Relationship between Molecular Structure and Electron Targets in the Electroreduction of Benzocarbazolediones and Anilinenaphthoquinones. Experimental and Theoretical Study

We report the synthesis and voltamperometric reduction of 5H-benzo[b]carbazole-6,11-dione (BCD) and its 2-R-substituted derivatives (R = −OMe, −Me, −COMe, −CF₃). The electrochemical behavior of BCDs was compared to that of the 2-[(R-phenyl)amine]-1,4-naphthalenediones (PANs) previously studied. Like PANs, BCDs exhibit two reduction waves in acetonitrile. The first reduction step for the BCDs represents formation of the radical anion, and the half-wave potential (E_1/2) values for this step are less negative than for that of the PANs. The second reduction wave, corresponding to the formation of dianion hydroquinone, has E_1/2 values that shift to more negative potentials. A good linear Hammett−Zuman (E_1/2 vs σ_p) relationship, similar to that for the PAN series, was also obtained for the BCDs. However, unlike the PANs, in the BCDs, the first reduction wave was more susceptible to the effect of the substituent groups than was the second wave, suggesting that the ordering of the two successive one-electron reductions in BCDs is opposite that in PANs. This is explained by the fact that the electron delocalizations in the two systems are different; in the case of BCDs there is an extra aromatic indole ring, which resists loss of its aromatic character. The electronic structures of BCD compounds were, therefore, investigated within the framework of the density functional theory, using the B3LYP hybrid functional with a double ζ split valence basis set. Our theoretical calculations show that the O₁···H−N hydrogen bond, analogous to that previously described for the PAN series, is not observed in the BCDs. Laplacians of the critical points (∇²ρ) and the natural charges for the C−O bonds indicate that the first reduction wave for the BCDs corresponds to the C₄−O₂ carbonyl, while in the PAN series the first one-electron transfer occurred at the C₁−O₁ carbonyl. Natural bond orbital analysis showed that, in all the BCDs, the lowest unoccupied molecular orbital (LUMO) is located at C₄, whereas for the PANs, the LUMO is found at C₁. The good correlation between the LUMO energy values and the E_1/2 potentials (wave I) established that the first one-electron addition takes place at the LUMO. Analysis of the molecular geometry confirmed that, in both series of compounds, the effect of the substituent groups is mainly on the C₄−O₂ carbonyl. These results explain the fact that reduction of the C₄−O₂ carbonyl (voltammetric wave II in the PANs and voltammetric wave I in the BCDs) is more susceptible to the effect of the substituent groups than is reduction of the C₁−O₁ carbonyl (wave I in the PANs and wave II in the BCDs).