Evaluation of the extent of conjugation in symmetrical and asymmetrical aryl-substituted acetophenone azines using electrochemical methods

The electrochemical behavior of a series of symmetrical and unsymmetrical aryl-substituted acetophenone azines (1-X/Y, where X and Y are 4-NO2, 4-CN, H, 3-OCH3, 4-OCH3, 4-CH3, and 4-N(CH3)(2)) was studied in acetonitrile and N,N-dimethylformamide (DMF) solution using cyclic voltammetry (CV). Compounds 1-X/Y, where neither X or Y are nitro substituents, undergo successive reduction to their radical anion (1-X/Y.-) and then dianion (1-X/Y2-), respectively. In all cases, the formation of the radical anion is completely reversible and the standard reduction potentials, E degrees (.-)(1-X/Y/1-X/Y) could be determined. The reversibility of the second electron transfer is substituent dependent with certain dianions sufficiently basic to be protonated under our conditions. Standard reduction potentials (E degrees (.-)(1-X/Y/1-X/Y)) for the formation of radical anions exhibit a large substituent effect with values differing by more than 0.66 V throughout the series going from 1-4CN/4-CN to 1-4-OCH3/4-OCH3; similar substituent effects were determined for the formation of the dianion. The nitro-containing azines deviate from the above-mentioned behavior. With the exception of 1-4-NO2/4-NO2, they exhibit single electron waves that have values of E degrees (.-)(1-X/Y/1-X/Y). within 40 mV of each other and thus the reduction is not subject to the same substituent effect as the other azines. 1-4-NO2/4-NO2 exhibits an E degrees at a similar potential, but is a two-electron reversible wave with features indicative of a reduction system containing two localized, nonconjugated redox centers. The reduction potentials of all the aryl azines were correlated with Hammett sigma parameters to look at variations in E degrees (.-)(1-X/Y/1-X/Y) vs SCE as a function of substituent. The small rho values in combination with the other electrochemical data provide support for single bond character of the N-N bond and evidence for a lack of strong electronic communication between the two aryl centers through the azomethine bonds, especially for those systems with electron-withdrawing groups.