Redox behaviour anion effects

It needs to be pointed out that E values may also be quite sensitive to the nature of the solvent and supporting electrolyte used for an electrochemical study. Apart from solvation effects of the non-specific type, solvent molecules may occupy coordination sites in either the starting complex or the products and hence influence redox behaviour (Fabbrizzi, 1985). Similarly, the nature of the anion present may also strongly influence the redox potential if it has ligating properties (Zeigerson etal., 1982). Because of such effects, caution needs to be exercised in attempting to compare electrochemical data which have not been obtained under similar conditions. 

The thermodynamic stability of the radical anions has been determined from log K and are displayed in Table 1.11. The tetrasubstituted derivatives showed lower values of K, thus indicating that steric factors are responsible for this behaviour. The substituent effect in positions 2 and 3 on the redox potentials of the A,AT-dicyano-1,4-anthraquinonediimine series can be correlated with the Hammet a constants. The Hammet plot of E i2 v5. (73 [133] gives a p value of 0.123 with a correlation coefficient of r = 0.971. The comparison of this value with that reported by Aiimuller and Hunig [32] for other simpler DCNQI series (p — 0.490) indicates that the substituent effect over the ease of reduction decreases as the conjugation is extended. 

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