An experimental and theoretical study of the substituent effects on the redox properties of 2-[(R-phenyl)amine]-1,4-naphthalenediones in acetonitrile

We synthesized and analyzed 19 compounds of 3'- (meta-) and 4'- (para-) sub stituted 2-[(R-phenyl)amine]-1,4-naphthalenediones (PANs) R = p-MeO, p-Me, p-Bu, p-Hex, p-Et, rn-Me, m-Et, H, p-Cl, p-Br, m-F, m-Cl, p-COCH3, m-CN, m- NO2, m-COOH, and p-COOH. Despite the fact that the nitrogen atom, which bin ds the quinone with the meta- and para-substituted ring, interferes with th e direct conjugation between both rings, the UV-vis spectra of these compou nds show the existence of an intramolecular electronic transfer from the re spective aniline to the p-naphthoquinone Moiety. In accordance with this do nor-acceptor character, the cyclic voltammograms of these compounds exhibit two, one-electron reduction waves corresponding to the formation of radica l-anion and dianion, where the half-wave potential values vary linearly wit h the Hammett constants (sigma(x)). The analysis of the different voltammet ric parameters (e.g., voltammetric function, anodic/cathodic peak currents ratio, and the separation between the anodic and cathodic potential peaks) show that with the exception of the carboxylic PAN derivatives, all compoun ds present the same reduction pathway. We investigated the molecular and el ectronic structures of these compounds using the semiempirical PM3 method a nd, within the framework of the Density Functional Theory, using the Becke 3LYP hybrid functional with a double zeta split valence basis set. Our theo retical calculations predict that, with the exception of the p-nitro compou nd, all the compounds are planar molecules where the conjugation degree of the nitrogen lone pair with the quinone system depends on the position and magnitude of the electronic effect of the substituent in the aniline ring. The Laplacians of the critical points (del(2)rho), for the C-O bonds, show that the first reduction wave corresponds to the carbonyl group in a-positi on to the aniline, and that the second one-electron transfer is due to the C-4-O-2 carbonyl reduction. Thus, the higher reaction constant value (rho) obtained for the second one-electron transfer is due to the fact that the d isplacement of the nonshared electrons of the amino nitrogen merely modifie s the electron density of C-4-O-2 bond. The positive correlation between th e LUMO energy values calculated for these compounds and the E-1/2 potential s corresponding to the C-1-O-1 carbonyl reduction show that the electron ad dition takes place at the lowest unoccupied molecular orbital, supporting t he fact that this wave is also prone to the substituent effect.