefulness of this method. Consequently, potentiometric and chronopotentiometric oxidations are most usually employed in these studies 21). 

The first attempt to investigate substituent effects by this oxidation technique was that of Kuwana and co-workers 38). They observed that the quarter-wave potential determined by chronopotentiometry varied considerably with the nature of the substituent. It was quite evident from their data that ferrocenes which contained electron-donating groups were more easily oxidized than ferrocene and those which contained electron-withdrawing groups were more resistant to oxidation than ferrocene itself. These results were not surprising since it was known that amino- (/) and hydroxyferrocene (5) suffered oxidation within a very short period when exposed to the atmosphere. 

Several amplifications of the results of Kuwana have since been published. Free-energy relationships between values and various substituent constants have been employed in an attempt to characterize more quantitatively the nature of the electronic effect transferred from the substituent to the reaction site, in this case the iron atom. Such studies have brought conclusions that the primary mode of transmittance of these effects involves inductive 38) and combined inductive-resonance parameters 25, 28, 29, 

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Substituent Effects on the Heteroatom. Oxidation-Reduction Potentials Interannular Electronic Effects. ... 

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